# Presentation: 283025

## 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* s*hort-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 s*ynthrophic acetogenic and homoacetogenic bacteria; *principal end products include acetate, CO2, H2.
**Methanogenesis:   **Reduction of CO2 and cleaved acetate by *Methanogens; e*nd 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

**ft****3**** biogas/cow/day                               **47-97
**Biogas energy content (Btu/ft****3****)         **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](mailto:Peter.Ciborowski@pca)_