Informative Articles: Benefits, Costs & Operating Experience

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.  The AgSTAR Program is sponsored by the USEPA, USDA and USDOE to encourage farm methane recovery from anaerobic digestion.  AgSTAR has provided technical assistance to over 14 farms to assist them through the development, installation, start-up and operating phases of their anaerobic digestion projects.   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 since January 1997 with AgSTAR technical assistance.  The farms and their digester systems are described.  The cost of the digester systems are summarized.  Biogas recovery and use in boilers or engine-generators is discussed for each farm.  Start up and operational lessons learned are presented.

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

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. 

AgSTAR is a voluntary program sponsored by the US Environmental Protection Agency (EPA), the US Department of Agriculture - Natural Resources Conservation Service (NRCS) and the US Department of Energy (DOE), to encourage methane capture as a part of manure management.  Since 1996, AgSTAR provided technical support to 14 AgSTAR Partner farms who were developing anaerobic digester systems.  The system installation, operation and benefits are described.  A summary table by type of digester is presented to allow comparisons of costs.

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 methane to maintain digester temperature.  Three complete mix digesters were built on farms with pull plug manure collection. The digesters are described, benefits enumerated and a summary table allows comparison of costs and benefits. All are heated with biogas heated water and mixed to maintain a high level of bacterial activity. All of the units are built in cold climates.  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 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 accomodate 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 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 flow.  The farm, originally a continuous flow facility, converted to all-in-all-out operations. Digester heating began in December 28, 1998.  January 24, 1999 in midwinter temperatures the boiler system was switched to biogas and has run full time with little or no maintenance since. The digester was 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 mill HDPE coated fabric are 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 from 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 nearly 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 sale 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 using natural gas to fuel the generator in August 6, 1999. 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 1.7 million cubic feet of biogas and producing 140,000 kWh of power 50 kW avg.  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 2,268,000 ft3 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 intertie requirements.  All generator equipment should be tested prior to being shipped to the site.

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 and contributing to the farm’s environmentally sound manure management strategy.  At SWUSA, the digester is 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 off 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 systems is estimated at $8,000 per year ($0.015/kWh). Table 1 summarizes the costs and benefits of these systems.

a.     Included in digester

b.     Breakdown not available

c.     Reconditioned equipment

PLUG FLOW DIGESTERS

Plug flow digesters are used to treat scraped manure from dairies.  Over the past 4 years, three 1,000 cow digesters have been built. 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.  It appears that the digested liquids produced a higher quality hay than previous manure nutrient management.   Pathogen concentrations were reduced.  Odors were reduced substantially as reported by the neighbors.

AA Dairy,  Candor, New York - 1000 cow digester

AA Dairy Farm of built and started up a at their 550 cow facility, completing 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 at their 480 cow facility while currently completing their expansion to 1,000 cows completing a boiler fired system 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 70 kWh and hot water to keep the milking parlor warm. Odor has been controlled.   Manure handling is much easier for the owner.

Table 2shows their costs and benefits.  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 and the balance of the cost from the owner. 

a.     Pump and wiring, no concrete

b.     Reconditioned, used equipment