anaerobic digestion of animal wastes: factors to consider
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DESCRIPTIONAnaerobic Digestion of Animal Wastes: Factors to Consider
- 1. Anaerobic Digestion of Animal ATTRA Wastes: Factors to ConsiderA Publication of ATTRA - National Sustainable Agriculture Information Service 1-800-346-9140 www.attra.ncat.orgBy John Balsam Anaerobic digestion is an alternative solution to livestock waste management that offers economicNCAT Energyand environmental benets. This publication provides an introduction to the technology of bio-gas,Specialist digester design considerations, and system costs with discussion of the digestion process, production,Updated by uses, and risks. Useful tables and further resources are included.Dave RyanNCAT EnergySpecialist Introduction R2006 NCAT ising energy prices, broader regu- latory requirements, and increasedContents competition in the marketplace are causing many in American agriculturesIntroduction ..................... 1 livestock sector to consider anaerobic diges-Digestion Process........... 2 tion of animal wastes. They view the technol-Bio-Gas: A Resourceogy as a way to cut costs, address environ-Requiring Care ................ 4mental concerns, and sometimes generateEnergy Content and new revenues.Relative Value ofBio-Gas ............................... 4Turkeys. Photo by Je Vanuga. While hundreds of anaerobic-digestion sys-Uses of Bio-Gas ............... 5tems have been installed in Europe and theRening Bio-Gas into U.S. since the 1970s, it was not until theBiomethane ...................... 61990s that better designed, more successfulRisks Associated withprojects started to come on line in the U.S.Bio-Gas ............................... 6 Today, an estimated 97 farm-scale projectsDigester Designare in operation, in start-up, or under con-Factors ................................ 6 struction on swine, dairy, and poultry farmsSystem Costs .................... 8 across the country. (1)Summary ........................... 9Assessment Key by-products of anaerobic diges-Resources .......................... 9 tion include digested solids and liquids,References ...................... 10 Sow with piglet. Photo by Scott Bauer. which may be used as soil amendmentsFurther Resources ........ 10or liquid fertilizers. Methane, the primary component of bio-gas, can be used to fuel a variety of cooking, heating, cooling, and lighting applications, as well as to gen- erate electricity. Capturing and using the methane also precludes its release to the atmosphere, where it has twenty-one timesATTRANational Sustainable more global warming potential thanAgriculture Information Servicecarbon dioxide. (2)is managed by the National Cen-ter for Appropriate Technology Despite the many benets, anaerobic diges-(NCAT) and is funded under agrant from the United States tion systems are not appropriate for all farmDepartment of Agriculturesoperations. A cooperative effort among theRural Business-Cooperative Ser-vice. Visit the NCAT Web siteCows and calves. Photo by Lynn Betts. U.S. Departments of Agriculture, Energy(www.ncat.org/agri. All photos this page courtesy of USDA/ARS.and the Environmental Protection Agencyhtml) for more informa-tion on our sustainableto promote bio-gas projects is known asagriculture projects.
2. AgStar. AgStar estimates that anaerobic Digestion Process digestion could be cost-effective on aboutAnaerobic digestion works in a two-stage 7,000 U.S. farms. (3) A critical issue is process to decompose organic material planning; each system needs to be designed(i.e., volatile solids) in the absence of oxy- to accommodate a variety of factors. This gen. Bio-gas is produced as a waste product publication provides an overview of those of digestion. In the rst stage, the volatile factors and identies resources for addi- solids in manure are converted into fatty acids by anaerobic bacteria known as acid tional detailed information. Several of these formers. In the second stage, these acids resources include computational analysisare further converted into bio-gas by more tools to help users determine whether anspecialized bacteria known as methane anaerobic digestion system could be a cost- formers. With proper planning and design, effective addition to their operation.this anaerobic-digestion process, which has Figure 1. Basic components of an anaerobic-digestion system (4) www.ext.colostate.edu/pubs/farmmgt/05002.htmlGas Receiver Gas Clean-upCompression Equipment Fuel Use or StorageMixer Manure SlurryHeatExchanger Effluent Storage or Disposal DigesterPage 2 ATTRAAnaerobic Digestion of Animal Wastes: Factors to Consider 3. been at work in nature for millions of years, indication that no single system is right forcan be managed to convert a farms waste- all or even most situations.stream into an asset.Starting the digestion process is not dif-There are several types of anaerobic digesters. cult, but it does require patience. Thedigester tank is lled with water and thenCovered lagoonsA pool of liquid manureheated to the