In a Sequencing Batch Reactor (SBR), the operational sequence within each cycle is:

Question

In a Sequencing Batch Reactor (SBR), the operational sequence within each cycle is:

1. A. Fill Aerate Settle Decant Idle (Correct answer)
2. B. Aerate Fill Decant Idle Settle
3. C. Settle Fill Idle Aerate Decant
4. D. Idle Aerate Fill Settle Decant

Correct Answer

Option A — Fill Aerate Settle Decant Idle

Detailed Solution & Explanation

The correct answer is Fill → Aerate → Settle → Decant → Idle.

Key Points

• A Sequencing Batch Reactor carries out the whole activated sludge process in one tank, separated in time rather than in space. Its cycle runs in a fixed logical order:
  • Fill — raw wastewater enters the reactor.
  • Aerate (React) — air is supplied and micro-organisms oxidise the organic matter.
  • Settle — aeration stops and the biomass settles under perfectly quiescent conditions.
  • Decant — clarified supernatant is drawn off the top.
  • Idle — a short pause before the next cycle, during which excess sludge is wasted.
• The sequence is forced by physics: you cannot settle before you aerate, and you cannot decant before the solids have settled.

Additional Information

• Because settling happens in a still tank with no inflow or outflow, an SBR usually achieves better solids separation than a continuous-flow secondary clarifier, where currents interfere.
• The SBR needs no separate secondary clarifier and no return sludge pumping, which saves both structures and energy.
• By varying the aeration within the react phase — alternating anoxic and aerobic conditions — the same tank can achieve biological nitrogen removal through nitrification and denitrification, and with an anaerobic phase, phosphorus removal too.
• Since flow arrives continuously but the reactor works in batches, plants normally provide two or more SBR basins operating out of phase, or an equalisation tank ahead of a single basin.