Sequential Batch Reactor (SBR)

Sequential Batch Reactor is a type of suspended growth activated sludge system. Bacteria’s are cultured in the tanks to digest the dissolved organic matter.

SBR is ‘fill and draw’ activated sludge system in which aeration, sedimentation and separation processes are carried out in the same tank but sequentially in alternate tanks.

SBR offers high flexibility, better control, easier monitoring, less prone to upset and higher organic removal efficiency for biological treatment.

SBR system consists of 5 processes phases as below

1.  Fill (Aerate)
At the beginning of the process, the reactor is at the minimum level and influent fills the reactor. Depending on the operating mode selected, oxygen can be supplied during the fill period or at the beginning of the react period.

2. React
        During this period, aeration starts until complete biodegradation of BOD and nitrogen is achieved. After the substrate is consumed famine stage starts. During this stage some microorganisms will die due to lack of food and will help reduce the volume of settling sludge. The length of the aeration period determines the degree of BOD consumption. 3. Settle
After the preset oxygen concentration or cycle is reached, the aerators are shut off and the settling period begins. At this stage, the reactor functions as a quiescent clarifier with no internal movement or fluid flow, solid separation takes place, leaving clear treated effluent above the sludge blanket.

For decant, the discharge valve opens and the effluent withdrawal mechanism collects the clarified effluent. This removal must be done without disturbing the settled sludge. When the minimum water level is reached the discharge valves close. 

The reactor is not in use at this stage, until the cycle is repeated. The time in this stage can be used to de-sludge if necessary The frequency of de-sludging ranges between once each cycle to once every 2 to 3 months depending upon system design.

Rationales and Advantages of SBR

Key advantages of the SBR biological treatment system.

  • No secondary clarifier is required, thus no floating sludge or sludge carry-over problems.
  • More effective sedimentation under stagnant condition resulting in better Suspended Solid, BOD and COD in final effluent results.
  • Longer mean cell retention time, less sludge yield rates and is equivalent to Extended Aeration System.
  • Batch system provides more data for better control, monitoring and trouble shooting.
  • Oxygen supply can be cut off when reaction is complete and reduce energy consumption.
  • Aeration time (Reaction Time) can be varied by changing the time allocated to each stage as required.
  • Batch system provides self-equalization for each batch, which will homogenize the loading. Less maintenance on SBR system because only PLC and pneumatic valves are involved.
  • As with BOD removal, nitrification can be accomplished in attached growth biological processes along with BOD removal in the same single process.
  • The modified SBR system is able to employ double anoxic denitrification process by using BOD in the influent wastewater and the aid of additional carbon source.
  • Nitrate removal may occurs in SBR during non-aerated settle and decant periods.