A number of Wisconsin livestock producers are consider-ing the installation of anaerobic digestion systems and theequipment necessary to generate electricity from the methaneproduced and collected by these systems. There are severalreasons these systems are being considered: decreasing on-farm energy costs by using methane to generate electricity;mitigating odor problems on livestock operations; andimproving the handling of manure nutrients and controlpathogens. In addition, electrical generator companies aremandated in some areas to increase the amount of “green”energy produced, and may pay a premium for it.

However, producing and managing methane requires asubstantial commitment of financial and managementresources on the part of the operator. Farmers need thoroughand accurate information before starting methane productionon their farm.

Anaerobic digestion and the production of methane gasare not new concepts. They have been used profitably formany years in the waste management industry (municipalwastewater treatment plants and sometimes landfills). Untilrecently however, not many farms were generating electricity.Recent changes in the rising cost of energy, improvements inthe technology used to convert methane to electricity and theenvironmental pressures agriculture is facing in terms of live-stock production, has increased interest in anaerobic diges-tion systems.

The purpose of this publication is to provide briefanswers to the common questions farmers often ask aboutanaerobic digestion and methane generation. It is not our goalto provide all the information needed to make a decision onwhether or not this technology will work on your operation.The goal of this publication is to provide information on thebasic questions producers have. Once you have answered

these questions and you desire additional information, moredetailed and specific guidance can be found in the recom-mended publications, web sites and other materials refer-enced by this publication.

In general, a well-designed and managed anaerobicdigester will have few safety concerns. Still, the gas-handlingcomponents of the digester and engine-generator must bedesigned to ensure safety. Inhalation of biogas can posehealth risks, and biogas is flammable and even explosive inconfined spaces.

There are only a few examples of methane digesters onwhich to base construction costs. Numbers vary, but someexamples indicate about $250,000 for a 1,000-cow dairyoperation. In many cases, farmers have found low-cost loansor even grants to assist in start-up. This $250,000 estimateincludes design, engineering, construction, generator andengine, and some electrical connections, depending on howsophisticated they need to be.

Anaerobic Digestion Systems

Wh at types of dige s t e rs are being used, and wh at arethe adva n t ages and disadva n t ages of each type?

An anaerobic digester is an enclosed tank that excludesoxygen. A specific population of naturally occurring bacteriabreak down manure into a variety of gases, includingmethane and carbon dioxide.

The digestion of manure occurs in four basic stages:hydrolysis, acidogenesis, acetogenesis and methanogenesis.The final stage, methanogenesis, is the step that breaks downthe intermediate compounds to produce methane. The gasreleased during the digestion process is captured and can beburned. This biogas contains 50 to 70 percent methane, 30 to50 percent carbon dioxide, and trace gases, including hydro-gen sulfide. Methane-producing bacteria are most active intwo temperature ranges, 95 to 105 degrees F and 130 to 135degrees F.

As a means to increasing methane production, somedigesters contain heating pipes that circulate hot water andheat and maintain the digester in the desired temperaturerange. A flexible, impermeable cover on the digester traps thebiogas, which is then either burnt in an open flame or passedthrough an electrical generator. There are several types ofgenerators. Most are driven by modified internal combustionengines. The electricity can be used strictly on the farm, or itmay enter the electrical grid that feeds the farm.

Anaerobic Digesters andMethane Production...Questions that need to be asked and

answered before investing your money

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Types of Digesters

C ove red Lagoon Dige s t e r: Frequently farmers covermanure lagoons to minimize odor problems. A c o v e r e dlagoon digester can be used on large-volume, liquid manurelagoons with less then 2 percent solids. The cover is usuallyan impermeable plastic membrane. The cover traps gas pro-duced during the decomposition of the manure. Collectingmethane involves a floating lagoon cover and may require agas pump to move the gas. This system can be used on bothswine and dairy operations and works optimally where themanure is handled as a liquid and the climate is temperateto warm year round. This is the least expensive of the threetypes of digestion systems, but may not function well incold climates.

