February 17, 2009 | General

Historical Perspective: Farm Digesters

BioCycle February 2009, Vol. 50, No. 2, p. 30
After various fits and starts over the past five decades, anaerobic digestion of manure is well-established and poised to grow.
Nora Goldstein

THE history of BioCycle’s coverage of anaerobic digestion can be traced back to its inaugural issue in Spring 1960, when the journal, originally named Compost Science, published “Composting Manure by Anaerobic Methods.” Dr. Clarence G. Golueke of the Sanitary Engineering Research Laboratory at the University of California, Berkeley – and a member of the founding Editorial Board – wrote the article, concluding that “reduction to a stable humus by anaerobic digestion seems to offer great promise for a relatively inexpensive and yet completely sanitary method of disposing of manure.”
Golueke described a simple process of “dumping or flushing all wastes into an airtight container in which bacteria can break down the organic matter to form humus and a combustible gas. The procedure provides sanitary treatment of organic wastes and results in a great reduction in flies. It also makes possible the efficient and economical recovery of some of the waste carbon as methane for fuel. It produces humus and nutrients for use on soils. Moreover, both liquid and solid wastes may be treated in one operation. Practicality of the process has been demonstrated by its successful use on European farms.”
Golueke noted that the “evolved gas is approximately two-thirds methane and one-third carbon dioxide,” which is pretty standard in the literature. He continued: “Thirty-five cubic feet of the gas compares to one quart of alcohol, 52.5 cubic feet of manufactured city gas, or 2.2 kilowatt hours (kWh) of electrical energy. The humus remaining after digestion is comparable to that obtained from digesting sewage sludge. It has a nitrogen content varying from one to two percent by dry weight…. Digester size and number will depend on the quantity of wastes available. For example, 1,400 pounds of cow manure without bedding and with normal moisture will require approximately one cubic yard of digester space. Space requirements for an equivalent manure naturally will increase according to the amount of bedding used.”
Clearly, the widespread adoption of concentrated animal feeding operations (CAFOs) was not on the radar screen at the time. “In estimating the volume of digester space required, it should be remembered that only a portion of the daily output of the cattle on a dairy farm will be available for the digesting plant. Some will be lost on the fields when the cattle are grazing.”
Figure 1 was included in the article. It shows a section of a manure digester with a floating cover serving as a gas holder; dimension depended on the volume of the manure to be treated. Concluded Golueke: “The initial cost of a digester plant may seem rather high. When this cost is amortized over a period of years, it will be found that such a plant will provide a cheap and effective means of treating farm wastes and controlling fly production as well as serving as an inexpensive source of fuel. The operating and maintenance costs are relatively insignificant. Loading and removal of the material is a matter of labor, part of which would be expended in the normal hauling of manure from barns to stack. Moreover, the effort required would be less than that needed to transport the untreated manure from the vicinity of the barns to a suitable storage site.”
Issues of Compost Science from the early 1960s through the mid-1970s contained a steady dose of articles and research reports on farm digesters (as well as a handful discussing digestion of municipal solid waste streams). In the January/February 1974 issue, a headline read, “Methane Digester Draws Farm Show Crowd.” The item described a methane gas digester designed and built by two staff members of the Pennsylvania University Cooperative Extension Service, which drew crowds of interested spectators when it was displayed at the “Agriculture Progress Days” in Hershey, Pennsylvania. The small digester, which produced about 2.5 cubic feet/day of methane from 30 gallons of animal manure and crop wastes, was built as a pilot project to show the public how organic wastes can be put to use.
Noted Dr. Don Harter, chair of the exhibit, the purpose of the digester was to “demonstrate the energy-producing potential of agricultural wastes, to encourage ‘futuristic’ interest in anaerobic digesters as part of a farm’s system for holding and disposing of manure, and as a reminder that the use of methane fuel can contribute to environmental improvement by helping to alleviate air pollution.” He projected that a system could be designed and built for $5,000 to $10,000 to digest manure from 30 to 40 cows/day, with a payback on the investment in five years.
A report in the July/August 1978 issue described an anaerobic digester developed by scientists at the University of Minnesota and installed at the Verlo Larson Farm in Anoka County, Minnesota. It had been operating since 1976 to treat manure from a 200-head swine unit. System components included a waste collection tank, digester tank, gas collection unit and generator, lagoon and series of pumps. “A total waste management system utilizing an anaerobic digester can provide pollution control, odor control, nutrient conservation as well as produce some energy for use on the farmstead,” explained Phillip Goodrich, an agricultural engineer at the university. “The primary objective of our research with the digester is to demonstrate the feasibility of integrating a methane-powered electrical generator into an electric distribution system on a Minnesota farm.”
In 1979, Mason Dixon Farms began operating what is thought to be the first plug flow digester at a commercial farm. “Energy self-sufficiency through methane production is what’s happening down on Mason Dixon Farms,” began the news item in the March/April 1980 issue of Compost Science. “Located near Gettysburg, Pennsylvania, this 2,675-acre dairy farm has facilities to convert manure from a herd of 1,700 dairy cows into methane gas which in turn provides fuel to produce electricity for the farm’s own generator.”

