Anaerobic Digestion in the Dairy Industry: Pollution Control Opportunities

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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 - PowerPoint PPT Presentation


  • Anaerobic Digestion in the Dairy Industry: Pollution Control Opportunities

    Peter CiborowskiMinnesota Pollution Control AgencyAir Innovations ConferenceAugust 10, 2004

  • Anaerobic Digestions of Farms Wastes1970s energy crisis: Anaerobic digestion as an energy source1980s-1990s: Anaerobic digestion as a pollution control measure2000s: ???

  • 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 FarmsWarm 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 DigestersTemperature35-40 CpH6.8-7.8VFA200-2000 mg/lOrganic loading rate3-7kg VS/d/m3 of digester volumeRetention time15-25 daysTotal solids content7-12%

  • Digester Size and Cost ParametersNeeded Digester volume 35 ft3 per cow

    500 head example dairy

    Manure Production30 tons/dayVolatile Solids Production 3.5 tons/dayDigester size17,500 ft3CH4 output13-32 Mcf/dayCapital investment(a)$250,000 ($1995)

    (a) At $500 (%1995) per cow

  • Environmental Problems Associated with Dairy Manure ManagementVolatile fatty acids productionH2S production and emissionAmmonia emissionOzone and PM2.5 precursor emission (e.g., soil NOx emissions)Methane emissionFine particles

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

  • Environmental Benefits and Drawbacks of Anaerobic Digestion

    Environmental Advantages

    Reduced biological oxygen demandOdor reduction (e.g., VFAs, phenol, mercaptan, H2S)Pathogen destructionReduced fly and rodent problemsIncreased nitrogen availability for crop growthImproved manure physical quality

  • Environmental Benefits and Drawbacks of Anaerobic Digestion (cont.)Environmental Advantages

    Reduced manure phosphorus imbalanceReduced methane emissionsAvoided emissions of fossil CO2 and Hg

    Environmental Drawbacks

    Possible increased ammonia volatilizationSOx emission

  • Measures of Control Potential of Anaerobic Digestion

    % reduction

    Manure solids20-40%Manure volatile solids20-45%Biological oxygen demand40-75%Chemical oxygen demand20-50%Volatile fatty acidshigh (a)Pathogens90%Methane90%

    (a) 75-95% for digestion of swine manure

  • Energy Production from Anaerobic Digestion of Dairy Manureft3 biogas/cow/day 47-97Biogas energy content (Btu/ft3) 600-650kWh (e) /cow/day 1.5-3.0Gal LPG-equiv-avail/cow/day 0.21-0.44

    kWhr (e) /animal/year 540-1,117Gal LPG-equivalent-available/yr 77-160

    kW(e)-equivalent animal numbers 8-16

  • Energy Production Potential: 500 Cow DairyGeneration capacity: 30 to 60 kW Annual electrical generation: 270,000 to 559,000 kwhGross annual energy production (MMBtu): 10 to 20

  • Sensitive Terms for Economic Analysis of Anaerobic DigestionManure solids contentCost of capitalMaterials costValue of replacement energyPurchase price of excess farm-generated electricityIRR hurdle

  • Herd Size Threshold for Economic Viability of Anaerobic Digestion

    dairy cowsPublished literature 1975-2000 500-800 2001 estimate for Minnesota (a) no policy intervention 650 $0.015/kwh production subsidy 350Best 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 importantAccounting 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 LagoonsInexpensiveMinimum herd size for economic deployment smaller by half that of medium-rate digestersWhere it can be deployed, the optimal use of anaerobic digestion

  • Anaerobic Digestion: Final WordAnaerobic 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


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