Anaerobic Digestion of Animal Wastes: Factors to Consider
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Abstract
 Turkeys. Photo by Jeff Vanuga, courtesy of USDA/ARS. |
Introduction
 Sow with piglet. Photo by Photo by Jeff Vanuga, courtesy of USDA/ARS. |
While hundreds of anaerobic-digestion systems have been installed in Europe and the U.S. since the 1970s, it was not until the 1990s that better designed, more successful projects started to come on line in the U.S. Today, an estimated 97 farm-scale projects are in operation, in start-up, or under construction on swine, dairy, and poultry farms across the country. (1)
Key by-products of anaerobic digestion include digested solids and liquids, which may be used as soil amendments or liquid fertilizers. Methane, the primary component of "biogas," can be used to fuel a variety of cooking, heating, cooling, and lighting applications, as well as to generate electricity. Capturing and using the methane also precludes its release to the atmosphere, where it has twenty-one times more global warming potential than carbon dioxide. (2)
 Cows and calves. Photo by Lynn Betts, courtesy of USDA/ARS. |
Digestion Process
Anaerobic digestion works in a two-stage process to decompose organic material (i.e., volatile solids) in the absence of oxygen. Biogas is produced as a waste product of digestion. In the first stage, the volatile solids in manure are converted into fatty acids by anaerobic bacteria known as "acid formers." In the second stage, these acids are further converted into biogas by more specialized bacteria known as "methane formers." With proper planning and design, this anaerobic-digestion process, which has been at work in nature for millions of years, can be managed to convert a farm’s waste-stream into an asset.There are several types of anaerobic digesters.
Covered lagoons—A pool of liquid manure topped by a pontoon or other floating cover. Seal plates extend down the sides of the pontoon into the liquid to prevent exposure of the accumulated gas to the atmosphere. Designed to use manure with two percent or less solid content, this type of digester requires high throughput in order for the bacteria to work on enough solids to produce gas. Most frequently used in warmer southern regions, where the atmospheric heat can help maintain digester temperatures, this is the least expensive of all designs to install and operate. About 18 percent of all digesters presently in use in the U.S. are covered-lagoon systems.
Complete mix—A silo-like tank in which the manure is heated and mixed, designed to handle manure with two to ten percent solids. This is the most expensive system to install and operate, but it’s particularly appropriate for operations that wash out manure. About 28 percent of all digesters in use in the U.S. are of this type.
Plug flow—A cylindrical tank in which the gas and other by-products are pushed out one end by new manure being fed into the other end. This design handles 11 to 13 percent solids and typically employs hot-water piping through the tank to maintain the necessary temperature. Most appropriate for livestock operations that remove manure mechanically rather than washing it out, the plug-flow system accounts for more than half of all digesters presently in use.
Fixed film—A tank is filled with a plastic medium that supports a thin film of bacteria called a biofilm. This design handles one to two percent solids, and uses a shorter retention time, as short as two to six days. (5) Only about one percent of systems currently installed in the U.S. are of this type.
Figure 1. Basic components of an anaerobic-digestion system (4) |
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There are also a number of hybrid systems being designed and installed, a strong indication that no single system is right for all or even most situations.
Starting the digestion process is not difficult, but it does require patience. The digester tank is filled with water and then heated to the desired temperature. "Seed" sludge from a municipal sewage treatment plant is then added to 20 to 25 percent of the tank’s volume, followed by gradually increasing amounts of fresh manure over a six to eight-week period until the desired loading rate is reached. Assuming that the temperature within the system remains relatively constant, steady gas production should occur in the fourth week after start-up. The bacteria may require two to three months to multiply to an efficient population. (6)
| Starting the digestion process is not difficult, but it does require patience. |
Temperature within the digester is critical, with maximum conversion occurring at approximately 95°F in conventional mesophilic digesters. For each 20°F decrease in temperature, gas production falls by approximately 50 percent. (7)
Even more significant is the need to keep the temperature steady. Optimal operation occurs when the methane formers use all the acids at approximately the same rate that the acid formers produce them. Variations of as little as 5°F can inhibit methane formers enough to tip the balance of the process and possibly cause system failure. (7)
Temperature is just one of the many important factors in successfully starting and operating an anaerobic-digestion system. (7) The other key factors include:
Loading rate. The system’s design will dictate loading rates and contents, but experience indicates that uniform loading, on a daily basis, of manure with 6 to 10 percent solids generally works best. The load’s retention time in the digester will typically range from 15 to 30 days.
