Field and material-specific simulated distribution system testing as aids to understanding trihalomethane formation in distribution systems

The evolution of disinfection by-products (DBP) in drinking water is triggered at the initial point of disinfectant application and, in most cases, continues to some extent throughout the distribution system. A comprehensive study using an extensive field monitoring program, together with an improved simulated distribution system (SDS) test, is presented, in which the effects of increased chlorine demand exerted by internal pipe walls are included in the evaluation of potential DBP exposure to the public. In the field study, comparing both pre-formed trihalomethane (THM) and THM formation potential among several distribution system locations eliminated inherent uncertainties associated with the complexities of network hydraulics, leaving exposure to the internal pipe environment as the primary factor of interest. Findings in this paper confirm the reliability of using the material-specific SDS (MS-SDS) test as a better representation of DBP evolution in a real distribution system. In the case study, the increased chlorine demand of unlined cast-iron results in lower levels of trihalomethanes than those dictated by testing the finished water alone. The MS-SDS test is readily adaptable for pilot-plant studies where real distribution system conditions are inaccessible.