Upflow anaerobic sludge blanket (UASB) technology, normally referred to as UASB reactor, is a form of anaerobic digester that is used in the treatment of wastewater

Wastewater

Wastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations...

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The UASB reactor is a methanogenic (methane-producing) digester that evolved from the anaerobic clarigester

Anaerobic clarigester

The anaerobic clarigester is a form of anaerobic digester. It is regarded as being the ancestor of the UASB anaerobic digester. A clarigester treats dilute biodegradable feedstocks and separates out solid and hydraulic retention times...

. A similar but variant technology to UASB is the expanded granular sludge bed (EGSB) digester. A diagramatic comparison of different anaerobic digesters can be found here.

UASB uses an anaerobic

Anaerobic digestion

Anaerobic digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen. It is used for industrial or domestic purposes to manage waste and/or to release energy....

 process whilst forming a blanket of granular sludge which suspends in the tank.