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Typical SBR contains five stages in the treatment process, naming   a consistent depth of supernatant disappears. Decant process
          a. Fill, b. React, c. Settle, d. Decant, e. Idle.      is done from the upper part of the reactor with the help of
                                                                 automatic valves. Decanter mechanisms are available where
          These cycle steps run in a sequence depending on the problem.   mechanical floating weirs are used for decanting.
          It may be a single tank or multiple tank process. The time of
          the complete cycle is between fill to end of the idle cycle in a   e. Idle:
          single tank system. In multiple tank system, the complete cycle is
          between beginnings of the fill phase in the first tank to end of the   The idle phase is an optional phase where settled sludge is wasted.
          idle phase in the last tank. The complete process is summarized   When two or more tanks are employed, this can be eliminated.
          below. The overall process is shown in the Fig-1.      Based on the design and operation of SBR, sludge wastage can
                                                                 occur at any stage during the react phase, during the decant
          a. Fill:                                               process, or during the idle phase. This phase takes place daily,
                                                                 weekly or every cycle [3,4,4,5].
          The fill phase is where the reactor is filled with wastewater. Fill
          can occur under aerated, unaerated, mixed or unmixed conditions.
          Quantity  of  wastewater  to  be  filled  depends  on  loading  rate,
          F/M ratio, HRT, and settling characteristics of the organisms.
          Duration of fill depends on the type of design and the pollutants
          targeted to be removed.


          b. React:
          React phase starts when the fill phase is complete. In this phase
          there is no flow of wastewater in the tank. It includes mixing,
          aeration of the influent and sludge is wasted. Aeration process       Fig-1: Treatment stages of SBR
          helps  in  oxidizing organic carbon, nitrifying  ammonia and
          promote uptake of phosphorous by microbes. When aeration   2.2.  Components of SBR:
          is done for consistent time aeration is stopped, during this   Typical sequencing batch reactor consists of the following
          condition denitrification happens. Duration of this phase is more   components. 1. Aeration system 2.Decanter system 3.Waste
          than any other phases. In this phase, alternating conditions of   sludge pump
          low dissolved oxygen concentrations and high dissolved oxygen
          concentrations may be required. Liquid level is maintained to the   2.2.1. Aeration Systems:
          maximum in this phase.
                                                                 Aeration systems are provided as an oxygen source for
          c.  Settle:                                            microorganisms for biological reactions to happen to degrade the
                                                                 organic content. Aeration is considered to be the most energy-
          Settle phase starts when react phase terminates. Sludge is not   intensive process at wastewater treatment plants as it consumes
          wasted in this phase. This phase serves for settlement of MLSS   up to 65% of a plant’s total energy need.  There are two different
          after aeration is done. Clearwater is found as a supernatant.   kinds of aeration devices used in Sequential batch reactors.
          Duration of this phase depends on settle ability of the sludge. The
          major advantage of this system lies in settling sludge in the same   a. Diffused aeration system: Diffused aeration is done by inducing
          aeration tank where volumes are much higher than conventional   air into the fine bubble diffusers through blowers from outside the
          clarifier systems.                                     tank. These diffusers can be made of the membrane, PVDF, ceramic
                                                                 material. There are tube diffusers and disc diffusers. Fig-2(a) shows
          d. Decant:
                                                                 the photograph of disc diffusers. Fig-2(b) represents tube diffusers.
          Treated wastewater from the reactor is decanted once the settle   These diffusers have the finest holes on the material covered where
          phase is completed. The decanting process is carried out until   they pass air bubbles from [6].



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