Biological Nitrogen Removal Database

A manually curated data resource for microbial nitrogen removal

Sequencing Batch Bioreactor

General Description

A sequencing batch reactor (SRB) is a fill and draw activated sludge system that is used for wastewater treatment. All processes - biological, oxidation, sedimentation, nitrification and denitrification occur in a single tank. Reducing biochemical oxygen demand (BOD) and chemical oxygen demand (COD) is achieved by bubbling oxygen through the wastewater, this produces high quality effluent with a low turbidity and nitrogen levels that are within regulated and stipulated effluent quality standards. Equalization, aeration, and clarification in accomplished in the SBR system in a timed sequence in a single reactor basin. Varying the operational strategy to achieve anoxic conditions that encourage the growth of most denitrifying microbes.

Basic Operation

Although the intimate details of the process are complex, the equipment and design of the SBR is fairly quite simple. The following is equipped in each tank, an influent diffuser, a jet aerator, size excluding based decanter and a wastewater sludge pump. As influent flows into the SBR, it is directed to the sludge blanket by a fiberglass reinforced polyester (FRP) or stainless steel influent diffuser. This helps facilitate a fast setting biological floc (low SVI) and at the same time prevents short-circuiting. Oxygen for the biological reaction is supplied by a jet aerator which is connected to a pump and blower. This enables high oxygen transfer and complete mixing to provide aerobic and anoxic conditions are required during biological nutrient removal. The constant on and off operations associated with an SBR system requires the jets with a robust design and large solids handing capability to be used. Discharging the highest quality effluent is achieved by using a fixed solids-excluding decanter. In some cases, an air block is included during all cycles and when the time to remove the treated effluent comes, the air is evacuated and clear water is discharged. In addition, the SBR system has been modified to use a separate wastewater pump and sludge line. This helps keep the mixed liquor suspended solids level constant in the reactor for optimum biological treatment.

Biofilm carriers commonly used

Alcohol gel (PVA-gel)

Kaldnes K1 Media




  • Potential to increase capacity within same footprint by 100% with low purchase, installation and operating costs

  • Provides more effective reduction of nitrogen and other contaminants

  • Requires a small footprint (minimal land use)

  • Compact footprint since treatment and clarification occur in the same tank; Separate clarifiers and associated yard piping are eliminated

  • Eliminates abusive odor (all treatment is underwater)

  • Provides the highest water quality surpassing regulatory requirements

  • Batch processing provides optimal performance and eliminates short circuiting

  • Requires minimal maintenance (few moving parts)

  • Requires minimum power (most treatment is during idle) and has a simple mechanical design and rapid start-up


Potential requirement for equalization after the SBR, depending on the downstream processes.

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) associated with more sophisticated controls, automated switches, and automated valves.

Potential plugging of aeration devices during selected operating cycles, depending on the aeration system used by the manufacturer.

Potential of discharging floating or settled sludge during the DRAW or decant phase with some SBR configurations.

Applications of the bioreactor system
  • Municipal wastewaters

  • Industrial wastewaters

  • Trialed Configurations/ Usage examples
    • Evoqua – Florida Keys, Marathon, USA: Historically they used septic systems to treat wastewater and recently they started using secondary treatment. The city decided to install the most cost effective option for wastewater treatment. The OMNIPAC SBR was chosen by the city council because of its small footprint. The systems enabled the city of Marathon to meet their new discharge parameters at the reduced cost. During the dry seasons around 20 to 40 percent of the treated water is recycled for community use, with the remaining water being pumped into injection wells which meant that no effluent was discharged into the Gulf of Mexico.

    • Evoqua, Bowling Green, Kentucky, USA: With its existing wastewater treatment plant approaching the end of its usefulness and no longer meeting the requirements for it to be re-licensed. The Bowling Green Municipal Utilities decided to rope-in Evoqua and install of the largest SRB systems with a design capacity of 10.6 million gallons per day, that meets effluent discharge requirements and environment goals.

    Further Reading