Digester Dimensions And Accessories

Many wastewater treatment facilities use a 3 in 12 (3 ft vertical, 12 ft horizontal) digester bottom slope (14°). The digester design can be based on a 15-ft minimum side wall height with 2 ft of freeboard. Medium and large waste-water treatment facilities should consider duplicate digestion units. A means for supernatant liquid drawoff should be provided for secondary digesters only if the primary digesters are the high-rate type. The digesters should be equipped with flame arresters and vacuum relief devices.

The gas holder (usually a sphere) is designed on the basis of the following relationship:

where:

FIG. 7.45.1 Types of anaerobic digesters. A, Conventional; B, High rate.

C = Capacity of the gas holding sphere (ft3 of gas at

14.7 psia and 60°F) V = Volume of sphere, ft3 P = Maximum operating pressure, psig P' = Minimum operating pressure, psig T = Temperature of stored gas, °F

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