Twenty years ago, Holsum Dairies installed the first anaerobic digester in Wisconsin. Part of a combined heat and power system, the digester mixed cow manure with heat and bacteria to produce biogas that fueled a generator.
It was a novel experiment at the time. The digester, and another the dairy installed a few years later, reduced the size of open lagoons of manure that release methane into the atmosphere. Wisconsin Public Service, the local utility, bought the electricity. But last year, Holsum Dairies changed course, upgraded the two digesters and began producing renewable natural gas for California’s burgeoning renewable fuels market.