Chlorine Decay and DBP Formation under Different Flow Regions in PVC and Ductile Iron Pipes: Preliminary Results on the Role of Flow Velocity and Radial Mass Transfer

A systematic experimental study was conducted using a pilot-scale drinking water distribution system simulator to quantify the effect of hydrodynamics, total organic carbon (TOC), initial disinfectant levels, and pipe materials on chlorine decay and disinfection by-product (DBP) formation. The first phase of the experiments focused on the variables of flow rate and pipe materials and their effects on the formation of trihalomethanes (THMs) a primary category of DBPs in chlorinated drinking water. Different from previously reported bench-scale investigations, this experimental study was to determine chlorine decay and DBP formation kinetics under simulated field conditions and to contrast the effects of new PVC and aged ductile iron pipe materials. In this paper, we report the experimental findings on the rate of THM formation under stagnant, laminar, transitional and turbulent conditions, and further attempt to address the effects of the pipe materials on the reaction kinetics. The results indicate that the second-order DBP formation model of Clark (1998) can sufficiently describe the variations in total trihalomethanes (TTHM) concentrations. The determined reaction constants are smaller under stagnant and turbulent flows in the new PVC pipes than the aged ductile iron pipe. The latter has a high rate of DBP formation accompanying with rapid chlorine residual loss. It is suggested that these observed differences are a result of the mass-transfer enhanced wall demand in the aged ductile iron pipe. Implications for re-chlorination in the distribution network operations are discussed.