Transcript Anaerobic Digestion in the Dairy Industry: Pollution

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
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Covered Anaerobic Lagoon
Cold/Cool Winter Climates
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Plug Flow Digester
Complete Mix Digester
Slurry-Loop Digester
Anaerobic Sequencing Batch Reactor (ASBR)
Operating Parameters for MediumRate Farm Digesters
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Temperature
pH
VFA
Organic loading rate
 Retention time
 Total solids content
35-40 C
6.8-7.8
200-2000 mg/l
3-7kg VS/d/m3 of
digester volume
15-25 days
7-12%
Digester Size and Cost Parameters
Needed Digester volume
35 ft3 per cow
500 head example dairy
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(a)
Manure Production
Volatile Solids Production
Digester size
CH4 output
Capital investment(a)
At $500 (%1995) per cow
30 tons/day
3.5 tons/day
17,500 ft3
13-32 Mcf/day
$250,000 ($1995)
Environmental Problems Associated
with Dairy Manure Management
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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
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(a)
Manure solids
Manure volatile solids
Biological oxygen demand
Chemical oxygen demand
Volatile fatty acids
Pathogens
Methane
75-95% for digestion of swine manure
20-40%
20-45%
40-75%
20-50%
high (a)
90%
90%
Energy Production from Anaerobic
Digestion of Dairy Manure
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ft3 biogas/cow/day
Biogas energy content (Btu/ft3)
kWh (e) /cow/day
Gal LPG-equiv-avail/cow/day
47-97
600-650
1.5-3.0
0.21-0.44
 kWhr (e) /animal/year
 Gal LPG-equivalent-available/yr
540-1,117
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
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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
 Published literature 1975-2000
 2001 estimate for Minnesota (a)
no policy intervention
$0.015/kwh production subsidy
 Best guess, current conditions
dairy cows
500-800
650
350
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 mediumrate 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
[email protected]