RELATIONSHIP BETWEEN WASTEWATER SLUDGE QUALITY AND ENERGY PRODUCTION POTENTIAL
YEE PONG, CHUA
SUPERVISORS: PROF. ANAS GHADOUANI, DR ELKE REICHWALDT, PROF. RAJ KURUP
SCHOOL OF ENVIRONMENTAL SYSTEMS ENGINEERING
FACULTY OF ENGINEERING, COMPUTING AND MATHEMATICS
THE UNIVERSITY OF WESTERN AUSTRALIA
JUNE 2013
Cover photo: Subiaco wastewater treatment plant (UWA, 2013)
This thesis is presented in partial fulfilment of the requirements of the Bachelor of Engineering (Environmental) at The University of Western Australia.
ABSTRACT
Wastewater Treatment Plant (WWTP) plays an irreplaceable role in the overall wellbeing and development of societies. Wastewater treatment is an ongoing process that requires high- energy consumption, and this demand contributes negatively to climate change. Nonetheless, there are options available for energy production and recovery in WWTPs during its treatment process, which can also reduce the negative environmental impacts. This study aims to investigate the potential of energy production and recovery at one WWTP, and the reduction of environmental impacts achieved.
The study site is a WWTP situated at Subiaco of Western Australia, operated by Water Corporation. Currently, the WWTP uses an activated sludge treatment system and aerobic sludge stabilisation system. This process does not allow for energy production and recovery. On the other hand, an anaerobic sludge treatment system can produce energy during its treatment process in the form of biogas that can be captured and converted into energy for treatment use.
The research evaluated sludge samples from the Subiaco WWTP at the UWA SESE laboratory for the characteristics of the sludge. Laboratory batch scale anaerobic digestion studies were also carried out to evaluate the efficiency of the system. The results of this study were then compared with data from the neighbouring WWTPs that use anaerobic treatment for sludge stabilisation. Further analyses were carried out to determine the economical values of the generated energy potential and the reduced environmental impacts.
The experimental results showed that sludge samples from the Subiaco WWTP had a biogas production capacity of 0.015 m3/L sludge or 0.6 m3/VS, with a potential energy production of 40.4 megawatt-hour (MWh) per day. The biogas conversion to electricity used a combined heat and power (CHP) unit with an assumed energy efficiency factor of 70 %, and results indicated that Subiaco WWTP has the potential to recover 78 % of its overall electricity consumption through anaerobic treatment, with a generated value of A$1,012,291 per year. The payback period of purchasing a CHP unit using this generated value alone is between 2.2 to 9.6 years in the Best Case scenario, and 4.2 to 12.5 years in the Worst Case scenario. The amount of avoided carbon dioxide (CO2) emission from the substitution of treatment system is 7,475 metric tons annually. This study had successfully demonstrated the sustainability and
i economical advantage of an anaerobic treatment process, and concluded that energy production and recovery is a feasible option for Subiaco WWTP.
ii ACKNOWLEDGEMENT
The completion of this undergraduate dissertation was possible with the support of several people. I would like to express my sincere gratitude to all of them. First of all, I am grateful to my research supervisors, Prof. Anas Ghadouani, Dr. Elke Reichwaldt and Prof. Raj Kurup for their help and academic support rendered to me throughout the research work. I thank Prof. Raj Kurup for his valuable guidance, scholarly inputs and consistent encouragement throughout the journey. I am much thankful and appreciative of the lessons they taught me along the way.
Besides my research supervisors, I would like to also give thanks to John Langan for his invaluable assistance and guidance with all the laboratory work associated with the research.
This research would also not be possible without the help I had received from Water Corporation, in particular the plant operators at Subiaco, Beenyup and Woodman Point wastewater treatment plant, for their patience and assistance in sampling and data collection.
The completion of this research was made less obstacle ridden because of the presence of a few special individuals. Firstly, I sincerely offer my gratitude to my parents for providing me with unwavering support and consistent encouragement at every stage of both my personal and academic life. Secondly, I thank Skyler Han, for playing a strong supportive role throughout the course of this research. She supported me in every possible way to the completion of this work and I thank her for volunteering her time to painlessly proofread and correct any grammatical mistakes in the writing.
Lastly, I thank all who had offered their good wishes to me throughout the period of this research.
iii Table of Contents
ABSTRACT ...... i
ACKNOWLEDGEMENT ...... iii
LIST OF FIGURES ...... viiii
LIST OF TABLES ...... ix
ABBREVIATIONS ...... x
1. INTRODUCTION...... 1
1.1. Background ...... 1
1.2. Popularity and Consequences of Fossil Fuels ...... 1
1.3. Climate Considerations ...... 1
1.4. Anthropogenic Contributions to Climate Change ...... 2
1.5. Emissions from Waste Treatment Facilities ...... 2
1.6. Energy Consumption, Generation and Recovery ...... 3
1.7. Purpose of Study ...... 3
2. LITERATURE REVIEW ...... 4
2.1. Wastewater Treatment Plants ...... 4
2.2. Wastewater Treatment Objectives ...... 4
2.3. Constituents of Wastewater ...... 5
2.4. Wastewater Treatment Processes ...... 5
2.5. Sludge Management...... 6
2.5.1. Types and Characteristics of Sludge ...... 7
2.6. Anaerobic Treatment and Processes ...... 8
2.6.1. Inhibition and Limiting Factors ...... 9
2.6.2. Principles of Methane Generation ...... 12
2.6.3. Enhancement of Anaerobic Digestibility ...... 14
iv 2.6.3.1. Optimisation of Process Conditions ...... 14
2.6.3.2. Pre-Treatment of Feed Sludge ...... 14
2.6.3.3. Staging Process and Higher Operating Temperature ...... 14
2.6.3.4. Digester-Mixing Regime ...... 16
2.7. Energy in Wastewater Treatment...... 16
2.8. Energy Recovery in Sludge ...... 18
2.9. Energy Generation Technologies ...... 19
2.9.1. Fuel Cells ...... 19
2.9.2. Microturbines ...... 20
2.9.3. Biogas Powered Reciprocating Engines ...... 20
2.9.4. Biogas Upgrade ...... 22
2.9.5. Defective Components in Biogas...... 23
2.10. Climate Change Benefits ...... 23
2.10.1. Mitigation efforts in Australia ...... 23
2.11. WWTP Energy Recovery in Western Australia ...... 24
2.11.1. Woodman Point WWTP ...... 25
2.11.2. Beenyup WWTP ...... 25
3. MOTIVATION ...... 26
4. AIMS AND OUTCOMES ...... 27
5. MATERIAL AND METHODS ...... 28
5.1. Field Sampling Site ...... 28
5.2. Sludge Samples ...... 28
5.3. Biogas Collection System ...... 28
5.4. Biogas Sampling and Monitoring ...... 28
5.5. Biogas Analysis ...... 30
v 5.6. Sample Analysis ...... 31
6. RESULTS AND DISCUSSION ...... 32
CHAPTER 1 ...... 32
6.1. Laboratory Results ...... 32
6.1.1. Total Solids Test ...... 32
6.1.1.1. Total Solids and Volatile Solids ...... 32
6.1.1.2. Fixed Solids ...... 33
6.1.2. pH Level...... 34
6.1.3. Biogas Production ...... 34
6.1.4. Biogas Composition ...... 36
6.2. Data Comparison of Anaerobic Digestion between WWTPs ...... 36
CHAPTER 2 ...... 37
6.3. Subiaco WWTP Analysis ...... 37
6.3.1. Biogas Parameters ...... 38
6.3.1.1. Biogas Production Per Day ...... 38
6.3.1.2. Wobbe Index ...... 39
6.3.1.3. Mass Flow Rate of Biogas ...... 40
6.3.2. Power Generation from Subiaco Biogas ...... 41
6.3.2.1. Power generation from CHP Technologies ...... 42
6.3.3. Economical Analysis ...... 43
6.3.3.1. Cost of Aeration Treatment ...... 43
6.3.3.2. Savings from Generated Power ...... 44
6.3.3.3. Case Scenarios ...... 44
6.3.4. Carbon Reduction Equivalent ...... 46
vi 7. CONCLUSION ...... 49
8. RECOMMENDATIONS ...... 50
9. REFERENCES ...... 51
vii List of Figures
Figure 1: Typical processes in a wastewater treatment plant (ANZBP, 2009) ...... 6
Figure 2: Effect of retention time on methane production (Appels et al., 2008) ...... 10
Figure 3: Influence of process temperature on residual COD in anaerobic digestion of sewage sludge (Casey, 2006) ...... 10
Figure 4: Processes to methane production by anaerobic digestion (Gavala et al., 2003) ...... 13
Figure 5: Electricity requirements for a typical wastewater treatment plant (Science Applications International Corporation, 2006) ...... 17
Figure 6: Illustration of a fuel cell (FuelCells.org, n.d.) ...... 19
Figure 7: Schematic of a microturbine process (Robbins, 2012) ...... 20
Figure 8: Components of a biogas engine (Clark Energy, n.d.) ...... 21
Figure 9: Processes for biogas upgrade (Handley, 2010) ...... 22
Figure 10: Gross national income with an without carbon price (Australia Goverment, 2013) ...... 24
Figure 11: Biogas collection system setup...... 29
Figure 12: Orsat gas analyser ...... 29
Figure 13: Cumulative biogas production over 36 days ...... 35
Figure 14: Daily gas volume produced during anaerobic digestion ...... 35
Figure 15: Gas composition of Subiaco samples produced during anaerobic digestion ...... 35
Figure 16: Theoretical biogas composition of Subiaco samples ...... 38
Figure 17: Comparison between aerobic and anaerobic treatment expenditure ...... 48
viii List of Tables
Table 1: Calorific values comparison of various fuels (Abbasi et al., 2012) ...... 3
Table 2: Advantages and disadvantages of anaerobic digestion (Demirbas, 2009) ...... 8
Table 3: Typical biogas composition (Biomass Energy, N.d.) ...... 9
Table 4: Comparison between mesophilic and thermophilic range (Demirbas, 2009)...... 15
Table 5: Biogas production of mesophilic and thermophilic process per input volatile solids (Dohanyos et al., 2004) ...... 15
Table 6: Relationship of specific biogas production and potential annual electricity production (Jenicek P. et al., 2012) ...... 18
Table 7: Advantages and disadvantages of CHP technologies (Robbins, 2012) ...... 21
Table 8: Experimental results from anaerobic digestion of sewage sludge ...... 33
Table 9: Reduced solids during anaerobic digestion ...... 33
Table 10: Comparison of treatment efficiency in WWTPs ...... 33
Table 11: Parameters of biogas production between Western Australia’s WWTPs ...... 36
Table 12: Biogas parameters of Subiaco WWTP ...... 40
Table 13: Performance and cost of energy generation technologies (U.S.E.P.A., 2007) ...... 42
Table 14: Capital and O&M scenarios for biogas technologies ...... 44
Table 15: Payback period of technologies using case scenarios ...... 45
Table 16: Contribution comparison between aerobic and anaerobic treatment technologies . 47
ix Abbreviations
AD Anaerobic Digestion
CHP Combined Heat and Power
DAFT Dissolved Air Floatation Thickener
FS Fixed Solids
GHG Greenhouse Gas kWh Kilowatt-hour
LHV Lower Heating Value
MWh Megawatt-hour
TS Total Solids
O&M Operation and Maintenance
VS Volatile Solids
WA Western Australia
WAS Waste Activated Sludge
WSP Wastewater Stabilisation Pond
WWTP Wastewater Treatment Plant
x 1. INTRODUCTION
1.1. Background
Our insatiable demand for energy has risen considerably throughout the twentieth century, propelled by economic and social advancements in modern society. According to International Energy Agency (2009), global energy demand will increase on average by 1.5 % per year between 2007 to 2030. The consumption of our predominant energy source, fossil fuels (coal, petroleum and natural gas), accounts for more than 75% of this increase. At the same time, the world’s population is projected to reach 8.9 billion in 2050, a rise of 47% in population from 2000 (Cakir and Stenstrom, 2005, Muradov and Veziroğlu, 2008). The growing prosperity, consumption and population rates have caused a strain on our current resources which would threaten the survival of our planet. Against this backdrop, it is certain that we are at a critical stage where the choices we make now will affect the future world we live in.
