Particulate contaminants and treatment decision-making: maximizing the value of raw water pathogen monitoring for drinking water safety.

Periodic evaluation of particulate contaminants in raw/untreated water is integral to assessing risk, establishing treatment requirements, and ensuring drinking water safety. However, pathogenic microorganisms and other discrete particles (e.g., microplastics) are not typically monitored with any regularity. When monitoring is required, recommended, or proactively used to evaluate the adequacy of treatment or assess treatment needs, there is a need for guidance on how to collect data and use them to maximize return on investment. The potentially increasing variability in source water quality associated with climate change emphasizes the importance of knowing contaminant concentrations to effectively manage risks. This work presents a framework to guide the development of monitoring protocols for particulate contaminants in water and the integration of monitoring data and quantitative microbial risk assessment into treatment decisions. The protozoa monitoring and risk-based compliance approach of a drinking water utility in Canada is presented along with 7 years of data. Guidance for determining sampling frequencies and locations is provided. It is shown that monitoring may be insufficient to inform treatment needs when cysts are more abundant in source water. This work underscores the importance of revisiting and enhancing monitoring practices for effective treatment and public health protection.