The direct filtration system is widely used in the treatment of source waters with low and stable turbidity. We have previously indicated the importance of optimizing agitation strength GR and time TR in rapid mixing tanks in order to decrease filter head loss and treated water turbidity in direct filtration. In the present study, we employ a batch-type coagulation experimental apparatus that incorporates a high-sensitivity particle counter, where the particulate concentrations are measured continuously after injection of coagulant, in order to clarify the fundamental coagulation and microfloc formation dynamics. Specifically, it is shown that, after injection of the coagulant, coagulation and microfloc formation occurs through distinct periods: an agglomeration preparation period, followed by an agglomeration progression period, and then finally an agglomeration stabilization period, and that optimization of the GR value is the most important consideration, although both the coagulant concentration and GR influence the time at which agglomeration begins in the preparatory period, the time at which agglomeration stabilizes after the progression period, and the concentration of initial particles with diameters of 1–3 μm at completion of agglomeration.

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