RO efficiency: how much water goes down the drain?

Read both sides of a ratio

A ratio labeled “waste to treated” means something different from “treated to waste.” Ask the provider to write both quantities as gallons for one gallon of drinking water. This avoids a surprisingly common comparison problem: two attractive numbers expressed in opposite directions.

Use a worked example

At 30% efficiency, 100 gallons of feed water produce 30 gallons of treated water and 70 gallons of concentrate. That is about 2.33 gallons of concentrate for each treated gallon. EPA’s product directory reports efficiency and translates it into the waste-to-treated ratio. This is arithmetic illustrating the rating, not a prediction for your household.

Reference: EPA WaterSense RO directory

Compare the right model

Check the current directory entry and performance sheet rather than relying on a brand-wide claim. Discuss incoming pressure, temperature, household demand and manufacturer requirements with the installer. Ask whether the quoted configuration matches the rated product. A certified result is more useful than an unexplained “zero waste” slogan.

Put efficiency in your decision

Estimate your actual drinking-water demand before comparing costs. A family using a few gallons a day has a different use pattern from a whole-house system. Include filters, membranes and service in the comparison too. Choose appropriate treatment first, then compare efficiency among suitable models; water savings alone do not justify buying equipment you do not need.