This is because dairy and swine manure management systems are often liquid or slurry based, which simplifies the necessary manure movement. Also, poultry manures contain a higher concentration of fine solids that can quickly fall out of suspension unless continuously agitated. If not kept in suspension, these solids can quickly reduce reactor volume and its ability to produce biogas.
The nation's first farm-based digester was initiated as a result of a now familiar problem--urban encroachment. The McCabe Farm built most of its hog production facilities between 1951-1953 on a rural site outside of the town of Mt. Pleasant, Iowa. By 1970, the town had expanded to the farm's border, and the McCabe family had to develop an odor-free system of managing swine manure. Initially, the McCabes converted their anaerobic lagoon into an aerobic system by adding an aerator. However, the buildup of organic matter over the winter took 6-8 weeks to stabilize in the spring, during which time a significant odor problem developed. Chemicals were added to the aerobic lagoon in early spring one year, and it helped control odors but did not eliminate them.
A new system was needed that would deodorize the manure all year and allow it to be spread according to the farm's schedule during good weather. With the assistance of the County Extension Service and others, Harold "Wiz" McCabe found what appeared to be a satisfactory solution in a theoretical article describing the AD of swine manure. The process promised to provide a gas that could be easily disposed of and would produce a stabilized sludge that could be spread anywhere. "Wiz" was an innovator and master mechanic, and he took a crash course in the design and construction of a complete-mix anaerobic digester. It took 2 years to locate and install the reactor, fabricate heat exchangers from old dairy equipment, convert an old dairy 10-horsepower upright boiler to operate on both biogas and propane, install the necessary control and safety equipment, and put all the pieces together. In early May 1972, the digester was seeded with 6,000 gallons (gal) of sludge from the town's municipal waste digester and two hours of manure flow from the swine facility. Over the next few days, digester seeding continued on a planned schedule. On 10 May, the fifth day after digester inoculation, excess biogas tripped a relief valve and the frrst farm-based digester in the United States carne to life.
During the energy crises of the mid- and late 1970s, the search for alternative energy resources Jed to investigation of small- and medium-scale anaerobic digesters developed in India and China to determine whether these technologies were directly transferable to farms in the United States. Unfortunately, although these technologies are useful in providing fuel for cooking and lighting in developing economies, most are much too small to be useful to most American farmers. For example, the typical small-scale digester daily produces about the same amount of energy as contained in 1 gal of propane.1
The greater energy requirements of the larger American livestock operations Jed to the design and installation of several demonstration projects that transferred state-of-the-art sewage treatment plant technology to the farm. Although complete-mix digesters can operate in the thermophilic temperature range, the demonstration projects at facilities such as the Washington State Dairy Farm in Monroe2 operated only in the mesophilic temperature range. At the Monroe project the digester was sized for the manure volume produced by a milking herd of 180-200 Holstein cows.
Although these frrst-generation complete-mix digesters generally produced biogas at the target design rate, they suffered from high capital costs and significant O&M requirements. In practical application on the farm, solids settling, scum formation, and grit removal often presented major problems.
Today's complete-mix digesters can handle manures with TS concentrations of 3%-10%, and generally can handle substantial manure volumes. The reactor is a large, vertical, poured concrete or steel circular container. The manure is collected in a mixing pit by either a gravity-flow or pump system. If needed, the TS concentration can be diluted, and the manure can be preheated before it is introduced to the digester reactor. The manure is deliberately mixed within the digester reactor. The mixing process creates a homogeneous substrate that prevents the formation of a surface crust and keeps solids in suspension. Mixing and heating improve digester efficiency. Complete-mix digesters operate at either the mesophilic or thermophilic temperatures range. with a HRT as brief as 10-20 days.
A fixed cover is placed over the complete-mix digester to maintain anaerobic conditions and to trap the methane-rich biogas that is produced. The methane is removed from the digester, processed, and transported to the site of end-use application. The most common application for methane produced by the digestion A modern dairy farm complete-mix digester that also recovers valuable fiber process is electricity generation using a modified internal combustion engine. Both the digester and the mixing pit are heated with waste heat from the engine cooling system. Complete-mix digester volumes range considerably from about 3,500-70,000 cubic feet (ft3) This represents daily capacities of about 25,000-500,000 gal of manure/digester. Larger volumes are usually handled by multiple digesters.
By the late-1970s researchers at Cornell University3 were able to reduce the capital costs and the operational complexities associated with the early complete-mix digesters by using a simple extension of Asian AD technology.
