Manure Digester Bio-Gas

SEPTEMBER 6, 2001                                                                                                                                                                                              ISSUE #M18

Due to renewed energy awareness there has been a surge in interest in manure digesters.  In these digesters the manure decomposes anaerobically (without Oxygen), releasing bio-gas. This bio-gas is collected and burned to release its energy.

These dairy systems have environmentally attractive features.  Energy is produced from a waste source.  Also, the digester action treats the manure, which addresses other problems:  run-off is controlled and odor from the dairy is reduced.

The bio-gas (methane with CO2, SO2 and other gasses) can be burned in a boiler to produce either steam or hot water for the farm.  Alternatively, this low BTU gas can be burned in a Waukesha or Caterpillar internal combustion engine.  These engines are coupled to induction generators that more than fill the farm's electricity requirements.

Besides internal combustion engines, mini gas turbines are being offered for use on biogas.  The metallurgy of the engine and other components is critical because of corrosive gases.

Two types of digesters predominate.  In both the gas is collected off the top.  The most common, a plug flow digester, is a long pit where the manure enters at one end and progresses slowly to the discharge.  Our presses are used with this type of digester at Fairgrove Farms in Michigan, High Plains Dairy in Kansas, and Iowa State University.

Mixed flow digesters using tanks are a second type of digester where extensive development has been done.  A.O. Smith Harvestore leads in this technology, and Cushman Farms in Franklin, Connecticut is a good installation to visit.  Manure solids flow off the top of the digester into our Model KP-10. 

Another type of mixed flow digester uses a pit instead of a tank.  A good example is under construction at Matlink Dairy in Clymer, New York.  This 660,000 gallon pit digester has propeller agitators on two sides.  Digested manure will flow over a weir to be pumped to a Vincent press.

The manure can be pressed in a screw press ahead of the digester, with the press liquor being directed into the digester.  However the more common arrangement has the press dewatering the sludge that comes from the digester.  The press cake, from either raw or digested manure, has about 70% moisture.  This is ideal for composting and producing a rich soil amendment, or for immediate use as barn bedding.

It is surprising how accurate this report is.  The trend has been toward mixed flow instead of plug flow, and gas generation and recovery technology has improved.  However politically incorrect unstated facts from ten years ago remain true:  No digester project goes ahead without a government grant because the investments do not pencil out.  And, most digester projects get abandoned.  For example, all five showcase digesters mentioned in this Pressing News have been shut down.