Impact of Organic Carbon Removal on Cancer Risks Posed by Drinking Water Chlorination
Creators: name: Black, Bryan D.
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 1994

description: The US Environmental Protection Agency is currently proposing regulations to further limit the concentrations of disinfection by-products in drinking water distribution systems. Certain disinfection by-products have been indicted as potential carcinogens in both toxicological and epidemiological studies. One important strategy for reducing by-product concentrations in water distribution systems is to limit the amount of organic carbon, a by-product precursor, at the point of disinfection. According to the regulatory impact analysis, over 4,000 surface water utilities (greater than 60 percent of total US surface water utilities) would use this strategy to comply with proposed regulations. The analysis presented in this report evaluates the impact of reducing organic carbon concentrations at the point of chlorination on cancer risks posed by trihalomethanes, a major class of disinfection by-products. In addition, costs and benefits are estimated for various water qualities, service populations, and treatment technologies to identify conditions in which a given treatment technology more cost-effectively reduces cancer risks. The ratio of source water bromide concentration to the applied chlorine dose affects the distribution of chemical species within a by-product class. Because the chlorine dose is typically varied proportionally with the organic carbon concentration, reducing the organic carbon concentration indirectly increases the ratio of bromide to aqueous chlorine. As this ratio increases, the more brominated by-products predominate. Many researchers indicate that reducing organic carbon concentrations at the point of chlorination results in increased concentrations of the more brominated by-products, due to the increase in bromide to aqueous chlorine ratio. If the more brominated by-products are particularly hazardous, the overall cancer risk could increase as organic carbon concentrations at the point of chlorine application are further limited. Alternatively, if the potencies of the more brominated by-products are less severe but still high relative to other by-products, organic carbon removal could result in little or no risk reduction. A methodology is presented to systematically evaluate the effects of water quality, in particular organic carbon and bromide concentrations, on both total risk and the potential for risk reduction. Empirical models are applied to a baseline water quality to predict trihalomethane concentrations. Concentrations are converted to risk using toxicological potency factors. The organic carbon removal performances of treatment technologies are simulated so that the risk reduction achieved by implementing the technology can be calculated. The costs and benefits associated with each technology are then estimated for various service population sizes. The results of this analysis indicate that risk generally decreases with decreasing organic carbon concentration at the point of chlorination, although the concentrations of the more brominated by-products typically increase. Waters with higher initial bromide concentrations generally show greater potential for risk reduction. Therefore, enhanced coagulation, GAC adsorption, and membrane filtration would more cost-effectively reduce risk when applied to waters with higher concentrations of bromide. The cost-effectiveness of GAC adsorption and membrane filtration generally improve for larger service populations. The large disparity in cost-benefit ratios, based on water quality and service population size, suggest that equitable regulations should consider site-specific characteristics. Numerical modeling as presented in this document could help ease the management burden of site-specific regulations. Enhanced coagulation, GAC adsorption, and membrane filtration have high cost-benefit ratios when considering reduction of risks induced by trihalomethanes only. When a worst-case scenario of risk for haloacetic acids is also considered, enhanced coagulation appears reasonably cost-effective, but GAC adsorption and membrane filtration appear cost-prohibitive. Mandating enhanced coagulation, therefore, is a reasonable risk mitigation measure in the presence of health effects uncertainties. However, GAC adsorption and membrane filtration appear cost-prohibitive when implemented solely for risk mitigation purposes, even under conditions most favorable to their implementation (worst-case scenario of health risks).