Precursor Size and Organic Halide Formation Rates in Raw and Coagulated Surface Waters

Abstract

The size distribution of organic carbon and organic halide precursors was determined for raw and coagulated waters from two Virginia reservoirs. Organic carbon removal by ferric sulfate coagulation was size dependent, with larger molecules (>5,000daltons) removed more effectively than smaller ones (<1,000daltons). In coagulated waters, the specific yields of organic halide and reaction rates of precursors with chlorine were low compared to the corresponding raw waters. These trends were explained by the selective removal of fulvic adds by coagulation. The dominant organic halide precursors in treated waters were small hydrophobic neutral molecules. Compared to fulvic acids, they had lower specific yields of organic halide and slower reaction rates with chlorine. Because this type of molecule was not removed by coagulation, their concentration ultimately determined whether coagulation alone was sufficient for trihalomethane control.