Reverse osmosis membranes are the separating element inside the long horizontal cartridge on your system: a sheet of semi-permeable film wound in a spiral around a central collection tube. Water is forced through the film under line pressure while most dissolved solids are rejected and carried out to drain.
Thin-film composite against cellulose triacetate
Almost every household membrane sold today is thin-film composite. It rejects a higher fraction of dissolved solids, works at lower pressure, and is destroyed by chlorine, which is why a carbon block always sits ahead of it. Cellulose triacetate membranes tolerate chlorine and are still used on some chlorinated supplies, at the cost of lower rejection and vulnerability to bacterial attack when chlorine is absent. Know which one your system takes before ordering, because they are not interchangeable in performance.
How membranes are rated
Two figures describe a membrane: its production rate in gallons per day at a stated test pressure and temperature, and its rejection rate, the share of dissolved solids it holds back. Both are measured under laboratory conditions your kitchen does not reproduce. Cold water is more viscous and cuts production sharply in winter, and low household pressure cuts it again. A membrane sized comfortably above your daily demand is the practical answer to both, and a booster pump is the fix where pressure is genuinely low.
What fouls a membrane and what destroys it
Sediment blinds the membrane face, hardness scales it, iron and manganese coat it, and biological growth colonizes it during long idle periods. All of those are fouling: gradual, and largely preventable with correct pretreatment. Chlorine is different, because it chemically attacks the thin-film layer and the damage is permanent. On hard water, a softener ahead of the system is genuine membrane protection. On a well, iron and sediment need handling before the water reaches the manifold at all.
Knowing when to change the membrane
A membrane rarely announces itself. The reliable test is a total dissolved solids meter reading on the feed water and on the product water: when the gap between them narrows, the membrane is passing what it used to hold. A tank that fills far more slowly than it did, with fresh prefilters and normal pressure, points the same way. Membranes commonly outlast several sets of cartridges, and they outlast them by more when the carbon prefilter is never allowed to run past its schedule.
Questions people ask about reverse osmosis membranes
How long does a reverse osmosis membrane last?
Several years is normal on well-pretreated water, far less if chlorine reaches it or the prefilters are neglected. Measure rejection rather than trusting the calendar.
Can a reverse osmosis membrane be cleaned?
Household spiral-wound membranes are not practically cleanable at home. Light scaling sometimes eases after a flush, but a fouled membrane is replaced, not restored.
Does a higher gallon-per-day membrane give faster flow at the tap?
Not directly. Tap flow comes from the storage tank. A larger membrane refills that tank faster, which matters for heavy use rather than for a single glass.