Process Control Cheat Sheets
Core formulas and setpoints operators use every shift to keep the process in range.
MCRT (Mean Cell Residence Time)
MCRT = (MLSS × Aeration Volume) ÷ (TSS wasted + TSS effluent per day).
F/M Ratio (Food-to-Microorganism)
F/M = BOD Loading (lbs/day) ÷ MLSS (lbs). Typical conventional range: 0.2–0.5.
DO Setpoints
Maintain 1.5–2.5 mg/L in aeration basins for healthy floc formation and nitrification.
Alkalinity & pH Control
Maintain alkalinity above 50 mg/L as CaCO3 to buffer nitrification's acid production.
SVI (Sludge Volume Index)
SVI = (Settled Volume, mL/L ÷ MLSS, mg/L) × 1,000. Under 100 mL/g is good settling.
WAS Pumping Rate / Wasting Calculation
Wasting rate is set to hit a target SRT, not a fixed flow — recalculate as MLSS changes.
Chemical Dosage (mg/L to lbs/day)
lbs/day = Dose (mg/L) × Flow (MGD) × 8.34 — the single most-used conversion in daily chemical feed calculations.
Detention Time
Detention Time (hours) = Tank Volume (gallons) ÷ Flow Rate (gph) — confirms a basin is sized for the process it's running.
Percent Removal Efficiency
Percent Removal = (Influent − Effluent) ÷ Influent × 100 — the standard way to report treatment performance.
Oxygen Transfer Efficiency (OTE)
The percentage of supplied air's oxygen content actually dissolved into the mixed liquor.
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