COMPLETE MIX DIGESTERS
The greater energy requirements of the larger American livestock operations led 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 Monroe 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 to 200 Holstein cows. Although these first-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 (total solids) concentrations of three to ten percent, 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 hydraulic return time (HRT) as brief as ten to 20 days. A fixed cover is placed over the complete-mix digester to maintain anaerobic conditions and to trap the methane-rich biogas 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 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 to 70,000 cubic feet (ft3). This represents daily capacities of about 25,000-500,000 gallons of manure/digester. Larger volumes are usually handled by multiple digesters.
PLUG FLOW EVOLUTION
By the late-1970s, Bill Jewell and his research colleagues at Cornell University 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 a the mesophilic temperature range, with a HRT from 20 to 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 to 13 percent by dilution with water. Many systems use a mixing pit with a capacity roughly equal to one day’s manure output to store manure before adding it to the digester.
COVERED ANAEROBIC LAGOONS
The 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 three percent. 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 captured 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. The Royal Farm’s digester used the manure from a l,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 Rand-leigh Dairy in 1988. The digester processed the manure from 150 dairy cows. The project objective was to educate dairy producers through practical demonstration 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 of biogas 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. 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 covered lagoon digester has several merits. First, it has good potential for widespread adoption in the United States, especially in the southeast and southwest regions, because many dairy and swine facilities use hydraulic flushing to collect manure and anaerobic lagoons to treat waste. Second, covered lagoon digester O&M is simple and straightforward compared to complete-mix and plug-flow digesters. Third, the capital costs for this type of digester can be less than those required for the complete-mix and plug-flow types of conventional digesters.
One digester type that might be considered by some to be a variant of the plug-flow digester is a slurry-based system. Unlike plug-flow systems that are used only on dairy farms and that require manure TS concentrations of 11 to 13 percent, slurry-based digestion systems can operate with much lower solids concentrations and can be used to treat a variety of animal manures. Slurry systems require no mechanical mixing and are often found as silo-type reactors or in a loop or horseshoe configuration. Several operators believe the slurry design can enable greater convective currents in the digester, thereby helping to avoid the solids crusting problem commonly associated with the plug-flow design when TS concentrations fall below design parameters.
REACTIONS AND FUTURE TRENDS
None of the farmers surveyed for the Casebook who had an operating anaerobic digester said that they regret their basic decisions. Most would have preferred to spend less money on design and installation, but they are unsure exactly how costs could have been cut. Many seek new ways to increase profitability by selling coproducts, primarily the digested solids. Conversion of agricultural residuals — animal manures in particular — into a renewable energy resource has been the focus of intensive research for more than two decades. Much has been learned about how manure and other feedstocks can be used as an energy and nutrient source. Several available digester systems are both cost-effective and easily managed. Not only will farmers benefit monetarily, AD will also help mitigate animal manure’s contribution to air, surface and groundwater pollution. Extending the AD process to recover methane has considerable potential beyond the farm to other industries with a waste stream characterization similar to livestock manures. Example industries include processors of milk, meat, food, fiber and pharmaceuticals. Future reports in this series of articles on anaerobic digestion will describe its applications for commercial and municipal waste streams throughout the world. By Phil Lusk.