Mixing. The loaded manure needs to be mixed regularly to prevent settling and to maintain contact between the bacteria and the manure. The mixing action also prevents the formation of scum and facilitates release of the biogas.
Nutrients. The best digestion occurs with a carbon to nitrogen ratio between 15:1 and 30:1 (optimally 20:1). Most fresh animal manures fall within this range and require no adjustment. Nutrient imbalance can occur, however, if excessive amounts of exposed feedlot manure become part of the load. Adding crop residues or leaves (both can be heavy in carbon) can improve digester performance.
Management. Anaerobic digesters require regular and frequent monitoring, primarily to maintain a constant desired temperature and to ensure that the system flow is not clogged. Failure to properly manage the digester’s sensitivity to its environment can result in a significant decline in gas production and require months to correct.
Safety. Working with anaerobic digester biogas, and especially with methane (the major component of the gas), warrants extreme caution. Methane, when mixed with air, is highly explosive. In addition, because digester gas is heavier than air, it displaces oxygen near the ground, and if hydrogen sulfide is still present, the gas can act as a deadly poison. It is critical that digester systems be designed with adequate venting to avoid these dangerous situations.
| The most common means of collecting and storing the gas produced by a digester is with a floating cover—a weighted pontoon that floats on the liquid surface of a collection/storage basin. |
Biogas: A Resource Requiring Care
Biogas produced in an anaerobic digester contains methane (60 to 70 percent), carbon dioxide (30 to 40 percent), and various toxic gases, including hydrogen sulfide, ammonia, and sulfur-derived mercaptans. Biogas also typically contains 1 to 2 percent water vapor.Energy Content and Relative Value of Biogas
At roughly 60 percent methane, biogas possesses an energy content of 600 Btu/ft3. For comparison, Table 1 presents the energy content of several other well-known energy sources.Uses of Biogas
Because of the extreme cost and difficulty of liquefying biogas, it is not feasible for use as a tractor fuel. Biogas has many other on-farm applications, however, including virtually anywhere natural gas is used—for cooking, heating (space heating, water heating, grain drying), cooling, and lighting. In most cases, the equipment designed to burn natural gas will require certain modifications to accommodate the slightly different burn characteristics of biogas.Biogas can also be used to fuel generators to produce steam and electricity. In some cases, the electricity can be sold to a local utility, possibly in a net metering arrangement. This option should be explored early, however, to make sure the utility is amenable to such arrangements.
North Carolina State University’s Cooperative Extension Service developed several specific examples of how biogas can be applied on-farm:
1. A well-insulated, three-bedroom home that requires 900,000 Btu/day for heating in cold weather could be served by 50 dairy cattle, 600 hogs, or 7,870 layers (assuming that around 35 percent of the biogas produced will be used to maintain the digester’s temperature).The number of animals required for a digester system to be cost effective depends upon your situation and upon what you wish to get out of the digester. Some dairy operations with as few as 100 cows have installed cost effective digester systems for odor control that also produce digested solids. (8)
2. A dairy using the national average of 550 kWh/cow/year could generate 70 percent of its electrical needs with biogas (assuming 20 percent generator efficiency and that around 35 percent of the biogas produced will be used to maintain the digester’s temperature).
3. A swine operation that uses about 55 kWh of electricity and 5.75 gallons of LP gas per hog per year (including feed mill and incinerator) could supply 40 percent of its energy needs with biogas (assuming 20 percent generator efficiency and that around 35 percent of the biogas produced will be used to maintain the digester’s temperature).
