COMMERCIALIZATION OF ANAEROBIC CONTACT PROCESS
FOR ANAEROBIC DIGESTION OF ALGAE
by
GUNJAN ANDLAY
Submitted in partial fulfillment of the requirements
for the degree of
Master of Science
DEPARTMENT OF BIOLOGY
CASE WESTERN RESERVE UNIVERSITY
May 2010
CASE WESTERN RESERVE UNIVERSITY
SCHOOL OF GRADUATE STUDIES
We hereby approve the thesis/dissertation of
______Gunjan Andlay
Master of Science candidate for the ______degree *.
Claudia M. Mizutani, Ph.D. (signed)______(chair of the committee)
Christopher A. Cullis, Ph.D. ______
Andrew K. Swanson, Ph.D. ______
David J. Burke, Ph.D. ______
______
______
April 2, 2010 (date) ______
*We also certify that written approval has been obtained for any proprietary material contained therein.
Dedicated to my parents, and family
2 Table of Contents List of tables ...... 5
List of figures ...... 6
ACKNOWLEDGEMENTS ...... 7
List of abbreviations ...... 8
Abstract ...... 9
1. INTRODUCTION ...... 10
2. ANAEROBIC DIGESTION ...... 12 2.1 Anaerobic Digestion at commercial level ...... 12
3. ANAEROBIC DIGESTION – THE PROCESS ...... 13 3.1 Stages of Anaerobic Digestion ...... 13
4. FACTORS AFFECTING PERFORMANCE OF ANAEROBIC DIGESTION ...... 17 4.1 Chemical Oxygen Demand ...... 18 4.2 C/N Ratio...... 19 4.3 pH and Alkalinity ...... 20 4.4 Volatile Solids ...... 20 4.5 Total Suspended Solids ...... 20 4.6 Fats, Oils and Grease ...... 21 4.7 Solids Residence time ...... 21 4.8 Hydraulic Retention time ...... 22 4.9 Agitation ...... 22 4.10 Temperature ...... 22 4.11 COD removal efficiency ...... 23 4.12 Inoculum growth rate ...... 23 4.13 Biogas output...... 24 4.14 Sludge output...... 24
5. INHIBITION OF ANAEROBIC DIGESTION PROCESS ...... 26 5.1 Ammonia ...... 26
3 5.2 Sulphide ...... 27 5.3 Light metals (Na, Al, K, Ca, Mg) ...... 27 5.4 Heavy metals ...... 28 5.5 Organic compounds ...... 29
6. ALGAL SPECIES AND ANAEROBIC DIGESTION ...... 31
7. ANAEROBIC DIGESTION TECHNOLOGIES ...... 34 7.1 Anaerobic Lagoon ...... 35 7.2 Anaerobic contact process ...... 36 7.3 Upflow Anaerobic Sludge Blanket Reactor ...... 37 7.4 Upflow/Downflow Anaerobic Filtration (Biofilm) process ...... 38 7.5 Anaerobic Expanded / Fluidized Bed Reactor ...... 39 7.6 Anaerobic Membrane Reactor ...... 40 7.7 Bulk Volume Fermenter ...... 41 7.8 Plug Flow Reactor ...... 41 7.9 Continuous Stirred Tank Reactor (Complete Mix reactor) ...... 42 7.10 Anaerobic Sequence Batch Reactor ...... 42
8. ANAEROBIC DIGESTION TECHNOLOGY MATRIX ...... 43 8.1 COD loading rate ...... 43 8.2 COD removal efficiency ...... 43 8.3 Influent solids concentration ...... 44 8.4 Theoretical biogas output ...... 44 8.5 Comparison of Anaerobic Contact Process with Other Processes ...... 46
9. COMMERCIAL FEASIBILITY OF ANAEROBIC CONTACT PROCESS ...... 49
10. CONCLUSION ...... 56
11. BIBLIOGRAPHY ...... 57
4 List of tables
Table 1: List of factors affecting anaerobic digestion performance ...... 18
Table 2: Estimated methane outputs of microalgal species ...... 32
Table 3: Anaerobic digestion of algal species at laboratory scale ...... 33
Table 4: List of anaerobic digestion technologies with advantages and disadvantages ... 34
Table 5: Anaerobic Digestion Technology matrix ...... 45
Table 6: Cost comparison of Anaerobic Contact process with other processes ...... 48
Table 7: CapEx and OpEx calculations for Anaerobic contact process ...... 53
5 List of figures
Figure 1: Anaerobic Lagoon ...... 35
Figure 2: Anaerobic contact process ...... 36
Figure 3: Upflow Anaerobic Sludge Blanket Reactor ...... 37
Figure 4: Upflow Anaerobic Filtration process ...... 38
Figure 5: Anaerobic Expanded/Fluidized Bed Reactor ...... 39
Figure 6: Anaerobic Membrane Reactor ...... 40
6 ACKNOWLEDGEMENTS
I am extremely grateful to Dr. Christopher A. Cullis, Chair, Department of Biology and
Director, MS Entrepreneurial Biotechnology, Case Western Reserve University for constantly guiding and supporting me at each and every step and continuously interacting with me to help in my gradual improvement as a biologist and an entrepreneur.
I extend my sincere gratitude to Dr. Andrew K. Swanson, Vice President, R&D, Phycal
LLC for guiding me throughout the internship at Phycal by being an excellent supervisor and my thesis advisor.
I am indebted to Mr. Martin L. Johnson, P.E. and Mr. Aaron B. Brister (Phycal) and Dr.
Mark W. Tenney, P.E. (Independent Consultant) for their extraordinary guidance and assistance on this project.
I am extremely thankful to the professors at Department of Biology, Case Western
Reserve University, Dr. David J. Burke, for serving on the thesis committee and Dr.
Claudia M. Mizutani, for being the Chair of the thesis committee.
I am very much contented after fruitfully completing this project and everyone who has helped me in any manner deserves the credit for its success.
GUNJAN ANDLAY
7 List of abbreviations
AD Anaerobic Digestion ATP Adenosine Tri-Phosphate Btu British Thermal Unit BVF Bulk Volume Fermenter CapEx Capital Expenditure COD Chemical Oxygen Demand CSTR Continuous Stirred Tank Reactor EDTA Ethylene Diammine Tetra Acetic acid EGSB Expanded Granular Sludge Blanket EPA Environmental Protection Agency FOG Fats, Oils and Grease HRT Hydraulic Retention Time IS Inert Solids kW Kilowatts MMBtu Million Btus MPN Most Probable Number MT Metric tonnes OpEx Operational Expenditure SBR Sequence Batch Reactor SCF Standard Cubic Feet SRT Solids Residence Time TSS Total Suspended Solids UASB Upflow Anaerobic Sludge Blanket VFA Volatile Fatty Acids VS Volatile Solids
8 Commercialization of Anaerobic Contact Process for Anaerobic Digestion of Algae
Abstract
by
GUNJAN ANDLAY
The commercialization of anaerobic contact process for the digestion of algal biomass has been shown to be feasible on a technical and economic basis. The selection of anaerobic contact process was done based on its superiority in a comparison among presently available commercial anaerobic digestion technologies. It is estimated that 60 anaerobic contact units would be required at a capital expense of $ 217 million, and it would operate at annual expenses of $ 3.2 million. This production facility would process
500 metric tonnes of algal biomass on a per day basis. The facility would yield $ 10.6 million in revenues on an annual basis, with more than 90% of those revenues coming from the sale of biogas to customers, who use it for power generation.
