How to use this calculator
- Enter the average flow and peak factor.
- Choose an overflow rate and side water depth.
- Set the number of clarifiers, then read diameter and checks.
- Add influent TSS and BOD for removal and sludge estimates.
How do you design a primary clarifier?
Area = Q ÷ SOR; D = √(4A ÷ π), rounded up to 0.5 m
HRT (h) = A × SWD ÷ Q × 24
Removal (%) = t ÷ (a + b × t)
Removal constants from Metcalf and Eddy: BOD a = 0.018, b = 0.020; TSS a = 0.0075, b = 0.014, with t in hours.
Worked example
With average flow 5,000 m³/day, peak factor 2.5, design overflow rate at average flow 40 m³/m²·day, side water depth 3.5 m, number of clarifiers 2, influent tss 250 mg/L, influent bod 200 mg/L:
Clarifier diameter: 9 m
The calculator also gives surface area per unit 63.6 m²; overflow rate, average 39.3 m³/m²·day; overflow rate, peak 98 m³/m²·day.
Assumptions and limits
- Domestic sewage. Industrial solids may settle differently; do a settling column test.
- Removal curve is an empirical average.
Frequently asked questions
What is the surface overflow rate of a primary clarifier?
Metcalf and Eddy suggests 30 to 50 m³/m²·day at average flow and 80 to 120 at peak hourly flow for primary clarifiers that are not receiving waste activated sludge.
What is the detention time of a primary clarifier?
Usually 1.5 to 2.5 hours at average flow. Longer times can turn sewage septic and cause odour.
How much BOD does a primary clarifier remove?
Typically 25 to 40% of BOD and 50 to 70% of suspended solids.
References
- Metcalf and Eddy, Wastewater Engineering: Treatment and Resource Recovery, 5th ed.
- CPHEEO, Manual on Sewerage and Sewage Treatment Systems (2013)