1.2. Popularity and Consequences of Fossil Fuels
Fossil fuels possess many attractive properties that establish itself as the universal choice for energy. Attributes include high caloric values, transmutation and usage versatility, ease of use on small and large scale applications, easy transportation, relatively inexpensive (Judkins et al., 1993) and most importantly, our mastery and exploitation of it have become so effective that it has become an indispensable resource. In the absence of other energy sources that are able to compete on the same scale and cost, fossil fuels would likely remain as a popular choice in the following decades to come (Lim et al., 2012). Yet, the disproportionate rates of fossil fuels consumption have led to undesirable consequences on the environment. Environmental problems include local and regional scales of acid deposition, urban air and waterways pollution to a global scale of climate change (Judkins et al., 1993, Khan et al., 2011). Most of the environmental problems that exist are solvable, within reasonable time and costs, to meet statutory limits. Climate change however, is a global phenomenal that no attainable solutions or technologies exist to nullify its effects (Fujii et al., 2012).
1.3. Climate Considerations
The most widely-known environmental consequence is the greenhouse effect (Cao and Shan, 2011). Its impacts have become apparent in recent years with increasing air and ocean
1 temperatures, widespread melting of snow and ice and rising global average sea levels. There is a scientific consensus that anthropogenic activities have resulted in increasing global temperatures and consequently, climate change (IPCC, 2007). The World Bank (2012) predicted a stark scenario where a global temperature increment by 4 °C can lead to the inundation of coastal cities, instability in food production, extreme climate patterns, water scarcity, increased cyclone intensity and the irreversible loss of biodiversity.
1.4. Anthropogenic Contributions to Climate Change
The main contributors to climate change are greenhouse gases (GHGs), notably carbon dioxide (CO2), methane (CH4), chlorofluorocarbons (CFCs), nitrous oxides (N2O) and ozone
(O3). Global temperatures are raised as a result of GHGs’ ability to absorb infrared radiation and trap them in the lower atmosphere (Judkins et al., 1993). In United States, the two largest
GHG sources from anthropogenic activities are CO2 and CH4, accounting for 93 % of total greenhouse emissions (U.S. Energy Information Administration, 2004). CH4 is a powerful greenhouse gas with a global warming potential 21-25 times of CO2 (Johari et al., 2012, Abbasi et al., 2012). A study by Abbasi et al. (2012) pointed that anthropogenic methane emissions sources arise from landfills, fossil fuel production, animal husbandry, agriculture, biomass burning and treatment and disposal of biodegradable liquid/solid wastes.
1.5. Emissions from Waste Treatment Facilities
GHGs emission from waste and wastewater treatments represents approximately 3 % of total global anthropogenic GHGs emission, of which CH4 accounts for 90 % of it (Bogner et al., 2008, Wang et al., 2011). Waste treatment processes contribute to GHGs through the production of CO2, CH4 and nitrous oxide (N2O) (Cakir and Stenstrom, 2005).
Despite its environmental concerns, CH4 represents a potential energy source as the ignition of CH4 does not give off any soot or odour, making it a clean valuable gas. Table 1 shows a comparison of calorific values between all the fuels. Biogas, with CH4 as its main constituent, can offer comparable calorific values as the other fuels on a per kg basis (Kurup, 2013). The capture of biogas is an attractive and sensible option because it occurs naturally in waste treatment facilities and has the advantages of generating energy while controlling global warming (Abbasi et al., 2012).
2 Table 1: Calorific values comparison of various fuels (Abbasi et al., 2012) Fuel Calorific Value Indirect emission factor (approximate) (kgCO2e/GJ) Petrol 10800 kcal per kg 12.51 Natural gas 8600 kcal per m3 5.55 (EU mix) Liquefied natural gas 13140 kcal per kg 20.00 Liquefied petroleum gas 10800 kcal per kg 8.00 Kerosene 10300 kcal per kg 13.34 Diesel 10700 kcal per kg 14.13 CNG 8600 kcal per m3 8.36 3 Biogas 5000 kcal per m 0.246 (Direct CO2 emission)
1.6. Energy Consumption, Generation and Recovery
The capacity to generate CH4 highlights the potential for energy production in all wastewater treatment facilities. There is a good opportunity for energy sustainability in wastewater treatment facilities as energy required to handle and treat waste can be recovered in its processes. In WWTPs, treatment processes produce by-products in the form of stabilised sludge that is a key contributor to energy production. By utilising this form of potential energy, WWTPs can positively contribute to achieving energy sustainability and GHG mitigation.
1.7. Purpose of Study
Given our current predicament, it is certain that energy prices will not decrease anytime soon. On top of that, our energy demands have contributed to climatic implications that impel us to adjust towards energy sustainability. Wastewater treatment is an important sector in society but it requires continual energy consumption that also contributes to GHGs. However, there lies a feasible path of energy offset and recovery during its treatment process. The first step in this study is to investigate the energy sustainability potential of one municipal WWTP through 1) The characteristics of sludge and its qualities, 2) Comparison with other similar WWTPs in the region and 3) Its generated energy potential.
Further analysis were carried out in the second step of this study to determine 4) its value of specific power consumption, 5) estimation on the self-sufficiency rate of electric power by power generation using digestion gas and 6) the amount of GHGs that can be offset .
3 2. LITERATURE REVIEW
2.1. Wastewater Treatment Plants
The term wastewater treatment plant (WWTP) is used to describe a facility designed to receive waste from domestic, commercial and industrial sources and through treatment processes, discharged water that meets environmental regulations back into receiving environmental systems. Wastewater collected must ultimately return to receiving waters, land or be reused. The aim of WWTPs, like all water treatment systems, is to reduce environmental impacts and health risks associated with untreated water. To achieve that, a series of treatments that combines physical, chemical and biological processes and operations are used to remove solids, organic matter and nutrients from wastewater. WWTPs can be found around the world as it plays a vital role in the wellbeing and overall development of societies. However, due to its high costs, maintenance and complex operations, developing countries prefer waste stabilisation pond (WSP) over WWTP because of its simple, low cost and maintenance method of wastewater treatment. Nevertheless, the ultimate goal of wastewater treatment is to ensure and provide a safe, abundant and affordable water supply to the general population.
A modern wastewater treatment plant has several functions:
. Remove detritus and other solid and gritty objects from the wastewater; . Remove organic solids and convert into useful products for reuse; . Remove dissolved constituents to meet water quality standards; . Remove nitrogen and phosphorus to meet environmental objectives; . Remove pathogens to protect public health; and . Prepare a reclaimed water stream for subsequent reuse.
2.2. Wastewater Treatment Objectives
Generally, the characteristics of raw water determine the treatment method. However, since public usage of water spreads across a wide spectrum from human consumption to gardening, the most important use of water (human consumption) defines the degree of treatment. The objective of wastewater treatment is to reduce the concentration of specific pollutants to the level at which the discharge of the effluent will not adversely affect the environment or pose
4 a health threat (Vesilind P. et al., 1994). To avoid the consequences of inadequate treatment, environmental regulations set the benchmark for effluent quality standards prior to its discharge into receiving systems. Wastewater treatment ameliorates sewage that consists of a wide range of contaminants through three broadly classified treatment methods:
1) Primary: Removal of settleable solids and scum; 2) Secondary: Degradation of biological contents via microorganisms; 3) Tertiary: Improve effluent quality prior to discharge.
Sludge is produced during all three stages of treatment, and further treatment at secondary and tertiary stages can be achieved through aerobic and anaerobic stabilisation, composting and drying for land application or disposal.
2.3. Constituents of Wastewater
Wastewater originates from a variety of domestic, commercial, industrial and non-point sources. It comprises mainly of suspended and dissolved chemicals, faecal microbes (viruses, bacteria and protozoan), nutrients (mainly nitrogen and phosphorus) and heavy metals (Metacalf and Eddy, 2003). The organic composition of municipal wastewater has approximately 50 % proteins, 40 % carbohydrates, 10 % fat and oils, and trace amounts of priority pollutants and surfactants (Ellis, 2004). Most of the suspended materials in wastewater are microbial floc and colloidal matter, and these particles forms the constituents of sludge during wastewater treatment (Shapally, 2012).
