The digester vessel is usually a round insulated tank, above or below ground and made from reinforced concrete, steel or fiberglass. Heating coils with circulating hot water can be placed inside the digester or, depending on the consistency of the feedstock, the contents can be circulated through an external heat exchanger to maintain desired temperatures. They can be mixed with a motor driven mixer, a liquid recirculation pump or by using compressed biogas. A gas tight cover (floating or fixed) traps the biogas. The complete mixed digester is best suited to process manure with 3–10 percent total solids. Retention time is usually 10 to 20 days. A dairy or swine farm using a flush manure management system in the barns and milking parlor could use the biogas created by the digester to heat the digester to the desired temperature. Excess biogas could be used to run an engine generator. Heat can be recovered from the engine generator and used for space or floor heating, water heating or producing steam to offset the cost of purchased electricity, propane, natural gas or fuel oil used on the farm for daily operations.
The complete mix digester at Cooperstown Holstein Corporation Farm in Cooperstown, NY has been in operation since 1985. The digester vessel is an above ground, insulated glass lined tank. The biogas produced powers an engine generator. The heat recovery from the engine generator maintains the digester temperature in the mesophilic range and heats sanitary wash water for the dairy. Solids are separated from the digested effluent for use as animal bedding. The remaining liquid effluent is stored in a slurry tank for land application at a later date on crop land.
Additional information on the Cooperstown Holstein Corporation Farm can be found in:
- Methane Recovery from Animal Manures — The Current Opportunities Casebook pg. 4-35, Case Study 4:11