9 1. INTRODUCTION
The rising global energy demand and depletion of fossil reserves has resulted in the need
to explore non-conventional renewable alternatives. The most prominent among these is
capturing energy in living organisms and using it as fossil energy substitutes. Since the
bulk of the energy is used for transportation, the use of bioenergy in this field is
inevitable. Therefore, researchers and businesses all around the world are examining
various biofuels for their potential in solving the energy issue. Algal biomass has emerged as a frontrunner due to its extraordinary productivity when compared to other fuel crops such as oilseeds and corn. Algae can be used to capture sunlight and CO2 and
convert to specific products by providing substrates and conditions to favor such
production. Algae have been shown to produce lipids that can be used as fuel oil in
boilers and can also be converted to biodiesel for use in transportation. The residual algal
biomass after oil extraction can be converted to biogas for power generation using
process such as anaerobic digestion.
The technical and commercialization feasibility of utilizing algae as a feedstock for
anaerobic digestion process has been studied. The technical feasibility included the study
of various process factors that impact the digestion along with evaluation of presently
available technologies. This comparison between technologies was carried out based on
significant performance parameters and subsequent cost analysis was done for four major
technologies deemed relevant for anaerobic digestion of algae. The reason behind
selection of these four technologies was their theoretical ability to process the algal
biomass, without encountering issues with the operation.
10 The commercial feasibility of anaerobic contact process as the technology best suited for
digestion of algae was conducted. The large scale production scenario was prepared,
where the anaerobic contact process is used for producing biogas from algae. The
production process involves the input of 500 MT of algal biomass per day resulting in a
9.2 million standard cubic feet of biogas output and 48,000 tons of residual solids output.
The capital expenditure required for building such a production plant amounts to $ 217.3 million and annual operating expenses of $ 3.2 million would be incurred.
This study shows that the use of algae in an anaerobic digester is both technically and
commercially feasible. However, this process has not been examined at the commercial
level. Therefore, it is recommended to conduct pilot testing using algae as the feedstock,
in order to prove the concept and then scale it up.
11 2. ANAEROBIC DIGESTION
Anaerobic digestion refers to the biochemical process wherein complex organic compounds are degraded to carbon dioxide and methane in the absence of oxygen. The process involves specific microorganisms that convert the organic matter to biogas. These microorganisms are mostly archaebacteria and eubacteria that can grow in anaerobic conditions and utilize organic compounds as substrates for their growth. The cellular biochemistry operates in a manner that the microbes either die or survive without growing on exposure to air/oxygen. The biochemical pathways get routed towards substrate consumption without the need for energy from aerobic respiration processes.
2.1 Anaerobic Digestion at commercial level
Anaerobic digestion has been utilized for the treatment of organic waste and production of biogas for generating power and other purposes such as providing manure to agricultural farms. One of the primary sources of organic waste is dairy farm, where the waste is part of the water stream coming out of the farm and this is generally transported to city wastewater plants, dumped into nearby water bodies or dried and dumped in a landfill. Though the waste gets operated on by microbes in all the above mentioned scenarios, landfill and the floor of water bodies essentially are sites of anaerobic activity.
The dairy farms are evaluated based on the number of cows and the corresponding amount of waste water generated. In addition, the biogas production for power generation has been computed depending on the size of the farm. For instance, waste from a dairy farm having 500 cows can yield about 42,000 cubic feet of biogas on a daily basis
(Wright, March 2001). Using this in a 70 kW generator, around 1390 kW/day of power
12 can be generated. Several commercial scale plants operate next to a dairy farm and produce biogas based on the above relationship. Though it is difficult to obtain 100% energy from this biogas, considerably efficiency generators can be used to power a portion of the total energy demand of the farm.
3. ANAEROBIC DIGESTION – THE PROCESS
3.1 Stages of Anaerobic Digestion
The conversion to carbon dioxide and methane is a three-step process, with different types of microorganisms operating at each step. The three steps are hydrolysis, acidogenesis and acetogenesis and methanogenesis. It should be noted here that the rate at which the organic substances are degraded is the same for all three stages. This can be significant in its effect on the overall methane production, resulting in a lower yield if the rates are not equal in each stage.
3.1.1 Stage 1 – Hydrolysis
This stage involves the breakdown of the complex polymeric substrates into simpler monomer compounds. The biological waste primarily is composed of carbohydrates, fats and proteins. The microorganisms, predominantly bacteria, excrete the enzymes such as lipases, cellulases, amylases, proteases etc. in order to break carbohydrates to glucose and other monosaccharides, proteins to amino acids and lipids to fatty acids. In addition,
13 during the cellular anaerobic respiration process, nucleotides can be converted into purines and pyrimidines (Verma, May 2002).
The following simple equations represent the hydrolysis stage:
Carbohydrates + Water Monosaccharides
Proteins + Water Amino Acids
Lipids + Water Fatty Acids
Hydrolysis of substrates
The biomass being digested by bacteria primarily consists of polymeric carbohydrates such as cellulose and hemicelluloses apart from other biological polymers like lignin, pectin and so on. The hydrolysis of each of these compounds is achieved by enzymes that break the bonds between each of the monomeric molecules. These monomers vary depending on the type of compound being hydrolyzed.
Cellulose hydrolysis to glucose is carried out by (cellulases excreted by) bacteria like
Trichoderma reesei, Thermobifida fusca, Clostridium thermocellum, Ruminococcus flavefaciens, Acetovibrio cellulolyticus, and many others (Chynoweth & Isaacson, 1987).
Hemicellulose is hydrolyzed to hexoses, pentoses and uronic acids by same species as ones that secrete cellulases (mentioned above), due to the fact that these bacteria require the presence of cellulose for digestion of hemicelluloses. Apart from these, some bacteria are good hemicelluloses digesters but weak cellulose digesters like Butyrivibrio
14 fibrisolvens, Ruminococcus albus and Bacteroides ruminicola (Chynoweth & Isaacson,
1987).
3.1.2 Stage 2 – Acidogenesis / Acetogenesis
The second stage of the anaerobic digestion process involves the conversion of hydrolysis
products to volatile fatty acids. This stage is coupled to the hydrolysis as a single stage by
most of the researchers and experts as noted in the literature. The microbes responsible
for this stage are commonly known as acidogenic bacteria. The hydrolysed substances are
converted to different volatile fatty acids (VFAs) such as acetic acid, propionic acid,
butyric acid and lactic acid. The byproducts of this conversion are ammonia, hydrogen,
carbon dioxide and other neutral compounds like methanol and ethanol.
The acidogenic bacteria are found to be less sensitive to changes in temperature when
compared to methanogenic bacteria. The acidogenesis is affected by changes in pH and
the maximum activity is shown to be around neutral pH, i.e. close to pH 7.0. Yu et al.
(2003) found that the VFAs were formed in highest quantity at a pH of about 6.0 – 6.5 for
protein-rich waste, while 5.0 to 5.5 was found be optimum for carbohydrate-rich waste.
Since the products of acidogenesis need to be fed to the methanogens, the environment
should be suitable to favor the production of those products required for methanogenesis.
At high pH, the process favors the production of acetate and butyrate more than
propionate.
15 The acetogenic bacteria specifically convert all the products of hydrolysis to acetate. This
is one of the mechanisms of providing substrates to the methanogens for conversion to
methane. The following equation represents one of the mechanisms of acetate formation:
2 CO2 + 4 H2 CH3COOH + 2 H2O
The byproducts of this stage are carbon dioxide and hydrogen, which are then utilized by the methanogens in the following stage.
