10 Digesters:

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To be presented at:  Eighth International Symposium and Exhibition on Animal, Agricultural and Food Processing Waste.  Marriot Conference Center, Des Moines, IA. October 9-12, 2000

BENEFITS, COSTS AND OPERATING EXPERIENCE
AT TEN AGRICULTURAL ANAEROBIC DIGESTERS

Mark A. Moser and Richard P. Mattocks[1]

ABSTRACT

Farmer motivation for building and operating anaerobic digesters has expanded from solely energy benefits to include manure treatment cost savings, nutrient conversion, odor and pathogen control, and byproduct recovery.  Over the last 3 years, three dairy plug flow digesters (NY, CT, OR), four covered pig manure lagoons (NC, VA, IA, MS) and three heated mixed pig manure digester (CO, IO,IL) have been placed in operation with AgSTAR technical assistance.  The farms and their digester systems are described.  The system installation, operation and benefits are described.   A summary table by type of digester is presented to allow comparisons of costs. Biogas recovery and use in boilers or engine-generators is discussed for each farm.  Start up and operational lessons learned are presented. The USEPA, USDA and USDOE sponsor the AgSTAR Program to encourage farm methane recovery from anaerobic digestion. 

Introduction

Anaerobic digestion is more extensively used outside of the US where treatment of animal waste has been  a concern for a longer time.  An anaerobic digester is a vessel designed to retain decomposing manure for sufficient time at the designed operating temperature to allow the growth of methanogenic bacteria in a “steady-state”.  Electricity and heat production are direct benefits of anaerobic digestion. The effluent of a digester has an earthy smell with some ammonia present. The first dairy digester systems in the US were installed principally to produce energy during the energy crisis.  The first pig manure digester systems in the US were installed principally to control manure odors. Today, farmer motivation for building and operating anaerobic digesters has expanded from direct energy benefits to include key non-energy benefits such as: odor control, improved manure handling, mineralization of organic nitrogen, weed seed destruction, pathogen reduction and byproduct production such as digested dairy solids. 

Complete Mix Digesters

Complete mix digesters are used to treat waste with 3 to 10% total solids and adequate volatile solids to produce enough biogas (60% methane, 40% carbon dioxide) to maintain digester temperature.  Three complete mix digesters were built on farms with pull plug manure collection in cold climates. All are heated with biogas-heated water and mixed to maintain a high level of bacterial activity. All units represent a cost savings over using an ambient temperature lagoon designed to Natural Resources Conservation Service (NRCS) or American Society of Agricultural Engineers (ASAE) standards to perform the same level of treatment for odor control.  Ambient temperature lagoons would be 10-20 times larger than these digesters to perform the same function.  The design and operation goal of the Apex Digester is consistent biological stabilization of waste and odor control, while the goal of Colorado Pork and Swine USA is optimization of biogas production and use while stabilizing the waste.

Apex Pork - 8,600 Head Pig Finishing Facility, Rio, IL

Apex Pork has 8,600 hogs in 9 buildings with pull plug manure collection.  Three years of seasonal odor episodes from the manure storage pond were not acceptable to downwind neighbors. The farm chose Resource Conservation Management, Inc. to design an innovative, low cost, heated, mixed covered earthen lagoon digester.  A bank-buried insulated floating cover is installed on the 36.3 x 48.5 x 5 meter (120 x 160 x 16.5 ft) lagoon which can hold roughly 20 days worth of manure. The digester produces enough methane to fuel a boiler to heat the digester and flares unused methane.  Stabilized digester effluent flows to the storage pond.  The pond no longer emits odorous gases. The primary benefits to the owners are odor reduction of stored and field applied manure. There have been no odor complaints since the digester was installed. Compliance investigations by state regulators stopped. The digester is able to accommodate the variation in manure loading rate that results from all-in-all-out operation.

The digester started up in about 30 days switching to biogas from propane in June 1998. By mid summer, 1998 the system was producing 1,020 m3  (36,000 ft3) of biogas per day. The unit was running well when a microburst damaged the gas collection cover and nearby buildings in August 1998.  The partial cover was replaced with a bank attached floating cover over the whole lagoon in October 1998.  Concurrently the farm was emptied of pigs to allow building renovations and a change of pig production technique.  The farm, originally a continuous flow facility, converted to all-in-all-out operations. Digester heating began in December 28, 1998.  The boiler system was switched to biogas January 24, 1999 and has run full time with little or no maintenance since. The digester gained temperature during the coldest times of the year. The digester has been through 3 grow-out cycles without problems.  The system is simple to operate and the owner only spends about 10 minutes daily to check the system.   Several lessons of this project were: 1. A bank-buried complete lagoon cover, though a more expensive first cost, is less expensive to maintain than a floating edge partial cover.  2.  Inexpensive cover materials such as 20 mil HDPE coated fabric can present problems in extreme weather.  3.  Pig operations can change rapidly and radically and present a completely different waste stream.

