SMALL-SCALE ELECTRICITY GENERATION USING COW
MANURE MICROBIAL FUEL CELLS
A Thesis
Presented to
The Faculty of Graduate Studies
of
The University of Guelph
by
CAIRAN O'TOOLE
In partial fulfilment of the requirements
for the degree of
Master of Science
May, 2008
© Cairan O'Toole, 2008 Library and Bibliotheque et 1*1 Archives Canada Archives Canada Published Heritage Direction du Branch Patrimoine de I'edition
395 Wellington Street 395, rue Wellington Ottawa ON K1A0N4 Ottawa ON K1A0N4 Canada Canada
Your file Votre reference ISBN: 978-0-494-42815-3 Our file Notre reference ISBN: 978-0-494-42815-3
NOTICE: AVIS: The author has granted a non L'auteur a accorde une licence non exclusive exclusive license allowing Library permettant a la Bibliotheque et Archives and Archives Canada to reproduce, Canada de reproduire, publier, archiver, publish, archive, preserve, conserve, sauvegarder, conserver, transmettre au public communicate to the public by par telecommunication ou par I'lnternet, prefer, telecommunication or on the Internet, distribuer et vendre des theses partout dans loan, distribute and sell theses le monde, a des fins commerciales ou autres, worldwide, for commercial or non sur support microforme, papier, electronique commercial purposes, in microform, et/ou autres formats. paper, electronic and/or any other formats.
The author retains copyright L'auteur conserve la propriete du droit d'auteur ownership and moral rights in et des droits moraux qui protege cette these. this thesis. Neither the thesis Ni la these ni des extraits substantiels de nor substantial extracts from it celle-ci ne doivent etre imprimes ou autrement may be printed or otherwise reproduits sans son autorisation. reproduced without the author's permission.
In compliance with the Canadian Conformement a la loi canadienne Privacy Act some supporting sur la protection de la vie privee, forms may have been removed quelques formulaires secondaires from this thesis. ont ete enleves de cette these.
While these forms may be included Bien que ces formulaires in the document page count, aient inclus dans la pagination, their removal does not represent il n'y aura aucun contenu manquant. any loss of content from the thesis. •*• Canada ABSTRACT
SMALL-SCALE ELECTRICITY GENERATION USING COW MANURE MICROBIAL FUEL CELLS
Cairan O'Toole Advisor: University of Guelph, 2008 Professor K. Farahbakhsh
Humanity is in the midst of a fossil fuel dependency that may leave it unable to adequately respond to future energy needs. As the number of inhabitants on earth increases, it is vital to find methods of energy generation coupled with organic waste utilization that are sustainable. This thesis is an investigation of manure microbial fuel cell (MFC) technology. Manure MFCs offer an additional opportunity to gain value with little reduction in the manure's soil building value, while concurrently reducing the manure's pollution potential. A variety of factors affecting power generation were investigated including ionic strength, temperature, suitable electrodes, and substrate consistency, all of which impact internal resistance. A scaleable MFC model was constructed based on knowledge gleaned from the pilot stage and was able to generated power densities as high as 5.46 mW/m2 with peak power of 85 uW. MFCs can be utilised to trickle charge ultracapacitors for battery operated applications. ACKNOWLEDGEMENTS
First and foremost I would like to thank my graduate advisor, Dr. Khosrow Farahbakhsh,
P.Eng., for valuable insight and mentoring into the development and framework of my research. His encouragement to participate in the first interdisciplinary degree between
Environmental Engineering and International Development Studies at the University of
Guelph is noteworthy. This has assisted in the broadening of my academic and practical vision to be of service to humanity.
I am also deeply indebted to Dr. Gordon Hayward, P.Eng. and Dr. Stefano Gregori for their invaluable assistance towards the understanding of microbiological functions and electrical circuitry respectively. It is important to thank Joanne Ryks, Allen Miller and
Ken Graham for assistance with practical laboratory analyses, circuitry troubleshooting and MFC construction.
Finally, to my family and friends, thank you for the encouragement to wage forward into the arena of research when things were frustrating and sharing in victories during this research process. Most importantly thank you to my dear wife Lua for her tireless support and patience through this endeavour.
"There is no field of human endeavour whether it be in industry or in agriculture, or in the preparation of food or in connection with the problems of shelter or clothing, or in the conservation of human and animal health and the combating of disease, where the microbe does not play an important and often dominant role"
Nobel laureate microbiologist Selman Waksman, 1942.
i TABLE OF CONTENTS
LIST OF TABLES viii
LIST OF FIGURES xi
1 Introduction 1
1.1 Statement of the Problem and Significance 1
1.2 Objectives and Scope of the Work 10
2 Analysis of the Current State of Knowledge of Microbial Fuel Cell
Technology 11
2.1 Introduction 11
2.2 Historical Background of Electricity and Fuel Cells 12
2.3 Principles of Operation of Microbial Fuel Cells 14
2.4 MFCs Compared to Enzymatic, Hydrogen and Methanol Driven Fuel Cells... 20
2.5 Calculation of Power Output and Current Density 21
2.6 Loses Impacting Voltage Production 23
2.7 Primary Mechanisms for Electron Transfer to Electrodes 24
2.8 Mediators and Redox Reactions 28
2.9 Mediatorless Microbial Fuel Cells 30
2.10 MFC Catalysts 32
2.11 External Resistance and Maximum Sustainable Power Generation 33
2.12 Internal Resistance, Electrode Spacing and Ionic Strength 35
2.13 Impact of Temperature on MFC Performance 38
ii 2.14 Impact of Electrode Material 39
2.14.1 Optimizing Electron Transfer at the Anode 42
2.14.2 Optimizing Reduction at the Cathode 44
2.15 Stacked MFCs Coupled with the Effect of Cell Reversal 47
2.16 Substrate 49
2.17 Electrochemically Active Microorganisms for Power Generation 51
2.18 Impact of Proton Exchange Membranes on Power Generation 53
2.19 Result of System Architecture on MFC Operation 55
2.20 Various Sources of Power for MFC Functioning 58
2.20.1 Rumen Fluid as a Substrate 58
2.20.2 Manure as a Substrate 59
2.20.3 Wastewater as a Substrate 60
2.20.4 Seafloor Sediments as a Substrate 62
2.21 Guidelines for MFC Comparison and Reporting 64
2.22 Limitations at Present in MFC Design and Operation 68
2.23 Challenges Associated with Scale up of MFCs 70
2.24 Practical Utilisation of Electricity Generated by MFCs 71
2.25 Summary and Conclusions 72 3 Optimization of Lab-Scale Production of Electricity Through the use of Cow Manure and Microbial Fuel Cell Technology 75
3.1 Introduction 75
3.2 Background Information 75
3.3 Materials and Methodology 77
iii 3.3.1 Measurement of Voltage 77
3.3.2 Manure Consistency and Preparation 77
3.3.3 Electrode Assembly 78
3.3.4 Cell Construction and Arrangement 81
3.4 Results and Discussion 85
3.4.1 Internal Resistance, Electrode Spacing and Ionic Strength 86
3.4.2 Usefulness of clay cups while increasing anode surface area 90
3.4.3 Effect of Altering Electrode Materials on Open Circuit Voltage 90
3.4.4 Effect of Altering Electrode Material Coupled with Dilution of Manure 91
3.4.5 Effect of Altering Anode and Cathode Surface Area in Manure MFCs... 92
3.4.6 Relationship between Power Production and Temperature 94
3.4.7 Performance of MFCs linked to Height of Water in the Cathodic Region95
3.4.8 Effect of Load on Power Production 97
3.4.9 Result of Increasing Clay Cup Size on Power Production 98
3.4.10 Rapid Opening and Closing of the Circuit 99
3.4.11 Effect of Increasing Ionic Strength of the Substrate 101
3.4.12 Comparison of Manure MFCs and 9 V Batteries under Constant Load. 101
3.5 Summary and Conclusion 103
4 Lab-Scale Production of Electricity Through the use of Cow
Manure and Microbial Fuel Cell Technology after Optimization of
Operational Parameters 104
4.1 Introduction 104
4.2 Materials and Methodology 104
iv 4.2.1 Polarization curve 107
4.3 Results and Discussion 107
4.3.1 Polarization Curve and Power Production 107
4.3.2 Effect of Molasses Addition on Power Generation 108
4.3.3 Increasing Substrate Ionic Strength Coupled with Aerobic Conditions. 109
4.4 Summary and Conclusion Ill
5 Conclusions and Recommendations 113
5.1 Physical Characteristics 113
5.2 Biochemical Characteristics 114
5.3 Operational Parameters 114
5.4 Recommendations for Future Research and Development 115
6 References 118
Appendices 131
A Background to Manure Microbial Fuel Cells 131
A.l Manure as an Energy Source 131
A.2 Manure Decomposition Processes 131
A.2.1 Aerobic Decomposition 132
A.2.2 Anaerobic Decomposition 133
A.2.2.1 Hydrolysis 133
A.2.2.2 Acidification 133
A.2.2.3 Biogas Formation 134
A.3 The Rumen of a Cow 135
A.4 Cow Manure 136
v A.4.1 Uses of Cow Manure 139
A.5 Ruminants 140
A.5.1 Ruminant Digestion and Microbes 141
B Calculation of Coulombic Efficiency and Energy Recovery...... 144
C The Potential of the Electrode and the Nernst Equation 145
D Economic Analysis of Alternatives to Power Generation with the use of Cow Manure 147
D.l Introduction 147
D.2 Background to Electricity Provision in Rural Areas 147
D.3 Challenges Associated with the Comparison of Different Energy Generation
Technologies 151
D.4 System 1: Drying and Direct Combustion of Manure 152
D.5 System 2: Small-scale Anaerobic Digesters 158
D.6 System 3: Microbial Fuel Cell Technology 166
D.7 Summary and Conclusions 169
E Dimensions of Electrode Materials 170
F Pictures of Electrode Material 173
G Details relating to Electrode Material Costs and Resistance 175
H Trial Configurations and Tests Conducted on each MFC 176
I Results from Trial Configurations 178
J Statistical Results 185
J.l Addition of Salt and Increasing Anode Surface Area 185
vi J.2 Usefulness of Clay Cups and Increasing Anode Surface Area 186
J.3 Altering Electrode Materials Coupled with Increased Dilution 186
J.4 Further Analysis of the Effect of Dilution 187
J.5 Temperature Effect on Power Generation 188
J.6 Electrode Spacing 188
J.7 Ionic Strength Effects 189
J.8 Electrode Material and Increasing Anode Surface Area 189
J.9 Altering Surface Area of Electrodes 190
J.10 Further Altering Surface Area of Electrodes 191
J.ll Height of Water in the Cathodic Region 191
J.12 Dilution Effect of Manure 191
J.13 Manure Consistency Effect 192
J.14 Effect of Load Resistor 192
J. 15 Location of Mixture on Interior or Exterior of Cup 193
J.16 Further Effect of Load Resistor 193
J. 17 Combination Effects of Load Resistor and Location of Mixture on Interior or
Exterior of Cup 193
Sample Calculations 195
K.1 Volume of a Cylinder 195
K.2 Surface Area of a Rectangular Prism 195
K.3 Price for cm2 of Electrode Material 195
K.4 Quantity of Salt Required to Achieved Desired Molarity 196
vii LIST OF TABLES
Table 1-1: Sources of light and their intensity 4
Table 2-1: Comparison of abiotic and microbial fuel cells 21
Table 2-2: Variety of possible MFC substrates 50
Table 2-3: Electrochemically active microorganisms 51
Table 3-1: Rectangular graphite felt dimensions for electrode configuration 80
Table 3-2: Graphite felt in cylindrical form in small cup 80
Table 3-3: Graphite felt in cylindrical form in large cup 80
Table 3-4: Dimensions of clay cups 85
Table A-l: Aerobic conditions and food source affect on microbial growth and by products 132
Table A-2: Anaerobic environment and food source affect on microbial growth and by products 133
Table A-3: Economic analysis of manure in Bangladesh and Canada 139
Table D-l: Sources of light and their intensity 150
Table D-2: Economic analysis of manure collection and sale in Bangladesh and Canada
154
Table D-3: Capital and Operating Costs of European Digestion Systems 163
Table D-4: Sample methane emissions calculations 163
Table D-5: Sample calculations of total expenditure 164
Table D-6: Net present value of the two alternatives 165
Table D-7: Capital and Operating Costs of microbial fuel cell 168
Table E-l: Woven carbon fibre 170
viii Table E-2: Uni-carbon fibre 170
Table E-3: Solid graphite 170
Table E-4: Uni-carbon fibre (increased surface area) 170
Table E-5: Woven carbon fibre (anode twice as large as cathode) 170
Table E-6: Uni-carbon fibre (anode twice as large as cathode) 170
Table E-7: Solid graphite electrodes 171
Table E-8: Solid graphite (for clay cup trial 2:1 ratio of anode to cathode) 171
Table E-9: Graphite felt (for clay cup trial 2:1 ratio of anode to cathode) 171
Table E-10: Woven carbon fibre (mini cup) 171
Table E-l 1: Woven carbon fibre (large cup) 171
Table E-12: Uni-carbon fibre (large cup) 171
Table E-13: Uni-carbon fibre (large cup, cathode Vi sized) 172
Table E-14: Woven carbon fibre (large cup, cathode twice size) 172
Table E-15: Uni-carbon fibre (large cup, cathode twice size) 172
Table H-l: Varying electrode material and salt addition 176
Table H-2: Temperature configurations 176
Table H-3: Ionic strength configurations 176
Table H-4: Further electrode material trials 177
Table H-5: Location of mixture on interior or exterior of clay cup 177
Table H-6: Manure dilution ratio 177
Table J-l: Statistical results of addition of salt and increasing anode surface area 185
Table J-2: Statistical results of usefulness of clay cups and increasing anode surface area
186
IX Table J-3: Statistical results of altering electrode materials coupled with increased dilution 186
Table J-4: Statistical results of further analysis of the effect of dilution 187
Table J-5: Statistical results of temperature effect on power generation 188
Table J-6: Statistical results of electrode spacing 188
Table J-7: Statistical results of ionic strength effects 189
Table J-8: Statistical results of electrode material and increasing anode surface area... 189
Table J-9: Statistical results of altering surface area of electrodes 190
Table J-10: Statistical results of further altering surface area of electrodes 191
Table J-ll: Statistical results of height of water in the cathodic region 191
Table J-12: Statistical results of dilution effect of manure 191
Table J-13: Statistical results of manure consistency effect 192
Table J-14: Statistical results of effect of load resistor 192
Table J-15: Statistical results of location of mixture on interior or exterior of cup 193
Table J-16: Statistical results of effect of load resistor 193
Table J-17: Further statistical results of combination effects 193
x LIST OF FIGURES
Figure 2-1: Various conversion methods of primary energy into electricity 11
Figure 2-2: Typical layout of a microbial fuel cell 14
Figure 2-3: Glucose metabolism and the electron transport chain of bacteria 17
Figure 2-4: Electron tower and voltage available 19
Figure 2-5: Potential losses during electron transfer in a MFC 24
Figure 2-6: Electricity production under four different mechanisms of electron transfer to the anode with glucose as the substrate 25
Figure 2-7: Images relating to bacterial nanowires 28
Figure 2-8: Power production for mediatorless MFCs based on published results 31
Figure 2-9: Various MFCs used in studies 56
Figure 2-10: Various MFCs used in continuous operation studies 57
Figure 2-11: Sediment MFC with reactions 62
Figure 2-12: Transmission electron micrograph of Geobacter on graphite anode 64
Figure 3-1: Connection between solid graphite electrode and wire 79
Figure 3-2: Cathode electrode before assembly (left) and after assembly (right) 80
Figure 3-3: Complete electrode assembly with wires attached and manure in place 81
Figure 3-4: Mason jar arrangement 82
Figure 3-5: Horizontal electrode arrangement 83
Figure 3-6: Clay cup arrangement with profile and top view 84
Figure 3-7: Large, small, and mini clay cup sizes 85
Figure 3-8: Electrode spacing and power production 87
Figure 3-9: Addition of sea salt to increase conductivity of catholyte 88
xi Figure 3-10: Ionic strength of the catholyte and resulting power production 89
Figure 3-11: Usefulness of employing clay cups while increasing anode surface area.... 90
Figure 3-12: Dilution (%) of manure with solid graphite and graphite felt electrodes 92
Figure 3-13: Results from altering anode and cathode surface area 93
Figure 3-14: Effect of increasing graphite felt and solid graphite anode surface area 94
Figure 3-15: Temperatures effect on power production 95
Figure 3-16: Results from altering height of water in the cathodic region 96
Figure 3-17: Effect of a variety of resistive loads on resulting power production 97
Figure 3-18: Results from various clay cup sizes in MFC operation 98
Figure 3-19: Open and closed circuited conditions for various surface areas 99
Figure 3-20: Rapid Vi hour open and closed circuited conditions 100
Figure 3-21: Effect of increasing ionic strength in substrate on open circuit voltage.... 101
Figure 3-22: Dilution (%) of manure compared to a 9 V battery 102
Figure 4-1: Exterior housing of the microbial fuel cell 105
Figure 4-2: MFC lid showing wire connections 105
Figure 4-3: Completed microbial fuel cell arrangement 106
Figure 4-4: Polarization and power-current characteristics of a manure MFC 108
Figure 4-5: Effect of various molasses to water ratios on voltage production 109
Figure 4-6: Increasing substrate ionic strength under anaerobic conditions 110
Figure 4-7: Effect of increasing ionic strength in substrate on power production 111
Figure A-l: Number of Different Rumen Microbes compared to Humans 135
Figure A-2: Rumen pH throughout a 24 hour period 136
Figure A-3: Cow manure 137
xii Figure A-4. Methane emissions in 1997 in Million Metric tons of Carbon Equivalents 138
Figure A-5: Drying cow manure for firewood 140
Figure A-6: Diagram of the digestive system of a cow 141
Figure A-7: Bacteria attacking a strand of fibre from the rumen of a cow 142
Figure A-8: Protozoan with a fungal spore on the side and rod-shaped bacteria underneath 143
Figure A-9: Protozoan during splitting into another cell 143
Figure A-10: Protozoal cell with cilia on the right side Source: (Russell 2005) 143
Figure D-l: Drying cow manure for firewood 153
Figure D-2: A plug flow anaerobic digester from a 500-cow dairy 161
Figure F-l: Woven carbon fibre electrode 173
Figure F-2: Uni-carbon fibre electrode 173
Figure F-3: Solid graphite electrode 174
Figure 1-1: Results from Vi hour open and closed circuited conditions 179
Figure 1-2: Results from 1 hour open and closed circuited conditions 180
Figure 1-3: Results from rapid 1 hour open and closed circuited conditions 181
Figure 1-4: Results from % and 1 Vi hour open and closed circuited conditions 182
Figure 1-5: Results from various open and closed circuited conditions of rectangular electrode 183
Figure 1-6: Results from various open and closed circuited conditions of exterior electrode 184
xni 1 Introduction 1.1 Statement of the Problem and Significance Societies demand enormous amounts of energy that result in greenhouse gas emissions which are linked to global warming. Since the first oil crisis of 1973 the worldwide perspective on energy has been altered (Blomen and Mugerwa 1993). Even at moderate costs energy production remains an important process in non-oil-producing countries (El-
Mashad et al. 2004). The vast majority of the developed world has attempted to reduce their dependency on oil through diversification of primary energy sources. As the number of inhabitants on earth approaches seven billion, it is vital to find methods of energy generation coupled with organic waste utilization that are sustainable for the future. "The building of a sustainable society will require reduction of dependency on fossil fuels and lowering of the amount of pollution that is generated. Waste treatment is an area in which these two goals can be addressed simultaneously. As a result, there has been a paradigm shift recently, from disposing of waste to using it" (Angenent et al. 2004).
Electricity provision is one of the major challenges experienced in economically developing countries (EDC), specifically in rural areas. These areas are traditionally the final locations within a country to receive electrification due to the expense of extending national power grids. In 2005 one quarter of the world's population had no access to electricity; eighty percent of which lived in rural areas of the developing world (Barnes et al. 1996). Without an adequate affordable energy supply it is almost impossible to conduct productive economic activity or improve health and education. This lack of
1 access to electrification frustrates economic development hindering improvement of the quality of life thereby condemning billions of people to a life of continued poverty.
Electricity provision to rural households in developing countries is an objective that has been on the agendas of EDC governments, countless donor agencies and international financing institutions for decades. One of the reasons of failure to achieve satisfactory results is the high cost of implementing power production facilities on the individual household scale and community level. According to Powers (2007) the capital expenditure cost of energy production from fossil fuel by conventional combustion processes, wind turbines, anaerobic digestion and chemical fuel cells is in the order of
$1.5 million per MW capacity installed.
It is worthwhile to mention that the power consumption pattern in these situations is often substantially lower than patterns of urban areas and hence the generating capacity required is significantly lower. The primary means of providing reliable energy is through grid electrification. The options available to governments to meet rural energy demands is to extend existing electrical grids, which can be extremely expensive when the distance is considerable; to create a small isolated grid that is powered by a small- scale generator; or to provide individuals households with batteries which are charged at a central stations powered by a small-scale generator (Siemons 2001). This investigation is a derivation of the latter option, to provide the home with batteries, however these batteries will not require charging at a central location since they produce electricity from
2 cow manure through the process of anaerobic decomposition. Refer to Appendix A for background information relating to manure MFCs.
Many different types of technology provide both economic as well as social benefits. The obvious advantage of alternate sources of energy to fossil fuel use, specifically for economically developing nations, is the decreased reliance on firewood for fuel which leads to the preservation of forests and the reduction in air pollution. The protection of forests has a direct effect on the water table level and reduced soil erosion. Deforestation forces those reliant on this type of energy to utilise less efficient sources of combustible energy, such as crop residuals, diverting these sources away from fertilizer applications.
Even though wood is a common source of fuel in developing countries, it is harmful to the user if burnt under inadequately ventilated conditions as is commonly the case. Non smoking women in Nepal and India who utilise biomass stoves for cooking purposes have higher-than-normal incidences of chronic respiratory diseases (Barnes et al. 1996).
Rural communities without access to electricity usually function with minimal amounts of lighting after sunset. The lighting in these areas is typically provided by candles or kerosene lamps while flashlights powered by batteries are used for intermittent portable use (Intermediate Technology Development Group 2006). World Bank rural energy surveys indicate that the majority of people in rural Bolivia spend a significant amount of money on candles, kerosene and batteries for lighting their homes. The report indicates the utilisation cost of a 20 W incandescent lamp would cost only a few dollars more and
3 would provide between 25 and 75 times more light than a candle (Barnes et al. 1996).
This is yet another way in which poverty is perpetuated.
Light is physically defined as electromagnetic radiation and liminous intensity in one direction is measured in candela (i.e. approximately the output from a standard paraffin- wax candle). The total luminous flux from a source is measured in lumens (lm). Table
1-1 outlines some common sources of light and their intensity. According to the Lighting
Research Center of the Rennselaer Polytechnic Institute in New York, 98 lumens are required at a minimum for reading when light emitting diodes (LEDs) are used for lighting. Two companies Cree, Inc. and Nichia Corp have developed white light LEDs with luminous efficacy of 131 lm/W and 150 lm/W at 20 mA respectively (Rennselaer
Polytechnic Institute 2002). With this type of LED the power required to achieve this lighting effect for reading purposes is 0.75 W and 0.65 W respectively, two orders of magnitude lower than that associated with a 100 W filament light.
Table 1-1: Sources of light Light Source Energy Source Luminous flux Efficiency dm) (lm/W) Candle Paraffin wax 1 0.01 Oil lamp Kerosene 1-10 0.01-0.1 Hurricane lamp Kerosene 10 - 100 0.1-0.2 Filament lamp - 3 W Electricity 10 3 Filament lamp - 40 W Electricity 400 10 Filament lamp -100 W Electricity 1300 13 LED - 1 W Electricity 150 150 Source: Adjusted from (Intermediate Technology Development Group 2006) and (Rennselaer Polytechnic Institute 2002)
The more important factor to the end user is often the cost per lumen as opposed to efficiency in lumens per watt. Although candle wax and kerosene are cheaper sources of
4 energy they produce poor quality light, large amounts of heat and a variety of combustion by-products. Even though kerosene lamps provide more light than candles they tend to be noisy, troublesome to light, pose a fire hazard, and potentially cause carbon dioxide poisoning. It is important to bear in mind that the price of kerosene is increasing in developing countries due to demand and is becoming increasingly scarce.
Biomass resources are a potential solution for this energy crisis particularly in developing countries. India, for example, relies heavily on biomass for their rural energy needs and the country's primary source of bio-resource is cow manure. This manure is a sustainable and readily available resource with almost 600 million tons of wet manure produced annually from 270 million head of cattle (Kishore et al. 2004). The usage of manure in a sustainable energy generation scheme has the potential to ease the fossil fuel dependency in rural areas to a limited degree.
Governments of economically developing countries have begun to recognize the usefulness and importance of energy provision to rural households to increase the standard of living as well as the quality of living. The Food and Agriculture Organisation
(FAO) of the United Nations has drawn parallels between the ability to have an alternate source of fuel and the improvement in cultural, recreation and spare time activities
(Marchaim 1992). The same study noted that the enhanced lighting effect of biogas combustion has allowed farmers in Jiangsu Province of China to be able to embroider, weave and tailor after dark thereby providing a source of secondary income. The mere availability of electricity would result in capacity building by alleviating the pressures on
5 children and women to gather fuel, thereby allowing them to participate in education and other forms of decent work. Many of these governments are working in collaboration with donor agencies and international nongovernmental organisations to achieve this goal.
Alternative sources of energy need to be explored and developed as energy from renewable sources may be a large portion of global energy production in the future. In modern day civilization batteries are the backbone of portable electronics which have become an integral component to our way of life. The limited lifetime of batteries is one of the major drawbacks associated with their use, having eventually to be recharged or replaced. An emerging technology called microbial fuel cells (MFCs) has the potential to generate electricity from material that is otherwise considered waste or of no inherent value. MFCs are bio-electrochemical transducers that convert the reducing power generated from the metabolism of organic substrates into electrical energy (Ieropoulos et al. 2005a). These MFCs convert biochemical energy into electrical energy, avoiding the use of heat which entails an increase in entropy.
Humanity as a whole is experiencing a decline in fossil fuel availability and the inevitable danger of environmental degradation is apparent. These MFCs have the ability to alleviate to a small degree the dependency on fossil fuels and provide clean energy.
These cells also pose a solution to global climate change concerns as they have the potential to be clean sources of energy since they are 'carbon-neutral', i.e. the oxidation of the organic matter only releases recently fixed carbon back into the atmosphere thereby not further exacerbating global warming through greenhouse gas emissions
6 (Lovley 2006c). These MFCs are similar to a battery except that MFCs are refuelled as opposed to being recharged. This refuelling makes these cells an attractive option since
MFCs recover the capacity to deliver power almost immediately while conventional batteries take several hours to recharge. Results indicate that MFCs are able to exhibit many of the desirable features of secondary storage batteries, including:
1. the ability to be recharged to their nearly original charge state following discharge;
2. no severe capacity fading on charge/discharge cycling;
3. the ability to accept fast recharge;
4. reasonable cycle life; and
5. low capacity loss under open circuit conditions as well as in prolonged storage
under idle conditions.
MFCs utilise live microorganisms as biocatalysts in order to metabolize organic substrates as fuel to obtain electron-bearing biological molecules such as NADH
(reduced nicotinamide adenine dinucleotide). In the absence of any regeneration of the substrate, the system will eventually run out of oxidizable fuel and the flow of electrical current will cease (Bennetto 1990b). Under these conditions the cell becomes a battery or basic galvanic cell, however if the supply of fuel is maintained continuously the cell becomes a fuel cell, which is capable of supporting electricity generation in the long-term.
Another form of microbial respiration has recently been discovered in which microorganisms conserve energy to support growth by oxidizing organic compounds to carbon dioxide with direct quantitative electron transfer to electrodes. Derek R. Lovely
7 recently coined the term "electricigen" for these microorganisms. Previously the terms anodophile and electrodophile have been used to refer to microorganisms associated with anodes but these terms refer to microorganisms that do not conserve energy for growth.
MFCs that utilise these electricigens have the potential of harvesting electricity from organic waste and renewable biomass. These MFCs can oxidize a wide variety of fuels such as marine sediments, wastewater and organic matter to generate power. These fuel cells operate in a self-sustaining manner, which removes the necessity for regular maintenance. Therefore these systems could be used as power sources in remote areas that are not serviced by centralised power grids and function with long-term stability. In some situations it is impractical, costly and time consuming to have to change batteries.
Hence MFCs would be a practical solution to this challenge utilising resources available at the locale where power generation is required.
Since various species of bacteria have different natures it is impossible for a single model of a MFC to work for all species. There are many different bacteria that live in a wide variety of habitats hence there is ample room for innovation. The two main classifications are mediator and mediatorless microbial fuel cells. It is common for MFCs to have a redox mediator to transport the electrons to the anode, which then travel through an external load to the cathode. The anode is immersed in the bacterial culture and typically kept anoxic, while the cathode can be submerged in liquid or exposed to air (Lovley
2006c). The fuel that feeds the cells is oxidized at the anode and the reduction of molecular oxygen takes place at the cathode. Proton exchange membranes (such as
8 Dupont Nafion ) are typically used to separate the anode and cathode chambers to inhibit oxygen transfer and allow for the movement of protons from the anodic region to the cathode electrode. The separation of oxygen from the bacteria intercepts the electron flow that would catalyze oxygen if it were present.
Electricity can be harnessed through the use of biodegradable material, such as cow manure, without the addition of chemicals if bacteria present in the material are utilised.
A number of iron-reducing bacterium, such as Shewanella putrefacians and Geobacter metallireducens, are capable of producing electricity. There are many other bacteria that are endogenously present in manure and wastewater that can also produce viable amounts of power. MFCs offer an additional opportunity to gain value through direct electrical generation with little reduction in the manure's soil building value, while concurrently reducing the manure's pollution potential (Powers 2007). Yokoyama et al. (2006) observed that after MFC utilisation the three essential fertiliser elements (nitrogen, phosphorous, and potassium) were retained at 84%, 70% and 91% respectively.
It is important to note there have been tendencies to skew results, such as recording the maximum voltage obtained when the cell is connected to a resistance as an indicator of performance instead of the steady state voltage which is typically a degree lower in magnitude. It is only recently becoming a common practise to report the value of the load resistance used leading to non-comparable MFCs and inconclusive arguments. Also a few researchers have only examined their MFC under a single load and imagine this result to showcase the MFC ability which may not be the case. It is challenging to compare the
9 performance of various MFCs since numerous approaches have occurred under a range of conditions. In particular there have been multitudes of types of electrodes, surface area ratios, working volumes, and conditions which further complicate the comparison.
It is important to provide power densities from polarization curves which outline the complex spectrum of MFC ability. Logan et al. (2006) provide a detailed explanation of how to obtain and analyse these curves. Furthermore many approaches to referencing power densities have emerged where some authors publish values based on mW/m3, others as mW/m2 of the total anode area, and others still as mW/m2 of the projected surface area of the anode. These values are non-comparable in their current units thereby increasing the need for standardization of reporting of MFC results.
1.2 Objectives and Scope of the Work
The objectives of the research project were to:
• investigate the potential for power production through the utilization of cow manure;
• evaluate the various properties of manure MFCs to eliminate factors that do not
directly impact the power generating potential and exploit those that increase power;
• improve the efficiency of MFCs by investigating limiting factors and stimulating
cellular metabolism of bacteria through the use of materials that promote cell growth;
• develop a manure MFC that would produce viable amounts of energy for a multitude
of low power applications such as LED lighting or rechargeable batteries;
• design a cell which could be maintained by persons of limited technical ability;
• and fabricate this MFC from locally available materials while minimizing the initial
investment and maximizing the lifetime of the system.
