On-farm Anaerobic Digestion Biogas Production in Pennsylvania - 30 Years

The recent resurgence of digesters in early 2000 have been installed mainly for electrical power production but also brings the added benefits of manure odor reduction and pathogen destruction.
On-farm Anaerobic Digestion Biogas Production in Pennsylvania - 30 Years - Articles

Updated: November 9, 2017

Introduction

Pennsylvania has the distinction of having four of the oldest, continuously operating farm-based anaerobic digesters (AD) in the country. These four digesters (two dairy, a poultry and a swine farm) have the combined operating experience of 99 years. It is also informative to note that these digesters were completely or almost completely built with farm supplied funds. They also had one or more farm personnel heavily involved with their design and operation. Anaerobic digesters and the use of biogas is not a new technology. On the contrary, anecdotal evidence indicates Assyria, in the 10th century BC, used biogas to heat bath water. The first actual digester plant was built in Bombay, India at a leper colony in 1859 (Meynell 1976). In 1895 biogas was used from a sewage treatment plant to fuel street lamps in Exeter, Devon, UK (Lusk 1998). The first Pennsylvania on-farm anaerobic digester was built in1978 at the Mason Dixon Farm in Gettysburg, PA and is still in operation today. As of 2005, Sweden put the world's first biogas fueled passenger train into service. The biogas is created in an anaerobic digester using the entrails of cattle (Franks 2005).

The energy crisis in the late 1970's made Americans think about their energy consumption. Farmers in particular were looking for ways to reduce the amount of electricity purchased from the power company. Anaerobic digesters not only reduce the odors from manure which could leave the farm as emissions, but, the biogas that is produced from the anaerobic digestion process can fuel an internal combustion engine combined with a generator to produce electricity to lower, meet or exceed the power requirements of the farm; was a very attractive option to the farmer.

In response to the need for producing energy from readily-renewable sources as an alternative from expensive and diminishing supplies of fossil fuels, The Pennsylvania State University built and operated a research/demonstration, two-stage, vertical anaerobic digester to produce biogas using the manure from 50 to100 milking cows at the Penn State Dairy Center (Persson et al. 1979). This digester was dismantled at the conclusion of the project in December 1977. As an early manure digester it provided information and hands-on experience for various subsequent on-farm digesters around the United States.

Since anaerobic digesters were not as widely used in this country as in others, the United States did not have experienced farm-based AD designers in the 1970's. This lead to more failed digesters than successful ones. Currently, in 2008 the U.S. has a few designers with 13 to 26 years of experience designing farm-based digester systems. Although, most of the digesters built during the energy crisis of the 70's and 80's received funding from the government, some of these digesters failed not only from design flaws, but from the farmer not having the necessary operating experience. Also, farms that still had an operating digester after the energy crisis was over; let the digester fail because the farmer had no incentive to keep the digester running.

Some reasons for the renewed interest in digesters over the past 8 years have occurred due to the encroachment of housing developments bordering farm land. These new residents complain about odors from the farms and crop fields after land application of manure containing vital nutrients that support crop growth. Farmers look for ways to continue to be good stewards of the land and combat odor complaints. The completion of de-regulation of Pennsylvania electrical utility companies in the very near future is another reason farms are looking at digesters as a way to cut down or eliminate electricity purchases from power companies. Farmers also face new and more restrictive regulations for nutrient discharges from non-point sources, nutrient management changes (phosphorus) and the growing concern of greenhouse gas emissions coming off the farm. An anaerobic digester system with power production is one tool to help farms meet these new regulations and keep their farms operating and profitable.

Grants and loans from various agencies and programs such as the U.S. Farm Bill, Pennsylvania Department of Environmental Protection (PADEP) Energy Harvest Grants, United States Department of Agriculture (USDA) Environmental Quality Incentives Program (EQUIP), Pennsylvania Department of Agriculture's Machinery and Equipment Loan Fund (MELF) and Pennsylvania Governor Rendell's Renewable Energy Portfolio Standards has made capital available for farmers to install an anaerobic digester into their manure handling system to virtually eliminate odor from manure, make a stable, nutrient rich effluent available to apply to crop land, reduce pathogens and weed seeds, produce renewable green energy, destroy methane (a very potent greenhouse gas) and produce by- products such as bedding and soil amendments which have value and can be sold to help offset the cost of the digester system. Every PA digester installed over the past eight years (as did the first four digester systems built during the energy crisis in the late 1970's) has made electric power production part of their system. This added component to the system adds significant costs to the overall digester project, but without the power production capability these anaerobic digester systems would not be cost effective at the current $1M plus price tag.

