Effectiveness of pre- and intermediate ozonation on the enhanced coagulation of disinfection by-product precursors in drinking water

This study evaluated the impact of pre- and intermediate ozonation coupled with enhanced coagulation in controlling halogenated disinfection by-product formation in, drinking water. Raw waters from utilities representing each of the nine elements of the enhanced coagulation matrix presented in Table I were examined. All testing was completed using bench-scale, batch experimental procedures. The various waters were analyzed for turbidity, total organic carbon, dissolved organic carbon, ultraviolet absorbance, trihalomethane formation potential, and haloacetic acid formation potential before and after ozonation. The results indicated that formation of trihalomethanes and haloacetic acids following enhanced coagulation decreased with both pre- and intermediate ozonation applications relative to the decreases observed by enhanced coagulation alone. The amount of trihalomethanes and haloacetic acids formed were lower for the waters that were pre-ozonated and then coagulated compared to those that were coagulated first and then ozonated. This comparison must be tempered by the fact that the settled waters treated by intermediate ozonation were not subjected to subsequent biofiltration which is commonly used in water treatment practice to remove additional DBP precursors. Strong correlations between disinfection by-product formation potentials and ultraviolet absorbance at 254 nm were observed for enhanced coagulation with and without pre- and intermediate ozonation.