10 2 Analysis of the Current State of Knowledge of Microbial Fuel Cell Technology 2.1 Introduction
Societies demand enormous amounts of energy that result in greenhouse gas emissions which have been linked to global warming. Since the first oil crisis of 1973 the worldwide perspective on energy has been altered (Blomen and Mugerwa 1993). The vast majority of the developed world has attempted to reduce their dependency on oil through the diversification of primary energy sources. There are multiple ways in which primary energy is converted into electricity as can be visualised in Figure 2-1.
Electrical Energy i—r~i—t t t t Solar Electro Thermo Thermo - batteries mechanical electric ionic MHD T
Mechanical
Heat I—J Plasma i—i——i—*L
Artificial Radio Fission Fusion activity I Solar Energy Nuclear Energy Figure 2-1: Various conversion methods of primary energy into electricity Source: Modified from Oniciu (1976)
11 An emerging technology called microbial fuel cells has the potential to generate electricity from material that is otherwise considered waste or of no inherent value. Early in the 20th century Potter (1911) stated that "the disintegration of organic compounds by microorganisms is accompanied by the liberation of electrical energy." This electrical energy can be harnessed in a MFC to power circuits.
Chaudhuri and Lovley (2003) observed MFCs of Rhodoferax ferrireducens to rebound to the original levels of current production after 36 hours of being left as an open circuit.
These results indicate that MFCs are able to exhibit many of the desirable features of secondary storage batteries as noted earlier. MFCs operate in a self-sustaining manner and could consequently be used in remote areas not serviced by centralised power grids and function with long-term stability. In some situations it is impractical, costly, and time consuming to have to change batteries; hence MFCs would be a practical solution utilizing resources available at the locale in which the power generation is required. This chapter will detail the development of microbial fuel cell technology to date.
2.2 Historical Background of Electricity and Fuel Cells In the eighteenth century the linkage between electricity and physiological processes in living organisms was first observed when Luigi Galvani established the theory of "animal electricity" after noticing electricity production in the legs of a frog (Piccolino 1998).
Alessandro Volta's voltaic pile in 1800 was the forerunner to the battery with fuel cell concepts dating from 1802 and can be ascribed to Sir Humphrey Davy who made a simple fuel cell (Oniciu 1976). It was Sir William Robert Grove who designed a voltaic battery in 1839 and is widely considered the "Father of the fuel cell" since Davy's work
12 was not sufficiently published at the time (The Hebrew University of Jerusalem 2002).
Grove stated that "every chemical synthetic action may, by a proper disposition of the constituents, be made to produce a voltaic current" (Blomen and Mugerwa 1993). The first fuel cell vehicle emerged 120 years after German scientist Christian Friedrich
Schonbein first outlined that fuel cells were viable by precisely detailing the science behind a hydrogen-oxygen fuel cell in the Philosophical Magazine in 1839 (Kite 2006).
At the turn of the twentieth century the first reported MFC was demonstrated by Potter
(1911) to produce electrical energy from living organisms such as Escherichia coli or
Saccharomyces with platinum electrodes. It was only in 1931, after scientists had already demonstrated how the enzymes in bacteria oxidise food that Potter's MFC, which was ignored or forgotten until that time, was revived by Cohen (1931). Later the concepts of
MFCs were further elaborated and expanded upon in the 1980s by Bennetto (1990b).
For almost half a century scholars have predicted the utilisation of MFCs in daily life under a variety of applications. Some have proposed that they could be used as household electrical generators or to power portable electronics, car or boats (Bennetto 1990a) while others have developed functional cells that convert sewage and other organic waste into useful electricity (Angenent et al. 2004; Logan 2005b). Presently these applications are not practical for widespread use as MFCs only produce sufficient power for monitoring devices in remote locations as outlined by Tender et al. (2002). Logan et al. (2006) speculate that one of the first applications could be the development of pilot-scale reactors at industrial locations where high quality and reliable influent is available.
13 2.3 Principles of Operation of Microbial Fuel Cells
MFCs are bioelectrochemical transducers that convert the reducing power generated from the metabolism of organic substrate into electrical energy (Ieropoulos et al. 2005a).
Bacteria store this energy in the form of adenosine triphosphate (ATP). Some bacteria are able to oxidize the reduced substrates and transfer the electrons to respiratory enzymes by
NADH which is the reduced form of nicotinamide adenine dinucleotide (NAD+) (Figure
2-2) (Logan and Regan 2006b). nAAAr
Bacterium Membrane Anode Cathode Figure 2-2: Typical layout of a microbial fuel cell Source: (Rabaey and Verstraete 2005)
14 All living cells utilise energy through different forms of respiration. A form of microbial respiration has recently been discovered in which microorganisms, termed "electricigens", conserve energy to support growth by oxidizing organic compounds to carbon dioxide with direct quantitative electron transfer to electrodes (Lovley 2006c). Previously the terms anodophile and electrodophile have been used to refer to microorganisms associated with electrodes but these terms refer to microorganisms that do not conserve energy for growth. Bacteria, unlike plants and animals, do not employ the use of organelles to produce energy rather they perform these reactions themselves. MFCs that utilise these electricigens have the potential of harvesting electricity from renewable biomass such as marine sediments and organic matter.
MFCs can be single or dual chambered with anodic and cathodic compartments separated.
They consist of two electrodes: a negative anode and a positive cathode, and are based upon extraction and transfer of electrons from microbial cells onto the anode electrode.
The anode is immersed in the bacterial culture and typically kept anoxic while the cathode can be submerged in liquid or exposed directly to air. Under anaerobic conditions the substrate is oxidized at the anode thereby producing carbon dioxide, protons and electrons while the reduction of molecular oxygen takes place at the cathode as outlined below in Equation 2-1:
Anode: C12H22011(aq) +13H2Q(1) °^ »*•*- )12C02(aq) +48H^ + 48e The anode replaces the typical electron acceptor and the cathode presides in a more aerobic environment thereby creating a potential difference causing electrons to flow 15 from the anode through an external electrical circuit to the cathode. The current generated in a MFC cannot occur at a rate greater than the rate which the bacteria can oxidize a substrate and transfer the resulting electrons to the surface of the electrode. The protons travel through the electrolyte to the cathode where both the electrons and protons are consumed by reducing oxygen to water as outlined below in Equation 2-2: + Cathode: 02(aq) + 4H (aq) + 4e"q) -> 2H20(1) 2-2 The electron carriers (Figure 2-3) transport their electron load to a protein that is imbedded within the bacterial membrane. Electrons are then transported from one protein to another progressively down the electron transport chain with each protein oxidizing the one before. Some of the proteins along this chain utilise the electron's energy to pump an H+ ion (proton) across the membrane and out of the cell. One protein that synthesises ATP allows the protons to flow back across the membrane. Therefore under aerobic conditions some cells reduce oxygen to water while under anaerobic conditions another electron acceptor must be utilised such as the anode (Dartnell 2006). Scientists have observed some bacteria that can exogenously transfer electrons called exoelectrogens through the process of electrogenesis (Logan 2008). 16 Outside of cell Figure 2-3: Glucose metabolism and the electron transport chain of bacteria Source: (Dartnell 2006) Under anaerobic conditions electrons may be diverted from the respiratory chain by an oxidation-reduction mediator. Mediators penetrate bacterial cells in their oxidized form and interact with reducing agents (such as reduced cytochromes) within the cell thereby becoming reduced themselves. The reduced mediator is cell permeable and is therefore capable of diffusing out of the cells to the anode surface where it becomes electrostatically oxidised. This oxidised mediator is therefore able to repeat this cycle of transportation of electrons. This cyclic behaviour is only possible as long as anaerobic conditions are maintained since under aerobic conditions oxygen will gather the electrons as it has a greater electronegativity than the mediator. A portion of the metabolic 17 reducing power is continually drained off due to the cycling in order to give electrical power at the electrodes. Mediator interactions and cell metabolism also release protons in the anodic region that travel to the cathode through the solution (Ieropoulos et al. 2005a). Figure 2-4 outlines the potential energy available from reactants ranging from NADH to oxygen with more energy released depending on the difference between the two reduction potentials. The maximum potential for this process is approximately 1.25 V on the basis of the difference between glucose (-0.43 V) and oxygen (0.82 V) under standard conditions. For example if acetate was utilised instead of glucose there would be 0.15 V less potentially available for capture. Further elaboration of the maximum potential voltage of a MFC can be observed in Appendix C. For more redox reaction potentials refer to Rabaey and Verstraete (2005) and Du et al. (2007). Equation 2-3 below outlines how it is possible to calculate the total energy released during a reaction: AG = -nFAE 2-3 Where: AG = total energy n = the number of electrons exchanged F = Faraday's constant (96,485 J*V"1*mol"1) AE = difference between the reduction potentials of the electron acceptor and donor 18 C02/glucose (-430 mV r - Fr • -~*3 -03 |- CO2/acetate(-280mV). • -i • • S°/H S(-280mV)- 2 -0? \- i Ftavoprotein i * 0.1 |- iron-sutfui proteins 00 i- DtiinofiB Cytochrome be i 01 & a 02 Cytochrome c •5 03 a 04 Cytochrome aa3 N037N02" (+420 mV) as h 0.6 NO37N2(+740mV). Fe3+/Fe2+ (+760 mV) 02//H20 (+820 mV) *i Figure 2-4: Electron tower and voltage available Source: Adapted from Logan and Regan (2006b) and Madigan et al. (2005) Three common types of MFCs as proposed by Ieropoulos et al. (2005a) are: 1. Fuel cells with artificial or synthetic redox mediators added to the anodic region. 2. Fuel cells with natural mediating properties such as metal-reducing bacteria like Shewanellaceae or Geobacteraceae which have cytochromes attached to their membranes. These bacteria are capable of transferring their electrons directly onto the electrodes with special membrane bound cytochromes (Scholz and Schroder 2003). 19 3. Fuel cells capable of oxidizing fermentation products like methanol onto electrocatalytic electrodes (Jardon 2005). These chemically modified electrodes are capable of efficiently oxidizing such metabolites (Scholz and Schroder 2003). The first type of fuel cell discussed above with artificial redox mediators has shown relatively favourable current densities yet suffers from the need to use synthetic and often toxic redox mediators that are nonrecoverable from the cells. The second type is suitable for open natural environments like marine sediments, but the growth rate of the bacteria and the resulting current densities are very low. The third category of MFCs yields the largest power densities while functioning with simple and readily available microorganisms such as E. coli. One major drawback associated with their use, as pointed out by Ieropoulos et al. (2005a) is that the electrocatalytic electrodes require expensive surface modifiers that make implementation more costly. 2.4 MFCs Compared to Enzymatic, Hydrogen and Methanol Driven Fuel Cells Interesting to note from the developed world context, as pointed out by Lovley (2006c), is that "the ubiquitous and innocuous properties of fuels for microbial fuel cells alleviates the need for the complex and highly regulated distribution systems that are required for hydrogen and methanol." There are a number of significant advantages that MFCs have over hydrogen and methanol driven fuel cells that indicate their versatility for use. Larminie and Dicks (2003) in their book Fuel Cell Systems Explained adequately detail abiotic fuel cells which can be compared to microbial fuel cells as seen in Table 2-1. 20 'able 2-1: Comparison of abiotic and microbial fuel cells Abiotic Fuel Cells Microbial Fuel Cells Oxidation Expensive catalyst Microorganisms itself Operational temperature Elevated temperatures1 Ambient temperatures Condition of fuel Often require purification 'dirty fuels'4 Other fuel factors Toxic or highly explosive Commonly available Source: Adapted from Larminie and Dicks (2003) 1 hundreds °C 2 function at any temperature that the microbial community can accept 3 to prevent poisoning the catalysts 4 sediment or organic waste As the name implies enzymatic fuel cells generate electricity through cell extracts and enzymes rather than the whole cell. Although these types of fuel cells are better developed to date and are able to deliver high power relative to their size, they are mainly utilised in sensor applications. This is because they typically only employ a small percentage of the electrons available in organic fuels while MFCs have been shown to extract over 90% of the available electrons from organic matter (Lovley 2006c). 2.5 Calculation of Power Output and Current Density Since the current produced by MFCs is low, it is typically calculated using the measured voltage (EMFC) across the external resistance (Rext) with Equation 2-4: r _ p'MFC 2-4 R„ Current density (J) in Amperes per anode surface area, Aan, can be calculated through Equation 2-5: J = 2-5 21 It is most straightforward to calculate power output of a MFC from the measured voltage (EMFC) across the load and the resistance of the load itself as outlined in Equation 2-6: E2 P=^M£_ 2-6 Rext The power output is often normalised to the anode surface area Aan to facilitate comparison as observed below in Equation 2-7: P* =1ME£- 2-7 \nRext The power output formula (Equation 2-8) proposed by Cheng et al. (2006a) can be used for a MFC with approximately 2% error: * V2R p = oc ext 2-8 Aw l^int + Rext ) Where: Voc = open circuit voltage Rim = internal resistance of the MFC The true power output of a MFC can be obtained by subtracting the power generated in the absence of microbes from the power generated in the presence of microbes. Power can be normalised to one side of the anode if the anode was pressed onto a surface allowing only one side to be utilised for power generation. It is also possible for the anode to be surrounded by the anodolyte in which case the total anode surface area would be used for power normalisation. If the anode is not the limiting factor or the relative surface area of the anode is very high compared to the cathode it is common to normalise 22 power density to the cathode surface area. This can lead to an inflation of the referenced power density since a much smaller area is employed in the calculation. Since 2003, it has become much more prevalent for researchers to quote power as volumetric power (mW/ra3) since this provides power production based on reactor volume. The convention in environmental engineering is to reference power relative to the total reactor volume yet some researchers have normalised their values to the anode liquid volume perhaps in an attempt to "boost" their apparent power production in the literature. However this method of reporting power does not accurately reflect the size of reactor required to achieve the power potential since the electrodes tend to take up substantial amounts of space. This practise has led to erroneous reporting such as the case of Ringeisen et al. (2006) where their MFC produced approximately 500 W/m3 in a small reactor measuring only 1.5 cm . This arrangement was fed by a 100 cm reactor where cells were grown and pumped into the MFC. 2.6 Loses Impacting Voltage Production Rabaey et al. (2005c) have been able to achieve the highest voltages during current generation of 0.62 V yet the largest Voc reported are 0.83 V by Cheng and Logan (2007). Figure 2-5 provides a visual representation of various losses during electron transfer which impacts Voc. Loss #1 is associated with bacterial electron transfer, loss #2 with electrolyte resistance, loss #3 and #5 with the electrodes, loss #4 with MFC and membrane resistance, while loss #6 with electron acceptor reduction (Rabaey and Verstraete 2005). The theoretical value of 1.25 V cannot be achieved due to four main types of losses experienced in MFCs: 23 1. ohmic losses: such as those associated with the resistance in the cation exchange membrane, electrodes, interconnections, and compartment solutions, 2. bacterial metabolic losses, 3. mass transport or concentration losses of different species, 4. activation losses: due to the energy needed to transfer the electrons in an oxidation/reduction reaction (typically below 0.1 mA/cm2). I—VWVVi +840 ¥ -320 Figure 2-5: Potential losses during electron transfer in a MFC Source: (Rabaey and Verstraete 2005) 2.7 Primary Mechanisms for Electron Transfer to Electrodes To date four mechanisms for microorganisms to facilitate the transfer of electrons to the anode have emerged: 1. indirect electron transfer (Figure 2-6a), 2. electron transfer using artificial mediators (Figure 2-6b), 24 3. microorganisms which are capable of manufacturing mediators (Figure 2-6c), 4. electron transfer through electrically conductive pilus-like appendage or nanowires (Figure 2-7c). I Electron shuttle (reduced) | Electron shuttle (oxidized) I Electron tran^ort proteins ©©H3 12H20 Air cathode Figure 2-6: Electricity production under different mechanisms of electron transfer to the anode with glucose as the substrate Source: (Lovley 2006c) 25 Indirect electron transfer (Figure 2-6a) occurs through the interaction between the anode and the reduced metabolic products. This method is not efficient in transferring electrons to the anode since there is an accumulation of organic products in the anode chamber and the reduced metabolic products do not readily interact with the electrodes unless they are further broken down. Attempts have been made by Schroder et al. (2003) to increase the reactivity of the anodes by modifying their composition with metabolic end products. However these electrodes tend to foul with oxidation products. Katz et al. (2003) observed that anaerobic metabolism does result in one reduced product, hydrogen sulphide, which reacts readily with electrodes. Some species like the Desulfovibrio species only incompletely oxidize their organic electron donors to acetate, which cannot interact with electrodes, further limiting efficiency (Lovley 2006c). MFCs that utilise artificial mediators are better able to generate increased power production as a result of the mediator's ability to transfer electrons from within the cell to the anode. These MFCs are often termed mediator-driven since the electron-shuttling mediator accepts electrons from cell constituents that have been reduced and abiotically transfer the electrons to the anode. MFCs that employ the use of E. coli, Pseudomonas, Proteus, and Bacillus require mediators since these species are otherwise unable to effectively transfer electrons. Stams et al. (2006) and Rabaey et al. (2005b) suggest humic substances such as riboflavin, phenazines, and quinones as suitable exocellular electron mediators. Katz et al. (2003) offer a number of other common electron shuttles such as thionine, benzylviologen, 2-hydroxy-l, 4-naphthoquinone, phenazines, phenothiazines, phenoxoazines, iron chelates and neutral red (Lovley 2006c). 26 A number of microorganisms are capable of manufacturing their own mediators to promote extracellular electron transfer (Figure 2-6c). Since these species do not require artificial mediators to ensure power generation they are often superior in terms of power production. A disadvantage of utilising these types of bacteria is that they incompletely oxidize their organic substrates thereby limiting effectiveness of electricity generation. Some bacteria that utilise this pathway are: Geobacter sulfurreducens, Aeromonas hydrophila (Pham et al. 2006), Geothrix fermentans (Bond and Lovley 2005), and Rhodoferax ferrireducens species (Chaudhuri and Lovley 2003). Other species are capable of directly transferring electrons to electrodes through electrically conductive pilus-like appendages or nanowires (Figure 2-7c). Gorby et al. (2006) observed Shewanella oneidensis to exhibit this behaviour when there was a limitation in the soluble electron acceptor. Other species such as oxygenic phototrophic cyanobacterium Synechocystis PCC6803 and the fermentative bacterium Pelotomaculum thermopropionicum behave similarly. Since the latter two species are not metal-reducing bacteria and portray this mechanism of transfer it outlines that this is a common bacterial strategy to facilitate electron transfer. Logan and Regan (2006a) observed nanowires to participate in interspecies electron transfer (Figure 2-7d). It has also been observed that mixed cultures of bacteria found in sewage act in a similar manner (Chaudhuri and Lovley 2003; Park and Zeikus 2003; Liu et al. 2004; Liu and Logan 2004). Figure 2-7a provides a visual representation of pilus-type nanowires that have grown under electron-acceptor limited conditions with the Shewanella oneidensis MR-1 species 27 while Figure 2-7c provides a detailed view of the anode under colonization. The ridges and troughs along the entire length are consistent with a bundle of wires (Figure 2-7b) indicating that a number of these nanowires may bind together. Figure 2-7: Images relating to bacterial nanowires Source: (Logan and Regan 2006a) 2.8 Mediators and Redox Reactions It is common for MFCs to have a redox mediator to transport the electrons to the anode since most microbial cells are electrochemically inactive. An electron shuttle or mediator is a very large complex molecule that is able to take electrons from a molecule, float over to the anode, and release the electrons at this location. They are often used in fuel cells to 28 increase efficiency and are often manufactured by the bacteria themselves. Only recently has research by Kim et al. (1999) outlined the fact that chemical mediators are unnecessary for the functioning of these types of systems. The disadvantages associated with most artificial mediators are cost and fact that most are often toxic. Also in open environments it would not be possible to utilise mediators since they would be washed away during use (Bond et al 2002). With other variables held constant the lower the redox of the anode compared to the cathode the higher the output Voc. Efficient transport by mediators requires the anodic mediator possess a standard redox potential (Eo) which is negative enough in comparison to the anode electrode to be oxidised at their surface, while sufficiently positive compared to the biological electron carrier to extract electrons (Ieropoulos et al 2005 a). Ieropoulos et al. (2005a) further explained that the lowest redox (highest negative value of Eo) is not necessarily the best choice of mediator. They observed this trend when neutral red (NR) is used with E. coli. Although it has the lowest redox (Eo = -0.325 mV) and would be expected to produce the highest voltage and current, the outcome is otherwise. This suggests the possibility that NR is not the most efficient mediator when competing for electron transfer within the cell. One reason is that the difference in redox potentials within the cell and the highly negative NR may be too small to facilitate efficient electron transfer. The converse was true for MB which had a less negative redox (Eo = -10 mV) than NR, yet generated greater power output. 29 Lam (2005) establish the following features of an ideal redox electron mediator for MFCs: 1. have a redox potential sufficiently positive to oxidize NADPH, 2. form redox couples reversibly enough to give up the electron to the anode, 3. be stable in both oxidized and reduced forms to endure long-term redox cycling, 4. be soluble in aqueous media near neutral or physiological pH, and 5. be able to be absorbed into or diffuse through cell or organelle membranes 2.9 Mediatorless Microbial Fuel Cells As mentioned earlier there are a number of bacteria that are able to produce electricity and function without the need of a mediator. These MFCs do not require mediators but rather utilise electrochemically active bacteria to transfer electrons to the anode. Such metal reducing bacteria belong primarily to the families of Shewanella, Rhodoferax, Geobacteraceae, and fermentative bacteria such as Clostridium butyricum (Oh and Logan 2006). It has been observed that G. sulfurreducens species form a monolayer directly on the anode and use this as the end terminal electron accepter under anaerobic conditions (Bond and Lovley 2003). Chaudhuri and Lovley (2003) also witnessed this unique ability in species such as R. ferrireducens. It has been observed that in the absence of an exogenous mediator several bacteria are able to generate electricity from chemicals such as glucose {R. ferrireducens) (Chaudhuri and Lovley 2003), benzoate (G. metallireducens) (Bond and Lovley 2003), acetate and hydrogen (G. sulfurreducens) (Bond and Lovley 2003). The family Geobacteraceae is versatile since they are able to directly transfer electrons to the anode with simple organic 30 acids such as acetate. Therefore they rely on fermentative microorganisms to produce their required electron donors from sugars (Chaudhuri and Lovley 2003). Mediatorless fuel cells hold the most promise for future development with conversion efficiencies of 95% already experienced by Ieropoulos et al. (2005a). In less than a decade the power production in mediatorless MFCs has increased by several orders of magnitude (Figure 2-8). Power production is clearly limited by systems that have the cathode immersed in water (triangles) and sediment MFCs (diamonds). Higher power production has been achieved with air-cathode designs (squares). The circles represent MFCs with glucose, acetate and cysteine. It is important therefore to proceed with designs that contain an air-cathode. 10000| lOOth O CM 100| 0 o A 10j 6 A "Ufa. 1] M 0.1 i 0.01 O 0,001 1998 2000 2002 2004 2006 2008 Year Figure 2-8: Power production for mediatorless MFCs based on published results Source: (Logan and Regan 2006a) 31 2.10 MFC Catalysts Cathode catalysts remain essential to maximizing power output in MFCs. This was confirmed when tripling the surface area of the cathode resulted in an increased power density of only 22%. Cheng et al. (2006b) observed that there are other advantageous cathode catalysts like perfluorosulfonic acid (Nafion™), which performed better as a platinum (Pt) binder than polytetrafluoroethylene (PTFE). Later research by Cheng et al. TM (2006) observed diffusion layers (DLs) made of Nafion or PTFE which are hydrophobic can potentially improve the performance of the cathode by decreasing the water flooding of the catalyst. Four DLs increased the maximum power density from 620 mW/m2 to 766 mW/m while further increasing DLs resulted in a decrease in maximum power density indicating 4 DLs as optimum for this particular arrangement. These DLs reduce oxygen permeability through the cathode thereby increasing the Coulombic efficiency. PTFE is highly hydrophobic therefore limiting their effectiveness as it tends to dry the environment of the catalyst. Cheng et al. (2006) also observed that replacing the Pt catalyst with cobalt tetramethylphenylporphyrin (CoTMPP) improved the cathode performance above 6000 raA/m2 but reduced performance to levels below 40 mV at lower current densities. The power densities with these catalysts were fourfold higher than without. This study further detailed that the cathode can contain as little as 0.1 mg/cm2 Pt or the Pt can even be replaced with non-precious and hence less expensive metal catalysts such as CoTMPP with only slight reductions of 12% in performance compared to 2 mg/cm2 Pt. Zhao et al. (2006) realised it was possible to reduce the amount of platinum in this case since the cathode was not the rate limiting factor. Had it been the limiting factor then the Pt loading would have impacted power generation. 32 Other research has indicated the possibility of replacing platinum catalysts with less expensive non-precious ones using a layered bioelectrocatalytic cathode of cytochrome cxytochrome oxidase couple (Katz et al 1999), pyrolyzed Fe III phthalocyanine (Rosenbaum et al 2005), and CoTMPP (Zhao et al 2006). Biocathodes are another possibility where the bacterial metabolism is utilised to accept electrons from the cathode as explored by He and Angenent (2006). It is important to note that the longevity of such materials is not well documented or studied at present. 2.11 External Resistance and Maximum Sustainable Power Generation There is a direct relationship between the load resistance and the power generated by a MFC. Experiments conducted by Ieropoulos et al. (2005a) show that reducing the load resistor from 10 kQ to 1 kQ resulted in a five-fold increase in the power output with G. sulfurreducens. Under relatively high external circuit resistance (>10 kQ) the equilibrium potential of the cell initially generates a current (<100 pi A) that is lower than the maximum sustainable rate of charge transfer to/from the current-limiting electrode. Under these conditions the cell potential adjusts to the external resistance and the power generation is sustainable but lower than it could be if the resistance of the external circuit was lower. The converse is true for conditions under relatively low external resistance (<500 Q.) where a high instantaneous current (>600 ^A) is generated which is higher than the maximum sustainable rate of charge transfer to/from the current-limiting electrode. Under relatively high external circuit resistance the cell potential decreases quickly to adjust to the rate of charge transfer thereby effectively decreasing the current in the external circuitry. Ohm's Law describes the relationship between the current, resistance 33 and voltage. When the resistance is very low the resulting instantaneous power achieved by the cell is very high. This high power is not sustainable since the cell cannot deliver elevated power output at very low loads for a substantial period of time. When excessively small electrical resistors are used it has the potential to lead to erroneous assessments of the capabilities of MFCs. The current-limiting electrode is the electrode that exhibits the slower charge-transfer kinetics. One of the most important tasks in the evaluation of the power generation potential of a MFC is selection of the external resistor. Steady state is achieved when the power produced is equivalent to the power consumed for an extended length of time. Menicucci et al. (2006) have proposed a method of comparison for MFCs by which the maximum sustainable power is recorded. They utilise data loggers to take readings of current and cell potentials at 10-second intervals in which the external load resistor is reduced at 0.5 kQ/min. The average current value for each external resistance is then used to compute the power. Varying the external resistance, the cathode appeared to be totally unaffected in this case while the anode experienced large variations in cell potential over the 10 kQ range. To select the external resistance for maximum sustainable power generation of the MFC they plotted the relative decrease in anode potential (since the anode was the current-limiting electrode) against the resistance. The relative decrease in anode potential (RDAP) was calculated as outlined in Equation 2-9 below: E - E T)T\AT) o,anodic anodic n Q o, anodic Where: E0,anodic= initial anodic potential 34 Eanodic = anodic potential at each resistance The curve of RDAP against resistance yielded two regions where electron delivery was limited because of internal and external resistance. These two regions were linear and the intersection of these two lines yielded the small range of resistance (3 - 3.5 kQ) that would generate sustainable power. This resistor value for sustainable power was verified experimentally to ensure accuracy resulting in a resistor of less value causing a drop in cell current with time and resistors of higher value not altering the value of the current. The current author finds that this is a suitable method to quantify the value of the resistor which will yield sustainable power. It is important to note the bacterial community structure could change if too long a time is utilised for measurement or too short a time to allow for equilibration to occur. Growth and decay continue constantly in time, hence it is important to conduct tests for extended periods to ensure the resistor value obtained from this procedure is indeed suitable throughout the period required for power generation. 2.12 Internal Resistance, Electrode Spacing and Ionic Strength Power generation is directly related to the internal resistance and a decreased internal resistance will increase the power generation potential. The majority of MFCs are operated at neutral pH to accommodate optimum bacterial growth conditions unless the bacterium requires a specific pH. However the concentration of protons is low at neutral pH thereby increasing internal resistance to a high value compared to chemical fuel cells. 35 Two ways to reduce internal resistance without changing the pH are to increase the ionic strength (IS) or to decrease the electrode spacing. Liu et al. (2005a) found that increasing ionic strength from 100 mM to 200 mM resulted in a decrease in internal resistance from 161 Qto91 Q when domestic wastewater was utilised. It is important to note that it is not possible to continuously reduce internal resistance in this manner. For example further enhanced IS from 300 mM to 400 mM altered the internal resistance to 79 Q. from 83 Q, a nominal change for a increase of 100 mM. Increasing ionic strength by a factor of 4, from 100 mM to 400 mM only increased the power density a factor of 2 from 720 mW/m2 to 1330 mW/m2. Ionic strength affected the anode and cathode inversely, increasing the cathode working potential while decreasing the anode working potential when carbon paper was used for both electrodes in a chamberless design. It was also determined that the Coulombic efficiency and energy recovery were increased with increasing ionic strength (refer to Appendix A.l for more information). The four main methods to calculate internal resistance are: the polarization slope, power density peak, electrochemical impedance spectroscopy (EIS) and the current interrupt method. The first two are quick yet less accurate while the latter two are the preferred methods and require the use of a potentiostat. Aelterman et al. (2006) propose a scan rate of 1 mV/s when a potentiostat is employed. When the internal resistance is calculated in a MFC it is imperative that a connection is maintained for a period of time to allow for electrode colonisation of the bacteria and then the disconnection of the terminals to allow for an open-circuit voltage to develop for several hours. 