Set the flow, influent BOD/TKN, MLSS and HRT — get the tank volume, F/M, oxygen demand and rotor
power, the anoxic/aerobic DO zoning, and a temperature-driven nitrification-SRT check. The extended-aeration
workhorse of many SEA municipal plants, sized for tropical 26–34 °C.
1Duty & reactor
Oxidation ditches are low-F/M, long-SRT reactors. HRT 10–40 h; MLSS 2.5–5 g/L; SRT 15–30 d.
Results — reactor & oxygen
Tank volume V
—m³
F/M ratio
—kg/kg·d
AOR
—kg O₂/d
Item
Value
Basis
2DO zoning & nitrification
Orbal runs outer channel anoxic → inner aerobic; Carrousel runs an upfront anoxic selector then the aerobic loop.
Zone
Volume
DO target
Aerator / rotor duty
Item
Value
Basis
Quote the ditch instruments at factory price
An oxidation ditch is instrument-dense: DO per channel, ORP in the anoxic zone, MLSS, NH₃-N, NO₃-N,
level and flow. We supply them at the manufacturer's ex-works price — margin from the export rebate only.
Nitrification SRT via Q₁₀=2: SRTₙ(T)=SRTₙ(20)·2^((20−T)/10), SRTₙ(20)≈3 d [WEF MOP-8].
[3]
Oxygen: carbon 0.9 kg O₂/kg BOD removed; nitrification 4.57 kg O₂/kg N; denitrification −2.86 kg O₂/kg N; biomass credit 1.42 kg O₂/kg VSS (VSS≈0.3·BOD removed).
[4]
Rotor/brush OAE ≈ 1.0–1.5 kg O₂/kWh (Orbal brush ~1.2, Carrousel rotor ~1.5); SAE for blowers 1.8–2.5.
Full derivation and worked example: Oxidation-Ditch-Calculation-Sheet.md. The ditch is the SEA-common alternative to A²/O — run it, then carry MLSS into the secondary clarifier tool.
