Sequencing Batch Reactor

Sequenced batch reactor (SBR)

What is Sequencing Batch Reactor (SBR)?

Sequencing batch reactors (SBR) or sequential batch reactors are industrial processing tanks for the treatment of wastewater. SBR reactors treat waste water such as sewage or output from anaerobic digesters or mechanical biological treatment facilities in batches. Oxygen is bubbled through the waste water to reduce biochemical oxygen demand (BOD) and chemical oxygen demand (COD) to make suitable for discharge into sewers or for use on land. While there are several configurations of SBRs the basic process is similar. The installation consists of at least two identically equipped tanks with a common inlet, which can be switched between them. The tanks have a “flow through” system, with raw wastewater (influent) coming in at one end and treated water (effluent) flowing out the other. While one tank is in settle/decant mode the other is aerating and filling. At the inlet is a section of the tank known as the bio-selector. This consists of a series of walls or baffles which direct the flow either from side to side of the tank or under and over consecutive baffles. This helps to mix the incoming Influent and the returned activated sludge, beginning the biological digestion process before the liquor enters the main part of the tank.

Executive Summary

Activated sludge reactors are aerobic suspended-growth type processes. Large amounts of injected oxygen allow maintaining aerobic conditions and optimally mixing the active biomass with the wastewater to be treated. Activated sludge systems are highly efficient for organic matter and nutrient removal, though pathogen removal is low. In the view of reuse of the effluent in agriculture, it is not beneficial to remove all nutrients while standards for pathogen removal are barely met.


  • Little land required.
  • High effluent quality.
  • Fully automatised.
  • Resistant against shock-loads and applicable for a large range of organic and hydraulic loading rates.


  • Requires continuous supply of energy.
  • Highly mechanised equipment (control panel).
  • Effluent and sludge might require further treatment.

The contents of this factsheet are results of the Indo-European Project NaWaTech- “Natural Water Systems and Treatment Technologies to cope with Water Shortages in Urbanised Areas in India”, co-financed by the EC and the DST – India.

Basic treatment process

In its most basic form, the SBR system is a set of tanks that operate on a fill-and draw basis. Each tank in the SBR system is filled during a discrete period of time and then operated as a batch reactor. After desired treatment, the mixed liquor is allowed to settle and the clarified supernatant is then drawn from the tank. The cycle for each tank in a typical SBR is divided into five discrete periods: Fill, React, Settle, Draw and Idle as shown in Fig.2. There are several types of Fill and React periods, which vary according to aeration and mixing procedures. Sludge wasting may take place near the end of React, or during Settle, Draw or Idle. Central to SBR design is the use of a single tank for multiple aspects of wastewater treatment.

Fill: The influent to the tank may be either raw wastewater (screened and degritted) or primary effluent. It may be either pumped in or allowed to flow in by gravity. The feed volume is determined based on a number of factors including desired loading and detention time and expected settling characteristics of the organisms. The time of Fill depends upon the volume of each tank, the number of parallel tanks in operation, and the extent of diurnal variations in the wastewater flow rate. Virtually any aeration system (e.g., diffused, floating mechanical, or jet) can be used. The ideal aeration system, however, must be able to provide both a range of mixing intensities, from zero to complete agitation, and the flexibility of mixing without aeration. Level sensing devices, or timers, or in-tank probes (e.g., for the measurement of either dissolved oxygen or ammonia nitrogen) can be used to switch the aerators and/or mixers on and off as desired.

React: Biological reactions, which were initiated during Fill, are completed during React. As in Fill, alternating conditions of low dissolved oxygen concentrations (e.g., Mixed React) and high dissolved oxygen concentrations (e.g. Aerated React) may be required. While Fig. 1 suggests that the liquid level remains at the maximum throughout react, sludge wasting can take place during this period as a simple means for controlling the sludge age. By wasting during React, sludge is removed from the reactor as a means of maintaining or decreasing the volume of sludge in the reactor and decreases the solids volume. Time dedicated to react can be as high as 50% or more of total cycle time. The end of React may be dictated by a time specification (e.g. the time in React shall always be 1.5 h) or a level controller in an adjacent tank.

  • All tank in civil with flexible positioning
  • Proven design
  • Systems available from 10 – 5000 kl/day and more
  • Fully / semi-automatic controls
  • Concert tank design enhance longevity
  • Flexible Positing of tanks
  • Expandable and flexible designs
  • Completely above ground installation.
  • Easy to install &expand.
  • High removal efficiency for nitrogen, BOD & COD,
  • Treated Water reuse in irrigation and grey water applications.
  • Minimum maintenance requirements.
  • Full technical, installation and operation support from trained and experienced AWT Team.
  • Proven Technology with reliable performance.
  • Reuse of Wastewater in gardening application.

Onsite Sewage Treatment Plants are ideal for the following applications:

  • Housing Project.
  • Schools & hostels.
  • Industrial Office &Parks.
  • Small Communities.
  • Recreational camps & travels trailer parks.
  • Shopping Centres.
  • Restaurants.
  • Resorts, Golf Courses and country clubs.
  • Rural Area Schools
  • Hospitals.

And Many more where sewage disposal & reuse is compulsory


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