Anaerobic Digestion in the Dairy Industry: Pollution Control Opportunities Peter Ciborowski Minnesota Pollution Control Agency Air Innovations Conference August 10, 2004

Anaerobic Digestions of Farms Wastes 1970s energy crisis: Anaerobic digestion as an energy source 1980s-1990s: Anaerobic digestion as a pollution control measure 2000s: ???

Stages of Anaerobic Digestion Hydrolysis: Liquefaction of complex organic compounds to simpler forms by hydrolytic bacteria; principal end products include soluble sugars, amino acids, peptides, long-chain fatty acids. Acidogenesis: Metabolization of the products of hydrolysis by Acidogenic bacteria; principal end products include short-chain volatile organic acids (propionic, butyric, acetic and formic acids), CO2, and H2.

Stages of Anaerobic Digestion (cont.) Acetogenesis: Reduction of CO2 and catabolization of short-chain fatty acids produced during acidogenesis by synthrophic acetogenic and homoacetogenic bacteria; principal end products include acetate, CO2, H2. Methanogenesis: Reduction of CO2 and cleaved acetate by Methanogens; end products include CH4, CO2,, trace Gases.

Anaerobic Digester Types for Dairy Farms Warm Winter Climates Covered Anaerobic Lagoon Cold/Cool Winter Climates Plug Flow Digester Complete Mix Digester Slurry-Loop Digester Anaerobic Sequencing Batch Reactor (ASBR)

Operating Parameters for Medium-Rate Farm Digesters Temperature 35-40 C pH 6.8-7.8 VFA 200-2000 mg/l Organic loading rate 3-7kg VS/d/m3 of digester volume Retention time 15-25 days Total solids content 7-12%

Digester Size and Cost Parameters Needed Digester volume 35 ft3 per cow 500 head example dairy Manure Production 30 tons/day Volatile Solids Production 3.5 tons/day Digester size 17,500 ft3 CH4 output 13-32 Mcf/day Capital investment(a) $250,000 ($1995) (a) At $500 (%1995) per cow

Environmental Problems Associated with Dairy Manure Management Volatile fatty acids production H2S production and emission Ammonia emission Ozone and PM2.5 precursor emission (e.g., soil NOx emissions) Methane emission Fine particles

Environmental Problems Associated with Dairy Manure Management (cont.) Biological oxygen demand (BOD) Manure phosphorus imbalance Water-borne pathogens

Environmental Benefits and Drawbacks of Anaerobic Digestion Environmental Advantages Reduced biological oxygen demand Odor reduction (e.g., VFAs, phenol, mercaptan, H2S) Pathogen destruction Reduced fly and rodent problems Increased nitrogen availability for crop growth Improved manure physical quality

Environmental Benefits and Drawbacks of Anaerobic Digestion (cont.) Environmental Advantages Reduced manure phosphorus imbalance Reduced methane emissions Avoided emissions of fossil CO2 and Hg Environmental Drawbacks Possible increased ammonia volatilization SOx emission

Measures of Control Potential of Anaerobic Digestion % reduction Manure solids 20-40% Manure volatile solids 20-45% Biological oxygen demand 40-75% Chemical oxygen demand 20-50% Volatile fatty acids high (a) Pathogens 90% Methane 90% (a) 75-95% for digestion of swine manure

Energy Production from Anaerobic Digestion of Dairy Manure ft3 biogas/cow/day 47-97 Biogas energy content (Btu/ft3) 600-650 kWh (e) /cow/day 1.5-3.0 Gal LPG-equiv-avail/cow/day 0.21-0.44 kWhr (e) /animal/year 540-1,117 Gal LPG-equivalent-available/yr 77-160 kW(e)-equivalent animal numbers 8-16

Energy Production Potential: 500 Cow Dairy Generation capacity: 30 to 60 kW Annual electrical generation: 270,000 to 559,000 kwh Gross annual energy production (MMBtu): 10 to 20

Sensitive Terms for Economic Analysis of Anaerobic Digestion Manure solids content Cost of capital Materials cost Value of replacement energy Purchase price of excess farm-generated electricity IRR hurdle

Herd Size Threshold for Economic Viability of Anaerobic Digestion dairy cows Published literature 1975-2000 500-800 2001 estimate for Minnesota (a) no policy intervention 650 $0.015/kwh production subsidy 350 Best guess, current conditions 500-600 (a) Free stall, flush parlor and feed apron and scrape rest, 10% IRR hurdle, $0.07/kwh farm electric rates, purchase price for excess electricity $0.015-0.04/kwh, no credit for pollution control.

Caveats Anaerobic digestion works best with new dairies and full integration of digester into dairy design (e.g., gravity manure flow, waste heat use, high manure total solids) Local conditions are important Accounting for the money-value of time is essential Lesson: Anaerobic digestion is not for everyone. Certainly anaerobic digestion is not for small producers The economics of anaerobic digestion depend critically about what you assume about the future cost of energy, materials and capital and future yields from alternative uses of investment dollars

Covering Preexisting Anaerobic Lagoons Inexpensive Minimum herd size for economic deployment smaller by half that of medium-rate digesters Where it can be deployed, the optimal use of anaerobic digestion

Anaerobic Digestion: Final Word Anaerobic Digestion: Cross-media pollution control at a profit Availability of: MPCA, Anaerobic Digestion of Livestock Manure for Pollution Control and Energy Production: A Feasibility Assessment, March 2001, contact MPCA at Peter.Ciborowski@pca.state.mn.us