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Schematic of a Complete Mix Digester

Environmental BenefitsCC Manure management using

enclosed digesters reducesodors.

CC Digestion converts organicnitrogen into ammonium

compounds, significantlyreducing the potential for ground and surface water contamination.

CC Digestion reduces BOD levels,reducing the potential for surface water contamination

CC Heated digestion significantlyreduces harmful pathogens.

United States Air and Radiation EPA 430-F-97-004Environmental Protection (6202J) February 1997

Agency

AgSTAR Technical Series: Complete Mix Digesters

Complete Mix Digesters - A Methane Recovery Option For AllClimates

Most confined or partially confined may prove essential to remain competitive Complete mix digesters are also heated tolivestock production facilities handle in today’s industry. Greater use of optimize methane production. This allowsmanure as liquids and slurries that are complete mix digesters as a waste complete mix digesters to be economicallystored in lagoons, concrete basins, tanks, management option contributes to sized and used in any climate.ponds, and other types of containment environmentally sound agri-business M ethane produced by the completestructures. These structures are typically growth and sustainable rural economic mix digester can offset on-farm energydesigned to comply with local and state development. purchases. An engine-generator fueled byenvironmental regulations and generally Complete mix digesters anaerobically the methane can produce most, if not all,add costs to production. By adding a decompose manure to produce methane of the electricity needed to run a livestockcomplete mix digester methane recovery gas and biologically stabilized effluent. farm. Thus, a complete mix digester can

system, dairies and swine farms can earn Complete mix digesters are versatile, able be a practical way to handle manure andfinancial and environmental benefits that to handle manure of 3 to 8% solids. increase productivity and profitability.

How do Complete Mix Digesters Work?Complete mix digesters are heated, digester vessel maintains anaerobic

constant volume, mechanically-mixed, conditions and traps the methane that istanks that decompose medium solids produced. The gas is collected through aswine or dairy manure (3-8% total solids) pipe supported above the manure. Theto produce biogas and a biologically amount of methane produced is about 5-8stabilized effluent. The basic complete ft /lb volatile solids (volatile solids are themix digester design is a vertical, poured organic portion of the manure and

concrete or steel circular container, with a represent about 8 to 11% of the totalgas-tight collection cover. The circular manure.)shape is the strongest and most The produced methane is removedeconomical design for the digester vessel. from the digester, processed, and

In the complete mix digester system, transported to the end use site. Themanure is collected daily in a mixing pit remaining effluent manure flows from thewhere the percent total solids can be digester and is stored in a storage tank oradjusted and the manure can be pre- lagoon until it can be land applied. Forheated. The manure then enters the dairy manure, solids may be separated outdigester vessel through either a gravity-flow of the digested effluent and sold as soilor pump system. Manure is intermittently amendments.mixed within the digester vessel to prevent

the formation of a surface crust and tokeep solids in suspension. The digestervessel is heated with waste heat from theengine cooling system.

The volume of the digester equals thevolume of the influent (manure and wastewater) times the average manureretention time, typically between 15 and20 days. (Shorter retention times areimportant for keeping the volume of thetank as small as possible.)

A gas-tight cover placed over the

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Mix Tank atCooperstown Farm

Complete Mix Digester at Valley Pork

Complete Mix Digester at Mutzu, Japan

Valley PorkComplete Mix Digester

Economics

Total Capital Costs $250,000

Annual Benefits $65,000Annual Operating Cost appx. $5,000

NPV $89,000Simple Payback 4-5 years

For more information aboutcomplete mix digesters andthe AgSTAR Program, write:

AgSTAR ProgramU.S. EPA, 6202-J401 M Street, SWWashington, DC 20460

or call:1-800-95AgSTAR

SuccessfulSystems

M utzu Farm, Japan : This 2,400 sow farrowto finish swine farm installed a completemix digester for odor control and BODreduction. The biogas powered engine-generator runs an aerobic treatmentsystem for the digester effluent,completing the treatment cycle. Thissystem has proven to be cost effective andefficient.

V alley Pork, Seven V all eys, PA : Valley Pork,a 1,500 sow farrow to finish facility,installed a mix digester in 1986 for odorcontrol and on-farm energy production.

The recovered biogas is used for electricityand to heat the farrowing rooms and

nurseries. The estimated annual electricity

Cooperstown H olstein Corporation Farm, home’s boilers to operate on biogas. TheCooperstown, N Y. In 1985, this 270 milkcow dairy installed a mixed tank digesteras a part of a manure management systemupgrade to allow growth in herd size. Athird of the recovered biogas is used toheat the digester while the remainder ispiped to a nearby retirement home to fueltheir boilers. The upgrade also includedthree manure pits, a pump and

underground manure lines from the barnsto the digester, an underground gaspipeline to the Meadows Retirement

benefits are $50,000 while the thermalbenefits are $15,000. The effluent is landapplied onto croplands producing corn,wheat, and soybeans. The capital cost of

this system, $250,000, was paid for in 4 to5 years.

Home, and retrofitting of the retirement

capital cost of the complete upgrade was$500,000, and the annual operating cost is$4,125. The farm sells 17,000 to 18,000gallons of fuel oil equivalent to theRetirement Home each year. In addition,

The energy savings are estimated as$10,600 per year. The digester effluent isstored in a slurry tank for land-applicationon corn. The effluent fertilizer value isestimated to be $25,000 per year.