The “power-generating concept” utilized a system marketed as Energy Harvester, designed by Sheaffer & Roland, Inc., a Chicago engineering and consulting firm. Continued the Compost Science report: “The Energy Harvester delivers 13,000 gallons/day of liquefied manure to a large tank facility with a plastic bubble that collects and retains methane as the manure is heated to 100°F. The manure is liquefied as a result of twice-daily flushing of Mason-Dixon Farms’ large cattle barn floors. The water used in this flushing process is eventually separated from the solids, clarified and reused for continued flushing of the cattle barn floors. Conversion of methane is accomplished by a diesel engine system that starts up on diesel until its exhaust heat raises the temperature of the manure to 100°F. Then, with production of methane gas, the engine automatically reduces its use of diesel fuel and begins utilizing methane trapped in the plastic bubble.”

Fred Roland, who was a partner in Sheaffer & Roland, Inc., says that his company built about 10 digesters during that time period. “We used a very simple plug flow design that had the single purpose of using the manure as an energy source,” says Roland, now with Agresource, Inc. in Massachusetts. “It essentially was a lagoon concept with a floating cover, and flow through of the manure.”
Sheaffer & Roland hired Mark Moser, now with RCM International in California, to work on the digester it had sold to Mason-Dixon Farms. “I had just finished graduate studies at Cornell University, where I had done research on anaerobic digestion with Professor Bill Jewel,” recalls Moser. “As a term assignment, Dr. Jewel gave us his gear – two Lucite tube digesters – and gave us the operating parameters. It gave me experience working with farm digestion and animal waste management. I supervised the start-up of the Mason-Dixon installation, and then did the troubleshooting because there was trouble to be shot.” Eventually, Moser started his own company. “I realized there was an awful lot of unknowns about how to make digesters work,” he adds. “I learned in the field. A key lesson from all my years of experience is that you need to make it work for the farm. We have a half-dozen different designs and we change what we will do or what it will look like depending on how the farm is operating. It needs to be an integral part of the farm’s business plan.”

