To filter or not to filter? Water quality impacts of carbon-based point-of-use (POU) filters installed in monochloramine systems

Both water consumers and water treatment professionals are becoming increasingly aware that it is difficult to answer the question, “Is the water safe to drink?”—this is a public health problem. In the US, activated carbon block (AC) point-of-use (POU) filters exist in almost every home, in every refrigerator, and under every sink, yet we still do not understand the basics regarding microbiological colonization within these systems and the risk tradeoffs we incur by using one. Installation of AC POU filters is a common lead (Pb) 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 those installed within chloraminated water systems. Over the past five years, from investigating the mysteriously high nitrite concentrations at school fountains while working at DC Water to my present laboratory research at USF, I have investigated nitrification within AC POU filters to help propose mitigation strategies. Installed filters can nitrify within the first month after installation and quickly exceed 2 mg/L-N nitrite, concentrations greater than double the regulatory threshold. Common nitrification mitigation strategies used in distribution systems, such as flushing and temporary disinfection switches to free chlorine, can have mixed results in reducing the nitrite concentrations in the AC POU filter effluent. In a recent set of experiments tracking the influence of the secondary disinfection switch to free chlorine on microbial activity, qPCR analysis revealed a general Nitrobacter dominance during and post-free chlorine conversion for two of the filters; however, one filter maintained AOB dominance. To further evaluate the drivers of microbial community dynamics within the filters, ongoing investigations use culture-independent methods to identify, quantify, and understand the extent to which nitrifying communities and opportunistic pathogens develop across various brands of AC POU filters with a focus on the presence/absence and thickness of a filter mesh. By quantifying biofilm composition over time, we will elucidate changes in microbial growth patterns for selected filters.