The Science of Global Change - American Chemical Society

Chapter 17

Global Significance of Biomethanogenesis

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D. P. Chynoweth Agricultural Engineering Department, University of Florida, Gainesville, FL 32611

Biological formation of methane is the process by which bacteria decompose organic matter using carbon dioxide as an electron acceptor in absence of dioxygen or other electron acceptors. This microbial activity is responsible for carbon recycling in anaerobic environments, including wetlands, rice paddies, intestines of animals, aquatic sediments, and manures. The mixed consortium of microorganisms involved includes a unique group of bacteria, the methanogens, which may be considered to be in a separate kingdom based on genetic and phylogenetic variance from all other life forms. Because methane is a greenhouse gas that is increasing in concentration, its fluxes from various sources are of concern. Biomethanogenesis may be harnessed for conversion of renewable resources to significant quantities of substitute natural gas and to reduce levels of atmospheric carbon dioxide.

In the United States, methane is a major energy source used in many homes for cooking and heating of water and indoor air and water. It is commonly known that some power plants and industries use natural gas as a source of energy for generation of electricity and process heat and that this methane is a fossil fuel obtained from gas wells and transmitted throughout the country by gas pipelines. Most people also know that methane bubbles up from polluted swamps where sedimented plant matter is undergoing decomposition. Because of odors from swamps, and the odor due to natural gas additives, methane is incorrectly considered malodorous. It is less commonly known that methane was one of the original atmospheric gases and is a normal product of the microbial decomposition of organic matter under anaerobic conditions. Bacteria involved in production of methane are unique in their metabolism and other properties. The balanced

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cycling of carbon is obligately dependent upon the activity of these organisms in the intestines of animals as well as in large anaerobic pools of organic matter, including sediments and flooded soils. Atmospheric methane is of major concern as a greenhouse gas that is increasing in concentration as a result of man's activities and is a potential cause of future global warming. Biomethanogenesis can be harnessed in the process known as anaerobic digestion to treat organic wastes in a manner that reduces their environmental impact and generates renewable energy. It can also be used to produce substitute natural gas from a variety of energy crops. This renewable methane would not only replace use of fossil fuels, but would also represent a balanced carbon cycle, and therefore not contribute to increases in atmospheric carbon dioxide. These lessknown aspects of methane are discussed in this paper. Methane Some properties of methane are outlined in Table I. Methane is a colorless and odorless hydrocarbon that is a gas at ambient temperatures and has a critical temperature of -82.2°C and a critical pressure of 45.8 atm. It is combustible when the oxygemmethane ratio exceeds 2.0 (ainmethane = 10.0) producing carbon dioxide and water; the heating value is 2350 J g" . Because of its abundance and high energy content, methane is widely used as a fuel, representing about 20% of the U.S. energy supply in 1988 (2,3). Currently, readily accessible deposits and reserves in the U.S. are estimated at 500-600 trillion cubic feet or a 35-40 year supply at current projected use rates. These estimates exclude a new technology increment estimated at 11 trillion cubic meters (3). Compared to oil and coal, methane is attractive as an energy source because it burns without production of oxides of sulfur and nitrogen and incomplete combustion products such as hydrocarbons. Methane combustion does however contribute to increased concentrations of the greenhouse gas carbon dioxide when derived from fossil resources. One limitation of methane is that it is difficult to liquify and thus can't be easily transported in large quantities as a fuel in vehicles unless maintained at high pressures and/or low temperatures. The concentration of atmospheric methane increased from 0.70 ppm to 1.68 ppm from 1787 to 1987 and is increasing by about 1.0% per year (4). This trend is based on direct measurements since 1978, prior infrared spectra, and recent analyses of air trapped in polar ice. This increase is of great concern because methane is considered a significant greenhouse gas. It not only absorbs infrared light directly, but also reacts photochemically to produce other greenhouse gases, including ozone, carbon dioxide and water vapor (5). Based on the current atmospheric concentration and rate of increase, the relative contribution of methane to the greenhouse effect (parts per billion volume basis) is estimated at 15% compared to 60% for carbon dioxide. Other gases 1

