So you want a digester

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So you want a digester

• ORS
• agrisa

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Why?

• Do you have an environmental issue?
• Are you looking to get rich?
• Did someone tell you it was a good idea?
• Do you think it will help you remain competitive?

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What does a digester do?

• Converts volatile solids to methane
• Reduces BOD and COD of the waste
• Reduces odor from the waste
• Converts nitrogen to ammonia
• Can provide pathogen reduction

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How does it work?

• Bugs!
• Anaerobic digestion is the natural decomposition
  of organic matter in an anaerobic environment
  (oxygen free).
• This degradation is accomplished by anaerobic
  micro-organisms under controlled conditions,
  including temperature, and leads to the
  production of biogas

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Is it really that simple?

• Mostly
• Methane Happens!
• The process requires at least two steps
• Acidogenesis
• organics converted to acetate and H2
• Methanogenesis
• Acetate and H2 converted to methane
• These steps rely on a diverse and interdependent
  group of bacteria to complete the process

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The Anaerobic Digestion Process
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Or Bacteria Make Biogas
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Energy Forms that can utilize Biogas

• Compressed Natural Gas
• Liquefied Natural Gas
• Liquefied Petroleum Gas
• Electricity
• Methanol
• Gaseous Hydrogen, Centralized Plant Production
• Gaseous Hydrogen, Refueling Station Production
• Liquid Hydrogen, Centralized Plant Production
• Dimethyl Ether
• Fischer-Tropsch Diesel

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AD Market Status

• Increased emphasis on farm, industrial
  municipal organic waste management
• Odor control
• Pathogen reduction
• Food security
• Volume reduction
• Farm nutrient balances
• But several limiting factors...

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Types of Digesters Used Depends

• On feedstocks
• Feedstock total solids (TS) concentration
• Facility location
• Management structure
• Other factors
• Bedding materials

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With Livestock Operations

• Break-point is manure management system
• Flushed barns have dilute TS (lt5)
• Scraped barns have higher TS (gt5)

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Flushing Example 1
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Flushing Example 2
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Low TS AD System Options

• Suspended growth systems
• SRT HRT
• Covered lagoons
• Complete mix or CSTR
• Attached growth systems
• SRT ? HRT
• Fixed film development
• Florida, Michigan State, New York, North Carolina
• Others in development pipeline

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Covered Lagoon Digester
Photo Credit RCM Digesters
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Covered Lagoon Liabilities

• Must be gt 15 ft. deep, usually 20-24 ft.
• Use in areas w/ high water tables impractical
• Lagoons must be lined in areas having highly
  permeable soils
• Long start-up period
• May take 1-2 years to achieve steady-state biogas
  production
• Digestion rate temperature dependent
• End-use applications problematic compared to
  other AD technologies

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Attached Growth Digester
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Attached Growth Media aka Bug Condos
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Attached Growth AD System Advantages

• Small footprint
• 2-6 day HRT v. 20 days for suspended growth
• Modular design
• Deployed at 30 less than suspended growth
• High CH4 yields
• Up to 80 CH4 v. 60 for suspended growth
• Very high destruction of volatile solids
• Up to 65 VSd
• Tank heated _at_ 35C
• Biogas production should not vary significantly
• Can meet an approved NRCS Conservation Standard
  Practice

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Hydraulic Flushing Summary

• The real problem in animal waste treatment is the
  water volume used
• Difficult to find cost-effective treatment
  options given large water volumes
• Wastewater reduction is imperative, but not a
  dimension usually considered

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Scraping Example 1
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Scraping Example 2
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Higher TS-Capable Digesters

• HRT SRT
• Slurry mix (6-10)
• Complete mix (3-15 TS)
• Plug-flow (11-13 TS)
• Dairy farms only
• No excess water
• No sand bedding

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CSTR (1985)
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Plug-Flow (1998)
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Small Centralized AD System
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Large Centralized AD System
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Hot Water Boiler
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Engine Generator Making Electricity
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Biogas Used as a Transportation Fuel
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Heavy Vehicle Fleet Fuel
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Screw-Press Separator Recovering Fiber
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Fiber Co-Product
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Fiber into Value-Added Products
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Filtrate Nutrient Recovery
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Does It Smell After AD?
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CAFO needs

