Digestion

The Roanoke WPCP uses a two-phase digestion process. In the primary digestion tanks, the sludge is heated and mixed to promote the anerobic consumption of organic material by bacteria contained within the tanks. First, acid-forming bacteria use the organic material as an energy supply to produce organic acids and carbon dioxide. A second group of bacteria, the gas formers, breaks down the organic acids to make methane and carbon dioxide gas. The sludge from the primary digesters is taken to secondary digesters for "polishing." Here the already partially digested sludge is allowed to separate from any water mixed with it (supernatant) and digestion is completed. The dystor domes shown below are used to collect the methane produced from the anerobic respiration. The methane is then burned as fuel for the raw sewage pumps and blowers. This represents a savings of $250,000 in energy costs. The digested sludge is pumped to the lagoons to be weathered to ensure total decomposition.

Methane-Collecting Dystors


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