Particulate and THM precursor removal with ferric chloride

Pilot-scale experiments were performed to investigate the effectiveness of enhanced coagulation in removing particles and trihalomethane (THM) precurs ors from two surface source waters: California State Project water and Colo rado River water. The removal of suspended particles and natural organic ma tter at various ferric chloride doses and coagulation pHs was assessed thro ugh source water and filter effluent measurements of turbidity, particle co unt, UV254, TOC, and THM formation potential. Overall, it was found that op timal removal of particles and THM precursors by enhanced coagulation with ferric chloride is obtained at high coagulant doses (>16 mg/L) and low pH c onditions. Generally, turbidity removal is more efficient and head loss is more moderate at ambient pH compared with pH 5.5. Additionally, filter effl uent particle counts were found to be consistent with residual turbidity da ta. The removal of THM precursors by enhanced coagulation is significantly enhanced at pH 5.5 compared with ambient pH. The reduction in THM formation potential is consistent with the trends observed for the THM precursor rem oval data (i.e., UV254 and TOC data). Furthermore, specific UV absorbance w as used to estimate the proportion of humic substances in the raw waters. E nhanced coagulation was found to be less effective for the source water wit h the lower specific UV absorbance.