Anaerobic digestion is a biological wastewater treatment process that is used for treatment and reduction of organic wastes such as organic sludge or concentrated organic industrial waste which contain solids. As the quantity of organic solids are decreased in the sludge after this process, the treated sludge is easier to dispose due to less volume. Further, microbial action during the process also helps in biogas generation (methane) which can be used as a clean energy source.

At micro level, the anaerobic sludge digestion process is carried out by microbes under anaerobic conditions (i.e., oxygen is not used in the degradation process reactions). This makes it different from aerobic digestion wherein oxygen is used. Anaerobic digestion can also be more economical than aerobic treatment / aerobic septic system as aeration process is not required for the anaerobic digestion.

The process of anaerobic digestion takes place in three stages. The first stage involves liquefying of solid material in the sludge. This process is called hydrolysis. The second stage is rapid and involves digestion of the soluble solids that resulted from the previous stage. This process is carried out at molecular level by acid (primarily acetic, propionic and butyric acid etc.) producing anaerobic bacteria. The microbes involved in this stage are facultative anerobes, are heterotrophic and found in soil. They belong to diverse genera such as Escherichia, Flavobacterium, Alcaligenes, Aerobacter, Psuedomonas etc. and can function in a large pH range.

At the final stage which is called the gasification stage, the organic acid produced in the previous stage is used by certain microbes as substrate and methane and carbon dioxide gases are produced as a result. This stage is slower and is also called methanogenesis as it leads to production of methane. Microbes involved in this process are also anaerobic and belong to the Methanococcus, Methanobacterium, and Methanoscarcina genera.

The anaerobic digestion optimally occurs between 30 to 38 degree Celsius and can be done in single stage digestion system or two-stage digestion system. In the prior, all the reaction stages occur in the same anaerobic digestor. So care must be taken so as not to overfeed or underfeed the digestor with new sludge. Overfeeding may quickly produce acids in digestor and it can inhibit the slower gasification stage. Anaerobic lagoons, for example, have all  reactions of anaerobic digestion in one area. These are economical but the rate of digestion can be very less. In the latter case, hydrolysis/liqueaction and acid generation happens in one digestor and the resultant is transferred to another digestor wherein gasification takes place. This allows for more control on the digestion process. Irrespective of single or two stage digestion system, in general, the frequency of pumping the sludge to the digestor needs to be standardized so that uniformity of digestion rate can be maintained. Further, mixing (using, for example, mechanical or gas mixers or recirculation pumps) is necessary to prevent stratification of sludge in the digestor which can affect performance.

In conventional digestors mixing is intermittent and, therefore, likelihood of stratification is more and the holding times can vary from 30 to 60 days. These have slower rate of digestion. In comparison to that, modern digestors have high turnover and digestion rate due to provision of continuous mixing and streamlined sludge feeding and withdrawal system. The holding times in such digestors can be as low as 15 days or even lesser. Many digestors also have floating covers to accommodate the gas produced. Such gas can be transferred to be used for heating the sludge to necessary levels. It can also be used elsewhere, for example, for domestic purposes.

In general, apart from being comparatively economical, the anaerobic digestion is beneficial for environment also. This is because it helps in reduction of solids in sludge which leads to reduction of final volumes of treated waste to be disposed and it also generates biogas which can be used as a clean source of energy.