Pennsylvania has a law known as ACRE (Agriculture Communities & Rural Environment) or Act 38. The first paragraph of the law states: ACRE creates a process for farmers to seek judicial review of ordinances believed to be restrictive of normal agricultural operations. Farmers will have the ability to request the Pennsylvania Attorney General to review an ordinance restricting agriculture that the farmer believes to be illegal (ACT 38 of 2005). The law was enacted to protect farmers from municipal ordinances that would interfere with traditional farming practices resulting in forcing the farm out of business or causing great financial burden. An example of a restrictive municipal ordinance could be Air Quality/"Noxious" Odors that would attempt to mitigate odor problems by establishing vague restrictions and guidelines for what constitutes a violation (ACRE 2005). Some municipalities have also tried to enact ordinances that limit farm size, who can own a farm, set water supply protections that require setbacks in excess of what is required by the Nutrient Management Act, and restrictions on construction of new buildings or roads that are necessary to operate the farm. Before ACRE, when a municipality passed an ordinance on agriculture and a farmer felt it was unlawful (in violation of state law) or jeopardized his operation the only recourse was to sue the municipality.

In a letter from PA Secretary of Agriculture, Dennis C. Wolff, to Friends of Agriculture he wrote: "This new law will address potentially restrictive local agricultural ordinances affecting the state's leading economic enterprise. I believe Act 38 strikes the proper balance between farmers and communities by creating a process for farmers to seek review of ordinances believed to be illegal and restrictive of normal agricultural or farming operations" (July 13, 2005). He also goes on to say, "Our goal was to provide agriculture the opportunity to grow and adapt to business changes, while also addressing the needs of communities to protect the environment. Act 38 helps accomplish that goal" (July 13, 2005). ACRE provides piece of mind to farmers who diligently are good stewards of land, water and air, who have filed and follow nutrient management requirements, federal air and water quality regulations and meet the right to farm law. An illegal municipal ordinance hurts all of us, farms go out of business, the supply of grains, dairy products, eggs and meat, we all take for granted, are lost and raises food costs across the board for everyone.

Farm-based Anaerobic Digesters

Pennsylvania currently has 16 known operating farm-based anaerobic digesters with two others in the design phase and one digester in the construction phase. All of these digesters are designed to operate at a mesophilic temperature, except one - Zimmerman Farms is designed to operate at a thermophilic temperature. All 16 farms are using biogas to power a combined heat & power unit (CHP) to make electricity & using engine and exhaust heat for various heating applications (table 1). There are seven digester systems in the planning stage.

Table 1: List of Pennsylvania Anaerobic Digesters operating, in design phase or under construction.
Farm Name
YearType of Digester
Animal Type
# Animals Contributing
CHP Unit Rating
Heat Recovery Utilization
Dairy
Brookside Dairy
2006Modified Plug Flow (Slurry Loop)
Dairy42585 kW
digester and hot water
Brubaker Farms
2007Complete Mix
Dairy900160 kWdigester and genset radiator air used to dry separated solids used for bedding
Dovan Farm
2006Plug Flow
Dairy400100 kWdigester and hot water
Four Winds
2006Plug FlowDairy500130 kWdigester and hot water
Hillcrest Saylors Farm
2006Plug FlowDairy750100 kWdigester and hot water
Main Farm
2006Complete MixDairy50090 kWdigester and hot water
Mason Dixon Farm
1978Modified Plug Flow (Slurry Loop)Dairy2985600 kWdigester and home heating
Oregon Dairy
1986Modified Plug Flow (Slurry Loop)Dairy38565 kWdigester and farmhouse domestic hot water and home heating
Penn England
2006Plug Flow
(Mixed Loop)
Dairy800160 kWdigester and hot water for milk parlor, genset radiator air used to dry separated solids used for bedding
Schrack Farms
2006Plug FlowDairy650200 kWdigester and hot water for milking parlor
Wanner Pride-N-Joy Farm
2007Plug Flow Circular (Slurry Loop
Dairy400130 kWdigester and hot water
Swine
Beaver Ridge Farm
Design 2008Complete MixFinishing3000130 kW
digester and hot water
David High
1998Vertical Plug Flow
Swine120022 kW
digester and hot water
Mathis Farm
Design 2008
Complete MixFarrow to Finish
27215 kW
digester and hot water
Pine Hurst Acres
2004Complete MixFinishing440047 kW
digester
Rocky Knoll LMD Partners
1985Complete MixFarrow to Finish and food waste
1000130 kW
digester and hot water
Beef and Poultry
Brendle Farm
1984Slurry Loop
Layers7200065 kW
digester, pre-heat egg wash water for egg processing, heat egg processing area and office
Zimmerman12007Complete MixBeef / Broilers
1000 / 120000
175 kW
digester and hot water
Food Waste
Fairview Swiss CheeseConstruction 2008mobilized film technologycheese whey and cone batterNAUnknownBoiler to produce steam and electricity for processing milk into cheese