desired temperature. Seedtopped by a pontoon or other oating cover.sludge from a municipal sewage treatmentSeal plates extend down the sides of theplant is then added to 20 to 25 percent ofpontoon into the liquid to prevent exposurethe tanks volume, followed by graduallyof the accumulated gas to the atmosphere.increasing amounts of fresh manure overDesigned to use manure with two percenta six to eight-week period until the desiredor less solid content, this type of digesterloading rate is reached. Assuming that therequires high throughput in order for thetemperature within the system remains rela-bacteria to work on enough solids to producetively constant, steady gas production shouldgas. Most frequently used in warmer south-occur in the fourth week after start-up. Theern regions, where the atmospheric heat canbacteria may require two to three months toShelp maintain digester temperatures, this ismultiply to an efcient population. (6) tarting thethe least expensive of all designs to installand operate. About 18 percent of all digest-There are two distinct temperature ranges digestion pro-ers presently in use in the U.S. are covered- most suitable for gas production, and differ- cess is notlagoon systems. ent bacteria operate in each of these ranges. dicult, but it doesMesophilic bacteria optimally function in require patience.Complete mixA silo-like tank in whichthe 90 to 110F range. Thermophilicthe manure is heated and mixed, designedbacteria are most productive in the 120to handle manure with two to ten percentto 140F range. Thermophilic digestionsolids. This is the most expensive systemkills more pathogenic bacteria, but it hasto install and operate, but its particularlyhigher costs due to maintaining higher tem-appropriate for operations that wash outperatures, and thermophilic digesters maymanure. About 28 percent of all digestersbe less stable. Bacterial digestion in cov-in use in the U.S. are of this type.ered lagoons at temperatures below 90F isPlug owA cylindrical tank in whichcalled psychrophilic. Psychrophilic meansthe gas and other by-products are pusheda preference for lower temperatures; how-out one end by new manure being fed intoever, digestion slows down or stops com-the other end. This design handles 11 topletely below 60 or 70F, so these digest-13 percent solids and typically employs hot-ers do not produce methane all of the time.water piping through the tank to maintainTemperature within the digester is criti-the necessary temperature. Most appro-cal, with maximum conversion occurring atpriate for livestock operations that removeapproximately 95F in conventional meso-manure mechanically rather than washing itphilic digesters. For each 20F decrease inout, the plug-ow system accounts for moretemperature, gas production falls by approx-than half of all digesters presently in use.imately 50 percent. (7)Fixed lmA tank is lled with a plasticEven more signicant is the need to keepmedium that supports a thin lm of bacteriathe temperature steady. Optimal opera-called a biolm. This design handles onetion occurs when the methane formers useto two percent solids, and uses a shorterall the acids at approximately the sameretention time, as short as two to six days.rate that the acid formers produce(5) Only about one percent of systems cur-them. Variations of as little as 5F canrently installed in the U.S. are of this type.inhibit methane formers enough to tip theThere are also a number of hybrid sys-balance of the process and possibly causetems being designed and installed, a strong system failure. (7)www.attra.ncat.orgATTRAPage 3 4. Temperature is just one of the many impor-a deadly poison. It is critical that digestertant factors in successfully starting and systems be designed with adequate ventingoperating an anaerobic-digestion system.to avoid these dangerous situations.(7) The other key factors include:Storage. Because of the high pressureLoading rate. The systems design will dic- and low temperature required, it istate loading rates and contents, but experi-impractical to liquefy methane for use asence indicates that uniform loading, on a a liquid fuel. Instead, the gas can be col-daily basis, of manure with 6 to 10 per-lected and stored for a period of time untilcent solids generally works best. The loadsit can be used. The most common meansretention time in the digester will typically of collecting and storing the gas producedrange from 15 to 30 days. by a digester is with a oating coveraweighted pontoon that oats on the liquidMixing. The loaded manure needs to besurface of a collection/storage basin. Skirtmixed regularly to prevent settling and toplates on the sides of the pontoon extendmaintain contact between the bacteria anddown into the liquid, thereby creating a sealthe manure. The mixing action also pre-and preventing the gas from coming intovents the formation of scum and facilitatesTcontact with the open atmosphere. High-he most release of the bio-gas.pressure storage is also possible, but is bothcommonNutrients. The best digestion occurs with more expen