Sequencing batch reactors (SBR) or sequential batch reactors are a type of activated sludge process for the treatment of wastewater. SBR reactors treat wastewater such as sewage or output from anaerobic digesters or mechanical biological treatment facilities in batches. Oxygen is bubbled through the mixture of wastewater and activated sludge to reduce the organic matter (measured as biochemical oxygen demand (BOD) and chemical oxygen demand (COD)). The treated effluent may be suitable for discharge to surface waters or possibly for use on land. There are five stages in the treatment process:

1. Fill
2. React
3. Settle
4. Decant
5. Idle

The inlet valve opens and the tank is being filled in, while mixing is provided by mechanical means (no air). This stage is also called the anoxic stage. Aeration of the mixed liquor is performed during the second stage by the use of fixed or floating mechanical pumps or by transferring air into fine bubble diffusers fixed to the floor of the tank. No aeration or mixing is provided in the third stage and the settling of suspended solids starts. During the fourth stage the outlet valve opens and the “clean” supernatant liquor exits the tank. Benefits and Features The SBR is an advanced biological aeration type treatment plant designed to produce a very high quality of final effluent in addition to overcoming several of the common problems Common problems, which normally occur, in other treatment plants are : Peak surges flow (i.e. Mornings and evenings) can force effluent through the

• Plant before it has had sufficient treatment time.
• Fixed film, types, which have a media matrix, can suffer from media blockage because of excessive bacteria growth. Replacement / maintenance of the media material may be required at regular intervals.
• Mechanical / electrical components within the tanks are difficult to maintain and can be a safety risk.
• The liquor is not spread evenly over the available bacteria preventing an adequate Oxygen supply.

The SBR overcomes these problems by : Operating on a storage and batching system storing the effluent at peak times in the primary settlement tank and treating in a small batches throughout rest of the day thereby ensuring that each batch receives the full treatment time. There is no fixed media within the SBR to become blocked up it operates on the activated sludge, principal where by the bacteria float in suspension in the reactor and form an active mixed liquor.