Determination of digester tank volume is the first important consideration in the design of an anaerobic digestion system. Various methods have been used to determine the size of the digester, including per capita basis, solids loading, solids retention time, volumetric loading, and volatile solids destruction. Other design considerations, such as mixing, heating, energy recovery, and chemical additions, should also be evaluated in the design of the digestion system. The discussion below is for mesophilic digestion, as it is the most common system in use. Some of the design criteria are also applicable for thermophilic digestion.
To design an anaerobic digestion system, it is important to know the quantity and characteristics of the feed sludge. The characteristics include percent total solids, percent volatile solids, and the ratio of primary to secondary sludge solids. Solids production can be calculated based on the solids mass balance of the treatment plant (see Chapter 2) or from the existing plant operating data. If there are significant contributions of industrial wastewater to the treatment plant, the feed sludge should be analyzed for the toxic materials described in Section 5.2.4.
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