These digesters can operate at a mesophilic or thermophilic temperature. The plug flow digester has no internal agitation and is loaded with thick manure of 11-14 percent total solids. This type of digester works well with a scrape manure management system with little bedding and no sand. Retention time is usually 15 to 20 days.
In theory, manure in a plug flow digester does not mix longitudinally on its trip through the digester but can be imagined to flow as a plug, advancing towards the outlet whenever new manure is added. When the manure reaches the outlet it discharges over an outlet weir arranged to maintain a gas tight atmosphere but still allow the effluent to flow out. In actuality the manure does not remain as a plug and portions of the manure flow through the digester faster than others and some settles or floats and remains in the digester. Biogas produced by the digester is used to heat the digester to the desired temperature. Excess biogas can be used to run an engine generator. Heat can also be recovered from the engine generator and used for space or floor heating, water heating or steam production to offset the cost of purchased electricity, propane, natural gas or fuel oil used on the farm for daily operations.
The Haubenschild Farms plug flow digester started operations in the Fall of 1999. Located near Princeton, Minnesota the Haubenschild Farms operates a 750 head dairy using a scraped manure system with high solids. Shredded newspaper is used for bedding. The digester is kept at a constant mesophilic temperature by using waste heat from the engine generator cooling jacket to heat suspended water pipes in the digester. Portions of the barn floor are also heated with the recovered heat. The biogas produces enough electricity to power the farm and the equivalent of approximately 75 homes.
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Digester vessels, liquid manure tanks and pump sumps are confined spaces that may be immediately hazardous to life and should only be entered using approperiate procedures and equipment.