Colorado Pork, Lamar, Colorado and SWUSA, Thayer, IA  -  5,000-sows farrow-to-wean each

The farms are very similar in size and operation.  Colorado Pork has about 600 more pigs growing as replacement gilts. However, the farm buildings and equipment are different.  Both facilities were built in 1997-8 on sites that had been range or pasture.  Environmental concerns of neighbors and state authorities prompted both owners to weigh waste management alternatives.  Both chose complete mix digesters with separate storage to: 1. biologically stabilize manure, eliminate odor and optimize methane recovery for electricity production. 2. reduce their investment in waste management and 3. avoid problems with regulators by installing an environmentally beneficial system.

Periodically, plugs are drained to a concrete collection tank and then pumped to the 2,100-2,200 m3 concrete tank digesters. The digesters are heated, mixed and covered by two biogas inflated plastic gas collection domes. Effluent at Colorado Pork flows into a nearby basin to be evaporated, while SWUSA effluent is stored in 2 concrete tanks for annual application to cropland.  Biogas fuels Caterpillar 3306 engines attached to 80 kW generators.  All electricity produced is used on the farms. Heat, recovered as hot water from the engine and exhaust, is used to maintain the appropriate temperature in the digester. Biogas is automatically flared when not being used by the engine (for example during maintenance).  The farms produce up to 60% of their own electricity needs.  The farms are not expected to have surplus electricity to sell to the utility and do not have sales metering.  The farms chose not to sell electricity rather than submit to uneconomical requirements and controls of the utility company. Future plans include using excess heat in the farrowing barns.

The Colorado Pork digester heating began August 6, 1999 using natural gas to fuel the generator. System operations on biogas began late September 1999. The digester has been problem-free. The engine has had typical startup mechanical problems, however, the unit has been operating continuously. In the first 5 months, the generator operated over 90% of the time, burning 48,160 m3 (1,700,000 ft3) of biogas and producing 140,000 kWh at and average load of 50 kW.  The system output increased to 67 kW after 2 bad sensors were found and fixed.

The SWUSA digester was filled and heated over a 90-day period and started biogas production in August 1999, after delays due to additional construction on the digester tank.  Full biogas yield was reached in September 1999.  The digester has been virtually trouble free.  The generator has operated 77% of the time during the first 6 months yielding 64,250 m3 (2,268,000 ft3) of biogas with an electricity output average of 67 kW. Typical startup issues of bad sensors cropped up.  The engine was rebuilt under warranty after a breakdown due to loss of coolant.  The system had a manure spill reach a ditch through a drain added by contractors. The local utility and the larger transmission and distribution utility were inconsistent on requirements for intertieing the farm to the utility grid.  Table 1 summarizes the costs and benefits of these systems.

The methane recovery system at both Colorado Pork and SWUSA has diminished farm costs of production by reducing the quantity of electricity purchased, while greatly improving site odors from manure storages which were objectionable to all who experienced them and contributing to the farm’s environmentally sound manure management strategy.  At SWUSA, the digester is annually producing about $46,600 worth of electricity at $0.09/kWh.  The annual value of electricity produced at Colorado Pork is estimated to be $34,800 based on $0.07/kWh.

Apex paid for all labor and material except AgSTAR assistance during cover installation, start-up and minor troubleshooting. Total cost of all work and changes was $152,300 or $17.75 per finisher space.  Operation and maintenance costs have been less than $1000 per year for electricity and boiler servicing. Colorado Pork paid all project costs except AgSTAR technical assistance.  The installed cost was $368,000 or $73.60 per sow.  SWUSA received a 100% grant for the project plus AgSTAR technical assistance. The undifferentiated installed cost was $576,000 or $115.20 per sow.   Given the similarities of the systems, the author has no explanation for the reported $208,000 difference in system costs. Maintenance for either system is estimated at $8,000 per year ($0.015/kWh).

a. Included in digester

b. Breakdown not available

c. Reconditioned equipment

Plug Flow Digesters

Plug flow digesters are used to treat with 11 to 13% total solids scraped manure from dairies. Plug-flow digesters are heated, unmixed, rectangular tanks. New waste is pumped into one end of the digester, displacing an equal portion of older material horizontally through the digester, and pushing the oldest material out through the opposite end of the digester.  Over the past 4 years, three 1,000 cow digesters have been built in the US by the author’s clients. The digesters are described, benefits enumerated and a summary table allows comparison of costs and benefits.

