FXSTD – Alternative Energy Center

By: Steve Evans
digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen. It is widely used to treat wastewater sludges and organic waste because it provides volume and mass reduction of the input material.

As part of an integrated waste management system, anaerobic digestion can also reduce the emission of landfill gas into the atmosphere. Anaerobic digestion is a renewable energy source because the process produces a methane and carbon dioxide rich biogas suitable for energy production helping replace fossil fuels. Also, the nutrient-rich solids left after digestion can be used as fertiliser.

Scientific interest in the gasses produced by the natural decomposition of organic matter goes back a long way. Biogas was first reported in the seventeenth century by Robert Boyle and Stephen Hale, who noted that flammable gas was released by disturbing the sediment of streams and lakes. In 1808, Sir Humphry Davy determined that methane was present in the gasses produced by cattle manure.

It has been reported on the web that the first anaerobic digester was built by a leper colony in Bombay, India in 1859. In 1895 the technology was developed in Exeter, England, where a septic tank was used to generate gas for street lighting. Also, web sources report that in England, in 1904, the first dual purpose tank for both sedimentation and sludge treatment was installed in Hampton. In 1907, in Germany, a patent was issued for the Imhoff tank, an early form of digester.

The biogas digester (anaerobic digestion plant -AD plant ) also, of course, has a big part to play in farming. An AD plant captures naturally occurring gas from manure and converts it into electricity. One recently publisher US News item declares that with their new AD system, they are generating up to 300,000 kilowatt-hours per year fo their farm. That is an value in power of about $40,000 a year!

The farming industry is developing a variety of AD Plant systems using differnet technologies. Placing a tarpaulin over manure ponds can provide a very basic biogas digester. Doing it eliminates the release of methane (a natural byproduct of manure) into the air. The amount that can be removed is small in overall emissions terms but could be significant. According to the 2003 U.S. Department of Energy Report on Greenhouse Gases, agricultural sources, primarily animal waste, account for approximately three percent of greenhouse gas emissions. The same source reports that a dairy cow can generate 120 lbs. of waste each day, totaling about 40,000 lbs. per year. Furthermore, using a digester wastes nothing as the solids separated from the waste are composted and reused as fertilizer, providing additional, and highly sustainable benefits.