These "plug-flow" digesters were adopted with some success in the cooler climate of the Northeast, where dairy farms primarily use scraping systems for manure removal. The 1979 project at the Mason Dixon Dairy Farms in Gettysburg, Pennsylvania, was the first plug-flow digester operated on a commercial farm. At the Mason Dixon project, the plug-flow digester was originally sized for a manure volume produced by a milking herd of 600 Holstein cows.
The basic plug-flow digester design is a long linear trough, often built below ground level, with an air-tight expandable cover. Manure is collected daily and added to one end of the trough. Each day a new "plug" of manure is added, slowly pushing the other manure down the trough. The size of the plug-flow system is determined by the size of the daily "plug." As the manure progresses through the trough, it decomposes and produces methane that is trapped in the expandable cover. To protect the flexible cover and maintain optimal temperatures, some plug-flow digesters are enclosed in simple greenhouses or insulated with a fiberglass blanket. Plug-flow digesters usually operate at the mesophilic temperature range, with a HRT from 20-30 days. An often vital component of a plug-flow digester is the mixing pit, which allows the TS concentration of the manure to be adjusted to a range of 11%-13% by dilution with water. Many systems use a mixing pit with a capacity roughly equal to 1 day's manure output to store manure before adding it to the digester.
Jhe complete-mix and plug-flow digestion technologies are not suited for use on farms that use hydraulic flushing systems to remove manure and anaerobic lagoons to treat waste. Hydraulic flushing substantially dilutes the manure, with TS concentrations often far less than 3%. An anaerobic lagoon is a popular method used to treat and store manure. A properly designed and operated anaerobic lagoon system, in which the HRT exceeds 60 days, may produce significant quantities of methane.
In the early 1980s, the concept of using a floating cover to collect biogas as it escapes from the surface of an anaerobic lagoon was transferred from industry to the·farm. The first successful floating cover that recovered biogas from an anaerobic lagoon operating in the psychrophilic range was sponsored by the California Energy Commission at the Royal Farm operation in Tulare, California.4 The Royal Farm's digester used the manure from a I, 600-sow farrow-to-finish farm.
The North Carolina Energy Division and North Carolina State University constructed the first full-scale covered anaerobic lagoon digester on the east coast at the Randleigh Dairy in 1988.5 The digester processed the manure from 150 dairy cows. The project used funds provided by the Southeast Regional Biomass Energy Program (RBEP), the North Carolina Agricultural Research Service, and the North Carolina Dairy Foundation. The project objective was to educate dairy producers through practical demoustration and outreach about the merits of a low-cost and easily maintained digester suitable for use on farms using hydraulic flush manure management systems. The project provided information about the amount ofbiogas that can be recovered, along with cost information from which the economic merit of the technology can be evaluated.
The methane produced in an anaerobic lagoon is captured by placing a floating, impermeable cover over the lagoon. The. cover is constructed of an industrial fabric that rests on solid floats laid on the surface of the lagoon. The cover can be placed over the entire lagoon or over the part that produces the most methane.
Once the cover is installed, the methane produced under the covered area of the lagoon is trapped. The biogas is harvested using with a collection manifold, such as a long perforated pipe, that is placed under the cover along the sealed edge of the lagoon. Methane is removed by the pull of a slight vacuum on the collection manifold (by connecting a suction blower to the end of the pipe) that draws the collected biogas out from under the cover and on to the end-use application.
The cover is held in position with ropes and anchored by a concrete footing along the edge of the lagoon. Where the cover attaches to the edge of the lagoon, an air-tight seal is constructed by placing a sheet of the cover material over the lagoon bank and down several feet into the lagoon, and clamping the cover (with the footing) onto the sealed bank. Seals are formed on the remaining edges with a weighted curtain of material that hangs vertically from the edge of the floating cover into the lagoon.
- Volunteers in Technical Assistance (1979). Design and Construction of a Three-Meter Anaerobic Digester. Mt Ranier, MD: VITA.
- Coppinger, E. et al. ( 1980). "Economics and Operational Experience of a Full-Scale Anaerobic Daily Manure Digester." Chapter 6 in Biogas and Alcohol Fuels Production. Emmaus, PA: The JG Press.
- Jewell, W. et al. (1979) "Low Cost Methane Generation on Small Fanns." Third Annual Symposium on Biomass Energy Systems. Golden, CO: Solar Energy Research Institute.
- Chandler, J. et al. (1983) "A Low-Cost 75-kW Covered Lagoon Biogas System." Paper at Energy from Biomass and Wastes VII, Lake Buena Vista, FL.
- Safley, L.M.; Lusk, P. (1990). Low Temperature Anaerobic Digester. Raleigh, NC: Department of Corrunerce, Energy Division.