Biogas Production from Anaerobic Digestion of Organic Matter

Anaerobic Digestion of Organic material

Anaerobic  digestion is the microbial degradation of organic material under anaerobic conditions. It is unique naturally occurring process, occurs in swamps, hydric soils, landfills, digestive tracks of ruminant animals and termites. During the process, carbon in organic molecules is fully reduced to the methane and functions through synergistic relationships between acid producing and acid consuming microorganism.

fig: Bio-gas production

Bio-gas through Bio-gas digester includes

• Methane ~ 50-70% 
• Carbon Dioxide ~ 30-50% 
• Hydrogen, Ammonia, Hydrogen sulfide ~ trace

Average calorific value of biogas is 20 MJ/m3 (4713 kcal/m3 ).

Alessandro Volta discovered methane in 1776 through studying swamp bottoms. 

We can consider digester as black box. 

fig: Bio digester as black box

fig: Bio-gas production  in Digester

Hydrolysis -Anaerobic digestion

Large organic compounds are broken down into monomeric compounds. In this process cell are allowed to assimilate materials. Hydrolysis of particulate biodegradable material is carried on by facultative bacteria (Eubacterium, Clostridium, Peptococcus etc.)

fig: Hydrolysis process

– Carbohydrates are broken down to simple sugars (Glucose…)

– Proteins broken down to amino acids (HOOC-R-NH₂ ).

– Lipids (include fats, fat-soluble vitamins, waxes, -glycerides, etc) broken down to long chain fatty acids. – No COD reduction in this step. Just shifts from solids into the liquid.

fig: Hydrolysis process

Acidogenesis -Anaerobic digestion

Long-chain fatty acids (LCFAs) are produced; generally produced from lipids.

fig: Acidogenesis process

• Sugars and amino acids are degraded using fermentative reactions leading to formation of intermediate compounds, such as pyruvate ( by glycolysis) propionic and butyric acid (CH₃ -R-COOH, these are volatile fatty acids - VFAs) as well as direct methane precursors, such as acetic acid (CH₃COOH) and hydrogen.

• Hydrogen production from fermentation is small, majority of hydrogen arises from oxidation of VFAs (up to 6 carbon acids) and long chain fatty acids to acetic acid, through the action of obligate anaerobes. This process is called anaerobic oxidation.

fig: Acidogenesis process

• Anaerobic oxidation:

– Electron acceptor is H+ and it is converted to hydrogen gas.

– Anaerobic oxidation is inhibited by hydrogen gas. For high hydrogen gas partial pressures, reaction becomes thermodynamically unfeasible. Final end product would be VFAs.

Acetogenic -Anaerobic digestion

Organic acids and alcohols are converted into acetic acid, hydrogen and carbon dioxide by acetogenic bacteria. It is important that the hydrogen produced is consumed by the methanogenic microorganisms, as too much hydrogen inhibits the formation of acetic acid.

fig: Acetogenic process

Propionic acid: CH₃(CH₂)COOH + H₂O → CH₃COOH + CO₂ + 3H₂

Methanogenesis

For successful operation of anaerobic digestion, need to continuously remove hydrogen.

fig: Methanogenesis 

• Methanogens are strictly anaerobes. There are mainly two groups of these organisms in digesters.

– H2 -oxidizing methanogens: Reduce CO2 using H2 as the electron donor and forming methane

(CO2 + 4H2 → CH4 + 2H2O).

– Acetoclastic methanogens: Cleave acetic acid into methane and CO₂

(CH₃COOH→ CH₄ + CO₂ ).

 – About 2/3 of methane produced is derived from acetate.

– Also methanol forming bacteria, may also use methanol

(CH₃OH + H₂ → CH₄ + H₂O )

– Virtually all COD into digester will end up as methane, and small amount in biomass, due to low yield Methanogenesis e.g. Mathanobacillus omelianskii, Methanobacterium formicicum, Methanosarcina barkerii, and Methanococcus vannielli

This step is Often the rate limiting step in anaerobic digestion of acids.