FREE ENGINEERING TOOL · BIOLOGICAL

Sequencing Batch Reactor (SBR)

One tank does it all — fill, react, settle and decant on a clock. Set the flow, the cycle count and the decant fraction: the tool takes the envelope of the hydraulic and the organic-load volume, builds the full cycle-time budget, and checks that the decanter does not outrun the sludge blanket.

1Duty & cycle structure

An SBR runs N discrete cycles per day. N = 4 gives a 6 h cycle — the usual choice for municipal plants; N = 2–3 is common for small package units, N = 6 for high-rate BNR configurations.

Avg flow Q^[1] (1k–200k)m³/d

BOD_inf^[1] (150–400)mg/L

TKN_inf^[1] (30–70)mg/L

Cycles N^[1] (2–6)/d

Basins^[1] (2–6)ea

Cycle T_c derivedh

2Sizing basis — two controls, one envelope

SBR volume is bounded by both hydraulics (how much you decant per cycle) and organic load (F/M). Real designs take the larger of the two; the tool shows which one governs.

Decant fraction f_d^[2] (0.15–0.40)—

MLSS^[1] (2.5–5)g/L

Target F/M^[1] (0.05–0.3)kg/kg·d

Top depth H^[2] (4–6)m

SVI^[3] (80–150)mL/g

Wastage^[1] (3–10)%/d

Results — reactor envelope

Total volume V

—m³

HRT

—h

Actual F/M

—kg/kg·d

Item	Value	Basis

3Cycle-time budget

The four phases must fit inside T_c = 24/N. Whatever is left over is idle time — your operating buffer for peak flow.

Fill t_f^[2] (0.5–2)h

React t_r^[2] (1–4)h

Settle t_s^[2] (0.5–1)h

Decant t_d^[2] (0.5–1.5)h

Phase	Time	Share

4Decant & settling check

In an SBR there is no separate clarifier — if the decanter draws faster than the sludge blanket falls, solids leave with the effluent. This is the check that most SBR upsets trace back to.

Item	Value	Basis

Oxygen & aeration

The SBR only aerates during the react phase — so the blower must be sized on kg O₂ per aerated hour, not per average hour. This is why an SBR's aeration hardware is materially larger than a continuous-flow plant of the same duty.

Item	Value	Basis

5Instrument schedule for this SBR

A batch plant is a time-driven plant: level and DO are the two signals the sequence logic cannot run without.

Point	Instrument	Why here

Quote the SBR instruments at factory price

Per-basin DO, MLSS, level, ORP and decanter position — all mappable to our catalogue and supplied at the manufacturer's ex-works price (our margin is the export rebate, not a markup on the device).

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Sources & parameter basis

[1]	METCALF & EDDY / EPA — SBR F/M 0.05–0.30 kg BOD/kg MLSS·d (0.05–0.15 when nitrifying/denitrifying), MLSS 2.5–5 g/L, SRT 10–30 d, basin count ≥2.
[2]	US EPA Wastewater Technology Fact Sheet: Sequencing Batch Reactors (EPA 832-F-99-073) — decant fraction typically 0.15–0.40 of top-water volume; cycle phases fill / react / settle / decant / idle.
[3]	Settling check: sludge-blanket fall velocity v_s ≈ 3.6/(SVI/100) m/h; the decant draw-down rate is held to ≈0.5·v_s so the blanket is never drawn into the decanter.
[4]	Oxygen: carbon 0.9, nitrification 4.57, biomass credit 1.42 kg O₂/kg VSS (VSS ≈ 0.3·BOD removed); blower SAE 1.8–2.5 kg O₂/kWh; air carries ≈0.28 kg O₂/m³.

Full derivation and worked example: SBR-Reactor-Calculation-Sheet.md. Try raising the cycle count N from 4 to 6 — the volume drops, but the decant draw-down rate climbs and you will hit the settling limit first. That tension is the whole art of SBR sizing.

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