Dunnette and O'Brien; The Science of Global Change ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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THE SCIENCE OF GLOBAL CHANGE

Table I. Properties of Methane

molecular formula heating value

4

2350 J g

ratio of 0 : C H required for combustion 2

CH

1

4

2.0

boiling point

-162°C

critical temperature

-82.25°C

critical pressure

45.8 atm

solubility in water at 35 C

17 mg L

combustion products

C0 , H 0 2

1

2

SOURCE: Adapted from réf. 1.

with significant effects include nitrous oxide, tropospheric ozone, CFC-11, and CFC-12. The significance of methane is influenced by the facts that the direct effect of methane is 25-fold greater than that of carbon dioxide on a molar basis and the decay time for methane is 10 years compared to 120 years for carbon dioxide (5). When corrected for decay time and indirect effects, the relative contribution of methane compared to carbon dioxide is 15:1. Compared to other greenhouse gases, methane is an excellent candidate for control. Because of its short atmospheric lifetime, emissions must only be reduced by 10% to stop the yearly increase (6). In contrast, reductions of 50 to 100 percent would be required to prevent increases in other greenhouse gases. Furthermore, the value of reducing methane would be realized in the near term compared to centuries for other gases. For example, a 10% reduction in methane emissions is equivalent to a 10% reduction in carbon dioxide emissions, even though the concentration of carbon dioxide is vastly higher. An estimate of the annual methanefluxinto the atmosphere can be calculated by adding the sinks and the annual increase. These data (Table Π) indicate that a flux of 375-475 trillion tons(Tg) per year would be required to account for an annual increase of 50-60 trillion tons (7). Estimates of sources of atmospheric methane indicate that up to 83% is biogenic in origin (8). The other abiogenic


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Table II. Global Losses and Production of Methane

Methane Tg 1

Sinks tropospheric reactions stratospheric reactions uptake on aerobic soils

290-350 25-35 10-30

Annual Increase (about 1%)

50-60

Total Production to Account for Increase

375-475

SOURCE: Adapted from ref. 7.

sources include fossil natural gas leaks, coal mine leaks, and methanefromincomplete combustion during biomass burns. Biomethanogenesis Microbial methanogenesis is a natural process occurring in anaerobic environments, such as ocean and lake sediments and animal digestive tracts, where organic matter has accumulated. In these environments, electron acceptors such as dioxygen, nitrate, and sulfate are depleted and replaced by carbon dioxide resulting in formation of methane (9). Biomethanogenesis is a process occurring only under strict anaerobic conditions where several populations of bacteria react in concert to decompose organic matter to methane and carbon dioxide. A n overall scheme of biomethanogenesis, shown in Figure 1, indicates that the principal substrates of methanogenic bacteria are acetate and hydrogen/carbon dioxide (or formate). The relative importance of formate versus hydrogen/carbon dioxide in the methane fermentation is not well documented because of exchanges which occur between these substrates. A different physiological population, the hydrolytic bacteria, are responsible for depolymerization of organic polymers and fermentation to products, including organic acids, alcohols, and the methanogenic substrates. Organisms that convert fermentation products, such as propionate, butyrate, lactate, and ethanol generally exhibit obligate proton-reducing metabolism, i.e. they produce dihydrogen as a fermentation product and this reaction is obligately dependent


342


Figure 1. Methanogenic decomposition of compounds in nature (Adapted from ref. 46)


