An industrial wastewater effluent is treated using a complete mix activated sludge process with a 2 secondary clarifiers following it. The effluent from the clarifiers contains 60 mg/L VSS (volatile suspended solids). The flow rate from the primary clarifiers is 60,000 m3/day and contains 100 mg/L VSS. The bioreactor (activated sludge tank) has a volume of 8,000 m3 and a biomass production rate of 0.5 kg/m3-d. The return VSS concentration is 12,000 mg/L. The specific gravity of the VSS is 1.0.
a) For a mixed liquor volatile suspended solids (MLVSS) of 1800 mg/L, what is the total return solids flow rate?
b) Assume the return solids flow rate is 5,000 m3/d and the MLVSS is 2200 mg/L; what is the wasted solids flow rate from each of the secondary clarifiers?
c) What solids must be wasted daily to maintain the MLVSS at constant concentration?