Coagulation Followed by Continuous Sand Filtration for Treatment of Graywater

Abstract

In a long-duration study, the performance of a physicochemical system for the treatment of raw graywater––namely, coagulation followed by continuous saturated-sand filtration––was investigated. Graywater coagulated using alum at the optimum dose was used as the influent to the filter. The filter was operated for 101 days in downflow mode at a flow rate of 10 L/day (hydraulic loading rate of 320 L/m2/day). The results showed that the quality of the raw graywater was highly variable, even though the source of the graywater was the same. The raw graywater had a mean turbidity and chemical oxygen demand (COD) concentration of 49 NTU and 157 mg/L, respectively. Coagulation-flocculation was very efficient at removing turbidity, COD, and fecal coliforms (FCs), achieving 94%, 65%, and 96% removal, respectively, but failed to meet the different reuse standards. The combined treatment of coagulation followed by sand filtration showed 99%, 91%, and 94% overall removal of turbidity, biochemical oxygen demand, and COD, respectively. The treatment also resulted in >99% removal of FCs, while phosphates were reduced to below detectable limits. The results of the study indicated that effluent quality was not significantly affected by variations in the influent characteristics, and the filter effluent met different reuse standards.