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Sequential Batch Reactor

The Sequencing Batch Reactor (SBR) is an activated sludge process designed to operate under non-steady state conditions. An SBR operates in a true batch mode with aeration and sludge settlement both occurring in the same tank. The major difference between an SBR and a conventional continuous-flow, activated sludge system is that the SBR tank carries out the functions of aeration and sedimentation in a time sequence rather than in the conventional space sequence of continuous-flow systems. In addition, the SBR system can be designed with the ability to treat a wide range of influent volumes whereas the continuous system is based on a fixed influent flow rate. Thus, there is a degree of flexibility associated with working in a time rather than in a space sequence.

SBR's produce sludge with good settling properties provided the influent wastewater is admitted into the aeration in a controlled manner. Controls range from a simplified float and timer based system with a PLC to a PC based SCADA system using either flow proportional aeration or dissolved oxygen controlled aeration. This is to lower the energy consumption and enhance the selective pressures for BOD, nutrient removal and control of filaments.

An appropriately designed SBR process is a unique combination of equipment and software. Working with automated control reduces the number of operator skill and attention requirement. Sequencing Batch Reactors operate by a cycle of periods consisting of fill, react, settle, decant, and idle.

Sequencing Batch Reactor Process Cycles:

The operating principles of a batch activated sludge process, or SBR, are characterized into five discrete periods:

  1. Fill: The influent wastewater is distributed throughout the settled sludge through the influent distribution manifold to provide good contact between the microorganisms and the substrate. The influent can be either pumped in or allowed to flow in by gravity. Aeration is started during filling period.
  2. React: During this period aeration continues until complete biodegradation of BOD and nitrogen is achieved.
  3. Settle: Aeration is discontinued at this stage and solids separation takes place leaving clear, treated effluent above the sludge blanket. During this clarifying period no liquids should enter or leave the tank to avoid turbulence in the supernatant.
  4. Decant: This period is characterized by the withdrawal of treated effluent from approximately two feet below the surface of the mixed liquor by the floating solids excluding decanter. This removal must be done without disturbing the settled sludge.
  5. Idle: The time in this stage can be used to waste sludge. The wasted sludge is pumped into an anaerobic digester to reduce the volume of the sludge to be discarded. The frequency of sludge wasting ranges between once every cycle to once every two to three months depending upon system design.

The duration, oxygen concentration, and mixing in these periods could be altered according to the needs of the particular treatment plant. Appropriate aeration and decanting is essential for the correct operations of these plants. The aerator should make the oxygen readily available to the microorganisms. The decanter should avoid the intake of floating matter from the tank.

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