• U.S. agriculture produces more than 350 million
  tons/year of manure
• Some farms are so large that they cannot
  efficiently dispose of the enormous amounts of
  manure created
• USDA reports that only 18 of large hog farms and
  23 of large dairy farms are applying manure on
  enough cropland to meet a nitrogen-based standard
  to protect water quality
• Current practices of treating manure in open-air
  lagoons, and spraying liquefied manure over
  fields is becoming an unacceptable option
• A serious problem occurs when the crops cannot
  use all the nitrogen and phosphorus from the
  sprayfield application of untreated manures
• Unassimilated nutrients are, in some cases,
  washed away into surface waters, and in other
  cases are leaching into groundwater
• Tremendous odor problems are also often the
  result
• Air pollution from untreated animal manure is
  associated with a number of health impacts in
  people living near farms
• Damages surrounding properties and lowers
  property values
• Main livestock odor sources are the barns and
  manure storage pits
• Regardless of farm size, what is needed is a
  manure treatment method that is both sustainable
  and cost-effective

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Actual Gas Production Depends On Many Factors

• Assume as initial guesses
• Cow gives milk at a rate of 80/day
• VS production _at_ 11 pounds/AU/day
• COD production _at_ 11 pounds/AU/day
• Organics destruction
• VS destruction _at_ 30 to 45
• Plug flows reported at 30 VSd
• COD destruction of 50 to 65
• CH4 generation
• 8.3 to 10 scf/pound VSd
• 5.60 scf/pound CODd
• Genset conversion efficiency of 30

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The Expected Range

• Expected range is 4.1-7.5 cows/kW
• 1000 cows can make 133 to 244 kW
• At 30 VSd, a cow produces 37 to 44 ft3/day of
  CH4 6.1 to 7.5 cows produce one kW
• At 35 VSd, a cow produces 43 to 52 ft3/day of
  CH4 5.3 to 6.5 cows produce one kW
• At 40 VSd, a cow produces 46 to 59 ft3/day of
  CH4 4.6 to 6.1 cows to produce one kW
• At 45 VSd, a cow produces 55 to 66 ft3/day of
  CH4 4.1 to 5.0 cows to produce one kW

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Gas Yields of Different Materials
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Misstatements from Industry

• You cant digest hog manure by itself.
• If you have a hydraulic retention time of less
  than 25 days, you will kill the methanogenic
  bacteria.
• Another little known advantage of biogas systems
  is that it breaks down the prions of BSE into
  harmless amino acids
• the wastewater from biogas systems can be dumped
  directly into waterways

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Why Farm Digesters Fail

• Bad design or installation
• Poor equipment and materials selection
• Use life-cycle costing
• Unrealistic expectations
• Farm management
• Operation needs to be screwdriver friendly

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Potential Greenhouse Gas Emission Reductions

• Open lagoons emit biogas to the atmosphere
• Biogas can be 60 to 70 methane
• Amount of biogas emissions from open lagoons
  dependent upon temperature low in winter, high
  in summer
• By capturing and combusting methane in an
  anaerobic digester methane emissions to the
  atmosphere can be reduced

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How do I know if a digester is right for me?

• Do you have a long term view?
• Do you need to look at nutrient management?
• Are odors causing you problems?
• Would you like to stop spending money on manure
  management?

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What information do I need?

• Type of operation dairy, swine, beef, food
  processor?
• How many animals? Size?
• How much waste is generated?
• Volume of water in waste?
• Whats in my waste?
• Current energy usage and cost?
• Any other economic considerations?

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How do I get started?

• Perform pre-feasibility analysis
• Based on general data
• Adjust data to your situation
• Based on type of farm and manure removal
• Provides GO/NO GO decision point
• Should provide mass and energy balance as well as
  projected economics
• Looks good, now what?

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The Missing Link

• Do I buy now?
• NO!
• Confirm preliminary results
• Test your waste
• Chemical analysis
• Biochemical methane potential
• Determine amount of water used for flushing
• Selling Electricity? Determine price and
  interconnect issues
• Determine actual cost and terms for construction
• How are you going to pay? Grants? Low interest
  loans? Third party financing?
• Confirm project economics
• Focus on regulations and nutrient management
  issues

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Implementation

• Financing
• Permitting
• Design/Build Contract
• Power purchase agreement
• Other by-product sales agreement
• Federal incentives
• Nutrient management issues
• Operations

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• David C. Palmer
• 1328 Kinnard Drive
• Franklin TN 37064
• 615-794-7124