In the two-stage approach, two vessels are used to maintain separate environments, one optimized for acetogenic bacteria (pH 5.0), and the other optimized for methano-genic bacteria (pH 7.0). Retention times are significantly lower and upsets are uncommon in this approach.

The fixed bed approach (for single- or 2-staged systems) utilizes an inert solid media to which the bacteria attach. Low solids wastes are pumped through columns of the bacteria-rich media. Use of such supported cultures allows reduced retention times since bacterial loss through washout is minimized. Organic degradation efficiencies can be quite high.

A number of proprietary engineered processes based on these types of systems are being actively marketed. Each has distinct features, but all utilize the fundamental anaerobic conversion to methane and carbon dioxide (Figure 11.17.5).

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