Sequencing batch reactor (SBR)

The Sequencing Batch Reactor (SBR) is a different configuration of the conventional activated sludge systems, in which the process can be operated in batches, where the different conditions are all achieved in the same reactor but at different times.


The treatment consists of a cycle of five stages: fill, react, settle, draw and idle. During the reaction type, oxygen is added by an aeration system. During this phase, bacteria oxidize the organic matter just as in activated sludge systems. Thereafter, aeration is stopped to allow the sludge to settle. In the next step, the water and the sludge are separated by decantation and the clear layer (supernatant) is discharged from the reaction chamber.


Depending on the rate of sludge production, some sludge may also be purged. After a phase of idle, the tank is filled with a new batch of wastewater. At least two tanks are needed for the batch mode of operation as continuous influent needs to be stored during the operation phase. Small systems may apply only one tank. In this case, the influent must either be retained in a pond or continuously discharged to the bottom of the tank in order not to disturb the settling, draw and idle phases. SBRs are suited to lower flows, because the size of each tank is determined by the volume of wastewater produced during the treatment period in the other tank.




1. FILL phase

FILL provides for the addition of influent to the reactor. During FILL, the influent wastewater is added to the biomass (i.e. mixed liquor suspended solids) which remained in the tank from the previous cycle. Depending upon the treatment objective, the fill may be static, mixed or aerated.
Static FILL (no mixing or aeration) results in minimum energy input and high substrate concentration at the end of this phase. Mixed FILL (mixing without aeration) results in denitrification, if nitrates are present, a subsequent reduction of BOD and energy input, and in the anoxic or anaerobic conditions required for biological phosphorus removal. Aerated FILL (mixing and aeration) results in starting of aerobic reactions leading to a reduction of cycle time, and holds substrate at lower concentrations, which may be important if biodegradable constituents present in wastewater are toxic at high concentrations.

2. REACT phase

With the reactor full, the REACT phase begins. . In general, vigorous aeration is the feature of this phase. However, as in FILL, the REACT phase may require to be carried out in high dissolved oxygen concentrations (aerated REACT), or in low dissolved oxygen concentrations (mixed REACT). The time allocated for REACT should be sufficient to achieve the desired level of effluent quality. The time dedicated to REACT phase can vary from a low of zero to more than 50% of the total cycle time. If only organics removal is desired, the aeration period can be as short as 15 minutes. However, longer aeration periods in the order of 4 hours or more, are normally required for long term stability of the process and nitrification. Where denitrification following nitrification is required, aeration during the REACT period is interrupted. Anoxic conditions would then prevail over a period of hours followed by a short period of aeration. This will strip away the nitrogen gas bubbles and aid in sedimentation.

3. SETTLE phase

The SETTLE phase allows for separation of biosolids from the treated effluent without any inflow or outflow, in the SBR reactor that may have a volume more than ten times that of a secondary clarifier used for conventional continuous-flow activated sludge plant. The major advantage of SBR is its use as a clarifier, which allows for truly quiescent sedimentation conditions.
4. DRAW or DECANT Phase

This is the withdrawal phase to discharge the clarified effluent from the reactor. There are several withdrawal mechanisms available. It may be as simple as a pipe fixed at some predetermined depth with the flow regulated by an automatic valve or a pump. In any case the withdrawal mechanism should be designed and operated in a manner that prevents floating matter from being discharged. The time dedicated for DRAW phase can range from 5% to more than 30% of the total cycle time. The time for DRAW should not be overly extended because of possible problems with rising sludge. One hour is the usual time period allowed for this phase of the operation.

5. IDLE Phase

IDLE is the phase between discharging the treated effluent and before filling the reactor again. This time can be effectively used to waste sludge. The frequency of sludge wasting is determined by the net solids increase in the reactor for each cycle, and the mixing and aeration equipment capacity. After sludge wasting, aeration and/or mixing can be provided, depending upon the overall system objectives.

Removal efficiency in SBR process:


Key parameters in SBR process design:



The SBR process is used in industries where wastewater is capacity or space is low.



Advantages of SBR process

• Equalization, primary clarification (in most cases), biological treatment, and secondary clarification can be achieved in a single reactor vessel.
• Little land required.
• Potential capital cost savings by eliminating clarifiers and other equipment.
• High effluent quality.
• Fully automatized.
• Resistant against shock-loads and applicable for a large range of organic and hydraulic loading rates.

Disadvantages of SBR process

• A higher level of sophistication is required (compared to conventional systems), especially for larger systems, of timing units and controls
• Higher level of maintenance (compared to conventional systems)
• Requires continuous supply of energy.
• Highly mechanized equipment (control panel).
• Effluent and sludge might require further treatment.