A membrane is the most expensive component in most household treatment systems and the one that fails least dramatically. It does not block, leak or make a noise. It simply rejects less than it used to, month by month, until the household is drinking water not much different from the feed while the system runs exactly as before.
The three things that shorten its life
Chlorine is the fastest and it is irreversible: thin film composite material degrades on contact, so the carbon prefilter is the only thing standing between a municipal supply and a ruined element. Scale is the most common, because the hardness being rejected concentrates precisely at the membrane face, which is why softening or antiscalant dosing ahead of it matters on hard water. Iron is the most stubborn, depositing a film no flush will lift, which is why iron treatment goes upstream on well water.
Measuring whether it is finished
Do not judge by production rate, because slow production is far more often a blocked prefilter, low line pressure or a storage tank that has lost its air charge. Judge by rejection: take a dissolved solids reading of the feed water and another of the treated water sampled before the tank, and compare. A healthy element leaves a wide gap between the two. As the gap narrows, the element is passing what it used to reject, and when the readings converge it is finished regardless of how much water it still makes.
Flushing, and what cleaning can and cannot do
Many systems have a flush arrangement that runs water across the membrane face with the drain open, which sweeps loose material away and can recover some performance on an element that has been sitting or is lightly scaled. Beyond that, proper chemical cleaning belongs to commercial systems built with the loop and instrumentation to do it and to verify the result. On a household cartridge, replacement is cheaper and more certain than an unverifiable attempt, and chlorine damage cannot be reversed by anything.
Storing a spare
Membranes are shipped wet or preserved for a reason: an element that dries out can suffer irreversible loss of performance. Keep a spare sealed in its original packaging, somewhere cool and out of sunlight, and do not open it until it is going in. If an installed element has to come out temporarily, keep it wet and covered rather than leaving it on a bench. Note the date it was fitted, since rejection is judged against its own history. Our whole house filter guide covers the stages that protect it.
Questions people ask about water filter membrane
How do I know a membrane is spent?
Compare a dissolved solids reading of the feed with one from the membrane output before the tank. Converging readings mean it is finished.
Does slow production mean the membrane has failed?
Usually not. Check the prefilters, the line pressure and the storage tank's air charge first, since all three mimic a failing element.
Can a household membrane be cleaned?
Not properly. Flushing helps a lightly fouled element; chemical cleaning belongs to commercial systems, and chlorine damage is permanent.
How should a spare be stored?
Sealed in its original packaging, cool and out of sunlight. An element that dries out can lose performance irreversibly.