36 The spacing of electrodes has a pronounced effect on power generation as examined by Liu et al. (2005b) with domestic wastewater. They observed that a decrease in spacing from 4 cm to 2 cm, while maintaining the ionic strength at 100 mM, increased the 2 2 maximum power generation of the cell from 720 mW/m (Rint=161 Q.) to 1210 mW/m (Rint=77 Q). Cheng et al. (2006a) observed that a decrease in electrode spacing from 2 cm to 1 cm, when glucose was utilised in a single chamber air cathode MFC, resulted in a decrease of power production per surface area from 811 mW/m2 to 423 mW/m2 even though the internal resistance decreased from 35 Q to 16 Q. This shows that there is a complex relationship between electrode spacing and internal resistance with no observable linear trends. Each MFC configuration and bacterium employed will yield various results when the electrode spacing and internal resistance are manipulated. Based on results from Liu et al. (2005a) and Liu et al. (2005b) it is clear that improvements in the potentials at the anode and cathode occur with a decrease in electrode spacing with low IS solution (100 mM) yet little improvement was observed with high IS solutions (400 mM). The same trend was observed for Coulombic efficiency and energy recovery. This was confirmed by Cheng et al. (2006a) when the ionic strength is high there was little result in modifying electrode spacing. Rabaey et al. (2003) achieved one of the highest power densities to date of 4310 mW/m with an internal resistance of only 3 CI. Min et al. (2005a) realised that the power densities associated with salt bridges 2.2 mW/m2 (Rint = 19920 Ci) are dramatically lower than those compared to a proton 37 2 exchange membrane (PEM) 41 mW/m (R;nt 1286 Q). The resulting low power output is attributed to the higher internal resistance of the salt bridge. A salt bridge is device used to connect the oxidation and reduction half-cells of a galvanic cell. It is possible to "fold down" paper electrodes to reduce the internal resistance of the MFC as was done by Ieropoulos et al. (2005b) where a 180 cm electrode was folded down to only 5 cm2 thereby decreasing the internal resistance of the cell. This folding of the electrode reduced internal resistance through a reduced surface area. Oh et al. (2004) observed that changing the size of ferricyanide cathodes from 22.5 cm2 to 2 cm2 (a factor of 11.3) increased the internal resistance by less than 2.2%. This indicates that under various compositions of electrodes and arrangements of the components of the fuel cell, the internal resistance can increase or decrease as a result of modifying the surface area of the electrodes. 2.13 Impact of Temperature on MFC Performance MFCs are commonly operated at elevated temperatures (30°C to 37°C) since temperature has a direct affect on bacterial kinetics, rate of mass transfer of protons and oxygen reaction rates. Lowering the operating temperature would significantly lower the cost of operation especially if the MFC is used in wastewater treatment. It is known that with each 10°C increase in temperature the chemical reaction rate coefficients are doubled. The effect of temperature on MFCs are seen to be independent of this relationship with only 9% reduction in power observed by Liu et al. (2005a) accompanying a decrease in temperature of over 10°C (from 32°C to 20°C ) with wastewater. 38 Hence temperature has not been observed to be the limiting factor in power generation with wastewater. Rather the performance of the cathode was the main factor affecting power generation at higher current densities (>2400 mA/m ). The same study noted that Coulombic efficiency are unaffected but energy recovery fluctuates with temperature. Bacteria often become inhibited at elevated temperatures as demonstrated by Powers (2007) where manure MFCs at 55°C ceased to produce viable power indicating that the electrochemically active bacteria are possibly inactivated at thermophilic temperatures. There is no optimum temperature range for MFC applications as each bacterial species has it own optimum conditions for growth. 2.14 Impact of Electrode Material Logan and Regan (2006b) point out that one of the greatest challenges at present to achieving substantially higher power densities in MFCs is the architecture of the cell and not the composition of the microbacterial communities. Therefore much of the current research is aimed at determining the optimal operating parameters and setup of the MFC. However it is vital that these investigations include a thorough study of specific microorganisms and bacterial flora since various parameters impact performance of MFCs in different ways. An integral component to the optimization of any MFC is the selection of suitable electrodes that maximize power production. Materials required for sustainable and efficient power generation need to be highly conductive yet non-corrosive with a high surface area per volume. It is vital to have an open structural arrangement to avoid fouling. Changing the anode from carbon paper to carbon cloth had negligible results in 39 wastewater applications examined by Liu et al. (2005a). However when the same was done for the cathode the maximum power density increased from 660 mW/m2 to 1114 mW/m2. The Coulombic efficiencies were similar under both types of electrode materials, however the energy recovery was greater at higher current densities for the carbon cloth electrodes. This implies that changes to one electrode cannot be implied of the other. Research conducted by Chaudhuri and Lovley (2003) revealed that increased power densities have been observed with increasing the surface area of electrodes. This study showed that current densities and cell densities remained roughly equivalent on graphite felt as graphite rods but the power density increased by a factor of three with a three-fold increase in surface area. However when porous graphite foam was used with the same geometric surface area as the graphite rods it produced 2.4 times more current. The porous material had almost three times as much concentration of cells. Therefore it is important to note that there is a difference between apparent surface area based on geometric calculations and volumetric surface area based on the volumetric calculations since some electrodes are porous. Park and Zeikus (2003) created Mn4+-graphite, Fe3+-graphite and neutral red woven graphite electrodes that proved remarkably efficient at power generation. When Mn4+- graphite was used as an anode and Fe3+-graphite was used as a cathode with sewage sludge bacteria the MFC produced 14 mA of current with current density values of 1750 mA/m2 and power density of 787.5 mW/m2. 40 The vast majority of research, such as that done by Chaudhuri and Lovley (2003) as well as Oh et al. (2004), have been done with similar sizes of anode and cathode. However a new generation of MFCs which have current densities on the order of one magnitude higher than those previously reported have been developed by Schroder et al. (2003). This was achieved through the use of a novel layered multifunctional anode consisting of a metallic electrocatalyst covered by an electrocatalytic conductive polymer and a regenerative-potential program applied to maintain the long-term electrode activity. The procedure utilised is to apply regular short oxidative-potential pulses to the anode. These pulses strip off the chemisorbed species thereby reactivating the surface of the metallic electrocatalyst. This procedure prevents the rapid diminution of anodic currents and increases the current densities to values of 15000 mA/m2, an order of magnitude higher than those observed by Park and Zeikus (2003). Power density is directly related to the square of the open circuit voltage, therefore effort needs to be invested to remedy defects causing lower electrode potentials. Often the anode potential is determined by the respiratory enzymes of the bacteria typically amounting to Ean = -300 mV and does not appear to vary substantially in different systems or with different substrates (Logan and Regan 2006b). However Ecat does vary depending on the oxidant and catholyte and is commonly around -500 mV when oxygen is utilised producing approximately 800 mV as the Voc. It is important to note that the Ecat is much lower than the -800 mV value from equilibrium calculations and even lower when there is a current flowing through (200 - 300 mV), therefore much work can be done in this area of MFC research. 41 2.14.1 Optimizing Electron Transfer at the Anode Anodes materials range from graphite rods (Bond and Lovley 2003), carbon cloth (Liu et al. 2004), carbon paper (Liu and Logan 2004), and woven graphite felt (Chaudhuri and Lovley 2003) to name a few. Some authors such as Shantaram et al. (2005) have toyed with the idea of sacrificial anodes in MFCs combined with the reduction of biomineralised manganese oxides. Such anodes can be composed of a magnesium alloy that dissolves according to Equation 2-10: MgM^Mg^)+2e^ 2-10 EpH=7.2 = -2.105 VSCE The potential of the above reaction is against a standard calomel electrode (SCE). The same study proved microbially deposited manganese oxides are superior to other cathodic reactants in MFCs. As mentioned earlier attempts to bioengineer electrodes have resulted in high power densities. It is important to remember that copper is not a suitable anode material since even trace copper ions are toxic to bacteria. Chaudhuri and Lovely (2003) observed that current increases with overall internal surface area in the order carbon felt > carbon foam > graphite. Rosenbaum et al. (2006) created a very successful anode based on tungsten carbide that produced current densities as high as 30000 mA/m2, more than double that experienced by Schroder et al. (2003). Further research will have to examine whether these improvements in performance outweigh the increased cost of production. Graphite fibre brushes, such as those patented by Logan (2005a), are an ideal candidate for anode electrodes due to their non-corrosive yet highly conductive properties as well as high surface area per volume combined with the open nature of their bristles which 42 reportedly minimises biofouling. Another property that makes them useful is the fact that individual graphite fibres are very small (7.2 fim), have a low resistance (0.00155 Q/cm) and are malleable enough to form good dispersal distribution of filaments. For example a brush 5 cm in diameter and 7 cm long has an area of 1.06 m2 and specific surface area of 9 -2 7170 m7mJ (Logan 2008). They can be made into any size or packing density but additional work is needed to optimize brush architecture (such as fibre density, length, and winds per length) to maximize power generation while minimizing the mass of material used (Logan et al. 2007). Using wastewater as substrate Oh and Logan (2006) realised that power production is a function of the surface area of the cathode and PEM relative to that of the anode. When a PEM smaller than the surface area of the electrodes is utilised, increasing the size of the anode with the cathode size held constant will not increase the power generated due to internal resistance. However with a PEM larger than the surface area of the electrodes 9 9 increasing the anode size from 6.5 cm to 22.5 cm (by 246%) increased the power from 0.26 mW to 0.39 mW (by 50%). For the anode to become the preferred electron acceptor it must have a higher or more positive potential than other substrates (Logan and Regan 2006b). Lower anode potentials initiate fermentation processes and cease electricity production. Reticulated vitrified carbon is a suitable candidate for anode electrodes since it is an excellent conductor at 200 S/cm (5 x 10"3 Q cm) with 97% porosity. The main disadvantage is in scale up since the material is quite brittle. While graphite granules have recently emerged as a popular choice for MFC tests they are not ideal due to their shape and bed porosity as well as the 43 fact that they are in contact only at small fractions of their total surface area reducing conductivity. 2.14.2 Optimizing Reduction at the Cathode Logan (2008) notes the design of the cathode is the single greatest challenge for making MFCs a useful and scaleable technology. Shantaram et al. (2005) realised the number of possible anodic reactions in MFCs is practically limitless while the number of cathodic reactions is much less diverse. Often abiotic reactions such as oxygen reduction, ferricyanide reduction and iron reduction occur there. Cathodes are commonly either platinum (Pt) coated carbon electrodes immersed in water that use dissolved oxygen as the electron acceptor or plain carbon electrodes in a ferricyanide solution (Oh et al. 2004). It is only more recently that cathodes exposed to air have begun to gain ascendency as a natural choice. Research by Liu et al. (2005a) observed that cathode performance is limited due to low temperature and proton concentrations combined with the nature of cathode material and their interaction with water. There are advantages in using dissolved oxygen in the cathode chamber as opposed to ferricyanide such as the ease of replenishment of oxygen by simple bubbling whereas ferricyanide must be replaced after it is depleted. However power generation is much higher when ferricyanide is used, rates as high as 7200 mW/m2 have been reported by Schroder et al. (2003). Logan et al. (2006) point out that the use of ferricyanide is restricted to fundamental laboratory studies as it is not a sustainable substance for MFC applications in practise. There is room for further improvements in cathode performance when utilising oxygen since the maximum potential, at a pH = 7, is 804 mV from the 44 Nernst equation. Present studies, such as those by Oh et al. (2004) have only achieved 268 mV. Cheng and Logan (2007) reported a maximum Voc of 830 mV with oxygen at the cathode but this was with addition of phosphate buffers to boost voltage. Zhao et al. (2006) found the reduction of molecular oxygen as the best choice for cathodic reactions in MFCs. Two reasons for this argument are that oxygen is readily available and has a positive redox potential making it superior as an electron acceptor. Research by Cheng et al. (2006b) note cathode performance as the limiting factor in the maximum power densities for air-driven MFCs in neutral pH mediums. This was attributed to the poor kinetics of oxygen reduction. The kinetics and thermodynamics of the electrocatalytic oxygen reduction are also influenced by other physical and chemical environmental effects such as temperature and concentration (Zhao et al. 2006). The cathode architecture is typically of two types: either a camberless cathode that is directly exposed to air on one side or a dual chamber design in which the cathodic region is filled with some dissolved electron acceptor such as ferricyanide. Air-cathodes without PEMs have been successful in wastewater applications by Park and Zeikus (2003) and Liu and Logan (2004) with power densities recorded at 788 mW/m2 and 494 mW/m2 respectively. Schroder et al. (2003) have achieved the highest maximum power density to date of 7200 mW/m by utilising ferricyanide as opposed to oxygen as the electron acceptor. Rabaey et al. (2004) have achieved the next highest values of 4300 mW/m2 by repeatedly feeding glucose to the reactor to allow the mediators to accumulate in the solution. In terms of air-cathode MFCs Cheng and Logan (2007) and Logan et al. (2007) have achieved 1970 45 mW/m and 2400 mW/m respectively. This was achieved through ammonia gas treatment of the electrodes coupled with elevation of temperature to 30°C and by utilising graphite brush electrodes. Research by Logan et al. (2007) also utilised ammonia treatment of the anode and the power density was normalised to the cathode surface area which was substantially smaller than the anode (1.06 m2 anode vs. 0.0007 m2 cathode). If the power was referenced to the anode it would have only been 1.6 mW/m2. There has been a tendency to reference power production according to the electrode with the smallest surface area to artificially raise the reported power density. Gil et al. (2003) indicate that graphite tends to experience poor oxygen reducing activity and hence might not be the optimum electrode for the cathode material. They observed oxygen limitations at DO concentrations of 6 mg/1 which is still quite high considering that a concentration of 5 mg/L DO is recommended for optimum fish health. Pham et al. (2004) observed that the addition of platinum helped reduce the required saturation level of DO to 2.0 mg/1. Other studies (Wang 2005; Park and Zeikus 2002; Cheng et al. 2006b; Rhoads et al. 2005) have shown there can be electrode modifiers utilised that not only reduce cost, as platinum is expensive, but increase power production. Counter to this argument however is research (Oh et al. 2004; Min and Logan 2004; Pham et al. 2004) indicating that when catholyte was sparged with pure oxygen amounting to 38 mg/L DO it did not increase the power output compared to a much lower level of only 7.9 mg/L. Pham et al. (2004) realised that as DO concentrations increase so did the rate of oxygen diffusion toward the anode hence instead of transferring the electrons through the electrode some of the substrate gets consumed by oxygen. As methods of preventing 46 oxygen diffusion to the anodic region are perfected then it will be worthwhile to increase DO concentrations in the cathodic region. MFCs that employ dissolved oxygen perform satisfactorily in aerated solutions such as seawater (Tender et al. 2002). However the costs associated with artificially aerating the cathodic solution is prohibitive in terms of scaling up and hence open air cathodes are the favoured option as noted by Min and Logan (2004) and Oh et al. (2004). The utilisation of other compounds besides dissolved or atmospheric oxygen as the cathodic electron acceptors have resulted in higher cell voltages being recorded such as with ferricyanide (361 mV) (Rabaey et al. 2003) and Mn02 (470 mV) (Logan and Regan 2006b). These voltages are elevated beyond that associated with oxygen since the concentrations of the compounds are increased. Rhoads et al. (2005) observed current densities (1000 mA/m2) almost 2 orders of magnitude higher than those associated with oxygen when biomineralised manganese oxides deposited by Leptothrix discophora were employed. The use of Mn02 or ferricyanide is unsustainable since soluble manganese is lost with time and ferricyanide must be externally regenerated. 2.15 Stacked MFCs Coupled with the Effect of Cell Reversal MFCs in parallel increase current with voltage being averaged while connections in series increase voltage with a common current throughout. When MFCs are operated in stacks the surface area of each cell contributes to the current while the voltage is dependent on the number of cells. When Aelterman et al. (2006) connected six MFCs of sludge in series and parallel the results were a six-fold increase in voltage and current respectively 47 as compared to a single MFC configuration. In parallel the cell achieved a current of 255 mA resulting in power densities of 140 mW/m2 and when operated in series achieved a voltage of 2.02 V resulting in 129mW/m2. Oh and Logan (2007) later reaffirmed what Aelterman et al. (2006) were the first to record, that maximum power outputs in MFCs were largely unaffected by parallel or series connections but more importantly individual MFCs could produce higher hourly averaged power output. This study observed unequal distribution of voltages at high currents (>30 mA) in series with voltage differences as high as 0.99 V between individuals cells. Aelterman et al. (2006) did not fully explore this situation or the long term effects on power production and thought cell reversal was primarily due to fuel starvation. These two issues were expounded on during recent experiments by Oh and Logan (2007) in which they document the causes and impact of prolonged reversal on power generation in air-cathode MFCs. This study is very important as it sheds insight into the performance of stacked MFCs of which there is little information in the literature. With high current in series some individual cells also changed polarity. This cell reversal is commonly experienced when drawing excessive current from a fuel cell at a rate higher than substrate delivery supports leading to an increased anode potential. Fuel starvation occurs with an inadequate supply of substrate and is another major cause of cell reversal. This can occur during a sudden change of fuel demand such as during start-up or change of load. Fuel delivery was not the cause of cell reversal in the particular case of Aelterman et al. (2006) rather it was a result of the limiting catalytic substrate conversion 48 properties of some microbial consortia. This process can also occur with insufficient oxygen at the cathode, impedance differences, and a lack of effective catalyst. Oh and Logan (2007) note the presence of bacteria was not necessary to experience cell reversal. Cell reversal is also experienced when very low external (<30 Q.) resistances are utilised. The main challenge in stack MFCs is to obtain useful power by avoiding cell reversal. This reversal can also occur if the voltages in the cells are not similar. One manner in which this challenge has been overcome is to employ the use of diodes in parallel to short circuit cells with reversed polarity. This will automatically remedy defective operation in an economic manner. It is also vital to ensure adequate substrate is available at the anode and oxygen at the cathode. Batch fed cycles have the propensity to encounter difficulties due to substrate depletion and hence it is recommended to operate stacked MFCs in continuous mode. Attempting to run stacked MFCs at high current densities (>1000 mA/m ) has led to reversal and maintaining lower current densities would avoid this pitfall. Further research must be done in the area of controlling cell reversal and varied approaches to the temporary isolation of the fuel cells that experience this defect. 2.16 Substrate Various researchers have outlined that MFCs can employ a wide range of substrates such as those found in Table 2-2. It is worth mention that a clear trend has been established between decreasing the proportion of substrate concentration at lower acetate concentration and voltage production (Min et al. 2005b; Oh and Logan 2007; Liu and Logan 2004). Others have also revealed that complex substrates can be utilised such as seafloor sediments (Tender et al. 2002), landfill leachate (Frew and Christy 2006), 49 lignocellulose (Rismani-Yazdi et al. 2006), and cysteine (Logan et al. 2005). For more substrates refer to Liu and Logan (2004). Tayhas and Palmore (2004) note that when compared on a volume basis glucose is the most energy-dense of the three common fuels for fuel cells: hydrogen, methanol and glucose. Table 2-2; Variety of possible MFC substrates Open Surfac Circuit e area Power Current Resistor Voltage Voltage (cm2)* density density Substrate (Q) (mV) (mV) § (mW/m2) (mA/m2) Reference Acetate* 218 2810 798 ± 7.1 506 1800 (Liu et al. 25 2005b) Bovine 1000 321 NA 7.1 354 1100 (Heilmann serum and Logan albumin 2006) Butyrate 1000 469* 795 ± 7.1 305 650 (Liu et al. 25 2005b) Domestic 1000 320(no 146 (no (Liu and wastewater PEM) PEM) Logan 145 28 2004) (PEM) (PEM) Glucose NA NA NA 50 3600 NA (Rabaey et al. 2003) Meat 1000 120" NA 7.1 139 1150 (Heilmann packing and Logan wastewater 2006) Peptone 1000 316* NA 7.1 269 850 (Heilmann and Logan 2006) Swine 200 189 320 11.25 261 1380 Min et al. wastewater 2005b V No PEM e Not provided so interpreted from current density and power density Projected surface area § Same surface area for anode and cathode NA means not stipulated in article Liu et al. (2005b) observed current densities with acetate (2200 mA/m2) are superior to butyrate (770 mA/m2) when wastewater is employed implying that faster bacterial uptake 50 is experienced by acetate. Also worthy of analysis is the fact that the polarisation curves for glucose, acetate, and butyrate are all similar in terms of Voc and cathodic cell potentials at low current densities (Logan and Regan 2006b). 2.17 Electrochemically Active Microorganisms for Power Generation Table 2-3 outlines a variety of electrochemically active microorganisms along with the electron donor and mechanism of transfer. The majority of phylum that comprise this list are proteobacteria such as Escherichia and iron reducing organisms such as Rhodoferax. Table 2-3: Electrochemically active microorganisms Microorganism Substrate Electrode Current Power Power Reference material (mA) (mW/m2) (W/m3) Axenic cultures Brevibacillus agri sludge graphite 255 140 258 (Aelterman granules etal. 2006) Erwina dissolvens glucose woven 0.7 0.27 NA (Park and graphite Zeikus 2003) Escherichia coli lactate woven 3.3 1.2 7.6 (Park and graphite Zeikus 2003) Escherichia coli lactate plain 2.6 91 3.6 (Bond and graphite Lovley 2003) Geobacter acetate plain 0.4 13 0.35 (Rabaey et sulfurreducens graphite al. 2005a) Pelotomaculum glucose plain 2.7 0.06' (Gorby et thermopropionicum graphite al. 2006) Proteus vulgaris glucose glassy 0.8 4.5 18 (Chaudhuri carbon and Lovley 2003) Pseudomonas glucose plain 0.1 88 8.8 (Chaudhuri aeruginosa graphite and Lovley 2003) Rhodoferax glucose plain 0.2 8 0.25 (Chaudhuri ferrireducens graphite and Lovley 2003) 51 Rhodoferax glucose woven 0.57 17 1.7 (Kim et al. ferrireducens graphite 2002) Rhodoferax glucose graphite 0.45 33 0.96 (Delaney ferrireducens foam et al. 1984) Shewanella lactate woven 0.04 0.00032 0.08 (Vega and putrefaciens graphite Fernandez 1987) Mixed culture Activated sludge wastewater woven 0.2 8 1.6 (Bond et graphite al. 2002) Activated sludge lactate plain 2.6 788 32 (Kim et al. graphite 2004) Activated sludge lactate woven 11 5.1 34 (Park and graphite Zeikus 2003) Activated sludge glucose woven 0.9 494 13 (Park and graphite Zeikus 2003) Mixed seawater acetate plain 0.23 10 NA (Liu and graphite Logan 2004) calculated from available data power output was calculated as an average Some of the most predominant species in highly anoxic conditions are from the Geobacteraceae family (Lovley 2006b) such as G. sulfurreducens (Bond and Lovley 2003), G. metallireducens (Bond et al. 2002) and the psychrotolerant Geopsychrobacter electrodiphilus (Holmes et al. 2004). Other bacteria from marine sediments that are iron reducing include the R. ferrireducens (Chaudhuri and Lovley 2003) and Desulfuromonas acetoxidans species (Bond et al. 2002). Bond and Lovley (2005) discovered that G. fermentans were able to completely oxidize organic compounds linked to electrode reductions. Many of these species have a wide range of operational temperatures such as G. electrodiphilus which Holmes et al. (2004) 52 showcased as able to transfer approximately 90% of the available electrons while experiencing growth at temperatures between 4 and 30°C. This versatility is important since it could be employed in a wide range of applications. Mixed bacterial communities are superior to pure cultures since these mixed communities actively take advantage of hydrolysis, fermentation, and anaerobic oxidation performed by other species to provide readily degradable substrates. This combination of fermentative microorganisms coupled with the oxidation of fermentation products allows these MFCs to have a more competitive process (Lovley 2006b). 2.18 Impact of Proton Exchange Membranes on Power Generation Proton exchange membranes typically separate the anode and cathode chambers to inhibit oxygen transfer and facilitate the movement of protons from the anodic region to the cathode electrode. The isolation of oxygen prevents the interception of the electron flow that would catalyze oxygen if present. There are some limitations to the oxygen impermeability of PEMs since a value of 9.3* 10~12 mol/cm*s has been recorded by Min et al. (2005a) implying minute infiltration of oxygen through the membrane. Liu and Logan (2004) measured the oxygen diffusion rate through a Nafion 117 membrane as 0.05 mg/h. A variety of methods have been proposed by Min et al. (2005a) to combat oxygen infiltration. Most notable was a chemical oxygen scavenger, 1-cysteine, to increase power production in a pure culture of G. metallireducens. PEMs in microbial fuel cells are typically composed of Dupont® Nafion™ which is hydrophilic yet capable of absorbing water, swelling by as much as 50% from their 53 original volume (Lam 2005). One major limitation with the use of Nafion PEMs is the high cost of $820/m2 - $l,380/m2 as well as the fact that Nafion™ membranes are sensitive to fouling as a result of ammonia. It is important to note that although most TM MFC experiments with PEMs have employed the use of Nafion materials Rabaey et al. (2004) note Ultrex cation exchange membranes (also classified as PEMs) yield superior results. With the substantially reduced cost associated with Ultrex membranes ($152/m ) and their improved performance it is an obvious choice to utilise them in MFCs. The main disadvantage associated with PEMs is the potential for loss of substrate due to aerobic oxidation by bacteria in the anode chamber from oxygen penetration of the PEM. PEMs have been recorded by Liu and Logan (2004) to lower power densities of MFCs. MFCs using wastewater had a power density of 146 mW/m2 without a PEM and only 28 mW/m2 with a PEM (5.2 times lower). Similar results by Liu et al. (2005b) indicate that PEMs reduced the power density generated from 506 mW/m2 to 328 mW/m2 for acetate and from 305 mW/m2 to 194 mW/m2 for butyrate when utilised with wastewater. The decreased power generation is because PEMs are a source of internal resistance to the system as they resist the flow of proton transfer to the cathode (Liu and Logan 2004). Oh and Logan (2006) realised when the anode and cathode surface area is fixed, the power density will increase with an increase in the size of the PEM. This study observed 2 2 2 2 an increase in the order 45 mW/m (APEM=3.5 cm ), 68 mW/ra (APEM=6.2 cm ), and 190 2 2 mW/m (APEM=30.6 cm ). The study also noted that PEM surface area limited power production when the surface area of the electrodes was larger than the PEM due to 54 increased internal resistance. Kim et al. (2005) have documented that the size of the PEM and the cathode can limit power generation in a dual chamber MFC. Zhao et al. (2006) were the first to discover that PEMs affect the pH gradient of a solution. They noted a significant pH gradient was established between the anode and the cathode which may inhibit the oxygen reduction potential at the cathode and negatively impact the microbial activity at the anode. Therefore it is recommended that MFC designs without PEM be further explored due to the above mentioned reasons. 2.19 Result of System Architecture on MFC Operation A multitude of MFC designs have emerged as portrayed in Figure 2-9 and Figure 2-10. A popular layout employs a PEM in a tube to separate two compartments (Figure 2-9F). This setup allows for gas or liquid to be present in the cathodic and anodic chambers. Logan et al. (2006) explain that this system typically produces low power densities and is best utilised for basic parameter research such as investigating what types of microbial communities arise during the degradation of specific substrates. This design is also useful for the examination of power production using new materials for electrode configurations. A common chamberless MFC design where the cathode is exposed to air on one side and the anode is sealed against a flat plate at the other in seen in Figure 2-9E. Other designs include a photoheterotrophic MFC (Figure 2-9D) where light is used as the energy source through the metabolic activity of R. sphaeroides (Rosenbaum et al. 2005). 55 The MFC designed by Rabaey et al. (2003) is comprised of four cells in which a PEM is employed to separated the chambers (Figure 2-9B). This design has achieved one of the highest power densities of 3600 mW/m2 using dissolved ferricyanide in the cathodic region. This design was later tweaked by Rabaey et al. (2005d) to create a MFC (Figure 2-9C) which has a continuous flow-through anode of granular graphite matrix with proximate anode-cathode placement. This arrangement was capable of generating a maximum power density of 1219 mW/m2 (1 kg COD/m3). Some MFCs utilise a salt bridge to connect the two compartments as designed by Min et al. (2005a) (Figure 2-9A). Figure 2-9: Various MFCs used in studies Source: (Logan et al. 2006) Efforts to increase power production have advanced these basic designs into continuous flow MFCs like those in Figure 2-10. The chamberless tubular continuous flow MFC (Figure 2-10A) proposed by Rabaey et al. (2005c) has achieved a maximum power density of 27.8 mW/m2 when used with acetate. The up-flow MFC (Figure 2-10B) 56 created by He et al. (2006) was operated under continuous flow and was capable of power densities as high as 636 mW/m2. MFCs have also been operated under plug flow conditions in a flat plate setup (Figure 2-IOC) with domestic wastewater and have achieved power densities of 72 mW/m2 (Min and Logan 2004). Liu et al. (2004) explored the effect of increasing the number of anodes to 8 while maintaining a single air cathode (Figure 2-10D). This arrangement was utilised with domestic wastewater and resulted in a maximum power density of 26 mW/m2 with an internal resistance of 69 ohms. Aelterman et al. (2006) have experimented with grouping MFCs into a stacked configuration (Figure 2-10E) with graphite granules electrodes achieving 146 mW/m2 with a hexacyanoferrate cathode. Figure 2-10: Various MFCs used in continuous operation studies Source: (Logan et al. 2006) 57 Single chamber design MFCs can be applied to marine sediments and are more practical and less expensive as there is no need for aeration. Park and Zeikus (2003) documented that similar amounts of electricity are generated from single chamber designs as from the more complex dual chamber designs when sewage sludge was utilised. Dual chamber MFCs are typically separated by a PEM and the main disadvantage that Liu and Logan (2004) associate is that the solution must be aerated to provide oxygen to the cathode. 2.20 Various Sources of Power for MFC Functioning 2.20.1 Rumen Fluid as a Substrate Researchers at Ohio State University have generated electricity from bacteria that occurs naturally in the rumen of a cow. The rumen is the first of four stomachs that break down grass into digestible mush. Rumen MFCs optimized through an extensive process of refinement and medium provision beyond normal conditions yield 66 mW/m (Rismani- Yazdi et al. 2007a). These cells were also stirred until a steady state current production was achieved. Catholyte was constantly replenished and pH adjusted to maintain a value of pH 6.8 since this was the value Hu et al. (2004) stipulated as the optimum pH for anaerobic degradation of cellulose by rumen microorganisms. Without the addition of external redox mediators rumen MFC yield a maximum power density of 55.3 mW/m An important observation by Rismani-Yazdi et al. (2007a) was that cathodic potentials did not change while the anodic potentials ranged from 201 mV to 13 mV under an external load of 20 Q to 1000 Q. They noted that initial external resistance of the fuel cells can affect the maximum sustainable power output of MFCs with 20 Q (66 mW/m2), 249 Q (57 mW/m2), and 1000 Q. (19 mW/m2). They attribute this strange behaviour to 58 the fact that various anodic potentials promote enrichment of a multitude of electrode- reducing bacteria when a consortium of microorganisms is used as the biocatalyst. This has been shown by Rabaey and Verstraete (2005) to affect the anodic microbial metabolism and electron transport mechanisms which directly effect power output. 