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Quantitative Microbial Risk Assessment of De Facto Water Reuse Practice: A Case Study of Trinity River, Texas

Abstract

Water reuse have been adopted in regions with limited natural water resources to alleviate water supply stress caused by rapid population and economic growth. The form of water reuse varies, can include non-potable, indirect potable and possibly direct potable reuse in the near future. Each category of water reuse has been highly regulated and controlled in the United States and other developed countries. However, there is an unofficial form of water reuse, sometimes called de facto reuse, where the drinking water source of downstream communities contains a significant portion of wastewater effluent from upstream communities. These effluents were treated to meet the EPA surface water discharge standards, which are not intended for use as the source for drinking water supplies. This study examined the health risk of de facto reuse by Quantitative Microbial Risk Assessment (QMRA) using Trinity River, Texas as a case example. The concentrations of Cryptosporidium and norovirus in wastewater effluents upstream of Trinity River were estimated using literature data that fitted by normal and bimodal distribution function, respectively. The exposure assessment considered the portion of wastewater effluent in drinking water source, pathogen decay during storage, and removal rates in water treatment plants. Health risks were computed using Monte Carlo simulation with 10,000 iterations. Results indicated that the annual infection risk of norovirus are exceeding the suggested safety level set by the U.S. EPA, while cryptosporidium risks can only meet the safety benchmark under some circumstances. The disease burden for both pathogens also exceed the WHO DALY-based tolerance level under some circumstances. This study risks concerns of de facto reuse in high portion of effluent and insufficient storage time.

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