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Management of cow manure is both a challenge and an opportunity. Livestock manure management has limited the growth of the dairy industry. Unstabilized biosolids can cause odor problems. Land application of the liquid fraction that is rich in nutrients can be restricted by environmental regulations. However, cow manure is a renewable energy source. The bioenergy conversion of cow manure, using anaerobic digestion, produces biogas: a mixture composed primarily of methane and carbon dioxide. Biogas can be beneficially used to produce electricity and hot water in a combined heat and power (CHP) system.

Anaerobic digestion is a proven stabilization process for biosolids that is used successfully at municipal and industrial wastewater treatment plants. Although there are a few successful anaerobic digestion systems at agricultural facilities, many have not met expectations. Some digesters at dairies have suffered from low budget designs that have not taken advantage of proven municipal design concepts. Reliable, resilient municipal digestion designs have not been applied at dairy farms due to their high capital cost.

Portland General Electric (PGE) is Oregon's largest utility and serves electricity to more than 750,000 customers in Portland, Salem and nearby communities. With the leadership of PGE in association with engineering firms, the cooperation of Cal-Gon Farms, and taking advantage of incentives (i.e., low interest loans and grants) an anaerobic digester and CHP facility has been designed for a dairy farm based on proven municipal digester design concepts.

This bioenergy conversion project progressed through planning, design, construction, and start-up over a two-year period. The facility has operated for roughly two years. Planning began by setting objectives, gathering data, comparing alternatives, and conducting research. The main objective was to determine the ability for a third party to install and operate a “utility-grade cost-effective” anaerobic digestion and CHP system at a 350–500 cow dairy farm. Alternatives included different types of anaerobic digesters and CHP prime movers. Both mesophilic and thermophilic operating temperatures were evaluated. Cylindrical digester configurations were compared to rectangular. A cylindrical concrete mesophilic digester was selected with provisions for thermophilic operation. An internal combustion engine-driven generator was selected for the CHP system.

Side-by-side bench-scale tests were conducted to compare the operating performance of mesophilic to thermophilic digestion of cow manure. Operating parameters were established, as shown in Table 1, to form the basis of design. Thermophilic digestion was considered for its potential cost saving due to higher achievable loading rates compared to mesophilic digestion. The provision to operate at thermophilic temperatures in the future provided the flexibility to handle future loading rates as the dairy grows.

The project design focused upon critical components (digester configuration, manure feeding, manure heating, biogas hydrogen sulfide (H2S) removal, and biogas utilization). Some components were installed “as-needed” rather than “just-in-case.” This methodology avoided over-design but had the risk of the system not operating ideally during start-up. A general contractor/construction manager (GC/CM) approach was taken. The design started with a municipal-style digester with attention to reducing costs by eliminating redundancy and non-essential components. A concrete cover was designed that allowed for adequate biogas pressures for operation of the CHP system without a booster blower. A biogas scrubber was included to reduce H2S concentrations and protect the internal combustion engine.

Design Criteria
Criteria Values Criteria Values
Manure Digester
 Cows 350–500  Active Volume, gallons 139,000
 Manure Rate, wet 1bs/day/cow (max.) 160  Diameter, ft 28
 Manure, wet tons/day 20–28  Side Water Depth, ft 28
 Flow, gpd 7,800–11,500  Hydraulic Retention Time, day 12–18
 Total Solids Concentration, % by mass 9–13  VS Loading Rate, 1b VS/cf/day 0.18–0.36
 Volatile Solids (VS) Concentration, % 75  Digested fiber, cy/day 7–8
 VS Reduction, % 30–35  Digested manure liquid, gpd 6,000–10,000
Biogas Combined Heat and Power (CHP)
 Production, scfm 12–24  Generator Set Capacity, kW 100
 Production Rate, scf/lb VS reduced 15  Electrical Efficiency, % 28
 Heat Content, Btu/scf (LHV) 600  Heat Recovery Efficiency, % 30
 Methane (CH4) Composition, % 60  Hot Water Supply, deg F 190
 Carbon Dioxide (CO2) Composition, % 40  Hot Water Return, deg F 180
 Hydrogen Sulfide Composition, ppmv 3,000  NOx Emissions, g/bhp 1.5
 Moisture Content, deg F (Saturated Gas) 95  CO Emissions, g/bhp 2.65

Implementation of the planning and design resulted in a bioenergy conversion system that operates unmanned while not interfering with the dairy operation. The CHP system achieved a better than 95 percent availability and generated electricity valued at 0.048 per kWh. The bioenergy system continues to operate with supervisory visits by PGE twice a week.

Operating this system reduces greenhouse gases by beneficially using methane that would otherwise be emitted to the atmosphere. Uncontrolled methane is a toxic greenhouse gas, 20

times more harmful to the atmosphere than carbon dioxide. Odors are reduced and 7 to 8 cubic yards per day of fiber is sold for making potting soil. Between 6,000–10,000 gallons of liquid fertilizer is produced as a beneficial byproduct. This bioenergy system reduces the buildup of manure solids in farm storage lagoons.

There is a need for additional research and improvements can be made on this initial “beta” test design. Cow manure has a high volatile solids content (approximately 80% of total solids) but a relatively low VS reduction percentage (e.g., 30–35% versus 60% at municipal facilities).

Higher VS reduction may be possible with co-digestion with municipal wastewater solids and/or increased hydrolysis (e.g., thermal, flashing to steam, acid/gas, ultrasonic treatment, etc.). With the concrete digester cover design that only has a small gas space, a gas holder that compensates for fluctuating gas production rates would be beneficial. This would provide a “wide spot” in the gas system and dampen out fluctuations. Constructing the digester out of steel instead of concrete may reduce capital costs and have a shorter yet acceptable life.

Bioenergy conversion systems with high-rate cylindrical digesters with IC engine CHP systems are technically feasible for installation at 350–500 cow dairy farms. They can be operated without continuous supervision. Cost effectiveness continues to be a challenge and lower construction costs, additional incentives, and/or higher operating savings (e.g., greater value on electricity) are needed for economic feasibility. Anaerobic digestion of cow manure reduces greenhouse gases by beneficially using the methane that would otherwise be released to the atmosphere.
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Document Type: Research Article

Publication date: 2005-01-01

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