3.1.3 Stage 3 – Methanogenesis
Methanogenesis, as the name suggests is the process of producing methane and this
occurs in strict anaerobic conditions. The bacteria convert the acetic acid from the
acetogenesis stage to carbon dioxide and methane using the hydrogen produced in
previous stages. The process is depicted by the following reaction:
CH3COOH CH4 + CO2
The methanogenic bacteria represent the only class of organisms that can use electrons as
H2 and convert the acetate and methanogens show a very high affinity towards H2. An
alternate way by which methanogens produce methane is by using carbon dioxide
directly and reacting with hydrogen present as a byproduct of the acidogenesis stage.
Some of the most common methanogenic bacterial strains include the species such as
Methanospirillum hungatei, Methanosarcina barkeri, Methanobacterium
thermoautotrophicum and Methanobacterium formicicum (Ye Chen, 2008).
16 Apart from acetic acid, the methanogens can produce methane by using substrates such as
formic acid, propionic acid, butyric acid, valeric acid, caproic acid, alcohols including
methanol, ethanol, propanol, butanol and pentanol (Buswell & Barker, 1956).
The equations are given as follows:
4 CH3CH2COOH + 2 H2O 7 CH4 + 5 CO2
2 CH3CH2CH2COOH + 2 H2O 5 CH4 + 3 CO2
2 CH3CH2OH 3 CH4 + CO2
4. FACTORS AFFECTING PERFORMANCE OF ANAEROBIC DIGESTION
Anaerobic digestion involves a multitude of factors that are critical to the
operation or are significant at a specific stage in the process. Some of them are useful in
determining the output of the process and the corresponding efficiency, whereas the
others help in understanding both positive and negative effects on the whole process. A
careful analysis and monitoring of these parameters is essential for developing the
process.
The various factors are listed in the following table, along with their definitions
and commonly used units. Each of the factors is subsequently described along with effects on the process due to their variation.
17 Table 1: List of factors affecting anaerobic digestion performance
Factor Units Definition Chemical Oxygen Amount of organic compounds that mg/L or g/L Demand can be oxidized in the feed Total elemental carbon to total Carbon-to-Nitrogen N/A elemental nitrogen present in the Ratio feed Measure of acidity or alkalinity of pH and alkalinity pH the reaction Amount of organic compounds lost Volatile solids % TSS on being heated in a furnace at 550 °C Amount of solids present in the Total suspended solids kg/day or tons/day feed insoluble in water Fats, Oils and Grease mg/L or g/L Amount of lipids present in the feed Time duration for which the solids Solids Residence Time days or hours are present in the reactor Hydraulic Retention Time duration for which soluble days or hours Time compounds stay in the reactor Agitation rpm Measure of mixing in the reactor Operating temperature in the Temperature °C or °F reactor COD removal Percentage of COD converted to % efficiency biogas Inoculum growth rate Bacterial growth rate in the reactor L per kg COD destroyed or scf Amount of gaseous mixture coming Biogas output per lb COD out of the reactor destroyed Amount of slurry coming out of the Sludge output kg/day or tons/day reactor
4.1 Chemical Oxygen Demand
Chemical Oxygen Demand (COD) represents the total amount of organics being
loaded in the anaerobic digester. In other words, it is a measure of the amount of
oxygen needed to oxidize the substances in a sample. The COD is measured by
exposing the samples to strong oxidizing agents like Potassium permanganate
18 (KMnO4), Potassium dichromate (K2Cr2O7) etc. and then, once the oxidation has
taken place, the oxidizing agent is titrated to find out the remaining amount and
thereby, oxygen consumed. The units for COD are the usual concentration units like
grams per liter or milligrams per liter.
The COD measurement is carried out prior to feeding the waste water stream to the
digester and depending on the COD value, the process options are explored. For
instance, a feed with a high COD value might require a digester like UASB that can
handle high COD input and that would incur larger costs. On the other hand, a feed
with a low COD value can be digested by simpler systems like lagoons or bulk
volume fermenters with much lower costs.
A digester feed with a higher COD generally can be translated into higher biogas
production. This is due to the fact that the feed contains high amount of oxidizable
compounds and therefore, they can be converted into biogas more than the feed where
these oxidizable compounds are in low quantity. Again, the choice of the process to
be used depends on the COD value and the desired biogas output from the feed.
4.2 C/N Ratio
This is the ratio of the total elemental carbon to the total elemental nitrogen present in
the waste water or the digester feed sample. The nitrogen part of the input material is
used by the bacteria for their growth and metabolism. The carbon portion gets
converted to Methane and Carbon Dioxide. The nitrogen values can be measured by
19 the Kjeldahl nitrogen test. A higher ratio is said to result in a higher biogas
production, though the ratio should not go beyond certain value. Moreover, it should
not be lower than a certain value such that the nitrogen does not reach a concentration
level toxic to the microbes, mainly in the form of ammonia.
4.3 pH and Alkalinity
pH is one of the important factors in the operation of the digester and the microbial
activity is highest in the pH range of 6.8 – 8.5 (Burke, 2001). Moreover, the alkalinity
of the digester should be maintained as close to the operating range as possible and
there is a need for addition of alkaline solutions in order to counter the lowering of
pH during the acetogenesis phase.
The pH fluctuations can affect the bacterial growth and their activity on the
degradation of organics in the digester.
4.4 Volatile Solids
These represent the total amount of solids that are lost on heating in a furnace at 550
°C. The biogas production takes place from the oxidation of these solids. These are
generally expressed as a percentage of the total solids being fed to the digester.
4.5 Total Suspended Solids
The total suspended solids (TSS) are the solids that are not soluble in water but
remain suspended in the feed stream. The amount of TSS in the feed stream affects
20 the systems wherein the feed is passed either through a membrane, a filter or even in
a UASB where the granular sludge would have limitations to the amount of solids
that can be handled. The units being used for expressing TSS levels is in kg per day
or tons per day.
4.6 Fats, Oils and Grease
Fats, oils and grease (FOG) are sometimes a part of the wastewater stream that is fed
to the digester. Their presence in small quantities has no effect on the process but a
high level of these compounds in the feed can reduce the efficiency. The potential
risks associated with the introduction of high FOG into the digester include fouling of
membrane and adverse effects on the growth of microbes due to reduced material
transfer between the media and the cells.
The inhibitory effects if fats oils and grease are discussed in the following section.
4.7 Solids Residence time
This provides a measure of the duration of time between which the solids are loaded
onto the digester and the solids are removed from the reactor. A typical digester has a
solid residence time of about 30 days. A higher residence time is deemed to be better
for the process resulting in a higher biogas output.
21 4.8 Hydraulic Retention time
The hydraulic retention time is the time for which a soluble compound remains in the
digester. In other words, it would be the time between which there is removal and
addition of water to the system. A higher hydraulic retention time would mean that
the process takes longer to degrade a given amount of feed. The high rate systems aim
for a low hydraulic retention time in order to make the process as continuous as
possible.
4.9 Agitation
The presence of an agitation system can increase the mass transfer and exposure of
the inoculum to the substrate. However, conventional systems like lagoons did not
have an agitator. Since almost all the systems employ agitation these days, this factor
is less likely to be a critical one for the operation.
4.10 Temperature
The anaerobic digestion process is generally operated under two ranges of
temperatures. When the digester is operated in the temperature range of 35–37 °C, the
operation is termed as mesophilic. The thermophilic operation involves maintenance
of the temperature in the digester at about 55–60 °C.
The thermophilic operation has been shown to produce more biogas than the
mesophilic operation. The difference in the biogas varies depending on the type of
feed and addition of some chemicals (J. Mata-Alvarez, 2000).