1. Information gathered by personal communication with the designer, David Wagner (2006 & 2007)

Anaerobic Digestion Brief Description

Man-made, anaerobic digesters have been used for hundreds of years. An anaerobic digester is an air tight, oxygen-free container that is fed an organic material; typically on farms this consists of animal manure and food waste. Naturally occurring bacteria in the organic waste under go a biological process and produce a mixture of methane and carbon dioxide gases, commonly known as biogas. The anaerobic process reduces odor by reducing volatile fatty acids, produces a nutrient-rich effluent, reduces pathogens and weed seeds, produces methane gas which can be converted into electrical or thermal energy, and improves the storage and handling characteristics of manure. The digester does not remove nutrients (nitrogen, phosphorus, potassium) and requires environmentally responsible manure storage and handling.

Electrical Power Interconnect

For the most part, Pennsylvania farmers have not had a very good experience with interconnection to the utility controlled, electric power grid. Large stand-by charges, stranded costs and negotiating good contracts with the power companies to purchase excess electricity have been an uphill battle. Recent legislation was introduced into the PA government, consisting of Energy Portfolio Standards, and Net Metering. Both pieces of legislation have been enacted into law. The Energy Portfolio Standards and Net Metering laws have allowed Pennsylvania farm-based digester electrical generators to produce "renewable energy" for the grid profitably. Net metering basically allows the farmer to sell its excess power to the grid at the retail generation rate.

Green Energy, Carbon Credits and Renewable Energy Credits (REC)

Anaerobic digesters produce and capture methane and carbon dioxide (biogas) which are greenhouse gases. Methane has 23 times the greenhouse effect as carbon dioxide. When the biogas is used in a combined heat and power unit (CHP) as fuel, the methane is prevented from escaping to the environment and is converted to heat, electricity and carbon dioxide. Excess methane can also be destroyed by using a flare. This destruction of the methane can be calculated into carbon credits. Some Pennsylvania farms have sold their estimated carbon credits for a 20 year period and have received a lump sum payment for those credits to help finance the digester project. One such company, Environmental Credit Corporation, certifies the farm's methane destruction and trades the carbon credits on the Chicago Climate Exchange. The sale of Renewable Energy Credits (REC) has also been a way for farms to get capital upfront to finance part of the digester system. These programs are in their infancy and long term success is unknown.

Case Study Summaries

A biogas website to communicate farm-based anaerobic digester information to the public has been created at The Pennsylvania State University, College of Agricultural Sciences, in the Department of Agricultural & Biological Engineering at University Park, PA. Information on: What is an Anaerobic Digester, History of Anaerobic Digestion, Types of Digesters (with case study examples from across the United States), AD Safety, Resource Listings of equipment, designers, financing and most recently, six Pennsylvania farm-based AD case studies were added to the biogas website.

These case studies include: Brendle Farm (poultry), Brookside Dairy, Oregon Dairy, Pine Hurst Acres (swine), Penn England Farm (dairy), and Schrack Farms (dairy). Information for the case studies was gathered from the farm owners and digester operators using a voluntary questionnaire, on farm visits and telephone follow-ups. A quick summary of the six case studies are listed below.

Brendle Farm

  • Type of farm: Belted, Caged
  • Layer Name of farm: Brendle Farm
  • County: Somerset Brendle Farm
  • Feasibility Study: 1982 by Wayne Bogovich of USDA Natural Resources Conservation Service (NRCS) (Bogovich 2004)
  • Digester designer: Bert and Dick Waybright, Gettysburg, PA
  • Digester installer: Brendle Farm
  • Construction start date: Spring 1984 (designed in 1983)
  • Date Digester became operational: June 1984
  • Number of animals contributing manure to the digester: 72,000 laying hens
  • Manure handling system: caged layers manure belt, augered to liquid mix tank
  • Type of digester: slurry loop
  • Digester cover: flexible
  • Digester temperature: mesophilic 95°F
  • Biogas uses: operate the CHP unit to produce electricity and heat
  • Biogas utilization equipment: engine generator
  • Heat Recovery Utilization: engine generator water jacket to heat the digester, pre-heat wash water for egg processing and to heat the egg processing area and the office.
  • Power Purchase Agreement: Yes
  • 2008 status of digester: operational
  • Information provided by: Robert and Michael Brendle, (2006 & 2007)

