Reverse osmosis is highly effective for PFAS removal but produces reject water — a concentrate stream containing removed contaminants — at a ratio of 2 to 4 gallons discarded per gallon purified in conventional systems. For environmentally conscious households, this water waste is a meaningful concern. Several approaches can significantly reduce reject water volume.
Understanding Why RO Produces Reject Water
RO membranes work under pressure, pushing water through while retaining contaminants. The retained contaminants must be flushed away continuously to prevent concentration buildup at the membrane surface that would impair performance. This flush stream is the reject or brine water. The ratio of product water to reject water — the recovery rate — depends on membrane design, operating pressure, and water quality.
High-Recovery RO Systems
Modern high-efficiency RO systems achieve recovery rates of 50% or higher, compared to 20 to 33% for older designs. Some residential permeate pump systems push recovery rates above 75%. These improvements come from optimized membrane configurations, pressure recovery devices, and smarter flush cycling. When shopping for an RO system, recovery rate is a meaningful specification to compare alongside PFAS removal performance.
Using Reject Water
RO reject water, while containing concentrated contaminants, is not hazardous for most secondary uses. In PFAS-affected areas, reject water should not be used for drinking or cooking, but it can appropriately be directed to toilet flushing, garden irrigation of non-edible plants, or other non-ingestion uses. Some households connect RO reject lines directly to toilet supply valves, effectively capturing all reject water for productive use.