22 4.11 COD removal efficiency
This is the percentage efficiency achieved by a particular technology in degrading the
COD in the feed stream. Most of the technologies can operate at efficiencies greater
than 60%. The systems like an anaerobic lagoon and CSTR can attain a COD removal
efficiency of close to 65-75% (Dr. Mark Tenney, Personal communication,
November 2009). The next higher efficiency systems include the bulk volume
fermenter and anaerobic sequence batch reactor that are able to remove almost 75-
85% of the feed COD to biogas. The highest efficiency ones include the anaerobic
contact, anaerobic membrane, UASB and the EGSB processes.
The higher the removal efficiency, the higher the biogas production and lower the
residual unreacted organics. The removal efficiency can be lowered by other factors
that might affect the overall process. This would, therefore, reduce the total biogas
output due to increased toxicity to the bacteria and their inability to optimally
carrying out the digestion.
4.12 Inoculum growth rate
The value for bacterial growth rate is dependent on the feed composition, the
concentration of inoculum and the type of bacteria used in the process. The feed
composition can be related to the C/N ratio and that decides the amount of nitrogen
available to the bacteria for the growth. The bacterial biomass concentration should
be in a range where it would be more than the required concentration to degrade the
COD. In this manner, it would be easy to drive the reactor towards higher COD
destruction.
23 4.13 Biogas output
Biogas is the gaseous mixture that is released from the anaerobic fermentation
process. The primary composition of biogas is methane and carbon dioxide and other
gases in negligible concentrations such as nitrogen, hydrogen, hydrogen sulphide and
oxygen if the microbes are active in small concentrations of oxygen in the reactor.
The amount of biogas produced during the anaerobic digestion is dependent on the
feed COD, the total volatile solids and also the C/N ratio. The higher values of each
of these result in a higher biogas output. The most prevalent composition of the
biogas has about 60% Methane and close to 35% carbon dioxide with the other gases
making up the rest.
4.14 Sludge output
Sludge represents the solid product of the anaerobic digestion. The sludge mainly
consists of the bacterial inoculum, the inert solids that could not be degraded and the
portion of volatile solids that were not converted to biogas. Since the bacteria use
nitrogen in the feed for growth and metabolism, the sludge is assumed to be of high
nitrogen content.
The solids disposed on land are classified by the EPA as Class A and Class B
depending on the pathogenic requirements. The pathogenicity is recorded in terms of
fecal coliform density.
24 Class A biosolids
Class A standards for pathogen reduction require a fecal coliform density of less than
1,000 MPN (most probable number) per gram dry weight, or a Salmonella density of less than 3 MPN per 4 grams dry weight. These standards may be met by a specific time-temperature combination treatment (55° C / 131° F for 20 days). The products of the thermophilic processes like anaerobic digestion operating at thermophilic conditions, might qualify for application as Class A solids. (Erickson, Fayet,
Kakumanu, & Davis, 2004)
These biosolids can be disposed to golf courses, agricultural fields, citrus growers, landscapers, fertilizers blenders, nurseries and parks. (Foess & Fredericks, 1995)
Class B biosolids
Class B standards require the density of fecal coliforms to be less than two million per gram dry weight. Class B standards are met by mesophilic (38° C / 100.4°F) processing of the biosolids.
These biosolids can only be used in agricultural fields and landfills.
25 5. INHIBITION OF ANAEROBIC DIGESTION PROCESS
The acidophiles and the methanogens operating in the anaerobic digestion process are
different in terms of physiology, substrate consumption, growth kinetics and sensitivity to
surrounding environment and related factors (Ye Chen, 2008). The wastewater feed can
vary in its composition and there are a few substances often found in the reactor that
interfere with the optimal growth of the inoculum and thereby, result in failure of the
process or instability or reduced output. The criteria for tagging a particular substance as
inhibitory include negative effects on the inoculum growth or complete inhibition of the
inoculum growth.
Factors inhibiting the anaerobic digestion process
5.1 Ammonia
Ammonia is produced on the degradation of protein and other nitrogenous matter like
+ urea present in the feed or in the reactor. Both free ammonia and ammonium ion (NH4 ) are responsible for negative effects on the inoculum. Free ammonia can pass through the permeable membrane and transfer of ammonium ions to the intracellular region can imbalance the ionic state, thereby affecting the potassium and sodium ionic balance.
The ammonia concentration of less than 200 mg/L is said to be beneficial to the digestion process. A reduction of up to 50% in biogas output has been mentioned in literature due to the ammonia concentration in the range of 1.7-14 g/L. This wide range of inhibiting concentration can be ascribed to variations in substrates, inoculums, pH, and temperature along with other factors.
26 In order to reduce or prevent ammonia inhibition, a few measures have been taken
and they have shown results. For instance, addition of zeolites as ionic exchangers for
ammonium ion can be used to reduce ammonia concentration by favoring formation of
ammonium ions and following adsorption by the zeolite. The stability of the process can
be increased by adding inert material like clay or activated carbon. The activated carbon
does not adsorb ammonia but gets rid of sulphide that would otherwise act in synergy
with ammonia to inhibit the process.
5.2 Sulphide
Sulfate present in the feed can disturb the reactor and result in reduced methane
production. This occurs due to the competition for organic substrates by the sulfate- reducing bacteria and the toxicity of sulphide (result of sulfate reduction) to the anaerobes. The sulfate reducing bacteria are able to degrade long chain fatty acids, alcohols, aromatics, and organic acids and thereby, compete with the methanogens, acetophiles, or other anaerobes present in the reactor.
Sulphide toxicity can be reduced by diluting the feed stream, however, that might be dependent on the handling capacity of the reactor. The other method would be adding a sulphide removal process, physical-chemical or biological, as a pretreatment step.
5.3 Light metals (Na, Al, K, Ca, Mg)
Salts of the light metals like Sodium, Potassium, Aluminum, Calcium and
Magnesium can be found in the feed stream as the degradation products of some organics or additions as nutrients for optimal growth of microbes. The growth of the microbes is
27 stimulated by the presence of these cations as some of them act as cofactors for metabolic
enzymes. However, excess concentrations can be deleterious to the microbes.
Aluminum competes with Iron and Manganese, and its adherence to the cell
membrane has been shown to affect microbial growth. Calcium accumulation can result
in formation of carbonates and bicarbonates, which can lead to scaling in the piping and
reactor surface, reduced activity of methanogens and loss of buffering capacity for the
digestion process. Magnesium has been shown to be non-toxic at a concentration below
400 mg/L and excess magnesium could lead to single-cell production. Single-cells are
undesirable in the acetogenesis due to the high chances of the cells getting lysed. Excess
of potassium ions in the extracellular space has been shown to increase the passive
movement of potassium across ion-channels and therefore, disturbs the membrane
potential of the cell. Moreover, addition of potassium has been linked to the loss of
essential micronutrients like Cu2+, Zn2+, Ni2+, Mo2+ and Co2+ and thus, reducing the methanogenic activity of the cells. Sodium is required for the formation of ATP in methanogens at low concentrations of about 100–200 mg/L, but can be inhibitory at concentrations more than 4000 mg/L.
5.4 Heavy metals
The anaerobic reactor can be disturbed by high concentrations of heavy metals present in the wastewater feed stream such as Chromium, Iron, Cobalt, Copper, Zinc,
Cadmium, and Nickel. These metals can build up in the reactor as they are not utilized or degraded by the microbes. Therefore, a negligible concentration in the feed stream can
28 result in an inhibitory concentration over a period of time. The mechanism of toxicity to the microbes is due to the binding of these heavy metals to enzymes or replacement of other metals at the metal-binding sites, thereby preventing the enzyme activity. The behavior of the heavy metals as a stimulating or inhibiting agent is dependent on total metal concentration in the reactor, chemical state of the metal, and other factors such as pH and redox potential prevalent in the process.
