Association between residual chlorine loss and HAA reduction in distribution systems

A goal of the proposed Stage 2 Microbial/Disinfection Byproducts Rules are to reduce peak concentrations of disinfection by‐products (DBPs) in distribution systems. Water utilities will be required to conduct an initial distribution system evaluation to select sampling locations that capture peak DBP concentrations. The selection of monitoring locations is difficult because trihalomethane (THM) and haloacetic acid (HAA) concentrations can vary over time and space in a distribution system. Maximum concentrations may not occur at the same location over time, and maximum THM and maximum HAA concentrations may not occur simultaneously at the same site. Although the maximum THM concentration often corresponds to the location with the maximum residence time, this does not hold true for HAA concentrations, which tend to reach a maximum value and then decline with increased residence time. Occurrence data for HAAs from several distribution system studies were compiled and analyzed to demonstrate variability in HAA concentrations. In these cases, under conditions of low residual chlorine or chloramine concentrations, HAA concentrations were low, even at locations with long residence times. In contrast, THM concentrations were generally high at these same sampling locations. HAA concentrations were often observed to decrease in storage tanks.