Methane is a potent greenhouse gas with a warming power about 21 times higher than carbon dioxide. At this dairy farm in Wisconsin, cows produce an average of 112 kilograms of methane per year in addition to their milk. Historically, all manure was washed into open lagoons and allowed to decompose directly into the atmosphere, but money from the sale of carbon offsets has allowed this dairy to build an anaerobic digester for the manure to be diverted into as cows take their turn on the milking carousel. The energy created powers the farm and surplus energy feeds into the local grid. In fact, Holsum dairies produce about enough electricity to meet the needs of the nearest town of 600 people.
The cow manure is collected at the Holsum Irish and the Holsum Elm dairy farms. First, the solid and liquid parts are separated. Next, the solids are sent through the anaerobic digester system where methane is collected in a tank. The biogas is composted to generate electricity. Residual products from this process can then be used sustainably on the farm. Dry manure is used as cow bedding and liquid manure as a low cost, locally sourced, natural fertilizer.
In addition to lowering emissions, the project contributes to sustainable development by: