The membrane is where reverse osmosis actually happens, and it is the component that outlasts every cartridge around it when the system is looked after and dies early when it is not. A membrane rarely fails on its own account: it is usually killed by chlorine that got past a spent pre filter, or fouled by sediment, or scaled by hard feed water.
How it works and why it needs the rest
A thin film composite membrane is wound in a spiral with pressure applied across it, and water is forced through the film while the dissolved solids are left behind and swept away in the reject stream. That is why an RO system produces waste water: the concentrate carries away what the membrane rejected, and without it the membrane would foul immediately. The flow restrictor on the drain line sets that balance, so a missing or wrong restrictor either wastes water or destroys the membrane.
The three things that kill it
Chlorine, first and fastest. Thin film composite material is degraded by chlorine, so the carbon pre filters are what keep the membrane alive, and a spent pre filter costs a membrane rather than a cartridge. Sediment, second: grit blinds the surface and no flush recovers it, which is why a sediment stage comes first. Scale, third: hard feed water precipitates calcium on the membrane surface, which is why softening ahead of a reverse osmosis system extends membrane life considerably on hard supplies.
Knowing when it is finished
Measure, do not guess. Take a TDS reading of the feed water and of the permeate, and the difference is the rejection rate. Record it when the system is new and check it periodically: a falling rejection rate is a failing membrane and shows up well before the water tastes different. The other symptom is production rate, since a fouled membrane makes water slowly and a tank that no longer refills overnight is a signal. Both should be checked before ordering a replacement.
Changing it and getting the life back
Change the pre filters at the same time, always, since fitting a new membrane behind an old carbon stage starts the damage over immediately. Check the flow restrictor and the automatic shutoff valve while the system is apart, sanitize the housings, and check the storage tank's air charge, which drops over time and is the most commonly overlooked item. Then take a fresh baseline rejection reading. Our membrane replacement guide covers the interval by feed water quality, which varies enormously.
Questions people ask about ro filter membrane
What kills an RO membrane fastest?
Chlorine reaching it through a spent carbon pre filter. Sediment fouling and scale from hard feed water are the other two causes.
How do I know the membrane is failing?
A falling rejection rate, measured by comparing feed and permeate TDS readings, and a slower production rate that leaves the tank short.
Why does an RO system waste water?
The reject stream carries away the dissolved solids the membrane rejected. Without it the membrane would foul almost immediately.
Should I change the pre filters at the same time?
Always. A new membrane behind an old carbon stage starts being damaged straight away, so the cartridges go with it.