Reverse Osmosis Waste Water Ratio: Recovery Rate and Water Efficiency

Volume I  ·  May 2026  ·  233 words

A reverse osmosis system produces purified water (permeate) and a concentrate stream (reject water) that carries away the contaminants rejected by the membrane. The ratio of reject to permeate — the waste ratio — is typically 3:1 to 4:1 for basic residential RO systems (3-4 gallons to drain per gallon of purified water), and 1:1 to 2:1 for systems with permeate pumps. The APEC ROES-50 has a waste ratio of approximately 3:1. A permeate pump uses the hydraulic energy of the reject stream to boost feed pressure to the membrane, improving net driving pressure and reducing the waste ratio without an electrical connection. At 3:1 waste ratio, a household filtering 3 gallons per day produces approximately 9 gallons of reject water — 3,285 gallons annually — which at average water rates costs $15-25 per year. The environmental concern with RO waste water is less about cost than about water conservation in drought-prone regions. For households on septic systems, the additional 9 gallons per day of RO reject water adds approximately 3% to the typical daily wastewater load — within the capacity of most residential septic systems.