2.4. Wastewater Treatment Processes
The treatment of wastewater is either accomplished on site (uncollected), or channelled to a centralised plant (collected). Depending on the environmental requirements, the treatment and discharge options can vary between countries and regions. Wastewater is treated typically in the following stages, shown in Figure 1. In primary treatment, the combination of physical and gravitational techniques allows for the removal of larger solids and the settlement of smaller particles. This is followed by the secondary treatment, where biological processes use microorganisms to enhance the biodegradation of organic content. The aim is to convert rich waste material into lower energy material with water and CO2 as by-products, and to reduce sludge loadings and by-product volumes. This includes aerobic and anaerobic
5 Figure 1: Typical processes in a wastewater treatment plant (ANZBP, 2009) approaches, such as trickling filters, stabilisation ponds and activated sludge reactors. Tertiary treatment is the final polishing step to clean the water from pathogens, contaminants and nutrients (nitrogen, phosphorous), prior to its discharge into waterways.
2.5. Sludge Management
The management of sewage sludge produced from wastewater treatment is one of the most difficult problems to deal with. Even though the volume of sludge produced amounts to only a few percent, sludge handling accounts for 30 to 50 % of the total operating costs (Fulton, 2010, Spinosa et al., 2011).
Disposal channels of sludge are limited to only air, water and land. The options for air and water are not feasible, as potential pollutions to air or aquatic systems represents another set of environmental challenges (Vesilind P. et al., 1994). Land disposal of sludge is by far the most popular option, and sludge as fertiliser has achieved reasonable success, particularly in countries with agricultural activities. In recent years however, the need to achieve a sustainable sludge management strategy has become a great concern. Stricter environmental
6 regulations have led to the restrictions of land applications of sludge (Metacalf and Eddy, 2003). Thus, sludge as fertiliser cannot be relied as the only option towards achieving sustainability as it has become necessary to maximise recovery of useful materials and/or energy. Sludge is considered an unfavourable by-product of wastewater treatment, but there is potential to be used for energy production (Jenicek P. et al., 2012).
2.5.1. Types and Characteristics of Sludge
Sewage sludge is the relatively concentrated suspension by-product of wastewater in the course of purification. The daily quantity of solids and composition are expected to vary, influenced by contributions from different sources. Most of the sludge have unstable organic nature and readily undergo active microbial decomposition with consequent generation of nuisance odours. It is usually in the form of liquid or semisolid liquid, and typically contains 0.25 to 12 % of solids by weight (Metacalf and Eddy, 2003).
Primary Sludge
Primary sludge is essentially raw waste from the bottom of primary clarifier. It contains a high portion of organic matter in 93 to 97 % liquid. In comparison with activated sludge, primary sludge generally contains more fat and protein and less carbohydrates (Sykes, 2003). As a result, gas yield is higher but the methane content of the gas is lower (Navaneethan, 2007).
Activated Sludge
Activated sludge is a product of secondary treatment. It contains a mixture of bacteria, fungi, protozoa and rotifers maintained in suspension by aeration and mixing. The excess sludge, or waste activated sludge (WAS), is a result of overproduction of microorganisms in the active sludge process and is more difficult to digest than primary sludge.
Digested Sludge
After digestion of primary and activated sludge, the residual product is digested sludge. The digested sludge has achieved high pathogen removal and a reduction in mass and odour. It is more easily dewatered than primary and activated sludge.
7 2.6. Anaerobic Treatment and Processes
Biological methods of sludge stabilisation, such as aerobic and anaerobic digestion, are widely used in wastewater treatment processes and will become even more important in the future, as they reduce the problems associated with sludge, such as odour and putrescence and the presence of pathogenic organisms (Vesilind P. et al., 1994, Dohanyos et al., 2004). In addition, the process reduces the volume of sludge to be disposed off while producing biogas (during anaerobic digestion). The result of these processes is a high quality stabilised sludge, or biosolids, that can be used for land application as fertilisation and as a carbon source for denitrification (Navaneethan, 2007). Between the two biological options, aerobic digestion is not economically viable for stabilising large volume of sludge due to the high energy and operational costs required to run the aerators in aerobic digesters. Table 2 gives an overview of the advantages and disadvantages of anaerobic digestion. Hence, anaerobic digestion is most often incorporated in WWTPs and is generally applied to the mixture of primary and secondary (waste-activated) sludge.
Anaerobic digestion (AD) involves the microbial metabolism of biodegradable organic matter, in the absence of oxygen, to biogas. Biogas can be produced in different environments, e.g., in landfills, WWTPs and biodigesters, and usually contains 55 to 70 % methane, and 30 to 45 % carbon dioxide (Table 3). The resulting proportions of methane and carbon dioxide from the breakdown of organic matter is represented in the following simplified reaction (Metacalf and Eddy, 2003):
Table 2: Advantages and disadvantages of anaerobic digestion (Demirbas, 2009) Advantages Disadvantages Biogas production High capital costs Sludge mass reduction Highly sensitive microorganisms Low odour content of digested solids Long retention times High rate of pathogen inactivation High nutrient composition of digestrate (biosolids) Small to large scale applications Lower life cycle cost (compared with aerobic)
8 Table 3: Typical biogas composition (Biomass Energy, N.d.) Component Formula Concentration (% by vol.)
Methane CH4 55-70 Carbon dioxide CO2 30-45 Nitrogen N2 0-5 Oxygen O2 <1 Hydrocarbons CnH2n+2 <1 Hydrogen sulfide H2S 0-0.5 Ammonia NH3 0-0.05 Water (vapour) H2O 1-5 3 Siloxanes CnH2n+1SiO 0-50 mg/m
Depending on the source, biogas can also contain other trace gases such as nitrogen, hydrogen sulfide, halogenated compounds and organic silicon compounds (Rasi, 2009). Several microorganism groups such as hydrolytic bacteria, fermentative acidogenic bacteria, acetogenic bacteria and methanogens (Archer and Kirsop, 1990) are involved in the AD process. These microorganism groups operate at three different temperature ranges: psychrophilic (ambient temperature), mesophilic (30-38°C) and thermophilic (50-57°C) (Appels et al., 2008, Cao and Pawlowski, 2012). The mesophilic process is consistently the most commonly used in practical application, mainly because of its combined benefits with acceptable energy consumption, reliable process operation and favourable process performances (e.g. sludge reduction and biogas generation) (Cao and Pawłowski, 2012).
2.6.1. Inhibition and Limiting Factors
The digestion efficiency and its stability can vary significantly depending upon various parameters, such as pH, alkalinity, temperature and retention times. The effect of retention time on biogas production is shown in Figure 2. It would take about 15 to 20 days to extract 80 % of the maximum gas production (Kurup, 2011).
Perhaps the most important parameter of all is temperature, as it plays an important role in the removal efficiency of organisms and pathogens because it governs the rate of microbial reactions. Temperature also influences biomass composition, nutrient requirements, the nature of metabolism and metabolic reaction rate, mainly because microorganisms are unable to regulate their own internal temperature, hence they are dependent on suitable external conditions to function (Mayo and Noike, 1996). Foley et al. (2011) observed a significant difference between summer and winter emissions of CH4 in sewer systems, suggesting that
9 temperature is an important parameter in CH4 formation in sewers. The influence of temperature on microbial reactions is shown in Figure 3.
Figure 2: Effect of retention time on biogas production (Appels et al., 2008)
Figure 3: Influence of process temperature on residual COD in anaerobic digestion of sewage sludge (Casey, 2006)
10 The incorporation of the effect of temperature on the degradation of COD or VS uses the Van’t Hoff-Arrhenius equation with upper and lower limits (ACM0014, 2010, Kurup, 2011);
0 if T2,m 283 K E *(T - T ) f exp 2,m 1 if 283 K T 303 K T,m 2,m R *T1 *T2,m 1 if T2,m 303 K
(1)
Where: fT,m = Factor expressing the influence of the temperature on the methane generation in month m E = Activation energy constant (15,175 cal/mol)
T2,m = Average temperature at the project site in month m (K)
T1 = 303.16 K = (273.16 K + 30 K) R = Ideal gas constant (1.987 cal/K mol) M = Months of year y of the crediting period
The above equation shows that the value of fT,m cannot exceed 1 and should be assumed to be zero if the ambient temperature is below 10°C. This methodology is applicable for a temperature range of 20 to 30oC and does not recognize the impact of rate increase beyond 30oC (Kurup, 2011).
The process of methanogenesis, responsible for methane production, is particularly sensitive to slight variations of parameter changes. Methanogenesis occurs at neutral pH; in the range between 6.5-7.2 (Appels et al., 2008). This is a narrow range and a slight variation in pH can affect the digestion process. Similarly, a temperature shift during methanogenesis can cause a build up of volatile fatty acids (VFA). This lowers the overall pH in the digester which can further lead to a vicious circle of negative feedback (Navaneethan, 2007). Furthermore, no methanogenic activity, and subsequently sludge volume reduction can happen at temperature below 15°C (Gloyna, 1971). Therefore it is important to maintain a stable operating temperature in the digester to prevent any disruptions that could influence bacterial activity.
11 AD has been used for centuries to stabilise and concentrate organic wastes from human civilisation. Despite its ancient use and widespread application, the detailed microbiology of AD is not yet fully understood due to the difficulty of using tradition culturing methods to isolate and identify the role of specific anaerobic bacteria in their active colonies (Cowgill, 2011). These gaps in knowledge sometimes result in the inexplicable digester failures today, even after long periods of stable operation (Weiland, 2010).
2.6.2. Principles of Methane Generation
Methane (CH4) is produced as several groups of microorganisms work collaboratively in the absence of oxygen to convert organic material into CH4 and CO2 through four basic steps, shown in Figure 4. It is important to note that all stages of AD must proceed at the same time, but each stage has a different range of kinetic constants (Gavala et al., 2003). The four basic steps of conversion are (Haandel and Lubbe, 2007):
1. Hydrolysis
(C6H10O5) n + 2H2O → n (C6H12O6)
Hydrolysis is a relatively slow process and generally limits the rate of methane formation. It involves the exo-enzymes conversion of macromolecules (i.e. carbonates, proteins and fats) into simpler, smaller molecules soluble in water (i.e. peptides, fatty acids).
2. Acidogenesis
n (C6H12O6) → n CH3COOH
Hydrolysed products are converted into molecules with low molecular weight, like volatile fatty acids, alcohols, aldehydes and gases like CO2, H2 and NH3. Acidifying bacteria are known to metabolise rapidly, typically reproducing within 12 hours.