2.20.2 Manure as a Substrate Manure MFCs by Yokoyama et al. (2006) produced only 0.34 mW/m2 with cow manure as the substrate while that of Scott and Murano (2007) achieved a value of 10 mW/m2 with 0.5 mg/cm Pt loading. This latter manure MFC had the typical optimizations i.e. platinum loaded electrodes as well as gas sparging of both the anodic and cathodic compartments. These electrodes were further enhanced with gas diffused PTFE suspensions onto the surface. Platinum coated electrodes increased power production 2 fold and increased the Voc to 700 mV from 410 mV. This platinum catalyst was replaced with nickel and caused a decrease in Voc to less than 250 mV. This platinum coating was applied to the anode causing peak power production to plummet to 0.65 mW/m . Therefore this implies that platinum while a suitable cathode catalyst should not be applied to the anode in manure MFCs. Scott et al. (2007) obtained a Voc of 510 mV and 530 mV with manure MFCs exposed to and covered from the atmosphere respectively. These cells achieved 2.5 mW/m (covered from the atmosphere) and only 1.9 mW/m2 under aerobic conditions implying that anaerobic conditions as a result of being covered from the air do not substantially impact power production. Scott et al. (2007) further experimented with a large coiled MFC and 2 obtained Voc values of only 470 mV and 30 mW/m . This increase in over an order of 59 magnitude was attributed to the greater cathode to anode area as well as the increased supply of air since the tubular design only employed the use of seawater at the cathode. Powers et al. (2007) observed power densities of 116 mW/m2 with undiluted manure but this peak occurred within the first 24 hours of operation and was not sustainable. That group also experimented with the combination of anaerobic digestion of manure prior to MFC application and found this to be a possible configuration if necessary. 2.20.3 Wastewater as a Substrate Oh and Logan (2006) have conducted experiments on anaerobic sludge as well as wastewater from a primary clarifier and found them to be suitable biocatalysts for electricity production providing 168 mW/m2. For a thorough review of electricity production from industrial and agricultural wastewater refer to Angenent et al. (2004). If seawater could be utilised to supplement wastewater as is the case in Hong Kong it may be feasible to utilise this increased ionic concentration to achieve higher power production (Liu et al. 2005a). Otherwise it would not be feasible to attempt to increase the ionic strength to achieve increased performance of the MFC. Even though there are advantages associated with reduced electrode spacing there are significant drawbacks in wastewater applications since a high void volume and greater electrode spacing is required to prevent clogging and fouling of the system as a result of build up of bacteria in the system (Liu et al. 2005b). Short-circuiting is also possible if the electrodes are placed too close to one another. 60 The majority of bacteria in anaerobic wastewater sludge consist of fermentative bacteria, methanogens, and sulphate reducers. Kim et al. (2005) have found that it is possible for non-electrochemically active bacteria to occupy space on the electrodes that are used in wastewater treatment during the initial inoculation step thereby preventing efficient power generation in MFCs by the electricigens. It is common to utilise acetate as a suitable electron donor for dissimulatory iron-reducing bacteria because it is nonfermentable as a substrate and since dissimulatory iron-reducing bacteria have been estimated to comprise as much as 3% of the total bacteria in activated sludge (Kim et al. 2005). Iron-reducing bacteria grown with ferric iron and acetate will compete with methanogens and limit methane production which would be disadvantageous if there was power generation through methane combustion at a later stage of the wastewater treatment process. Continuous flow MFCs are not suitable wastewater treatment applications, since mediators and bacteria will flow out of the system. Therefore, the anode compartment would need to use an efficient biofilm on a large anode (Rabaey et al. 2005d). These experiments have reported up to 50% removal of COD from the influent water while generating useful power without the necessity of aeration or the complicated equipment that tends to accompany biogas processing. Liu et al. (2004) observed that power output is directly proportional to the influent COD concentration for single chamber MFCs. The maximum power density achieved in this system (26 mW/m2) was attributed to the organic matter being broken down by processes that did not generate electricity. 61 2.20.4 Seafloor Sediments as a Substrate Recently applications of MFCs have been attached to the seafloor where the thick layer of organic carbon can be oxidized by microorganisms (Figure 2-11) by MFCs known as Benthic Unattended Generators. The conditions naturally inherent within the sea and the organic carbon lend themselves to energy production. For example the high salinity of seawater increases ionic conductivity between the electrodes. The applications of sediment MFCs for long-term power supply for oceanographic equipment are tremendous. The linear gradient of organic matter decreases the closer to the surface and this natural difference in organics removes the necessity of a semi-permeable membrane to separate the two electrodes. B Cathode reaction: -8 Water ii v q[|n %Jfjp i%n*"W"*i -* i ! Sfriimenippp Anode ft: Anode* rooction; Sediment - ——»- Acetate C2H4O2 + EHgO -•" Fernu'itdl ui* ,_ . _ + 2C02 + 8H + 8e^ organic (C-:h,jO?) matter Figure 2-11: Sediment MFC with reactions Source: (Lovley 2006a) One substantive advantage of this type of set up is that it requires very little maintenance. This is especially true of the 'bottle brush' cathodes that were employed by Hasvold et al. (1997). However it is important to note that power density is partly limited to low organic matter in most marine sediments (Rezaei et al. 2007). This value can be as low as 0.4 % 62 organic matter by weight in some case (Rezaei et al. 2007). Sediment MFCs originally provided 10 mW/m2 (Tender et al. 2002) but recent modifications by Lowy et al. (2006) achieved values 10 fold higher (105 mW/m2). These modifications were made with mediators and metals bound to the electrodes. Logan (2008) established that such devices could sustain power generation at a level of 50 mW/m essentially indefinitely. The Geobacteraceae species are frequently the predominant Fe3+ reducing microorganisms in marine sediments with over half of the microorganisms on the anode belonging to that species. In saline water Bond et al. (2002) observed it is the Desulfuromonas species that predominate while Lovley (2006c) noted in freshwater environments it is the Geobacter species (Figure 2-12). Under Fe3+ reducing conditions in marine sediments microorganisms that are capable of producing their own electron shuttles can be expected to have a competitive disadvantage because the shuttles will be lost rapidly from the site of release. Lovley (2006c) notes that the Geobacteraceae species have a significant advantage since they establish contact with Fe + oxides for direct electron transfer. Bond and Lovley (2003) note this species has a competitive advantage over other bacteria in anoxic marine sediments since they are able to oxidize acetate, the primary organic intermediate in the degradation of organic matter, with an electrode as the sole electron acceptor. It is important to remember that biosynthesizing an electron shuttle is energetically expensive and therefore an electron shuttle must be recycled many times in order to recoup this energy investment (Lovley 2006c). 63 Figure 2-12: Transmission electron micrograph of Geobacter on graphite anode Source: (Lovley 2006a) Bond and Lovley (2003) realise that the medium in the anode chamber of a MFC with G. sulfurreducens can be replaced without affecting power production. This indicates that the cells attached to the anode are responsible for power generation and are independent of electron shuttles since they would have been removed during medium exchange. 2.21 Guidelines for MFC Comparison and Reporting It is important to note there have been tendencies to skew results, such as recording the maximum voltage obtained when the cell is connected to a resistance as an indicator of performance instead of the steady state voltage which is typically a degree lower in magnitude. It is only recently becoming a common practise to report the value of the load 64 resistance used leading to non-comparable MFCs and inconclusive arguments. Also a few researchers have only examined their MFC under a single load and imagine this result to showcase the versatility of their design. It is challenging to compare the performance of various types of MFCs because numerous approaches have occurred under a range of conditions. In particular there have been multitudes of types of electrodes, surface area ratios, working volumes, and conditions which further complicate the comparison. One challenge associated with results published to date is that authors are often vague about which electrode surface area was utilised in the computation of the power density. According to Larminie and Dicks (2003) in their book Fuel Cell Systems Explained they indicate that the comparison of fuel cell electrodes and electrolytes must entail the comparison of current densities at a specific operating voltage. The specific operating voltage for fuel cells is typically 0.6 or 0.7 V, however MFCs are often unable to produce such voltages under loads and hence this value will have to be lower to facilitate MFC electrodes and electrolytes comparisons. The potential of the electrodes can only be accurately reported by measuring the voltage against a reference electrode with a known potential. The silver-silver chloride (Ag/AgCl) reference electrode is a common choice. However it is important that researchers also report the pH of the solution since electrode potentials frequently depend on pH and this is often overlooked in work that publishes electrode potentials. 65 It is advised to harvest the biofilm from an existing active anode rather than start from an un-colonised electrode as sufficient colonization can take weeks to occur. Zhang et al. (2006) have seen E. coli electrochemically evolving within the fuel cell environment to naturally select the bacterial communities to reside on the anode. It has been observed that the addition of a secondary anode, on another circuit, to culture electrochemically active bacteria was counter productive since the cathode could not support the additional reactions that occurred under an increased anode surface area. To implement this strategy one would likewise need to increase the cathode surface area to ensure colonization occurred on the secondary anode. The vast majority of MFCs reported have undergone extensive inoculations to ensure electrodes are covered with the desired bacteria and are well established. It is therefore important that comparisons also take into consideration the time permitted for colonization as this will severely impact power generation. In reporting results it is important to provide the polarization curve including the Voc, to ensure that the complex spectrum of MFC ability is portrayed. The polarization curve must be obtained after prolonged open circuit mode to ensure full establishment of Voc. In the process of developing the polarization curve there have been a wide variety of times utilised at each resistance ranging from less than a minute (Finkelstein et al. 2006), to a few minutes (Menicucci et al. 2006), to a quarter of an hour (Logan et al. 2007), to 7 hours (Heilmann and Logan 2006), and to multiple days (Cheng and Logan 2007). There is no established procedure at present to ensure the time is not too long since the bacterial community structure could change or too short to allow sufficient time for equilibration to occur. Refer to Logan et al. (2006) for a more thoroughly explanation of these curves. 66 Furthermore many approaches to referencing power densities have emerged where some authors publish values based on mW/m3, others as mW/m2 of the total anode, and others still as mW/m2 of the projected surface area of the anode. These values are non- comparable in their present units thereby increasing the need for standardization of reporting MFC results. Power density should be calculated based on the electrode where the biological activity occurs, often the anode. The cathode projected surface area is used infrequently and the manner of calculation should be detailed to ease comparability. Research by Logan et al. (2007) normalised the power density of their MFC to the cathode surface area which was substantially smaller than the anode (1.06 m2 anode vs. 0.0007 m2 cathode). Had the power been referenced to the anode it would have been 1.6 mW/m2 instead of the reported value of 2400 mW/m2. Another reporting scheme that is very misleading was employed by Ringeisen et al. (2006) where their MFC produced approximately 500 W/m3 in a small reactor measuring only 1.5 cm3 yet this arrangement was fed by a 100 cm3 reactor where cells were grown and pumped into the MFC. This reactor volume should be included in the volumetric power calculations to be representative of the system. If this reactor was included the value is only 7 W/m . Since 2003 some researchers have held a preference to quote volumetric power (mW/m ) as opposed to power density in an attempt to facilitate comparisons between MFC technology and other modern technology such as chemical fuel cells. These units will ease comparison and elucidate whether the power outputs noted to date are significant for utilisation in practise. Units of volumetric power assist in engineering calculations for 67 sizing and costing of materials. Further details regarding evaluation of MFCs according to volumetric basis can be obtained from Logan et al. (2006). The vast majority of journal papers have not detailed the range of values achieved during duplicates. Rather they present the average value which is useful for comparison but not indicative of the degree of variability experienced. One recent paper by Rezaei et al. (2007) is among the first which detail the range of power production (± 29 mW/M2) and internal resistance (+ 901 Q.) explicitly. These variations appear substantial but the present author believes that if researchers were to publish their data variability these results would actually appear quite common. This amounts to almost 50% variability of the internal resistance and power production. Some of the most notable detailed reviews of MFCs to date include (Logan et al. 2006; Rabaey and Verstraete 2005; Lovley 2006b; Pham et al. 2006; Chang et al. 2006; Bullen et al. 2006). 2.22 Limitations at Present in MFC Design and Operation Three main drawbacks encountered with MFCs to date are detailed by Chaudhuri and Lovley (2003) as: the necessity of mediators for many types of bacteria, incomplete oxidation, and lack of long-tem stability. Mediators consume some of the energy they are assisting in generating and hence make these fuel cells further inefficient. Oh and Logan (2006) note that the recent discovery of Rhodoferax ferrireducens by Chaudhuri and Lovley (2003) is important since it avoids the necessity of an electron mediator thereby increasing the efficiency of the fuel cells. Incomplete oxidation is another cause of inefficiency within a system. Service (2003) 68 details how glucose can yield 24 electrons yet some fuel cells are capable of releasing just two of these electrons. Chaudhuri and Lovley (2003) found R. ferrireducens as capable of releasing 20 of the available electrons from glucose while the remaining electrons create ATP. Therefore this bacterium is able to grow and multiply while creating electricity and solve the previously mentioned drawback associated with MFCs showcasing their versatility in MFCs applications. Further research by Logan (2004) noted roughly 2/3 of the electrons remain as fermentation products such as acetate leaving only 1/3 of the electrons for current generation. There are currently a number of challenges that are affecting microbial fuel cell design and functionality. Even though PEMs are designed to allow protons to pass but to block the larger oxygen molecules, oxygen is still able to cross the membrane to the anode where it takes electrons that would have otherwise been used for current flow in the circuit reducing power generation (Holzman 2005). Power density is a challenge to overcome since conventional fuel cells are measured in W/cm2 but microbial fuel cells are commonly measured in mW/m2. This lower power density means that electrodes need to be large and therefore more costly. Numerous studies utilizing the same substrate and microbial consortia have resulted in power densities differing over one order of magnitude (Kim et al. 2007). This indicates that further research is required into reactor configurations to minimize internal resistance and other impedance factors. Research potential exists in increasing degradation rates and 69 more efficient electron-transfer mechanisms as well as microbial structure which have increased electrical conductance of the biofilm matrix (Logan and Regan 2006b). Du et al. (2007) suggest that efforts be invested into overcoming the inherent metabolic limitations of microbes for MFC applications. Lovely noted that it is possible to increase, by four orders of magnitude, the current flow if Geobacter was assisted to transfer electrons to the anode at the same rate that is does to ferric iron, the natural electron acceptor (Holzman 2005). MFCs for wastewater applications need to produce more than 160 W/m3 (based on the wet volume of the anode chamber) to compete with anaerobic digesters for electricity production after conversion of methane gas to electricity via a generator (He et al. 2005) yet we are not close to this value at present. 2.23 Challenges Associated with Scale up of MFCs The most limiting factor to MFC scale up is the cost of electrode materials. According to Powers (2007) the capital expenditure cost of energy production from fossil fuel by conventional combustion processes, wind turbines, anaerobic digestion and chemical fuel cells is in the order of $1.5 million per MW capacity installed. Rabaey and Verstraete (2005) put the cost of the same production potential by MFCs at an order of magnitude higher. This substantially increased cost is associated with the low power densities achieved in comparison. As research continues in this emerging field the costs are suspected to decrease dramatically and the power densities increase. Angenent et al. (2004) are not as optimistic stating that even when optimization is achieved it remains to be seen whether MFCs will become economically viable. 70 It is important to note that electrodes do not often 'scale up' linearly, i.e. if the surface area is doubled that does not imply that the power will similarly double. Larminie and Dicks (2003) state that the reasons for this are varied and often not well understood but relate to issues such as even delivery of reactants and removal of products from the face of the electrode. Clearly there is a need for large surface areas to support electrochemically active bacterial biofilm and as Logan and Regan (2006b) point out some materials such as carbon paper are not scaleable due to their inherent lack of structural strength. While carbon granules could be easily modified to the appropriate volume they are heavy and could clog as a result of their inherent low porosities. Graphite fibre brushes are an obvious choice for scale up since they have very high specific surface areas and are extremely conductive. 2.24 Practical Utilisation of Electricity Generated by MFCs To overcome the relatively low energy generation obtained in MFCs researchers have employed the use of ultracapacitors to store energy. One particular ultracapacitor utilized by Shantaram et al. (2005) had a very low leakage current of roughly 0.28 |iA/h at 25°C. Furthermore a DC-DC converter was utilised to achieve increased potential of the cell. The current density of this MFC was very low only producing 0.0029 mA/m2. With this ultracapacitor they were able to monitor environmental changes and transmit this data through pulses to a receiver wirelessly. A more practical approach will be to utilise this meager yet constant power source for recharging batteries. 71 2.25 Summary and Conclusions There is a direct relationship between the load resistance and the power generated by a MFC. The method proposed by Menicucci et al. (2006) is suitable to quantify the value of resistor which will yield maximum sustainable power. It is important to note bacterial growth and decay continues constantly in time and resistor values may pose a limiting factor in the MFC. Therefore it is important to conduct tests for extended periods to ensure the resistor value obtained from this procedure is indeed suitable in the long term. The power generation of a fuel cell is directly related to the internal resistance and decreased internal resistance will increase the power generation capability. Since the external resistance can be varied but the internal resistance is fixed it is therefore a limiting factor in most MFCs and advances in designs will often address this challenge. Two ways in which it is possible to decrease internal resistance without changing the pH of the solution are to increase the ionic strength (IS) and/or to decrease the electrode spacing. Under various compositions of electrodes and arrangements of the components of the fuel cell, the internal resistance can increase or decrease as a result of modifying the surface area of the electrodes. Changing the electrode material of the anode may yield no result, but when the same is done for the cathode the result has shown to almost double the power production. It cannot be implied that the effect of electrode material of the anode will impact the cathode in a similar manner and vice versa. Worthy of mention is the fact that power production is proportional to cathode surface area when the PEM is of sufficient size in wastewater applications. Research has detailed that the cathode can contain as little as 0.1 72 mg/cm Pt or the Pt can even be replaced with non-precious and hence less expensive metal catalysts such as CoTMPP with only slight reductions of 12% in performance. An integral component to the optimization of any MFC is the selection of suitable electrodes that maximize power production. It is important to remember that the materials required for sustainable and efficient power generation need to be highly conductive yet noncorrosive with a high surface area per volume. It is vital to also have an open structural arrangement in order to avoid fouling. Although most artificial mediators are toxic to humans and costly, they are often used in fuel cells to increase efficiency and are often manufactured by the bacteria themselves. Of all the MFCs configurations mediatorless fuel cells hold the most promise for future development with conversion efficiencies of 95% already experienced by Ieropoulos et al. (2005a). Since the combined voltage and current densities are similar to the addition of individual MFCs it implies that stacked MFCs are unable to deliver higher power densities than individual MFCs. However it is important to note that they are able to produce averaged power at more useful currents and voltages. This emerging technology has the potential to generate electricity from material that is considered waste or of no inherent value. The future looks promising when the fact that these MFCs recover the capacity to deliver power almost immediately is coupled with the knowledge that there have been tremendous advancements in the development of MFC technology. Power production has increased by several orders of magnitude in mediatorless MFCs within less than a decade. It is important to note that there have been 73 numerous studies utilizing the same substrate and microbial consortia which have resulted in power densities differing over one order of magnitude (Kim et al. 2007). This indicates that further research is required into the reactor configuration to minimize internal resistance and other factors of impedance. There is great work to be done with the architecture of MFCs since various species of bacteria have diverse natures and it is impossible for a single model of a MFC to work for all species. The number of possible anodic reactions in MFCs is practically limitless yet the number of cathodic reactions is much less diverse. Therefore there is ample work for research in these areas. One of the greatest challenges to achieving substantially higher power densities at present is the architecture of the cell and not the composition of the microbacterial community. Therefore much of the current research is aimed at determining the optimal operating parameters and setup of the MFC. Once the intricacies of these factors have been ascertained then researchers should more deeply investigate specific microorganisms and bacterial flora. Future research should be geared towards experimenting with structurally stronger materials that are more practical for scale up. 74 3 Optimization of Lab-Scale Production of Electricity Through the use of Cow Manure and Microbial Fuel Cell Technology 3.1 Introduction Usage of manure in a sustainable energy generation scheme has the potential to ease this fossil fuel dependency to a limited degree with the measured calorific value of 82 kJ/g on dry basis (Scott and Murano 2007). Manure is biologically alive with bacteria and other microorganisms that subsist on energy within the manure itself. With cellulose C6H10O5 as one of the most ubiquitous renewable sources of energy on earth there is ample room for variation in this process. The main challenge associated with the implementation of cellulose as an energy source is that it is not easily hydrolyzed or dissolved under normal conditions due to its complex structure. However microorganisms in the rumen of mammalian herbivores excel at cellulosic biodegradation (Hu et al. 2004). There are no known microorganisms that can both metabolise cellulose and transfer electrons to solid extracellular substrates. Therefore the conversion of cellulosic biomass to electricity requires a syntrophic microbial community (Ren et al. 2007). 3.2 Background Information Manure MFCs are advantageous over pure culture MFCs since there is higher resistance to process disturbance, higher substrate consumption rates, smaller substrate specificity, and higher power output (Rabaey et al. 2005 a). Clostridium or Clostridium like species predominate in rumen MFCs accounting for over 36.5% of the entire bacterial population on the anode (Rismani-Yazdi et al. 2007b). Interestingly suspended bacteria in the anode compartment of cellulose-MFC held only 5.5% Clostridium or Clostridium like species. 75 This shows that the bacteria self select for electrochemically active microorganisms on the anode. Zhang et al. (2006) observed similar behaviour where E. coli electrochemically evolve within the fuel cell environment to naturally select the bacterial communities to reside on the anode. Power production in manure MFCs is a result of a number of factors beyond those usually associated with MFCs such as electrode spacing, ionic strength, electrode surface area; it also involves factors relating to the manure itself. The microbial flora in manure will depend in large part on the feed that the cow receives which has a direct effect on the power production of the MFC. Cow feed for the vast majority of experiments (Rismani- Yazdi et al. 2005; Rismani-Yazdi et al. 2007a; Rismani-Yazdi et al. 2007b; Rismani- Yazdi et al. 2006; Rismani-Yazdi et al. 2007) is that of a mixed ration which contained alfalfa silage, corn grain, soybean meal, as well as a vitamin and mineral mixture. There has also been variability in manure dilution further complicating comparability. Yokoyama et al. (2006) utilised a manure slurry composed of manure and urine diluted 10 fold with distilled water. Manure utilised by Scott et al. (2007) was dried blended farm manure which required reactivation by hydration (100% dilution with water). Anaerobic rumen fungi are able to transfer electrons to the anode under antibiotic treated conditions since eukaryotes are not affected by penicillin or streptomycin antibiotics (Hu et al. 2004). Therefore if the cow was treated with antibiotics it is not likely to adversely affect power production adding to the robust nature of manure microbial fuel cells. 76 This chapter will detail the multitude of factors affecting power generation in manure MFC technology to date. It will detail the effect of physical characteristics such as the effect of electrode spacing, dilution of manure, electrode material, temperature, and water depth in the cathodic region on power production. A host of biochemical characteristics were also explored such as the response to addition of molasses in the substrate, the potentiality of rumen fluid as a substrate, the result of varied consistencies of cow manure, and ionic strength effects on power generation. An assortment of operational parameters such as effect of open circuiting on power production, increasing anode and cathode surface area, and the effect of load as well as the location of substrate in the anodic or cathodic region were also examined. For the purposes of these experiments manure microorganisms are the biocatalysts with the cellulose as the electron donor. 3.3 Materials and Methodology 3.3.1 Measurement of Voltage The voltage was measured with a Fluke 175 multimeter at regular intervals with the regularity of the timings decreasing as the experimental conditions stabilised. 3.3.2 Manure Consistency and Preparation Manure was either collected from the Elora Dairy Research Station (EDRS) or the Department of Animal and Poultry Science (DAPS) of the University of Guelph. Cattle at the Elora Dairy Research Station are fed a total mixed ration (TMR) twice daily. This ration consists of haylage, corn silage, high moisture corn, protein supplement, and mineral premix. The cows at the Department of Animal and Poultry Science are fed six flakes (9 kg) of second cut hay with low nutrient content each day. Minerals are added to 77 supplement the low nutrient intake as the weight of each of the cows is being monitored. The manure was prepared according to one of the methods outlined below: Method 1: Manure from EDRS at a ratio of 1:3.75 manure to water by weight Method 2: Manure from DAPS at a ratio of 1:1 manure to water by volume Laboratory deionized (DI) water was used instead of tap water to eliminate the negative impact of residual chlorine on microbial growth. Salt was added to DI water to replicate similar conductivity as tap water which was recorded as 685.7 (xS/cm at 20.9°C using an Accumet Excel conductivity meter (Fisher Scientific, Accumet Excel XL 60). The conductivity was referenced to the specific temperature of the intended MFC experiment; hence to achieve the similar conductivity as tap water, 460.4 mg of sea salt had to be added to 1 L of deionized water at 30°C. This water was used for manure slurry preparations while evaporation was supplemented with DI water. 3.3.3 Electrode Assembly Several different electrode materials were tested including solid graphite, graphite felt, woven carbon and uni-carbon fibre electrodes of varying dimensions and surface areas. For further details relating to electrode dimensions and visualization refer to Appendices C and F. A small hole, slightly wider than the wire, was drilled 1 cm deep into the top of the solid graphite electrode for the connection of the wire. Approximately 15 cm of insulated wire (24 AWG) was joined to the tip of the electrode through the hole (Figure 3-1). Roughly 5 mm of the wire was stripped and connected with conductive epoxy to ensure a low-resistance connection between the electrode and the wire. Conductive epoxy was used to fill the hole and cover the length of exposed wire as well as a few millimeters 78 of insulation. After the epoxy had dried non-conductive epoxy was liberally applied to the connection to protect it from short-circuiting on contact with water as well as to provide mechanical stability. This procedure was repeated for the fibre electrodes. Figure 3-1: Connection between solid graphite electrode and wire The graphite felt was cut into 4 strips of appropriate sizing according to Table 3-1 and fastened together with nonconductive epoxy to form rectangular electrodes. The epoxy was only applied to a small area on the periphery of the electrode to minimize the loss of conduction. The circular electrodes were cut according to the specification in Table 3-2 and Table 3-3 as seen in Figure 3-2. The wire was stripped as mentioned above and attached to the top of the electrode in a similar manner. Each of the electrodes was tested with a multimeter to ensure it had a good electrical connection. Figure 3-3 provides a visual representation of a completed electrode with anode and cathode. 79 Table 3-1: Rectangular graphite felt dimensions for electrode configuration Dimension Anode Cathode Thickness (cm) 1.30 1.30 Width (cm) 1.57 1.57 Length (cm) 9.50 9.50 SA (cm2) 58.61 58.61 Table 3-2: Graphite felt in cylindrical form in small cup Dimension Interior of cup Exterior of cup Thickness (cm) 0.3175 0.3175 Width (cm) 9.7 12.8 Length (cm) 8.60 9.30 SA (cm2) 128.46 252.11 Table 3-3: Graphite felt in cylindrical form in large cup Dimension Interior of cup Exterior of cup Thickness (cm) 0.3175 0.3175 Width (cm) 14.20 16.3 Length (cm) 9.80 10.30 SA (cm2) 293.56 352.67 Figure 3-2: Cathode electrode before assembly (left) and after assembly (right) 80 Figure 3-3: Complete electrode assembly with wires attached and manure in place 3.3.4 Cell Construction and Arrangement Cells were constructed in a variety of arrangements outlined below. The wires between the anode and the cathode were connected with a 1 k£2 resistor unless otherwise indicated. Arrangement 1: The cell was constructed in mason jars according to the configuration stipulated in Appendix H, filled % full and operated in a fume hood at 22°C (Figure 3-4). A half centimeter ventilation hole drilled into the lid for methane gas to escape. 81 Figure 3-4: Mason jar arrangement Arrangement 2: These cells have the anode electrode at the bottom of a 1000 ml beaker in a horizontal manner so the anode was maintained as anaerobic as possible (Figure 3-5). The cathode was likewise hung horizontally just below the surface of the water. 82 Figure 3-5: Horizontal electrode arrangement Arrangement 3: When clay cups were employed the anode was placed vertically into the centre of a clay cup while the cathode electrode was placed vertically in the surrounding liquid adjacent to the cell (Figure 3-6). The clay cups were filled with the specific dilution of manure while the anode was in place so that the manure mixture surrounded it. Anaerobic conditions were maintained in the anode compartment by using a thin layer of plastic wrap covering the top of the clay cup to inhibit oxygen transfer. A small hole was cut into the plastic to allow for the wire from the electrode to exit the cell. This opening was taped to further inhibit oxygen transfer. This arrangement was utilised with various electrode surface areas specified in Appendix E. Electrodes for this arrangement were graphite felt unless otherwise noted. The electrode spacing was maintained as uniformly as possible over all the trials to decrease error and allow for comparability between trials. 