Assessing the Impact of Stochastic Demands and Fixture Purging on Free Chlorine Residuals in Premise Plumbing Systems

In this study, we used modeling with stochastic demands to assess free chlorine concentrations in premise plumbing systems. Different occupancy scenarios and the use of automated purging devices at the fixtures were assessed. Chlorine concentrations were highly variable, so three novel metrics were proposed to assess the temporal distribution of chlorine residuals: the initial chlorine concentrations at fixtures for each use, the percentage of time a fixture was exposed to low chlorine concentrations (below 0.1 mg/L), and the durations of the low chlorine events. Showers, particularly those with lower occupancy, often had near-zero chlorine residuals. The chlorine residuals were below a 0.1 mg/L threshold 75% of the time for the one-person household but only 55% in a two-person household. 12-h scheduled purging reduced the percentage of time with low chlorine to 29% for the one-person household. Lower chlorine residuals in the water supply, smaller pipes, and longer service pipes resulted in substantially lower median chlorine concentrations at the fixtures. The choice of decay coefficients also had a large impact on the percentage of time with low chlorine. Overall, this research provides a new methodology to quantify and assess the highly variable chlorine concentrations in premise plumbing systems and the impacts of purging devices.