A Sequencing Batch Reactor (SBR) is a fill-and-draw activated sludge system where biological reaction (aeration/anoxic mixing) and secondary clarification occur within the same tank on a timed sequential cycle. Unlike conventional continuous-flow plants—which require separate aeration basins, secondary clarifiers, and return activated sludge (RAS) pumping stations—an SBR eliminates secondary clarifiers and RAS pumps, achieving complete carbon removal, nitrification, and biological phosphorus removal in a compact footprint.

The 5 standard cycle phases are: (1) Fill (raw wastewater enters while mixing or aerating); (2) React (aeration for carbonaceous BOD oxidation and nitrification, or anoxic mixing for denitrification); (3) Settle (quiescent clarification under zero inflow or aeration, allowing sludge to form a blanket); (4) Decant (a mechanical surface decanter withdraws clarified supernatant effluent); and (5) Idle (optional buffer time before the next fill, during which waste activated sludge WAS is pumped out).

The Decant Exchange Fraction (DEF) is the ratio of treated batch volume decanted per cycle to the total liquid volume of the SBR basin: DEF = V_fill / V_total. If DEF is too high (>35%), the decanter descends too close to the settled sludge blanket, causing sludge scour and high effluent suspended solids. If DEF is too low (<15%), the basin must be excessively oversized, increasing concrete construction costs without process benefit.

AOR accounts for both carbonaceous heterotrophic respiration and autotrophic nitrification: AOR = a * (BOD_removed) - b * (VSS_produced) + 4.57 * (TKN_oxidized). Each kilogram of BOD removed requires approximately 1.0 to 1.2 kg O2, while each kilogram of ammonia-nitrogen oxidized to nitrate consumes 4.57 kg O2. The AOR is then converted to Standard Oxygen Requirement (SOR) by correcting for clean-water alpha, beta, temperature, elevation, and dissolved oxygen driving force.

Daily WAS production is governed by the target Solids Retention Time (SRT or sludge age): P_x,VSS = (Total Biomass in Basin) / SRT. The net dry solids wasted per day is WAS = (V_total * MLSS) / (SRT * 1000) minus effluent TSS losses. For year-round complete nitrification, SRT is maintained between 15 and 25 days, producing 0.6 to 0.85 kg dry sludge solids per kg of BOD removed.