The Dependence of Chlorine Decay and DBP Formation Kinetics on Pipe Flow Properties in Drinking Water Distribution

摘要

Simultaneous chlorine decay and disinfection byproduct (DBP) formation have been discussed extensively because of their regulatory and operational significance. This study further examines kinetic variability of the water quality changes responding to hydrodynamics in drinking water distribution. The variations of kinetic constant for overall chlorine decay (kE), trihalomethane (THM) formation were determined under a set of physiochemical and flow conditions using three devices of different wall demand and two types of natural organic matters (NOM) in water. The results from the comparative experiments and modeling analyses show the relative importance of wall demand (kw), DBP-forming chlorine decay (kD), and other bulk demand (kb′) for pipe flow Re=0–52500. It is found that chlorine reactivity of virgin NOM is the overriding factor. Secondly, for tap water NOM of lower reactivity, pipe flow properties (Re or u) can significantly affect, the THM yield (T), formation potential (Y), and the time to reach the maximum THM concentration (tmax) through their influence on kinetic ratio kD/(kb′+kw). These observations, corroborating with turbidity variations, cannot be explained alone by chlorine dispersion to and from the pipe wall. Mass exchanges through deposition and scale detachment are most likely the other mechanism depending on the flow velocity and shear stress on pipe walls. Thus for the simultaneous occurrence of chlorine decay and DBP formation, model considerations of NOM reactivity, pipe types (wall demand), flow hydraulics, and their interactions are essential.