Zones in a clarifier
- Inlet and feed well: dissipate energy, encourage flocculation, spread flow.
- Clear water zone: the top few feet; clarified effluent moves to the weirs.
- Settling (hindered) zone: floc settles as a mass.
- Blanket and thickening zone: sludge compacts before being drawn off.
- Collector: scrapers or suction headers move sludge to the draw-off.
Loading rates
Surface overflow rate (SOR) = Q / surface area, gpd/ft². It controls clarification: the upward velocity of water must be less than the settling velocity of the floc. Typical design values for activated sludge are about 400 to 800 gpd/ft² average and 1,000 to 1,200 peak, depending on settleability.
Solids loading rate (SLR) = (Q + RAS) x MLSS x 8.34 / area, lb/d/ft². It controls thickening. Typical design values are about 20 to 30 lb/d/ft² average, up to about 40 to 50 at peak with good settling.
Weir overflow rate = Q / weir length, gpd/ft. Usually less critical with modern baffles.
White River example
Four 90 ft clarifiers, 25,447 ft² total. At design average 8.0 MGD:
SOR = 8,000,000 / 25,447 = 314 gpd/ft²
SLR = (8.0 + 4.0) x 3,000 x 8.34 / 25,447 = 11.8 lb/d/ft²
At peak hour (about 24 MGD design), SOR is about 943 gpd/ft² and SLR about 27.5 lb/d/ft².
Sludge blanket
The blanket is your best real time indicator. Most plants aim for 1 to 3 ft (White River alarms at 4 ft). Measure every clarifier at the same time each day with a core sampler, sludge judge or blanket meter. A rising blanket means solids are arriving faster than they leave.
State point analysis
State point analysis compares the clarifier's thickening capacity (a flux curve built from settling tests) with the operating conditions (overflow and underflow lines). It shows whether the clarifier is underloaded, critically loaded or overloaded, and what RAS rate would help.
Try it with your own data in the state point tool.
Common problems
- Rising blanket: thickening overload, low RAS, high SVI. Guide
- Floating sludge: denitrification or septic sludge. Guide
- Washout: hydraulic overload in storms. Guide
- Cloudy effluent: pin floc, dispersed growth. Guide
- Short circuiting: uneven weirs, density currents, missing baffles.
- Mechanical: collector breakdowns, plugged suction headers, scum system failures.
Good habits
- Measure blankets every shift.
- Keep weirs level and clean.
- Balance flow and RAS between clarifiers.
- Run state point when SVI changes or storms are forecast.
- Inspect collectors and drain each clarifier on a schedule.