To Filter or Not to Filter? Water Quality Impacts and Stagnation Effects of Carbon-Based Point-of-Use (POU) Filters Installed in Monochloramine Systems

Point-of-use (POU) filters are increasingly used in commercial buildings, schools, and within homes worldwide. Installation of activated carbon block POU filters is a common lead risk mitigation measure used by utilities and state regulatory agencies; however, the potential for unintended consequences exists and further evaluation of microbial activity and byproducts from these filters in use is needed, especially for chloraminated water systems due to the additional nitrogen source. Two in-line, under the sink POU filters were installed and monitored for 41 weeks. The installed filters began to nitrify shortly after installation, with weekend-long stagnation samples exceeding 1.5 mg/L-N nitrite within 3 months, concentrations 50% greater than the regulatory threshold. These stagnant nitrite concentrations were not always mitigated by simply flushing the filters, with post-flush nitrite for one filter type regularly exceeding 0.5 mg/L-N. DNA sequencing of the biofilms forming on the filter sediment mesh outer layers and the outer surface of the carbon block differed depending on the filter manufacturer and component variability. In both filters tested, opportunistic premise plumbing pathogens were detected in filter biofilms.