Complete mix digesters process manure in a heated tank above or below ground. A mechanical or gas mixer keeps the solids in suspension. However, complete mix digesters are expensive to construct and cost more than a plug-flow digester to operate and maintain. Plug-flow digesters are suitable for ruminant animal manures having a solids concentration of 11 percent to 13 percent. In a plug-flow digester, raw manure slurry enters one end of a rectangular tank and decomposes as it moves through the tank.
New material added to the tank pushes older material to the opposite end.
Coarse solids in ruminant manure form a viscous material as they are digested, limiting solids separation in the digester tank. Anaerobic digestion of the manure slurry releases gas as the material flows through the digester. A flexible, impermeable cover on the digester traps the gas. The anaerobic digester at Craven Farms is an example of a plug-flow digester. A plug-flow digester requires minimal maintenance.