Trihalomethane and haloacetic acid disinfection by-products in full-scale drinking water systems

Trihalomethane (THM), haloacetic acid (HAA5), and total organic carbon (TOC) data provided by the Missouri Dept. of Natural Resources for drinking water treatment systems in the State of Missouri was analyzed for the years 1997-2001. These data indicated that a significant portion of systems exceeded the current regulatory limits of 80 and 60 mu g/L for THM and HAA5 in these years. The vast majority of the treatment plants exceeding the regulatory limits were small plants with service populations less than 10,000 people. No significant temporal trend in either THM or HAA5 was noted for the years 1997-2001. This work suggests that the proposed use of a locational running annual average may have a significant effect on compliance. The use of chloramines (combined chlorine) versus free chlorine (HOCl/OCl-) as a residual disinfectant was shown to significantly reduce both THM and HAA5 in systems that treat their own water (primary systems), but did not have a significant effect in systems which purchase their water from primary systems (secondary systems). Comparison of finished water at the treatment plant versus in the distribution system suggested that a majority of THM and HAA5 may be produced within the plant as opposed to the distribution system. Hence, reducing these chlorinated disinfection byproducts within the treatment plant itself should be a key focus for achieving compliance, and supports Environmental Protection Agency disinfection byproducts compliance guidelines using enhanced coagulation.