Impacts of setting a numeric minimum disinfectant residual for public water system distribution systems

US Environmental Protection Agency (EPA) requires public water systems (PWSs) that disinfect to maintain a detectable disinfectant residual throughout the distribution system (DS) to protect consumers against risks from microbial pathogens. The EPA is tasked with proposing revisions to the Microbial, Disinfectant, and Disinfection Byproduct rules by July 2025. Potential revisions could include a numeric minimum disinfectant residual level in place of the “detectable” requirement. Twenty-three states have defined a required DS numeric minimum disinfectant residual level. Only three such states, Louisiana, Colorado, and Pennsylvania, have made recent changes and have sufficient data available to assess implications. These states were used as case studies to assess the impact this regulatory change had on disinfectant residual levels and microbial and disinfection byproduct (DBP) occurrence in PWS DSs. Results included increases in disinfectant residual levels, decreases in total coliform positive occurrences and violations, and temporary increases in DBP occurrence and violations.