What is anaerobic digester sludge?

 Anaerobic digestion of sludge refers to the digestion technology in which biodegradable organic matter in sludge is decomposed into CH4, CO2, H2O and H2S by facultative bacteria and anaerobic bacteria under anaerobic conditions. The organic matter is decomposed anaerobic, and along with the stabilization of the sludge, a large amount of biogas with high calorific value is generated as an energy source, which makes the sludge resource. It can remove 30% to 50% of the organic matter in the waste and stabilize it. It is one of the common methods for sludge reduction and stabilization. It is the most economical sludge treatment method for large sewage plants.


Biochemical process of anaerobic digestion

The anaerobic digestion of sludge is a complex multi-stage process. There are two-stage theories, three-stage theories and four-stage theories for the biochemical process of anaerobic digestion. The three-stage theory refers to the need to go through three stages, namely hydrolysis, acidification stage, acetic acid stage, and methanation stage. Each stage is both interconnected and influenced by each other, and each stage has its own characteristic microbial community.


Hydrolysis and acidification stage

Generally, the hydrolysis process occurs in the initial stage of anaerobic digestion of sludge, and the non-water-soluble macromolecular organic substances in the sludge, such as carbohydrates, proteins, fats, cellulose, etc., are hydrolyzed into soluble substances under the action of microbial hydrolase. Under the action of facultative bacteria and anaerobic bacteria, the hydrolyzed substances are converted into short-chain fatty acids, such as acetic acid, propionic acid, butyric acid, etc., as well as ethanol and carbon dioxide.


Acetation stage

The simple soluble organics produced in the hydrolysis stage are further decomposed into volatile fatty acids (such as propionic acid, acetic acid, butyric acid, long-chain fatty acids), alcohols, ketones, aldehydes, carbon dioxide and hydrogen under the action of hydrogen-producing and acid-producing bacteria. Acetobacter and methanogen are symbiotic in this process.


Methanation stage

The methanation stage occurs in the later stage of anaerobic digestion of sludge. During this process, methanogens convert acetic acid (CH3COOH), H2, and CO2 into methane, respectively, as follows:

2CH3COOH→2CH4↑+ 2CO2↑

4H2+CO2→CH4+ 2H2O


In the whole anaerobic digestion process, the methane produced from acetic acid accounts for about 2/3 of the total, and the methane converted from CO2 and H2 accounts for about 1/3 of the total.


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