Impact of Dual Alum and Polyaluminum Chloride Coagulation on Filtration

The process of treating drinking water with a combination of alum and polyaluminum chloride (dual coagulant) has been performed by at least one utility along the Colorado Front Range since the early 199Os. Previous bench-scale research with water containing low concentration of natural organic matter (NOM) showed the dual treatment developed larger non- settleable floc than alum and these floc re-aggregated faster than polyaluminum chloride (PACL) or alum after experiencing induced shearing. Pilot-scale studies showed longer filter run volumes with low-NOM waters treated with dual coagulant. Full-scale testing using reservoir water with stable water quality parameters, including relatively low levels of NOM, was conducted to determine if bench-scale and pilot-scale studies were indeed indicative of full-scale performance. The coagulants were administered at typical, plantestablished doses and finished water quality was continuously monitored to ensure that the overall water quality was not sacrificed. Turbidity, filter loss of head, and filter run length were monitored during the sample periods. Total organic carbon (TOC) samples were collected and analyzed during each of the coagulant monitoring periods. Full-scale data shows that the dual treatment provided significantly longer filter run times than alum or PA% alone. However, TOC removal for alum treatments proved better than for PACL or the dual coagulant. Bench-scale experiments were conducted with high-NOM water to further the understanding of the mechanisms involved for the dual coagulants improved performance and to determine if high-NOM water treated with dual coagulant performed similarly as low-NOM water. The same three coagulant regimes as previously mentioned were examined and their optimum doses were determined based on turbidity and TOC removal. A photometric dispersion analyzer was used to study the effects of shearing.