The Sequencing Batch Reactor (SBR) is a fill-and-draw, non-steady state activated sludge process in which one or more reactor basins are filled with wastewater during a discrete time period, and then operated in a batch treatment mode. The SBR accomplishes equalization, aeration and clarification in a timed sequence, in a single reactor basin, whereas a conventional continuous flow process requires multiple structures and extensive pumping and piping systems. A single cycle for each reactor consists of five discrete periods, Fill, React, Settle, Draw, and Idle. The purpose of each period is described in the illustrations below.

Varying the operating strategy enables aerobic, anaerobic or anoxic conditions to be achieved. Precise control of these conditions allows Organism Selection to take place–the proliferation of specific desireable microorganisms is encouraged, while the growth of undesireable mircoorganisms is inhibited. Microorganisims can also be acclimated to a wide range of industrial and chemical processing wastes.