Controlling Disinfection Residual Losses in Drinking Water Distribution Systems: Results from Experimental Studies

It has become generally accepted that water quality can deteriorate in a distribution system through reactions in the bulk phase and/or at the pipe wall. These reactions may be physical, chemical and/or microbiological in nature. Perhaps one of the most serious aspects of water quality deterioration in a network is the loss of the disinfectant residual that can weaken the barrier against microbial contamination. Studies have suggested that one factor contributing to the loss of disinfectant residuals is the reaction between bulk phase disinfectants and pipe wall material. Free chlorine loss in corroded metal pipes, subject to changes in velocity, was assessed during an experiment conducted under controlled conditions in a specially constructed pipe loop located at the U.S Environmental Protection Agency's (EPA's) Test and Evaluation (T&E) Facility in Cincinnati, Ohio (USA). These studies demonstrated that in older unlined metal pipes, the loss of chlorine residual increases with flow. Additional experimental studies are currently being conducted by the USEPA. They are intended to study the effect of hydrodynamics on disinfectant residual wall demand as well as the effect of total organic carbon, initial disinfectant levels, and pipe materials on chlorine and chloramine decay and disinfection by-product (DBP) formation. These experiments are being conducted in parallel using both unlined metallic and new polyvinyl chloride (PVC) pipe. The first phase of these experiments (which will be reported here) has focused on the effect of flow rate and pipe materials on the loss of chlorine disinfectant residuals. Results from these studies indicate that there is significant disinfectant wall demand in unlined metallic pipe even under stagnant and laminar flow conditions and that increases in flow rate can increase this demand. Wall demand in the PVC pipe, however, was found to be virtually nonexistent. Studies were also conducted on the rate of trihalomethanes (THM) formation under stagnant, laminar, transitional and turbulent conditions. Planned experiments include studies on chloramines and further attempts to isolate the effects of materials on disinfectant reaction kinetics. It is believed that results from these studies can be used to develop strategies to help maintain adequate disinfectant levels in distribution system networks.