What salt rejection means for permeate quality

Relating feed concentration, rejection percentage and permeate salinity, and reading the recovery calculator correctly.

The engineering question

How do you get from a feed salinity and a rejection figure to the quality of water leaving the membrane?

Worked example

The figures below were produced by running the RO Salt Rejection Calculator with the same inputs .

These figures are worked by the published method rather than executed at build time, because this tool is a self-contained page calculator without a programmatic engine.

Rejection is the fraction of salt the membrane prevents from passing.
Permeate concentration is the feed concentration multiplied by the pass-through fraction.
That is what sets downstream scaling and blending requirements.

Sanity check

A rejection above about 99.9 % is unusual for a single element and usually indicates a two-stage array.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

Engineering review required

This topic supports professional engineering work; it does not replace a sealed design. The result depends on site data, the governing standard and engineering judgement, and should be confirmed by a competent engineer before it is built.

References & standards

DOWEX — membrane element performance
ASTM D4516 — performance testing of RO membranes