Craven Dairy Farm, Cloverdale, Oregon – 1,000-cow digester

Craven Farms completed a heated, unmixed, plug flow digester sized for the daily manure production of 1000 cows in December 1996. The winter startup was not a difficult problem.  The digester performed without incident.  The used engine generators presented many challenges and required a lot of small repairs.  However, the used generators were $50,000 cost saving over new units.  The digester peaked when treating manure from 750 cows on site and 250 cows worth of manure from neighboring dairies.  In November 1999 the farm was sold.  The owner is expected to move in and begin operations in summer of 2000.  

The digester produced about $ 24,000 of electricity and $ 30,000 of digester fiber yearly.  The value of digested solids is twice the original estimates.  The digester eased manure handling and reduced the cost of application.  Pathogen concentrations were reduced  two orders of magnitudes based on 3 sets of  samples. 

AA Dairy, Candor, New York - 1000 cow digester

AA Dairy Farm built and started up a at their 550 cow facility as a boiler fired system in October 1997.  The digester is sized for planned expansion.  AgSTAR provided technical assistance in all phases of the project.  Issues with New York State Electric and Gas Co. over a used intertie panel previously approved in Pennsylvania delayed engine-generator startup until June 1998.  Modifying the previously approved panel to conform to NYSEG wishes cost the owner $21,000.  Additional issues have arisen over NYSEG punitive insurance requirements that are being resolved.

The digester has operated without problems.  The engine has had occasional outages for repair.  In total the generator is operating over 90% of the time. The system is currently producing 70 kWh, hot water and about $60/day of digested fiber. Odor has been controlled.   The cost of manure application is substantially reduced.

Haubenschild Dairy,  Princeton, Minnesota - 1000 cow digester

Haubenschild Dairy Farm started up a digester at their 480 cow while concurrently completing their expansion to 1,000 cows completing startup of their generator in October 1999.  The major challenge was from the generator company who delayed equipment delivery, hence system startup for months. The utility, Steele-Waseca Cooperative has been a promoter of and asset to the project.  The project has been well received and promoted within the state.  The dairy has chosen to wait to install a separator to recover solids for resale.

The digester has operated without problems.  The engine has had occasional outages for adjustment.  The generator is operating over 95% of the time. The system is currently producing 85 kWh and hot water to keep the milking parlor warm.  Manure handling is much easier for the owner.

Table 2 shows the costs and benefits of these three systems.  The cost variation between the systems depends mostly on equipment that the farms already owned and the engine-generator.  The benefits depend on the value of electricity and the value of byproduct fiber and heat. AA Dairy funding included a $90,000 grant from the local Soil Conservation District to improve manure management with the balance of the cost paid by the owner.

a.      Pump and wiring, no concrete

b.     Reconditioned, used equipment

Covered Lagoon Digesters

Covered lagoon digesters are most successful in warmer climates south of the Mason-Dixon line.   A properly sized lagoon receives dilute either flush or pull plug collected manure and decomposes the material resulting in year round biogas production.  Gas production varies seasonally.

Barham Hog Farm,  Zebulon, NC - 4000 sow farrow to wean

Barham Hog Farm has 5 buildings with pit recharge.   The farm was built with a single cell treatment and storage lagoon.  The project installed a separate covered lagoon prior to the existing lagoon. AgSTAR provided design, installation and troubleshooting support and worked with NRCS to design the lagoon.  Lagoon construction began in July 1996.  The lagoon cover, 400,000 Btu boiler and a 120 kW generator were installed in December 1996.  Biogas use for heating water began in January 1997.  Lagoon cover manufacturing problems limited biogas recovery and the production of electricity, however the boiler has operated almost continuously, providing hot water for pig mats under farrowed pigs.  The owner was refunded his money and has purchased a new 40 mil HDPE cover.  The 18 month average for biogas recovery is 632 m3/d (22,300 ft3/d) of biogas.  Much of the year the generator is operated 12 hr/d at up to 90 kW during the daytime and during nighttime 12  hours a boiler operated to produce hot water for Keeping baby pigs warm . Odor is virtually non-existent. Cheng (1999) found that 30% of the total Kjeldahl nitrogen (TKN), 75% of the P and 20% of the K was retained in the covered cell. Cheng (1999) found pathogen reduction to be 2 to 3 orders of magnitude. 

The farm has been limited to offsetting about $18,000 per year in electricity and  about $12,000 per year in propane purchases.  The local utility is not in favor of farm cogeneration.  However, odor control benefit is very important to the owner because large subdivisions are being built within one mile of the farm.  Also, the improved biological stabilization and nutrient mineralization in the digester resulted in the effluent from the storage lagoon containing 60% less nutrients than before.  Consequently, the farm manure treatment and nutrient application complies with the 1997 manure management regulations without additional investment.