Complete Mix Dige s t e r: A complete mix digester isused for manure containing 2 to 10 percent solids, whichincludes dairy manure or swine manure collected by a flushsystem. Above- or below-ground tanks are heated in thissystem. A mechanical or gas-mixing system keeps the solidsin suspension, which accelerates the digestion process.These digesters are expensive to construct and cost morethan covered lagoon digesters tooperate and maintain. One con-cern of this system is that exces-sive water addition increases thedigester size requirements andthe cost of construction andoperation, without increasingbenefits.

P l u g - Fl ow Dige s t e r: P l u g -flow digesters are suitable forruminant animal manure (cows),collected by scraping and con-taining a solid concentration of11 to 14 percent. This system isnot appropriate for manure withlower solids concentration, suchas swine manure or dairy manure

collected through a flush system. In aplug-flow digester, the solids inmanure with lower than desirablesolids concentrations are poorlydigested because they gradually settleto the bottom of the tank. Under opti-mal conditions, the plug passes com-pletely through the digester in 15 to20 days. This system has few movingparts and requires minimal mainte-n a n c e .

Te m p e rat u re-Phased Anaero b i cD i gestion (TPAD): The T PA Dmethod, developed at Iowa StateU n i v e r s i t y, combines traditional typesof digestion technology in a two-stage reactor. The first stage operatesat high (thermophilic) temperatures of55 degrees C/135 degrees F, while thesecond stage digests at lower(mesophilic) temperatures of 35degrees C/95 degrees F. The system

is a complete mix, which works best with the more dilutemanures coming from dairy farms operating their parlorsmore or less continuously. A variety of other organic prod-ucts can be added to increase methane yield during theprocess. Research data indicates improved odor control,reduced foaming, additional volatile solids destruction, andimproved dewatering characteristics compared with conven-tional plug-flow designs. The high heat process destroysClass A level pathogens, thus producing a higher qualitybiosolids product. Although the T PAD system holds prom-ise, very little data is available on field-scale systems.

Wh at types of manu re management practices – wat e rcontent, bedd i n g, storage, and tre atment – are re q u i re dfor anaerobic dige s t i o n ?

In general, methane generation systems are most appro-priate where most manure is recoverable (e.g., completeconfinement), where manure is regularly scraped, and sandbedding is not used.

Water content: Anaerobic digestion systems are avail-able for wet or dry manures. Maintaining constant watercontent is important for consistent processing. Collecting

Covered Lagoon Digester, Wisconsin

Complete Mix Digester, Colorado

manure through a flush system (using water to flush manureto storage) usually makes the manure too diluted for eff e c-tive processing.

B e dd i n g : Mixed with feces and urine, bedding becomesanother feedstock for the digester. The chemical and physi-cal properties of the bedding must be considered. Researchhas shown that shredded newspaper is not only easilydigested, but it increases the amount of methane produced.Other sources of bedding (straw, corn stalks, etc.) can aff e c tthe digestion process. Anaerobic digestion systems are notadaptable for livestock housing systems that use sand as thebedding source because the sand accumulates and clogs theflow of nutrients in any of the digestion systems.

M a nu re Storage : In most cases the digester serves asthe storage system for the fresh manure. After anaerobicdigestion, manure can either go to a storage system directly,or go through a separator to separate the solids from the liq-uids. The biosolids remaining after the digestion processhave to be stored but are smaller in volume and usuallyhave much less odor. Depending on how producers want or

need to handle the nutrients inthe biosolids, manure storageand treatment systems must bemodified appropriately. Heatrecovered from thedigestion/electricity generationprocess can be used to dry thes o l i d s .

P re - t re at m e n t : Ideally thedigester should be set up toaccept manure as it is pro-duced. In some cases watermay be added if needed. Mostoperations will have a manurecollection pit that can handlemanure for a pre-determinedamount of time, which feedsfresh manure into the anaero-bic digester.

A re there any special re q u i rements for handling theb i oga s ?

Biogas is primarily a mixture of methane and carbondioxide, with some trace gasses; including ammonia, hydro-gen sulfide, mercaptans (strong odors) and other noxiousgases. Biogas is combustible and must be handled to avoidsparking when mixed with air. When the gas is thoroughlycombusted, most of the organic compounds are oxidized tocarbon dioxide. Unfortunately, the hydrogen sulfide is con-verted to sulfuric acid, making it very corrosive. The hydro-gen sulfide can either be removed (scrubbed) from the gasbefore combustion, or combusted in the generator engines.Scrubbers typically decrease maintenance requirements andextend the useful life of generator engines.

