Influence of Alum Coagulant Dose and Influent Turbidity on Floc Blanket Growth Rate, Steady-State Suspended Solids Concentration, and Turbidity Removal

Floc blankets are concentrated, fluidized beds of particles utilized in some water treatment plants. Floc blankets occur in vertical-upflow sedimentation tank configurations and are reported to enhance removal of suspended solids. In this research, floc blankets were formed in the laboratory at an upflow velocity of 1.2 mm/s and at varying alum coagulant dosages with influent turbidities of 10 nephelometric turbidity units (NTU), 100 NTU, and 500 NTU. Image analysis was utilized to quantify floc blanket suspended solids concentration and floc-water interface height over time. Blankets formed with higher influent turbidities required a lower ratio of coagulant dose to influent turbidity, formed more quickly, had higher steady-state suspended solids concentrations, and achieved better suspended solids removal efficiency. Increasing alum coagulant dose decreased steady-state floc blanket suspended solids concentration and resulted in lower effluent turbidities. At an influent turbidity of 100 NTU, effluent turbidity was strongly correlated with coagulant dose at coagulant doses of 1 to 7.5 mg/L alum. Diminishing improvement in turbidity removal was observed for coagulant doses above 15 mg/L alum for both 100 and 500 NTU. A higher ratio of coagulant dose to influent turbidity could be used by water treatment plant operators to allow floc blankets to form quickly, and after formation of the floc blanket, the coagulant dose could be reduced without significant impact on the removal of suspended particles.