Removal of Disinfection By-Product Precursors by Lime-Soda Softening
Creators: name: Thompson, Jeffery David
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 1995

description: The purpose of this research was to evaluate the removal of natural organic matter (NOM) by the lime-soda ash softening process from both a scientific and a regulatory perspective. The relationships between hardness removal, NOM removal, and certain chemical characteristics of NOM were explored and the implications of the enhanced softening portion of the proposed Disinfectants/Disinfection By-Products (D/DBP) Rule were assessed. Total organic carbon (TOC) is a commonly used surrogate parameter for NOM, which is the principal precursor to the formation of disinfection by-products. The fraction of TOC that passes through a 0.45 μm filter is defined as the dissolved organic carbon (DOC) content, which was used as a surrogate for NOM in this study. Raw waters were collected from nine randomly selected softening utilities across the United States to provide a diversity in source waters and inorganic material. For each raw water, the distribution between hydrophobic and hydrophilic organic carbon was experimentally determined and softening jar tests were conducted. In addition, raw, settled and filtered water samples were collected from the participating utilities so removal of TOC at the plant-scale could be evaluated. The results of this study show that softening with lime and soda ash followed by settling can remove modest amounts of DOC from most raw waters, depending upon the pH of softening. The precipitation of calcium carbonate (CaC03) removed DOC to some degree, but the precipitation of magnesium hydroxide (Mg(0H)2) was found to greatly enhance the removal of DOC. For most of the waters examined in this study, the amount of DOC removed per unit increase in pH was found to increase considerably at the pH where magnesium precipitation dramatically increased. In addition, the amount of DOC removed showed a strong correlation with the amount of magnesium precipitated, with a semi-logarithmic model providing an excellent fit to the data for most waters. The slope of this semi-logarithmic model was found to be highly correlated with the initial DOC concentration of the raw water, which indicates that the absolute amount of DOC removed by Mg(0H)2 increases with increasing DOC concentration. A Freundlich adsorption isotherm adequately described the relationship between DOC removal and Mg(0H)2 precipitation for five of the nine waters examined. This suggests that removal of DOC by Mg(0H)2 involves an adsorption process but it may be too complex to be adequately described in all cases by a simple adsorption model such as the Freundlich isotherm. DOC removal by softening was found to be most effective for the raw waters with the highest percent hydrophobic organic carbon and least effective for the raw waters with the lowest percent hydrophobic organic carbon. However, the specific UV absorbance of the raw water (SUVA), which is the UV absorbance at 254 nm divided by the DOC concentration, was found to be a better predictor of the effectiveness of DOC removal by softening than the percentage of hydrophobic organic carbon. Considering the enhanced softening requirements of the proposed D/DBP Rule, it appears that for most waters, the removal of 10 mg/L of magnesium hardness as CaC03 can be achieved with lower chemical doses than the required percent removal of TOC. For some waters, significantly more than 10 mg/L of magnesium hardness removal is necessary to meet the required percent TOC removal.