• Hydrogen-producing acetogenic bacteria (catabo-lized products of hydrolytic bacteria, e.g., fatty acids and neutral end products)

• Homolactic bacteria (catabolized multicarbon compounds to acetic acid)

• Methanogenic bacteria (metabolized acetic acid and higher fatty acids to methane and carbon dioxide)

Figure 11.17.4 is a schematic diagram of biological reaction in an anaerobic system.

Strict anaerobics require totally oxygen-free environments and an oxidation reduction potential of less than —0.2 V. Microorganisms in this group are known as methanogenic consortia and are found in anaerobic sedi ments or sewage sludge digesters. These organisms play an important role in reductive dehalogenation reactions, ni-trosamine degradation, reduction of epoxides to olefins, reduction of nitro-groups, and ring fission of aromatic structures.

Available anaerobic treatment concepts are based on approaches such as the classic well-mixed system, the two-stage system and the fixed bed system.

In a well-mixed digester system, a single vessel is used to contain the wastes being treated and all bacteria must function in that common environment. Such systems typically require long retention times, and the balance between acetogenic and methanogenic populations is easily upset.

Initial substrate

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