What is a defining characteristic of a sequence batch reactor (SBR)?

• A. Fill, react, settle, decant, idle.
• B. Continuous flow with aeration throughout.
• C. Step feed with multiple feeds to equalize F/M.
• D. A fixed-film trickling approach.

Answer: (A)

Sequence batch reactors operate in a batch, time-based cycle within a single tank rather than in continuous flow. The defining stages are filling the reactor, reacting with aeration to treat the waste, allowing solids to settle, decanting the clarified effluent, and then an idle period before the next cycle. This setup enables treatment, clarification, and cycle reset all in one vessel, with aeration typically active during the reacting phase and paused during settling and decanting.

The continuous-flow with aeration throughout describes a system where wastewater moves steadily through a tank with aeration ongoing, which isn’t how an SBR operates. The idea of step feeding to equalize the Food-to-Microorganism ratio points to a control strategy rather than the fundamental batch sequencing of an SBR. A fixed-film trickling approach refers to a different treatment method entirely, not a batch process in a single tank.