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Keeping The Neighbors Happy - Reducing Odor While Making Biogas

Mark A. Moser, Richard P. Mattocks, Dr. Stacy Gettier, Kurt F. Roos

Abstract

Controlled anaerobic digestion with biogas recovery is one method to stabilize manure, reduce odor, and produce an energy resource that offers a return on investment. Substituting a digester for the treatment portion of a lagoon results in a very low cost supply of methane for use on the farm. The installation cost of treatment volume at an Iowa sow farm collecting manure at 3% total solids is approximately $21/sow in a 12' deep unlined lagoon, or $35/sow in a concrete, heated, mixed digester. The lagoon produces no benefit as propane replacement while the digester would produce $9/sow/yr. Cost and benefit comparisons for nursery and finishing farms are included with supporting assumptions.

The calculated conclusions are supported by previously built digester systems. Digesters at Rocky Knoll Farms of Lancaster, PA (1982 - 800 sows farrow-to-finish) and Valley Pork in 7 Valleys, Pennsylvania (1986 - 1,500 sows, farrow-to-finish) stabilized manure, collected usable biogas and satisfied the odor objections of the neighbors, town council and state regulators. The systems had simple cash paybacks in less than 5 years.

In the last 2 years, at least a dozen farms have installed anaerobic digesters. Seven farms were assisted by the EPA, USDA, and DOE sponsored AgSTAR program. AgSTAR provided technical assistance through out the development, installation, start-up and operating phases of these anaerobic digestion projects. Three dairy plug flow digesters (NY, CT, OR), three covered pig manure lagoons (NC, VA, IA) and one heated mixed pig manure digester (IL) have been placed in operation since January 1997 with AgSTAR technical assistance. The farms and their digester systems are described. The cost and benefits of the digester systems are summarized.

Keywords: Biogas, methane, odor, anaerobic digestion, digester, covered lagoon, nutrient management, pathogen

Mark A. Moser - Resource Conservation Management, Inc., P.O. Box 4715, Berkeley, CA 94704
Richard P. Mattocks - Environomics, 5700 Arlington Blvd., #17A, Riverdale, NY 10471
Dr. Stacy Gettier - P.O. Box 474, Waverly, VA 23890 Kurt F. Roos, AgSTAR Program Manager, USEPA, 401 M St. SW, 6202J, Washington DC 20460

Introduction

Manure treatment and storage requirements have changed, requiring larger investments for manure management. In the interest of reducing capital costs, many manure storage pits, ponds and basins have been built for pig farms. Unfortunately, the storage facilities often yield odors that are offensive to neighbors and passers by. The odor issue leads to conflicts, lawsuits and administrative actions. Quite a bit of research is being conducted on odor control additives and alternative treatment technologies. The question to be answered is: Can anaerobic digesters be a cost effective waste treatment alternative?

Anaerobic Digestion for Odor Control

Much has been written about odor from swine manure. For simplicity of understanding, let us acknowledge that anaerobic bacterial activity breaks down manure in liquid or slurry form. Short term or "incomplete anaerobic digestion" produces intermediate products such as volatile organic acids which are effusive and disagreeably odiferous. Long term anaerobic action which can be called "complete anaerobic digestion" produces a stabile, odorless product, methane and carbon dioxide.

An anaerobic digester is a vessel designed to contain decomposing manure for sufficient retention time at the designed operating temperature in order to allow the growth of methanogenic bacteria in a "steady-state". The effluent of a digester has an earthy smell with some ammonia present. A paper by Wilkie et al (1) summarizes odor control benefits of anaerobic digestion. Several different digester configurations can be used and are described briefly at the beginning of sections of AgSTAR demonstration projects.

Older Successful Digesters

The basic design concepts proven by successful digesters built in the 1980's are applicable today. The first pig manure digester systems in the US were installed principally to control manure odors. The odor control goal was successfully met. Anaerobic digestion is more extensively used outside of the US where concern for treatment of animal waste has been a concern for a longer time. In today's world, odor and pathogen control are important.

The primary goal of digesters at Rocky Knoll Farms of Lancaster, PA (800 sows farrow to finish) and Valley Pork in 7 Valleys, Pennsylvania (1,500 sows, farrow-to finish) was to reduce manure odors. Both farms were located within sight of clusters of non-farm residences. Both were located within a mile of towns and subdivisions. Valley Pork had an acute odor problem prior to installation of the digester system. The heated, mixed digester stabilized the manure, collected usable gas and satisfied the objections of the neighbors, town council and state regulators. The Rocky Knoll digester which is currently operating, cost $ 325,000 and produces up to $ 100,000 worth of usable energy annually. The Valley Pork digester cost which is not currently operating due to the rebuilding of the farm, cost $ 250,000 and produced about $ 65,000/ year worth of energy. (2)

Roy Sharp has operated a covered lagoon digester at his at Royal Farms for 20 years (1,700 sows, farrow-to finish) in the desert climate of Tulare, CA. He invested about $180,000 and has been producing approximately $ 50,000/year worth of energy.