3. Acetogenesis
Acidification products are further converted into acetic acids, CO2 and H2 by the acetogenic bacteria. The first three steps, known as acid fermentation, involve no removal of organic material; it is merely the transformation into a form of substrate suitable for the subsequent process of methanogenesis.
12 4. Methanogenesis
3 n CH3COOH → n CH4 + CO2
Waste stabilisation is achieved when the products of acid fermentation (mainly acetic acid) are converted into CO2 and CH4. Organic material is removed as the produced methane gas will largely desorbed from the liquid phase. Stable methanogenesis requires between 4 to 10 days for bacteria to reproduce. The methane produced in the liquid phase of an anaerobic reactor can subsequently (Foley and Lant, 2008):
. Remain dissolved in the liquid phase (possibly at super-saturated concentrations); . Be stripped to the gas phase by natural mass transfer and/or aggravation (mechanical aeration);
. Undergo continued oxidation to CO2 by methanotrophic bacteria in aerobic environments; or . Be further utilised as the carbon and energy source for heterotrophic denitrification in anoxic zones.
Figure 4: Processes to methane production by anaerobic digestion (Gavala et al., 2003)
13 2.6.3. Enhancement of Anaerobic Digestibility
2.6.3.1. Optimisation of Process Conditions
The manner and frequency of reactor feeding is important especially in intensive and high loaded processes. Dohanyos et al. (2004) found that with higher frequency of lesser amount of fed sludge, and sufficient homogenisation of primary with WAS prior to feeding, a higher stability and better efficiency of AD could be achieved. A similar effect can be reached by improved mixing within the digesters to get a better distribution of substrate to anaerobic biomass, or through the enhancement of sludge input concentration by activated sludge thickening before feeding. Another consideration would be to preheat the input sludge to avoid any temperature differences while feeding into the digesters (Dohanyos et al., 2004).
2.6.3.2. Pre-Treatment of Feed Sludge
The pre-treatment process is an additional step in the sewage sludge treatment technology that have been developed to improve subsequent sludge treatment and final output quality (Muller, 2000). The AD process is limited by the rate of hydrolysis of organic matter, and it is of particular importance as it causes the delay of methane formation. Through effective pre-treatment processes, floc structures within sludge can be more easily hydrolysed, optimising the methanogenic potential of the waste treated. There are several methods used in sludge pre-treatment such as ultrasonic, chemical, thermal, enzymatic and mechanical disintegration. The objective is to accelerate the digestion of input sludge, raise the degree of degradation and thus decrease the amount of sludge to be disposed off (Dohanyos et al., 2004).
2.6.3.3. Staging Process and Higher Operating Temperature
Staging and a higher operating temperature can remarkably intensify the process of anaerobic digestion (Ramakrishnan and Surampalli, 2013). Process optimisation can be achieved through the dual-stage mesophilic/thermophilic process, or temperature phased anaerobic digestion (TPAD), because it combines the advantages of thermophilic systems in terms of pathogen control and VS reduction, and is still economical to operate because the bulk of digestion takes place in the mesophilic stage (Coelho et al., 2011). Kiyohara et al. (2000) proposed that the reasons for better performance of dual-stage TPAD could be the setting of optimal conditions for two bacterial populations (mesophilic methanogens and thermophilic
14 Table 4: Comparison between mesophilic and thermophilic range (Demirbas, 2009) Mesophilic (30-40°C) Thermophilic (50-60°C) Less energy for temperature maintenance Increased solid reduction Less odour potential Higher metabolic rate Lower bacterial death rate Increased destruction of pathogens Lower effluent VFA concentrations Higher specific growth rate of methanogens Higher stability Reduced retention times
Table 5: Biogas production of mesophilic and thermophilic process per input volatile solids (Dohanyos et al., 2004) Operational Temperature Thermophilic 55°C Mesophilic 35°C Specific biogas production (m3/kg) 0.71 0.54 Standard conditions (Nm3/kg) 0.61 0.48
hydrolytic/acidogenic bacteria) in terms of pH, temperature and residence time. Also, Kobayashi et al. (1989) found that some compounds that are inhibitory in mesophilic systems such as phenol or unsaturated fatty acids, becomes less inhibitory in thermophilic systems.
For any specific wastewater, evaluating the potential of thermophilic digestion process requires the assessment of whether it has real advantages over mesophilic digestion system (Ramakrishnan and Surampalli, 2013). Table 4 shows the advantages and disadvantages between mesophilic and thermophilic range of temperature. Studies on removal efficiencies of thermophilic compared to conventional mesophilic process show better performance (Barr et al., 1996, Kosseva et al., 2001, Rozich and Bordacs, 2002), moderate performance (Kurian et al., 2005, Krzywonos et al., 2008) to poor performance (Tripathi and Allen, 1999, Suvilampi and Rintala, 2002). Abeynayaka and Visvanathan (2011) found that studies by Krzywonos et al. (2008) indicated a three to five fold reduction in sludge yields at thermophilic temperature over mesophilic temperature, yet studies by Suvilampi and Rintala (2002) and LaPara et al. (2000) indicated no difference in sludge yields between these two temperatures. As such, the literature provides no satisfactory conclusion that can be drawn for the performance of thermophilic over mesophilic temperatures.
However, other studies have concluded that thermophilic digestion have higher VSS removal efficiency and yield more biogas compared to mesophilic digestion (Table 5) (De La Rubia et
15 al., 2002, Dohanyos et al., 2004, Coelho et al., 2011, Abeynayaka and Visvanathan, 2011, Ramakrishnan and Surampalli, 2013).
2.6.3.4. Digester-Mixing Regime
The digester-mixing regime can either promote growth of AD bacterial colony by supplying microorganisms with fresh substrate, maintaining a stable temperature throughout the digester, moving products of metabolism to receiver organisms, separating biogas from the liquid phase and moving it out of the digester, breaking up floating or submerged layers of sludge and scum, and preventing undigested solids from entraining with the discharge sludge (Cowgill, 2011). It can also impede growth and biogas production through excessive mixing or by simply killing the shear-sensitive bacteria (Deublein and Steinhauser, 2008). Deublein and Steinhauser (2008) suggested that in general, a continuous, intensive but careful mixing action should be used.
2.7. Energy in Wastewater Treatment
Wastewater treatment is an energy-intensive operation. In an era where concerns about increased energy consumption due to the volatility of fuel supplies, cost of energy and stringent treatment levels required, the focus of WWTPs’ designs and operations are increasingly shifting towards improving the efficiency of energy use and reduction in treatment costs (Metacalf and Eddy, 2003). While primary treatment is relatively standard among different WWTPs, a wide range of secondary treatment alternatives exist, and the energy consumption of these facilities is highly variable (Menendez and Black & Veatch, N.d.). A typical energy requirement of WWTP is shown in Figure 5. As illustrated, about 50 % of total plant energy is used for aeration. This process has been identified as the single most energy consuming in wastewater treatment (Liu et al., 2011). Though aeration is an effective means of biological treatment, the continuous energy demand to create air bubbles necessary for biological activity is significant. Considering that the typical life expectancy of a WWTP is about fifty years (possibly longer) and wastewater treatment is a daily operating process, any forms of energy reduction or recovery available can result in significant savings throughout its operational lifespan.
16 Figure 5: Electricity requirements for a typical wastewater treatment plant (Science Applications International Corporation, 2006)
Energy self-sufficiency is a possible goal that is achievable in WWTPs. For instance, the As- Samra municipal WWTP in Jordan has a capacity of 270 mega litres per day (270 ML/d), serving 2.7 million persons currently (Myszograj and Qteishat, 2011). It receives 80 % of its electricity needs through the combination of hydraulic and gas turbines powered by biogas (Net Resources International, 2012). Nowak et al. (2011) gave further examples of two municipal WWTPs in Austria that had a production of an overall surplus electricity of 6.3 % and 7 % respectively.
Another successful example can be found in a starch manufacturing plant in Thailand. Previously, open-system WSPs were used to treat starch wastewater with extremely long retention times of more than a year, and local air and water quality suffered as a result. With the installation of an anaerobic WWTP, the plant is utilising the captured biogas for energy production and had reduced its fossil fuel use by 80 %. (South Pole, 2011). The change in wastewater treatment method from WSP to WWTP had brought positive socio-economic and
17 environmental benefits to the community, which include improved water quality that allows for fish farming and irrigation to nearby farms, fertilisers for farmers in the form of biosolids and a general reduction in odour and water consumption rate. The success of the starch plant in Thailand had highlighted the benefits of a closed treatment system (WWTP) over an open treatment system (WSP), and has proven that there is potential for energy recovery across all sectors and industries that utilises AD treatment.
2.8. Energy Recovery in Sludge
Since sludge is initially a suspension, it must be dewatered before the energy in sludge can be applied for useful purposes. The moisture in sludge provides a certain binding strength and this limits dewatering. The energy production of the suspension is low in this case, with an effective heat value of 0.16-0.8 MJ/kg-sludge (Lee and Tay, 2004). This suspension is usually used as feed for anaerobic digestion. Lee and Tay (2004) found that with the removal of moisture, a 90 % volume reduction was achieved and the dewatered sludge has an increased effective heat value of 2.4 – 6 MJ/kg-wet cake. The cake is a raw material that further undergoes thermal drying, incineration or pyrolysis.
The difference in sludge quality and AD technology used can cause very wide intervals of specific biogas production. Since raw sludge have a higher calorific value that waste activated sludge (WAS), the fluctuation of either sources influences the amount of methane production. Typically, a high primary sludge, low activated sludge feed ratio produces a higher biogas yield (Bouallagui et al., 2010). Table 6 shows the theoretical data of biogas production per person and year.
Table 6: Relationship of specific biogas production and potential annual electricity production (Jenicek P. et al., 2012) Specific biogas production Annual biogas production Potential annual electricity (L/kg VS) (m3/person) production (kWh/person) 300 6.1 15.8 400 8.2 21.1 500 10.2 26.4 600 12.3 31.6 700 14.3 36.9
18 2.9. Energy Generation Technologies
Biogas is increasing viewed as a valuable, renewable fuel for decentralised power generation in urban areas (Cowgill, 2011). The recovery of energy from sludge, through combined heat and power (CHP) technology such as a microturbine, fuel cell or biogas powered reciprocating engine, can produce electricity on site to offset a plant’s electricity cost, at the same time provide additional heat for heating purposes.
