Sequencing batch reactor

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Courtesy of IWA Publishing

Sequencing batch reactors (SBR) or sequential batch reactors are industrial processing tanks for the treatment of wastewater. SBR reactors treat waste water such as sewage or output from anaerobic digesters or mechanical biological treatment facilities in batches. Oxygen is bubbled through the waste water to reduce biochemical oxygen demand (BOD) and chemical oxygen demand (COD) to make suitable for discharge into sewers or for use on land. While there are several configurations of SBRs the basic process is similar. The installation consists of at least two identically equipped tanks with a common inlet, which can be switched between them. The tanks have a “flow through” system, with raw wastewater (influent) coming in at one end and treated water (effluent) flowing out the other. While one tank is in settle/decant mode the other is aerating and filling. At the inlet is a section of the tank known as the bio-selector. This consists of a series of walls or baffles which direct the flow either from side to side of the tank or under and over consecutive baffles. This helps to mix the incoming Influent and the returned activated sludge, beginning the biological digestion process before the liquor enters the main part of the tank.


During the early 1900’s, the basic principles of biological degradation processes using activated sludge were established by Ardern, Lockett and Fowler amongst others. These researchers operated “fill-and draw” processes on crude sewage at Manchester in the UK and established the concept of sequencing batch reactors (SBR’s) operating a single biological reactor basin using repetitive cycles of aeration, settlement and discharge of the treated effluent. These original fill-and-draw variable-volume SBR systems were capable of achieving excellent treated effluent quality but suffered many operational difficulties which favored the development of fixed volume continuous-flow activated sludge processes which incorporate two separate units, one for aeration and one for settlement. Further developments of the SBR process did not occur until the 1950’s when Pasveer and co-workers incorporated interrupted and continuously fed batch treatment principles in their variable volume activated sludge system. Further development took place in the 1970’s mainly in Australia and the United States, and with grant aid from the EPA and publication of the EPA’s SBR Design Manuals in 1986 and 1992, led to the wide scale application of the technology worldwide. The earlier operational difficulties have been resolved by technological improvements, particularly reliable microprocessor control systems, aeration equipment and mechanically actuated valves. Through process performance monitoring and variations / modifications of the original SBR process, the modern generation of SBR’s have found application in large scale municipalities (up to 1 million population equivalent), as well as the modular expansion and up-rating of existing wastewater treatment facilities.

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