Why do dige s t e rs fa i l ?

The primary reason for the failure of anaerobic digestersis bad design and/or installation. The second most commonreason for digester failure is the selection of poor equip-ment and materials. The third reason for digester failure,

Plug-Flow Digester, New York

Heated Mixed Covered Lagoon, Illinois

and an important reason, is poor farm management. Adigestion system should be engineered specifically for theindividual farm operation, and then properly installed. It iscritical the farm operator understands that a digestion sys-tem is similar to other farm equipment. The system must beoperated and maintained by a producer who understandshow the system should work and watches it closely just likeany other piece of farm equipment (tractor, chopper, etc.).

Another factor in digester failure can be specific com-pounds in livestock diets passing through the manure,which can cause problems with the anaerobic digestionprocess. Excessive ammonia concentration, produced fromanimal urine, can inhibit or kill the methane bacteria.Rumensin or similar products affect the microbes andchange the digestion process. An anaerobic digester relieson organisms to digest the manure and produce methane.Care must be taken to insure the desired organisms can sur-vive and thrive in the digester.

Methane Generation andElectricity Production

H ow many cows or animal units do I need to makemethane ge n e ration economical?

The Ag STAR Program (a joint program of theEnvironmental Protection A g e n c y, the U.S. Department ofE n e rg y, and the U.S. Department of Agriculture) suggeststhat a farm should have at least 300 head of cattle.H o w e v e r, there are examples of successful methanedigesters on smaller farms. Since initial digester start-up

can be time-consuming and inefficient, the digester needs tobe constantly and regularly supplied with fresh manure tomaintain the methane-producing bacteria. Therefore, it isimportant there be a year-round supply of fresh manure.Economies of scale include both the anaerobic digester andthe electrical generation system.

In determining whether or not an anaerobic digestionsystem could be financially successful, one of the mostimportant factors is whether or not your local electric com-pany is willing to buy your biogas and if so, at what price?If your goal or intention is to build a digester on the hopesof selling the biogas, it is important to contact the localelectric utility and determine what your options are andtheir availability for handling green gas.

H ow mu ch electricity can I produce? Wh at types ofge n e rat o rs and electrical connections are used for elec-trical production from bioga s ?

As a rule of thumb, if the anaerobic digester is workinge ff i c i e n t l y, a dairy farm can produce two to three times theelectricity it normally uses. For example, a typical dairymilking an average of 500 cows per day can produce30,000 - 50,000 cubic feet of biogas daily. With this volumeof gas, a 70 kW generator can produce 1000 - 1,400 kWh ofelectricity per day, along with significant heat recoveryfrom the engine. The recovered heat can be used to heatw a t e r, to heat the digester or for space heating.

When deciding on the size of generators, each operationshould do their own evaluation based on animal numbers,the type of digester and other factors unique to the farm.H o w e v e r, as broad guidelines, a 70 kW generator has beenrecommended for a 500-cow herd, and one 2,300-cow dairyinstalled a 750 kW g e n e r a t o r.

Modifying engines to burn methane is not difficult. T h egenerator engine, rather than the generator itself, requiresthe most maintenance. Maintaining the generation equip-ment usually requires 15 to 30 minutes per day, and entailschecking the oil and monitoring gauges. The farm operatorcan normally do this common routine maintenance.Monthly maintenance includes engine oil change, valveadjustment and spark plug cleaning. Estimated engine main-tenance for an on-farm biogas generator engine, includingperiodic engine overhaul, was $3,700 per year in one case,for about 1,800 milking cows.

Electrical equipment and connections in areas whereflammable or explosive gases may develop require specialadaptations to avoid spark ignition.

H ow mu ch more time will it take to produce methanethan my typical manu re handling re q u i re s ?