The methods employed to detoxify anaerobic reactors from heavy metals are precipitation, sorption and chelation using both organic and inorganic compounds. One of them is Ferrous Sulphide. The heavy metals displace ferrous ion and precipitate as sulphides, and ferrous ion does not affect the reactor in concentrations of a few hundred mg/L. Addition of chelating agents like EDTA (Ethylene Diammine Tetracetic Acid), aspartate and citrate have been shown to reduce the toxicity of the heavy metals.
5.5 Organic compounds
The anaerobic digestion can be inhibited by a variety of organic compounds. Some of these include those insoluble in water and bind to solids in the sludge, which might build up to toxic concentration levels. Some non-polar compounds can disrupt the ion channels in the bacterial membrane and result in cell lysis. Most of the toxic organic agents are aromatics including benzene and several of its derivatives. Chlorophenols have been studied as a class of organics that affect the digestion process by interfering with the membrane proton gradient and energy transduction of the bacterial cells. Halogenated hydrocarbons are another class that can be toxic to the bacterial cells. These include
29 chloroform (CHCl3), carbon tetrachloride (CCl4), Trichloroethane and other chlorinated aliphatics. Chloroform has been found to be toxic in concentrations of even 0.15 mg/L and the toxicity is attributed to the intermediates that are formed during its digestion.
Alcohols and other organic solvents are found to be toxic to the microbial population in the anaerobic digester, though toxicity levels could not be found out.
Sometimes, the anaerobic processes have restrictions to the amount of FOG levels
(greater than 800 mg/L) that they can tolerate and this is due to the long-chain fatty acids.
The toxicity concentration of long-chain fatty acids is different for Gram positive and
Gram negative strains, with the latter not inhibited at low concentrations. The fatty acids affect the cells by adsorbing to the cell membrane, and disrupting the transport and protective functions. Oleic acid and Lauric acid have been reported to be toxic at a concentration of around 1,215 mg/L and 861 mg/L, respectively. Moreover, the thermophiles are said to show a higher sensitivity to the fatty acids than the mesophiles, and this can possibly be attributed to the cell membrane composition.
In order to use anaerobic digestion process with high lipid-containing waste stream, it is suggested to acclimatize the microbes first and then the microbes might be able to degrade these as a carbon source into methane and carbon dioxide.
30 6. ALGAL SPECIES AND ANAEROBIC DIGESTION
The use of algae as a feedstock for anaerobic digestion has been studied at a laboratory scale by several researchers. One of the first ones was Golueke et al. (1957), who studied the use of algae as the source of carbon for conversion into methane gas. They concluded that the digestion of algae was slower than sewage sludge and the COD destruction was only around 60-70% of the COD destruction of the sewage sludge. As a result, the overall biogas yield was lower in the case of algae on comparison with sewage sludge. Moreover, the degradation of algae was more rapid at thermophilic temperatures than mesophilic, and they suggested using a highly dilute algal feed with short residence times of at least
11 days. They noticed a consistent biogas composition of about 60% methane and about
30% carbon dioxide.
Samson et al. studied the anaerobic digestion of the algal species Spirulina maxima and obtained about 350 L of methane per kg of COD destroyed (R. Samson, 1983). They reported a C/N ratio of about 4 – 5 for pure algal biomass as algae contained huge quantities of proteins, thereby increasing the total elemental nitrogen concentration of the algal biomass. Some of the other results from the literature have been organized for comparison against different algal species in the following table.
In 2006, Yen and Brune studied co-digestion of algae and paper as a combined carbon source for assessing the possibility of higher biogas output due to higher C/N ratio of the digester feed. They showed that algal sludge had a C/N ratio close to 5 while paper had
31 C/N ratio of almost 2000. At a volatile solids loading concentration of about 4 g/L/day, algal sludge digestion (C/N ratio 6.7) resulted in about methane output of 573 mL/L/day while the combination of algal sludge and the combination of half algal sludge and half paper (C/N ratio 18) resulted in methane output of 1170 mL/L/day, which is more than double than pure algal sludge. Therefore, in order to obtain increased biogas output, the combination of high C/N ratio feed along with low C/N ratio algal biomass must be used in the digester, while there is a need to optimize the ratio to get highest possible biogas output.
Table 2: Estimated methane outputs of microalgal species (Feinberg, 1984) Methane Methane Methane Methane Algal species (m3/kg COD (MJ/kg) (Btu/lb) (scf/lb) destroyed)
Botryococcus braunii 14 6020 6.02 0.373
Ankistrodesmis falcatus 11 4730 4.73 0.293
Isochrysis sp. 9.5 4085 4.085 0.253
Dunaliella salina 7.5 3225 3.225 0.200
Chlamydomonas sp. 10.5 4515 4.515 0.280
Cyclotella Cryptica 9.5 4085 4.085 0.253
Chlorella sp. 11 4730 4.73 0.293
Nannochloropsis salina 14 6020 6.02 0.373
32 Table 3: Anaerobic digestion of algal species at laboratory scale
Biogas Methane values Methane generated (m3 / kg COD / day) Algal species value (m3 (scf/lb COD) Reference Assuming 1.13 g per kg VS) Assuming 60% COD / g VS Methane Gracilaria - 0.21 5.62 (Gunaseelan Macrocytis - 0.31 8.29 1997) (Samson and Spirulina - 0.35 9.36 Leduy 1983) (Chynoweth, Fannin and Laminaria - 0.52 13.91 Srivastava 1987) Palmaria 0.453 0.401 10.72 Porphyra 0.442 0.391 10.46 Fucus 0.442 0.391 10.46 Laminaria 0.442 0.391 10.46 (Briand and digitata Morand Laminaria 0.41 0.363 9.70 1997) saccharina Ulva 0.4 0.354 9.47 Enteromorpha 0.389 0.344 9.21 Himanthalia 0.379 0.335 8.97
Assumptions 1. Conversion factor = 1.13 g COD per g VS 2. Biogas = 60% Methane, 40% Carbon Dioxide
33 7. ANAEROBIC DIGESTION TECHNOLOGIES
Some of the technologies being evaluated for the digestion process are listed in the following table with each of them being evaluated based on their advantages and disadvantages (Totzke, 2009):
Table 4: List of anaerobic digestion technologies with advantages and disadvantages
Technology Advantages Disadvantages Anaerobic Lagoon • Simple, Robust system • Low biogas output • High biogas output • Low hydraulic retention Anaerobic Contact • time • High solids residence time Anaerobic Sequence • Sequential removal • Low biogas output Batch Reactor
Upflow Anaerobic • High COD loading • Total suspended solids Sludge Blanket acceptance limitations Reactor • High biogas output • Granule stability dependent
Complete Mix/CSTR • Efficient mass transfer • Low biogas output Bulk Volume • Simple, robust system • Low biogas output Fermenter
• High COD loading Membrane Separation acceptance • Membrane fouling Anaerobic Treatment • High biogas output
• High COD loading • Reactor operation; Plug Flow reactor acceptance conditions maintenance • High TSS levels acceptance difficult
Upflow Anaerobic • Wide range of COD loading • Biofilm fouling by high Biofilm Reactor acceptance TSS levels
34 7.1 Anaerobic Lagoon
The lagoon system is the simplest and easiest to construct of all the digestion
technologies. This comprises a large pit dug into the ground (liner can be added to
prevent any kind of seepage into the surrounding land) and the digestion input material is
dumped into it. Water is added to the lagoon and the waste undergoes settling. The
digestion process continues for around 50 – 100 days. The biogas is collected from the
bubbles rising through the liquid at the top of the lagoon. The lagoon area is in the range
of about 2 – 3 acres and can go up to a depth of about 20 feet. A lagoon is generally suitable for a COD loading of 0.5 – 2 kg per m3 per day and can achieve about 65 – 75 %
COD removal efficiency. The efficiency of the process can be increased by adding a sludge recycle stream, agitator, heater and a cover to capture the maximum possible biogas.