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Refining Biogas into Biomethane
The biogas produced in the methane digester is primarily methane and carbon dioxide, with traces of hydrogen sulfide, and other gasses. Biogas by itself can be used as-is for heating and for cooking. However, use of raw biogas in heating equipment and in internal combustion engines will cause early failures because of the corrosive nature of the hydrogen sulfide and water vapor. Carbon dioxide in the biogas lowers the heating value of the gas. It should be noted that the biogas from the digestion of animal wastes does not have some of the contaminants of biogas from landfills or municipal waste water treatment plants and is therefore easier to clean up.Hydrogen sulfide is corrosive and smelly. It can be removed from the biogas by injecting less than six percent volume of air into the biogas in the gas reservoir, by adding iron chloride to the digester influent stream, or by flowing the biogas through wood chips impregnated with iron oxide (iron sponge) or through activated carbon. Carbon dioxide can be removed by bubbling the biogas though water in a vertical column packed-bed scrubber. Finally, moisture can be removed by flowing the biogas through a refrigerated coil. )
Risks Associated with Biogas
While methane is a very promising energy resource, the non-methane components of biogas (hydrogen sulfide, carbon dioxide, and water vapor) tend to inhibit methane production and, with the exception of the water vapor, are harmful to humans and/or the environment. For these reasons, the biogas produced should be properly "cleaned" using appropriate scrubbing and separation techniques.In addition, the methane itself represents a serious danger, as it is odorless, colorless, and difficult to detect. Methane is also highly explosive if allowed to come into contact with atmospheric air at proportions of 6 to 15 percent methane. For these reasons, it is recommended that buildings be well ventilated; motors, wiring, and lights should be explosion-proof; flame arrestors should be used on gas lines; and alarms and gas-detection devices should be used.
Digester Design Factors
Digesters are installed primarily for economic and/or environmental reasons. Digesters represent a way for the farmer to convert a waste product into an economic asset, while simultaneously solving an environmental problem. Under ideal conditions, an anaerobic-digestion system can convert a livestock operation’s steady accumulation of manure into a fuel for heating or cooling a portion of the farm operation or for further conversion into electricity for sale to a utility. The solids remaining after the digestion process can be used as a soil amendment, applicable on-farm or made available for sale to other markets. Unfortunately, such ideal conditions seldom exist, in part because of faulty planning and design.For anyone considering an anaerobic-digestion system, the single most important point to understand is that each farmer’s situation is unique, and as such, requires careful consideration of many factors. Anaerobic-digestion systems can be quite costly to install, so the owner should fully understand the purpose of the system and its economics.
The size of the system is determined primarily by the number and type of animals served by the operation, the amount of dilution water to be added, and the desired retention time. The most manageable of these factors is retention time; longer retention times mean more complete breakdown of the manure contents, but require a larger tank. Table 4, developed by North Carolina State University’s Cooperative Extension Service, presents one set of recommended loading rates and dilution ratios for different animals. Other sources provide similar yet different recommendations, underscoring the importance of working with an individual experienced in designing anaerobic-digestion systems.
Digesters must be airtight and situated so that they can be heated, usually with hot-water piping running in and out of the digester tank. It may be possible to heat the water using the methane produced by the digester. The tank should also be insulated to help it retain optimal operating temperatures. Many practitioners take advantage of the soil's insulating effect by at least partially burying the digester tank in a pit or piling the soil up against the tank’s sides.
6) Purdue University’s Cooperative Extension Service suggests that the mechanical mixer works well, as long as a good air seal is maintained. Purdue Extension also provides the following formula to determine the horsepower needed to mix the digester contents:
hp = .185 x % total solids x liquid capacity (in 000s of ft3)
As an example, a 10,000-ft3 digester containing waste with 6 percent solids would require an 11.1-hp mixer (.185 x 6% x 10).
