Characterizing the Pipe Wall Demand for Free Chlorine in Metallic Pipes

Abstract

As water moves through a distribution system water quality deterioration can take place in the bulk phase or through interactions at the pipe wall. These transformations can be physical, chemical or microbiological in nature. Perhaps one of the most serious aspects of water quality deterioration is the loss of disinfectant residual that may take place in the network and which can weaken the barrier against microbial contamination (Sarin et al., 2 003). A factor frequently cited as contributing to the loss of disinfectant residuals is internal corrosion of the pipe wall material. Recent studies have suggested that with older unlined metal pipes chlorine wall demand varies significantly with flow r ate. In order to systematically study the loss of free chlorine in corroded metal pipes subject to changes in velocity, the authors conducted a controlled study in specially constructed pipe loops l ocated at EPA’s Test and Evaluation (T&E) Facility in Cincinnati, Ohio. The distribution system simulator (DSS) is designed to maintain continuous flow conditions through unlined ductile iron pipe. It consists of two sets of three individual 6 -inch diameter cast iron pipe loops (a total of six loops). The pipe s in the loops have been in service for a number of years and there is considerable corrosion and tubercle buildup on the inside surface. For purposes of this study, one loop was selected for the set of experiments discussed in this paper and modified so that it could be operated over a wide range of flow rates. A total of 24 experimental runs were performed in the investigative phase of this study. Each run was performed at a constant recirculation flow rate, and three consecutive replicate tests were p erformed at each of the following flow rates: 2 gallons per minute (gpm), 6 gpm,15 gpm, 30 gpm, 60 gpm, 90 gpm, and 120 gpm. Three experiments were conducted at 0 gpm. Results from the