Why RO sends water to drain
The membrane separates feed water into permeate and a more concentrated stream. Continuous cross-flow helps carry rejected salts away from the membrane. Eliminating the reject stream altogether would change the process and encourage concentration/scaling.
Efficiency versus waste ratio
EPA WaterSense defines efficiency as the percentage of incoming water that becomes treated water. A 50% efficient unit makes 50 litres of product water from 100 litres of feed. Its reject-to-product ratio is 1:1. A 25% efficient unit uses 100 litres to make 25 litres and rejects 75 litres: 3:1.
At 50% efficiency, 8 L of product water uses about 16 L feed and creates about 8 L reject.
What changes real efficiency?
Feed pressure, temperature, membrane design, flow restrictors, pump control, tank back-pressure and flushing routines all matter. A headline ratio measured under ideal laboratory conditions may not equal your winter kitchen performance.
Tank versus direct-flow
Traditional tank systems can become less efficient as tank back-pressure rises. Modern pumped direct-flow designs can achieve better recovery, although they use electricity and still need appropriate drain arrangements.
Can reject water be reused?
It contains the salts rejected from the product stream, so it is not equivalent to clean RO water. Some households collect it for suitable non-potable uses, but avoid creating hygiene, cross-connection or overflow problems. Never plumb an improvised reuse arrangement in a way that risks backflow into drinking-water pipework.
UK regulations matter
The Water Fittings Regulations aim to prevent waste, misuse, undue consumption and contamination. RO and treatment units producing waste discharge are specifically relevant to notification requirements. Check with the water undertaker before a plumbed installation.
Sources and further reading
For factual claims that matter to health, regulation or product performance, we prefer primary and official sources. Useful references for this guide include:
- US EPA WaterSense — Point-of-use RO efficiency
- Water Supply (Water Fittings) Regulations 1999 — legislation.gov.uk
- NSF — NSF/ANSI 58 reverse osmosis systems
Related guides
Last reviewed: 6 October 2026. This guide is written for a UK audience. Always follow current supplier, regulator, manufacturer and healthcare advice where it applies.