AWWA JAW69829

This study was undertaken because the literature on enhanced softening, specifically with respect to the role of magnesium in removing natural organic matter (NOM), has been limited. In evaluating the fundamental chemistry of precipitate characteristics and their effect on NOM removal, this research showed that NOM removal before magnesium hydroxide precipitation occurs from adsorption onto calcium carbonate (and mixed magnesium-calcium carbonate) precipitates as they form and possibly also through direct precipitations as a magnesium fulvate or humate. The authors identify three potential strategies that softening plants can use to enhance NOM removal and reduce disinfection byproduct formation. Some facilities might be able to operate at a slightly higher lime dose, avoiding magnesium hydroxide precipitation while achieving slightly greater removal of total organic carbon. Depending on source water characteristics, plants could enhance NOM removal by adding magnesium. If NOM removal is to be maximized, soda ash (if required) should be added in the second stage of a two-stage softening process. Includes 37 references, tables, figures.