83 top view Original to the field of MFC research is the implementation by the present author of a clay cup manufactured by CoorsTek, Inc. There were three different sizes of clay cups utilized in these MFCs trials as detailed in Table 3-4 and visually represented in Figure 3-7. These cups are inexpensive at $24.03 (large), $23.15 (small) and $14.04 (mini) substantially lower than the cost of $152/m2 associated with the most inexpensive proton exchange membranes (PEMs) such as the Ultrex membranes. This clay cup performs many of the same functions as these PEMs such as separating the cathode and anode into separate compartments. These clay cups simultaneously maintain a porous environment which does not facilitate rapid oxygen transfer since the air entry value to displace water from the pores is provided as greater than 80 psi by the manufacturer (CoorsTek 2006). Standard atmospheric pressure is 14.7 psi therefore water will not be displaced by air to penetrate the cells making it an ideal choice as a PEMs. 84 Table 3-4: Dimensions of clay cups Size of cup Mini Small Large Diameter (cm) (external) 2.5 4.0 5.2 Diameter (cm) (internal) 2.0 3.35 4.6 Height (cm) (external) 8.0 9.3 10.3 Height (cm) (internal) 7.5 8.6 9.8 Volume (cm3) 23.56 75.80 162.93 Average pore diameter (u.m) <0.5 <0.5 <0.5 Apparent porosity (%) 38.5 38.5 38.5 Absorption (%) 21.0 21.0 21.0 Figure 3-7: Large, small, and mini clay cup sizes 3.4 Results and Discussion Power production for the MFC outlined in this chapter are lower than that achieved in previous MFC studies with well-adapted cultures, modified electrodes, or specially designed compartments (Gil et al. 2003; Min and Logan 2004; Park and Zeikus 2003; Pham et al. 2004; Rabaey et al. 2003). It is however comparable to those employing soluble and low molecular weight organic compounds in two-chambered MFCs under 85 analogous operating conditions (Bond and Lovley 2003; Lee et al. 2003; Logan et al. 2005; Kim et al. 2004; Phung et al. 2004). Power generation in the present author's MFCs have been recorded without delay while experiments by other researchers Scott & Murano (2007) and Scott et al. (2007) have allowed for over a week of fermentation to occur to allow for anaerobic conditions to become firmly established before MFC trials commence. It usually takes 2-3 days for biogas formation to occur under anaerobic conditions. Other researchers (Rismani-Yazdi et al. 2005; Rismani-Yazdi et al. 2007a; Rismani-Yazdi et al. 2007b; Rismani-Yazdi et al. 2006; Rismani-Yazdi et al. 2007) have allowed over a month of bacterial colonization on electrodes to occur before considering their experiments to have begun thereby skewing results and challenging comparability. 3.4.1 Internal Resistance, Electrode Spacing and Ionic Strength As could be expected the power production for the 8 cm spacing was the lowest while the 5 cm spacing had the maximum power production for the series (Figure 3-8). Liu et al. (2005b) recount a similar experience with domestic wastewater where they observed a two fold increased in the maximum power generation of the cell by decreasing the spacing from 4 cm to 2 cm, while maintaining the ionic strength. It is important to note that the 5 cm and 6.5 cm spacing are not significantly different (refer to Appendix J for full statistical analysis of all results). 86 0 25 50 75 100 125 150 175 200 225 250 275 Time (hours) | —•—5 cm spacing -»-6.5 cm spacing -*-8 cm spacing | Figure 3-8: Electrode spacing and power production During the 50 hour period of (100 hours to 160 hours) the cell ceased to produce any viable power and this can be attributed to evaporation of water in the substrate. This lack of water caused a reduced ability to facilitate the transfer of electrons and protons between the cathode and the anode heightening internal resistance. It is vital to note that the typical spacing employed in these manure MFCs of 3.5 cm (Figure 3-9) performed better and hence was the recommended minimum electrode spacing to proceed with. 87 0 25 50 75 100 125 150 175 200 225 250 Time (hours) | -Si-no addition of salt —I—addition of 5 g sea salt ] Figure 3-9: Addition of sea salt to increase conductivity of catholyte Altering the ionic strength of the catholyte is another way in which to increase power production (Figure 3-9). The addition of 12.3 g sea salt per litre caused the conductivity of the slurry to increase from 0.86 \iS/cm to 26.03 mS/cm. The addition of salt results in a statistically significant difference in power production. Interestingly salt also lengthened the duration of the peak power generation while boosting power production values by approximately 33%. The enhanced duration of power production is attributed to the effect of salt amplifying the concentration of ions in solution and hence the addition of salt is seen as advantageous. The control experiments (data not shown) with water achieved 1000 fold less power production indicating that the manure and bacterial community are responsible for power generation. The manure method for these experiments was that of Method 1 under arrangement 1 at 22°C with electrode surface 88 area of 124.80 cm2 (anode) and 62.40 cm2 (cathode) (further electrode dimensions can be observed in Appendix E). Supplementary experimentation was conducted at four different levels of ionic strength in the catholyte to investigate the optimum concentration conditions (refer to Appendix H for information relating to the amount of salt to replicate the conductivity). Results from Figure 3-10 indicate that the power production at 100 raM was the lowest while the 700 raM had the maximum power production for the series. A similar trend was observed by Liu et al. (2005a) when they increased ionic strength by a factor of 4 (100 mM to 400 raM) and only enhanced the power density by a factor of 2. Statistically, each of the ionic strengths is significantly different than the 700 mM trial while they are otherwise not significantly different from each another (refer to Appendix J for further analysis). 0 25 50 75 100 125 150 175 200 225 250 275 Time (hours) | -»-100mM -»-300mM -A-500mM -X-700mM | Figure 3-10: Ionic strength of the catholyte and resulting power production 89 3.4.2 Usefulness of clay cups while increasing anode surface area Results from Figure 3-11 indicate clay cups substantially enlarge and lengthen the duration of power production. Doubling anode surface area in clay cups did not achieve twice as much power production since there was less manure to immerse the anode in and hence less substrate available. The Voc of clay cups was over 4 times as high as without a clay cup indicating potentially higher power production. Results from this point forward employ the use of clay cups since they have been proven to increase power production. This experiment was conducted with Method 1 under arrangement 3 at 22°C. 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Time (hours) -•—Double Anode —•—Equal surface area -*-Equal surface area in Clay Cup -3R-Double Anode in Clay Cup Figure 3-11: Usefulness of employing clay cups while increasing anode surface area 3.4.3 Effect of Altering Electrode Materials on Open Circuit Voltage A range of materials such as solid graphite, graphite felt, woven carbon and uni-carbon fibre laminates were investigated for their ability to enhance power generation. Trials 90 with woven carbon and thin uni-carbon laminate produced similar Voc as graphite felt configurations. Graphite however yielded 5 orders and 3 orders of magnitude higher power density. This implies that open circuit voltage is only a partial indicator of MFC capabilities rather power production is a more informative indicator of performance. The manure method for these experiments was that of Method 1 under arrangement 2 at 22°C. For a complete explanation of the tests conducted in this trial refer to Appendix H. 3.4.4 Effect of Altering Electrode Material Coupled with Dilution of Manure Concentration has a dramatic influence on rates of reactions; therefore it was determined to examine the effect of manure dilution on power production. Results from Figure 3-12 indicate that solid graphite electrodes may be less suited to manure MFC applications than corresponding graphite felt electrodes regardless of the dilution factor. As dilution increases the power production decreases indicating that the cells are able to produce power longer when there are greater amounts of manure in relation to water. These results suggest graphite felt should be employed for the remainder of manure MFC trails. 91 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 Time (hours) -•-Solid-100% -*-Solid-200% -X-Solid - 300% -*-Solid - 400% -•-Felt-100% —4— Felt -200% Felt-300% -—Felt-400% Figure 3-12: Dilution (%) of manure with solid graphite and graphite felt electrodes 3.4.5 Effect of Altering Anode and Cathode Surface Area in Manure MFCs It was important to identify which electrode is limiting in manure MFC configurations in order to optimize that electrode. Results indicate that the peak Voc is highest when the anode and cathode surface areas were equivalent (data not shown). Doubling and tripling the anode surface area decreased the peak Voc by more than 100 mV. Increasing cathode surface area, although achieving a lower Voc, was able to produce substantially higher power. Therefore Voc is not singly an adequate representation of the potentiality of voltage production. Results from Figure 3-13 indicate that the cathode is the limiting factor. This implies that the oxygen availability for the cathodic reactions that is the true inhibiting factor to increased power generation. 92 0.14 -m 0.12 0.10- 0.08 ! S 0.06 o OH 0.04 -H 0.02 o.oo- 10 20 30 40 50 60 70 80 Time (hours) -Double Cathode - Double Anode -Triple Cathode Triple Anode Equal Figure 3-13: Results from altering anode and cathode surface area While keeping the cathode surface area constant there appears to be an upper limit to increasing power production by enlarging anode surface area (Figure 3-14). A two fold and three fold increase in anode surface area produce more power production than a four fold increase. This is because the increased reactions at the anode could not be supported without increasing the cathode surface area. It is important to note that the three fold and four fold increase are not statistically significantly different from one another. When uni- carbon laminates were employed maintaining the anode at the same surface area while increasing the cathode area four fold generated 25 times more power as a result of greater surface area for oxygen contact (data not shown). 93 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Time (hours) | -•- Four fold -felt -m- Three fold - felt -A-Two (old - (elt M-One fold - felt -*- Two fold - solid | Figure 3-14: Effect of increasing graphite felt and solid graphite anode surface area 3.4.6 Relationship between Power Production and Temperature Results from Figure 3-15 indicate that increasing temperature increases power production. Power production at 37°C is only slightly statistically different to that at 30°C (Pr < 0.0713) but it does provide more power so the decision to increase the temperature would be made on a case by case basis. Similarly, MFCs using wastewater are seen to be independent of temperature effect, to an extent, with only a 9% reduction in power observed by Liu et al. (2005a) accompanying a decrease in temperature of over 10°C (from 32°C to 20°C ). To maintain manure MFCs at 30°C is not seen as a challenge in the majority of developing countries where the ambient temperature is often at this level. Vital to note is the fact that bacteria can become inhibited at elevated temperatures as experienced by Powers (2007) where MFCs with cow manure operated at 55°C ceased to produce viable power. This indicates that the electrochemically active bacteria are 94 possibly inhibited or killed at thermophilic temperatures. The 30°C temperature was employed from this point forward for all manure MFC trials. 0 3 5 8 10 13 15 18 20 23 25 Time (hours) | -+-20°C -*-25°C -*-30°C -*-37°C | Figure 3-15: Temperatures effect on power production 3.4.7 Performance of MFCs linked to Height of Water in the Cathodic Region To the knowledge of the author there have been no MFC experiments analysing the effect of height of water in the cathodic region. Since one of the factors which have a bearing on power generation is the oxygen availability at the cathode electrode, it was important to investigate this relationship. Research in the literature suggests that aqueous oxygen is not the ideal candidate for power production rather oxygen in air is a better choice. Results from Figure 3-16 indicate there is greater power production with decreasing water height in the cathodic region. Further examination reveals these trials had 95 oscillations of voltage generation which are a result of evaporation of water within the clay cup without the hydration effect by water being present on the exterior of the cup. When the voltage generation started to decrease sharply this coincided with a decrease in liquid within the cup. The voltage rebounded immediately with the addition of water in the anodic region to facilitate electron transfer to the anode. To combat this cyclic activity it is recommended to maintain the water level in the anodic region. It is clear to achieve higher voltage potential it is useful to employ a configuration with very little (i.e. 1 cm height of water) or no water at all. This small height of water in the cathodic region was applied to all further MFC experiments from this point forward. 4.0 3.5 ^ -^5*,M* 3.0 •- V ,1101 .kldi-d l« Ml| pL'inenl L'V.ipni.Hinn & iMii es spiko (»l \i>ll S 2.0 I \' '• . -.: ' • o -Vie PN 1.5 — 1.0 * "^ Twl1 '/ '\\ 0.5 "^ llll • Wfr ' ^"^—A 4 7 0.0 25 50 75 100 125 150 175 200 Time (hours) -0 cm -1 cm 3 cm -5 cm -7 cm -9 cm Figure 3-16: Results from altering height of water in the cathodic region 96 3.4.8 Effect of Load on Power Production The resistive load on a MFC is an integral component to power generation as observed from Figure 3-17 where a range of resistors values were utilised. The 680 Q load was substantially greater in power production than loads higher as well as lower than itself. Each of the loads was statistically different from one another with the exception of the 220 Q. vs. 1000 Q, loads. Experiments conducted by Ieropoulos et al. (2005a) observed the effect of reducing the load resistor from 10 kQ to 1 kQ resulted in a five-fold increase in the power output with G. sulfurreducens. This trend was not reciprocated by the manure bacteria where a decrease in load from 10 kQ to 10 Q resulted in a ten-fold decrease in power output. 0 50 100 150 200 250 Time (hours) [-afr-10ohm —I—220 ohm -^—680 ohm -•-1000 ohm -X-8200 ohm -•-10000 ohm Figure 3-17: Effect of a variety of resistive loads on resulting power production 97 3.4.9 Result of Increasing Clay Cup Size on Power Production It is important to explore the relationship between void space and surface area in MFCs. Results from Figure 3-18 and statistical analysis imply that whether the cell employs the use of a large or small clay cup the power generation is approximately similar. However when this knowledge is combined with the above mentioned advantages of increased surface area, particularly that of the cathode, it would be well worth the slight increased cost ($0.88) to utilise a large clay cup to achieve a greater cathode surface area. The main reason that these values are two orders of magnitude higher than previously noted is the use of an air-cathode arrangement whereas prior arrangements relied on aqueous oxygen from water in the cathodic region. 0 25 50 75 100 125 150 Time (hours) —X— large cup —*—small cup U mini cup Figure 3-18: Results from various clay cup sizes in MFC operation 98 3.4.10 Rapid Opening and Closing of the Circuit The relationship between open circuited and closed circuited conditions was explored as it is important to know the effect of rapid utilization of power and then idle conditions. Open and closed circuit timings of Vi hour, % hour, 1 hour, and 1 Vi hours were employed. Severe voltage fluctuations during open or closed circuit conditions are associated with a decrease in water in the anolyte as detailed earlier. Figure 3-19 indicates that for the first half of the trial the rectangular shaped electrodes (Table 3-1) produced more voltage under open circuit conditions yet lower voltage under load. This relationship changed to the electrode surrounding the clay cup (hereafter referred to as the "exterior" electrode) (Table 3-2) performing substantially better under both open and closed conditions. Open circuit cells achieved ~500 mV while those which had been repeatedly opened and closed achieved ~650 mV as a result of better electrode colonization due to power production. 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 Time (hours) cc: closed circuited | • oc - exterior -•—oc- rectangular —constantly oc-rectangular Figure 3-19: Open and closed circuited conditions for various surface areas 99 Results from Figure 3-20 indicate these cells have an inherent ability to rapidly rebound to original levels of voltage potential immediately after open circuiting. Further opened and closed circuiting results can be observed in Appendix I. A quick comparison of this supplementary material reveals that exterior electrodes achieve a Voc of -660 mV approximately 100 mV more under open or closed circuit conditions than the rectangular electrodes. However these are very close under power density considerations. Clearly the increased power production is associated with an increase in surface area. Therefore the electrode configuration completely enveloping the exterior of the clay cup is not a more suitable arrangement in terms of kinetics but rather in terms of increased surface area. 450 10 15 20 25 40 45 rapid: oc & cc at 30 min intervals Time (hours) -1/2 hour-exterior -1/2 hour-rectangular Figure 3-20: Rapid Vi hour open and closed circuited conditions 100 3.4.11 Effect of Increasing Ionic Strength of the Substrate Previous experiments relating to ionic strength by the present author centered on the concentrations in the cathodic chamber. It is important to ascertain the effect of altering the ionic strength within the substrate in the anodic region. The configurations employed during this experiment with 167% dilution of manure were four different levels of ionic strength with rectangular felt electrodes and two different strengths with exterior felt electrodes. Results in Figure 3-21 indicate that 200 mM is more suitable IS than a stronger concentration of 400 mM for circular electrodes. 0 5 10 15 20 25 30 35 Time (hours) -A-circular electrodes - 200 mM -^-circular electrodes - 400 mM -8E-100 mM -•-300 mM —1—500 mM —— 700 mM Figure 3-21: Effect of increasing ionic strength in substrate on open circuit voltage 3.4.12 Comparison of Manure MFCs and 9 V Batteries under Constant Load To facilitate comparison between manure MFC technology and 9 V batteries both were placed under a 1000 Q, load for over 250 hours to test their resilience to power production under constant loading. Results (Figure 3-22) indicate that manure MFCs are capable of 101 producing more power under continuous load conditions than a 9 V alkaline battery in the very long term. However it is vital to point out that 9 V batteries will provide much more power in the short term. This is very promising since a 9 V battery is considered a useful option for non-grid dependent electrification. As mentioned earlier these fluctuations in voltage are associated with evaporation of anode liquid to facilitate electron transfer. This is the first time to the knowledge of the present author that these types of comparisons have been conducted in the literature. 0 25 50 75 100 125 150 175 200 225 250 Time (hours) I -*- 9 V battery —•—100% dilution -*- 200% dilution | Figure 3-22: Dilution (%) of manure compared to a 9 V battery 102 3.5 Summary and Conclusion The systems outlined in this chapter have great potential for application in electricity generation from cellulosic wastes and the following conclusions can be drawn: 1. MFCs left open circuited achieve lower peak Voc than those which had repeatedly undergone opened and closed circuit conditioning. Open circuit voltage is only a partial factor in overall power production and is not singly an adequate representation of the potentiality of voltage production. 2. Clay cups more than doubled the power production in manure MFC applications portraying their versatility in use and comparability to PEMs. 3. Air-cathodes performed exceedingly well as long as there was sufficient hydration within the clay cups and access to convection currents in air. 4. Manure MFCs performed substantially better than 9 V batteries after 50 hours under constant load applications showcasing their ability to meet power demands in the long term. 5. Circular electrodes are able to achieve three fold more power production than rectangular electrodes after altering ionic strength in the substrate. The ionic strength in the catholyte does not have a significant effect on power production below 500 mM. 6. Maintaining cathode surface area constant there appears to be an upper limit to increasing power production by enlarging anode surface area. 103 4 Lab-Scale Production of Electricity Through the use of Cow Manure and Microbial Fuel Cell Technology after Optimization of Operational Parameters 4.1 Introduction This chapter will detail a manure MFC battery after optimization of operational parameters. The main objective was to take into consideration the learning to date and design a battery which could function with little maintenance in the long term. This arrangement must be a practical application of manure MFC technology whereby these cells could be easily transported to the locale in which power generation is required and provide structural stability. The necessity to maintain cells anaerobically was explored to observe whether this is a strict requirement. It was also useful to investigate the result of molasses addition on power production. The effects of biochemical characteristics such as ionic strength of the substrate were also investigated. This battery was designed to easily form a stack of fuel cells in parallel or series combinations as desired. 4.2 Materials and Methodology The materials and methodology is similar to the previous chapter (refer to Section 3.3) with only a few modifications. This arrangement employed the use of graphite felt electrodes with dimensions outlined in Table 3-3 placed on the interior and exterior of a large clay cup as indicated detailed. Six of these completed cells were utilized in this MFC arrangement as observed in Figure 4-1, Figure 4-2, and a finalized visual representation in Figure 4-3. Each of the six individual cells has a small volume of water in their cathodic region to ensure adequate hydration of the cathode to ensure drying does 104 not inhibit power production as previous research indicated. These cells employed the use of laboratory deionized water throughout except for the substrate ionic strength trial. Figure 4-1: Exterior housing of the microbial fuel cell Figure 4-2: MFC lid showing wire connections 105 _ *»»- - . ~. . I •|> r itdlillwhlf* « •* • *»*•• *' ' V Figure 4-3: Completed microbial fuel cell arrangement It was advantageous to maintain anaerobic conditions in the anode compartment. This was achieved by using a cover (Figure 4-2) for each cell through which a small hole was cut to allow the wire from the electrode to exit the cell. The electrode spacing was maintained uniformly with this arrangement to decrease error and allow for comparability between trials. These MFCs were operated at 30°C in an incubator. The wires between the anode electrode and cathode electrode were connected with a 1 kQ resistor to complete the electrical circuit unless otherwise indicated. A small hole was drilled through the container lid surface to facilitate the ease of water replenishment to the anode to supplement evaporation as this was one of the factors affecting power production. 106 4.2.1 Polarization curve It is important to provide the polarization curve including the open circuit voltage (Voc) to ensure that the complex spectrum of MFC ability is portrayed. To determine the polarization curve 24 resistors ranging from 12 Q, to 1,000,000 Q were employed. The polarization curve was obtained after prolonged open circuit mode to ensure full establishment of Voc. For the purposes of this MFC experiment the cells were allowed 15 minutes to develop Voc conditions which when examined was sufficient time to rebound to original Voc. They were also allowed to remain under closed conditions for 15 minutes to ensure there was ample time to develop steady state conditions. 4.3 Results and Discussion 4.3.1 Polarization Curve and Power Production From the polarization curve and power-current characteristics visible in Figure 4-4 it is clear that the Voc is about half that of the previous arrangements. The first reason is the lag time associated with initial power production from un-inoculated electrodes. Secondly, the electrodes were unutilised before meaning they would be less porous to manure bacteria than after being utilised a number of times. As pointed out in the previous chapter MFCs left open circuited achieve lower peak Voc than those which have repeatedly undergone opened and closed conditioning. Therefore it would be advantageous to open and close condition these cells, a process that was not employed in this trial but would be expected to increase power production. Each MFC was capable of producing a maximum sustainable power density of 1.57 mW/m with a 266 Q, resistor. 107 Figure 4-4: Polarization and power-current characteristics of a manure MFC In Figure 4-4 there are regions of activation losses, ohmic losses and mass transport losses. Activation losses are due to the energy needed to transfer the electrons in oxidation/reduction reactions. These losses can be remedied by utilising improved catalysts at the cathode and improving electron transfer at the anode. Ohmic losses are associated with the resistance in the cation exchange membrane (clay cup), electrodes, interconnections, and compartment solution while the mass transport or concentration losses are linked to the bacteria involved in oxidation/reduction reactions. 4.3.2 Effect of Molasses Addition on Power Generation The addition of molasses did not improve power production (Figure 4-5). The addition of 5 ml of molasses at the beginning and middle of the trial caused the voltage production to plummet to negative values, a process called cell reversal. Important to note is that the 108 second rise associated towards the latter half of this trial is caused by the dilution of molasses with the repeated addition of water to supplement evaporation and not the bacterial changes to accommodate molasses addition. Time (hours) -•-1:1 -«-1:5 -*-1:10 Figure 4-5: Effect of various molasses to water ratios on voltage production The reason for cell reversal in this case is a combination of drawing current at a rate faster than fuel delivery supports as well as an inadequate supply of fuel since it was not an appropriate substrate to utilise. It is also the case that there may have been limiting catalytic substrate conversion properties of some of the microbial consortia. 4.3.3 Increasing Substrate Ionic Strength Coupled with Aerobic Conditions Experiments in the preceding chapter indicate that altering the ionic strength of the substrate had a substantial impact on power production. Therefore increased ionic strength was implemented in this larger lab-scale MFC battery. Results in Figure 4-6 109 indicate that increasing the concentration of ionic strength enhanced Voc. It is important to note that these trials were conducted in small clay cups and not manually maintained anaerobic with a plastic film on the top of the cell. The battery arrangements were in large clay cups with a concentration of only 8 mM (equivalent to tap water) under anaerobic conditions. Regardless of ionic strength concentration the cells exposed to air have higher Voc- Therefore it is advantageous to allow the cells to create their own anaerobic conditions as previously noted by Scott et al. (2007). 0 5 10 15 20 25 30 35 Time (hours) | —•— battery 460 mg/l closed -»- battery 460 mg/l open —A- exterior cell 200 mM -X- exterior cell 400 mM | Figure 4-6: Increasing substrate ionic strength under anaerobic conditions Results from Figure 4-7 clearly indicate that the power production for the battery arrangement with larger electrodes generated approximately 3-4 times more power than the smaller clay cup configurations. The increased size of clay cup and corresponding increase in electrode surface area did not enhance power density but rather power 110 production as a result of larger contact area. It is important to indicate that the power production peaks associated with the small cup configurations were not sustainable while those associated with the battery configuration were sustainable for over 100 hours. ' S 3 Z 2 O SjMWP iMSlSil o-P 0 50 100 150 200 250 300 Time (hours) -battery 460 mg/l closed -battery 460 mg/l open —9K— exterior cell 200 mM —I—exterior cell 400 mM Figure 4-7: Effect of increasing ionic strength in substrate on power production 4.4 Summary and Conclusion The systems outlined in this chapter have great potential for application in electricity generation from cellulosic wastes and the following conclusions can be drawn: 1. This MFC arrangement is capable of producing an average of 0.085 mW per cell amounting to a maximum sustainable power density 2.90 mW/m2 under 1000 Q. 2. Cells are able to produce more power when left open to air rather than under strict anaerobic conditions. Ill 3. These cells have been operated for over 700 hours without the need for maintenance beyond periodic addition of water to supplement evaporation. 4. The addition of molasses had a detrimental effect on power production in manure MFCs. 5. Although the Voc of increased ionic concentrations were higher, the power production potential was higher with a lower concentration under load conditions. 6. The increased size of clay cup and corresponding increase in electrode surface enhanced power production as a result of larger contact area. 112 5 Conclusions and Recommendations The results of this research have demonstrated that manure microbial fuel cells could successfully be operated under extended periods of time and function without the need for regular maintenance. These MFCs perform similarly to a battery except that MFCs are refuelled as opposed to being recharged. This refuelling makes these cells an attractive option since MFCs recover the capacity to deliver power almost immediately while conventional batteries take several hours to recharge. Results from these investigations indicate that MFCs are able to exhibit many of the desirable features of secondary storage batteries, including: 1. the ability to be recharged to their nearly original charge state following discharge; 2. no severe capacity fading on charge/discharge cycling; 3. the ability to accept fast recharge; 4. reasonable cycle life; and 5. low capacity loss under open circuit conditions as well as in prolonged storage under idle conditions. The most significant findings with respect to physical and biochemical characteristics as well as operational parameters are summarised below. For a complete economic analysis of alternatives to power generation with the use of cow manure refer to Appendix D. 5.1 Physical Characteristics 1. There is a direct but complex relationship between the load resistance, internal resistance and the power generated by a MFC. 2. Decreasing electrode spacing will decrease internal resistance and increase power. 113 3. Cells are able to produce more power when left open to air rather than under strict anaerobic conditions. 4. Clay cups doubled the power production in manure MFC applications portraying their versatility in use and comparability to PEMs. 5. The increased size of clay cup and corresponding increase in electrode surface area enhanced power production. 6. Air-cathodes performed exceedingly well in this study as long as there was sufficient hydration within the clay cups and on the exterior of the cells. 5.2 Biochemical Characteristics 1. Elevated power production will accompany an increase in ionic strength as long as the bacterial community can tolerate these conditions. 2. The addition of molasses had a detrimental effect on power production in manure MFCs. 5.3 Operational Parameters 1. MFCs left continuously open circuited achieve lower peak Voc than those which had repeatedly undergone opened and closed conditioning. Voc is only a partial factor in overall power production under load conditions and is not singly an adequate representation of the potentiality of voltage production. 2. Manure MFCs performed substantially better than 9 V batteries after 50 hours under constant load applications showcasing their ability to meet power demands in the long term. 3. Manure should be interior to the clay cup for maximum sustainable power production. 114 4. These cells have been operated for over 700 hours without the need for maintenance beyond periodic addition of water to supplement evaporation. 5.4 Recommendations for Future Research and Development This emerging technology has the potential to generate electricity from material that is otherwise considered waste or of no inherent value. These manure MFCs systems have great potential for application in electricity generation from a variety of industrial and agricultural cellulosic wastes. Such source of waste include crop reside and paper. The future looks promising when the fact that these MFCs recover the capacity to deliver power almost immediately is coupled with the knowledge that there have been tremendous advancements in the development of MFC technology. Power production has increased by several orders of magnitude in mediatorless MFCs within less than a decade. The method of evaluation of maximum sustainable power generation as outlined by Menicucci et al. (2006) is recommended as a standard approach to be able to accurately compare results from experiments in this emerging field of technology. It is important to note that bacterial growth and decay continues in time constantly and resistor values may pose a limiting factor in the MFC. Hence it is important to conduct tests for extended periods to ensure the resistor value obtained is indeed a suitable value. Worthy of mention is the fact that mixed bacterial communities appear to be superior to pure cultures. These mixed microbial communities actively take advantage of hydrolysis, fermentation, and anaerobic oxidation performed by other species to provide readily degradable substrates. This combination of fermentative microorganisms coupled with 115 the oxidation of fermentation products helps to make these MFCs more efficient. Therefore it would be useful to explore the effect of combining manure bacteria with other bacterial communities. It is important to note that there have been numerous studies utilizing the same substrate and microbial consortia which have resulted in power densities differing over one order of magnitude (Kim et al. 2007). This directly indicates that further research is required into the reactor configuration to minimize internal resistance and other impedance factors. There is great work to be done with the architecture of the MFC since various species of bacteria have diverse natures and it is impossible for a single model of a MFC to work for all species. There is ample work for research in these areas since the number of possible anodic reactions in MFCs is practically limitless while the number of cathodic reactions much less diverse. One of the greatest challenges to achieving substantially higher power densities, at present, is the architecture of the cell and not the composition of the microbacterial community. Therefore much of the current research is aimed at determining the optimal operating parameters and setup of the MFC. Once the intricacies of these factors have been ascertained then researchers should more deeply investigate specific microorganisms and bacterial flora. Future research should be geared towards experimenting with structurally stronger materials that are more practical for scale up. There are also a number of other recommendations relating to configuration techniques to be examined in future research such as: 116 1. The effect of series and parallel configurations is an important area that will require further research in order to ascertain whether it is possible to stack manure MFCs under the current arrangement. 2. It would be useful to reduce the thickness of the clay cup to explore that relationship to power potential since the internal resistance is a factor of this thickness. 