What is a sequencing batch reactor (SBR)?

A sequencing batch reactor (SBR) is a treatment process that consists of a sequence of steps that are carried out in the same containment structure, usually a tank reactor. They are also referred to as “fill-and-draw” systems. Although SBR systems exist that do not use aeration (anaerobic SBRs), a typical SBR system is designed to include aeration in the treatment step. A typical sequence for a SBR system is: (1) FILL, when the tank is filled with fresh wastewater, (2) REACT, when aeration and mixing are used to promote microbial removal of waste constituents, (3) SETTLE, when aeration and mixing devices are turned off to allow settling of suspended solids, and (4) DRAW, when clear effluent is drawn from the top of the reactor.

Waste solids can be removed from the reactor after the DRAW stage from the bottom of the tank, or during the REACT stage while the wastewater is completely mixed. The SBR treatment process requires a liquid waste input, so it is more suitable for flush systems than for scrape or pit-storage systems.

The advantages of the SBR system over other aerated technologies are: (1) that all steps are contained in one tank and (2) a great deal of flexibility in operating conditions allows for optimizing different treatment objectives. This flexibility has allowed successful treatment of a wide range of wastewater types, from very strong to very dilute. In particular, aeration during the REACT stage can be cycled on and off to provide a suitable environment for complete ammonia conversion to harmless N2 gas. Operation can also be manipulated to achieve biological phosphorus removal.

The disadvantages of the SBR system include: (1) expense of providing aeration, (2) possible requirement for more than one treatment unit to accommodate cleaning schedules, and (3) requirement for monitoring and operating expertise to manage the reactor in such a way that will achieve desired treatment results. Many technology providers will also provide service to minimize disadvantage No. 3.

Author: Sarah Liehr, North Carolina State University