Anaerobic treatment is very effective due to the ability of anaerobic bacteria to tolerate and decompose high concentrations of organic materials without dilution. Another advantage is that anaerobic treatment is relatively inexpensive compared to aerobic treatment and relatively small amounts of sludge are obtained at the end of the process.

Anaerobic bacteria decompose organic materials in a three-stage process. In the first stage, anaerobic bacteria degrade complex organic materials into simpler compounds, namely, polysaccharides and polyphenols, which are converted to their monomers (monosaccharides and phenols, respectively). During the second stage, acetogenic bacteria convert the phenols and monomers into organic acids such as acetic, lactic and formic acids, and alcohol. Finally, in the third stage, methanogenic bacteria, which are characterized by their sensitivity to pH, convert the organic acids into biogas (a mixture of 60-80% methane and other gases, mainly carbon dioxide).

The UASB is an empty tank filled with anaerobic sludge, which has a good settling property . The wastewater is fed into the bottom of the tank, where it comes into contact with sludge. The anaerobic degradation of organic substrates occurs in the sludge-bed where biogas is produced. The combined flow of the wastewater and the released biogas results in expanding some of the sludge solids to the top part of the reactor. A three-phase separator is fitted at the top of the reactor in order to separate the biogas, water, and sludge. The biogas is trapped and released from the top of the reactor, while the solid matter settles and returns to the sludge bed. The treated wastewater overflows from the top of the reactor.