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on hydrogen removal by methanogenic or other hydrogen-using bacteria (10). In addition, there are organisms present which form acetate and other C-3 or higher volatile acids via back reactions with dihydrogen and carbon dioxide (10). The fermentation is dependent upon a delicate balance between activities of bacteria that form acids, carbon dioxide, and dihydrogen (or formate) and methanogenic bacteria which utilize these substrates. Overproduction of electrons resulting from overfeeding or underutilization of electrons by methanogenic bacteria results in the accumulation of fermentation products to inhibitory levels, leading to cessation of decomposition. During the past 10 years, literature on the microbiology and biochemistry of methanogenic bacteria has increased exponentially with isolation of several new species and development of more rapid techniques for identification. Beginning with the discovery of interspecies hydrogen transfer in 1967 (11), several species of hydrogen-producing acetogenic bacteria have been isolated. Although the overall niches of these microbial groups are beginning to be unveiled, it is important to realize that knowledge of the physiology, metabolism and genetics of these organisms is in its infancy. Hydrolytic bacteria are the least studied organisms in the methane fermentation even though hydrolysis is apparently the overall ratelimiting step in many of the methanogenic environments. With the exception of the rumen (12,13), knowledge of the microbial ecology of methanogenesis in anaerobic environments, including anaerobic digesters, is at best superficial (9,14,15). The methane bacteria are such a unique group of organisms that they have been placed into a new evolutionary group (separate from eucaryotic plants and animals and procaryotic bacteria) referred to as Archaebacteria (76). Archaebacteria also include a few other species of extreme halophilic and thermophilic bacteria. Placement into a separate taxonomic group is related to the variance of their genetic makeup from that of other living organisms, including bacteria. This difference is illustrated in the diagram shown in Figure 2 which is based on analysis of ribosomal R N A of numerous organisms, including most of the available pure cultures of methanogenic bacteria. These genotypic differences are reflected in numerous phenotypic characteristic s unique to this group, including metabolism, coenzymes, and cell membrane lipids. Methanogenic bacteria produce methane from acetate, methanol, dihydrogen/carbon dioxide, formate and mono-, di-, and tri-methylamines (Figure 3). Methane production from acetate, formate, and dihydrogen/carbon dioxide are the most important reactions in nature. These organisms have unique coenzymes for electron transfer, CoM and F (17). The fluorescence property of F has been used to locate methanogenic colonies and enumerate methanogens in mixed culture (18,19). The property of possessing ether-linked instead of ester-linked lipids in their cell membranes (20) has been proposed as a method to distinguish methanogens from other microorganisms in the environment. Biomethanogenesis plays a major role in the cycling of carbon in nature. It has been reported in a broad variety of environments where organic decomposition 420

420


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THE SCIENCE OF GLOBAL CHANGE EUKARYOTES

EUBACTERIA

ciliates

animals

plants flagellates

mlcrosporidia

extreme halophites methanogens extreme thermophiles ARCHAEBACTERIA

Figure 2. Universal phylogenetic tree determined from rRNA sequence comparisons. A matrix of evolutionary distances (99) was calculated from an alignment (260) of representative 16S RRNA sequences from each of the three urkingdoms. The length of the lines is proportional to the phylogenetic difference. (Reproduced with permissionfromret 16. Copyright 1987. American Society for Microbiology.)

