Process
Sequencing Batch Reactor (SBR)
A time-based activated sludge process that carries out aeration, reaction and clarification sequentially in a single tank — no separate secondary clarifier required.
Process Description
The Sequencing Batch Reactor is a fill-and-draw activated sludge system. Instead of moving wastewater continuously through separate aeration and clarification tanks, an SBR performs all steps in one tank on a repeating timed cycle. Because settling and decanting occur in the same vessel, no separate secondary clarifier or return-sludge pumping is needed.
Cycle timing is fully adjustable, which makes the SBR well suited to variable flows and to plants that need integrated biological nitrogen removal. Two or more tanks are commonly operated in parallel, out of phase, so the plant can accept continuous inflow.
The SBR Cycle
Fill
Wastewater enters the tank. Can be static, mixed or aerated depending on the nutrient-removal strategy.
React
Aeration drives BOD removal and nitrification; programmed anoxic periods allow denitrification within the same tank.
Settle
Aeration and mixing stop. Activated sludge settles under quiescent conditions — the clarification step, in the same tank.
Decant
Clarified supernatant is withdrawn from the top by a decanter without disturbing the settled sludge blanket.
Idle
Waiting period before the next cycle; waste activated sludge is removed to control sludge age.
Key Parameters
Typical MLSS
Suspended growth
DO in React (aerobic)
For nitrification
Effluent BOD target
UWWTD secondary standard
Typical Cycle Length
Site-specific
Engineering Note
Frequently Asked Questions
An SBR is an activated sludge process that carries out all treatment steps — fill, react (aeration), settle and decant — sequentially in a single tank on a timed cycle, rather than in separate tanks flowing continuously. Because settling happens in the same tank, no separate secondary clarifier is required.
A typical SBR cycle has five phases: Fill (wastewater enters the tank), React (aeration drives BOD removal and nitrification, with anoxic periods for denitrification), Settle (aeration stops and sludge settles), Decant (clarified effluent is withdrawn from the top), and Idle (waiting for the next cycle, with sludge wasting). Cycle timing is adjustable to the treatment goal.
Advantages: flexible operation, good handling of variable flows, integrated nitrogen removal, and no separate clarifier. Limitations: batch operation needs equalization or multiple tanks for continuous inflow, decanter and control systems add mechanical complexity, and peak aeration demand can be high. SBR suits small-to-medium plants and sites with fluctuating loads.
Yes. By programming aerated and non-aerated (anoxic) periods within the react phase, an SBR can achieve both nitrification and denitrification in the same tank. This makes it capable of biological nitrogen removal without additional reactors, provided sludge age and temperature allow stable nitrifier growth.
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