Methane fermentation is a versatile biotechnology capable of converting almost all types of polymeric materials to methane and carbon dioxide under anaerobic conditions. This is ach­ieved as a result of the consecutive biochemical breakdown of polymers to methane and car­bon dioxide in an environment in which varieties of microorganisms which include fermentative microbes (acidogens); hydrogen-producing, acetate-forming microbes (aceto — gens); and methane-producing microbes (methanogens) harmoniously grow and produce reduced end-products (Fig. 10—11). Anaerobes play important roles in establishing a stable environment at various stages of methane fermentation.

Methane fermentation offers an effective means of pollution reduction, superior to that ach­ieved via conventional aerobic processes. Although practiced for decades, interest in anaero­bic fermentation has only recently focused on its use in the economic recovery of fuel gas from industrial and agricultural surpluses.

The biochemistry and microbiology of the anaerobic breakdown of polymeric materials to methane and the roles of the various microorganisms involved are discussed here. Recent progress in the molecular biology of methanogens is reviewed, new digesters are described and improvements in the operation of various types of bioreactors are also discussed.

Methane fermentation is the consequence of a series of metabolic interactions among vari­ous groups of microorganisms. A description of microorganisms involved in methane fer­mentation, based on an analysis of bacteria isolated from sewage sludge digesters and from the rumen of some animals,. The first group of microorganisms secretes enzymes which hy­drolyze polymeric materials to monomers such as glucose and amino acids, which are sub­sequently converted to higher volatile fatty acids, H2 and acetic acid (Fig. 10). In the second stage, hydrogen-producing acetogenic bacteria convert the higher volatile fatty acids e. g., propionic and butyric acids, produced, to H2, CO2 and acetic acid. Finally, the third group, methanogenic bacteria convert H2 CO2 and acetate, to CH4 and CO2 (Nagai et al., 1986).

4% 24% ^ ► H2 ^ 28% COMPLEX 76% r HIGHER ORGANIC % 1 CH4 ORGANICS ACIDS TP f ACETIC 20% ACID TZvo METHANO- HYDROLYSIS ACETQGENESIS GENESIS AND AND ACIDOGENESIS DEHYDROGENATION STAGE 1 STAGE 2 STAGE3 Figure 10. The main steps for production of methane gas
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