In an SBR cycle, which stage follows the Fill stage?

• A. Idle
• B. Decant
• C. Settle
• D. React

Answer: (D)

After filling, the next stage is the React stage. This is when aeration and mixing are applied to the mixed liquor to foster biological treatment, allowing microorganisms to oxidize organics and nutrients. The Fill step simply brings the wastewater into the tank; it doesn’t provide time for the biological processes to occur. Following React, the system needs time for solids to settle, which happens during the Settle stage, then clarified liquid is removed during Decant, and Idle may be used if there’s a planned pause before the next cycle. So React is the natural next phase because it provides the necessary environment for treatment to begin before solids separation and effluent removal.