= Total heat requirement, Btu/hr = Overall heat transfer coefficient (a value of 140 is used in the absence of other information) Btu/hr/°F/ft2 = Heat exchange surface area, ft2 = Diameter of sludge pipe, in = Logarithmic mean of the temperature difference / AT1 - AT2 \ 1 loge AT1/AT2J

Digester Sizing

The following equations predict the VSS destruction: Conventional digesters:

High-rate digesters:

where:

These equations specifically apply to sludge with an initial VSS content of 65 to 70%. The quantity of solids remaining in the primary digester at the end of the digestion period can be calculated as follows:

where:

MTt = Solids remaining in digester, lb

MTw = Daily solid waste input into digester, lb t1 = Digestion period, days

%V = VSS in the sludge received by the digester, %

The following equations calculate the required primary digester volume:

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