2.9.1. Fuel Cells
A fuel cell (Figure 6) operates similar to a battery but does not run out or requires recharging. It consists of a polymer electrolyte membrane sandwiched between two electrodes, and electricity, water and heat are generated via the transportation of hydrogen and oxygen electrons (Robbins, 2012). The usage of fuel cells produces zero pollutant emission as no combustion occurs during the reaction process. The demonstration of energy production from various fuel cell technologies such as the molten carbonate fuel cell, phosphoric acid fuel cell, and solid oxide fuel cell were exhibited in the Aichi Japan Expo (Kurup, 2005).
Figure 6: Illustration of a fuel cell (FuelCells.org, n.d.).
19 2.9.2. Microturbines
A microturbine is a versatile energy system that can be fuelled by natural gas, biogas, or other types of fuel. Inside the turbine, a generator is powered by fuel to produce electricity, and the hot exhaust air that was created in the process can be recovered for heating needs. The schematic process is shown in Figure 7.
Figure 7: Schematic of a microturbine process (Robbins, 2012)
2.9.3. Biogas Powered Reciprocating Engines
The biogas produced from AD can be used as fuel for the internal combustion of reciprocating engines, which run the generators to produce electricity. Hot exhaust air created during this process can be recovered for heating needs. The components of a gas engine are shown in Figure 8. In San Diego, California, the Point Loma WWTP has achieved energy self-sufficiency by using biogas reciprocating engines, and excess energy in the form of electricity were sold to the grid (U.S. Department of Energy, 2004). The gas is used to provide space heating and cooling, and the CH4 produced powers two reciprocating engines that run generators with a total capacity of 4.5 MW. The heat produced by the operation of the engine is utilised to maintain optimum conditions for gas production. The city of San Diego was able to save more than US$3 million in operational energy costs, and had sold US$ 1.4 million worth of excess power to the grid.
20
Figure 8: Components of a biogas engine (Clark Energy, n.d.)
The EPA CHP Partnership compelled various data and information regarding CHP technologies. A summary of the advantages and the disadvantages from the three types of technologies is presented in Table 7.
Table 7: Advantages and disadvantages of CHP technologies (Robbins, 2012) CHP Technology Advantages Disadvantages . High power efficiency . High maintenance costs . Fast start-up . Limited to lower temperature . Relatively low investment cost cogeneration applications Reciprocating Engines . Can be overhauled on site . Relatively high air emissions with normal operators . Must be cooled even if . Operate on low-pressure gas recovered heat is not used . High levels of low frequency noise . Small number of moving parts . High costs . Compact size and lightweight . Relatively low mechanical Microturbines . Low emissions efficiency . No cooling required . Limited to lower temperature cogeneration applications . Low emissions and low noise . High costs . High efficiency over load . Low durability and power Fuel Cells range density . Modular design . Fuels requiring processing unless pure hydrogen is used
21 2.9.4. Biogas Upgrade
Biogas can be processed into a high quality, CH4-rich product known as biomethane. Raw biogas undergoes separation and removal of CO2 and other trace gases using various technologies, and the final product can be compressed and stored for further utilisation. Figure 9 shows the typical types of processes used during biogas upgrade.
Figure 9: Processes for biogas upgrade (Handley, 2010)
22 2.9.5. Defective Components in Biogas
Though biogas consist mainly of CH4 and CO2, it also contains other contaminants including
H2S, sulfur compounds, and a variety of corrosive gases that evolves from chemical products in waste (U.S.E.P.A., 2007). Contaminants present in the biogas can cause erosion and corrosion to the generation equipment, so before biogas can be utilised in any application, some minimal amount of gas cleaning is required. Specific contaminants that cause operational problems include (U.S.E.P.A., 2007):
. Solids can cause erosion of critical surfaces or plugging of orifices. . Water retards combustion and can cause erosion, corrosion, or catastrophic damage to critical surfaces or components. . Non-methane fuel components (butane, propane, carbon monoxide, hydrogen) can change combustion characteristics; if present in liquid form can cause physical damage. . Sulfur and sulfur compounds can cause corrosion in engines, increase maintenance requirements (more frequent overhauls and oil changes), and poison catalyst materials.
. CO2 reduces heating value and combustibility. . Siloxanes create a glassy deposition on high-temperature surfaces; particles can break off and damage working parts.
2.10. Climate Change Benefits
CHP systems offer considerable environmental benefits in comparison to purchased electricity and onsite-generated heat. Through heat capture and utilisation that would otherwise gone wasted in energy production, CHP requires less fuel than equivalent but separate heat and power systems to produce the same energy (U.S.E.P.A., 2007). The use of biogas as energy, rather than fossil fuels, reduces GHGs emission because less fuel is combusted.
2.10.1. Mitigation efforts in Australia
Australia generates about 1.5 % of global GHG emission (Carbon neutral, 2011). On a per capita basis, Australia is one of the world’s largest polluters, with a per capita CO2 emission of more than four times the world average (Carbon neutral, 2011). The waste sector accounts
23
Figure 10: Gross national income with an without carbon price (Australia Goverment, 2013) for 2 % of Australia’s national inventory and annual emissions have increased 0.3 % in 2011 – 2012 (Commonwealth of Australia, 2013). The Australia government have recently implemented measures to control and mitigate the effects of GHGs, by placing a price on carbon pollution. The pricing mechanism will apply to the biggest polluters in the country, where they will pay for each tonne of pollution release into the atmosphere (Australia Goverment, 2013). Through this, the government hopes to create economic incentives to reduce the pollution (Figure 10). Calculations of GHG emissions factor can be found in DCCEE (2012).
2.11. WWTP Energy Recovery in Western Australia
Energy production and recovery through AD has been utilised in some WWTPs in Western Australia (WA). Below are two examples of such WWTPs, chosen for comparison because of its proximity and similarities.
24 2.11.1. Woodman Point WWTP
Woodman Point WWTP is a mixed wastewater treatment facility located at South Fremantle, WA, owned and operated by Water Corporation. It receives a wastewater inflow of 132 mega litres per day (132 ML/d), providing sewerage services to approximately 600,000 persons. The inflow has a mixture of 80 % municipal and 20 % industrial source (Francis, 2013).
After primary treatment, influent passes through four sequencing batch reactors (SBRs), where aeration is carried out for two hours followed by one hour of solids settlement before proceeding into the anaerobic digesters. The sludge fed into the digesters has a typical ratio (primary sludge: WAS) of 1 : 1, but that can fluctuate depending on the quality of incoming influent. The sludge are held in three, 38-metre high egg-shaped digesters under anaerobic conditions at 37 °C and 3 kPa for 20 days. The digesters are mixed mechanically to ensure constant temperature throughout its profile and the sludge are fed in a continuous process. Biogas produced in the digesters consists about 57 to 64 % of methane. Currently, biogas captured during the AD process provides half of the plant’s power usage onsite (Francis, 2013).
2.11.2. Beenyup WWTP
Beenyup WWTP is a municipal wastewater treatment facility located at Craigie, WA. The plant is the largest wastewater treatment facility in the region owned and operated by Water Corporation. It receives a wastewater inflow of 135 ML/d, providing sewerage services to approximately 660,000 persons.
Similar to Woodman Point WWTP, the sludge fed into the digesters is a combination of primary sludge and WAS, with a typical ratio (primary sludge: WAS) of 1 : 2. The sludge is held in six 6000 m3 cylindrical digesters under anaerobic conditions at 37 °C for 20 days. The digesters are mixed by compressed air supplied from the bottom of the tank to ensure a constant temperature throughout its profile. The biogas produced is stored in a seventh digester where they are used for the heating of the digesters. Biogas produced from the six digesters is collected and used to generate heat to maintain the temperature within the digesters. Excess biogas left after heating is flared off. All treatment processes operates on grid electricity, except for the heating of the digesters.
25 3. MOTIVATION
Subiaco WWTP is a water treatment facility receiving a mixed of industrial and domestic sources. It serves the city of Perth and its neighbouring suburbs, receiving an influent of 61 ML/d.
The current biological treatment method of Subiaco WWTP uses an aerobic dissolved air floatation system (DAFT), where dissolved air is continuously pumped into the tanks during the treatment process. This requires a continual consumption in energy and contributes GHGs through the demand for electricity to operate the aeration system, making wastewater treatment a costly and energy-intensive process. Adopting an aerobic treatment system over an anaerobic treatment system overlooks the opportunities for recovering energy during the anaerobic treatment process. This can in turn allow for cost reduction and ultimately minimises GHGs production.
Firstly, even though anaerobic treatment system may periodically consume some energy for in-tank mixing, the energy consumption is not as intense as compared to the continuous requirement for aerobic systems. Secondly, anaerobic systems produce biogas where it can be harnessed and utilised for onsite energy demands. This offsets the energy that would otherwise be supplied by grid. Additionally, excess energy harnessed during treatment process could be sold back to the grid, providing some economical incentives to the WWTP.
26 4. AIMS AND OUTCOMES
The objective of this study is to investigate the technical feasibility of energy production and recovery from wastewater treatment processes, towards achieving sustainable energy consumption. The specific aims of this study are;
1) Develop an understanding on the sewage sludge characteristics of Subiaco WWTP;
Subiaco WWTP receives a different set of sewage sludge characteristics as compared to other regional treatment facilities, mainly because of the differential in population size and variation from incoming sources. The investigation will facilitate the understanding of incoming sewage sludge characteristics into Subiaco WWTP, and provide further information on the behavioural attributes and the energy potential of the sludge.
2) Investigate the potential energy production of Subiaco WWTP;
Literature suggests that anaerobic digestion of sewage sludge can be a net positive energy contributor during wastewater treatment process. Biogas recovered from anaerobic digestion process can be quantified into energy potential and this investigation will explore the energy potential of biogas gas produced in Subiaco WWTP through laboratory experiments.
3) Conduct an energy feasibility study on the proposed treatment process.