1. Hydrogen 4 H + C0 2

2. Acetate CHgCOOH a

4

4

2

0

2

Formate CH + 3 C0 4

2

+ 2 H 0 2

Methanol

4 CH3OH

3 CH + CO + 2 H 0 4

5. Trimethylamine 4 (CHafeN + 6 H 0 2

6

2H

CH + C0

4 HCOOH 4.

CH +

2

Dimethylamine 2 (CH^NH + 2 H 0 2

7. Monomethylamine 4 (CH3)NH + 2 H O 2

z

z

2

9 CH + 3 C0 4

2

+ 4 NH

3

3 C H + C O + 2 NH

3

3 CH + C0

3

4

4

z

2

+ 4 NH

Figure 3. Reactions of methanogenic bacteria. (Adapted from ref. 47) Dunnette and O'Brien; The Science of Global Change ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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has depleted oxygen and other inorganic electron acceptors, including aquatic sediments and the intestines of many varieties of animals, including ruminants, non-ruminant mammals, reptiles, and insects (21-26). In non-animal environments, decomposition would cease in the absence of methanogenesis due to buildup of inhibitory fatty acids and other fermentation products. Methanogenesis insures removal of electrons and continued degradation of the biodegradable fraction of organic residues releasing the carbon and electrons back into the biosphere. The other fraction of organic matter (about 25-30%), including lignin and other compounds, is refractory to anaerobic catabolism and therefore becomes sequestered in anaerobic sediments (27). Thisfractionrepresents the origin of peat, oil, coal, and natural gas. Without biomethanogenesis there would be a rapid accumulation of biomass sequestered in acidic, odorous sediments resulting in an imbalance in the carbon cycle and cycling of other elements such as Ν, P, and S contained in organic matter. Many animals are dependent on methanogenesis for their nutritioa In ruminants and certain wood-eating insects such as termites, effective digestion and fermentation of lignocellulosic matter is dependent upon an efficient methane fermentation for depolymerization and hydrogen/formate metabolism. In such animals, nutrition is obtainedfromlow molecular weight bacterial metabolic products (1Z13). Although methanogenesis has been reported in man and other mammals with simpler diets, its role in these environments is not known and is not considered significant. The absence of an extensive methane fermentation in these animals may be attributed to washout caused by the short intestinal retention time and the long generation time of methane bacteria. Concerns over atmospheric methane as a greenhouse gas and the large contribution of biomethanogenesis as a source of this gas make it important to determine the relative significance of various components of this activity. A recent paper (8) summarized estimates (28-30) of source fluxes of atmospheric methane based on several carbon isotopic studies and presented new data on natural sources and biomass burning. These data (Table ΠΙ) show that of a total flux of 594 million tons (Tg) per year, 83% is produced via biomethanogenesis from a combination of natural (42%) and anthropogenic (41%) sources. The principal anthropogenic sources of atmospheric methane arericepaddies, cattle and other ruminants, and organic waste processing. The significance of wood-degrading termites may be significant and is debatable (24-26). Flooding of soils rich in organic matter leads to anaerobiosis and methane production. Methane is released directly and transported through plants such as rice to the atmosphere. Increased demands for rice by exploding populations will result in increasing fluxes from this source. Cattle and other ruminants numbering 1.3 billion generate about 13% of the global methane flux from their digestive tracts. These animals generate wastes which contribute further to the methane pool. Most of the organic wastes associated with man's activities are deposited in landfills where decomposition via a methane fermentation releases about 8% of the methane


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Table HI. Source Fluxes of Atmospheric Methane

1

Source

Tg y

Natural Sources (tundra, bogs, swamps)

246 ±86

42

120 78 45 50

±50 ±12 ±14 ±20

20 13 8 8

35 ±? 20 ±10

6 3

594 ±192

100

Anthropogenic Sources rice paddies livestock biomass burning landfills, organic waste oil and natural gas processing coal mining Total Flux

% of total

SOURCE: Adapted from ref. 8.

emission pool. Considerable reduction in these fluxes might be realized through improved rice and animal production practices, and waste handling techniques to capture the methane for use as fuel. These reductions, however, are likely to be offset by increased production in each category resulting from demands of our exploding population and its continued development. Other activities of man resulting in non-biological methane release include coal mining, natural gas mining and transmission leaks, and biomass burning; these sources contribute 17% of the emission pool (8). Applications of Biomethanogenesis Biomethanogenesis has been harnessed in the process known as anaerobic digestion in which this microbial process is optimized for conversion of organic wastes and energy crops to methane, carbon dioxide, and compost. The earliest use of this process was the treatment of domestic and animal wastes (31). It was mostly used in the United States and other developed countries for domestic sludge treatment Less developed countries like China and India have employed small-scale digesters for treatment of sewage and animal wastes (32,33). In these applications the methane produced is used for cooking, lighting, and operation of small engines, and the residues are applied in agriculture as a compost. As greater cost and impending depletion of fossil fuels became apparent in the 1970's and early 1980's, the search for renewable alternative fuels resulted


