In contrast to most widely propagated aerobic degradation processes, the anaerobic conversion of organics down to methane and carbon dioxide is limited to the work of three different groups of bacteria (see Table 8.1 and Fig. 8.1 in the sections below):

1. Acidogenic bacteria produce extracellular enzymes (exoenzymes) for the hydrolysis of all organic solid particles and those dissolved colloids and molecules which are too large to diffuse through cell walls and membranes. Carbonhydrates are hydrolyzed down to mono- and disaccharides, proteins into amino acids and lip-ids into fatty acids. These compounds are transformed to acetate and longer chain fatty acids as well as CO2 and H2.

2. Acetogenic bacteria transform lower fatty acids such as butyrate and propionate into acetate, CO2 and H2. It is of great importance that H2 is oxidized by other anaerobic bacteria. Otherwise, propionate concentrations would continually increase.

3. Therefore, hydrogen and acetate must be utilized by methanogenic bacteria. They exhibit two main products of catabolic metabolism. Methane is not very soluble in water and carbon dioxide is in equilibrium with HCO3- and CO32- as a function of pH (see Fig. 4.2). Most of the CO2 and nearly all the methane produced are desorbed, forming biogas bubbles which can be recovered for utilization.

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