Roland recalls the early 1980s as a time that “we made farmers believe – we convinced them – that their manure was gold,” he says. “It was the future energy source of America. A couple of things happened to spur construction of anaerobic digesters. There were some grant monies, and some states had incentives that if a farm built a digester, they would get 8 to 9 cents/kWh for the electricity. A lot of digesters were built with that idea and revenue stream. They installed engine generators. Farmers could go to the bank to get a loan to put in a system because their land also was gold. Then, two to three years down the road, the premium rate paid by utilities ended and the electricity market bottomed out. The AD boom pretty much ended by the late 1980s.”
The federal government, primarily through the U.S. Department of Agriculture (USDA) and its various equipment grants, and state energy and agricultural agencies have supported development of farm digestion systems over the past 20 to 25 years. In 1994, the U.S. Environmental Protection Agency, in collaboration with USDA and the U.S. Department of Energy, established the AgStar program specifically to encourage the use of methane recovery (biogas) technologies at CAFOs that manage manure as liquids or slurries.
Since AgStar began 15 years ago, the number of operational digester systems has grown to 121 systems across the United States, yielding approximately 256,000 MWh equivalent of energy generation. According to a December 2008 AgStar report, these include 93 installations on dairy farms and 20 at swine operations. In 108 of the 121 operational systems, the captured biogas is used to generate electricity, with many farms recovering waste heat for on-farm use. The remaining 13 systems use the gas in boilers, upgrade the gas for injection into the natural gas pipeline or flare it.
One of the early supporters of farm digesters was the California Energy Commission, which helped fund development of biogas/biomass technologies through a series of initiatives including the Biomass Demonstration Program (BDP), Energy Technologies Advancement Program (ETAP) and, most recently, the Public Interest Energy Research (PIER) Program. “The BDP (1979-1984) was a result of the Senate Bill 771, the State Agricultural and Forestry Residue Utilization Act of 1979,” says Zhiqin Zhang, who is in CEC’s Energy Research and Development Division. “The program goal was to promote the immediate development and implementation of residue conversion as an energy-generating technology by providing funds to encourage development and demonstration.”
The ETAP (1985-1996) was a result of the Rosenthal/Naylor Act to make energy technologies more efficient and cost-effective and to develop new alternative sources of energy. The current PIER Program, started in 1997, was established by California AB1892 and SB90 to support energy-related research including the relevant core subject areas of environmental enhancements, end use efficiency, environmentally preferred advanced generation technologies, renewable technologies and other strategic energy research. Funding for anaerobic digestion projects in the state is provided through this program, as is research.
Despite this support, only a handful of California farms employ anaerobic digesters. “There are over 1,900 dairies but only 12 dairies installed digesters,” says Zhang. “Most of the systems installed in California used traditional designs such as covered lagoon, plug flow and complete mix digesters, and internal combustion engines.” While growth in the number of installations is small, Zhang adds that existing centralized and on-site digesters in California “have set the stage” for adoption of digesters by farms in California. “We learned that greater collaboration is required among dairy operators, utilities, permitting agencies and funding and financing authorities to encourage efficient plant operation and resolve existing issues on air and water emission analysis and permits, net metering (such as paying the farmer both energy and demand charges for excess energy delivered), setting reasonable renewable energy prices and carbon credits,” she says.
Zhang highlights some research questions that still need to be addressed to help in future industry development. Areas include feedstock logistics, to reduce costs of handling; digester system, to optimize gas production; engine-generator, to increase operating time, net power production and reduce emissions; pollution control to meet air and water quality standards; gas clean-up equipment, to extend equipment life and provide greater flexibility in engine exhaust after-treatment, to reduce air emissions from power generation, and to improve gas upgrading for pipeline injection.
Focus On Energy in Wisconsin provides research grants, technical assistance and financial incentives to help implement renewable energy projects, including farm digester systems. “Until a few years ago, people told us they were interested in farm digesters as a means of nutrient management,” says Larry Krom, the bioenergy and large wind energy project manager for Focus on Energy’s Renewable Energy Program. “Over the last couple of years, having another revenue stream from energy production started to gain equal footing with nutrient management.”
Krom started the anaerobic digester component of Focus on Energy in 2000. Working with digester equipment providers and energy markets has provided valuable insights into critical factors for program success. “First, keep it simple,” he says. “Complicated systems are difficult to control unless there is someone on the farm who is very knowledgeable. Second, select an engine generator that is designed for biogas that has modern air fuel mixture controls. Third, have a clear understanding of the technology needed to fractionate the effluent stream. There are two drivers here – one is energy production and the other is nutrient management. Anaerobic digestion doesn’t make nutrients disappear; it changes the form from organic to inorganic.”
More research is needed to determine how a digestion system can be integrated into overall farm operations, he adds. “There is a national protocol for assessing performance of digester systems on farms, but it doesn’t include anything beyond the digester system itself. Farms are incredibly complicated systems so there needs to be a protocol to evaluate how the digester interacts with everything else on the farm. For example, what are the benefits of further fractionation of the effluent for the soil types and agricultural practices?” Krom also says a better understanding is needed of gas clean up systems. “It would be nice to evaluate them in a less complicated setting than a farm. We know hydrogen sulfides have to be removed, and for some engines, removing the water appears to be the issue.”
To improve digester economics, many farms are considering projects that involve the addition of off-farm substrates, such as food processing residuals and FOG (fats, oils and grease). “Right now, you really have to look at taking in substrates to make digesters financially feasible – unless a farm needs a digester just to start up a large dairy operation,” says Roland. “You can get about 50 percent of the capital costs covered with grant money, and the rest has to come from revenue streams like substrates and energy sales.”
Key to taking off-farm substrates is having an aggressive mixing system, he adds. “Fats and oils tend to float, so once they get separated out, you get accumulation of hard scum, or a crust, that reduces digester capacity and overall performance.”
DODA USA has seen growing interest in its digester mixers as new and existing projects evaluate substrate additions. DODA, based in Italy, has been developing components for digester operations for many years. “The founder of DODA developed a chopper pump in the 1960s, and then people started asking for other products to address challenges they were having,” says Rich Miller of DODA USA. “Today, our equipment addresses all aspects of digester operations -gathering and homogenizing slurry on the front-end, mixing in the digester, and solids separation on the back end, including dewatering. In Europe, the bulk of our business is in retrofitting existing systems, whereas in North America, we’ve been mostly servicing new operations.”
The company’s experience in Europe led them to recognize that one of the biggest challenges with farm digesters is that they fill up with solids. “There has to be a good mixing system involved, and there have been many different ideas of how to do that, from mixing with gas to installing turbine-type pumps inside the digester,” explains Miller. “The reality is that just using a good bladed type of mixer provides a homogenous mix so that there aren’t dead spots. The key is to have a soft agitation, as you don’t want to bring more air into the digester.”
Effective methods to heat digesters also were gained from European experiences. “Historically, a lot of digesters used grids like radiators that were installed right in the unit,” he adds. “That became a settling place for solids. With trial and error, more effective approaches were developed, such as running coils around the perimeter and heating the floor.”
Today, with the intense focus on developing cost-effective and local sources of renewable energy, increasing attention is being paid to farm digester systems. The experiences of the past 50 years leave little doubt that excellent groundwork has been laid for increased adoption of anaerobic digesters as a tool to manage not just manure, but municipal and industrial organic waste streams as well.

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