Ellen Perlman was a GOVERNING staff writer and technology columnist.E-mail: firstname.lastname@example.org
Manure lagoons, bad. Manure digesters, good. That is the argument that Texas' Gulf Coast Industrial Development Authority used in floating $60 million in revenue bonds to finance four digester machines that will help keep farm runoff from polluting land, water and air.
In many places, manure from dairy farms goes into big lagoons where it sits for a period of time, releasing bio-gas into the atmosphere and contributing to the production of greenhouse gases. Digesters process manure, preventing methane from being released into the atmosphere. The end product is a natural gas that can be sold commercially.
With digesters, manure goes into a stainless steel tank and is heated, producing 60 percent methane, 35 percent carbon dioxide and trace sulfur compounds. Scrubbers remove the carbon dioxide and sulfur. The treated manure then can be spread on farmland where it can act as a nutrient without producing polluting gases. By keeping untreated manure out of lagoons, heavy rainfall is prevented from washing it down to rivers and streams.
California is also testing a digester. In this case, the source of raw material is food scraps from Bay Area restaurants. Rather than heading for a landfill, the waste is to be hauled to a digester at the University of California, Davis, where it will be processed into fertilizer and biogas. The biogas will be used to fuel garbage trucks and other commercial vehicles.
Digesters have been around for several years but none have been commercially successful. The California project is a joint venture between UCDavis and a private partner.