AWWA ACE63032

Compliance needs for the Stage-2 Disinfection Byproducts (DBPs) rule is prompting many water treatment utilities to explore additional DBP precursor removal beyond enhanced coagulation. The use of biologically active granular activated carbon (GAC) filters is now becoming popular as an effective means of lowering DBP levels. To minimize DBP levels by biologically active GAC filter, many water treatment facilities are now abandoning the practice of pre-chlorination. Two large water treatment utilities, one located in Texas and the other located in Arizona, have implemented or will soon be implementing such practices. Elimination of pre-chlorination, however, causes iron/manganese (Fe/Mn), if present in source water, to remain in soluble form (due to non-oxidation) passing through filtration processes. Upon chlorination of post-filtered water and/or inadequate mixing during caustic addition have caused dissolved Fe/Mn to precipitate causing yellow or reddish coloration, staining submerged surfaces and/or increasing the turbidity in the finished water. To avoid these water qualities related issues several treatment strategies were investigated in bench and full-scale settings for these two utilities. Proposed strategies included a preliminary assessment of the fate and transport of Fe/Mn from raw to finished water and evaluation of conventional (chlorine, permanganate) and alternative (chlorine dioxide) oxidants to minimize Fe/Mn related issues for the finished water. Includes figures.