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

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

3. 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.

4. 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.

5. 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)

6. Operating Parameters for Medium-Rate Farm Digesters
Temperature C pH
VFA mg/l
Organic loading rate 3-7kg VS/d/m3 of digester volume
Retention time days
Total solids content 7-12%

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

8. 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

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

10. 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

11. 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

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

13. Energy Production from Anaerobic Digestion of Dairy Manure
ft3 biogas/cow/day
Biogas energy content (Btu/ft3)
kWh (e) /cow/day
Gal LPG-equiv-avail/cow/day
kWhr (e) /animal/year ,117
Gal LPG-equivalent-available/yr
kW(e)-equivalent animal numbers

14. 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

15. 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

16. Herd Size Threshold for Economic Viability of Anaerobic Digestion
dairy cows
Published literature
2001 estimate for Minnesota (a)
no policy intervention
$0.015/kwh production subsidy
Best guess, current conditions
(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 $ /kwh, no credit for pollution control.

17. 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

18. 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

19. 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