WWTPs as PFAS Pass-Through Systems
Municipal wastewater treatment plants (WWTPs) process billions of gallons of sewage daily using biological treatment processes optimized for removal of organic matter, nutrients, and pathogens. These processes were not designed to remove per- and polyfluoroalkyl substances—and largely don’t. PFAS enter WWTPs from multiple sources: household products (personal care items, food packaging, nonstick cookware leachate); industrial pretreatment discharges; stormwater inflows; and landfill leachate from co-managed disposal. Most WWTP effluent contains detectable PFAS at concentrations that can significantly impact receiving waters—making WWTPs one of the most significant continuous PFAS discharge pathways in developed watersheds.
PFAS Mass Loading Through WWTPs
Studies measuring PFAS mass loading through WWTPs consistently find that significant fractions of incoming PFAS pass through treatment without removal. A comprehensive 2021 EPA study of 20 US WWTPs found: median total PFAS in plant influent: 430 ng/L; median total PFAS in plant effluent: 350 ng/L—a removal rate of less than 20%. Long-chain PFAS (PFOS, PFOA) showed modestly higher removal (20–40%) due to sorption to biosolids, but short-chain PFAS showed near-zero removal. For a typical large WWTP treating 100 million gallons per day (MGD) with 350 ng/L effluent PFAS concentration, this represents a PFAS mass discharge of approximately 130 kg/year—equivalent to the annual PFAS use of a substantial industrial facility—continuously released to receiving surface waters.
EPA Effluent Limitation Guidelines (ELGs) for PFAS
EPA’s Effluent Limitation Guidelines establish technology-based effluent standards for specific industrial categories under the Clean Water Act. In 2024, EPA finalized ELGs for PFAS manufacturing facilities—setting numeric effluent limits for PFAS at fluorochemical production plants for the first time. These limits (0.01 µg/L for PFOA, 0.01 µg/L for PFOS, 0.01 µg/L for PFNA, 0.01 µg/L for PFHxA, 0.01 µg/L for PFHxS, and 10 µg/L for fluoropolymer manufacturing) apply to direct dischargers and require pretreatment for indirect dischargers (those discharging to WWTPs). Pretreatment requirements for industrial PFAS dischargers are critical for reducing PFAS loading to municipal WWTPs—but these ELGs apply only to PFAS manufacturing facilities, not to the hundreds of industrial sectors that use PFAS in their operations.
WWTP PFAS Treatment Retrofits
Some large WWTPs are evaluating PFAS treatment retrofits for their effluent—either voluntarily or in response to state NPDES permit conditions requiring PFAS reduction. Treatment options for WWTP-scale PFAS removal: GAC polishing of secondary-treated effluent (effective for long-chain PFAS; higher EBCT required than drinking water applications due to complex matrix); membrane bioreactor (MBR) systems can reduce some PFAS through enhanced sorption to the biofilm matrix; ozone and advanced oxidation applied after biological treatment shows limited PFAS removal for terminal acids but can degrade some precursors. The economics of retrofitting PFAS treatment at WWTP scale are challenging—billions of dollars of investment nationally—and the regulatory framework requiring such treatment has not yet been established. This represents a significant future investment cycle for the water sector.
WWTP PFAS effluent represents the largest continuous unregulated PFAS discharge pathway in developed countries—a gap in the regulatory framework that will require ELG expansion, pretreatment requirements, and ultimately WWTP-level PFAS treatment to close.