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Type of Activated Sludge Process

Description of different types of Activated Sludge Process

Type of ASP


Complete mix Activated Sludge Process Settled waste water and recycled activated sludge are introduced at several points in the aeration tank. MLSS concentration and oxygen requirement are uniform throughout the tank.

Conventional plug flow Settled wastewater and recycled activated sludge enter the front end of the long narrow aeration tank and are mixed either by diffused air or mechanical aeration system. The aeration system is designed to match the oxygen demand along the length of the tank by tapering the aeration rates, i.e., applying higher rates in the beginning and lower rate at the end of the tank.
Contact Stabilization The system uses two separate tanks, one for the treatment of wastewater and the other for the stabilization of the activated sludge. The stabilized activated sludge is mixed with the influent wastewater in a contact zone (tank). The contact zone detention time is relatively short (30 to 60 min). The detention time of the stabilization zone is 1 to 2 hours. Contact stabilization requires much less aeration volume than complete mix or conventional plug flow.
Extended Aeration (EA)

The extended aeration process is similar to the conventional plug flow process except that the aeration time is longer in the order of 15 to 24 hours. Generally primary sedimentation tank and sludge digester are not used in the extended aeration system. Oxidation ditch functions according to the principles of extended aeration.

Sequencing Batch Reactor (SBC)

SBR is a fill-and-draw type reactor system involving a single complete-mix reactor in which all steps of the ASP occur. An SBR goes through a number of cycles per day; a typical cycle may consist of 3-h fill, 2-h aeration, 0.5-h settle and 0.5-h for withdrawal of supernatant. The detention time for SBRs generally ranges from 18 to 30h, based on influent flow rate and tank volume used.

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