Moving the manure to the digester, checking gas pro-duction, removing solids from the digester are all part ofthe process. The plug-flow system is the most appropriatesystem for cow manure collected by scraping. With a herdof 500 cows, this system requires about 45 minutes of rou-tine operation per day. This includes system inspection,mixing and pumping manure into the digester twice a day,and checking and recording gauges to measure biogas andelectricity output. On average, routine daily maintenancerequires less than one hour, depending on herd size.

Methane being safely flared off of a digester.

Wh at fa c t o rs determine how mu chm o n ey I can save or make in usingmethane ge n e rat i o n ?

The simple answer to this questionis that it depends on the amount ofmethane produced. There are severalfactors affecting the amount of gas pro-duced, including the feed ration, solidscontent of the manure, constant temper-ature, length of time manure is in thedigester and bedding materials. High-e n e rgy diets can more than double themethane potential of manure comparedto manure from animals fed a lowere n e rgy diet. The higher the solids con-tent, the greater the biogas productionper gallon of manure, because there aremore energy-rich organic materials inthat gallon.

The frequency and regularity ofmanure collection is important, becausethis creates a stable environment for theorganisms. Maintaining a constant temperature throughoutthe digester is very important for microbial activity. Thelonger the manure remains in the digester, the more methanewill be produced. The pH of the manure can also be a factor.The pH should be as close to 7.0 (neutral) as possible.Higher or lower pHs can kill or inhibit the methane-generat-ing bacteria. Addition of readily decomposed organic materi-als in bedding (like shredded newspaper) can increase biogasproduction, although they are not required.

Wh at issues re l ate to connection to the electric gr i d ?

A number of important issues must be addressed if thefarm decides to go “on the grid” and sell its power to theutility company. In all cases, the utility company should beconsulted very early in the process to get the specific infor-mation needed to evaluate this option. Issues include the fol-lowing: (1) The alternating current cycle (phase) of thefarm’s generators must always be matched with those of theline coming into the farm. (2) The electrical service line sys-tem to the farm must be large enough to handle additionalload from the farm when it is selling significant quantities ofpower. (3) Interruption of current from either side – the farmor the outside line – must be handled by the system in place.

With up-to-the date anaerobic digesters the equipmentused to connect the generator to the public electric grid canensure that the connection is both safe and reliable. Controlsare built into the system that balance the output from thedigester system with the line current.

Environmental Benefits and theHandling of Biosolids

Wh at are the ch a racteristics of the residual by p ro d-ucts? Wh at is the market for them?

The biosolids produced from anaerobic digestion can bean excellent soil amendment, either for direct use or whenblended with other materials. Anaerobic digestion typically

decreases the manure solids volume by 50 - 60 percent, butthere is little effect on total volume because the slurry is 90- 95 percent water. The biosolids contain higher concentra-tions of N, P, K, and trace elements than manure. They alsocontain relatively high concentrations of ammonium-N,making the N rapidly available (more like commercial fer-tilizers) once it is spread on fields. However, to avoidammonia losses and odor problems the material must behandled appropriately.

Because manure digestion is anaerobic, most weedseeds and pathogens are killed during the process. T h enumbers of most pathogenic organisms are reduced bymore then 90 percent by the digestion process. Fecal col-iform bacteria numbers in the biosolids are only about 1percent of those in fresh and stored manure, lowering thepotential for this source of water pollution. Pathogens caus-ing some of the common food production diseases, can bepractically eliminated from the manure by anaerobic diges-tion. Fly eggs are killed during anaerobic digestion, elimi-nating this important disease vector and general pest.

The liquid portion of the digested manure has a highvalue as fertilizer. The liquid effluent from the digestedmanure has been used as a dietary supplement, and report-edly contains 40 to 50 percent crude protein on a dry matterb a s i s .

Some livestock operations have installed digesters andadditional equipment to separate suspended solids from theliquid. The separated fibers (solids) contain 20 - 30 percentof the total nutrients compared to the original manure. Forexample, if a farm with approximately 500 head of cattleinstalls a digester and a liquid/solid separation system, thenutrients contained in the solids would be worth approxi-mately $40,000 per year compared with commercial fertil-izer prices. Depending on the market and price received fore l e c t r i c i t y, the value of the nutrients in the solids is verycomparable to the value of the electricity produced.

