Manure is probably dairy’s single best opportunity for significant greenhouse gas reduction. When the industry made its reduction pledge, it had its eye on anaerobic digesters to harvest the manure methane, or biogas. It’s a double bonus, capturing methane that would otherwise enter the atmosphere, but also turning it into energy, preventing further emissions from burning coal or natural gas.
“It’s one of the largest tools we have to really limit emissions,” says Rebecca Larson, a CALS/UW-Extension biological systems engineering professor and bio-waste specialist who is leading the Dairy CAP manure team.
A visit to the Dane Community Digester in Waunakee, operated by Clear Horizons LLC, reveals this potential future. Tucked between three dairies in rolling landscape north of Madison, it operates at industrial scale. One steel tank accepts 100,000 gallons of liquid manure pumped daily from nearby farms. Waste from animal bedding goes into what the operator calls the “jacuzzi,” a massive liquefying tank, churning and steaming in the cold. A smaller tank stores food waste delivered by local food processors.All three waste types are mixed together and pumped into one of three 1.25 million-gallon digestion tanks where bacteria do the real work. The methane bubbles up, feeding two generators, each the size of a mobile home. At full capacity, the plant produces two megawatts of power, enough to power about 2,400 homes.
There’s more. Each day about one semi-load of spent fiber is filtered from the tanks and sold for use as livestock bedding or soil amendments. Liquid waste is returned to the farmers, who can then use it to fertilize their fields. This facility is also an emergency option for farmers. A local farmer whose retention pond was failing, for example, off-loaded 100,000 gallons in one weekend, averting disaster. And the operation is testing increasing phosphorus removal to help with local water-quality problems.