117 6 References Aelterman, P., Rabaey, K., Pham, H. T., Boon, N., and Verstraete, W. (2006). "Continuous electricity generation at high voltages and currents using stacked microbial fuel cells." Environmental Science & Technology, 40(10), 3388-3394. Aggie Horticulture Network. Composting. Texas A & M University, Texas. Angenent, L. T., Karim, K., Al-Dahhan, M. H., Wrenn, B. A., and Domfguez-Espinosa, R. (2004). "Production of bioenergy and biochemicals from industrial and agricultural wastewater." Trends in Biotechnology, 22(9), 477-485. Animals Australia. (2006). "Dairy Cows." http://www.animalsaustralia.org/default2.asp?idLl=1273&idL2=1301 (URL accessed 04/02, 2007). Annamalai, K. (2006). "Biogas and Clean Air." Texas A & M Engineer Research Magazine, (Premiere Issue). Barnes, F. D., Van der Plas, R., and Floor, W. (1996). "Rural energy and development: improving energy supplies for two billion people." Development in Practice Series, The World Bank, Washington. Bennetto, H. P. (1990a). "Bug power - the generation of microbial electricity." Frontiers of Science, A. Scott, ed., Blackwell Publishing, Cambridge, USA, 60-82. Bennetto, H. (1990b). "Electricity generation by microorganisms." Biotechnology Education, 1(4), 163-168. Blomen, L. J., and Mugerwa, M. N. (1993). "Fuel Cell Systems." Plenum Press (New York). Bond, D. R., and Lovley, D. R. (2005). "Evidence for involvement of an electron shuttle in electricity generation by Geothrix fermentans." Applied and Environmental Microbiology, 71(4), 2186-2189. Bond, D. R., Holmes, D. E., Tender, L. M., and Lovley, D. R. (2002). "Electrode- reducing microorganisms that harvest energy from marine sediments." Science, 295(5554), 483-485. Bond, D. R., and Lovley, D. R. (2003). "Electricity production by Geobacter sulfurreducens attached to electrodes." Applied and Environmental Microbiology, 69(3), 1548-1555. Bullen, R. A., Arnot, T. C, Lakeman, J. B., and Walsh, F. C. (2006). "Biofuel cells and their development." Biosensors & Bioelectronics, 21(11), 2015-2045. 118 Burke, D. A. (2001). Dairy Waste Anaerobic Digestion Handbook: Options for Recovering Beneficial Products from Dairy Manure. Environmental Energy Company, Washington. California Energy Commission. "Cogeneration Optimization." http://www.energy.ca.gov/process/pubs/cogen.pdf (URL accessed 04/03, 2007). Chang, I. S., Moon, H., Bretschger, O., Jang, J. A. E. K., Park, H. O. I. L., Nealson, K. H., and Kim, B. H. (2006). "Electrochemically active bacteria (EAB) and mediator-less microbial fuel cells." Journal of Microbiology and Biotechnology, 16(2), 163-177. Chaudhuri, S. K., and Lovley, D. R. (2003). "Electricity generation by direct oxidation of glucose in mediatorless microbial fuel cells." Nature Biotechnology, 21(10), 1229-1232. Cheng, S., Liu, H., and Logan, B. E. (2006). "Increased performance of single-chamber microbial fuel cells using an improved cathode structure." Electrochemistry Communications, 8 489-494. Cheng, S., and Logan, B. E. (2007). "Ammonia treatment of carbon cloth anodes to enhance power generation of microbial fuel cells." Electrochemistry Communications, 9(3), 492-496. Cheng, S., Liu, H., and Logan, B. E. (2006a). "Increased power generation in a continuous flow MFC with advective flow through the porous anode and reduced electrode spacing." Environmental Science & Technology, 40(7), 2426-2432. Cheng, S., Liu, H., and Logan, B. E. (2006b). "Power densities using different cathode catalysts (Pt and CoTMPP) and polymer binders (Nafion and PTFE) in single chamber microbial fuel cells." Environmental Science & Technology, 40(1), 364-369. Cohen, B. (1931). "The bacterial culture as an electrical half-cell." The Journal of Bacteriology, 21(1), 18-19. Converse, J. C. (2001). "Fundamentals of Biogas Production from Dairy Manure." Wisconsin Ad Hoc Anaerobic Digestion Committee Meeting DATCP, University of Wisconsin, Wisconsin. CoorsTek. (2006). "Chemical and Scientific Labware." Rep. No. 8510-1097 Rev. C, Technology Materials Manufacturing Assembly, Golden, Colorado. Dartnell, L. (2006). "Sparks of life." http://www.ucl.ac.uk.cerberus.lib.uoguelph.ca/~ucbplrd/sparks_middle.htm (URL accessed 08/20, 2006). Directorate of Economics & Statistics. (2006). "Half Yearly Price Bulletin of Average Retail Prices in Rural Markets of Tripura - July 2005 to December 2005." Government of Tripura, Agartala. 119 Du, Z., Li, H., and Gu, T. (2007). "A state of the art review on microbial fuel cells: A promising technology for wastewater treatment and bioenergy." Biotechnology Advances, 25(5), 464-482. Duffield, T., Plaizier, J. C, Fairfield, A., Bagg, R., Vessie, G., Dick, P., Wilson, J., Aramini, J., and McBride, B. (2004). "Comparison of techniques for measurement of rumen pH in lactating dairy cows." Journal of Dairy Science, 87(1), 59-66. Edwards, M. (2007). "Personal Communication." University of Guelph - Academic Services 06 July. El-Mashad, H. M., Zeeman, G., Van Loon, W. K. P., Bot, G. P. A., and Lettinga, G. (2004). "Effect of temperature and temperature fluctuation on thermophilic anaerobic digestion of cattle manure." Bioresource Technology, 95(2), 191-201. Environment Canada. (2003). "A Primer on Environmental Citizenship." http://www.atl.ec.gc.ca/msc/as/acidfaq.html (URL accessed 02/26, 2007). Environmental Protection Agency. (2004). "Market Opportunities for Biogas Recovery Systems A Guide to Identifying Candidates for On-Farm and Centralized Systems." Rep. No. 430-8-06-004, Environmental Protection Agency. Environmental Protection Agency. (1999). "U.S. methane emissions 1990-2020: inventories, projections, and opportunities for reductions." Rep. No. EPA 430-R-99-013, Environmental Protection Agency, Washington, USA. Environmental Protection Agency. (1998). "NOx - How nitrogen oxides affect the way we live and breathe." Rep. No. EPA-456/F-98-005, Office of Air Quality Planning and Standards, North Carolina. Finkelstein, D. A., Tender, L. M., and Zeikus, J. G. (2006). "Effect of electrode potential on electrode-reducing microbiota." Environmental Science & Technology, 40(22), 6990- 6995. Food and Agriculture Organization of the United Nations. (2006). "Livestock's long shadow." Frame, D., Madison, F., Jarrell, W., Johnson, J., Steenlage, S., and Hayes, S. (2001). "Anaerobic Digesters and Methane Production." Rep. No. A3766, University of Wisconsin, Wisconsin. Frew, B., and Christy, A. D. (2006). "Use of landfill leachate to generate electricity in microbial fuel cells." presented at 2006 AS ABE Annual International Meeting, Oregan, USA, ASABE Paper No. 067064. Gil, G. C, Chang, I. S., Kim, B. H., Kim, M., Jang, J. K., Park, H. S., and Kim, H. J. (2003). "Operational parameters affecting the performance of a mediator-less microbial fuel cell." Biosensors & Bioelectronics, 18(4), 327-334. 120 Gooch, C. A. (Cornell University. 2006). "Anaerobic digestion in the United States." http://www.manuremanagement.cornell.edu. (URL accessed 06/22, 2006). Gorby, Y. A., Yanina, S., McLean, J. S., Rosso, K. M., Moyles, D., Dohnalkova, A., Beveridge, T. J., Chang, I. S., Kim, B. H., and Kim, K. S. (2006). "Electrically conductive bacterial nanowires produced by Shewanella oneidensis strain MR-1 and other microorganisms." Proceedings of the National Academy of Sciences 103, (30) (25 Jul 2006): 11358-11363. National Academy of Sciences, 11358. Haith, D., Crone, T., Sherman, A., Lincoln, J., Reed, J., Saidi, S., and Trembley, J. (2001). "Co-Composter Description and User's Manual." 1. Haren, M. V., and Fleming, R. (2005). "Electricity and Heat Production Using Biogas from the Anaerobic Digestion of Livestock Manure - Literature Review." Prepared for: Ontario Ministry of Agriculture, Food and Rural Affairs, Ridgetown College - University of Guelph, Ridgetown, Ontario, Canada. Hasvold, 0, Henriksen, H., MelvD r, E., Citi, G., Johansen, B. 0, Kj0nigsen, T., and Galetti, R. (1997). "Sea-water battery for subsea control systems." Journal of Power Sources, 65(1-2), 253-261. He, Z., and Angenent, L. T. (2006). "Application of bacterial biocathodes in microbial fuel cells." Electroanalysis, 18(19-20), 2009-2015. He, Z., Minteer, S. D., and Angenent, L. T. (2005). "Electricity Generation from Artificial Wastewater Using an Upflow Microbial Fuel Cell." Environmental Science & Technology, 39(14), 5262-5267. He, Z., Wagner, N., Minteer, S. D., and Angenent, L. T. (2006). "An upflow microbial fuel cell with an interior cathode: assessment of the internal resistance by impedance spectroscopy." Environmental Science & Technology, 40(17), 5212-5217. Heilmann, J., and Logan, B. E. (2006). "Production of electricity from proteins using a microbial fuel cell." Water Environment Research, 78(5), 531-537. Holmes, D. E., Nicoll, J. S., Bond, D. R., and Lovley, D. R. (2004). "Potential role of a novel psychrotolerant member of the family Geobacteraceae, Geopsychrobacter electrodiphilus gen. nov., sp. nov., in electricity production by a marine sediment fuel cell." Applied and Environmental Microbiology, 70(10), 6023-6030. Holzman, D. C. (2005). "Microbe power!" Environmental Health Perspectives, 113(11), A754-A757. Hu, Z., Wang, G., and Yu, H. (2004). "Anaerobic degradation of cellulose by rumen microorganisms at various pH values." Biochemical Engineering Journal, 21(1), 59-62. Hydro Quebec. (2004). "Comparison of Electricity Prices in Major North American Cities." Rep. No. 2004G792A,25M, Direction, Quebec. 121 Ieropoulos, I. A., Greenman, J., Melhuish, C, and Hart, J. (2005a). "Comparative study of three types of microbial fuel cell." Enzyme and Microbial Technology, 37(2), 238-245. Ieropoulos, I. A., Greenman, J., Melhuish, C, and Hart, J. (2005b). "Energy accumulation and improved performance in microbial fuel cells." Journal of Power Sources, 145(2), 253-256. Intermediate Technology Development Group. (2006). "Rural lighting." Practical Action Publishing, UK. International Energy Association. (2007). "Energy balances of non-OECD countries, 2004 - 2005." Organization for Economic Cooperation and Development, France. Iqbal, M. T. (2002). "Design of a hybrid energy system for households in Newfoundland." Institute of Electrical and Electronics Engineers, St. John's Newfoundland. Jardon, M. (2005). "Microbial fuel cells from Rhodoferax ferrireducens." The Science Creative Quarterly, Appendix(Issue 2). Katz, E., Shipway, A. N., and Wilner, I. (2003). Handbook of Fuel Cells-Fundamentals, Technology, and Application, W. Vielstich, A. Lamm, and H. A. Gasteiger, eds., John Wiley & Sons, Ltd., Chichester, 355-381. Katz, E., Willner, I., and Kotlyar, A. B. (1999). "A non-compartmentalized glucose 02 biofuel cell by bioengineered electrode surfaces." Journal of Electroanalytical Chemistry, 479(1), 64-68. Kim, B. H., Park, D. H., Shin, P. K., Chang, I. S., and Kim, H. J. (1999). "Mediator-less biofuel cell." Mediator-Less Biofuel Cell, (US Patent 5,976,719). Kim, B. H., Chang, I. S., and Gadd, G. M. (2007). "Challenges in microbial fuel cell development and operation." Applied Microbiology and Biotechnology, 76(3), 485-494. Kim, B. H., Park, H. S., Kim, H. J., Kim, G. T., Chang, I. S., Lee, J., and Phung, N. T. (2004). "Enrichment of microbial community generating electricity using a fuel-cell-type electrochemical cell." Applied Microbiology and Biotechnology, 63(6), 672-681. Kim, J. R., Min, B., and Logan, B. E. (2005). "Evaluation of procedures to acclimate a microbial fuel cell for electricity production." Applied Microbiology and Biotechnology, 68(1), 23-30. Kishore, V. V. N., Bhandari, P. M., and Gupta, P. (2004). "Biomass energy technologies for rural infrastructure and village power - opportunities and challenges in the context of global climate change concerns." Energy Policy, 32(6), 801-810. 122 Kite, S. (2006). "The birth of the fuel cell." http://www.forbesautos.com/news/features/2006/nextfest/articles/03-cell.html (URL accessed 10/26, 2007). Lam, K. (2005). "MEMS microbial fuel cells and photosynthetic electrochemical cells.". Larminie, J., and Dicks, A. (2003). Fuel Cell Systems Explained. John Wiley & Sons Ltd, West Sussex. Lee, J., Phung, N. T., Chang, I. S., Kim, B. H., and Sung, H. C. (2003). "Use of acetate for enrichment of electrochemically active microorganisms and their 16S rDNA analyses." FEMS Microbiology Letters, 223(2), 185-191. Leggett, J. A., Lanyon, L. E., and Graves, R. E. (1996). "Biological Manipulation of Manure: Getting What You Want from Animal Manure." Rep. No. G 87, Pennstate College of Agricultural and Biological Engineering, Pennsylvania State University. Liu, H., Cheng, S., and Logan, B. E. (2005a). "Power generation in fed-batch microbial fuel cells as a function of ionic strength, temperature, and reactor configuration." Environmental Science & Technology, 39(14), 5488-5493. Liu, H., Cheng, S., and Logan, B. E. (2005b). "Production of electricity from acetate or butyrate using a single-chamber microbial fuel cell." Environmental Science & Technology, 39(2), 658-662. Liu, H., and Logan, B. E. (2004). "Electricity generation using an air-cathode single chamber microbial fuel cell in the presence and absence of a proton exchange membrane." Environmental Science & Technology, 38(14), 4040-4046. Liu, H., Ramnarayanan, R., and Logan, B. E. (2004). "Production of electricity during wastewater treatment using a single chamber microbial fuel cell." Environmental Science & Technology, 38(7), 2281 -2285. Logan, B. E. (2005a). "Materials and configuration for scalable microbial fuel cells." PST 2091 8(PSU 2006-3 173), Penn State University. Logan, B. E. (2005b). "Simultaneous wastewater treatment and biological electricity generation." Water Science & Technology, 52(1), 31-37. Logan, B. E. (2004). "Biologically extracting energy from wastewater: biohydrogen production and microbial fuel cells." Environmental Science & Technology, 38(9). Logan, B. E., Murano, C, Scott, K., Gray, N. D., and Head, I. M. (2005). "Electricity generation from cysteine in a microbial fuel cell." Water Research, 39(5), 942-952. Logan, B. (2008). Microbial Fuel Cells. John Wiley & Sons, Hoboken, New Jersey. 123 Logan, B., Cheng, S., Watson, V., and Estadt, G. (2007). "Graphite fiber brush anodes for increased power production in air-cathode microbial fuel cells." Environmental Science & Technology, 41(9), 3341-3346. Logan, B. E., Hamelers, B., Rozendal, R., Schroder, U., Keller, J., Freguia, S., Aelterman, P., Verstraete, W., and Rabaey, K. (2006). "Microbial fuel cells: methodology and technology." Environmental Science & Technology, 40(17), 5181-5192. Logan, B. E., and Regan, J. M. (2006a). "Electricity-producing bacterial communities in microbial fuel cells." Trends in Microbiology, 14(12), 512-518. Logan, B. E., and Regan, J. M. (2006b). "Microbial fuel cells - challenges and applications." Environmental Science & Technology, 40(17), 5172-5180. Lor, J. (2007). "Heating with wood." http://www.tdc.ca/wood.htm (URL accessed 04/01, 2007). Lovley, D. R. (2006a). "Microbial Energizers: Fuel Cells That Keep on Going." Microbe, 1(7), 323-329. Lovley, D. R. (2006b). "Microbial fuel cells: novel microbial physiologies and engineering approaches." Current Opinion in Biotechnology, 17(3), 327-332. Lovley, D. R. (2006c). "Bug juice: harvesting electricity with microorganisms." Nature Reviews Microbiology, 4(7), 497-508. Lowy, D. A., Tender, L. M., Zeikus, J. G., Park, D. H., and Lovley, D. R. (2006). "Harvesting energy from the marine sediment-water interface II Kinetic activity of anode materials." Biosensors and Bioelectronics, 21(11), 2058-2063. Madigan, M. T., Martinko, J. M., Dunlap, P. V., and Clark, D. P. (2005). Brock Biology Of Microorganisms. Benjamin Cummings, California, USA. Marchaim, U. (1992). Biogas processes for sustainable development. MIGAL Galilee Technological Centre Kiryat Shmona, Israel. Mark Kantrowitz. (2007). "FinAid!" http://www.finaid.org/loans/scripts/interest.cgi (URL accessed 04/03, 2007). McBean, E. (2006). "Engineering in the International Context." Course Notes - ENGG 6970 Summer 2006. Menicucci, J., Beyenal, H., Marsili, E., Veluchamy, R. R. A., Demir, G., and Lewandowski, Z. (2006). "Procedure for determining maximum sustainable power generated by microbial fuel cells." Environmental Science & Technology, 40(3), 1062- 1068. 124 Merriam-Webster's Collegiate Dictionary. (2001). Merriam-Webster's Collegiate Dictionary. Merriam-Webster, Incorporated, USA. Midwest Rural Energy Council. (2005). "Anaerobic Digestion and Bio-Gas." http://www.mrec.org.cerberus.lib.uoguelph.ca/anaerobicdigestion.html (URL accessed 03/14, 2007). Min, B., Cheng, S., and Logan, B. E. (2005a). "Electricity generation using membrane and salt bridge microbial fuel cells." Water Research, 39(9), 1675-1686. Min, B., Kim, J., Oh, S., Regan, J. M., and Logan, B. E. (2005b). "Electricity generation from swine wastewater using microbial fuel cells." Water Research, 39(20), 4961-4968. Min, B., and Logan, B. E. (2004). "Continuous electricity generation from domestic wastewater and organic substrates in a flat plate microbial fuel cell." Environmental Science & Technology, 38(21), 5809-5814. Mukhtar, S., and Capareda, S. (2006). "Manure to Energy: Understanding Processes, Principles and Jargon." Rep. No. E-428, Agricultural Communications, Texas A & M University. National Agriculture Statistics Service, United States Department of Agriculture. (2007). "Milk cows and production." http://www.nass.usda.gov (URL accessed 21/06, 2007). Natural Resources Conservation Service. (2006). "Cow manure showing proper fiber and moisture content indicating balanced feed." http://photogallery.nrcs.usda.gov/Index.asp (10/07, 2007). Nesbitt, M. (2006). "Dung cakes being prepared for fuel on the Ile-de-Brehat, Brittany, c. 1900." http://fixedreference.org/2006-Wikipedia-CD-Selection/wp/rn/Manure.htm (10/08, 2007). Oh, S., Min, B., and Logan, B. E. (2004). "Cathode performance as a factor in electricity generation in microbial fuel cells." Environmental Science & Technology, 38 4900-4904. Oh, S. E., and Logan, B. E. (2007). "Voltage reversal during microbial fuel cell stack operation." Journal of Power Sources, 167(1), 11-17. Oh, S., and Logan, B. E. (2006). "Proton exchange membrane and electrode surface areas as factors that affect power generation in microbial fuel cells." Applied Microbiology and Biotechnology, 70(2), 162-169. Ohio State University. (2005). "It's electric: cows show promise as power plants." http://www.physorg.com/news6141.html (URL accessed 07/24, 2006). Oniciu, L. (1976). Fuel Cells. Abacus Press, Romania. 125 O'Toole, C. (2007). "Economic analysis of alternatives to power generation with the use of cow manure." University of Guelph. Park, H. M. (2006). Univariate Analysis and Normality Test Using SAS, STATA, and SPSS. Information Technology Services, Stat/Math Center, Indiana University. Park, D. H., and Zeikus, J. G. (2002). "Impact of electrode composition on electricity generation in a single-compartment fuel cell using Shewanella putrefaciens." Applied Microbiology and Biotechnology, 59(1), 58-61. Park, D. H., and Zeikus, J. G. (2003). "Improved fuel cell and electrode designs for producing electricity from microbial degradation." Biotechnology and Bioengineering, 81(3), 348-355. Perlack, R. D., Wright, L. L., Turhollow, A. F., Graham, R. L., Stokes, B. J., and Erbach, D. C. (2005). "Biomass as feedstock for a bioenergy and bioproducts industry: The technical feasibility of a billion-ton annual supply." Oak Ridge National Laboratory, 66, Oak Ridge, Tennessee. Pham, T. H., Jang, J. K., Chang, I. S., and Kim, B. H. (2004). "Improvement of cathode reaction of a mediatorless microbial fuel cell." Journal of Microbiology and Biotechnology, 14 324-329. Pham, T. H., Rabaey, K., Aelterman, P., Clauwaert, P., De Schamphelaire, L., Boon, N., and Verstraete, W. (2006). "Microbial fuel cells in relation to conventional anaerobic digestion technology." Engineering in Life Sciences, 6(3), 285-292. Phung, N. T., Lee, J., Kang, K. H., Chang, I. S., Gadd, G. M., and Kim, B. H. (2004). "Analysis of microbial diversity in oligotrophic microbial fuel cells using 16S rDNA sequences." FEMS Microbiology Letters, 233(1), 77-82. Piccolino, M. (1998). "Animal electricity and the birth of electrophysiology: the legacy of Luigi Galvani." Brain Research Bulletin, 46(5), 381-407. Plambeck, J. A. (1999). "Institute sciences - chemical sciences electrochemistry: introduction to the Nernst equation." http://www.intute.ac.uk/sciences/reference/plambeck/chem2/p02104.htm (URL accessed 08/28, 2006). Potter, M. C. (1911). "Electrical Effects Accompanying the Decomposition of Organic Compounds." Proceedings of the Royal Society of London. Series B, Containing Papers of a Biological Character, 84(571), 260-276. Powers, C. A., Richardson, R. E., and Scott, N. R. (2007). "Microbial Fuel Cell Operation and Use with Anaerobic Digestion for Power Production from Dairy Manure." 2007 Annual International Meeting, AS ABE, Minneapolis, Minnesota, Paper 074144. 126 Powers, C. A. (2007). "Microbial fuel cell operation and use with anaerobic digestion for power production from dairy manure." PhD thesis, Cornell University, New York. Rabaey, K., Boon, N., Siciliano, S. D., Verhaege, M., and Verstraete, W. (2004). "Biofuel cells select for microbial consortia that self-mediate electron transfer." Applied and Environmental Microbiology, 70(9), 5373-5382. Rabaey, K., Lissens, G., Siciliano, S. D., and Verstraete, W. (2003). "A microbial fuel cell capable of converting glucose to electricity at high rate and efficiency." Biotechnology Letters, 25(18), 1531-1535. Rabaey, K., Lissens, G., and Verstraete, W. (2005a). "Microbial fuel cells: performances and perspectives." Biofuels for fuel cells: Biomass fermentation towards usage in fuel cells, P. N. Lens, P. Westermann, M. Haberbauer, and A. Moreno, eds., RVA Publishing, USA. Rabaey, K., Boon, N., Hofte, M., and Verstraete, W. (2005b). "Microbial phenazine production enhances electron transfer in biofuel cells." Environmental Science & Technology, 39(9), 3401-3408. Rabaey, K., Clauwaert, P., Aelterman, P., and Verstraete, W. (2005c). "Tubular microbial fuel cells for efficient electricity generation." Environmental Science & Technology, 39(20), 8077-8082. Rabaey, K., Ossieur, W., Verhaege, M., and Verstraete, W. (2005d). "Continuous microbial fuel cells convert carbohydrates to electricity." Water Science & Technology, 52(1-2), 515-523. Rabaey, K., and Verstraete, W. (2005). "Microbial fuel cells: novel biotechnology for energy generation." Trends in Biotechnology, 23(6), 291-298. Rasby, R. (2007). "Effects of Three Hay Feeding Methods on Cow Performance and Wintering Cost." http://beef.unl.edu/stories/200703120.shtml (04/01, 2007). Rashid, M. M., Roy, B. C, Asaduzzaman, M., and Alam, M. M. (2007). "Study of the Dairy Cattle Management Systems at Farmer's Level in Jessore District of Bangladesh." Pakistan Journal of Nutrition, 6(2), 155. Rausch, J., and Sohngen, B. (1999). "An Economic Comparison of Three Manure Handling Systems." Rep. No. AE-5-99, Ohio State University Extension, Ohio. Ren, Z., Ward, T. E., and Regan, J. M. (2007). "Electricity production from cellulose in a microbial fuel cell using a defined binary culture." Environmental Science & Technology, 41(13), 4781-4786. Rennselaer Polytechnic Institute. (2002). "Lighting applications guideline for LEDs." Lighting Applications Guideline for LEDs, New York. 127 Rezaei, F., Richard, T. L., Brennan, R. A., and Logan, B. E. (2007). "Substrate-enhanced microbial fuel cells for improved remote power generation from sediment-based systems." Environmental Science & Technology, 41(11), 4053-4058. Rhoads, A., Beyenal, H., and Lewandowski, Z. (2005). "Microbial fuel cell using anaerobic respiration as an anodic reaction and biomineralized manganese as a cathodic reactant." Environmental Science & Technology, 39(12), 4666-4671. Ringeisen, B. R., Henderson, E., Wu, P. K., Pietron, J., Ray, R., Little, B., Biffinger, J. C, and Jones-Meehan, J. M. (2006). "High power density from a miniature microbial fuel cell using Shewanella oneidensis DSP10." Environmental Science & Technology, 40(8), 2629-2634. Rismani-Yazdi, H., Christy, A. D., Dehority, B. A., and Tuovinen, O. H. (2005). "Bioconversion of cellulose into electricity using rumen microorganisms as biocatalysts in a microbial fuel cell." 234th ACS National Meeting, American Chemical Society, Boston, Massachusetts, Prepr. Pap.-Am. Chem. Soc, Div. Fuel Chem. 2005. Rismani-Yazdi, H., Christy, A. D., and Tuovinen, O. H. (2007a). "Power output assessment of cellulose-based microbial fuel cells operating under different external resistance." 234th ACS National Meeting, American Chemical Society, Boston, Massachusetts, Prepr. Pap.-Am. Chem. Soc, Div. Fuel Chem. 2007, 52-1. Rismani-Yazdi, H., Christy, A. D., Tuovinen, O. H., and Dehority, B. A. (2007b). "Direct Generation of Electricity from Renewable Biomass Using Rumen Microorganisms as Biocatalysts in a Microbial Fuel Cell." Ohio State University, Agricultural Research and Development Center and the College of Food, Agricultural and Environmental Sciences, Food, Agricultural, and Biological Engineering. Graduate student poster competition, 2007. Rismani-Yazdi, H., Christy, A. D., Dehority, B. A., and Tuovinen, O. H. (2006). "A microbial fuel cell coupling anaerobic degradation of agricultural Lignocellulose wastes to electricity generation." Paper number 067115, 2006 ASAE Annual Meeting. Rismani-Yazdi, H., Christy, A. D., Dehority, B. A., Morrison, M., Yu, Z., and Tuovinen, O. H. (2007). "Electricity generation from cellulose by rumen microorganisms in microbial fuel cells." Biotechnology and Bioengineering, 97(6), 1398-1407. Rosenbaum, M., Schroder, U., and Scholz, F. (2005). "In situ electrooxidation of photobiological hydrogen in a photobioelectrochemical fuel cell based on Rhodobacter sphaeroides." Environmental Science & Technology, 39(16), 6328-6333. Rosenbaum, M., Zhao, F., Schroder, U., and Scholz, F. (2006). "Interfacing electrocatalysis and biocatalysis with tungsten carbide: a high-performance, noble-metal- free microbial fuel cell." Angewandte Chemie, 45(40), 6658-6661. Russell, J. B. (2005). "Ruminations - questions & answers and other thoughts on the role of rumen microbes in dairy cattle." USDA Agricultural Research Service, Madison, WI. 128 Scholz, F., and Schroder, U. (2003). "Bacterial batteries." Nature Biotechnology, 21(10), 1151-1152. Schroder, U., NieBen, J., and Scholz, F. (2003). "A generation of microbial fuel cells with current outputs boosted by more than one order of magnitude." Angewandte Chemie, 42(25), 2880-2883. Scott, K., and Murano, C. (2007). "A study of a microbial fuel cell battery using manure sludge waste." Journal of Chemical Technology & Biotechnology, 82(9), 809-817. Scott, K., Murano, C, and Rimbu, G. (2007). "A tubular microbial fuel cell." Journal of Applied Electrochemistry, 37(9), 1063-1068. Service, R., F. (2003). "Microbes sweet on making power." Science, 301(5639), 1453. Shantaram, A., Beyenal, H., Veluchamy, R. R. A., and Lewandowski, Z. (2005). "Wireless sensors powered by microbial fuel cells." Environmental Science & Technology, 39(13), 5037-5042. Siemons, V. R. (2001). "Identifying a role for biomass gasification in rural electrification in developing countries: the economic perspective." Biomass and Bioenergy, 20(4), 271- 285. Stams, A. J., de Bok, F. A., Plugge, C. M., van Eekert, M. H., Dolfing, J., and Schraa, G. (2006). "Exocellular electron transfer in anaerobic microbial communities." Environmental Microbiology, 8(3), 371-382. Tayhas, G., and Palmore, R. (2004). "Bioelectric power generation." Trends in Biotechnology, 22(3), 99-100. Taylor, G., Barkley. (1917). "Experiments on determination of cow manure in milk; moisture content and solubility of cow manure." Journal of Dairy Science, 1(40), 303- 312. Tender, L. M., Reimers, C. E., Stecher III, H. A., Holmes, D. E., Bond, D. R., Lowy, D. A., Pilobello, K., Fertig, S. J., and Lovley, D. R. (2002). "Harnessing microbially generated power on the seafloor." Nature Biotechnology, 20 821-825. The Hebrew University of Jerusalem. (2002). "Sir William Robert Grove." http://chem.ch.huji.ac.il/history/grove.htm (10/26, 2007). University of Illinois Extension. (2001). "Heating with Wood Needs Care and Consideration." http://www.urbanext.uiuc.edu/hortihints/0110b.html (URL accessed 04/02, 2007). US Microbics Inc. (2007). "Number of different rumen microbes compared to humans." http://www.bugsatwork.com/XYCLONYX/MICROBES_101/sld022.htm (URL accessed 10/22, 2007). 129 Wang, B. (2005). "Recent development of non-platinum catalysts for oxygen reduction reaction." Journal of Power Sources, 152 1-15. Yokoyama, H., Ohmori, H., Ishida, M., Waki, M., and Tanaka, Y. (2006). "Treatment of cow-waste slurry by a microbial fuel cell and the properties of the treated slurry as a liquid manure." Animal Science Journal, 77(6), 634-638. Zhang, T., Cui, C, Chen, S., Ai, X., Yang, H., Shen, P., and Peng, Z. (2006). "A novel mediatorless microbial fuel cell based on direct biocatalysis of Escherichia coli." Chemical Communications, (21)(21), 2257-2259. Zhao, F., Harnisch, F., Schroder, U., Scholz, F., Bogdanoff, P., and Herrmann, I. (2006). "Challenges and constraints of using oxygen cathodes in microbial fuel cells." Environmental Science & Technology, 40(17), 5193-5199. 130 Appendices A Background to Manure Microbial Fuel Cells A.l Manure as an Energy Source Manure is biologically alive with bacteria and other microorganisms that subsist on energy within the manure itself. The bacteria in these MFCs mediate the oxidation of organic compounds, such as acetate or glucose, and generate electricity by transferring the resulting electrons to the anode electrode (Rismani-Yazdi et al. 2007b). They utilise this energy as a natural response to decomposition creating a range of by-products. Various methods of storage, handling and application facilitate the managements of nutrients, odour control and create marketable products. With cellulose as one of the most ubiquitous renewable sources of energy on earth there is ample room for innovation in this process. The U.S. Departments of Agriculture and Environment estimates that there is annually 1.3 billion dry tons of biomass feedstock in the United States which would offset over 30% of the present petroleum consumption if utilised (Perlack et al. 2005). Usage of manure in a sustainable energy generation scheme has the potential to ease this fossil fuel dependency to a limited degree with the measured calorific value of manure at 82 kJ/g on dry basis (Scott and Murano 2007). A.2 Manure Decomposition Processes The decomposition of manure occurs through two means: aerobic and anaerobic. As mentioned earlier the production of electricity through microbial fuel cells requires the latter process of decomposition. 131 A.2.1 Aerobic Decomposition This most common form of decomposition refers to the decomposition of organic matter in the presence of oxygen. A large amount of heat is released during the oxidation of carbon to CO2. The organic material is initially colonized by mesophilic (25 - 40°C) organisms followed by organisms that are thermophilic (>45°C) in nature. Oxidation at thermophilic temperatures occurs much more rapidly than at mesophilic temperatures. Hence there is less time required for complete decomposition to occur at these elevated levels. Odour is indicative of a process that is partially or completely anaerobic, therefore aerobic decomposition will result in no objectionable odour. If the material is not aerated anaerobic bacteria will take the lead in the decomposition and odour will result (Aggie Horticulture Network). Table A-l provides a reference guide to the affect on microbial growth and by-products as a result of varied aerobic environmental conditions and nature of the food source. Table A-l: Aerobic conditions and food source affect on microbial growth and by-products Microorganisms Environment Food Source By-products of growth • available oxygen readily available C02, H20, • adequate but not organic matter such NH3, Aerobic bacteria excessive water as organic acids and stable humus • narrow range of pH sugar conditions (not acidic) • dry, warm environment complex organic C02, H20, • available oxygen matter such as fats, NH3, Actinomycetes • pH >5.0 proteins, and stable humus • low nitrogen content cellulose such as acids and sugar • available oxygen complex organics C02, H20, • wide range of pH such as cellulose, stable humus Fungi conditions hemicellulose, fats, • adequate but not lignin, organic excessive water nitrogen 132 A.2.2 Anaerobic Decomposition Decomposition that occurs in the absence of oxygen results in anaerobic conditions which follow a three stage decomposition process: hydrolysis, acidification, and biogas formation. Table A-2 provides a reference guide to the affect on microbial growth and by-products as a result of varied anaerobic environment and food source. Table A-2: Anaerobic environment and food source affect on microbial growth and by product^ Microorganisms Environment Food Source Byproducts of growth Acid-forming • no available oxygen complex simple organic acids bacteria • wide range of organic (often odorous), alcohols, temperature and pH compounds CO2, H2S, stable humus Methane- • no available oxygen simple biogas (C02, CH4, forming bacteria • constant temperature organic acids NH3, H2S), stable humus • pH between 6.2 and 7.2 A.2.2.1 Hydrolysis The first stage of anaerobic decomposition involves the breakdown of proteins by proteases that are secreted by fermentative bacteria. These enzymes function to convert polypeptides (proteins) to peptides (amino acids). Depolymerisation is accomplished through hydrolysis, where a water molecule is inserted between the two amino acids that are bonded together, breaking the bond between the two by capping each free reactive end with an hydrogen (H+) and hydroxide (OH") ion. This results in the long chain protein being reduced to individual amino acids. A.2.2.2 Acidification The second stage of anaerobic decomposition of manure involves a group of bacteria referred to as acetogens which break down the amino acids generated during hydrolysis into acetic acid, hydrogen gas, and carbon dioxide gas. Interestingly oxygen is required for this stage and is obtained from the substrate's fluid or from the substrate's structure 133 itself. Although these acetogens are classified as anaerobic bacteria they are capable of surviving in oxygen. Equation A-l outlines the decomposition of amino acids: 2C3H7N03(aq) + 02(1) -4 2CH3COOH(aq) + 3H(aq) + N2 + 2C02(aq) A-l serine (amino acid) + oxygen —> acetic acid + hydrogen + nitrogen + carbon dioxide This acetic acid which is formed as one of the by-products of acidification causes the pH of the organic matter to fall to 4 or 5. The pH rises as the acetogens die and the increase in pH allows for the influx of methanogens. Since the pH is heavily influenced by the balance between ammonia and volatile acids from microbial decomposition, the pH can increase or decrease depending on the conditions created by the method of management. It is important to note that the methane forming bacteria are particularly sensitive to pH (Leggett et al. 1996). The composition of the feed has a direct effect on microbial activity with anaerobic conditions encouraged by high protein diets. A.2.2.3 Biogas Formation The final stage of anaerobic decomposition involves bacteria called methanogens that produce methane gas a by-product. The characteristics of these bacteria include being very anaerobic as well as sensitive to pH. These bacteria combine hydrogen gas with the acetic acid produced during acidification to produce by-products as seen in Equation A-2. CH3COOH(aq) + 4H2(aq) + C02(aq) -* 2CH4(g) + 2H20(aq) + C02(aq) A-2 acetic acid + hydrogen + carbon dioxide —> methane + water + carbon dioxide 134 A.3 The Rumen of a Cow A cow's stomach is composed of four compartments. The largest of which is the rumen which holds roughly 150 litres of food and water. This digestion vessel is home to billions of single-celled protozoa and bacteria (Figure A-l) that synthesize the cow's food into components which can be assimilated later in the digestion process. The principle by-products of microbial activity in the rumen are amino acids and fatty acids. 