This study reports basic research on the effectiveness of raising the rapid agitation strength (velocity gradient, or GR value) in improving the treatment efficacy of coagulation and sedimentation. Coagulation and sedimentation treatment with a normal velocity gradient of 100-200 s-1 reduces turbidity to less than 1 mg/L but leaves a significant amount of coagulant residue and thus does not reduce the suction time index (STI) to a desirable level. By contrast, higher GR values yield improvements in the turbidity of the treated water, as well as marked improvements in the STI. Good coagulation and sedimentation treatment is also related to reductions in filtration head loss.

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