As with fresh manure, the digested solids must bestored and handled appropriately to minimize risk to theenvironment. Ammonia can be lost to the air throughvolatilization, and ammonia in the air is a pollutant.

Maintaining the digester engine is a key to success in this process.

Maintaining a crust on the storage pond or reducing its sur-face area can reduce nitrogen losses, although this may bemore difficult after the materials have been digested thanwith undigested manure. When applying the digestedmanure to fields, incorporating the digested manure into thesoil rather than leaving it on the soil surface can minimizenitrogen loss.

Air emission from combusting biogas is also a concerneven though it is cleaner then burning coal. The combustinggas contains small concentrations of sulfur dioxide andother trace gases. The generation system must be designedto handle the exhaust properly.

H ow will my odor management be improved by going tomethane ge n e rat i o n ?

Anaerobic microorganisms break down many odor- c a u s-ing compounds in the manure as it moves through thed i g e s t e r. This basically eliminates odor problems. Researchhas shown anaerobic digestion reduced odor by 97 percentover fresh manure. In farm expansion projects, odor controlcan be a primary reason for installing a digester, especiallycovered lagoon systems. Where the only objective is odorcontrol, the gas produced is simply burned (flared off ) .H o w e v e r, using the biogas to produce energy offers a sig-nificant economic return depending on the farm size.

Wh at are other env i ronmental benefits of methaned i ge s t i o n ?

On farms where manure is stored in manure pits orlagoons, methane is generated and released into the atmos-phere. Methane is 21 times more potent than carbon dioxidein causing global warming. By capturing and burning themethane produced from animal manure, anaerobic digesterslower the rate of global warming. Where manure manage-ment systems result in aerobic decay of manure, such asgrazing systems and dry manure packs, significant amountsof methane are not produced.

Where can I go for additionali n f o r m a t i o n ?

G e n e ral re fe rences covering many aspects of methanege n e ration and use:

• A G S TAR, US-EPA: www. e p a . g o v / a g s t a r /• The Minnesota Project: www. m n p r o j e c t . o rg• US Dept of Energ y. 1995. Methane Recovery from

Animal Manures: A current opportunities casebook.Washington, DC 20505

• Methane generation from Livestock Waste: PurdueUniversity Cooperative Extension Service, A E - 1 0 5 ,w w w. a g c o m . p u r d u e . e d u / A g C o m / P u b s

• Commercial infrastructure takes shape for anaerobicdigestion, by Jerome Goldstein, Biocycle, Nov. 2001.

Case studies:• Haubenschild Farm report, from Minnesota:

w w w.epa.gov/agstar/library/haub.html andw w w. m n p r o j e c t . o rg

• Long-term digester operation case study:w w w. r c m d i g e s t e r s . c o m

• Case studies of anaerobic digestion projects:w w w. b i o g a s w o r k s . c o m

• Craven Farms, Oregon:w w w. e n e rg y. s t a t e . o r. u s / b i o m a s s / d i g e s t e r / c r a v e n . h t m

S c reening tests:• Agricultural Utilization Research Institute:

w w w. a u r i . o rg / R e s e a r c h / d i g e s t e r /

Odor contro l :• Case study:

w w w. r c m d i g e s t e r s . c o m / p u b l i c a t i o n s / i n d e x . h t m

Prepared for Discovery Farms under funding by UW-Extension. Developed by Dennis Frame and Fred Madison, co-direc-tors; Wes Jarrell, senior scientist; Justin Johnson and Sarah Steenlage, technical assistants; and Shannon Hayes, communica-tions coordinator.

An EEO/Affirmative Action employer, University of Wisconsin-Extension provides equal opportunities in employmentand programming, including Title IX and A D A requirements. Request for reasonable accommodation for disabilities or lim-itations should be made prior to the date of the program or activity for which it is needed. Publications are available in alter-native formats upon request. Please make such request as early as possible by contacting the Discovery Farms Office @ 715-983-5668 so proper arrangement can be made.

A3766 A n a e robic Dige s t e rs and Methane Production...Questions that need to be asked and answe red befo rei nvesting your money. Copyright 2001.