Using Analytical Hierarchy Process for Excess-Chlorine Risk Assessments in a Water Distribution Network: A Case Study

Drinking water distribution systems (DWDS) are some of the critical components of a drinking water system, providing water supply services to our growing populations. To maintain the safety of the drinking water supply, the water distributed through these systems needs to be regularly disinfected to ensure that it remains free of pathogens and to minimize the risk of contamination during distribution. In DWDS, balancing the risks of excessive chlorine and microbial contamination is crucial for public health due to chlorine additions in the storage tanks of these systems. Proposing effective risk-mitigation strategies requires simulations of plausible contamination scenarios and input from local experts, given the challenge of maintaining an appropriate residual chlorine level that is effective but not excessive. In addition, excessive discharge of chlorine into the environment can result in increased water pollution levels and the production of harmful disinfection byproducts, such as trihalomethanes, posing a threat to the health of waterbodies and having implications for total maximum daily loads (TMDLs). Here, we presented a framework that couples the analytical hierarchy process (AHP) with water quality simulations for decision making related to mitigating the risk of contamination to DWDS for extreme chlorine levels. We demonstrated this framework on a section of a DWDS in a highly populated city, where an accidental overapplication of chlorine resulted in the chlorine level in the tank being excessively high. Various local experts provided input about important decision criteria, their relative importance, and potential risk-mitigation strategies. Using the input in our framework, we determined important decision criteria and identified geographic zones of the DWDS at risk of chlorine contamination. The presented framework can be used in preventing chlorine contamination risks in DWDS located in other population centers. It was found that the areas closest to the tank, in terms of distance or time of travel, are more vulnerable due to the potential for rapid dispersion of contaminants. The results demonstrated the importance of incorporating expert opinions into the decision-making process and highlighted the capability of AHP in risk assessments. Future research can explore other decision-making methods, consider demographic characteristics of affected populations, and apply our presented framework to entire drinking water systems to further improve risk-mitigation strategies for preventing chlorine contamination risks in DWDS located in other population centers.