Improvement on the paiticle sepaiation efficiency through coagulation, sedimentation and filtration which used STI as an index to the treatment

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1)In most cases, only the turbidity has been used as an evaluation index of water treatment. However, the evaluation of the coagulation and sedimentation treatment should be done by adopting the suction time ratio STR (=STI) which has a strong correlation to the residual coagulant quantity. In our research, the usefulness of such method was verified experimentally.2)In the conventional water purification method in which increased coagulant dosing under G_R value of 150 s^<-1> and T_R value of 1min was made, it was demonstrated that when the STR reduction was insufficient, the shortening of filtration time occurred, and the much volume of sludge arose when the high coagulant dosing rate was set.3)In order to reduce both the turbidity and the STR of water treated by the coagulation and sedimentation processes (T_R value is fixed at 5min), it is important to raise G_R values at the optimum value for the coagulant used. When the PAC1 is used as a coagulant, G_R values of 450 s^<-1> and 100 … More 0 s^<-1> were optimum for the reduction of turbidity and STR respectively. On the other hand, When the PSI-3 is used, G_R values of 650 s^<-1> and 1350 s^<-1> were found to be optimum for reducing the turbidity and the STR respectively.4)The following were measured right after the rapid stirring : diameter, circular rate and zeta-potential of the micro flock. As the result, it was confirmed that the diameter of micro flock was decreased and the circular rate was increased, as the G_R value approaches the optimum stirring condition. And, it was verified that the zeta-potential of the micro flock containing much coagulant, and the dense micro flock was formed. It was also shown that such change of the property of micro flock was strongly related to the treatment improvement.5)The direct sand filtration experiments were carried out in order to examine the treatability of the micro flock formed in the conditions in which G_R values and T_R values widely were varied. The results have indicated that the best filtration results on both the filter head loss and the filtered water turbidity were obtained in G_R value of 750 s^<-1> and T_R value of 7min (G_R・T_R value=315,000) where the STR of the raw water right after the rapid stirring became minimum. Less