Brookside Dairy

  • Type of farm: Dairy
  • Name of farm: Brookside Dairy Farm
  • County: Indiana
  • Digester designer: Jim Resh Engineering/Keller Engineering, Altoona, PA
  • Digester Installer: Jim Resh Engineering & Brookside Dairy
  • Construction start date: June 2005
  • Date Digester became operational: April 2006
  • Number of animals contributing to the digester: 425 milking cows
  • Type of barn: 2 freestalls
  • Manure handling system: continuous mechanical alley scrapers
  • Type of bedding: sawdust/wood shavings
  • Type of digester: modified plug flow (slurry loop)
  • Digester cover: flexible
  • Digester temperature: mesophilic 95°F
  • Biogas uses: operate the CHP unit to produce electricity and heat
  • Biogas utilization equipment: engine generator set
  • Heat recovery utilization: engine generator and exhaust jacket to heat digester
  • Power Purchase Agreement: Yes
  • 2008 Status of Digester: operational
  • Information provided by: Aaron George, Jim Resh, (2006 & 2007)

Oregon Dairy Farm LLC

  • Type of farm: Dairy
  • Name of farm: Oregon Dairy Farm LLC
  • County: Lancaster
  • Digester designer: Richard and Burt Waybright, Gettysburg, PA
  • Digester Installer: Subcontractors with Farm as General Contractor
  • Construction start date: 1985
  • Date Digester became operational: 1986
  • Number of animals contributing to the digester: 385 milking cows
  • Type of barn: freestall
  • Manure handling system: alley scraped
  • Type of bedding: sawdust
  • Type of digester: modified plug flow (slurry loop)
  • Digester cover: hard (concrete)
  • Digester temperature: mesophilic 90°F to 100°F
  • Biogas uses: operate the CHP unit to produce electricity and heat
  • Biogas utilization equipment: engine generator set
  • Heat recovery utilization: engine generator and exhaust jacket to heat digester and farmhouse domestic hot water and heat
  • Power Purchase Agreement: No
  • 2008 Status of Digester: operational
  • Information provided by: George Hurst, (2006)

Penn England Farm

  • Type of farm: Dairy
  • Name of farm: Penn England Farm
  • County: Blair
  • Digester designer: RCM Digesters (Mark Moser) Berkeley, CA
  • Digester Installer: Penn England Farm acted as General Contractor
  • Construction start date: October 8, 2005
  • Date digester became operational: August 17, 2006
  • Number of animals contributing manure to the digester: 720 milking + 80 dry
  • Housing system: freestalls
  • Type of bedding: dried digested separated solids
  • Manure handling system: alley scrapers to auger
  • Type of digester: mixed loop
  • Digester cover: insulated flat flexible
  • Digester temperature: mesophilic 100°F
  • Biogas uses: operate the CHP unit to produce electricity and heat
  • Biogas utilization equipment: engine generator set and flare
  • Heat recovery utilization: heat digester, hot water for milking parlor, genset radiator air used to dry manure solids
  • Power Purchase Agreement: Yes
  • 2008 status of digester: operational
  • Information provided by: Frederick England and Ben Postles, (2006 & 2007)

Pine Hurst Acres

  • Type of farm: Swine
  • Name of Farm: Pine Hurst Acres
  • County: Northumberland
  • Digester designer: Schick Enterprises, Kutztown, PA (800) 527-7675
  • Digester installer: Schick Enterprises
  • Construction start date: November 2003
  • Date digester became operational: October 2004
  • Number of animals contributing manure to the digester: 4,400 grow/finish hogs
  • Housing system: 2 barns, 4 auto-sort pens per barn, 550 head per pen
  • Manure handling system: slotted floor, deep pit under floor
  • Type of digester: complete mixed, below ground, heated
  • Digester cover: flexible
  • Digester temperature: mesophilic 95°F Biogas uses: operate the CHP unit to produce electricity and heat
  • Biogas utilization equipment: engine generator set and flare
  • Heat recovery utilization: heat digester
  • Power purchase agreement: no
  • 2008 status of digester: operating
  • Information provided by: Robb Meinen (Sr. Extension Associate), Stanley and Richard Crone (2006 & 2007)

