RO Membrane Salt Rejection and Flow Test Basics
Salt rejection and permeate flow are the two numbers that tell you whether a finished element performs. This guide explains what they mean, the formula, and how to run the test.
- Salt rejection% salts removed
- Permeate flowProduct water rate
- Tells youElement performance
- Pairs withLeak test
Key Takeaways
- Salt rejection = how much dissolved salt the element removes.
- Permeate flow = how much product water it makes.
- Both depend on test conditions — standardise them.
- Performance testing complements the vacuum leak test.
The Two Numbers That Define Performance
A finished element is judged on two things: how well it removes salt (rejection) and how much water it produces (flow). The vacuum leak test proves it is sealed; this performance test proves it actually works. Together they are the final quality gate before an element ships.
Both readings only mean something against fixed test conditions — feed concentration, pressure, temperature and recovery. For the terms used here, see the manufacturing glossary.
How Salt Rejection Is Calculated
Rejection compares the salt concentration in the feed with that in the permeate:
How to Run the Test
A typical performance test — follow your test bench’s validated standard conditions for your element type.
Set standard conditions
Use the validated feed concentration, pressure, temperature and recovery for the element type, so results are comparable.
Load and stabilise
Install the element in the test vessel, start the feed and let pressure and flow stabilise before reading.
Measure feed and permeate
Record feed and permeate conductivity (or TDS) and the permeate flow rate once readings are steady.
Calculate rejection
Apply the rejection formula and compare both rejection and flow against your pass criteria.
Record and act
Log the results against the batch in your QC checklist, and route any fail to review.
What Moves the Numbers
If rejection or flow looks off, it is often the test conditions, not the element. Temperature strongly affects flow; pressure affects both; and a leak or a rolling defect shows up as low rejection. Always confirm conditions before judging an element.
Choosing or upgrading a test bench? See the testing equipment selection guide.
Factors to control
- Feed concentration (TDS)
- Operating pressure
- Temperature
- Recovery rate
- Stabilisation time before reading
Untested Performance Is a Guess
An element can be perfectly sealed and still under-perform. Only the performance test confirms it does the job.
Low Rejection Ships
Without the test, a poor-rejection element reaches the customer and fails in service.
Unfair Comparison
Different conditions give different numbers — results are meaningless without a standard.
Missed Root Cause
Without records, a recurring performance issue is hard to trace and fix.
Setting up performance testing?
We supply test benches and can help you set validated conditions and pass criteria for your element types, free of charge.
Salt Rejection & Flow FAQ
How is RO membrane salt rejection calculated?
Salt rejection (%) = (1 - permeate concentration / feed concentration) x 100, usually measured as conductivity or TDS. For example, a permeate of 7.5 ppm against a feed of 1500 ppm gives 99.5% rejection.
What is permeate flow?
Permeate flow is the rate of product water the element produces, measured as a volume per time (such as L/h or GPD) at standard test conditions. It is read alongside rejection during the performance test.
Why do test conditions matter so much?
Rejection and flow change with feed concentration, pressure, temperature and recovery. Without fixed standard conditions, two readings are not comparable, so every element must be tested the same way.
Does performance testing replace the leak test?
No. The vacuum leak test confirms the element is sealed; the performance test confirms it removes salt and produces water. Both are needed for a complete final check.