Recent advances in coagulation have emphasized the importance of rapid mixig. It is generally accepted that the principal design parameters of rapid mix are velocity gradient and detention time, although several parameters such as chemical and other geometric factors also influence. Optimum mixing can be obtained if sufficient mixing energy is added and the mixing time is as short as possible on the basis of coagulant reaction kinetics and fluid mechanics.
In terms of velocity gradient and detention time for rapid mixing system, domestic design criteria have not been established exactly in details and not standardized in practice. Moreover it is considerably different from recent works or trends of advanced water treatment techniques. It is, therefore, desirable to investigate design and operating criteria of rapid mixing.
The objectives of this study were to establish reasonable velocity gradients and detention time through the experimental investigation over specific pH and alum dose conditions.
From the results of this study, the following conclusions were reached:
1) At pH 7.0 as general operation condition, optimum values of detention time and velocity gradient were respectively 1 second and $100s^{-1}$
2) For the other conditions studied, velocity gradient value of 1000 - $1500s^{-1}$ and detention time 1 second were observed as being optimal except for only pH 4.8, alum dose 30-40mg/l and pH 7.8, alum dose 30-50mg/l conditions.
3) Two stage rapid mixing system showed a better turbidity removal than one step rapid mixing system. Secondary mixing time of 30 seconds was required with a relatively high velocity gradient value of $200-300s^{-1}$
4) In this study, the optimum design and operation criteria for rapid mixing was as follows; G values of $1000-1500s^{-1}$ for detention time of 1 second in general rapid mixing system: G values of $200-300s^{-1}$ for 30 seconds as the secondary mixing following above initial mixing in two stage rapid mixing system.