Schrack Farms

  • Type of farm: Dairy
  • Name of farm: Schrack Farms
  • County: Clinton
  • Digester designer: RCM Digesters (Mark Moser) Berkeley, CA
  • Digester Installer: Schrack Farms acted of the General Contractor
  • Construction start date: April 2005
  • Date Digester became operational: August 2006
  • Number of animals contributing to the digester: 650 milking cows
  • Type of Barn: freestall
  • Manure handling system: alley scraped to tank
  • Type of Bedding: sawdust
  • Type of digester: plug flow (straight)
  • Digester cover: flexible
  • Digester temperature: mesophilic 100°F
  • Biogas uses: operate the CHP unit to produce electricity and heat
  • Biogas utilization equipment: engine generator set, auto flare
  • Heat recovery utilization: engine generator water and exhaust jackets to heat digester, hot water for milking parlor
  • Power Purchase Agreement: Yes
  • 2008 Status of Digester: operational
  • Information provided by: Jim Harbach (2006 & 2007)

Unique items and problems that have occurred on PA farms installing digesters

Brendle Farm - liquid chicken manure

Where most farms are looking to limit the amount of water put into the manure before it enters the digester, Brendle Farm wanted to be able to irrigate the digested manure slurry onto their fields. The dry chicken manure is augered to a mixing tank. Water is added and mixed with the manure to make a slurry, which then is pumped into the pre-heat tank. The effluent from the pre-heat tank becomes the influent for the digester at the desired total solids content.

Brendle Farm - pre-heat tank

A pre-heat tank was added early in operation as a limestone grit and feather removal mechanism to prevent the anaerobic digester from clogging.

Brookside Dairy - emergency biogas relief valve

The designer of this AD system uses an emergency biogas relief assembly that is attached directly to the flexible cover to prevent over pressurization in the event the biogas piping gets clogged.

Raw biogas is piped through eight inch diameter, PVC piping to the engine generator set. At the lowest point in the biogas piping a moisture trap/pressure regulating drip pot removes condensate and acts as a pressure regulator to maintain a 0.8" water column pressure under the flexible cover (Resh 2006).

Brookside Dairy - divided receiving pit

Sometimes major re-configuration of existing structures needs to take place in order to accommodate new manure handling patterns. An example of this is: the manure from the Brookside freestall barn is thick and needs to be mixed with manure collected from the dry cow barn and milking parlor waste water pit which is more dilute to achieve the desired 8 - 10% solids before entering the digester. To achieve this, the farm installed a manure pump at the dry cow barn/milking parlor pit to pump manure to the freestall receiving pit. The freestall manure pit was retrofitted with a dividing wall in order to have a way of mixing the two manures together to get the required percent solids needed for the digester influent. A second pump in the receiving/mix pit mixes the two manure concentrations. A heat exchanger in the receiving/mix pit preheats the manure. A diverting valve was also installed to direct the mixed, preheated manure to the digester.

Penn England Farm - flat cover

When we visited Penn England Farm the summer of 2006, we saw for the first time a flat flexible cover on the complete mixed digester. The digester operator stated the designer had specified this type of cover due to the high winds coming across the hill where the digester is located. Since then we have seen two other newly operating digesters in PA with this same type of flat flexible cover. In Pennsylvania, this designer has used a flat flexible cover design on other large diameter, complete mix digesters.

Penn England Farm - separated solids drying system

Penn England also had another feature we had not seen incorporated into an AD manure handling system. A conveyor belt system is being used to move separated manure solids across the solids storage area for even distribution across the floor. There is hot air from the radiator off the engine-generator set being piped under the concrete slotted floor to dry the separated manure solids to be used as bedding for the cows.

Schrack Farms - concrete and bolt delay (1 yr.)

A major problem with the concrete sub-contractor building the plug-flow digester tank on Schrack Farms delayed the entire project for almost a year. The groove along the entire top edge of the digester was not cast properly. Also, bolts of two different sizes were not cast in the proper locations.

Schrack Farms - offal pit

The original design for the Schrack Farms digester included receiving offal, equivalent to the manure from 200 cows, from a local packing plant. This plant burnt down during digester construction, but the Schrack's hope to add this additional feedstock in the future.