Nearby treatment facilities like Woodman Point and Beenyup have incorporated some forms of energy recovery through anaerobic digestion into their wastewater treatment process. The study conducts an energy evaluation of Subiaco WWTP and examines the feasibility of energy recovery in its treatment processes. Energy production from Subiaco WWTP will be compared with neighbouring treatment facilities, and further analyses on the sustainability and environmental prospects will be carried out.
27 5. MATERIAL AND METHODS
5.1. Field Sampling Site
The study site is a municipal wastewater treatment facility located at Subiaco, WA. It is owned and operated by the Water Corporation, and commenced operations since 1927 to provide sewerage services to the residents of Perth CBD and its western suburbs. The plant treats a wastewater flow of 61 ML/d, serving approximately 310,000 persons, with 7 to 8 % of influent comes from industrial sources.
5.2.Sludge Samples
The major component of the samples were WAS mixed with a small portion of primary sludge, collected from the sludge blending tank process. Special care was taken during the extraction process to ensure that a sample representative of the sludge in the tank was removed. The samples were collected in 4 L clean polyethylene (PE) containers and brought back to the laboratory within 30 minutes. The samples were immediately stored at 4 °C in a refrigerator until further use. The samples were allowed to equilibrate to laboratory temperature before it was used for the experiment.
5.3. Biogas Collection System
A batch system was set up to monitor biogas generation from the samples (Figure 11). A 1 L glass vacuum filtering flask was used as an anaerobic reactor for sludge incubation in a water bath set at a constant mesophilic temperature of 37 °C and sealed with a thick, black rubber stopper. The side arm is connected with a 50 cm rubber tubing hose (Ø 7.9 mm) for gas movement to an inverted graduated gas collector (i.e. 500 ml plastic container). The collector acts as a sampling port used to sample and release biogas. 600 ml of sludge samples were used in each reactor. All the experiments were carried out in triplicate and the results were expressed as means. The system was inspected for any gas leakages before the commencement of the experiment.
5.4. Biogas Sampling and Monitoring
A sampling port was created by drilling a hole and sealing it with a rubber membrane at the bottom of the graduated gas collector. Gas samples, extracted using a 50 ml air locked syringe, were transferred and stored in 1L CEL scientific gas sampling bags. Gas
28 measurements were monitored daily by recording the downwards displacement of water. The incubation time was approximately 36 days.
Sludge Incubation
Gas Collection with Extraction Port
Figure 11: Biogas collection system setup
Gas Inlet
Reagents
Figure 12: Orsat gas analyser
29 5.5. Biogas Analysis
Collected gas samples were analysed using the Orsat gas analyser (Figure 12). The composition of biogas in terms of methane (CH4) and carbon dioxide (CO2) were determined by volumetric method found in Standard Methods for the Examination of Water and Wastewater. A measured amount of biogas was first passed through a potassium hydroxide
(KOH) solution to remove CO2 and next through an alkaline pyrogallol solution to remove O2. The volume of gas remaining was measured at the end of each step, which gave the relative percentage of volume of each component in the mixture. The remaining gas was assumed to be CH4 with H2S. The composition of gas can be found as follows:
(2)
(3)
(4)
Where:
= Percentage of present in biogas sample.
= Percentage of present in biogas sample.
= Percentage of present in biogas sample.
V1 = Initial volume of biogas.
V2 = Volume of biogas after passing through KOH solution.
V3 = Volume of biogas after passing through alkaline pyrogallol solution.
30 5.6. Sample Analysis
The determinations of total solids (TS), volatile solids (VS) and fixed solids (FS) were carried out in triplicates as proposed by Standard Methods for the Examination of Water and Wastewater. TS indicate the mass that remains after drying the sludge sample at 105 °C for 48 hours, and is expressed as a percentage of the total wet mass. The VS content was obtained by measuring the mass loss after heating the TS fraction at 550 °C for 1 hour. The mass remaining is the fixed solids (FS). VS and FS were expressed as a percentage that can be referred to the wet mass or to the TS. All samples were weighed on AND ER-180A electronic balance (± 0.2mg). The pH level of the samples was taken using TPS WP-80D dual pH-mV meter.
Total Solids (TS) = Volatile Solids (VS) + Fixed Solids (FS) (5)
31 6. RESULTS AND DISCUSSION
The following section will be presented in two chapters. The first chapter will discuss on the results and findings obtained from the laboratory experiment. Results from the first chapter will be used further in the second chapter for energy, economical and environmental calculations and analyses of Subiaco WWTP.
CHAPTER 1
6.1. Laboratory Results
The following sections will discuss the results and findings of Subiaco sludge samples under anaerobic conditions. Section 6.1.1 will discuss the results of the Total Solids test, Section 6.1.2 will examine the influence of pH levels during treatment, Section 6.1.3 will discuss the biogas production, Section 6.1.4 will further describe its characteristics and composition and Section 6.2 will compare AD results of Subiaco WWTP with neighbouring WWTPs. The performance data of one kg of Subiaco sludge in an anaerobic reactor is summarised in Table 8.
6.1.1. Total Solids Test
6.1.1.1. Total Solids and Volatile Solids
The efficiency of the anaerobic digestion of the sludge samples was evaluated in terms of TS and VS reduction. The results of total solids content of the sludge sample before and after digestion are presented in Table 9. A 25.36 % reduction in TS and a 14.49 % reduction in VS was observed during the experiment. This noticeable drop in total solids content between pre and post digestion demonstrated the effectiveness of solid reduction during AD. The reduction of organic matter was measured by the volatile solid reduction, indicating the completeness of digestion. It is important to note that the TS reduction is dependent on the amount of moisture content in the samples, which in turn influences the FS content. A comparison of Subiaco biogas production with other WWTPs is shown in Table 10.
32 Table 8: Experimental results from anaerobic digestion of sewage sludge Summary of results Mass of sewage sludge used (kg) 1 Retention time (days) 36 Incubation temperature (°C) 37 Total volume of gas generated (L) 15.12 Peak volume of gas (L) 0.48 TS (%) 2.62 VS (%) 2.36 VS/TS (%) 78.63 pH 6.68 Biogas production (m3 kg-1 VS added) 0.53 Biogas production (m3 kg-1 VS destroyed) 0.60 Biogas production (m3 L-1 sludge) 0.015
Table 9: Reduced solids during anaerobic digestion % Pre-digestion sludge % Post-digestion sludge % difference Total Solids (TS) 3.51 2.62 25.36 Volatile Solids (VS) 2.76 2.36 14.49 Fixed Solids (FS) 0.36 0.27 25 pH 6.89 6.68 -
Table 10: Comparison of treatment efficiency in WWTPs WWTP m3 kg-1 VS destroyed Subiaco (Perth, Australia) 0.6 Malabar (Sydney, Australia) (Cowgill, 2011) 0.64 Haridwar (Uttarakhand, India) (Malik and Bharti, 2009) 0.6
6.1.1.2. Fixed Solids
A 25 % reduction of fixed solids (FS) content was observed at the end of the experiment. Conclusions in some of the published studies were based on the assumption that FS content in sewage sludge would remain unchanged, however the results suggest that this may not always be true. Patni and Jui (1987) suggested that a possible explanation for the apparent loss in FS content could be due to the increase in the volatile proportion of TS at the end of the incubation period. For example, some non-volatile carbonates and sulfates might have been transformed into volatile matter during the incubation period. These compounds might be initially stable at 105 °C, but volatise at 550 °C. In addition, though effort had been made
33 to ensure that the experiment was carried out according to guideline standards, experimental and human errors could still exist. A variation in the FS content might be influenced by the moisture content when both the initial and final samples were not dried completely, or when samples still had excess heterogeneity present in them. Further research is necessary to determine the exact cause of variation in FS content.
6.1.2. pH Level pH value is an important indicator of the anaerobic reactor’s activity. The pH within the reactor can fluctuate greatly during the digestion process and the main factors affecting the pH level are the alkalinity and acid content. A high acid content will result in a drop in pH level of the system, which can subsequently inhibit methanogenic activity and cease biogas production. The pH level of the sample can indicate the current phase of digestion, and therefore be used as a diagnostic parameter to determine actions required to maintain a conducive biogas environment. Since the reactors were operating in a closed batch system, the pH could not be monitored during the incubation period and it was not known if the pH was constant throughout. However, final pH measurements in Table 8 showed a value of 6.68 that is within the optimal range of 6.5-7.2 for biogas production.
6.1.3. Biogas Production
The cumulative volume of biogas over the retention time is shown in Figure 13. It can be seen that biogas was produced from day 1 and production remained constant until day 20, yielding about 10 L of biogas. Figure 14 shows the daily gas production over the retention time, and a slight dip in biogas production was observed from day 21 onwards. At the end of the 36-day retention period, a cumulative 15.12 L (0.015 m3) of biogas was produced from the sludge samples. This suggests that optimal biogas production reaches its peak at around the 20 day mark, and this is further supported by observations made by Bouallagui et al. (2003). In general, a 20-day retention period is a reasonable amount of time for AD and most WWTPs, including Beenyup and Woodman Point, have adopted a retention period of between 15 to 20 days.
34 18 16 14 12 10 Beaker 1 8 Beaker 2 6 Beaker 3 4 Beaker 4 2 Cumulative biogas biogas Cumulative volume(L) 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 Time (days)
Figure 13: Cumulative biogas production over 36 days
Figure 14: Daily gas volume produced during AD
Biogas composition
30%
Carbon dioxide (CO2) Methane (CH4) & other gases 70%
Figure 15: Gas composition of Subiaco samples produced during AD
35 6.1.4. Biogas Composition
The biogas composition of the sludge samples is shown in Figure 15. The composition of
CO2 is 30 % and the composition of CH4 is 70 %, together with other trace gases (i.e. H2S). Due to laboratory limitations regarding the availability of test reagents, the individual composition of other trace gases was not able to be determined at the time of the experiment. However, theoretical estimates of these gases, the main concern being contaminant gases, will be taken into account in further discussion.
6.2.Data Comparison of Anaerobic Digestion between WWTPs
Laboratory results of sludge samples from Subiaco WWTP will be compared with available data from the Woodman Point and Beenyup WWTPs. Table 11 shows a comparison of sludge characteristics and biogas production between Subiaco, Woodman Point and Beenyup WWTP. From the table, it can be seen that all three WWTPs have achieved a similar VS reduction. However, the TS reduction for Beenyup WWTP (3.20 %) is noticeably higher than Subiaco (2.62 %) and Woodman Point (2.35 %) WWTP. The same is observed for biogas production per VS destroyed, with Beenyup WWTP (1.05 m3 kg-1 VS destroyed) having a higher production rate than Subiaco (0.60 m3 kg-1 VS destroyed) and Woodman Point (0.79 m3 kg-1 VS destroyed) WWTP.
