Arsenic removal from drinking water by enhanced coagulation

The human health risk due to exposure to arsenic is expected to drive the current MCL of 50 {dollar}mu{dollar}g/L down to 5 {dollar}mu{dollar}g/L or less in the municipal water supplies of the United States. The potential application of enhanced coagulation for arsenic removal from surface water supply sources that have a low background level of arsenic has received much attention in recent years.; A research program was conducted to address the issues of enhanced coagulation on arsenic removal from surface water supply sources. Bench-scale studies on arsenic-spiked water samples were conducted by using standard jar test apparatus. The experimental data were used to (1) develop coagulation diagrams, (2) establish arsenic removal isotherms, (3) assess arsenic accumulation in coagulation sludge, and (4) determine the effect of preozonation of removal of arsenite species by coagulation.; The results showed the high capability of enhanced coagulation for arsenic removal. By using computer linear interpolation method, coagulation diagrams were created for turbidity and arsenic removal by enhanced coagulation. A strong pH dependence of total arsenic removal in settled water was observed. The relationship between total arsenic removal and turbidity removal clearly indicated the importance of effective particle separation and firmly supported two-step arsenic removal mechanisms.; Based on the experimental data, arsenic removal isotherms were obtained by use of regression analysis. These isotherms showed the high immobilization potential of arsenic onto amorphous ferric hydroxide. Several generalized equations were also derived from these isotherms. These equations can be used to calculate the concentrations of arsenic remaining in the finished water.; The production of coagulation sludge was experimentally studied on both a mass and volume basis. By use of mass balance analysis, a concentration expression for arsenic in sludge was established. A high level of arsenic in coagulation sludge is expected. With enhanced coagulation, however, the arsenic concentration in sludge is significantly reduced.; The experimental results showed that As(III) was removable by coagulation without preozonation. However, the removal rate of As(III) was much lower than that of As(V). With preozonation, the removal of As(III) approached that of As(V) without preozonation.