RO membranes are the working part of any reverse osmosis system: a thin semi-permeable sheet, wound in a spiral around a central tube inside a sealed element, that lets water molecules through under pressure while rejecting most dissolved ions. Everything else in the system exists to protect it or to make its output usable. Understanding the membrane is what explains why prefilters matter so much and why output falls off over time.
How the element is built
A household membrane element is a spiral wound thin film composite: layers of membrane sheet and mesh spacers rolled around a perforated permeate tube and sealed into a cylinder. Feed water enters at one end and travels along the outside of the roll under pressure. Some of it passes through the membrane and spirals inward to the permeate tube, and the rest continues along the element carrying the rejected dissolved solids with it and leaves as concentrate. That cross-flow is what keeps the surface swept clean, and it is why reverse osmosis always sends some water to drain: without a concentrate stream the membrane would foul almost immediately.
Thin film composite versus cellulose
Two membrane chemistries turn up in household systems and they behave very differently in the presence of chlorine. Thin film composite is the modern standard: high rejection, good output, and destroyed rapidly by chlorine, which is precisely why every system built around one puts carbon stages in front of it. Skip or overrun that carbon on a chlorinated municipal supply and the membrane can be ruined in a fraction of its expected life. Cellulose triacetate tolerates chlorine but gives lower rejection and is fussier about temperature and pH. If you are unsure which is in your system, assume thin film composite and treat the carbon prefilter as mandatory.
What the ratings mean in your house
Membranes are sold with a daily output rating and a rejection figure, both measured at standard test conditions: a specified feed pressure, temperature and dissolved solids level. Your kitchen is unlikely to match those. Cold winter water produces markedly less than the same membrane in summer, low incoming pressure produces less again, and high dissolved solids in the feed lowers effective rejection. A flow restrictor on the concentrate line is sized to match the membrane and keeps the ratio of permeate to drain where it should be, so replacing a membrane with a different rating without changing the restrictor is a common way to get disappointing results.
Life, failure and replacement
A membrane that has been properly protected lasts years rather than months, and the prefilter schedule is the single biggest factor in that. The signs of a failing element are a slow drift rather than a sudden stop: production falls off so the tank takes longer to fill, and treated water dissolved solids creep up towards the feed level. A cheap handheld meter comparing feed and permeate is the honest way to track it, because taste changes far too late to be useful. Fouling by hardness scale, iron, silica or biofilm is the usual cause, and where the feed water is hard, treating that upstream extends membrane life considerably. Our reverse osmosis system installation guide covers the sequence for a swap.
Questions people ask about ro membranes
How long do RO membranes last?
Years rather than months when the carbon and sediment prefilters are changed on schedule. Skipping prefilter changes, especially on chlorinated water, is what shortens membrane life dramatically.
Does chlorine damage an RO membrane?
It destroys the common thin film composite type. That is why a carbon prefilter sits ahead of the membrane on any chlorinated supply, and why letting that prefilter run past its capacity is expensive.
How do I know the membrane needs replacing?
Watch two things: how long the tank takes to fill, and the dissolved solids reading of the treated water compared with the feed. Both drift before there is any noticeable change in taste.
Can an RO membrane be cleaned instead of replaced?
Commercial systems are chemically cleaned, but for a household element replacement is simpler and usually cheaper. Preventing fouling upstream is far more effective than trying to reverse it.