Making the Most of Manure
Complaints from odor-offended neighbors and a desire to reduce greenhouse gas emissions have prompted some dairy farmers to integrate anaerobic digestion systems into their operations. Although it's not for everyone, using manure to generate power and produce a nutrient-rich soil amendment is something that should seriously be considered.
The environmental benefits provided by anaerobic digester systems exceed those of conventional liquid and slurry manure management systems that use storage tanks, ponds and lagoons. Dairy farmers and agricultural experts agree that the primary benefits of anaerobic digestion are odor control, improved soil nutrient management and the reduction of greenhouse gas emissions. The process also allows for the capture of methane and carbon dioxide, commonly known as biogas, which can be sold as clean-burning electricity.
It's often the environmental benefits, rather than the digester's electrical and thermal energy generation potential, that motivate most farmers to use digester technology. This is especially true in areas that enjoy low electric power costs.
Although anaerobic digestion can help farmers manage manure it won't fix poor management practices. "Manure does require a certain level of management and a digester is not going to replace poor management," says Amanda Bilek, energy and program associate for The Minnesota Project. "If the management on the farm is good then the management with the digester will remain good." The Minnesota Project is a nonprofit organization founded in 1979 by former U.S. Sen. Mark Dayton, D-Minn. The project is focused on renewable energy, farm practices and policy, and the production and consumption of local and sustainable-produced foods.
Anaerobic digestion occurs when the biomass, such as animal manure, is sealed in an airtight container called a digester. A biochemical process occurs where different species of bacteria digest biomass in an oxygen-free environment at temperatures similar to those in a cow's stomach. The different types of bacteria produce biogas by working together to break down complex organic wastes. "Anaerobic digestion is more akin to a natural process," says Larry Krom, project manager for Wisconsin's Focus on Energy program. "We're essentially working with nature and we're providing an environment to enhance the process to produce more biogas for domestic energy use." Focus on Energy is a statewide organization that helps residents and businesses install cost-effective, energy-efficient and renewable energy projects. The program also works with the U.S. DOE and other grant providers on the regulatory front to, for example, reduce barriers to anaerobic digestion by utilizing better buy-back rates, feasibility grants, business and marketing grants, implementation grants and equipment grants to jump-start projects.
Depending on the waste feedstock and the system design, biogas is typically 60 percent methane, and 40 percent carbon dioxide, water vapor and trace amounts of hydrogen sulfide. The biogas can be used in the form of electricity, steam or heat to reduce natural gas and/or coal consumption. The electricity can also be plugged into local utility grid.
Dennis Haubenschild, owner of a 1,000-head dairy farm in Princeton, Minn., installed an anaerobic digestion system in 1998 and knows first-hand the benefits of a digester. "The environmental advantages of the digester are that every 100 cows produce approximately a barrel of oil equivalent of energy per day," he says. "Agriculture should and could be supplying 50 percent of our domestic energy using the tools that are already available."
The Minnesota Project conducted a case study on Haubenschild's dairy farm in 1998 to quantify the economic and environmental benefits of his anaerobic digester. The most noticeable improvements were the elimination of farm odors and the removal of harmful greenhouse gases, such as methane and carbon dioxide. "It's the methane, which is 21 times more damaging to the atmosphere than carbon dioxide, that we're really trying to utilize in these systems," Krom says. The capturing of hazardous gases for use as domestic energy offsets the environmental impacts of fossil fuel generation, provides clean, renewable domestic power and enables a dairy farm to lower its carbon footprint. The carbon credits that are earned can be sold on the Chicago Climate Exchange, which is something Haubenschild has been doing for two years. "Any time you can lower your carbon footprint, you're doing what you need to do," he says. "My main goal is sustainability. The closer to zero [carbon emissions] the better, then I've achieved my goal."
Anaerobic digestion systems haven't necessarily improved in efficiency or complexity. Today's systems are merely variations of digesters developed years ago. However, there are different types of digester systems. The two types primarily being employed today are the thermophilic and mesophilic systems. A thermophylic system utilizes temperatures of about 130 degrees Fahrenheit, whereas a mesophilic system operates at about 100 degrees Fahrenheit. Before installing an anaerobic digester, the type of system, the specific needs of the operation and the geographic location must be considered, Krom says. "Theoretically you can produce more biogas with thermophilic systems," he says. "However, the heat loss in the northern climate is going to be much greater, especially if an above-ground tank is used. That means you're going to have to increase the amount of insulation and make sure that you have enough tank heating available."
Safeguarding Land and Animal Health
Aside from the environmental benefits, anaerobic digestion systems also provide dairy farmers with a soil amendment and a product that can help them manage their cow herd health.
Once the manure has been processed through the anaerobic digester and biogas is captured for power generation, the digestate can be separated from its solid (organic) and liquid forms. The liquid effluent can be used as a fertilizer. For long-term storage, it can be pumped into a facility similar to a lagoon. The digester effluent is composed mainly of nitrogen, phosphorus and trace amounts of potassium-also called "macro nutrients." Nitrogen is the chief ingredient that enhances crop growth. Although digester effluent is widely used among today's dairy farmers, soil scientists warn that misapplication can severely damage the environment. "I think anaerobic digestion is great but it has some additional management concerns that you need to be aware of when you're spreading these liquids," says Eric Cooley, research coordinator and outreach specialist for Discovery Farms, a Wisconsin-based research organization that takes a real-world approach to finding economical solutions to overcome challenges and environmental regulations placed on farmers. It's important to closely monitor the phosphorus content in the effluent because that's the limiting agent, he says. The nutrient undergoes a slight conversion from its particulate state to its soluble or dissolved state, which is ortho phosphorus, he says. The phosphorus in the ortho or soluble form can cause problems, Cooley says. Phosphorus can run off into water and form algal blooms, which degrade water quality. Applying effluent during the winter is especially dangerous as nitrogen in its organic form is ammonia. Ammonia has positively charged ions that have a high affinity to water and can harm fish because of its high toxicity. Digester effluent can be spread safely on hay ground, however, because hay is an exceptional crop for taking up nutrients like phosphorus, Cooley says.
While the rate of application is crucial, timing is everything when applying manure or digester effluent. "There's always the danger of misapplication whether farmers are applying raw manure or the effluent from an anaerobic digester," Krom says. "In other words, you don't want to apply materials on frozen fields. If you have a very mineralized form of the nutrient, you want to be able to match that with the uptake cycles of the plants to the best of your knowledge."
In addition to enhanced crop management practices, anaerobic digesters can eliminate the flow of lethal microorganisms such as E. coli, fecal streptococcus, Krohn's disease and Johnes disease that if not managed properly can infect cattle. One way to manage diseases is to use the proper bedding materials. The cost of animal bedding material is considerable over a year's time and is the second largest cash flow item for farmers, Krom says. On farms with anaerobic digestion systems, the digestate that's separated from the solid content of the manure can be dried and be used for bedding.
Despite its many benefits, most experts agree that the specific needs of the operation must drive the decision to invest in an anaerobic digestion system. "Anaerobic digestion is going to be great for some people and it can be terrible for others," Cooley says. "It just depends how that system is managed and how that farm is run. There are a lot of different factors that go into whether it's a good idea or not."
Bryan Sims is a Biomass Magazine staff writer. Reach him at firstname.lastname@example.org or (701) 746-8385.