Anaerobic Digestion

Anaerobic Digestion

A number of bacteria are involved in the process of anaerobic digestion. Anaerobic organisms digest the initial volatile organic matter, which undergoes a four stage microbial processes converting it to intermediate molecules including sugars, hydrogen & acetic acid before finally methane - biogas is produced.  The substrate mix fed to the digester results in a specific biogas composition and biogas yield

 

The first step is the hydrolysis. Hydrolysis of particulate biodegradable material is carried out by facultative bacteria. Carbohydrates are broken down to simple sugars. Proteins are broken down to amino acids and lipids are broken down to long chain fatty acids. The hydrolysis of lipids is the rate limiting step. At this point no COD reduction takes place. It just shifts from solids into the liquid phase.

 

After that acidogenic bacteria start working on the amino acids, sugars and long chain fatty acids. The process is a fermentative reaction, where both the electron donor and electron acceptor are organic compounds, leading to formation of intermediate compounds (volatile fatty acids), such as propionic and butyric acid.

 

Actogenic bacteria start metabolizing the intermediary metabolites - volatile fatty acids - converting them into acetic acid, carbon dioxide, and hydrogen

Anaerobic oxidation at this point is inhibited by hydrogen gas. With the a high hydrogen gas partial pressures the reaction becomes thermodynamically unfeasible, resulting in an increasing amount of volatile fatty acids (VFA) in the surrounding liquid phase.

 

For successful operation of anaerobic digestion there is a need to continuously remove hydrogen - this is what methanogens do. Methanogenisis is the last step in microbial biogas production. There are two groups of Methanogens, all of them are obligate anaerobes.

One group - H2-oxidizing methanogens - reduce CO2 using H2 as the electron donor to form methane. The second group of methanogens cleaves acetic acid into methane and CO2. About 2/3 of the methane produced, comes from this reaction.

 

Nearly all volatile organic matter entering a digester will end up as methane. Small amounts will be incorporated in newly formed biomass.