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Chapter Chapter 33

MicrobiologyMicrobiology of of

Anaerobic DigestionAnaerobic Digestion

Response to free molecular O2

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Electron transport

Electron transport molecules (e- acceptors) used in degradation of

soluble substrate

Electron transport

Depending upon the

molecule used

(O2, NO3-, SO4

2-, CO2,

or soluble cBOD)

by bacteria to remove

electrons from the cell,

a variety of substrates

are produced.

Sulfate

reduction

Denitrification

Methanogenesis

Ferm

entation

Aerobic

oxidation

e-e- e- e- e-e-

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ORP vs Cellular activity

Final ElectronAcceptor (carrier)

Energy yield and sludge production

Final ElectronAcceptor (carrier)

kg VSS Produced per kg COD Degraded

Methane producing archaea

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Microorganisms in AD

Higher FattyAcids, Alcohols

Monosaccharides, Amino acids

Intermediates (Propionate,

butyrate, alcohols)

CH4 + CO2

H2 + CO2 Acetate

1

2

2

2

3 3

4 4

3

1

2

Complex Organics(Carbohydrates, proteins, lipids)

H2S + CO25

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5

1. Hydrolytic bacteria

2. Fermentation bacteria

3. Acetogenic bacteria4. Methanogic archaea

5. Sulfate reducing bacteria

Hydrolysis

Cell membrane

Cell wallInsoluble

substrates

Extracellularenzymes

Intracellularenzymes

Soluble substrates

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Hydrolysis of cellulose

Cellulose degrading bacteria

Hydrolysis of proteins

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Extracellular (exo)enzymes

Enzymes used in AD

Stage of AD Enzymes

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Fermentation

Fermentative bacteria

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Substrates of methanogens

acetoclastic methanogens

hydrogenotrophic methanogens

methyltrophic methanogens

Habitats of methanogens

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Acetoclastic Methanogens

Methanosarcina acetivorans

Methanosaeta sp.Methanosaeta concilii

Methanogens

Mixed cultureSubstrate: Sucrose

MethanosaetaSubstrate: Acetate

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Competition: SRB vs Methanogens

Sulfate reducingbacteria

Methanogenicarchaea

Competition: SRB vs Methanogens

Model of kinetic and thermodynamic competition among sulfatereducing bacteria and methanogenic archaea

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Minimum Minimum doubling times of major doubling times of major

microbialmicrobial groups in ADgroups in AD (Mosey, 1989)(Mosey, 1989)

Days2.6Methanogens growing on acetate

Days2.5Acetogenic bacteria fermenting propionate

Days1.4Acetogenic bacteria fermenting butyrate

Hours6Methanogens growing on hydrogen

Min30Sugar-fermenting, acid forming bacteria

Sludge Granulation

Anaerobic sludge granules from a UASB reactor treating effluent from a recycle paper mill (Roermond, The Netherlands).

Millimeter paper indicating the

size of the granules.

Gas vents in the granules,

where biogas is released

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Spaghetti theory of granulation

I) Disperse methanogens (filamentous Methanosaeta)II) Floc formation via entanglementIII) Pellet formation (spaghetti balls) andIV) Mature granules, with attachment of other anaerobic

microorganisms onto the pellet.