Figure 1: Anaerobic Lagoon (www.ces.purdue.edu/pork/images/lagoon2.gif)
One of the key issues that need to be considered when building and operating a lagoon-
based digestion system is the permeability to nearby soil and water. In order to overcome
35 this, the latest lagoons have a wall around and this helps in keeping the process fully
contained.
7.2 Anaerobic contact process
This system consists of an agitated reactor and a solids settling tank for recycling into the
reactor. The system has a better contact between the biomass and the substrate though it
has a higher residence time compared to the lagoon (50 – 200 days). The COD loading
rate that the system can handle is in the range of 1 – 8 kg/m3/day and can obtain COD
removal efficiency of around 85 – 95 %. There is no buildup of solids and the reactor can operate with high levels of Total Suspended Solids and Lipid levels. The operation can be either mesophilic or thermophilic. Dissolved Air/Nitrogen Flotation can be used for
concentrating the solids in the settling tank for recycling into the reactor. This process is
considered as one of the best for digestion materials with high suspended solids.
Figure 2: Anaerobic contact process
36 7.3 Upflow Anaerobic Sludge Blanket Reactor
The UASB reactor involves the use of a bed of activated granules containing the bacterial inoculums and a blanket of flocculant. The wastewater runs in the upward direction and the organic materials are decomposed by the bacteria as they move. The biomass to substrate contact is carried out by the biogas movement through the bed. This process is a high-rate system and can handle high COD loadings in the range of 15 – 25 kg/m3/day.
The COD removal efficiency rates are around 85 – 95 %.
The process is not compatible with waste that has high Total Suspended Solids and the
Lipids levels due to the clogging issues. This is generally sold as a package containing the influent distribution system and the activated granules. This system can be difficult to operate if there are high total suspended solids in the feed stream.
Figure 3: Upflow Anaerobic Sludge Blanket Reactor (http://www.epa.gov/nrmrl/pubs/625r00008/html/html/tfs5fig4.gif)
37 7.4 Upflow/Downflow Anaerobic Filtration (Biofilm) process
This process comprises a bed of inert media, primarily polystyrene beads and this acts as the attachment on which the bacteria can grow. The wastewater flows upwards or downwards and the solubles in the wastewater stream are trapped in the bed. The clean water flows out of the reactor and the solids are degraded in the bed.
Figure 4: Upflow Anaerobic Filtration process (http://www.epa.gov/nrmrl/pubs/625r00008/html/html/tfs5fig3.gif)
This system can digest a COD loading rate in the range of 5 – 20 kg/m3/day and has operating issues with high Total Suspended Solids and Lipids levels. The efficiency of the process is dependent on the input flow rate and the type of media used. This system, too, cannot handle high amount of Total Suspended Solids.
38 7.5 Anaerobic Expanded / Fluidized Bed Reactor
This process consists of a fluidized bed of the inert media and the biomass grows on this bed. The key factor to be considered here is the fluidization from distribution of the influent as improper fluidization would result in a loss in the efficiency of the system.
This system is very similar to the filtration system and experiences similar problems.
These include the solids handling and high lipid levels limitation. This is also known as
Expanded Granular Sludge Blanket Reactor (EGSB).
This process is also a high throughput system and can handle a COD loading rate between 20 – 40 kg/m3/day. This system can be coupled with a UASB reactor, resulting in a hybrid system that can handle higher COD loadings and accordingly, produce higher biogas per unit mass of COD destroyed.
Figure 5: Anaerobic Expanded/Fluidized Bed Reactor (http://www.epa.gov/nrmrl/pubs/625r00008/html/html/tfs5fig5.gif)
39 7.6 Anaerobic Membrane Reactor
This process involves a complete mix reactor where the biomass and the substrate waste water are added together. This mix is then routed to a downstream reactor containing membranes where the solids are retained for digestion and the treated water goes out of the system. The biogas is collected from this reactor and the solids are recycled to the mixing reactor for the next pass. The average COD loading rate for the system is in the range of about 2 – 22 kg/m3/day with the COD removal efficiency of 85 – 95 %.
Figure 6: Anaerobic Membrane Reactor
The membrane cleaning is the most important factor here and it requires utmost care in preventing any kind of clogging and fouling of the membrane. Therefore, it cannot handle high solids or lipids levels as they might result in fouling and clogging. The membrane is a highly expensive component and, therefore, its use in treating algae is likely to meet with problems due to the total suspended solids limitation and high costs of maintenance for the membrane. This reactor might require nutrient recycling.
40 7.7 Bulk Volume Fermenter
The bulk volume fermenter is similar to the lagoon described previously, though the size of this fermenter is smaller compared to the lagoon. This operates on the same principle of dumping the waste inside the reactor and then running it for about 30-100 days. The reactor can be agitated and heated, based on whether the reactor is operating in the range of temperatures for a mesophilic or thermophilic process. The fermenter can handle a
COD loading rate of about 1 – 2 kg/m3/day with COD removal efficiency between 75 –
85 %.
This is a simple and robust system, and is proven on a commercial scale. The advantage with this system is that it can be adjusted for different flow rates of the input stream and the maintenance in terms of cleaning the reactor is low.
7.8 Plug Flow Reactor
This system comprises a long and narrow tube through which the digestion feed flows and the movement of the feed is carried out by pushing it with additional feed at regular intervals. This system is generally used with waste containing about 10 – 15 % solids.
There is no mixing inside the reactor longitudinally. The biogas collected can be used to heat the system for a thermophilic operation. Most of these systems are horizontal and the prominent ones are the DRANCO process.
41 7.9 Continuous Stirred Tank Reactor (Complete Mix reactor)
This consists of an agitated reactor where the feed water containing the substrate for the biomass is continuously mixed. This mixing results in a higher contact between the biomass and the substrate. However, the disadvantage of using only a complete mix reactor is that there is no solids recycle thereby, reducing the process efficiency. In other words, the effluent from the system might contain solids that have never been digested or have stayed in the vessel for a considerably longer duration than the average residence time. This system can handle a COD loading rate of 1 – 5 kg/m3/day and obtain COD removal efficiency of 65 – 80 %.
This system has been made more efficient by the anaerobic contact process that adds the solids recycle part to the process.
7.10 Anaerobic Sequence Batch Reactor
This process involves a sequence of batch reactors and the effluent from one reactor is the influent for the next in the row. This system can handle COD loading rate of 1.2 – 2.4 kg/m3/day with a COD removal efficiency of 75 – 85 %. This process would involve large operating expenses in running and maintaining one or more reactors used for the batch operation.
42 8. ANAEROBIC DIGESTION TECHNOLOGY MATRIX
The presently available anaerobic digestion technologies are summarized in a matrix,
where they are compared against each other on the basis of the differences in technical
parameters. The comparison and analysis yielded the following results.
8.1 COD loading rate
As evident from the matrix, the range of COD loading rate among all the technologies is
between 0.5 – 25 kg/m3/day. The technologies characterized by values more than 10-15
are considered to be the high rate systems. These include the UASB, EGSB, Anaerobic
Membrane and Anaerobic Filtration. Others like Bulk Volume Fermenter, Lagoon,
Anaerobic Contact, Anaerobic CSTR and Plug-Flow reactor are considered to be low-rate systems as they can handle lower amounts of COD loading.
