The Reject Water Problem
Reverse osmosis membranes achieve PFAS removal by rejecting contaminants into a concentrated waste stream — the reject or brine water. Standard residential RO systems operating at 50 to 75 psi typically produce 3 to 4 gallons of reject water for every gallon of filtered product water. In PFAS-contaminated supply areas, this reject water carries concentrated PFAS and requires responsible disposal rather than simply draining to the sewer.
High-Efficiency RO Technology
Advanced RO systems using permeate pumps, pressure-assisted membranes, or closed-loop recirculation designs can significantly improve water efficiency ratios. High-efficiency residential RO systems now achieve 1:1 or 2:1 reject-to-product ratios — substantially better than conventional designs. These improvements reduce household water consumption and minimize the volume of PFAS-concentrated reject water requiring disposal, addressing both economic and environmental concerns.
Reject Water Disposal Considerations
Standard practice routes RO reject to the municipal sewer system, which is acceptable in most jurisdictions. However, in areas with PFAS-contaminated source water, reject water contains elevated PFAS concentrations that pass through municipal wastewater treatment largely untreated, re-entering surface water and potentially groundwater. Some regulators and researchers have raised questions about whether highly concentrated PFAS reject from large-scale RO systems requires secondary treatment before discharge.
Permeate Flushing and Storage
RO systems that store filtered water in pressure tanks can face water quality degradation during extended storage periods. Permeate flushing — automatically clearing the membrane and first-draw water before dispensing — maintains product water quality. Storage tank sanitization using compatible disinfectants and appropriate tank material selection (avoiding materials that leach PFAS or other contaminants) are important operational considerations for long-term RO system performance.