1E+12 1E+10 1E+08 lllllllllll BMfl^W^^^MteMjjjB^ 1E+06 ITT flBffla|i)llll|i|||)lll ::-y 10000 iau.x —„,_ Jt • 100 — v • flggggsBgill 1 Bacteria Protisis Fungi Mycoplasma Viruses Humans Figure A-l: Number of Different Rumen Microbes compared to Humans Source: (US Microbics Inc 2007) Figure A-2 outlines the variability experienced in the pH of the rumen of a cow throughout a 24 hour period. The range in pH is almost a value of 1 with the minimum around 9 p.m and a maximum around 7 a.m. 135 ej 6.5 HfJ a, 6.3 5 IK* 6.1 5.9 0 2 3 5 7 8 10 11 13 15 16 18 19 21 23 Time of Day (homs) Figure A-2: Rumen pH throughout a 24 hour period Source: (Duffield et al. 2004) A.4 Cow Manure Manure (Figure A-3) is the outcome of digestions of herbivorous matter which has been acted upon by symbiotic bacteria thriving in the rumen. This resultant faecal matter is plentiful in minerals and the colour can range from blackish to greenish. Cow manure is readily available in the majority of rural areas worldwide. This manure is a sustainable and readily available resource in India with almost 600 million tons of wet dung produced annually from 270 million head of cattle (Kishore et al. 2004). In the developed countries animal confinements are ideal sources of manure and potentially form concentrated bioenergy feedstocks. There are currently over 9 million head of cattle in more than 75,000 total dairy operations in the United States alone (National Agriculture Statistics Service, United States Department of Agriculture 2007). On average each cow in North America produces 50.8 kilograms of manure per day according to the EPA (1999) amounting to over 180 millions tons of diary manure per year. 136 Figure A-3: Cow manure Source: (Natural Resources Conservation Service 2006) Manure has long been valued as a fertilizer and soil amendment, but can cause significant air and water pollution challenges as well. Excess nutrients (nitrogen and phosphorous), organic matter, sediments, hormones, antibiotics, and pathogens can leach or runoff impairing water bodies. The release of odour, carbon dioxide, methane, ammonia, and nitrous oxide can also degrade air quality. Livestock manure storage is estimated to be responsible for two percent of greenhouse gas emissions worldwide (Food and Agriculture Organization of the United Nations 2006). Most of this is in the form of methane which has a global warming potential 21 times that of carbon dioxide. In the United States, the EPA estimates methane emissions from livestock manure management accounts for 10% of total 1997 U.S. methane emissions (Figure A-4) (Environmental 137 Protection Agency 1999). This number continues to increase due to the increasing size of farms and use of liquid storage for manure management. "Livestock's contribution to environmental problems is on a massive scale and their potential contribution to the solution is equally large. The impact is so significant that it needs to be addressed with urgency" (Food and Agriculture Organization of the United Nations 2006). Manure 10% {17.0 MMTCE) Coal 10% |^^ Other 4% .... ^^T-. Enteric m - Fermentation • Wfi'- - L Natural Gas » * W -. J" •>• ** . ..** i 19% and Oil • *" ii i* » . n * inr^T.,(,p mi 20% * i ' 1 ' - • '' • ' 1-5- -a, •»*•*• ;• * * n- , i_ .y*ir *;**?¥<» *,*•• * <* - - -••- - • 'it. , i Landfills 37% Total = 179.6 MMTCE Figure A-4. Methane emissions in 1997 in Million Metric tons of Carbon Equivalents Source: (Environmental Protection Agency 1999) According to the Pakistan Journal of Nutrition, the price of cow manure in Bangladesh on a kilogram basis is currently less than one cent Canadian (Rashid et al. 2007), very similar to the price of less than half a cent per kilogram here in Canada according to the Canadian Ministry of Agriculture, Food and Rural Affairs (Haren and Fleming 2005). Table A-3 outlines a comparison between Bangladesh and Canada based on manure production in each country. There are obviously economies of scale involved here as well as price differences between the feed costs in these two countries. The breed of cow has a direct influence on manure production as noted by the range experience in Canadian manure production. Also if the cow was fed insufficient provisions then the quantity of manure would obviously substantially decrease. 138 Table A-3: Economic analysis of manure in Bangladesh and Canada Bangladesh Canada Feedstuff per year $236.24 $115.42 Manure production 15 kg 38 - 62 kg Sale of manure per day $0.12 $1.45 Source: (O'Toole 2007) The characteristics of cow manure are heavily dependant on their immediate environment and diet. The density of cow manure has been recorded as 0.99315 kg/1 (Haith et al. 2001). Extensive experiments conducted in 1917 by Taylor (Taylor 1917) revealed that the average moisture content of cow manure was 83 %. More recent studies portray this value to be slightly higher at 87 % (Haith et al. 2001). This value depends on diet and availability of sufficient water for hydration. A.4.1 Uses of Cow Manure Since manure is composed of organic matter it has therefore been used extensively for centuries as a fertilizer and soil amendment in agriculture. This is especially true for developing countries where the comparative cost of artificial fertilizers is prohibitive. Manure enhances the soil fertility by supplementing organic matter and nutrients such as nitrogen and phosphorus. It has also been utilised as a fuel throughout history. Dung, dried manure of a cow, has been and is still utilised as a major source of fuel in countries such as India. It amounts to over 42% of total energy sources in India when combined with fuel wood, dung cakes, and crop residue (International Energy Association 2007). During the 19th century as migrants were traversing the Oregon Trail in wagons to settle new parts of the United States they utilised "buffalo chips", dried buffalo manure, for fuel in lieu of scarce firewood. Figure A-5 portrays an image from the late 19th century where cow manure is being dried in France for combustion later. 139 Figure A-5: Drying cow manure for firewood Source: (Nesbitt 2006) More recently dung has been collected and utilized for biogas generation for heat and electricity. Cow manure is a rich source of methane and can be a stable source of energy provision. The faecal matter of cows is also utilized in developing countries to line the walls and floors of dwellings owing to its insect repellent nature. It also acts as an inexpensive thermal insulator for walls and is resistant to disintegration. Other uses include as a seed protector to help protect seeds against pests, as a heat source through natural biodegradation, or even as a mosquito repellent when smouldering. A.5 Ruminants Webster's dictionary describes ruminants as "any of a group of four footed, hoofed, even toed, and cud chewing mammals as the cattle, buffalo, bison, goat, deer, antelope, camel, 140 giraffe, llama, etc., which have a stomach consisting of four divisions or chambers, the rumen, reticulum, omasum, and abomasum; the grass etc. that they eat is swallowed unchewed and passes into the rumen or reticulum from which it is regurgitated, chewed and mixed with saliva, again swallowed, and then passed through the reticulum and omasum into the abomasum where it is acted on by gastric juice" (Merriam-Webster's Collegiate Dictionary 2001). Figure A-6 graphically outlines the digestive system in cattle. Figure A-6: Diagram of the digestive system of a cow Source: (Russell 2005) A.5.1 Ruminant Digestion and Microbes The food is mixed with saliva and forms two distinct layers of liquid and solid matter in the first two chambers. The solid matter clumps together to form the cud which is then regurgitated, slowly chewed to ensure that saliva completely mixes with the solids to 141 break down the size of the particles. Cellulose and hemi-cellulose is broken done by bacteria, protozoa and fungi into volatile fatty acids, acetic acid, propionic acid, and butyric acid. In these chambers the fermentation of protein and non-structural carbohydrate occurs. The rumen microbes consist of three main groups: bacteria, protozoa and fungi. Bacteria are responsible for the digestions of starch, fibre, protein, and sugars for the cow. Protozoa digest bacteria, some fibres, and starch granules. Only a small fraction of the microbial population of the rumen is composed of fungi but they are vital especially if the forage quality is poor (Russell 2005). The vast majority of microbes in the rumen are anaerobic and additional microbes are produced as old ones pass through the cow's digestive tract. Visual representations of a selection of these bacteria and protozoa are included in Figure A-7, Figure A-8, Figure A-9, and Figure A-10. Figure A-7: Bacteria attacking a strand of fibre from the rumen of a cow Source: (Russell 2005) 142 Figure A-8: Protozoan with a fungal spore on the side and rod-shaped bacteria underneath Source: (Russell 2005) Figure A-9: Protozoan during splitting into another cell Source: (Russell 2005) Figure A-10: Protozoal cell with cilia on the right side Source: (Russell 2005) 143 B Calculation of Coulombic Efficiency and Energy Recovery Power production is a paramount concern in MFC research but it is also important to extract as many electrons as possible, in the form of current, that are stored in the biomass. This recovery of electrons is termed Coulombic efficiency and can be calculated from Equation B-l: CE=— B-l Where: CE = Coulombic efficiency Cp = total coulombs calculated by integrating current over time CT = theoretical amount of coulombs that can be obtained from substrate (refer to Equation B-2) T M Where: F = Faraday's constant (96,485 C/mol of e") b = number of moles of electrons produced per mole of substrate S = substrate concentration (g/L) v = liquid volume (L) M = molecular weight of substrate Overall energy recovery can be calculated from Equation B-3: (E \ E = —p- *100% B-3 E F Where: EE = overall energy recovery Ep = total energy calculated by integrating power over time Ej = theoretical amount of energy that can be obtained from substrate (refer to Equation B-4) ET=^ B-4 T M Where: AH = enthalpy change of the reaction of substrate converting to carbon dioxide and water 144 C The Potential of the Electrode and the Nernst Equation The Nernst equation for an electrode in fundamental form outlined in Equation C-l (Plambeck 1999) connects the actual or measurable potential (in volts) of an electrode, E, to the standard potential of the electrode couple, E°: [RED] E = E°- — In C-l zF [OX] Where: R = universal gas constant (8.314472 J K"1 mol"1) T = absolute temperature (Kelvin) z = charge number of the electrode reaction (number of moles of electrons involved in the reaction as written) F = Faraday constant (9.6485309 x 104 C mol"1) [RED] = chemical activities of all of the species which appear on the reduced side of the electrode reaction [OX] = chemical activities of all of the species which appear on the oxidized side of the electrode reaction Since pure solids and liquids under standard conditions have a value of unity in chemical thermodynamics the Nernst equation for an electrode at 25°C is detailed in Equation C-2: 0.05915 f[RED]^ E = E°-\ In C-2 [OX] . This form of the Nernst equation can be applied to estimate the potentials of individual electrodes and the potential differences across cells. Now considering an electrode that is composed of O2 (g), Pt/fbO (Pt refers to platinum) the half-reaction is seen in Equation C-3 and the Nernst equation is detailed in Equation C-4: 02(g)+4H;q)=4e-aq)^2H20(1) C-3 Therefore; 145 [RED] E = E" In . [OX] t 0.05915N| ( [H 0f £ = 1.2288- 2 Jlog| + + i4 VL[0^2 (g)][H ] + E = 1.2288 + 0.0148 log[02(g)] + 0.05915 log[i/ ] £ = 1.2288 + 0.0148 log p[02 (g)] - 0.05915/?H C-4 From the above equation the actual potential of the oxygen electrode varies by the partial pressure of the oxygen gas (p) as well as the acidity of the solution with both exerting influence independently on the potential. Hence the maximum theoretical open circuit voltage (Voc) that a MFC, utilising glucose as substrate, can exhibit is when electrons are transferred from glucose to oxygen yielding 1.25 V since AE = (+0.82V) - (-0.43V). 146 D Economic Analysis of Alternatives to Power Generation with the use of Cow Manure D.l Introduction Societies at present demand enormous amounts of energy, which result in greenhouse gas emissions that have been linked to global warming. Humanity is experiencing a decline in fossil fuel availability and the inevitable danger of environmental degradation is apparent. Alternative sources of energy are being explored and developed at present. Energy from renewable sources may be a large portion of global energy production and usage. It is critical to be able to quantify the economic benefits of alternatives to electrification to implement cost effective measures at different stages of individual economic potential. This chapter outlines the quantitative economic analysis and the capital budgeting methods associated with the implementation of three distinct systems of power generation from material that is otherwise considered waste. D.2 Background to Electricity Provision in Rural Areas Electricity provision is one of the major problems experienced in economically developing countries (EDC), specifically in rural areas. These areas are traditionally the last locations within a country to receive electrification due to the expense of extending national electrical power grids. It is estimated that in 2005 one quarter of the world's population, effectively 1.6 billion people, had no access to electricity. Eighty percent of these people lived in rural areas of the developing world (Barnes et al 1996). Without an adequate affordable energy supply it is almost impossible to conduct productive economic activity or improve health and education. This lack of access to electrification 147 frustrates economic development hindering improvement of the quality of life thereby condemning billions of people to a life of continued poverty. It is worthwhile to mention that the power consumption pattern in these situations is often substantially lower than the patterns of urban areas and hence the generating capacity required is significantly lower. The three systems outlined in this chapter are the drying and combustion of manure, small-scale anaerobic digesters, and microbial fuel cell technology. Each of these systems is dependant on cow manure as the source of fuel for power generation. This waste material is readily available in both economically developed and economically developing countries. These three systems are not directly economically comparable since the scale of electricity production from small-scale anaerobic digestion is much greater than that of microbial fuel cell technology. However the three systems can be analysed in terms of their cost per electrical output of energy. For the purposes of this chapter it is worthwhile to be informed of the challenges associated with the comparison of different energy technologies. Methods are often advantageous on a societal level but do not have the host of secondary benefits that would be derived from some sort of dual application. The obvious advantage of alternate sources of energy to fossil fuel use, specifically for economically developing nations, is the decreased reliance on firewood for fuel which leads to the preservation of forests and the reduction in air pollution. The protection of forests has a direct effect on the level of the water table in an area and reduced soil erosion. Deforestation forces those reliant on 148 this type of energy provision to utilise less efficient sources of combustible energy, such as crop residuals, diverting these sources away from fertilizer applications. Rural communities without access to electricity usually function with minimal amounts of lighting after sunset. The lighting in these areas is typically provided by candles or kerosene lamps while flashlights powered by batteries are used for intermittent portable use (Intermediate Technology Development Group 2006). For example World Bank rural energy surveys indicate that the majority of people in rural Bolivia spend a significant amount of money on candles, kerosene and batteries for lighting their homes. The report proceeds to state that the utilisation cost of a 20 W incandescent lamp would cost only a few dollars more and would provide between 25 and 75 times more light than a candle (Barnes et al. 1996). This is yet another way in which poverty is perpetuated. Light is physically defined as electromagnetic radiation and light intensity in one direction is defined in candela or commonly referred to as "candle-power" (i.e. the output from a standard paraffin-wax candle). The rate of flow of one candela from a source is measured in lumens (lm). Table D-l outlines some common sources of light and their intensity. A standard 100 watt incandescent light bulb emits approximately 1700 lm in North America and around 1300 lm in 220V areas of the world. According to the Lighting Research Center of the Rennselaer Polytechnic Institute in New York, 98 lumens are required at a minimum for reading when light emitting diodes (LEDs) are used for lighting. Two companies Cree, Inc. and Nichia Corp have developed a white light LED with efficacy of 131 lm/W and 150 lm/W at 20 mA respectively (Rennselaer 149 Polytechnic Institute 2002). With this type of LED the power required to achieve this lighting effect for reading purposes is 0.75 W and 0.65 W respectively, one fold lower than that associated with a 100 W filament light. Table D-l: Sources of light and their intensity Light Source Energy Source Intensity Efficiency (lumens) (lumens/W) Candle Paraffin wax 1 0.01 Oil lamp Kerosene 1-10 0.01-0.1 Hurricane lamp Kerosene 10-100 0.1-0.2 Filament lamp - 3 W Electricity 10 3 Filament lamp - 40 W Electricity 400 10 Filament lamp -100 W Electricity 1300 13 LED - 1 W Electricity 150 150 Source: Adjusted from (Intermediate Technology Development Group 2006) and (Rennselaer Polytechnic Institute 2002) As expected the more important factor to the end user is often the cost per lumen as opposed to efficiency in lumens per watt. Candle wax and kerosene are cheaper sources of light than any other form of electricity provision yet produce poor quality light and large amounts of heat. Even though kerosene lamps provide more light than candles they tend to be noisy, generate substantial amounts of heat, troublesome to light, and pose a fire hazard. It is important to bear in mind that the price of kerosene is increasing in developing countries due to demand and is becoming increasingly scarce. Biomass resources are a potential solution for this energy crisis particularly in developing countries. India, for example, relies heavily on biomass for their rural energy needs and the country's primary source of bio-resource is cow dung. This manure is a sustainable and readily available resource in India with almost 600 million tons of wet dung produced annually from 270 million head of cattle (Kishore et al. 2004). The usage of 150 manure in a sustainable energy generation scheme has the potential to ease the fossil fuel dependency in rural areas to a limited degree. Governments of economically developing countries have begun to recognize the usefulness and importance of energy provision to rural households to increase the standard of living as well as the quality of living. The Food and Agriculture Organisation (FAO) of the United Nations has drawn parallels between the ability to have an alternate source of fuel and the improvement in cultural, recreation and spare time activities (Marchaim 1992). The same study noted that the enhanced lighting effect of biogas combustion has allowed for farmers in Jiangsu Province of China to be able to embroider, weave and tailor after dark thereby providing a source of secondary income. The mere availability of electricity would result in capacity building by alleviating the pressures on children and women to gather fuel, thereby allowing them to participate in education and other forms of decent work. Many of these governments are working in collaboration with donor agencies and international nongovernmental organisations to achieve this goal. D.3 Challenges Associated with the Comparison of Different Energy Generation Technologies Even though the above mentioned benefits are not directly economically based, they are integral to the improvement of society. It is impossible to precisely quantify them into monetary terms and hence will not be considered in the following economic feasibility analysis. The economic tools that are employed in this analysis include net present value, internal rate of return and payback period of an investment. 151 Firstly, the net present value is one of the standard methods for economic evaluation of an undertaking in the long-term. It is used extensively in economic analysis as it measures the excess or shortfall of cash flows in terms of present value and is commonly used for capital budgeting. Secondly, the internal rate of return (IRR) is a capital budgeting method that is utilised by companies to come to a decision in regards to a long- term investment in a specific undertaking. The IRR is basically the yield on the investment and if it is greater than the rate of interest that could have been earned by an alternate investment then it is considered economically advantageous to pursue this undertaking. Lastly, the payback period is the amount of time required for the return on an investment to "repay" the initial investment. It is important to realise that the adoption and popularization of any technology, at least in economically developing countries and by those who have a very limited budget, is heavily dependent on the economic benefit to the end user. If the technology is of limited benefit, financially, then there will be resistance to adoption, whereas if the profitability is high there will be a more rapid adoption once the value is known. The environmental benefit is typically not a driving force in economically developing countries or for those of constrained financial means. Clearly it is the economic factors that are dominant in the decision making process of adoption of a specific technology. D.4 System 1: Drying and Direct Combustion of Manure The first system that will be explored is that of drying and direct combustion of manure. This process has been employed throughout history as a source of fuel for cooking and heating. Figure D-l portrays an image from the late 19th century where cow manure is 152 being dried in France for combustion later. The dried manure of a cow is commonly called dung and is a vital source of fuel for some economically developing countries such as India and Ethiopia based on the author's personal experience. Experiments conducted by the Texas Agricultural Experiment Station (2006) note the heating value of manure as 18,700 BTU/kg (Mukhtar and Capareda 2006). BTU stands for the British thermal unit of energy. One BTU of energy is defined as the amount of heat that is required to raise the temperature of one pound of water by one degree Fahrenheit. For comparison purposes the average heating value of wood is 19,140 BTU/kg which is roughly equivalent to manure (University of Illinois Extension 2001). This shows that manure, as a solid, does burn as well as wood and it has sufficient heat value to be employed as an adequate source of fuel for cooking purposes and basic heating requirements. Figure D-l: Drying cow manure for firewood Source: (Nesbitt 2006) 153 According to the Pakistan Journal of Nutrition (2007), the price of cow manure in Bangladesh on a kilogram basis is currently less than one cent Canadian (Rashid et al. 2007). According to the Canadian Ministry of Agriculture, Food and Rural Affairs (2005), the price of manure is worth two and a half cents per gallon here in Canada (Haren and Fleming 2005). Since this cost is very similar to that in Bangladesh, an average of the two was taken as $0.0175/kg to provide an accurate representation between economically developed and economically developing nations. The average cost of feeding a cow in Bangladesh was $236.24 per year which resulted in 15.1 kg of manure produced (Rashid et al. 2007). This value is on the lower end of the spectrum in regards to manure production. In Canada the average quantity of manure produced was 57.8 kg (Haren and Fleming 2005), almost 4 times as much as in Bangladesh. The average cost of feeding a cow in Canada was approximately half as much as Bangladesh at $115.42 per year (Rasby 2007). There are obviously economics of scale involved here as well as price differences between the feed costs in these two countries. Table provides a comparison between Bangladesh and Canada based on manure production in each country. Table D-2: Economic analysis of manure collection and sale in Bangladesh and Canada Bangladesh Canada Expenditure: Feedstuff per year $236.24 $115.42 Income: Manure 15 kg 57.8 kg Sale of manure per day $0.12 $1.45 Sale of manure per year $43.80 $527.43 Net Present Value $544.17 $6,552.72 Source: (O'Toole 2007) 154 The Net Present Value formula used in this calculation is outlined in Equation D-l: ,=I (1 + rJ Where: t - the time of the cash flow n - average lifespan of a cow = 20 years (Animals Australia 2006) r - the discount rate here is 4.52% (Mark Kantrowitz 2007) Q - the net cash flow (the amount of cash) at time t. Co - the capital outlay at the beginning of the investment time (t = 0) For the purposes of this comparison it was assumed that the cow was already in the possession of the owner for other functions such as for dairy or for meat purposes. Therefore there was no capital cost associated with the animal and hence there was no payback period connected with this system. The internal rate of return for both are well over 100% which is much more than the vast majority of opportunities will offer as a return on an investment. The cow produces manure as a natural occurrence of digestion and it was assumed that the animal was being fed before their manure was collected for combustion and hence the food for digestion would not be required as an economic input for this specific analysis. The output of manure is entirely dependent on the quantity of foodstuff inputted. To achieve the upper value from the literature of 57.8 kg of manure production (Haren and Fleming 2005) it would require a considerable quantity of grass or grain feed. If the cow was fed insufficient provisions then the quantity of manure would substantially decrease. The quality, in terms of the heat value, would be unaffected as it is independent of the quantity of foodstuff. 155 With the cost of firewood at $3.38/kg (Directorate of Economics & Statistics 2006) and $0.15/kg (Lor 2007) in Bangladesh and Canada respectively, there is further savings experienced by the combustion of manure. Firewood is approximately 340 times as expensive as manure in Bangladesh based on a kilogram basis. When the heating value of manure is compared to wood and the cost of manure compared to wood, the saving on a kilogram basis is $3.37 per kilogram in Bangladesh. In Canada however it is more cost effective to buy wood to burn according to the price difference between the two. This provides ample evidence that it is more useful for economically developing countries or those countries with limited wood available to employ the practise of manure combustion if manure is readily available at an inexpensive rate. Table D-2 portrays the consequence of a farmer increasing the quantity of feed provided to the animal. With such savings experienced in Bangladesh it would be beneficial to divert some resources into additional foodstuff for the cow in order to produce more waste material for sale. Although there will be an enlarged cost associated with this new approach, there will be increased income from the sale of manure. As a result of the diversion of cow manure for burning purposes, the quality of soil nutrients will gradually decline if this practice is taken to excess. This will have severe economic repercussions in the future that are as yet unclear in their totality and beyond the scope of this chapter. The $43.80 per year income from the sale of manure for combustion in Bangladesh is actually a significant source of income for an impoverished individual. UNICEF has stated in the Millennium Development Goals that roughly 600 156 million children live on less than a dollar a day. Therefore this increased income would go a far way in impacting this statistic and increasingly the livelihood of the poor. Recent developments at the University of Texas have outlined a clear trend that the burning of cow manure is not merely a case of direct economic benefit but of secondary economic benefit as well. One of their leading researchers and experts of the combustion process has discovered that cow manure is a powerful reducer of the pollutant nitric oxide (NOx). They have been able to remove as much as 90 percent of nitric oxide from the stack gases of coal-fired power plants (Annamalai 2006). The economic savings of this is the reduction of acid rain since NOx is one of the main compounds in acid rain. The implications of acid rain are lower productivity of forestry, agriculture and fisheries. The impacts of acid rain translate into lower profits and fewer job opportunities for those industries that are affected. These implications also stretch out into the animal kingdom and kill off insects and aquatic life. In the human world the damage is primarily to building and impacts on human health through water and food ingested. Environment Canada has estimated that acid rain causes $1 billion worth of damage in Canada each year (Environment Canada 2003). They also recount that a large portion of the salmon habitat in the Maritimes has been lost as well as a significant proportion of Canada's eastern forests have been damaged. Interesting to note is that more than 80% of all Canadians live in areas with high acid rain-related pollution levels according to Environment Canada (Environment Canada 2003). 157 The Environmental Protection Agency (EPA) of the United States notes that 27% of NOx emissions are a result of utilities (Environmental Protection Agency 1998). It would be fair to draw a parallel between the damages caused by acid rain from NOx emissions and analyse this cost based on the percentage contributed by power generation facilities. This would amount to $270 million in damages to buildings alone. With the integration of the combustion of manure, 90% of the emissions of NOx from power generation utilities could be reduced. This would amount to a potential saving of $243 million in the repairs of buildings based on a percentage analysis. This is a tremendous indirect saving to the municipalities and governments involved. This technology is yet in infancy and will become more applicable with further refinement during research and development. D.5 System 2: Small-scale Anaerobic Digesters A naturally occurring process of bacterial decomposition of organic material is anaerobic digestion which occurs in the absence of oxygen. This process can be mechanically replicated with a digester which simulates the conditions necessary for anaerobic decomposition. Small-scale anaerobic digesters utilise the energy stored in the organic matter of manure by converting it to biogas. Biogas is formed by the activity of anaerobic bacteria and is comprised of roughly 65% methane (Midwest Rural Energy Council 2005). Hence the heating value of biogas is about 65% of natural gas or a quarter of propane. Storage of biogas is not practical as a result of the low energy content and corrosive nature. Biogas can be combusted to provide mechanical energy to rotate a microturbine which results in electricity production. 158 There are secondary benefits from these turbines since the waste heat that is produced could be harnessed to heat water or for conventional heating of room temperature. When biogas facilities are considered, especially in regards to economic analysis, they can be classified into two groups. The first of which, from an environmental and ecological perspective, would require significant economic costs associated with the disposal and handling of the manure. The second classification groups those in which the economic cost would be negligible. The disposal of manure from an intensive livestock facility would require a biogas facility of the first type. The quantity of manure that would necessitate disposal from such a facility would demand that environmental and ecological factors be taken into account and would thereby increase the cost of the system. The second category of biogas facilities are those such as would be implemented on a community scale in rural areas without large concentrations of cows. There are a host of benefits both socially and economically from biogas production and utilization. The economic benefit is a reduction in fuel expenses since there is less demand for the purchase of firewood, coal or electrification. With decreased demand for the collection of firewood, farmers can utilise the time saved to increase additional production which would directly raise their income. It is useful to note as mentioned earlier that the enhanced lighting effect of biogas combustion has allowed for farmers in Jiangsu Province, China to be able to embroider, weave and tailor after dark (Marchaim 1992). This allows for secondary income to supplement the earnings from farming. 159 A biogas facility, regardless of the scale, requires the biogas pit to be constructed as well as a system for gas storage. The majority of the capital cost is for the raw materials, labour as well as for the system for gas collection and distribution. In this particular scenario the output is fuel and fertilizer while the input is the cost of construction, equipment, labour, maintenance and operation (Rausch and Sohngen 1999). According to the EPA swine and dairy operations could generate 6.3 million MWh of electricity each year, a value equivalent to 722 MW of electrical energy from a municipal grid. The United States Department of Energy calculated the average cost of electricity in Ontario to be $0.1034/kWh in 2004 (Hydro Quebec 2004). These operations could potentially generate over $650 million annually at this rate of production. One of the most important factors in the economic feasibility of small-scale anaerobic digesters (Figure D-2) is whether or not the local electricity company is willing to purchase the electrical energy produced by the system and if so at what price. A functional anaerobic digester can produce two or three times the electricity that a typical dairy farm would use. A typical dairy farm for the purposes of this calculation was assumed to be 500 cows. A study completed by the University of Wisconsin (Converse 2001) found that a typical dairy of 500 cows can produce 850,000 - 1,400,000 litres of biogas daily (Frame et al. 2001). This volume of gas is capable of producing 1000 -1,400 kWh of electricity per day with an engine. 160 Figure D-2: A plug flow anaerobic digester from a 500-cow dairy Source: (Gooch Cornell University. 