Anaerobic manure digesters, also known as methane digesters, collect manure and convert the energy stored in its organic matter into biogas. This gas consists primarily of methane, carbon dioxide and other trace gases. Anaerobic digestion is a biochemical degradation process in which organic matter is decomposed by bacteria in the absence of oxygen. The digester must be airtight for anaerobic digestion to occur.

The methane is used to produce gas or electric energy for on- or off-farm use. In general, manure for digesters should have a solids concentration of about 14 percent or less. Also, the manure should not contain soil, sand, stones or fibrous bedding materials, although it can be processed to remove those materials.

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Animal wastes, dairy manure in particular, can be ideal feedstocks for anaerobic digesters. Food processing wastes and wastewater often can be excellent feedstocks to add to the manure, but the digester must be designed or adjusted for it. Food wastes typically have high energy content because of their high ratios of volatile to total solids. Cornell University research found that cheese whey, for example, with its high percentage of organic matter, is more purified than animal manure. The methane potential varies, but the methane content of biogas with food waste is typically higher.

Anaerobic digesters yield numerous environmental benefits. Odors are vastly reduced. Flies are diminished. Greenhouse gas emissions are limited. Due to heat, the potential for pathogens to enter the surface or groundwater when spread on crops is considerably reduced, thereby improving water quality. Manure management is easier. Also, the effluent from the digester for crop use is richer in nutrients and has fewer weed seeds. Animal bedding can be reused. The heat and/or electricity generated saves money for the farm. Plus, the excess energy can be sold, producing income for the farm. Handling wastes from other entities, such as other farms and food processors, also can be an income source.

Cost is the biggest disadvantage. Anaerobic digesters are so expensive that feasibility studies are typically recommended prior to any serious consideration. In addition, the design must be suitable for the farm’s particular operation and activities, management skills, future plans, geographic location, availability of funding and other factors. Government, private and university sources urge technical research before undertaking an anaerobic digester project.

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Experts advise talking with owners and operators who have tried or currently use a digester. Patrick Topper, who manages the poultry manure gasification plant at Gettysburg Energy & Recovery Facility in York Springs, Pennsylvania, said to view a digester as manure management. Odor reduction is a primary reason for digesting manure. Keep in mind that digesters do not make manure disappear – the same nutrients are retained. Topper advises farmers contemplating a manure digester to not think of the digester as a power plant. “You’re not going to make much electric energy,” he said. Topper strongly urges a power purchase agreement with the power company. Hiring knowledgeable people is critical, he added, noting that early failures of the digesters in the ’70s and ’80s were caused by people dabbling in technology without expertise.

The first digester for a swine operation nationally was started at Rocky Knoll Farms in Lancaster, Pennsylvania, by Harlan Keener, now deceased. His widow, Shirley, recalls that he was very much involved in researching the technology with extensive travel overseas, including China. She reports that their farm was very satisfied with the digester. “It kept down odors,” she said. Rocky Knoll also added waste milk products from several dairy processing plants to the digester. They sold their excess energy to the grid.

Mike Brendle had 80,000 laying hens when he operated Brendle’s Egg Farm near Somerset, Pennsylvania. He advocates getting help from experienced people and recommends a digester if handling the liquid effluent is feasible. The limestone in feed created sediment, which demanded maintenance. But odor and flies were decimated, and he saved $2,000 per month on electricity costs.

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The digester at Oregon Dairy Farms in Lititz, Pennsylvania, owned by George Hurst since 1985, was expanded in 2010. “It works well for us,” he said. It produces energy for the farm’s 435 milking Holsteins operation, their full-line grocery store, restaurant, gift shop and farm buildings. They added another tank and a larger engine in 2010. His partner-son monitors the computerized controls and changes oil every couple of weeks. Grit from the cows’ hooves has required maintenance to drain every two years. Food waste from a beef processing plant nearby is added to the digester. A big advantage is significantly less odor in the lagoons.

The generator at Penn England Farm, Williamsburg, Pennsylvania, has produced electricity and heat for the dairy operation since 2006. Members of the England Farm family researched and visited digester systems in California, New York and Wisconsin as well as Pennsylvania for several years prior to initiating their manure digester project.

Environmental benefits

The Environmental Protection Agency (EPA) estimates the benefits from digesters of avoiding greenhouse emissions in 2014 to be 3 million metric tons of carbon dioxide equivalent. Plus, the EPA’s estimate of energy generated during 2014 was 948 million kilowatt hours equivalent.

Of the 247 anaerobic digesters operating on livestock farms, 202 accept dairy manure, 39 hog manure, eight beef manure, seven poultry manure and eight mixed. (Some of the 247 accept more than one animal type.)

However, the EPA’s AgSTAR program estimates that biogas recovery systems are technically feasible at over 8,000 large dairy and hog operations. Potentially, those farms could generate more than 13 million megawatt-hours of energy annually and displace about 1,670 megawatts of fossil fuel-fired generation.

New technologies have spurred growth. But, although technically feasible, economic feasibility varies. In the EPA’s assessment of the market potential for biogas energy projects at dairy and hog farms, the agency estimates twice as many swine operations as feasible candidates than dairy farms. Of the top 10 states for electricity generating potential from manure biogas recovery systems in dairy farms, the EPA includes over 100 dairy operations in New York state.

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Of the different technologies used in 2015, 42 percent used a plug flow design system, 38 percent used a complete mix and 15 percent used a covered lagoon. The AgStar Anaerobic Digester Database on the website, http://www.epa.gov/agstar, identifies the design features of U.S. livestock farms, and importantly, features stories from the farms that highlight operator experiences. This data can help a prospective farmer evaluate different design systems for his own operation.

While recovered biogas can generate electricity to fuel boilers, and create pipeline quality gas or compressed natural gas that can be used as fuel, the most frequent usage is electricity generation or combined heat and power.

Information sources

As noted, anaerobic digesters are costly. Recognizing the numerous benefits to the environment, both federal and state programs assist with information and financial incentives. The EPA’s AgSTAR site has links to a wealth of data. The Natural Resources Conservation Service (NRCS) national website, http://www.nrcs.usda.gov, has links to financial assistance and technical assistance.

The AgSTAR site includes highly descriptive case studies. Many were prepared by Topper.

The Database of State Incentives for Renewable Energy (DSIRE), searchable by ZIP code at http://www.dsireusa.org, presents state, local, utility and selected federal incentives for renewable energy. The list specifies when it was last updated, which can be valuable, for example, noting that the Commonwealth of Pennsylvania has been changing its net metering regulation.

The National Conference of State Legislatures database on renewable portfolio standards (RPS), http://www.ncsl.org/research/energy/renewable-portfolio-standards, reports the requirements for utilities. New York, New England and Pennsylvania have created RPS to stimulate market demand.

Feed-in tariffs encourage renewable electricity by mandating a set price for providing energy to the grid. Maine, Rhode Island and Vermont have this measure.

Washington State University has scheduled a webinar series on anaerobic digestion from February through April. The topics include dairy nutrient recovery technologies, biochar production potential, agronomic evaluation of recovered fertilizers, enterprise budget calculator and a decision support tool for gaseous emissions and nutrient management. To register, visit http://www.csanr.wsu.edu/webinars/anaerobic-digestion/signup.

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