Pine Hurst Acres - under the barn digested manure storage

Under the floor pits, for storage of raw manure, in hog houses is a common design, but Pine Hurst Acres under the floor pits included three pits for storage of digested manure under each of the two hog barns. The odor reduced, nutrient rich, effluent gravity flows back to the two barns under floor pits. The under floor pits in each barn were divided into two raw manure sections and three digested manure sections. The digested manure is stored in the under floor pits until ready for land application of the nutrients. Gas concentration monitoring has been performed on the air quality of the atmosphere at hog elevation. Ammonia, hydrogen sulfide, methane and carbon dioxide gas concentration levels are monitored and were not found to exceed recommended maximum concentrations for either animals or humans (Meinen 2008).

Conclusion

Many of Pennsylvania's farms have grown from less than a hundred cows to over 500 cows, along with the growth comes more responsibility for the farmer in odor reduction, long term liquid storage of manure, nutrient management, better handling of manure, water and air quality concerns, a higher cost of production and meeting any new regulations that come in the future. Pennsylvania farmers will continue looking for new and improved technologies to make their farms more energy efficient and productive, such as digesting food waste along with the animal manure. For now, one of those technologies is anaerobic digesters with power production. The number of Pennsylvania digesters in the Keystone State is growing. The end of 2008 could very well see the number rise from 16 to 22 operating digesters.

Continued efforts are planned to make farm visits and gather information from additional Pennsylvania farms with anaerobic digesters. A digester owner/operator user group is also being formed.

Acknowledgements

Support for the biogas website and this project was made possible by: Mid-Atlantic Regional Water Program USDA CSREES, Department of Agricultural and Biological Engineering, College of Agricultural Sciences and Transferring Innovative Manure Management Technology in the Northeast USDA.

References

  • Bogovich, W. M. 2004. Long term operation and maintenance of a digester at the Brendle farms poultry operation, NABEC Paper No. 04-0042, University Park, PA, USA: NABEC.
  • Brendle Farms. 2006. Personal Contact: Robert Brendle and Michael Brendle
  • Brendle Farms. 2007. Personal Contact: Michael Brendle
  • Brookside Dairy. 2006. Personal Contact: Aaron George, Kevin George, Keith George and Jim Resh
  • Brookside Dairy. 2007. Personal Contact: Aaron George
  • Franks, Tim 2005. Cows make fuel for biogas train. BBC News/Science/Nature. Accessed 9 May 2008.
  • Lusk, P. 1998. Methane Recovery from Animal Manures The Current Opportunities Casebook. NREL/SR-580-25145, Golden, CO
  • Meinen, R. J. 2008. Evaluation of economic feasibility and animal performance for a novel manure collection and anaerobic digestion system at a commercial swine finisher enterprise. MS thesis. University Park, Pennsylvania: The Pennsylvania State University, Department of Dairy and Animal Science.
  • Meynell, P.J. 1976. Methane: Planning a Digester. New York, NY: Schocken Books. pp3.
  • Oregon Dairy. 2006. Personal Contact: George Hurst
  • Penn England Farm. 2006. Personal Contact: Fredrick England and Ben Postles
  • Penn England Farm. 2007. Personal Contact: Ben Postles
  • Pennsylvania Department of Agriculture. 2005. ACRE (Agriculture, Communities and the Rural Environment) ACT 38 of 2005. Accessed 22 May 2008.
  • Pennsylvania Department of Agriculture. 2005. ACRE (Agriculture, Communities and the Rural Environment) ACT 38 of 2005. Local Ordinances. Accessed 28 May 2008.
  • Persson S.P.E., H.D. Bartlett, A.E. Branding, R.W. Regan. 1979. Agricultural Anaerobic Digesters Design and Operation. Bulletin 827, November 1979
  • Pine Hurst Acres. 2006. Personal Contact: Robb Meinen, Stanley Crone and Richard Crone
  • Pine Hurst Acres. 2007. Personal Contact: Richard Crone
  • Resh, J. 2006. Utilization of Manure Digesters on Large Dairy Operations to Establish a Distributed Generation Network-Final Report
  • Schrack Farms. 2006. Personal Contact: Jim Harbach
  • Schrack Farms. 2007. Personal Contact: Jim Harbach
  • Wagner, David. 2006. Personal Contact Wagner Anaerobics
  • Wagner, David. 2007. Personal Contact Wagner Anaerobics
  • Wolff, Dennis C. 2005. PA Sec. of Agriculture, Correspondence to Friends of Agriculture. July 13, 2005. Accessed 22 May 2008.

Prepared by Deborah A. Topper, Research Technician, Agricultural and Biological Engineering, Patrick A. Topper, Senior Research Technologist, Agricultural and Biological Engineering, and Robert E. Graves, Professor, Agricultural and Biological Engineering