2006) An average dairy cow in Canada excretes approximately 70 litres of manure per day (Haren and Fleming 2005). This typically contains seven kilograms of volatile solids which have a methane productivity of 468 ± 6 litres of methane per kilogram of volatile solid (Haren and Fleming 2005). Therefore for an average cow this would be approximately 3,300 litres of methane available for combustion daily in theory per day. However according to a study completed by the Midwest Rural Energy Council the biogas produced is typically lower at approximately 2,800 litres of methane available for combustion each day per cow (Converse 2001). Each cubic foot of methane yields 1,000 BTU of available energy (Haren and Fleming 2005). Each cow employed in this system would result in an average of 98,000 BTU of energy generated each day. This would produce 28.7 kilowatt-hours (kWh) per day at 100% efficiency, which is impossible to achieve. A more realistic efficiency would be at 25% with a resulting 7.2 kWh per day generation capacity per cow. A typical house requires 68.4 kWh per day to function which would necessitate nine cows per house based on this 161 average (Iqbal 2002). The above values are averages that will vary according to diet, overall health of the animal and quantity of manure produced. A single dairy cow is capable of producing 7 kWh of power each day when this system is implemented. This can be a tremendous economic saving for a farm which employs electrification from cow manure as opposed to the municipal grid. There are also extensive savings enjoyed by the owner of a farm that makes use of an anaerobic digester due to the reduced volume of solid waste disposal required. The literature has a wide range of prices for an anaerobic system for complete waste management. An extensive study that was completed of facilities in the United States has recorded the cost of $500 to $800 per cow (Burke 2001). It is important to note that the capital cost per cow does not adequately represent the entire cost of operation since the system would not completely be represented financially in that estimate. A review of a number of studies completed by Burke (2001) put the adjusted capital costs at $590 to $944 per cow roughly 1.18 times the average cost. The size of farm will be a direct factor as a consequence of the economies of scale involved as a result of a larger volume of manure available to larger farms while simultaneously decreasing associated costs on a per cow basis. As with economies of scale if the system is small it may provide only immediate benefits in terms of waste management and may not be economically viable for the production of electricity. The payback period on average has been stated as roughly ten years (Midwest Rural Energy Council 2005) but this does not factor into account the secondary savings resulting from 162 the production of fertilizer as well as the savings involved in no longer having to pay for adequate disposal of animal waste according to local environmental regulations. Table D-3 below outlines the capital and operating costs of typical European digestion systems. To be fair the capital, maintenance and operation costs are significantly lower in the United States than they are in Europe but the income from the sale of solid is also much lower in the United States. For example the capital cost of dairy waste systems in Idaho are in the range of $2,700 per KW to $6,000 per kW which are only 30% to 60% of the European costs (Burke 2001). Table D-4 provides a breakdown of the quantity of carbon dioxide emissions that are reduced and displaced through burning of methane gas. Table D-3: Capital and Operating Costs of European Digestion Systems Large 5000 Cow Facility Small 125 Cow Farm Capital Cost $9,113,000 $500,000 Annual Operating Cost $643,000 $8,800 Power Sale Rate $/kW $0.06 $0.06 Heat Sale $/kW $0.01 $0.01 Solid Sales $700,000 $20,000 Source: (Burke 2001) Table D-4: Sample methane emissions calculations Factors for an anaerobic digester Methane emission reductions Number of cows 500 Average live weight, lb/cow 1,400 Total volatile solids (VS) excretion rate, lb/1000 lb live weight-day 8.5 Maximum methane producing capacity (Bo), ft3/lb VS 3.84 Methane conversion factor (MCF) 0.292 Methane density, lb/ft3 0.041 Methane emissions*, tons/y 50 Methane emission reduction from biogas capture and utilization0, tons/yr 50 Equivalent reduction in carbon dioxide emissions, tons/yr 1,048 Displaced emissions from utility electric generation Methane production, ft3/yr @38.5 ft3/cow-day 7,026,250 Electricity generation potential0, kWh/yr 467,838 Reduction in utility carbon dioxide emissions8, tons/yr 526 163 Total greenhouse gas emission reductions as carbon dioxide, tons/yr 1,574 Source: (Environmental Protection Agency 2004) Methane emissions = number of cows * average live weight * VS excretion rate * 1/1000 * B0 * MCF * methane density * 365 days/yr * ton/2000 lb * Methane has approximately 21 times the heat trapping capacity of carbon dioxide. n Generation, kWh/yr = methane production * 1,010 BTTJ/ft3 of methane * kWh/3,413 BTU * 0.25 (methane to electricity conversion efficiency) * 0.9 (on-line efficiency). § Assuming 2,249 lb of carbon dioxide emitted per MWh generate from coal The following sample calculation combines Table D-3 and Table D-4 to generate the following extrapolations as seen in Table D-5. Sources of income are from the sale of electricity, heat, solids as well as sale of carbon credits from the combustion of methane gas per the Kyoto protocol (McBean 2006). Here labour costs are not include as it is assumed that the farm already has labourers and this would be integrated into their regular schedule of activities and hence not incur an additional charge. Table D-5: Sample calculations of total expenditure Large 5000 Cow Facility Small 125 Cow Farm Income Sale of electricity 4,678,380 kW @ $0.06 /kW 116,959kW@$0.06/kW $280,702.80 $7,017.57 Sale of solids 4,678,380 kW @ $0.01 /kW 116,959 kW@ $0.01/kW $46,783.80 $1,169.60 Kyoto - carbon credits 10,480 tons @ $21 /ton 262 tons @ $21 /ton $220,080.00 $5,502.00 Sales of solids $700,000.00 $20,000.00 Total income $1,247,566.60 $33,689.17 Expenditure Capital Cost $9,113,000.00 $500,000.00 Annual Operating Cost $643,000.00 $8,800.00 Total expenditure $9,756,000.00 $508,800.00 Loss after first year $8,508,433.40 $475,110.83 It is challenging to outline in detail the economics of establishing a small-scale anaerobic digester since many factors affect the costs as well as variation in circumstances between 164 countries. The presence of a renewable energy policy that allows for the sale of electricity from these systems will have a tremendous influence into the feasibility of such a venture. For the example elaborated in Table D-5 the sale of electricity amounts to one fifth of the total revenue. Therefore the amount that the electricity provider offers for buying electricity is a very important factor in this analysis. Table D-6 details the net present value and internal rate of return calculations of these two alternatives. Here the lifespan of an anaerobic small-scale digester is taken as 20 years (California Energy Commission). Table D-6: Net present value of the two alternatives Discount rate Small 125 Cow Farm 5% -$180,786.60 4.52% -$169,157.68 2.5% -$109,267.01 1% -$50,357.59 0% -$2,216.60 The IRR for this specific system is approximately 0% Discount rate Large 5000 Cow Facility 5% - $1,503,584.63 4.52% - $1,207,852.24 4% -$862,252.51 3% -$115,121.95 2.8% $48,655.71 The IRR for this specific system is roughly 2.85% Many other opportunities would provide a similar return on an investment as these two alternatives outlined in Table D-6. The IRR of a small farm of 125 cows is 0% which implies neither a substantial loss nor a substantial gain on the investment. The decision to implement this system will then have to include other economic factors such as employment opportunities or savings from what would have to be done with the manure otherwise. The payback periods are 16 years and 20 years respectively for the two 165 systems. Since the lifespan of an anaerobic system is estimated at 20 years it would only pay itself off and not incur profits for the small cow farm. D.6 System 3: Microbial Fuel Cell Technology An emerging technology called microbial fuel cells technology has the potential to generate electricity from material that is otherwise considered waste or of no inherent value. MFCs are bio-electrochemical transducers that convert the reducing power generated from the metabolism of organic substrates into electrical energy (Ieropoulos et al. 2005a). These MFCs have the ability to alleviate to a small degree the dependency on fossil fuels and provide clean energy because they are carbon-neutral i.e. the oxidation of the organic matter only releases recently fixed carbon back into the atmosphere thereby not further exacerbating global warming (Lovley 2006c). These MFCs operate in a self-sustaining manner, which removes the necessity for regular maintenance, and therefore these systems could be used as power sources in remote areas that are not serviced by centralised power grids and function with long-term stability. In some situations it is impractical, costly and time consuming to have to change batteries; therefore MFCs would be a practical solution to this challenge using resources available at the locale in which the power generation is required. For the purposes of this essay the MFCs will utilise cow manure as the substrate for energy production. Researchers at Ohio State University have been able to generate electricity from the bacteria that naturally occurs in the rumen of a cow and is passed out in the manure (Ohio State University 2005). Research conducted by the author of this chapter with manure 166 MFCs has achieved values of approximately 670 mV under open circuit conditions. A typical AA battery is 1.5 V which has just over twice as much voltage than these fuel cells. An interesting observation has been that these MFCs performed better than a standard 9 V battery under a constant 1000 Q. load. Although these cells produce about half as much as a normal AA battery it is important to remember this is a free source of energy that could be used in remote locations or those sites off of the municipal electricity grid. The power generation achieved to date has been 0.060 mW (3.33 mW/m2) with the total surface area of the electrode as 128.46 cm2 as one combination and 0.032 mW (5.46 mW/m2) as another. This latter arrangement has a higher per unit surface area of power production. In order to power a 44 mW LED which would provide over 10 times the light as a single candle it would require 8.06 m2 of electrode. However since the light from 1 candle is often what an individual suffices with this would only require 0.80 m2 of electrode which is small and costly at only $116.50/m2. Therefore to power this type of LED it would cost $93.20 for the electrode material which is a minimal sum of money considering it will provide power generation in the long term. The clay container that the manure is housed in costs $24.03, the beaker to the cell costs less than a dollar and could be any container readily available at the locale in which power generation is required. The total cost to produce one of these cells is therefore $93.20 (if the clay could be manufacture locally which is very plausible) or $117.53 if all the materials are to be provided. 167 The lifecycle of these cells is rated at 10,000 hours or 1.14 years at constant power generation. The clay cup would require replacement at this point and a subsequent $24.03 would be expended at that time. Table D-7 outlines the costs associated with these microbial fuel cell systems. With each batch of manure this system is capable of producing electricity at the rated power density for over 50 hours. Therefore the operating costs are simply the emptying and refilling of the cell with fresh manure, a procedure that takes three minutes every two days. These cells are capable of holding only 80 grams of manure which corresponds to $0.11 per batch of substrate for the fuel cells. Table D-7; Capital and Operating Costs of microbial fuel cell Potential power generation per year, kWh * 10"4 2.81 Expenditure: Capital Cost $117.53 Cost of manure per batch $0.11 Cost of manure per year $20.08 Income: Power savings from cell run for one full year (@ $0.000029 $0.1034/kWh) (Hydro Quebec 2004) Loss encountered each year -$137.72 There is annual operating cost in Table D-7 since it is assumed that this practise would be an acceptable cost for free power generation. From Table D-7 it is possible to evaluate the usefulness of this technology at present. As with most ground breaking technologies there is initially a loss encountered with their use but research and development will improve their functionality and the economic competitive nature of this technology will be exposed. This technology is at present geared toward small-scale electrification through the use of light-emitting diode (LED) lighting. This would be a useful way in which allow for the creation of secondary income as was mentioned earlier in Jiangsu 168 Province, China where farmers were able to embroider, weave and tailor after dark as a result of low cost lighting (Marchaim 1992). D.7 Summary and Conclusions This chapter has outlined the quantitative economic analysis and capital budgeting methods associated with the implementation of three distinct systems of power generation from material that is otherwise considered waste. Through this analysis the economic benefits of various alternatives to electrification were quantified. The reader is able to observe instances where the implementation of each system would be profitable and advantageous. It is imperative to bear in mind that the host of other benefits from a technology tend to be overlooked in basic economic analyses such as the economic savings from the reduction of acid rain from decreased NOx emissions. The drying and combustion of manure is seen as a profitable source of secondary income for the rural poor with income levels of $43.80 in Bangladesh and $527.43 in Canada. Small-scale anaerobic digester systems would be cost effective on a large scale (5000 cows) with the payback period of 16 years. It would break even on a small scale (125 cows) after 20 years of operation. At present microbial fuel cell technology with cow manure is not economically viable since there is a net loss each year of roughly $138. As scientific breakthroughs in MFC technology advance, the costs associated will be reduced and the price per electrical output will similarly drop. With the quantification of the economic benefits of each alternative the reader is able to adopt these measures at their specific economic potential. 169 E Dimensions of Electrode Materials Table E-l: Woven carbon fibre Dimension Anode Cathode Thickness (cm) 0.033 0.033 Width (cm) 0.635 0.635 Length (cm) 12.70 8.255 Wet length (cm) 6.50 2.50 SA (cmz) 8.73 3.38 Table E-2: Uni-carbon fibre Dimension Anode Cathode Thickness (cm) 0.018 0.018 Width (cm) 0.953 0.953 Length (cm) 12.70 8.255 Wet length (cm) 6.50 2.50 SA (cm2) 12.66 4.89 Table E-3: Solid graphite Dimension Anode Cathode Thickness (cm) 1.30 1.30 Width (cm) 1.57 1.57 Length (cm) 15.37 9.50 Wet length (cm) 6.50 2.50 SA (cm7) 41.39 18.43 Table E-4: Uni-carbon fibre (increased surface area) Dimension Anode Cathode Thickness (cm) 0.018 0.018 Width (cm) 0.953 0.953 Length (cm) 20.00 20.00 Wet length (cm) 18.85 18.85 SA (cmz) 36.64 36.64 Table E-5: Woven carbon fibre (anode twice as large as cathode) Dimension Anode Cathode Thickness (cm) 0.033 0.033 Width (cm) 0.635 0.635 Length (cm) 51.00 23.25 SA (cm2) 68.18 31.10 Table E-6: Uni-carbon fibre (anode twice as large as cathode) Dimension Anode Cathode Thickness (cm) 0.018 0.018 170 Width (cm) 0.953 0.953 Length (cm) 51.00 24.50 SA (cm2) 99.08 47.61 Table E-7: Solid graphite electrodes Dimension Anode Cathode Thickness (cm) 1.30*2 1.30 Width (cm) 1.57*2 1.57 Length (cm) 10.16*2 10.16 SA (cm2) 124.80 62.40 *2 implies 2 anode electrodes Table E-8: Solid graphite (for clay cup trial 2:1 ratio of anode to cathode) Dimension Anode Cathode Thickness (cm) 1.30*2 1.30 Width (cm) 1.57*2 1.57 Length (cm) 9.50 *2 9.50 SA (cm2) 117.22 58.61 *2 implies 2 anode electrodes Table E-9: Graphite felt (for clay cup trial 2:1 ratio of anode to cathode) Dimension Anode Cathode Thickness (cm) 1.30*2 1.30 Width (cm) 1.57 *2 1.57 Length (cm) 9.50 *2 9.50 SA (cm2) 117.22 58.61 *2 implies 2 anode electrodes Table E-10: Woven carbon fibre (mini cup) Dimension Anode Cathode Thickness (cm) 0.033 0.033 Width (cm) 0.635 0.635 Length (cm) 7.50 7.50 SA (cm2) 10.06 10.06 Table E-ll: Woven carbon fibre (large cup) Dimension Anode Cathode Thickness (cm) 0.033 0.033 Width (cm) 0.635 1.27 Length (cm) 10.00 23.25 SA (cm2) 13.08 60.67 Table E-12: Uni-carbon fibre (large cup) Dimension Anode Cathode Thickness (cm) 0.018 0.018 171 Width (cm) 0.635 1.27 Length (cm) 10.00 24.50 SA (cm2) 13.08 63.16 Table E-13: Uni-carbon fibre (large cup, cathode Vi sized) Dimension Anode Cathode Thickness (cm) 0.018 0.018 Width (cm) 0.635 0.635 Length (cm) 10.00 24.50 SA (cm2) 13.08 32.02 Table E-14: Woven carbon fibre (large cup, cathode twice size) Dimension Anode Cathode Thickness (cm) 0.033 0.033 Width (cm) 0.635 1.27 Length (cm) 10.00 51.00 SA (cm2) 13.08 132.99 Table E-15: Uni-carbon fibre (large cup, cathode twice size) Dimension Anode Cathode Thickness (cm) 0.018 0.018 Width (cm) 0.635 1.27 Length (cm) 10.00 51.00 SA (cm2) 13.08 131.42 172 F Pictures of Electrode Material -"7M 1 Figure F-l: Woven carbon fibre electrode Figure F-2: Uni-carbon fibre electrode 173 r I*JV -" * -»*JtfrS i m|yf Tlt^futufrl frl ^^ * * - "Bill t^ Figure F-3: Solid graphite electrode 174 G Details relating to Electrode Material Costs and Resistance The solid graphite electrodes were obtained from Asbury Wilkinson (http://www.asbury.com/) at $0.09209/cm2 (for uniform 1.57 cm width). The unit resistance of this material was: 81.3 Q/cm The woven carbon fibre and uni-carbon fibre electrodes were obtained from Aerospace Composite Products (http://www.acp-composites.com) at $0.03305/cm2 (for uniform 0.635 cm width and 0.033 cm thickness) and $0.05348/cm2 (for uniform 0.953 cm width and 0.017 cm thickness) respectively. The unit resistance of these materials was: 76.8 Q/cm and 90.3 Q/cm respectively. Graphite felt electrodes were obtained from National Electrical Carbon Products (http://www.nationalelectrical.com/) at $0.01165/cm2 (for uniform 0.3175 cm width). The unit resistance of this material was: 5.3 Q/cm (dry) and 14.2 Q/cm (wet) 175 H Trial Configurations and Tests Conducted on each MFC Each of the cells in Table H-l were conducted with a manure:water ratio of 1: 3.75 by weight unless otherwise stipulated. At 100 hours and 500 hours 5 g of sugar were added to each of the cells to provide substrate for the bacteria to grow in an experiment to test the result of the addition of sugar. Table H-l: Varying electrode material and salt addition Electrode material Setup Test 1 Woven carbon fibre Control - water only - 2 Solid graphite Control - water only - 3 Uni-carbon fibre Control - water only - 4 Woven carbon fibre No initial addition of salt/food 5 Solid graphite No initial addition of salt/food 6 Uni-carbon fibre No initial addition of salt/food 7 Woven carbon fibre Initial addition: 5 g NaCl 8 Solid graphite Initial addition: 5 g NaCl 9 Uni-carbon fibre Initial addition: 5 g NaCl Uni-carbon fibre Initial addition: 5 g NaCl 10 5 g sugar 11 Uni-carbon fibre Control to test trial 10 12 Uni-carbon fibre 2 : 1 ratio of anode : cathode 13 Solid graphite 2 : 1 ratio of anode : cathode 14 Woven carbon fibre 2 : 1 ratio of anode : cathode Solid graphite 2 : 1 ratio of anode : cathode 15 with initial addition of 5 g NaCl Table H-2: Temperature configurations Conductivity of tap water and salt required to replicate conductivity Temperature Conductivity (nS/cm) Salt (mg/L) Location 20°C 688.2 384.1 Water bath 25°C 767.2 434.4 Water bath 30°C 866.9 460.4 Incubator 37°C 972.3* 468.8 Incubator *this value was extrapolated since the Accumet Excel conductivity meter could not reference higher than 30°C Table H-3: Ionic strength configurations Ionic Strength Conductivity (mS/cm) Salt(g/L) 100 mM 12.34 5.84 200 mM 23.23 11.67 176 300 mM 34.11 17.53 400 mM 43.67 23.38 500 mM 53.23 29.22 700 mM 71.29 40.91 Table H-4: Further electrode material trials Electrode material Setup Cup size 1 Graphite felt 4 cups Small 2 Graphite felt 3 cups Small 3 Graphite felt 2 cups Small 4 Graphite felt 1 cup Small 5 Solid graphite 2 cups Small 6 Woven carbon fibre 1 cup Mini 7a Uni-carbon fibre lcup Large 7b Woven carbon fibre 1 cup Large 8 Uni-carbon fibre 1 cup Large 9a Uni-carbon fibre 1 cup Large 9b Woven carbon fibre lcup Large Table H-5: Location of mixture on interior or exterior of clay cup Substrate Location Resistor Cup size Replicate 1 Manure Inside 1000 ohm Large Triplicate 2 Manure Inside 680 ohm Large Triplicate 3 Manure Outside 680 ohm Large Triplicate 4 Manure Inside 1000 ohm Large Triplicate Rumen fluid and Outside 1000 ohm Small Duplicate 5 manure Rumen fluid and Inside 1000 ohm Small Duplicate 6 manure 7 Rumen fluid Outside 1000 ohm Small Duplicate Table H-6: Manure dilution ratio Substrate Manure Rumen fluid Water 1 Manure 1 1 2 Manure 1 1 3 Manure 1 1 4 Manure 1 1 Rumen fluid and 1 2.5 5 manure Rumen fluid and 1 2.5 6 manure 7 Rumen fluid 2.5 1 177 I Results from Trial Configurations 178 ON r- •B o w .'3s u t3 V o a c & e I* s o I CM o o o o o o o o o o o o o o o in in in in in in in m o o o o•>* o o o r-. r- CD co in m ^- CO CO CM CM •»~ T~ 2J (Aui) aBenoA 2s en 700 Joc&ccl 100 200 300 400 500 600 700 Time (hours) • 1 hour - exterior -•— 1 hour - rectangular Figure 1-2: Results from 1 hour open and closed circuited conditions 180 450 closed rapid B circuit 400 Jou will! 300 > 250 o a •5 200 150 100 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 Time (hours) • 1 hour - exterior —•— 1 hour - rectangular Figure 1-3: Results from rapid 1 hour open and closed circuited conditions 181 0 100 200 300 400 500 600 70O Time (hours) I—l—3/4 - rectangular ——1 1/2 - rectangular Figure 1-4: Results from 3A and 1 Vi hour open and closed circuited conditions 182 700 -MM 100 200 300 400 500 600 700 Time (hours) -o/c- rectangular •1/2 hour - rectangular • 1 hour - rectangular •1 1/2 - rectangular •constantly o/c - rectangular Figure 1-5: Results from various open and closed circuited conditions of rectangular electrode 183 700 100 200 300 400 500 600 700 Time (hours) • o/c - exterior • 1/2 hour -exterior X 1 hour - exterior Figure 1-6: Results from various open and closed circuited conditions of exterior electrode 184 J Statistical Results All calculations of parameters such as mean or median values, standard deviations and data ranges were done using SAS v9.1. This software package was also used to conduct ANOVA tests. To ensure normality of the data sets prior to ANOVA tests, the 'PROC UNIVARIATE' and 'PROC MIXED' procedures were utilised to compile numerical descriptive statistics, as outlined below. It was the Kolmogorov-Smirnov tests that were used as arbiter of normality. This test was the obvious choice since it is particularly suited for capturing departures from normality in small-sized data sets between 50 and 5000 (McBean 2006) and (Park 2006). Even if data was not normally distributed it was still possible to conduct ANOVA tests if the Brown and Forsythe's test for homogeneity of variance was observed (Edwards 2007). Finally the differences of Lease Squares Means would be analysed to determine if something was statistically significant at the a = 0.05 level. J.l Addition of Salt and Increasing Anode Surface Area Table J-l: Statistical results of addition of salt and increasing anode surface area Test Significantly Pr value different water only vs. cow manure Yes* <0.0001 5g salt addition at 1:1 vs. no salt No 0.0871 anodexathode of 1:1 vs. 2:1 without salt in either Yes* <0.0001 anodexathode of 1:1 vs. 2:1 with salt in either Yes* <0.0001 (abridged data - 0 to 250 hours) 5g salt addition at 2:1 vs. no salt No 0.2679 5g salt addition at 2:1 vs. no salt slightly 0.00318 (abridged data - 0 to 240 hours) under log transformation In summary: • Cow manure is significantly different to water only 185 The addition of 5g salt does not appear to significantly affect power production in 1:1 and 2:1 anode to cathode ratios A ratio of 2:1 of anode to cathode is significantly different to a 1:1 whether there is an addition of salt or not J.2 Usefulness of Clay Cups and Increasing Anode Surface Area Table J-2: Statistical results of usefulness of clay cups and increasing anode surface area Test Significantly Pr value different anodexathode of 1:1 vs. 2:1 with clay cup Yes <0.0001 anodexathode of 1:1 vs. 2:1 without clay cup Yes* 0.0003 1:1 with clay cup vs. 1:1 without clay cup Yes* <0.0001 2:1 with clay cup vs. 2:1 without clay cup Yes* 0.0005 * under log transformation In summary: • A ratio of 2:1 of anode to cathode is significantly different to a 1:1 whether there is a clay cup or not • Trials with clay cups are significantly different than trials without J.3 Altering Electrode Materials Coupled with In creased Dilution Table J-3: Statistical results of altering electrode materials coup ed with increased dilution Test Significantly Pr value (ratios refer to manure:water) different (Solid) 1:1 vs. 1:2 Yes 0.0001 (Solid) 1:1 vs. 1:3 Yes <0.0001 (Solid) 1:1 vs. 1:4 Yes <0.0001 (Solid) 1:2 vs. 1:3 Yes 0.0066 (Solid) 1:2 vs. 1:4 Yes 0.0083 (Solid) 1:3 vs. 1:4 No 0.9278 (Felt) 1:1 vs. 1:2 Yes* <0.0001 (Felt) 1:1 vs. 1:3 Yes* 0.0006 (Felt) 1:1 vs. 1:4 Yes* <0.0001 (Felt) 1:2 vs. 1:3 No* 0.1019 (Felt) 1:2 vs. 1:4 No* 0.3441 (Felt) 1:3 vs. 1:4 No* 0.4835 Is there a significant interaction between the type of graphite Yes 0.006 186 and the ratio of manure to water Solid vs. Felt Yes 0.0435 1:1 vs. 1:2 for both types of graphite Yes <0.0001 1:1 vs. 1:3for both types of graphite Yes <0.0001 1:1 vs. 1:4 for both types of graphite Yes <0.0001 1:2 vs. 1:3 for both types of graphite Yes 0.0118 1:2 vs. 1:4 for both types of graphite No 0.3515 1:3 vs. 1:4 for both types of graphite No 0.0929 * under log transformation In summary: • For the solid graphite electrode each of the ratios of manure to water was significantly different than each other except for the 1:3 vs. 1:4 ratio • For the graphite felt electrode each of the ratios of manure to water of 1:1 to the others was significantly different with the other ratios not significantly different than the other • There is a significant difference between the types of graphite irrespective of the manure to water ratio • Only the 1:2 and 1:3 vs. 1:4 ratios were significantly different depending on graphite type J.4 Further Analysis of the Effect of Dilution Table J-4: Statistical results of further analysis of the effect of dilution Test Significantly Pr value (ratios refer to manure:water) different 1:1 vs. 1:0.5 vs. 1:0 Yes <0.0001 Cut manure vs. uncut manure Yes 0.0570 Interaction between ratio and whether or not the manure was No 0.1404 cut 1:1 vs. 1:0.5 Yes 0.0483 1:1 vs. 1:0 Yes <0.0001 1.0.5 vs. 1:0 Yes <0.0001 In summary: • Each of the ratios is significantly different to one another 187 • It appears as though whether the manure is cut up into small pieces or not is significantly different (Pr = 0.0570). Under some criteria this would be considered not significantly different • There is no significant interaction between ratios and whether manure is cut or not J.5 Temperature Effect on Power Generation Table J-5: Statistical results of temperature effect on power generation Test Significantly Pr value different 20°C vs. 25°C Yes* 0.0120 20°C vs. 30°C Yes* <0.0001 20°C vs. 35°C Yes* <0.0001 25°C vs. 30°C Yes* 0.0600 25°C vs. 35°C Yes* 0.0025 30°C vs. 35°C No* 0.2012 under log transformation In summary: • Each of the temperatures is significantly different from one another with the exception of 30°C vs. 35°C • When 30°C vs. 35°C was run from the time they deviate the Pr value is 0.0713 which is slightly significant. The difference is clearly perceptible by visual discrimination of the graph of power production against time J.6 Electrode Spacing Table J-6; Statistical results of electrode spacing Test Significantly Pr value different 5 cm vs. 6.5 cm No* 0.4253 5 cm vs. 8 cm Yes* <0.0001 6.5 cm vs. 8 cm Yes* <0.0001 under log transformation In summary: • Whether the spacing is 5 cm or 6.5 cm there is no significant difference 188 The spacing of 5 cm compared to 8 cm is significantly different The spacing of 6.5 cm compared to 8 cm is significantly different J.7 Ionic Strength Effects Table J-7: Statistical results of ionic strength effects Test Significantly Pr value different 100 mM vs. 300 mM No 0.9548 100 mM vs. 500 mM No 0.7675 100 mM vs. 700 mM Yes 0.0213 300 mM vs. 500 mM No 0.8111 300 mM vs. 700 mM Yes 0.0242 500 mM vs. 700 mM Yes 0.0406 In summary: • Each of the ionic strengths is significantly different than the 700 mM • Otherwise they are not significantly different from one another J.8 Electrode Material and Increasing Anode Surface Area Table J-8: Statistical results of electrode material and increasing anode surface area Test Significantly Pr value different (felt) 4 anodes vs. 3 anodes No* 0.1015 (felt) 4 anodes vs. 2 anodes No* 0.1204 (felt) 4 anodes vs. 1 anode Yes* <0.0001 (felt) 3 anodes vs. 2 anodes Yes* 0.0032 (felt) 3 anodes vs. 1 anode Yes* <0.0001 (felt) 2 anodes vs. 1 anode Yes* <0.0001 (solid) 2 anodes vs. (felt) 2 anodes Yes* 0.0006 (woven carbon fibre) SA of cathode: 10 cm2 vs. 63 cm2 No 0.0834 (woven carbon fibre) SA of cathode: 10 cm2 vs. 133 cm2 Yes 0.0227 (woven carbon fibre) SA of cathode: 63 cm vs. 133 cm No 0.5483 (uni-carbon fibre) SA of cathode: 63 cm2 vs. 32 cm2 No 0.3018 (uni-carbon fibre) SA of cathode: 63 cm2 vs. 132 cm2 No 0.7333 (uni-carbon fibre) SA of cathode: 32 cm2 vs. 132 cm2 No 0.1743 * under log transformation In summary: 189 • 4 anodes is only significantly different than the single anode combination when graphite felt is employed • solid graphite and graphite felt are significantly different in a 2 anode combination • increasing the SA of the cathode six fold in the woven carbon fibre combination did not significantly impact the power production • increasing the SA of the cathode 13 fold in the woven carbon fibre combination did significantly impact the power production • increasing the SA of the cathode 2 fold in the uni-carbon fibre combination did not significantly impact the power production • increasing the S A of the cathode 4 fold in the uni-carbon fibre combination did not significantly impact the power production J.9 Altering Surface Area of Electrodes Table J-9; Statistical results of altering surface area of electrodes Test Significantly Pr value (anode:cathode) different (felt) 1:2 vs. 2:1 Yes <0.0001 (felt) 1:2 vs. 1:3 Yes <0.0001 (felt) 1:2 vs. 3:1 Yes <0.0001 (felt) 2:1 vs. 1:3 Yes <0.0001 (felt) 2:1 vs. 3:1 No 0.4456 (felt) 1:3 vs. 3:1 Yes <0.0001 (solid) 1:2 vs. 2:1 Yes* 0.0002 felt 2:1 vs. solid 2:1 Yes* 0.0001 felt 1:2 vs. solid 1:2 No 0.3391 * under log transformation In summary: • each of the felt combinations is significantly different from one another with the exception of the 2:1 vs. 3:1 combination. This implies that increasing the number of anodes from 2 to 3 while maintaining a single cathode had no significant effect 190 • a 1:2 vs. 2:1 ratio is significant in solid graphite • there is a significant difference for each type of graphite regardless of ratio J. 10 Further Altering Surface Area of Electrodes Table J-10: Statistical results of further altering surface area of electrodes Test Significantly Pr value (anode: cathode) different 1:2 vs. 2:1 Yes* 0.0245 1:2 vs. 1:1 Yes* <0.0001 2:1 vs. 1:1 Yes* <0.0001 * under log transformation In summary: • Each of the combinations are significantly different than one another J.ll Height of Water in the Cathodic Region Table J-ll: Statistical results of height of water in the cathodic region Test Significantly Pr value different 1 cm vs. 3 cm Yes* 0.0008 1 cm vs. 5 cm Yes* <0.0001 1 cm vs. 7 cm Yes* <0.0001 1 cm vs. 9 cm Yes* <0.0001 3 cm vs. 5 cm Yes* <0.0001 3 cm vs. 7 cm Yes* <0.0001 3 cm vs. 9 cm Yes* <0.0001 5 cm vs. 7 cm Yes* <0.0001 5 cm vs. 9 cm Yes* <0.0001 7 cm vs. 9 cm Yes* <0.0001 * under log transformation In summary: • Each of the combinations are significantly different than one another J.12 Dilution Effect of Manure Table J-12: Statistical results of dilution effect of manure Test Significantly Pr value (manure:water) different 1:0.5 vs. 1:1 Yes <0.0001 191 1:0.5 vs. 1:2 Yes <0.0001 1:0.5 vs. 1:3 Yes 0.0012 1:0.5 vs. 1:4 Yes 0.0093 1:1 vs. 1:2 Yes 0.0135 1:1 vs. 1:3 Yes 0.0468 1:1 vs. 1:4 Yes 0.0082 1:2 vs. 1:3 Yes <0.0001 1:2 vs. 1:4 Yes <0.0001 1:3 vs. 1:4 No 0.4847 In summary: • Each of the ratios of manure to water are significantly different besides the ratio of 1:3 vs. 1:4 J.13 Manure Consistency Effect Table J-13: Statistical results of manure consistency effect Test Significantly Pr value different outer vs. inner Yes* <0.0001 outer vs. combo Yes* 0.0051 inner vs. combo Yes* <0.0001 large vs. small No 0.2745 large vs. mini Yes <0.0001 small vs. mini Yes <0.0001 under log transformation In summary: • Each of the manure types is significantly different than one another • Whether or not a large vs. small cup is utilised it is not significantly different • A mini cup is significantly different than both a large and small cup. J.14 Effect of Load Resistor Table J-14; Statistical results of effect of load resistor Test Significantly Pr value different 1000 Q vs. 8200 Q Yes* 0.0004 1000 Q vs. 10000 Q Yes* <0.0001 8200 Q. vs. 10000 ft Yes* 0.0106 220 Q vs. 1000 Q. No* 0.4231 * under log transformation 192 In summary: 10 Q is significantly different to the other loads 220 Q. is significantly different to the other loads except the 1000 Q load 680 Q is significantly different to the other loads 1000 Q is significantly different to the other loads 8200 Q. is significantly different to the other loads 10000 Q is significantly different to the other loads J.15 Location of Mixture on Interior or Exterior of Cup Table J-15: Statistical results of location of mixture on interior or exterior of cup Test Significantly Pr value different Inside vs. outside Yes* 0.0011 rumen fluid vs. manure on outside Yes <0.0001 * under log transformation In summary: • Each of the configurations is significantly different from one another J.16 Further Effect of Load Resistor Table J-16: Statistical results of effect of load resistor Test Significantly Pr value different 1000 Q. vs. 680 Q when manure inside Yes 0.0001 1000 Q. vs. 680 Q, when manure outside Yes 0.0453 In summary: • Regardless of the resistor value the configurations are significantly different than one another J.17 Combination Effects of Load Resistor and Location of Mixture on Interior or Exterior of Cup Table J-17: Further statistical results of combination effects Test Significantly Pr value 193 different Manure outside (680 D.) vs. rumen and manure outside No* 0.0599 Manure outside (680 Q) vs. rumen outside Yes* 0.0007 Manure outside (1000 Q) vs. rumen and manure outside Yes* 0.0066 Manure outside (1000 Q) vs. rumen outside Yes* <0.0001 Rumen and manure outside vs. rumen outside No* 0.0667 Manure inside (1000 Q) vs. rumen inside Yes 0.0217 Manure inside (680 Q.) vs. rumen inside No 0.9734 * under log transformation In summary: • With a 680 Q resistor whether the substrate is manure or manure and rumen fluid it has no significantly different power production • With a 680 Q resistor whether the substrate is manure or rumen fluid has a significant difference on power production • With a 1000 Q resistor whether the substrate is manure or rumen fluid or manure and rumen fluid it has a significant difference on power production • Whether the substrate is rumen fluid or manure and rumen fluid it has no significant difference on power production 194 K Sample Calculations K.1 Volume of a Cylinder V = 7tr2h Where: r2 = radius of cylinder (cm) h = height of cylinder (cm) So: V=7tr2h V = 7t{2.3x(2.3 cm)2 9.8 cm V = 162.87 cmJ K.2 Surface Area of a Rectangular Prism SA = {2LH) + (2WL) + {2WH) c b SA of rectangular prism= 2ab + 2bc + 2ac 13 Where: SA = surface area (cm2) L = length of object (cm) W = width of object (cm) H = height of object (cm) So: SA = (2* L* H) + (2*W * L)+{2*W * H) SA = (2*6.50 cm* 1.37 cm) + (2* 1.59 cm*6.50 cm) + (2* 1.59 cm* 1.37 cm) 5A = 42.52 cm2 K.3 Price for cm2 of Electrode Material 50.8 cm * 50.8 cm slab costs $237.65 per unit width of 1.57 cm 2580.64 cm2 costs $237.65 1 cm2 costs $0.09209 per unit width of 1.57 cm 195 Quantity of Salt Required to Achieved Desired Molarity molar mass NaCl =58.44 V , /mol 0.1 mol NaCl requires X g NaCl per litre of solution ( 5SA4gNaCl X g NaCl =(0.1 mol NaCl)* 1.0 mol NaCl 196