The Microbiology of Direct Potable Reuse Systems

Regions facing water scarcity are expanding their water portfolios to augment supplies and meet future demand. Direct potable reuse, the deliberate introduction of advanced treated municipal wastewater into a drinking water treatment facility or distribution system, has gained considerable traction in the United States in recent years. The high concentrations of pathogens and chemical contaminants in municipal wastewater necessitates a high degree of treatment to meet drinking water standards and protect public health. Therefore, in potable reuse systems, municipal wastewater is typically treated by conventional wastewater treatment processes (e.g., sedimentation and activated sludge) followed by “advanced” treatment processes (e.g., microfiltration, reverse osmosis, and advanced oxidation). To date, the main research focus on microbial water quality in potable reuse systems has been quantifying the removal of pathogens (primarily enteric viruses and protozoa) present in municipal wastewater. While essential, this research does not provide insight into how advanced treatment alters the entire microbial community or the potential for microbial growth in treated water. Moreover, the management of microbial risk and water quality in distribution systems remains poorly studied in the context of direct potable reuse./nIn this dissertation, improved microbial monitoring methods were applied to two advanced wastewater treatment facilities and a lab-scale simulated direct potable reuse distribution systems to evaluate the impact of advanced treatment on microbial numbers, viability, and growth potential in direct potable reuse systems. Microbial water quality through the treatment and distribution systems was evaluated by measuring changes in total and potentially viable prokaryotic cells, total and intracellular ATP, organic carbon, several primary and opportunistic pathogens, and two antibiotic resistance genes. Enhanced microbial analysis methods such as flow cytometry for cell counts, ATP analysis, and dead-end ultrafiltration for quantitative PCR analyses were applied and evaluated as research tools and/or routine monitoring methods for potable reuse systems./nThrough this work I showed that the introduction of advanced treated water into a drinking water distribution system holds the potential for improving microbial water quality, but growth during the distribution of advanced treated water must still be properly managed. The microbial water quality of drinking water distribution systems may improve following the introduction of advanced treated water by a lower potential for microbial growth, lower microbial viability, and greater stability of a chlorine residual during simulated distribution. Specifically, chlorine residual decay and the numbers of potentially viable cells (i.e., intact cell counts measured by flow cytometry) were lower in distribution systems fed with advanced-treated water as compared to those fed with 100% conventional water. The prevalence of the opportunistic pathogens Legionella pnuemophila and Mycobacterium avium complex were unaffected by the use of advanced treated water. Furthermore, assimilable organic carbon concentrations (i.e., a common metric for microbial growth potential) in advanced treated water was similar to or less than values reported for finished and distributed drinking waters in the United States. The transportation, conditioning, and storage of reverse osmosis permeate without a chlorine residual resulted in large increases in microbial numbers, but no more than those typically seen in drinking water with a degraded residual. The relative abundance of sul1 remained unchanged by the handling of RO permeate, leading to high increases in absolute sul1 gene counts. Regardless, the application and maintenance of a chlorine residual substantially suppressed microbial numbers during distribution of advanced treated water; however, the passage of ammonia through advanced treatment could cause significant variability in the performance of chlorine disinfection./nI also demonstrated the utility of flow cytometry, ATP analysis, and dead-end ultrafiltration (to concentrate water for quantitative PCR analyses) as informative research and monitoring methods for potable reuse systems. In particular, these tools were used to successfully quantify microbial cells in matrices with low biomass (e.g., reverse osmosis permeate) with relatively good precision. Furthermore, flow cytometry and ATP analysis showed promise as routine/continuous tools to monitor the performance of treatment processes such as microfiltration, reverse osmosis, and chlorination. These tools also provided additional insights into how the chlorine residual impacted microbial viability (e.g., changes in intact cell counts) in simulated distribution systems./nOverall, this body of work contributes to a better understanding of engineered water systems and will ultimately inform more comprehensive water quality monitoring strategies for water treatment systems. Further work is needed to characterize microbial water quality in different types of advanced treatment trains, and utilities that implement direct potable reuse should use conventional and improved monitoring methods to evaluate full-scale distribution systems before and after the introduction of advanced-treated water.

Advisor: Nelson, Kara L