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in an expanded interest in anaerobic digestion to include industrial wastes, municipal solid waste, and biomass energy crops as feedstocks. During this period several novel high-rate digester designs were commercialized for industrial wastes, predominantly in the food industry (34,35). These industries realized the benefits of treating their wastes by a process that eliminates the cosdy aeration requirement and generates a fuel that can off-set a portion of the energy requirements of their operations. Although a few animal waste digesters were placed into operation in the U.S. and other developed countries, the absence of strict environmental regulations for these wastes and prevailing low energy prices stifled their development Research on anaerobic digestion of municipal solid waste also blossomed, resulting in new digester designs for high solids feedstocks. Two of these designs are barely commercial in Europe (36,37). Low tipping fees and plunging energy prices have stifled commercialization. Several research programs investigated energy crops coupled with anaerobic digestion for generation of renewable substitute natural gas, including marine algae (38), grasses (39), and woods (40,41). These programs have integrated research on crop production and harvesting, conversion by anaerobic digestion, and systems analysis. Resource potential estimates for these feedstocks (Table IV) have been reported at 7 E J (10 joules or about 10 Btu) for wastes and 30 EJ for terrestrial biomass (42). The potential for marine biomass is about 100 EJ (43). However, that value has many uncertainties which are related primarily to design of offshore farms. The cost of methane from these renewable energy systems was significantly higher than fossil-derived energy and interest in their continued funding dwindled with a continuation of energy gluts and depressed prices in the late 1980's. Several factors have recently stimulated a new interest in renewable energy resources and their conversion to methane via biomethanogenesis. Heading the list is the threat of global warming resulting from accumulation of greenhouse gases. The major greenhouse gas is carbon dioxide which is increasing as a direct result of combustion of fossil fuels. Combustion of coal and oil also results in acid rain, urban smog, and stratospheric ozone depletion caused by emissions of oxides of sulfur and nitrogen. Methane produced from anaerobic digestion of renewable energy crops and organic wastes could replace a major fraction of fossil fuels resulting in a direct decrease in the rate of increase of carbon dioxide (15,44). Although combustion of biomethane would also release carbon dioxide, an equivalent amount would be fixed by the plants from which it was produced. Furthermore, about 30% of the biomass feedstock is not converted and can be returned to the soil where it remains sequestered as refractory carbon. Because of biomethanogenesis decomposes organic matter with production of a useful energy product, anaerobic digestion of organic wastes is receiving increased attention. With increased levels of waste production, limited area for landfilling or application, and increased awareness of environmental impact, alternative methods for treatment of solid and agricultural wastes are being sought. A n attractive option for treatment of the organic fraction of these wastes is to separately treat 18

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Table IV. Energy Potential From Biomass and Wastes in the United States

Resource

municipal solid waste sewage sludge and sewage-grown biomass biodegradable industrial wastes crop residues logging residues animal manures energy crops payment-in-kind (PIK) land (32 Hectares) equivalent to PIK area devoted to energy crops marine biomass (e.g. macroalgae)

Total (excluding marine)

EJ y

1

1.5 0.8 0.4 4.1 0.3 0.4

11.0 11.0 100.

29.5

SOURCE: refs. 42,43

this fraction by composting and apply the stabilized residues on land as a soil amendment. The residues would reduce water needs and prevent erosion. This scheme, however, requires effective separation of undesired components such as metals, glass, plastics, and toxic compounds which affect the quality of residues more than the conversion process. Although aerobic composting is a more popular process for stabilization of these wastes, anaerobic composting has the advantages of methane production and lack of need for aeration or mixing (45). Currently these wastes release undesired methane into the atmosphere due to anaerobic conversion in landfills, lagoons, or stock piles. Treatment and recovery of this gas in reactors would reduce this source of atmospheric methane. Conclusion In spite of the significance of biomethanogenesis to the carbon cycle, as a source of a major greenhouse gas, and as a process for deriving substitute natural gas from renewable resources, the microbiology and ecology of methanogenesis are poorly understood and application of the process for derivation of renewable energy