The selection of systems for anaerobic digestion requires evaluation of the feed for the
COD value. Once the COD value of the digester input is known, the appropriate systems can be evaluated further for performance and efficiency.
8.2 COD removal efficiency
The difference among various technologies in terms of the efficiency of COD removal achieved is found out to be in the range of 65-95%. The high rate and newer systems are apparently more efficient at degrading the COD than the traditional systems like covered lagoon and anaerobic CSTR. However, the traditional systems have been well studied and are supposedly more robust and are easier to handle for continually varying feeds.
43 8.3 Influent solids concentration
Most of the systems are not build to handle total solids concentration of more than 10% due to the fact that the high solid concentration can result in plugging of membranes, filters and reactors. There are some systems where researchers have used solid-state anaerobic digestion for biogas production; however, they are fewer in number compared to the liquid-state digesters. The systems that were not considered for this analysis due to the influent solids limitations included the UASB, Anaerobic Filter and Anaerobic
Membrane reactor.
8.4 Theoretical biogas output
The theoretical biogas output for any anaerobic digestion process is 0.39 m3/kg COD
destroyed (Personal communication, Martin Johnson, November 2009). This value has
been used in order to arrive at the theoretical biogas outputs for each of the processes.
Therefore, they follow the similar trend as the COD values. The next filtering of
technologies was based on biogas output and COD removal efficiency, where CSTR,
Lagoon and Anaerobic sequence batch reactor.
Some of the technologies were not considered for application to algae due to TSS and
FOG limitations. The technology chosen for commercial feasibility analysis was
Anaerobic Contact. However, it was compared on a technical and cost basis with Bulk
Volume Fermenter, Plug-Flow reactor and Anaerobic Biofilm reactors and the total
information has been summarized in the technology matrix.
44 Volumetric Biogas output Hydraulic Solids Percent Removal Organic Loading Detention Retention COD Technologies Rate Time (HDT) Time (SRT) (scf/lb COD loaded)
(kg COD/m3/day) (%) Min Max (days) (days)
Complete Mix/CSTR 1.0 – 5.0 65 – 80 8.57 10.55 15 – 30 15 – 30
Anaerobic Contact 1.0 – 8.0 85 – 95 11.21 12.52 0.5 – 5 50 – 200
Anaerobic SBR 1.2 – 2.4 75 – 85 9.89 11.21 0.25 – 0.50 50 – 200
UASB 15 – 24 85 – 95 11.21 12.52 0.25 – 0.50 40 – 100
Anaerobic Lagoon <0.5 – 2.0 65 – 75 8.57 9.89 30 – 50 50 – 100
Bulk Volume Fermentor 1.0 – 2.0 75 – 85 9.89 11.21 0.5 – 10 30 – 100
Anaerobic Membrane 2.0 – 22.0 85 – 95 11.21 12.52 0.5 – 15 30 – 160
Plug flow reactor 2.0 – 12.0 75 – 85 9.89 11.21 n/a 30
Anaerobic Filtration 0.3 – 20 60 – 90 7.91 11.87 0.5 – 4 14 – 30
Table 5: Anaerobic Digestion Technology matrix (Personal Communication, Dr. Mark Tenney, November 2009)
45 8.5 Comparison of Anaerobic Contact Process with Other Processes
The anaerobic contact process can be considered the best among all the available technologies, though there is a need to compare it to the other technologies like bulk volume fermenter, plug-flow reactor and anaerobic biofilm reactor. The comparison is shown based on significant technical parameters.
8.5.1 COD loading rate acceptance
Here, the bulk volume fermenter has the lowest COD range that it can accept,
followed by anaerobic contact process. The higher ones include plug-flow and the
anaerobic biofilm reactors. If COD loading rate is the only metric, anaerobic biofilm
reactor would be the preferred system.
8.5.2 COD removal efficiency
Here, the anaerobic contact is the clear winner as it can achieve efficiency of up to
95%, 5% more than the anaerobic biofilm reactor. The bulk volume fermenter and the
plug-flow reactor have lesser removal efficiencies of 65-80% and 75-85%,
respectively.
8.5.3 Biogas output
Since anaerobic contact process can degrade COD with the highest efficiency, it
possesses the ability to produce the highest biogas output, among the other
technologies.
46 8.5.4 Hydraulic Retention Times
The anaerobic biofilm reactor has the shortest retention time among all the
technologies with a maximum of 3 days, within which the solubles are recycled,
though the anaerobic contact is almost comparable. The bulk volume fermenter takes
almost double the time to recycle the solubles.
8.5.5 Solids recycle
Among the four technologies, only the anaerobic contact process involves the
recycling of solids back into the reactor. This results in reduced downtime as the
recycled solids contain the inoculum and the time taken to grow the inoculum to a
considerable level is avoided. This is the reason for the higher efficiency of this
system.
8.5.6 Cost comparison
The technologies namely, anaerobic contact reactor, bulk volume fermenter, anaerobic
biofilm reactor and the plug-flow reactor were compared on a cost basis, taking into
consideration both the capital and operating expenditures. The analysis was carried out
with the help of Dr. Mark Tenney (Independent consultant with Phycal), Mr. Aaron
Brister (Research Scientist, Phycal) and Mr. Martin Johnson (Systems Integration
Engineer, Phycal), and the analysis was done for a pilot-scale anaerobic digester for digestion of algae.
47 Table 6: Cost comparison of Anaerobic Contact process, bulk volume fermenter, plug flow reactor, upflow anaerobic biofilm reactor
Pilot scale algae anaerobic digestion Technologies
CapEx OpEx per year
Anaerobic Contact $981,000 $336,500
Bulk Volume Fermentors (BVF) $899,500 $304,100
Plug flow reactor $2,750,000 $389,000
Upflow Anaerobic Biofilm Reactor $3,240,000 $300,000
48 9. COMMERCIAL FEASIBILITY OF ANAEROBIC CONTACT PROCESS
While evaluating the commercial feasibility of the anaerobic digestion technology that can best process the algal biomass as the input, the cost of the system in terms of the capital expenditure (CapEx) and operational expenditures (OpEx) need to be investigated. Each of the systems mentioned in the technology section have been scaled at the commercial level and optimized with different kinds of feed streams, predominantly industrial and sewage waste water. However, none of the technologies have been studied at pilot or commercial scale using algal biomass as the sole input to the digester. It is assumed that the algal biomass would behave in a similar manner as previously used feed streams and produce comparable biogas outputs. The economics of the process essentially boils down to the quantity of biogas produced and its use in generating power that can be sold or utilized in other processes.
An ideal scenario for the commercialization of anaerobic digestion using algae as the feed stream would be as a part of an integrated bio-refinery for obtaining energy from algae. Several companies are on their way of commercially producing algal oil to feed the ever-rising oil demand for the world. The utilization the algal biomass after oil extraction in a profitable way still remains a question for these algal oil companies. Anaerobic digestion is a proven process for generating energy from waste and deserves to be explored for this application also. The products, biogas and sludge, can be a source of revenue. The companies can decrease production cost of algal oil by recycling energy from biogas within the production process and selling sludge as an agricultural fertilizer.
49 Alternatively, the biogas can be stripped to provide CO2 to grow algae and methane in
power generators or steam boilers.
9.1 Assumptions
The present analysis has been conducted assuming that the biogas can be sold
directly to external customers and the customers would strip CO2 to use methane in the power generator. An alternate scenario would be using methane for powering internal systems in the bio-refinery and stripping carbon dioxide to be recycled for growing algae, though that has not been analyzed here.