The general cause for these discrepancies between the three plants, despite serving the same region, could mainly be attributed to the different types of influent that each WWTP receives. Subiaco and Woodman Point WWTP both receive an influent that contains varying amounts of industrial sources, which generally contains a high concentration of inorganic content. On the other hand, Beenyup WWTP receives a domestic influent that generally contains a higher organic content. It is known that during the AD process, microorganisms producing biogas degrade organic content in the influent. Since influent into Beenyup WWTP contains more organic matter than the other two plants, the TS reduction and biogas produced during AD is
Table 11: Parameters of biogas production between Western Australia’s WWTPs WWTP m3 / L sludge m3 kg-1 VS destroyed % TS % VS Subiaco 0.015 0.60 2.62 89 Woodman Point (Francis, 2013) 0.020 0.79 2.35 86.43 Beenyup (Cosa, 2013) 0.019 1.05 3.20 87
36 noticeably higher. Another reason could be attributed to the ratio of primary and activated sludge fed into the digesters by individual WWTPs. As discussed earlier, primary sludge usually contains more organic content than activated sludge, thus a feed with a higher primary sludge content will generally produce more biogas. However, it is operationally impossible to maintain an optimal ratio of sludge feed all the time, because that ratio depends heavily upon the influent. Therefore, it is with little surprise that the ratio that each WWTP receives varies, which may explain for the differences noted.
In terms of biogas produced per litre of sludge, both Woodman Point (0.020 m3 / L sludge) and Beenyup WWTP (0.019 m3 / L sludge) achieved similar results, whereas Subiaco samples had a lower yield (0.015 m3 / L sludge). This is likely due to the difference in the type of treatment systems used. In Woodman and Beenyup WWTPs, the AD process is carried out under a continuous system, where a known volume of sludge enters and exits the reactor every day. The daily supply of new sludge provides the microorganisms a continuous source of food and a continuous stable operating condition, meaning that less acclimatisation is needed. The samples from Subiaco were incubated in a batch system, where a known volume of sludge remained in the reactor throughout the experiment. Microorganisms may initially need to acclimatise to the new operating conditions before peak biogas production can take place. Since there was no new source of sludge, the biogas production was expected to decline after some time too, and this can be observed in Figure 13 and 14.
CHAPTER 2
6.3. Subiaco WWTP Analysis
The following sections will discuss the potential outcomes of AD in Subiaco WWTP. Section 6.3.1 will discuss the parameters of biogas characteristics, Section 6.3.2 will investigate the potential energy production using different biogas technologies, Section 6.3.3 will consider the economical aspects and Section 6.3.4 will quantify the environmental benefits.
In consideration that Woodman Point and Subiaco WWTP do share similar traits (same region, both receive mixed influent source), and that Woodman Point WWTP already has an AD system already in place, theoretical calculations made for Subiaco WWTP will therefore draw references from Woodman point WWTP.
37 6.3.1. Biogas Parameters
The laboratory experiment had determined the CO2 content of Subiaco samples to be 30 %,
but due to laboratory limitations, CH4 content was indistinguishable with other contaminant
gases (i.e. H2S). Woodman Point WWTP has a H2S content of approximately 1800 ppm, or 1.8 g/L biogas (1.8 %) (Charles et al., 2006). Using that as a reference with the consideration
of variability, Subiaco samples were assumed to have a H2S content of 5 g/L biogas (5%).
The specific composition of the biogas was thus presume to contain 65 % CH4, 30 % CO2
and 5 % H2S. The new biogas composition can be seen in Figure 16, and a summary of biogas parameters is presented in Table 12.
6.3.1.1. Biogas Production Per Day
In data provided by Francis (2013), Woodman Point WWTP receives an average inflow of 132 ML/d between the period of October 2012 to March 2013, of which 0.68 % of the total flow enters the digesters as sludge.
Biogas composition
5%
30%
Carbon dioxide (CO2) Methane (CH4) 65% Hydrogen Sulfide (H2S)
Figure 16: Theoretical biogas composition of Subiaco samples
38 For Subiaco WWTP with an average inflow of 61 ML/d, the amount of sludge entering the digesters is equivalent to;
Subiaco samples yield a biogas production of 0.015m3 / L sludge (Table 8). Hence, the total biogas produced in a day is;
6.3.1.2. Wobbe Index
The Wobbe Index is used as an indicator on the interchangeability of fuel gases, and is
particularly useful for evaluating fuels in a combustion engine. A 65 % CH4 content has a lower heating value (LHV) of 20.2 MJ/kg and a density of 1.2 kg/Nm3 (Biowrite, 2007). The Wobbe Index of biogas can be calculated using the following equation;
Wobbe Index = (5)
Where:
= Lower heating value of biogas, MJ/kg
= Relative density, dimensionless
The relative density can be calculated using;
(6)
Using Equation 5 and 6, the results are;
39
Wobbe Index:
6.3.1.3. Mass Flow Rate of Biogas
The mass flow rate is defined as the mass of substance that passes through a given surface per unit time. The mass flow rate of biogas is required to determine the power production, and can be calculated as;
(7)
Equation 7 yields:
Table 12: Biogas parameters of Subiaco WWTP Biogas Parameters Methane composition (%) 65 Carbon dioxide composition (%) 30 Hydrogen sulfide composition (%) 5 Biogas production per day (m3/d) 6222 Calorific value, lower (MJ/Nm3) (Biowrite, 2007) 23 Calorific value, lower (MJ/kg) (Biowrite, 2007) 20.2 Density of biogas (kg/Nm3) (Biowrite, 2007) 1.2 Density of methane (kg/Nm3) 0.66 Relative density 0.928 Density of methane (kg/Nm3) 0.66 Wobbe index(MJ/Nm3) 24.78 Wobbe index(MJ/kg) 21.77 Mass flow rate (kg/d) 7466.4
40 6.3.2. Power Generation from Subiaco Biogas
Biogas can be used for the production of electricity. To calculate the amount of energy production, the following assumptions were made;
1 Watt (W) = 1 joule second-1
1 Watt-hour (Wh) = 1 x 3600 joules
1 kilowatt-hour (kWh) = 3,600,000 joules (3.6 MJ)
3 1 m of CH4 = 36 MJ
36 MJ = 10 kWh
3 1 m of CH4 = 10 kWh
3 Biogas production is 6222 m per day. Taking 65 % as CH4 content, the energy equivalent is;
The U.S. Department of Energy (2004) estimated that as a rule of thumb, biogas produced from a WWTP can generate up to 35 kW from processing an influent of one mega gallon per day. This assumption is used to verify the accuracy of the calculation above. Since Subiaco WWTP receives an influent of 61 ML/d, the equivalent would be 16.10 MG/d (1 gallon = 3.79 litres).
41 The rule of thumb estimated that Subiaco WWTP can generate up to 13.52 MWh (per day), and the calculated value for the biogas production of 40.04 MWh (per day) is valid as it is of the same order of magnitude.
6.3.2.1. Power generation from CHP Technologies
Power generation from CHP technologies are able to achieve high electric efficiency because of the ability to recover heat and power from one system (U.S.E.P.A., 2012). Since the total electric efficiency for CHP captures both the value of electrical and thermal outputs, the heat produced (in Btu) can contribute to electric production resulting in the typically high values of electric efficiency of between 55 to 80 %. Recovered heat is particularly useful to maintain a mesophilic temperature during AD. The electric efficiency is the main indicator for energy production from biogas, and the exact value will depend on the model of CHP selected. It is important to consider other factors such as the technical, economical and site suitability aspects of purchasing a CHP unit while making a decision. Table 13 shows a summary of CHP technologies and their associated values.
Table 13: Performance and cost of energy generation technologies (U.S.E.P.A., 2007) Technology Recip. Engine Microturbine Fuel Cell Effective electrical 70-80% 50-70% 55-80% efficiency Typical capacity 0.01 – 5 MW 0.03 – 0.5 MW 0.005 – 2 MW Typical power to heat 0.5 – 1 0.4 – 0.7 1 – 2 ratio CHP installed costs 1,100 – 2,200 2,400 – 3,000 5,000 – 6,500 ($/kW) O & M costs ($/kWh) 0.008 – 0.022 0.012 – 0.025 0.032 – 0.038 Electric heat rate 8,758 – 12,000 13,080 – 15,075 8,022 – 11,370 (Btu/kWh) Hours to overhaul 25,000 – 50,000 20,000 – 40,000 32,000 – 64,000 Start-up time 10 sec 60 sec 3 hours – 2 days Fuels Natural gas, biogas, Natural gas, biogas, Hydrogen, natural gas, propane, landfill gas propane, oil propane, methanol
42 A low electric efficiency will make it difficult to justify the investment. So assuming Subiaco WWTP utilises a CHP technology with an electric efficiency of 70 %, the useful energy output will be;
(per day)
(per year)
Subiaco WWTP will be able to produce 10,330 MWh of useful electricity per year on a CHP technology with 70 % electric efficiency. The total electrical consumption of Subiaco WWTP in 2012 is 13,200 MWh. From the generated energy output, Subiaco WWTP can achieve;
Generated energy recovery:
The recovered energy can generate an equivalent of 78 % of its total energy consumption in a year.
6.3.3. Economical Analysis
Subiaco WWTP operates on the electricity provided by Western Power. Electricity pricing generally does not fluctuate, however there is a peak electricity cost when a certain usage threshold is exceeded. The exact pricing was not disclosed due to commercial confidentiality reasons, so estimates were made to determine the charges. For simplicity sake, the electricity pricing for Subiaco WWTP is assumed to remain fixed, and the electricity usage is assumed to be below the usage threshold, hence peak electricity cost is ignored. The aeration treatment process using dissolved air floatation thickeners (DAFT) contributes an estimated 264,390 kWh. The total electricity expenditure for that period was A$1,294,021, which works out to be A$0.098 per kWh (A$98 per MWh).
