SLR is the mass of solids applied to a clarifier per unit area. It controls thickening.
Clarifier Solids Loading Rate
Formula
SLR, lb/d/ft² = (Q + RAS, MGD) x MLSS, mg/L x 8.34 / Area, ft²
Inputs used
- Influent flow (MGD)
- RAS flow (MGD)
- MLSS (mg/L)
- Clarifier diameter (ft)
- Clarifiers in service (units)
- Or total area (overrides diameter) (ft²)
Worked example (White River WRRF, fictional)
- Solids applied = (8.0 + 4.0) x 3,000 x 8.34 = 300,240 lb/d
- Area = 25,447 ft²
- SLR = 300,240 / 25,447 = 11.8 lb/d/ft²
Tips
- Include RAS in the flow: (Q + RAS) x MLSS.
- Compare with state point limiting flux for the current SVI.
Results are estimates for operator use. Check units and inputs, and confirm decisions with plant data and procedures.
This calculator also appears on ActivatedSludge.com, our consulting site. Results are for training and operator use; always follow your plant’s procedures.