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is poorly developed. The principal bacteria involved have not been isolated in total from a single methanogenic fermentation. A limited understanding of the behavior of the overall reactions and factors influencing them has permitted design and optimization of anaerobic digesters for conversion of a number of different feedstocks. Application of this process has been stifled by the perceived instability of the fermentation and high costs of producing methanefromrenewable sources compared to those of competing fossil fuels. A concentrated effort to understand the fundamentals of the process should lead to the development of methods to improve process stability. As fossil fuels become depleted and their use curtailed because of their contribution to increased atmospheric carbon dioxide, incentives to turn to renewable methane should be increased. Attention should be directed toward development of better estimates of source fluxes of atmospheric methane and methods to reduce these fluxes. In particular, those fluxes related to man's activity should be addressed. Acknowledgments The author greatfully acknowledges Dr. Ann Wilke for her thorough review of this chapter and several valuable suggestions for its improvement. Literature Cited 1. The Merck Index; Merck and Co., Inc.: Rahway, NJ, 1983; pp 852-853, 2. Woods, T. J., "The Long-Term Trends in U.S. Gas Supply and Prices: The 1989 G R I Baseline Projection of U.S. Energy Supply and Demand to 2010," Gas Research Insights (GRI Publ.), Gas Research Institute: Chicago, IL, 1990. 3. Kerr, R. A. Science 1989, 245, 1330-1331. 4. Blake, D. R.; Rowland, F. W. Science 1988, 239, 1129-1131. 5. Rodhe, H. Science 1990, 248, 1217-1219. 6. Gibbs, M. J.; Hogan, K. EPA Journal 1990, 16(2), 23-25. 7. Bingemer, H. G.; Crutzen, P. J.J.Geophys. Res. 1987, 92, 2181-2187. 8. Stevens, C. M.; EngelkemeirJ.Geophys. Res. 1988, 93, 725-733. 9. Anaerobic Digestion ofBiomass; Chynoweth, D. P.; Isaacson, R.; Eds; Elsevier Applied Science: London, 1987. 10. Boone, D.; Mah, R.; In Anaerobic Digestion of Biomass; Chynoweth, D. P.; Isaacson, R.; Eds.; Elsevier Applied Science: London, 1987; pp 35-48. 11. Bryant, M. P.; Wolin, E. A.; Wolin, M. J.; Wolfe, R. S. Arch.Mikrobiol.1967, 59, 20-31. 12. Hungate, R. The Rumen and Its Microbes; Academic Press: New York and London, 1966. 13. The Rumen Ecosystem; Hobson, P. N., Ed.; Elsevier Applied Science: London and New York, 1988.


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14. Smith, P. H.; Bordeaux, F. M . ; Wilkie, Α.; Yang, J.; Boone, D.; Mah, R. Α.; Chynoweth, D.; Jerger, D.; In Methane From Biomass: A systems Approach; Smith, W. H.; Frank, J. R., Eds., Elsevier Applied Science Publishers: London, 1988, pp 335-353. 15. Smith, P. H.; Bordeaux, F. M.; Goto, M.; Shiralipour, A.; Wilkie, A.; Andrews, J. F.; Ide, S.; Barnette, M . W.; In Methane From Biomass: A systems Approach; Smith, W. H.; Frank, J. R.; Eds.; Elsevier Applied Science Publishers: London, 1988, pp 291-334. 16. Woese, D. R. Microbiol. Rev. 1987, 51, 221-271. 17. Daniels, L.; Sparling, R.; Sprott, G. D. Biochemica et Biophysica Acta 1984, 768, 113-163. 18. Peck, M . W.; Archer, D. B. Internat. Indust.Biotechnol.1989, 9(3), 5-12. 19. Peck, M . W.; Chynoweth, D. P. Biotechnol. Letts. 1990, 12, 17-22. 20. deRosa, M.; Gambacorta, A.; Gliozzi, A. Microbiol. Rev. 1986, 50, 70-80. 21. Zeikus, J. C.; Bacteriol. Rev., 1977, 41, 514-541. 22. Zehnder, A. J. B.; Ingvorsen, K.; Marti, T.; In Anaerobic Digestion 1981; Hughes, D. E.; Stafford, D. A.; Wheatley, B. I.; Baader, W.; Lettinga, G.; Nyns, E. J.; Verstraete, W.; Wentworth, R. L.; Eds.; Elsevier Biomedical Press: Amsterdam, 1982; pp 3-22. 23. Breznak, J. A.; Symbiosis 1975, 24, 559-580. 24. Zimmerman, P. R.; Greenberg, J. P.; Wandiga, S. O.; Crutzen, P. J. Science, 1982, 218, 563-565. 25. Rasmussen, R. A.; Khalil, M . A. K. Nature 1983, 301, 700-703. 26. Collins, Ν. M . ; Wood, T. G. Science 1984, 224, 84-86. 27. Kelly C.A.;Chynoweth, D. P.; In Native Aquatic Bacteria: Enumeration Activity, and Ecology, ASTM 695; Costerton, J. W.; Colwell, R. R., Eds.; American Society for Testing and Materials: Philadelphia, PA, 1979; pp 164-179. 28. Holzapfel-Pschorn, A.; Seiler, W. J. Geophys. Res. 1986, 91, 11803-11814. 29. Crutzen, P. J.; Aselmann, I.; Seiler, W. Tellus 1986, 38B, 271-284. 30. Hitchcock, D. R.; Wechsler, A E.; Biological Cycling of Atmospheric Trace Gases; Final Report; NASA Contract NASA-CR-126663; 1972; pp 117-154. 31. McCarty, P. L., In Anaerobic Digestion 1981; Hughes, D. E. et al.; Eds.; Elsevier Biomedical Press: Amsterdam, 1982; pp 3-22. 32. Ke-yun, D.; Yi-zhang, Z; Li-bin, W.; In Anaerobic Digestion 1988; Hall, E. R.; Hobson, P. N.; Eds.; Pergamon Press: Oxford, 1988; pp 295-302. 33. Ward, R.; In Anaerobic Digestion 1981; Hughes, D. E. et al.; Eds.; Elsevier Biomedical Press: Amsterdam, 1982, pp 315-344. 34. Pohland, F. G.; Harper, S. R.; In Anaerobic Digestion 1985; Quangzhou China, China State Biogas Association, 1985; pp 41-82. 35. Wilke, A.; Colleran, E.; In International Biosystems III; Wise, D. L.; Ed.; CRC Press: Boca Raton, FL, 1989; pp 183-226. 36. DeBaere, L.; Van Meenen, P.; Deboosere, P.; Verstraete, W. Resources and Conservation 1987, 14, 295-308.