Are These Older Digesters Unusual?

Rocky Knoll, Valley Pork and Royal Farms succeeded due to adequate professional design assistance and good management. Royal Farms succeeds with flush manure digestion due to the proper lagoon size and the relatively warm temperatures. In colder climates, Rocky Knoll and Valley Pork succeed because the manure was collected using underfloor scrapers yielding manure with about 4% total solids. Other digesters are not operating due to either poor design, poor economics, or poor management.

Anaerobic digesters are considered to be expensive when compared to other manure treatment or storage options. Many people cannot overcome the thought that digesters are costly. This is true when digester cost is compared to a manure management option such as a storage basin. However, storage basins, often mistakenly called lagoons, are not designed with biological activity or odor control in mind. The cost of a digester should be compared with the cost of other treatment options that biologically stabilize waste.

Lagoons vs. Heated Digesters - Sizing

Lagoons are used to biologically stabilize and minimize odors from manure. For Ames, Iowa, the Natural Resource Conservation Service(3) recommends sizing a single cell lagoon to receive 4 pounds of volatile solids (VS) per 1000 ft3 of dedicated lagoon treatment volume per day. This is a loading rate of 0.004 lb/ft3. A more recent innovation is a 2 cell lagoon system with a covered first cell to capture methane and reduce odor. The first lagoon is designed as a constant volume cell with its volume dedicated to treatment, while the second cell is sized for storage only. The second cell is emptied seasonally. NRCS Practice Standard 360 allows the covered treatment cell to be sized at 6 pounds of volatile solids (VS) per 1000 ft3 in Ames because the separate treatment volume maintains a higher bacterial mass year round and a floating cover results in a slightly warmer lagoon.

A heated digester performs the same biological function as a lagoon in a much shorter time because of the higher temperature. For a heated anaerobic digester, the minimum total solids concentration in the influent to sustain self heating is 3% with a VS concentration of 2.4% or 1.5 lb VS/ ft³. This concentration can be achieved by underfloor scrapers or managed pull plug manure collection. A conservatively designed heated mixed digester is sized for a 20 day retention time. Therefore, the loading rate is 0.075 lb VS/ ft³ (1.5 lb/ft³/20 days). By comparison of the loading rates above one can see that a heated digester requires about 1/20 of the treatment volume required in a lagoon.

Lagoons vs. Digesters - Costs

Per unit of volume, a heated concrete digester will cost more than a lagoon. However, a heated digester requires a much smaller volume than a lagoon. Costs vary between regions, soil types and digester types. Costs from recent AgSTAR projects(4) were reviewed, and gas use costs such as generators were removed. Unlined lagoon construction cost $0.07/ft³(4) while a HDPE lined lagoon cost about $ 0.16/ft³.(5) Concrete tank digesters including boilers to maintain digester temperature cost $1.90 to $2.25/ft³.(4) A digester project under construction using a clay lined, partially concrete lined lagoon will cost about $1.09/ ft³. (5)

Tables 1 and 2 compare the estimated costs of treatment and storage for two cases in Iowa, a 5,000 head continuous flow finishing operation and a 2,500 sow farrow to wean operation. Key assumptions are:

1. manure is collected with pull plugs at 3% total solids
2. excavation is $ 1.50/yd3
3. lagoon lining costs are $ 0.70/ft2
4. concrete costs average $ 200/yd3 placed
5. storage period is 210 days

Table 1. Estimated Costs of Treatment Options for 5,000 Finish Hogs (Iowa)

Table 2. Estimated Costs of Treatment Options for 2,500 Sows (Iowa)

The estimates suggest that a complete mix digester in a manure treatment/storage plan can be cost competitive. Substitution of a digester investment for lagoon investment yields excess methane which is essentially a free energy source. Additional savings are realized by reduction of lagoon surface area and land application costs. Also, the stored effluent is of uniform quality and will not be an odor source when land applied.

AgSTAR

AgSTAR is a voluntary program that promotes recovery and use of methane from animal manure. AgSTAR is one of many voluntary initiatives under the US Climate Change Action Plan to Reduce Greenhouse Gases. The program provides technical support, and compiles and distributes information. AgSTAR has supported development of standards for anaerobic digestion systems and decision support software. AgSTAR enrolls farms and industry in the AgSTAR program with a signed Memorandum of Understanding (MOU) as AgSTAR Partners and Allies. In some cases, Partners and Allies may be supported with direct technical assistance. In addition, AgSTAR provides speakers, conducts training workshops and exhibits a display booth as part of its outreach and education component.

