Design Criteria and Sizing the Plug-Flow Digester
Location. If a manure pump is installed to pump the 12% solids manure, the digester can be located
within a 300 ft radius of the mix tank at a convenient location with good access.
Mix tank. The mix tank can be round, square, or rectangular. A pump may be required to move
manure to the plug flow digester.
Hydraulic retention time and sizing of plug-flow digester. A plug-flow digester will function with an
HRT from 12 to 80 days. However, an HRT between 15 and 20 days is most commonly used to
economically produce 70% to 80% of the ultimate methane yield.
Dimensions. The depth of a plug-flow digester can be between 8 feet and 16 feet depending upon soil
conditions and the required tank volume. The width:depth ratio is usually greater than 1 and less than
2.5. The length:width ratio should be between 3.5 and 5.
Heat exchanger: An external heat exchanger or an internal heat exchanger is required to maintain the
digesting mixture at the design temperature. Hot water circulated through the heat exchanger is
heated using biogas as a fuel for a boiler or waste heat from a biogas fueled engine-generator.
Operating temperature. The daily temperature fluctuation should be less than 1o F. Most plug flow
digesters operate in mesophilic range between 95o to 105o F with an optimum of 100o F. It is
possible to operate in the thermophilic range between 135 to 145o F, but the digestion process is
subject to upset if not closely monitored.
Insulation. A plug flow digester surface may be insulated to control heat loss.
Construction materials. The digester can be constructed as a lined trench or as a reinforced concrete
or block tank.
Methane recovery system and covers. See discussion of methane recovery system above under
complete mix digesters.