Enhanced Softening: Effects of Source Water Quality on Nom Removal and Dbp Formation

Abstract

Jar tests were conducted on six source waters from softening plants throughout the United States to evaluate the extent of softening, natural organic matter (NOM) removal, and reduction in disinfection by‐product (DBP) formation achieved by varying lime doses. Results demonstrated the effect of source water characteristics, e.g., specific ultraviolet absorbance (SUVA), bromide, and total organic carbon (TOC) on the potential for softening to remove NOM and reduce DBP formation. The source water exhibiting the lowest SUVA value had the lowest degree of NOM removal, and the source water with the highest bromide concentration showed substantial formation of bromine‐substituted DBPs. These results confirmed previous observations that enhanced softening improves removal of dissolved organic carbon, decreases SUVA values, and decreases formation of dissolved organic halogen. The research also showed that both calcium carbonate and magnesium hydroxide are responsible for NOM removal in softening. With the exception of Lake Austin (Texas) source water, the waters studied would require softening to the point of magnesium hydroxide precipitation to meet required TOC removals and (in some cases) to meet the maximum contaminant level for total trihalomethanes and the sum of five haloacetic acids with lime softening alone.