1996-1998 AgSTAR Projects

From 1996 - June 1998, AgSTAR assisted in the realization of 7 farm scale digesters that have desirable environmental performance. 3 ambient temperature covered hog lagoon and 4 heated plug flow digesters were built. AgSTAR will assist 5 new projects in 1998. Electricity and heat production are direct benefits of the projects. Key non-energy benefits and byproducts from anaerobic digestion of manures such as digested dairy solids, contributed significantly to the desire of farm owners to install anaerobic digesters. Reduction of methane emissions, odor control, mineralization of organic nitrogen, weed seed destruction, pathogen reduction and decomposition of manure to increase pumpability are non-monetizable benefits demonstrated by existing digestion systems.

Barham Hog Farm - Barham Farm of Zebulon, NC is a 4000 sow farrow to wean pig farm with pit recharge. AgSTAR provided design, installation and troubleshooting support and worked with NRCS to design the lagoon. Lagoon construction began in July 1996. The lagoon cover was installed in December 1996. Biogas use for heating water began in January 1997. The lagoon cover manufacturing problems have limited the production of electricity, however a 400,000 btu boiler has operated almost continuously, providing hot water for pig mats under farrowed pigs. Odor is virtually non-existent, the effluent is stable and nutrient content of the second lagoon has been reduced substantially.

Martin Family Farm - Martin Family Farm of South Boston, VA is a 600 sow farrow to feeder pig farm with recycle flushing. The farm covered the first cell of a two cell lagoon in 1993, began engine operations in spring 1994 with a matching grant from the Southeast Regional Biomass Energy Program. Due to problems with the original cover, SERBEP and AgSTAR provided additional support in 1997 and 1998. Methane recovery has been continuous throughout the project, though gas use has not been. The farm is converting to a hot water boiler for pig mats under farrowed pigs. A boiler will more closely match the farm labor skill and availability. Odor is virtually non-existent, the effluent is stable and nutrient content of the second lagoon has been reduced substantially.

Apex Pork - Apex Pork of Rio, IL is a 8,900 head continuous flow finishing facility in Rio, IL with pull plug manure collection. The farm has built and in June 1998, started up an innovative digester - a heated, mixed covered lagoon, for sole purpose of operating the system to biologically stabilize waste prior to discharge to the existing storage. AgSTAR is providing assistance with startup and operations. Odor is reduced substantially.

Boland Farm - Boland Farm of Williamsburg, IA installed a low cost cover over their 2,400 head nursery storage basin to capture gases. Gas use will not be attempted and a flare has been installed to combust the gases. The project budget was $15,000 with grant participation from Iowa State University. The cover has almost eliminated odor from the covered basin. AgSTAR provided technical assistance.

Craven Dairy Farm - Craven Farms of Cloverdale, Oregon finished a heated, unmixed, plug flow digester sized for the daily manure production of 1000 cows in December 1996. Funding included $ 75,000 of farm monies, a $ 77,500 grant from the Pacific Northwest and Alaska Biomass Program, and a $ 98,000 loan from the Oregon State Energy Office. AgSTAR provided technical assistance during construction and startup. The farm is currently producing about $ 24,000 of electricity and $ 30,000 of digester fiber yearly.

AA Dairy - AA Dairy Farm of Candor, New York built and started up a 1000 cow digester at their 550 cow facility, completing a boiler fired system in October 1997. AgSTAR provided technical assistance in all phases of the project. Due to issues with New York State Electric and Gas Co. engine-generator startup was delayed until June 1998. Funding included a $90,000 grant from the local Soil Conservation District to improve manure management and the balance of the $250,000 cost from the owner. The system is currently producing 70 kWh, hot water and about $60/day of digested fiber.

Freund Dairy - Freund Dairy of East Canaan, CT installed a plug flow digester at their 200 cow dairy during a complete rebuild of the waste management system. The digester started up in Fall 1997. Gas is used in a boiler or flared. Hot water is used to keep the digester warm and 6,000 gallons of 180 degree hot water are available and or is used to supply heat to existing greenhouses on the farm. Digested solids are separated for sale. A cost/benefit summary is not available at this time.

1. Wilkie, A.C., et al., "Anaerobic Digestion for Odor Control", in: "Nuisance Concerns in Animal Manure Management: Odors and Flies", Florida Cooperative Extension, University of Florida, Gainesville, FL, 1995

2. Lusk, P., Methane Recovery from Animal Manures: A Current Opportunities Casebook, Regional Biomass Energy Program, USDOE, August 1995, DOE/EE-0062

3. USDA, SCS, Animal Waste Management Field Handbook, 1992

4. Moser, M.A. and K.F. Roos, "AgSTAR Program: Three Commercial-Scale Anaerobic Digesters for Animal Waste", Making a Business from Biomass, Proceedings of the 3rd Biomass Conference of the Americas, R.P. Overend and E. Chornet, editors, 1997, Elseveir Science Inc., Tarrytown, NY

5. Unpublished file data

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