Enhanced particle destabilization and aggregation by flash-mixing coagulation for drinking water treatment

In water treatment plants (WTPs), the operation of rapid mixing in coagulation units dominates particle destabilization and the formation of flocs. The goal of this study was to investigate the effect of enhanced rapid-mixing intensity (velocity gradient [G]) on the performance of coagulation for natural turbid water treatment. Two surface-water samples of low and high turbidity were coagulated by using polyaluminum chloride (PACl), with G values ranging from 200 to 800 s^-1, and the destabilization of particles and dissolved organic matter (DOM) was determined, followed by measurement of filterability of the supernatant. Furthermore, the effect of high rapid mixing on the coagulation efficiency was studied by using a pilot-scale, in-line instantaneous flash mixer (IFM) which could generate much higher G than a conventional mechanical mixing of WTP. The results of Jar test showed that the enhanced reduction in turbidity was accompanied by the improved filterability of the supernatant, which was due to the formation of larger flocs with strong structure in response to higher rapid-mixing intensity, especially for low-turbidity water. It was found that the fractal dimension (D_f) of coagulated flocs is inversely proportional to the recovery factor of broken flocs. The in-line IFM with intensive mixing (G > 5000 s^-1) substantially improved the removal of particles and DOM, as a result of the formation of large flocs.