Investigating Full-Scale Distribution System Flushing Practices for Better Drinking Water Quality Management

Maintaining the integrity of water quality in drinking water distribution networks, which requires upkeep of infrastructure and hydraulic operation, is critical to public health. Distribution systems harbor and can release into the water a variety of contaminants. Bulk water interaction with the distribution network environment, including the pipe material and accumulated matter, is the underlying cause of most drinking water quality degradation. Distribution system flushing improves water quality by reducing flow stagnation and removing accumulated material from pipes. Although flushing often aims to prevent negative aesthetic water quality events (e.g., odor), some utilities also use it to limit microbial nitrification in chloraminated systems. Flushing decreases nitrite levels by limiting nitrification, which can deplete chloramine concentrations that safeguard the water from microbial contamination. However, peer-reviewed research rigorously analyzing the efficacy of full-scale flushing programs to impact water quality is lacking. Furthermore, the extent to which water quality monitoring is integrated into flushing programs and the experience of staff who implement those programs have not been deeply studied. In this dissertation, I present three research aims with the goals of improving our understanding of how current flushing programs function to help maintain safe drinking water quality and developing recommendations for how they can improve. Through two complementary studies, I investigated the efficacy of a full-scale repeated conventional flushing program in Ann Arbor, MI, to prevent chloramine degradation and nitrification in the distribution system. First, I used time series analysis and statistical methods to describe trends in water quality (total chlorine and nitrite levels) observed over five months of repeated weekly flushing. In addition, I explored the potential impacts of bulk water turnover and removal of accumulated material on water quality. Short-term water quality improvements were achieved consistently during flushes through bulk water turnover. However, continued degradation between flushes at some sites suggested that lasting impacts from removing accumulated material were variable. In my second study, I explained potential reasons for the heterogeneous water quality trends observed across sites that experienced similar weekly flushing. Multivariate mixed effects linear regression analysis was used to model theoretically-expected relationships between accumulated material, nitrification, and chloramine degradation that also accounted for other water quality and site-related factors. Both flushing-induced turbidity and nitrite levels had significant effects on total chlorine levels in the model. Counter to expectation, nitrite level was not a partial mediator of turbidity’s effect on total chlorine concentration. In addition, several covariates significantly modified the effects of turbidity and nitrite on total chlorine. Results suggest that heterogeneous water quality trends observed were due to: i) a combination of total chlorine improvements from removing accumulated material and total chlorine degradation from persistent nitrification not impacted by flushes, ii) complex factors related to the nature of flushing-induced hydraulics and accumulated material. The final study aimed to understand flushing practices beyond the City of Ann Arbor and the extent to which those practices relied on water quality monitoring and evaluation. Semi-structured interviews with utility workers at a variety of Ann Arbor’s peer utilities in Michigan were conducted, coded, and thematically analyzed. Conversations highlighted the diversity and importance of flushing programs for maintaining water distribution systems, but rigorous water quality monitoring practices are needed. Flushing practices would benefit from complementing experience-based operations with a more scientific approach to ensuring the preservation of safe water quality for customers.PhDEnvironmental EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/176573/1/vedrin_1.pd