Dairy digesters send fossil fuels out to pasture
With massive revenues and climate change mitigation, dairy digesters are steering dairy farms in the right direction.
By Leah Hochberg
What do renewable energy, reduced greenhouse gas emissions, job creation, and improved air and water quality have to do with cow poop? As Vanderhaak and Edaleen Dairy are proving with dairy digesters, the answer is: everything! During a visit to the Lynden dairies last week for WSU’s Dairy Digester Field Day, farmers displayed advanced anaerobic digestion systems that mitigate the climate impacts of dairy farms and increase dairy revenues.
Traditionally, cow waste is stored in aerobic lagoons where it ferments until farmers spray the lagoon concoction (poop, pathogens, and all) onto their crops as fertilizer. It’s a pretty crappy method. Aside from being disgusting, the process degrades soil quality by applying excess nutrients to the soil and pollutes water sources by leeching phosphates and nitrates into the water supply. Meanwhile, greenhouse gases, odorous volatiles, ammonia, and nitrous oxide are released into the atmosphere, accounting for some of the 20 percent of U.S. greenhouse gases emitted by dairy operations.
Cue dairy digesters. For only $330 per cow per year, an anaerobic digestion system converts potentially harmful cow waste into soil amendments, fertilizer, renewable energy, and water. The initial investment for VanderHaak was $1.2 million, but with $112,828 in annual revenue, the farm should be seeing profits within ten years of implementation. With proper maintenance, the digesters should last indefinitely, earning VanderHaak an additional million dollars every decade. (VanderHaak and Edaleen received grants through the USDA Rural Development Program and the Paul G. Allen Family Foundation, covering 25-35 percent of the investment.) Each digester also creates 15 to 20 construction jobs and two to three permanent jobs, further adding to the economic benefits.
The waste is first pumped into a digester, which is basically a giant manmade stomach. During a 22-day process, the waste is heated and mixed in the air-free digester, which converts the solid carbon in the waste into biogas–a combination of carbon dioxide and methane. The biogas is collected, cooled, and used to power an electric generator, creating electricity for farmers to sell electricity to the local grid. This opens up another revenue stream for the farm.
As of 2010, there were four dairy digester operations in Washington, producing 22,513 MWh electricity per year13,507,800 homes. At the 2010 price of 6.6 cents/kWh, Washington dairy farms earned $148,585,800. As of May 2013, there were twice as many digester operations in Washington, so revenues continue to increase. Digesters are powered and heated using this electricity as well, so farmers don’t lose money on electricity purchases.
During the next step in the digestion process the methane in the biogas is burned and the gas is released as carbon dioxide, which is 20 times less potent than methane. Also, the digester carbon dioxide emissions are much lower than those emitted from traditional fossil fuel electricity production. In fact, in 2009 U.S. dairy digester operations reduced greenhouse gas emissions by 1.1 million metric tons, the equivalent of reducing oil consumption by 2.7 million barrels.
After the biogas is collected, the remaining waste material is sent to a separator, where water is extracted from the solid waste, turning it into a fibrous soil amendment or peat moss replacement. This is another potential source of revenue for VanderHaak and Edaleen, who use 60 percent of the solids as animal bedding and sell the rest. At the end of the process, the water that enters the lagoon is virtually odorless and contains 99 percent fewer pathogens than regular lagoon water.
As if they weren’t already milking their resources, VanderHaak and Edaleen take dairy digestion one step further with moo-trient – er, nutrient – recovery. While they are separating out the waste components, they recover phosphorous and nitrogen in the form of ammonia, which they then turn into sulphate fertilizer. It can be applied much more precisely, does not need to be transported to the farm, and when applied to crops, VanderHaak farmers noticed significantly faster growth and larger, greener yields. For them, this means larger annual revenue from crop sales.
VanderHaak and Edaleen plan on implementing a biogas scrubber, which turns biogas into renewable natural gas fuel. This fuel is not only renewable, it be used for cars and trucks and is half the price of diesel, suggesting huge potential shifts in transportation fuels in the near future. The facility costs only $10,000 to construct, and the EPA estimates that it has potential yearly revenue of $567,284. Dairies are also researching the creation of potable water from dairy digesters. With massive revenues and climate change mitigation, dairy digesters are steer-ing dairy farms in the right direction
Want to see more on turning poop into power? Check out our Solutions Stories Four Generations of Green video:
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