The Waste Stream Problem with RO Treatment
Reverse osmosis achieves excellent PFAS removal from product water—but where do the rejected PFAS go? RO systems produce a concentrated waste stream (reject or concentrate) containing the PFAS removed from feed water at 3–10 times the feed water concentration. Properly managing this concentrated PFAS waste stream is a critical but often overlooked component of responsible RO implementation.
Rejection Rates and Concentrate Volumes
Typical residential RO systems reject 65–75% of input water as concentrate, with each gallon of product water generating 2–4 gallons of PFAS-concentrated reject water. Municipal RO systems have better recovery ratios (typically 75–85%) but still generate substantial concentrate volumes containing highly concentrated PFAS requiring management. A utility removing 95% of PFAS from 10 MGD produces concentrate containing 19 times the source water PFAS concentration in 1.5 MGD.
Concentrate Disposal Options and Limitations
RO concentrate is commonly discharged to municipal sewers (if the POTW can accept it), surface water bodies under NPDES permits, or deep injection wells. As PFAS effluent standards for POTWs and surface water tighten, concentrate disposal options are becoming more restricted. Utilities treating PFAS with RO must incorporate concentrate management planning as a core component of system design, not an afterthought.
Zero Liquid Discharge and Destruction
Some utilities are pursuing zero liquid discharge (ZLD) approaches for RO concentrate, combining evaporation, crystallization, and thermal destruction to eliminate liquid waste streams. ZLD concentrates PFAS in solid residuals requiring secure landfill disposal or high-temperature incineration. These approaches add cost but provide the most definitive management of PFAS-containing concentrate streams in sensitive receiving environment contexts