Influences of Al-Based Coagulants with Different OH/Al Ratios on Drinking Water Treatability

Abstract

This research investigated the performance of Al -based coagulants of differing OH/Al mole ratio in terms of turbidity removal, pH depression, residual aluminium, z potential, and water temperature effect. When liquid aluminium sulfate (LAS, OH/Al=0) was replaced by a coagulant with high OH/Al mole ratio, the conventional dosing system would need modification to optimize performance. Silicated -poly aluminium sulfate (Si -PAS, OH/Al=0.75) was more effective for turbidity removal in comparison to LAS, although Si -PAS was more sensitive to water temperature and pH. At high pH, rapid mixing improved the quality of the settled water. From the result of this study on investigating the correlation between residual aluminium and physico -chemical factors such as (residual turbidity, alkalinity, and mixing intensity), residual aluminium was found to be significantly influenced by residual turbidity. This indicates that residual aluminium can be reduced by more efficient filteration. The increase of alkalinity in raw water resulted in the faster hydrolysis and the decrease of removing rate of particles. The increase of both OH/Al mole ratio and mixing intensity minimized residual aluminium and improved the incorporation of the microflocs. Moreover, the measurements of z potential of colloids indicated that an overdosed fraction of Si -PAS slightly broke up flocs, while excessive LAS gave a severe negative effect on flocs.