Turbid waters, containing suspended and colloidal particles, are normally treated by coagulation–flocculation followed by clarification. The process usually consists of the rapid dispersal of a coagulant into the raw water followed by an intense agitation commonly defined as rapid mixing. Mixing is influenced by chemical and physical parameters, and the result of the whole treatment depends on these steps. It is concluded that the proper process of mixing is able to not only improve outlet water quality, but also helps reduce energy dissipation, coagulant dosage and operating cost. It is evidenced, however, that the traditional theory of mixing should be modified. Based on hydrodynamics analysis and experimental results, a mixing model for turbulent flow is presented for the first time. The speed of macroscopic mixing is governed by turbulent diffusion, and the speed of microcosmic mixing is governed by molecule diffusion. The mixing process is dominated by macroscopic mixing. The Euler number Eu can be used as the design parameter that determines the mixing efficiency.

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