6.3.3.1. Cost of Aeration Treatment
The cost of operating the DAFT is;
(per year)
43 6.3.3.2. Savings from Generated Power
The electrical production was used to provide an estimate on how much savings Subiaco WWTP can expect from a technology with 70 % electric efficiency. The savings are;
(per year)
Total cost: (per year)
Since aeration treatment for sludge treatment will be obsolete with AD system in operation, the cost for aeration treatment can be added towards cost savings. In total, Subiaco WWTP will be able to achieve a cost savings of A$1,038,163 per year with a 70 % electric efficiency.
6.3.3.3. Case Scenarios
Three case scenarios (data from Table 13) anticipating the variability of the capital and the operational and maintenance (O&M) for the biogas technologies are shown in Table 14. The Best Case scenario anticipates a low capital and O&M cost, the Base Case scenario anticipates a typical capital and O&M cost, and the Worst Case scenario anticipates a high capital and O&M cost.
The recoverable useful energy for a CHP technology with 70 % electric efficiency is 28.3 MWh per day, so a CHP unit with one MW capacity will be suitable for Subiaco WWTP. Taking in account of another CHP unit as backup, the total MW capacity required will be two.
Table 14: Capital and O&M scenarios for biogas technologies Best Case Base Case Worst Case (Capital – O&M) (Capital – O&M) (Capital – O&M) Recip. Engine $1,100/kW - $1,650/kW - $2,200/kW - $0.008/kWh $0.015/kWh $0.022/kWh Microturbine $2,400/kW - $2,700/kW - $3,000/kW - $0.012/kWh $0.018/kWh $0.025/kWh Fuel Cell $5,100/kW - $5,750/kW - $6,500/kW - $0.032/kWh $0.035/kWh $0.038/kWh
44 The payback period can be calculated using the case scenarios as a reference. The overall cost does not take in account the additional cost for any pre-treatment equipment or specific engine modifications requirements, as this evaluation is beyond the scope of this project. The payback period uses only the recouped value generated from the recovered energy; in other words, the payback period can be said to be the amount of time needed for the CHP technology to generate a value to breakeven with the purchased cost.
The payback period is calculated as;
= (8)
Not included in this economic analysis are the inflation rate, future adjusted values and assumptions for additional labour for the biogas technology. Ideally, plant operators will be skilled to operate and perform routine maintenance on the technology, though a possibility exists where increased person-hours will be needed. This is a factor for consideration for Subiaco WWTP.
Table 15: Payback period of technologies using case scenarios Capacity (MW) 2 2 2 Electric efficiency (%) 70 70 70 BEST CASE Recip. Engine Microturbine Fuel Cell CHP installed costs (A$) 2.2 million 4.8 million 10 million CHP O&M costs ($/kWh) 16 24 64 70 % efficiency payback period 2.2 years 4.6 years 9.6 years BASE CASE Recip. Engine Microturbine Fuel Cell CHP installed costs (A$) 3.3 million 5.4 million 11.5 million CHP O&M costs (A$) 30 36 70 70 % efficiency payback period 3.2 years 5.2 years 11.1 years WORST CASE Recip. Engine Microturbine Fuel Cell CHP installed costs (A$) 4.4 million 6 million 13 million CHP O&M costs (A$) 44 50 76 70 % efficiency payback period 4.2 years 5.8 years 12.5 years
45 Table 15 is a simplified comparison of three technologies offering the same electric efficiency. In reality, all the three technologies are able to achieve that level of electric efficiency, if not more. Given that respective suppliers will determine the actual costs of individual technologies, the table is best used as a guide.
Between the three technologies, the reciprocating engine offers the lowest overall cost and the shortest payback period to operate a two MW capacity CHP unit among all the case scenarios. Reciprocating engine is a widespread and well-known technology, and since CHP systems has traditionally been the most prevalent on-site generation application, users can be rest assured of the technology of a CHP reciprocating engine (Energy and Environmental Analysis, 2008). Typical, a WWTP has a life expectancy of about fifty years, of which can be further extended if routine maintenance and upgrades of the equipment was carried out. The payback period for all the technologies is still within the operating lifespan of a WWTP, even in the Worst Case scenario.
6.3.4. Carbon Reduction Equivalent
The electricity reduction of kWh can be expressed in avoided units of CO2 emission through the following factor provided by U.S.E.P.A. (2013a):
(9)
Where: Energy consumed = Energy consumption of technology in kWh
= Emission factor in metric tons CO2 per kWh
The aeration treatment process using DAFT was estimated to consume 264,390 kWh in 2012. The replacement of DAFT with biogas technology could potentially offset its carbon emission by;
= 187 metric tons CO2 (per year)
46 The electricity recovered through biogas technology could further offset the electricity that would otherwise be drawn from the grid. The recovered electricity, as discussed in the earlier section, is dependent on the electric efficiency of the biogas technology. Since this recovered electricity does not contribute to carbon emission, the avoidance of CO2 emission from the displaced electricity is;
Avoided CO2 emission by AD:
= 7,288 metric tons CO2 (per year)
Total avoided CO2 emission: (per year)
In total, 7,475 metric tons of CO2 emission could be avoided just by switching from aerobic to anaerobic treatment system. This is equivalent to the amount of carbon sequestered by 6,127 acres of U.S. forests annually (U.S.E.P.A., 2013b). A cost comparison between aerobic and anaerobic technologies is presented in Table 16. From the table, it can be seen that there are neither any electricity nor monetary value that can be recovered from using the existing DAFT system. However, it is possible using an anaerobic treatment system coupled with a CHP unit. It can be expected that as the efficiency of the CHP engine increases, the energy recovered as well as the generated value will increase too. A proportional decrease in CO2 emission is expected too, since the recovered energy serves as a substitution for the energy that would otherwise be supplied by the grid. The generated value can be viewed in two ways: 1) As the amount of money saved from purchasing electricity from the grid, or 2) As the amount of money that can be profited by selling recovered electricity to the grid. Either way, both views stem from the idea of being able to achieve monetary and environmental benefits from wastewater treatment.
Table 16: Contribution comparison between aerobic and anaerobic treatment technologies DAFT CHP 70% MWh (per day) -0.72 +28.3 MWh (per year) -264 +10,330 Generated value ( A$ per year) -25,872 +1,038,163
CO2 emission (metric ton) +186.5 -7,475
47
Figure 17: Comparison between aerobic and anaerobic treatment expenditure
Figure 17 shows the projected expenditure between using aerobic and anaerobic technology in a twenty-year period. The projected cost of operation is calculated based on present operational values. It can be seen that energy recovery has a sustainable impact on the operational expenditure. The ability of the anaerobic technology to generate onsite electricity can result in a significant reduction in cost expenditure for Subiaco WWTP, since it offsets the need to purchase grid electricity.
48 7. CONCLUSION
Anaerobic digestion represents a commercially viable process that converts sewage sludge to methane gas, a useful energy source. The overall results of anaerobic digestion of sewage sludge suggest that gas capture is a promising process to harass energy yield with high efficiency in terms of degradation yield and biogas productivity. This report demonstrated that anaerobic treatment is a feasible option for Subiaco WWTP and it can serve as a replacement for their current aerobic treatment system, providing additional benefits without compromising on treatment quality. Laboratory results obtained are comparable to data from neighbouring WWTPs using anaerobic treatment, indicating that Subiaco WWTP will be able to reap the same benefits as enjoyed by the other WWTPs.
This project also undertook efforts to characterise the information relating to the sludge and its biogas production. This is helpful for Subiaco WWTP to understand the behaviour of its influent under an alternative treatment process, and at the same time providing the necessary information needed for decision makers when considering a suitable purchase relating to sludge and biogas technology. The economical analysis showed a favourable projection of using anaerobic treatment system. Significant electricity and heat are recoverable on site through a CHP unit that can be used as a substitute for grid source, reducing the cost of overall expenditure. The purchase of the CHP technology was calculated to be repaid solely by revenue generated from biogas recovery, with the payback period evaluated under different scenarios. Even in the Worst Case scenario, the payback period is still within the operational lifespan of the WWTP.
Another additional benefit of switching to anaerobic treatment system is the potential to avoid and reduce CO2 emission, the main contributor to climate change. It has been widely accepted that climate change is indeed an unavoidable scenario in the near future. Even though the contributions from Subiaco WWTP may be considered insignificant on a global scale, it is important to realise that a collective effort can mitigate as well as contribute to the severity of the situation. Wastewater treatment is and will still be an irreplaceable sector in the foreseeable future, and the number of WWTPs will only increase in time to come. Thus, it is reasonable to claim that even though the completed removal of GHG contribution from the wastewater industry is impossible, achieving a reduction in GHG contribution is still a plausible target. The solution for the wastewater industry lies in anaerobic treatment and CHP
49 systems, both proven technologies used in wastewater treatment that provides economical incentives while accomplishing its purpose.
8. RECOMMENDATIONS
The aim of this thesis was to assess the feasibility of energy production and recovery at Subiaco WWTP, towards achieving sustainable energy. This study has indicated that by switching from an aerobic to an anaerobic treatment system, there is a potential for Subiaco WWTP to recover energy from its treatment process. The overall results from this study support the initiative of using recovered energy to produce electricity and heating for various treatment requirements. However, further research will be needed to increase the confidence of the results. The following recommendations will be useful to assist future research directions.
1. Further studies will require the usage of a Gas Chromatography to validate the composition of produced gas during anaerobic digestion. This will provide the important information to CHP technology suppliers on the content of harmful pollutants, such as hydrogen sulfide and siloxanes, present in the biogas that may reduce the operational lifespan of the equipment. 2. Economical analysis suggests that procuring an energy generation technology fuelled by biogas at Subiaco WWTP is highly feasible. Subiaco WWTP should contact CHP technology suppliers for a quote on the capital and O&M cost to verify the savings and the economic viability to generate energy on site. 3. Further analyses could be carried out for two aspects. An economical analysis can be furthered by taking in account of the given budget, loans, ongoing operational and maintenance cost of the anaerobic treatment system and adjustment for inflation. A technical analysis can be furthered by researching into the suitable models of CHP technologies to determine the most suitable model for Subiaco WWTP in terms of electric efficiency, its cost and generated benefits. 4. Subiaco WWTP can consider upgrading the captured biogas to biomethane, where it can be piped and sold as a direct fuel supply for vehicles.
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