What is Biogas?
Biogas is a mixture of methane (CH₄), carbon dioxide (CO₂), and depending on the feedstock used, trace gases such as nitrogen, ammonia (NH₃), sulfur dioxide (SO₂), hydrogen sulfide (H2_S), and hydrogen. Biogas is produced when certain bacteria decompose biological matter in an anaerobic (no oxygen is present) environment. This process is referred to as anaerobic digestion (AD).

AD is a proven technology for effectively treating the organic fraction of waste. The treatment of waste through AD has many benefits; it leads not only to a cleaner and healthier environment, but also produces a renewable energy source, methane.

SERC Conducts a Feasibility Study on Anaerobic Digestion Technology for Humboldt County Dairy Farms

Dairy cows at pasture on a Humboldt County dairy farm

In the spring of 2002 SERC began a yearlong biogas study funded by the State of California Community Development Block Grant # 01-EDBG-782. The purpose of the study was to consider the feasibility of implementing anaerobic digestion (AD) technology as a means of dairy manure management on Humboldt County dairy farms. The following is an excerpt from the final report, submitted to the Humboldt County Economic Development Office.

Executive Summary
Current trends in milk production have forced dairies to intensify their operations. The larger herd numbers required by today's dairy operators in order to stay in business have led directly to an increase in manure production. The volume of manure has become a social and environmental issue. Dairies also consume significant amounts of energy in their daily operation. Anaerobic digestion of manure is a promising technology that has been shown to effectively address many of the problems associated with manure management while providing a reliable energy resource. AD technology has the ability to offer substantial benefits to dairy operators. In many cases, without the implementation of AD technology on U.S. farms, many farmers would have been forced to cease their operation.

There are a variety of AD systems used on U.S. farms, including covered lagoon digesters, complete mix digesters, and plug flow digesters. Choosing the appropriate system depends on many factors including local weather conditions, local water tables, manure collection technique, manure storage capacity, and end use of AD products. The U.S. Environmental Protection Agency's AgStar Handbook offers five preliminary screening questions that should be considered to determine whether AD technology is a suitable manure management technique for Humboldt County. Based on the screening questions, it would appear that many Humboldt County pasture-based dairies are not currently ready to consider implementing AD technology as part of their manure management plan. Some of the larger dairies in the county may be able to benefit from a plug flow AD system. In order to conduct an analysis, a suitable Humboldt County pasture-based dairy is defined. In summary, the dairy would have at least 400 cows that are housed in freestall barns for a portion of the year. A more extensive description of these requirements are listed on page 9. A dairy meeting these criteria could consider a plug flow digester as part of their manure management plan, as long as benefits besides energy production can be realized.

A digester on such a Humboldt County dairy could produce between 4 million and 6.4 million cubic feet of biogas and 124,000 to 198,000 kilowatt-hours of electricity annually, leading to potential annual avoided electrical costs of $11,100 to $23,800. Valuable thermal energy for water or space heating could be recovered from the engine-generator set displacing between 3,400 and 5,500 therms of natural gas per year, leading to a potential annual savings of $2,800 to $4,500 on heating costs. The digester would also produce enough fiber each year for an additional annual income of $4,800 to $8,000. Manure pit maintenance costs could be reduced by $9,000 per year. In summary, anaerobic digester products and avoided operation and maintenance (O&M) on current manure storage systems have a capacity to generate up to $45,000 for a suitable Humboldt County dairy.

One of the main issues discouraging dairy operators from utilizing AD technology in their manure management plan is the high capital investment necessary for installing the system. A plug flow system for a suitable Humboldt dairy would cost between $500 and $1,000 per cow, leading to an installed cost of $200,000 to $400,000 for the complete system. Expected operations and maintenance costs of an AD project may be another deterrent. Based on costs reported for existing systems, O&M costs for a typical Humboldt County digester designed to handle the manure from 400 dairy cows are estimated to be $10,000 to $20,000 annually.

In the best case scenario, an AD system installed on a Humboldt County dairy without funding assistance has an economic payback period of approximately 5.6 years. This length of time is often considered too long for a project to be considered viable solely on an economic basis. In the worst case scenario, the project will not pay for itself during the system's expected lifetime. Without significant financial assistance, AD systems are only a viable option for Humboldt County dairies whose operators place significant value on intangible benefits such as odor control, decrease in fly populations, and environmental stewardship.

Fortunately, there are many programs designed to help dairy operators fund and install AD systems on their farms. With these types of assistance, implementation of AD technology on a suitable Humboldt County dairy would become more feasible than previously indicated. The best case payback period is reduced to 2.6 years with outside financial assistance. The Dairy Power Production Program (DPPP) offers up to 50% of project capital cost or up to $2,000/kW capacity. The remaining portion of the project cost could be partially offset with PG&E's Self-Generation Incentive Program (SGIP) that would pay a rate of $1,500/kW or up to 40% of the projects' capital cost. The available financial assistance can significantly decrease the simple payback period of an AD system installed on a suitable Humboldt County dairy. These results, combined with the project's intangible benefits, could make AD technology a viable option for a local dairy's manure management system. The USDA's Rural Development Program also offers funding for purchase of renewable energy systems, including biogas generators, by agricultural producers.

Anaerobic digestion is a technology that has been shown to effectively manage dairy manure while yielding resources with significant financial and intangible value. The smaller pasture-based dairies of Humboldt County initially appear to be ill-suited for the implementation of this technology, but under the right circumstances, a suitable Humboldt County dairy could effectively use a plug flow digester as part of their manure management plan.