Haubenschild Farms Final Report - The Minnesota Project

The Minnesota Project August 2002
Plug-Flow Digester
Plug-flow digesters are suitable for ruminant animal manure having a solids concentration
of 11 to 14 percent, such as cow manure collected by scraping. A flush system for manure
collection is not appropriate for this system, since this would reduce the total solids content
of the manure below specified levels. In manure with lower solids concentrations, such as
swine manure, solids cannot stay in solution and tend to settle to the bottom of the tank,
limiting their digestion. A plug-flow digester has few moving parts and requires minimal
maintenance.
Figure 1: Anaerobic Plug-Flow Digester System
GAS
UTILIZATION
(electric generation and
hot water production)
BARN
MIXING
TANK
DIGESTER
STORAGE
POND
Before entering the digester, raw manure is mixed in a mix tank. It then enters one end of
the plug-flow digester, a rectangular tank, and decomposes as it moves through the digester.
New material added to the digester tank pushes older material to the discharge end. Coarse
solids in ruminant manure form a thick sticky material as they are digested, limiting solids
separation in the digester tank. As a result, the material flows through the tank in a “plug”.
Anaerobic digestion of the manure slurry creates biogas as the material flows through the
digester. A flexible, impermeable cover on the digester traps the biogas. For optimal
digestion, it should take about 15 to 20 days for a plug to pass completely through the
digester.
Inside the digester, suspended heating pipes allow hot water to circulate and heat the
digester. The heating pipes also serve to mix the slurry through convection. Recovered
heat from an engine fueled with digester gas usually provides the hot water required for
heating the digester. Figure 1 shows how a plug-flow digester system works.
There is a variation on the basic plug-flow design called a slurry loop digester. It works on
the same principle as a plug-flow, except the digester tank is designed in a U-shape or
circular configuration, so that the discharge end of the tank is near the point of entry. This
design is used in Wisconsin, at Gordondale Farms near Stevens Point. The Gordondale
digester represents a hybrid of complete mix and plug-flow designs, as collected biogas is
injected into the digester to further mix the slurry.
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