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37. Bonhomme, M.; In Energy From Biomass and Wastes, XI; Klass, D.; Ed.; Institute of Gas Technology; 1988; pp 721-730. 38. Seaweed Cultivation for Renewable Resources; Bird, K. T.; Benson, P. H.; Eds.; Elsevier: Amsterdam, 1987. 39. Methane From Biomass: A Systems Approach; Smith, W. H.; Frank, J. R.; Eds.; Elsevier Applied Science Publishers: London, 1988. 40. Kenney, W. A.; Sennerby-Forsse, L.; Layton., P. Biomass 1990, 21, 163-188. 41. Turick, C. E.; Peck, M . W.; Chynoweth, D. P.; Jerger, D. E.; White, Ε. H.; Zsuffa, L.; Kenney, W. A. Biomass, in press, 1991. 42. Legrand, R.; Warren, C. S.; "Biogas generation from community-derived wastes and biomass in the U.S.;" Paper presented at the Tenth Annual Energy-Sources Technology Conf. and Exhib.; ASME; Dallas, TX, 1987. 43. Chynoweth, D. P.; Fannin, K. F.; Srivastava, V.; In Seaweed Cultivation for Renewable Resources; Bird, K. T.; Benson, P. H.; Eds.; Elsevier: Amsterdam, 1987; pp 285-303. 44. Wilkie, A. C.; Smith, P. H.; In Microbiology of Extreme Environments and Its Potential for Biotechnology; da Costa, M . S.; Duarte, J. C.; Williams, R. A. D.; Eds; Elsevier Science Publishers: London, 1989; pp 237-252. 45. Chynoweth, D. P.; Bosch, G.; Earle, J. F. K.; Legrand, R.; Liu, K. Applied Biochemistry and Biotechnology 1991, 28/29, 421-432. 46. Ghosh, S.; "Microbial Production of Energy: Gaseous Fuels;" Lecture presented at the Seventh International Biotechnology Symposium; New Delhi, India, February 1984. 47. Ferguson, T.; Mah, R.; In Anaerobic Digestion of Biomass; Chynoweth, D. P.; Isaacson R.; Eds.; Elsevier Applied Science: London, 1987; pp 49-64. RECEIVED July 23,

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