Anaerobic Digestion Process Theory Operations and Maintenance Seminar April 12, 2005

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Anaerobic DigestionProcess TheoryOperations and
Maintenance SeminarApril 12, 2005

• Dr. Michael Robinson, P.E.
• Department of Civil Engineering
• Rose-Hulman Institute of Technology

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Anaerobic digestion is

• a complex biochemical process of biologically
  mediated reactions by a consortia of
  microorganisms to convert organic compounds to
  methane and carbon dioxide.
• a stabilization process achieving odor, pathogen,
  and mass reduction.

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Steps in the Anaerobic Process
Biodegradable Particulates
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1
Proteins and Carbohydrates
Lipids
1
1
1
Volatile Acids (propionic, butyric)
Long chain fatty acids
Amino acids and simple sugars
2
2
2
H2 and CO2
Acetic acid
3
4
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CH4 and CO2
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Steps in the Anaerobic Process
Biodegradable Particulates
Proteins and Carbohydrates
Lipids
Volatile Acids (propionic, butyric)
Long chain fatty acids
Amino acids and simple sugars
H2 and CO2
Acetic acid
CH4 and CO2
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Steps in Anaerobic Digestion
Hydrolysis
Acidogenesis
Acetogenesis
Methanogenesis
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Steps in Anaerobic Digestion
Hydrolysis
Particulates solubilized and large polymers
converted to simpler monomers
Acidogenesis
Simple monomers converted to volatile fatty acids
Acetogenesis
Volatile fatty acids converted to acetic
acid, CO2 and H2
Methanogenesis
Acetate converted into CH4 and CO2 while H2
consumed
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Steps in Anaerobic Digestion
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Step 1 - Hydrolysis
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Hydrolysis

• Particulates made soluble and large polymers
  converted to simpler monomers
• Carbohydrates, fats, and proteins
• Large molecules (polymers) broken down into
  smaller molecules (monomers)
• Allow passage through bacterial cell wall
• Facultative anaerobes and anaerobes
• May be rate limiting step in process if high
  concentration of particulate organic matter.

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Carbohydrates

• A macromolecule (polymer)

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Protein

• A macromolecule (polymer)

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Fats (Lipids)
C

• Molecule composed of
• fatty acids and alcohols

O
Fatty Acids Long-chain hydrocarbon molecule
capped by a carboxyl group (COOH)
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Step 2 Acidogenesis
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Step 2 - Acidogenesis

• Sugars, amino acids, and fatty acids converted to
  C3 and C4 volatile fatty acids (76), H2 (4),
  and acetic acid (20)
• Optimum growth rate occurs near pH 6
• Volatile fatty acids generally not significant
  consumer of alkalinity
• CO2 significant consumer of alkalinity
• NH3 produced from amino acids

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Volatile Fatty Acids

• "short-chain" or volatile fatty acids are 2 to
  4-carbon molecules

propionic acid
ethanoic acid (acetic acid / vinegar)
O H
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Acetogenesis
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Step 3 - Acetogenesis

• Volatile fatty acids converted to acetic acid
  (68) and H2 (32)
• Sensitive to H2 concentration
• Syntrophic (mutually beneficial) relationship
  with the methanogens

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Step 4 - Methanogenesis
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Methanogenesis

• Obligate anaerobes methanogens
• Tend to have slower growth rates
• H2 utilizing methanogens use H2 to produce
  methane removing H2 from system
• Limited pH range 6.7 to 7.4
• importance of alkalinity in system
• Sensitive to temperature change

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Routes to Formation of Methane
Hydrogenotrophic methanogens
CO2 4 H2 ? CH4 2 H2O
Acetotrophic methanogens
4 CH3COOH ? 4 CO2 2 H2
Methylotrophic methanogens
4 CH3OH 6 H2 ? 3 CH4 2 H2O
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Background Concepts

• Enzymes
• Environmental Conditions
• Alkalinity
• Respiration

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Catalysis Enzymes

• Catalysis - the acceleration of a chemical
  reaction by some substance which itself undergoes
  no permanent chemical change.
• Catalysts of biochemical reactions are enzymes.
• Responsible for bringing about almost all of the
  chemical reactions in living organisms. Without
  enzymes, the reactions take place at a rate far
  too slow for metabolism.
• Extracellular and intracellular
• Note Bacteria have specific enzymes for specific
  tasks and not all bacteria have all enzymes.

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Bacteria Environmental Conditions
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Bacteria
Hydrolysis
Robust
Acidogenesis
Acetogenesis
Sensitive
Methanogenesis
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Important Note

• Not all bacteria have same environmental
  tolerance.
• Not all bacteria have same optimum environmental
  conditions.

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Effect of Temperature on Biological
Sustainability
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Response of Mesophilic Bacterial Growth to
Temperature
sour digester
35 o optimum CH4 production
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Alkalinity

• Buffer that prevents rapid change in pH
• Provided by the carbonate system
• CO3-2 and HCO3-1

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CO3-2
HCO3-1
H2CO3
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H2CO3
CO3-2
HCO3-1
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Alkalinity as Performance Indicator

• Decrease in alkalinity and CH4 production
• methanogens may be inhibited
• accumulation of fatty acids
• Decrease CH4 production but not alkalinity
• methanogens and acidogens may be inhibited

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Respiration
Cellular degradation of substrate to obtain
energy and carbon.
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Electron Transfer
cell
e -1
A
e -1
e -1
B
C
e -1
D
D
terminal electron acceptor
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Respiration and TER
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Phased (Staged) Digestion

• Digestion process separated into multiple
  reactors to optimize process
• Environmental conditions optimized for specific
  microorganism population
• acid-forming
• methane-forming
• thermophilic
• mesophilic

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Acid / Gas Phased Digestion

• Digester 1
• Low solids retention time to promote growth of
  acidogens (acid forming bacteria)
• pH lt 6.0 (optimum growth rate)
• Digester 2
• High solids retention time to promote growth of
  methanogens
• pH 7.0

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Temperature Phased Digestion

• Thermophilic generally 4 times faster than
  mesophilic
• Mesophilic phase enhances stability

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Thank you for your attention!!