Effect of Demand Stochasticity on Water Quality in Premise Plumbing Systems

Water quality begins declining once treated water enters the water distribution system. In particular, it declines in premise plumbing systems due to the longer stagnation times, smaller pipe sizes, different pipe materials, and more favorable temperatures. This can lead to a variety of problems, including metal leaching, chlorine dissipation, and microbial regrowth. Some of the microorganisms that proliferate in premise plumbing systems, particularly in biofilms, are opportunistic pathogens, which can infect immunocompromised individuals.

A unique aspect of premise plumbing systems is the highly variable stagnation times in pipes, where the flows may be zero for hours or days. This results from the highly stochastic water demands at fixtures and appliances. Long stagnation times, or water ages, can have important impacts on water quality, but they have not been systematically studied. In this dissertation, I used mathematical modeling to explore the effect of demand stochasticity on water quality of premise plumbing system, especially focusing on microbial quality. First, I proposed a new way to statistically characterize water ages, which I used to assess the impact of different home occupancies and fixture flow rates. Then, I used a similar modeling approach to study the impact of water purging devices on water age. Since water age is not directly related to water quality, and not a measurable parameter, I then assessed the effects of demand stochasticity and purging devices on chlorine residuals. I proposed three novel metrics to quantify the time distributions of chlorine residuals, including the distributions of chlorine residual at the moment of use, the percentage of time the chlorine residual is below a threshold value, and the distribution of durations with chlorine is below the threshold. Finally, I developed and used a novel mathematical model to quantify biofilm growth, considering intermittent demands, chlorine dissipation, diffusion of substrates from the bulk water to the biofilm, leaching of biodegradable organic matter from pipes, and planktonic bacterial growth.

These results provide new ways to characterize and compare water ages and chlorine residuals in different systems. I also found that purging devices were an effective means to prevent excessive water ages and maintain chlorine residuals. I used the novel model I developed to compare biofilm development in chlorinated and non-chlorinated water systems, allowing factors leading to biofilm proliferation in premise plumbing systems to be better understood. My research can help develop better management practices to reduce the incidence of opportunistic pathogens in premise plumbing systems.