uasb is a combination of physical and biological processes; the main feature of the former process is the separation of solids and gases from the liquid and that of the latter, degradation of decomposable organic matter under conditions which do not require oxygen. The treatment tank (see figure: Cross-section of the uasb reactor) consists of an upflow reactor with a feed inlet distribution system at its bottom and a gas-solid-liquid (three phase) separator at the top. The wastewater enters the reactor from the bottom and flows upwards through the sludge bed in the digestion compartment. The sludge bed contains microorganisms capable of thriving in an oxygen-deficient environment. The bed traps the solid material, which is degraded by these organisms, producing methane or biogas and carbon dioxide in the process. The biogas provides a gentle mixing in the sludge bed promoting breakdown, and a reduction in levels of SS and BOD. The methane is collected at the top of the reactor in a gas collector from where it is withdrawn, while the water-sludge mixture enters a settling tank where the sludge settles down and flows back into the bottom of the reactor.

Retention of bacteria-containing sludge is one of the most important features of the uasb process; the bacteria in the sludge keep on with their function of treating the incoming effluent. The continuous bacterial presence and activity enables retention time in the reactor to be reduced to about six to eight hours compared to at least 30 hours that is required in conventional sewage systems.

The water is collected in gutters and discharged out of the reactor. The sludge is periodically shifted into drying beds to be used as a soil enricher. The methane generated can be used as gas for domestic or industrial use; also, after appropriate dehydration and cleaning, it can be used to generate electricity for running the plant. The process can be reactivated even after the plant has remained shut for months, or after power breakdowns and interruptions in wastewater supply.