What is a key characteristic of sequential batch reactors (SBRs)?

• A. Filled, reacted, settled, decanted, idle.
• B. Continuous flow operation with no idle time.
• C. Reliance on fixed-film media for treatment.
• D. Immediate discharge after a single reaction step.

Answer: (A)

SBRs are batch systems that process wastewater through a defined sequence in one tank: fill, react, settle, decant, and idle. The tank is filled to begin the cycle, then mixed and aerated for biological treatment, followed by allowing solids to settle, removing the clarified liquid, and finally a pause before the next cycle. This stepping through stages in a single reactor is what sets sequential batch reactors apart from continuous-flow systems, since there is intentional idle time between cycles. The concept also explains why you don’t discharge immediately after a single reaction step—the settling and decanting steps are essential to separate the treated effluent from the suspended sludge. Fixed-film media and continuous operation with no idle time aren’t defining features of SBRs.