Impacts of unplanned de facto wastewater reuse on disinfection byproduct formation at downstream drinking water treatment plants

De facto reuse (DFR) refers to the unplanned inclusion of treated wastewater in drinking water supplies due to upstream wastewater treatment plant effluents. Nearly half of drinking water treatment plants (DWTPs) in the USA are impacted to some extent by DFR, with maximum estimated DFR percentage streamflow approaching 90% in some cases. DFR is not unique to the U.S. but been reported globally in Asia, Europe, Africa, and others. Treated wastewater discharged to surface waters can contain significant levels of inorganic (e.g. bromide and iodide) and organic compounds (e.g. micropollutants, extracellular products, and nitrogen-containing organic matter) that serve as precursors to both regulated and unregulated disinfection by-products (DBPs) in downstream DWTPs. Although identified as a national concern over a decade ago, the lack of standardized methodologies for quantifying and reporting DFR hinders comparative assessments and regulatory decision-making. We believe DFR is underappreciated or outright ignored as compared to the water community focus on highly managed and regulated indirect and direct potable reuse—although risks from DFR are just as real and important to address. This review explores how DFR contributes to DBP risks at DWTPs and discusses strategies for monitoring, modeling, and managing these risks considering recent research.