A more direct design criterion in determining digester volume is the volatile solids loading rate. It is the mass of volatile solids fed to the digester per unit volume and time. A typical range of loading rates is shown in Table 5.3. Design loading criteria generally are based on sustained loading conditions: typically, peak month solids production. It should be recognized that excessively low volatile solids loading criteria would result in a large digester system, which can be expensive to operate because it may not produce sufficient gas to provide the energy needed to heat the digester content to its desired temperature.
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