The following assumptions relating to the process, flows, capital and operating costs were made for the analysis:
1. The solid algal biomass feed to the reactor is about 500 metric tonnes per day.
2. The total solids concentration is 4% (w/w feed stream).
3. The percentage of volatile solids in the feed stream is 90% (w/w solids concentration)
4. The reactor is agitated.
5. The addition of water is calculated to be 96%*500/4% = 12,000 metric tonnes per
day. This translates to 3.16 million gallons per day.
6. The digester is an anaerobic contact reactor coupled with a solids separator using
gravity settling.
7. The solids residence time (SRT) has been assumed to be 30 days, after which fresh
biomass would be added to the reactor.
50 8. The hydraulic retention time (HRT) has been assumed to follow an SRT/HRT ratio of
3, which is considered optimum for digestion process, and is 10 days. Therefore, the
water in the reactor is recycled every 10 days.
9. The volume of reactor needed to process daily algae load is about 15,000 m3. This is
assuming the reactor has an operating volume (volume of water in the reactor) of
close to 80%. 20% is head space. Since the water in the reactor is recycled every 10
days (HRT), total volume needed for 10 days worth of algae load is 150,000 m3. This
volume is divided into 60 units of 2,500 m3 each running in parallel.
10. COD value has been assumed to be 800 mg/L for pure algae.
11. Theoretical methane output is calculated using the stoichiometric relation, where 390
L of biogas is released per kg of COD destroyed.
12. Aspect ratio for the digester has been assumed to be 2.5:1, i.e. Height of reactor is 2.5
times the diameter; Height of reactor = 21.5 m; Diameter of reactor = 8.6 m.
13. COD removal efficiency for anaerobic contact reactor is assumed to be 90% as it is a
high rate system.
14. The composition of biogas is assumed to be 60% Methane (CH4) and 40% Carbon
Dioxide (CO2).
15. The heating capacity for methane is 1000 Btus per scf and 12.4 million Btus
(MMBtus) per kg COD destroyed.
16. The biogas is assumed to be directly sold to a customer by a pipeline with no
processing downstream of the reactor.
17. There is a sludge dewatering unit downstream of the reactor. The cost is assumed to
be $ 60,000 per unit.
51 18. The CapEx and OpEx cost numbers have been obtained from the data provided by
Dr. Mark Tenney, on consultation for anaerobic digestion with Phycal, LLC.
19. The scaling factor in the calculation is arrived at by assuming a discount of 20% on
the total capital expenditure for 60 units.
20. The cost of methane is assumed to be $ 7 per 1000 cubic feet. The cost of biogas is
assumed to be $ 3 per 1,000 cubic feet due to the fact that the customer would need to
invest capital in stripping CO2 and then sell methane at $ 7 per 1000 cubic feet.
21. The cost of selling sludge to agricultural farms or golf courses as Class A biosolids
has been assumed to be $ 10 per metric tonne.
The calculated capital expenditure for an anaerobic contact digester with a feed stream consisting of algal biomass at 500 MT per day is $ 217.3 million. The operating expenditure is found out to be $ 3.2 million per annum.
52 Table 7: CapEx and OpEx calculations for Anaerobic contact process
Pilot scale Production Equipment (Dr. Mark Tenney) scale Equalization Basin $30,000 $140,810 Influent Pumping $10,000 $46,937 Anaerobic Reactor $180,000 $844,859 Solids Removal - Clarifier using Dissolved Nitrogen $225,000 $1,056,074 Floatation Heating $80,000 $375,493 Chemical Addition $10,000 $46,937 Control Building $50,000 $234,683 Biogas Equipment (Gas $5,000 $23,468 meter)
Construction Subtotal $590,000 $2,769,261
Electrical $73,750 $346,158
Electronic Control Module $147,500 $692,315 Start Up $5,900 $27,693 Contingency $147,500 $692,315 Total $964,650 $4,527,741 Number of units 60 Scaling cost factor 0.8 Total CapEx $217,331,587
Pilot scale Production Item (Dr. Mark Tenney) scale Chemicals $37,500 $515,744 Maintenance $20,000 $275,063 Energy $88,000 $1,210,278 Personnel $90,000 $1,237,785 Total $235,500 $3,238,870
Total OpEx per year $3,238,870
53 9.2 General Design Information
The system would be designed for digesting 500 metric tons of dry algal mass (500,000 kg) with 4 % biomass concentration in the feed stream. This feed is equivalent to a daily
flow rate of 15,000 m3.
COD = 800 mg/L of algae
VS (Volatile Solids) = 90 %
IS (Inert Solids) = 10 %
Dry algal mass of 500,000 kg has a COD equivalent = 500,000 x 0.90
= 450,000 kg COD
9.3 Biogas output
The table containing the methane values obtained from anaerobic digestion of algae at the laboratory scale show that it is safe to assume the theoretical methane output of 390 L per
kg of COD destroyed and correspondingly 650 L of biogas produced per kg of COD
destroyed.
The COD destruction is assumed to be 90 % and 390 L methane per kg COD destroyed.
Total Methane production = 450,000*0.90*390 L
= 157,950 m3 = 5,577,951 ft³
Assuming that biogas is constituted by around 60% methane,
Total biogas yield = 5,577,951/0.6 = 9,296,585 ft3
Total sales = $ 3*9,296,585 /1000 = $ 27,890/day
Annual sales = $ 10.2 million
54 9.4 Residual Solids
The inert solids are the part of total solids that cannot be converted to biogas. The untouched solids are the part of the volatile solids that are not converted to biogas. The synthesis solids represent the biomass and it is assumed to be 10% of the mass converted to biogas. The masses have been calculated for the whole year assuming that the plant is operational 24 hours a day and 7 days a week. The loss in effluent is the percentage that is assumed to be lost in the effluent wastewater and the value is around 20% of the reactor volume.
Inert solids = 500,000 x 0.10 x 365 x 10-3 = 18,250 tonnes/yr
Untouched = 500,000 x 0.90 x 0.10 x 365 x 10-3 = 16,425 tonnes/yr
Synthesis = 450,000 x 0.90 x 0.10 x 365 x 10-3 = 14,783 tonnes/yr
Total (produced/collected) = 49,458 tonnes/yr
Subtract (loss in effluent) = 0.2 x 15,000 x 365 x 10-3 = 1,095 tonnes/yr
Solids Requiring Disposal (dry weight) = 48,363 tonnes/yr
Assuming a price of $ 10 per metric ton for Class A biosolids, this would fetch annual revenues of $ 483,630.
On the whole the digestion process of 500 metric tonnes per day of algae can provide annual revenues of $ 10.7 million.
55 10. CONCLUSION
The analysis shows that anaerobic contact process for anaerobic digestion of algal biomass can be a source of revenue to the order of almost $ 10.6 million every year. The production plant used for this purpose would consist of 60 units, each of 2,500 m3, running in parallel to accept feed into 6 reactors every day. The digestion would be run for 10 days before the water is recycled and this can be run in a continuous mode too, where in a portion of the culture is taken out and clarified and replaced by equal quantity of the feed stream. The capital cost of setting up such a production plant with 60 units is
$ 217 million with an operating cost of $ 3 million each year. The production plant must be set up only after exhaustive analysis of algal biomass anaerobic digestion since it has never been done on a commercial level. This can be done by carrying out pilot trials and then scale them up to the production level. The development of anaerobic contact process for the digestion of algae is recommended.
56 11. BIBLIOGRAPHY
1. Burke, D. A. (2001). Dairy Waste Anaerobic Digestion Handbook. Olympia:
Environmental Energy Company.
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