Enhanced softening: factors influencing DBP precursor removal

Abstract

Bench‐scale tests demonstrate the importance of magnesium hydroxide precipitation and NOM characteristics on precursor removal by softening. Relationships of hardness removal, natural organic matter (NOM) removal, and selected chemical characteristics of NOM were explored, and the implications of the enhanced softening portion of the proposed Disinfectants/Disinfection By‐products Rule were assessed. For most of the nine raw water sources examined, the amount of dissolved organic carbon (DOC) removed (both on an absolute and a percentage basis) correlated strongly with the amount of magnesium precipitated. A semilogarithmic model provided a good fit to the data for six of the nine waters ( R 2 values >0.8). Greater absolute removal of DOC was achieved for raw waters with higher initial DOC concentrations. DOC removal by softening was most effective for the raw water sources with the highest percentage of hydrophobic organic carbon. Greater specific ultraviolet absorbance values also corresponded to greater DOC removal.