| The identification of increasing numbers of potentially hazardous trace organic contaminants in drinking water supplies has raised considerable public concern. Lime softening, a commonly used water treatment process, is capable of removing certain soluble organic contaminants; but the types of contaminants that can be taken out and the factors influencing such removal are not well defined. This research was undertaken to investigate the mechanism responsible for the removal of trace organic contaminants by lime softening, to identify the molecular characteristics of removable compounds, and to explore modifications to the process to enhance removal efficiency. The study proceeded in two phases. In phase I removal mechanisms and process modifications were investigated using a fulvic acid extracted from a local groundwater supply. In phase II the molecular characteristics of contaminants pertinent to their removal were examined by selecting a wide range of natural and synthetic organic substances and testing their removal by softening, their adsorption onto calcium carbonate, and their ability to bind calcium ions.;Conditions suitable for organic contaminant removal were high reaction pH, high calcium concentration, low carbonate concentration, and finely divided calcium carbonate solids with poor crystallinity. Jar tests demonstrated that the softening process can be modified to foster a more favorable environment for the removal of organic matter without compromising the removal of hardness.;Removal of organic contaminants by softening was determined to result from coprecipitation in which organic contaminants adsorbed onto softening sludge (calcium carbonate) at an early stage of the reaction and precipitated together with the sludge solids. Except in the case of cationic polymers, the adsorption step involved specific chemical forces (chemisorption) and only those compounds able to coordinate with calcium ions were removed. Compounds more strongly adsorbed by calcium carbonate were better removed. Molecular characteristics influencing adsorption included molecular charge, functional groups, degree of polymerization, molecular geometry, and hydrophilicity. Polymeric organic compounds possessing acidic functional groups, such as carboxyl, phenol/enol, phosphoryl, and sulfuryl, were removable. Simple monomeric molecules were not taken out to any significant extent unless polymerizable during softening or possessing phosphorus-containing groups such as phosphoryl and phosphonyl. |