Martin Family Farm, South Boston, VA - 600 sow farrow to feeder pig

Martin Family Farm covered the first cell of a two cell lagoon receiving flushed manure in 1993 and began engine-generator operations in spring 1994 with a matching grant from the Southeast Regional Biomass Energy Program (SERBEP).  The first cover slowly sank and collected less gas each year.  In 1997, Engineered Textile Products of  Mobile, AL and Seamens Corporation contributed a demonstration XR-5 modular cover system to replace the original failed cover design.  Martin Farms installed the new cover and replaced the corroded lagoon heat loop with radiators.  Methane recovery has been continuous throughout the project, though gas use has not been. The farm has produced up to 397 m3/d (14,000 ft3/d) of biogas and 600 kWh/d during the summer.  Winter gas production drops off to less than 170 m3 (6,000 ft3/d) and use has been problematic.  The farm has planned and purchased 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.

The major benefit to the farm has been odor control and elimination of objections by neighbors. The farm has produced several thousand dollars worth of electricity.  A secondary benefit from the two cell approach has been nutrient reduction in the second lagoon and ease of effluent management in sprinklers on fields that are closer to the neighbors than the farm is.

Boland Farm, Williamsburg, IA - 2,700 Head Hog Nursery

Boland Farm installed a low cost Permalon cover in May 1998 over their manure storage basin to capture odorous gases. The cover is a complete, bank to bank design with trench buried edges. The basin produces combustible methane as predicted. Gas is flared.

The cover was designed for owner installation.  The owner, one AgSTAR support person and 6 neighbors assembled and installed the cover over a two day period.  The owner later installed a flare with solar powered ignitor.  Combustible gas is produced and burned in August and September. The lagoon filled for the first time in spring 1999 and was pumped out with a pto pump that was backed down the bank and through a hatch in the cover. Rainfall collected on the cover has been occasionally pumped off.  The cover has almost eliminated odor from the basin.  The owner states that they are now able to hang the wash outside for the first time in years.

Cal Poly University Dairy, San Luis Obispo, CA, 400 cows

The dairy is located adjacent to the California Polytechnic State University campus in San Luis Obispo, California.  The dairy presently milks 180 cows with a total population of over 350 animals. Most of the herd is housed in freestall barns.  About 90 percent of the manure is deposited on concrete and flushed with fresh or recycled water to the lagoon. The remaining 10 percent of the manure is deposited in the corrals and is only collected seasonally.  Solids are separated from the flushed wastewater prior to storage in a single cell lagoon.  This lagoon has a volume of 19,000 m3 (670,000 ft3), which translates to 50 to 90 days of storage, depending upon the water used by the dairy. A new primary lagoon was located next to the existing lagoon, and has approximate surface dimensions of 80 x 65 m (265 x215 ft). The depth varies from 5.2 to 3.7 m (17 to 12 ft) with 2:1 side slopes.  In May 1999, the new lagoon was completed with partial cover in place. The gas handling system including gas blower, meter and the flare were installed and operational. Preliminary gas measurements indicate approximately 130 cubic meters of biogas produced daily from the partial cover of less than 50% of the total lagoon surface area. The biogas is being continuously flared and is maintaining a self-supporting flame. The principle benefit to date has been reduced odor and some reduction of nutrients in the storage lagoon water.  The university plans to cover the balance of the lagoon surface and install a microturbine to use the biogas to produce electricity.

Table 3 summarizes the costs and benefits of the covered lagoons.  These projects were completed with some matching funds, but not more than 50%

SUMMARY

Ten digesters are reported in this paper.  All digesters have started up and functioned well.  All have recovered biogas as expected.  There have been some modifications to gas uses.  Some of the covered lagoon projects have required new floating covers due to materials assembly or cover design problems. 

Electricity and heat production are direct monetary 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.  Odor control, mineralization of organic nitrogen, weed seed destruction, pathogen reduction and improved manure handling are non-monetizable benefits demonstrated by existing digestion systems.  These factors are increasingly important in sustaining farm viability in rural areas and are appreciated and desired by farm owners.

References

Cheng, J., et al.1999. Evaluation of Alternative Swine Waste Treatment Systems in

Comparison with Traditional Lagoon System. In preparation for: Proceedings of the Eighth International Symposium on Animal, Agricultural, and Food Processing Wastes, American Society of Agricultural Engineers, 2950 Niles Rd., St. Joseph, MI  49085, Des Moines, IA, October 9-11, 2000

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