System Costs
The cost of an anaerobic-digestion system can vary dramatically depending on its size, intended purposes, and sophistication. Covered lagoon system cost can be as low as $25,000 for 150 animals (swine) and as high as $1.3 million for 5,000 animals (dairy). Plug flow digesters range from $200,000 for 100 dairy cows, to $1.8 million for 7,000 dairy cows. (10)These costs, of course, must be weighed against revenue streams developed with digestion’s by-products. In 1998, Mark Moser, Richard Mattocks, Stacy Gettier, PhD, and Kurt Roos—all highly regarded experts in the anaerobic-digester field— studied the economic returns of seven AgSTAR digester projects. Revenues came from electric generation, and sale of digested fiber for compost, and from reduced costs for natural gas and propane, as well as reduced bedding costs. Costs and annual revenues of four of these projects are available from the Minnesota Project. Of the remaining three projects, two were developed primarily for odor control rather than financial payback, and the third experienced problems that prevented it from realizing its expected revenues. (11)
The AgSTAR Program evaluators believe anaerobic digestion can be cost-competitive relative to conventional waste-management practices (e.g., storage tanks, storage ponds, lagoons). When the biogas produced by the system is put to work, digesters can reportedly have payback periods of three to seven years, substantially more attractive than the sunk costs typically associated with conventional approaches. (12)
| Construction Costs and Annual Benefits | |||
|---|---|---|---|
| Barham Farm | Covered Lagoon | $289,474 | $46,000 per/year |
| Martin Family Farm | Covered Lagoon | $95,200 | $16,000 per/year |
Summary
 Cow and calf. Photo by Lynn Betts, courtesy of USDA/ARS. |
If done right, however, this decision is not a simple one. It should involve careful planning and design, preferably with input from an engineering professional and/or someone well experienced with anaerobic-digestion systems. This planning process must consider a long list of factors.
Factors to Consider
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Assessment Resources
Because anaerobic digesters are expensive to install and manage, the above considerations and many others should be researched and then factored into an economic-feasibility assessment. A number of resources have been developed to guide a prospective system owner through this assessment process:- AgSTAR Program, the premier U.S. resource for information and assistance relating to methane digesters.
- Manurenet, the leading Canadian resource that also includes projects and providers in the U.S. and other countries.
- Various sources offer self-evaluation forms to estimate the potential of a successful digester system installation. The Cooperative Extension Service at Purdue University’s Department of Agricultural Engineering offers a complete evaluation with a full example of how it should be used. Though somewhat dated (published in 1980), the steps in the worksheet and most of the values used should still be valid. Only some of the dollar values, such as the current price of energy, will need to be updated. Another evaluation tool can be found at Environomics.
References
1. AgStar Digest. Winter 2006.2. Environmental Protection Agency Methane Web Page
3. AgStar—Market Opportunities for Biogas Recovery Systems [PDF/3.05MB]
4. Hansen, R.W. 2001. Methane Generation from Livestock Wastes. Publication #5.002. Colorado State University Cooperative Extension Service. Ft. Collins, CO. 6 p.
5. AgStar Digest. Winter 2003.
6. Jones, Don D., John C. Nye, and Alvin C. Dale. 1980. Methane Generation from Livestock Waste. Publication #AE-105. Purdue University Cooperative Extension Service, West Lafayette, IN. 15 p.
7. Barker, James C. 2001. Methane Fuel Gas from Livestock Wastes: A Summary. Publication #EBAE 071-80. North Carolina State University Cooperative Extension Service,
Raleigh, NC. 10 p.
8. The Minnesota Project. Anaerobic Digester Systems for Mid-Sized Dairy Farms [PDF/739KB].
9. Sustainable Conservation. Biomethane from Dairy Waste: A sourcebook for the Production and Use of Renewable Natural Gas in California [PDF/315KB].
10. U.S. Environmental Protection Agency. Guide to Operational Systems [PDF/1.12MB]. AgSTAR Program. 4 p.
11. Moser, Mark A., Richard P. Mattocks, Dr. Stacy Gettier, and Kurt Roos. 1998. Benefits, Costs and Operating Experience at Seven New Agricultural Anaerobic Digesters [PDF/115KB]. U.S. Environmental Protection Agency. 7 p.
12. U.S. Environmental Protection Agency. 2002. Managing Manure with Biogas Recovery Systems: Improved Performance at Competitive Costs [PDF/4.33MB]. 8 p.
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