Water Quality Improvment By Enhanced Coagulation Using Combined Coagulants And Aid Coagulants

In Tanggu water treatment plant, the conventional coagulant of FeCl₃ is used and the turbidity residual is around 1 NTU. Because of bad ability to remove turbidity of the filter, the produced water can not satisfy the water quality standard for urban water supply, especially in the period of the source water fluctuates visibly. So it heavily endangers safety of water supply. This paper did lots of jar test and production test with combination of coagulants under different source water qualities. Mechanisms of coagulation with combination of coagulants were also discussed.Under the customary operation condition (stirring for 3min at 200r/min, stirring for 2min at 40r/min and settling for 10min), the best combination way for enhanced coagulation of low temperature and turbidity source water is adding PAFC first at the speed of 200r/min following with FeCl₃ 5s-20s later. The best mass ratio of PAFC/ FeCl₃ is 3:1. In the study of enhanced coagulation with high temperature and algae source water, the best combination way is adding PAFC first at the speed of 200r/min following with FeCl₃ 5s-20s later, then adding HCA about 1 min later. The best mass ratio of PAFC/ FeCl₃ is 1:2 and the best dosage of HCA is 0.3mg/L.The result indicated that the combination of coagulant not only decreased turbidity residual to about 0.5NTU, but also has superiority in ferron residual (below the detect limit of 0.05mg/L) and algae removal efficiency compared to dosing FeCl₃. With comparing filtered water index, the combination of coagulant could reduce COD_Mn for 30%, UV₂₅₄ for 15%. It will save $0.6 million per year in using the combination of coagulant for a 100 thousand ten water purifying plant.