Coagulation and flocculation processes are used to separate the suspended solids portion from water. However, coagulation–flocculation and sedimentation only are not sufficient in modern practices as nowadays every process is being viewed through an economic point of view. The present work aims to study the impact of pre-sedimentation on optimum coagulant dosage of raw water taken from a lake located at Tezpur University. Alum was used with the aim of determining its optimum doses at optimum pH and alkalinity. The influence of seasonal variation of pH and alkalinity on the coagulation dosages was studied and conditions were optimized corresponding to the best removal of turbidity. For raw and settled water the optimum alum dose obtained was 35–40 and 25–30 mg/L, respectively, clearly showing the impact of pre-sedimentation on optimization of alum dose and thereby economizing the water treatment process. Results indicated up to 98.67% removal of turbidity of water sample which had undergone pre-sedimentation process. A correlation has been developed between overall suspended solids removal and overall turbidity removal which can help in quickly estimating overall suspended solids removal using turbidity data.

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