High turbidity water during the rainy season has long been a challenge to the waterworks in Taiwan. Coagulation is the critical process for solving this problem; however, the dispersion of coagulants in the concentrated solid phase is very difficult. In this study, the effects of rapid mixing on the PACl coagulation of high turbidity water were investigated in terms of turbidity and particle count. Operational parameters of coagulant dosage, coagulant concentration and mixing intensity in the rapid-mixing step exhibited great impacts on the residual turbidity and particle count of the simulated system. Aggregation degree, as evaluated by the Photometric Dispersion Analyser (PDA), was influenced by the intensity of rapid mixing. Poor rapid mixing failed to induce aggregation to effectively coagulate the small and hard to settle particles.

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