RO Membrane Replacement: The TDS Test That Proves It's Dead

Q: What TDS reading means my RO membrane is bad?

No single ppm fits everyone because it depends on your feed. Convert to rejection: divide product by feed, subtract from one. Below about 85% rejection means the membrane is failing, whatever the raw numbers are.

Q: My product TDS is high but the membrane looks new, why?

Often it isn't the membrane. A waterlogged tank, a failing auto-shutoff, or a clogged flow restrictor keeps the system from building the pressure the membrane needs to reject properly. Check those before replacing.

Updated July 2026 · Editorial team · Topic: water filtration / cartridges

RO Membrane Replacement: The TDS Test That Proves It's Dead — Filter Cartridges

The membrane is the one part of a reverse osmosis system you can't judge by looking at it. Sediment cartridges turn brown, carbon smells spent, but a failed membrane looks exactly like a healthy one — same white spiral inside the same housing. The difference lives entirely in a number, and a fifteen-dollar meter settles the argument in about thirty seconds.

Short answer: Prove a membrane's condition with a rejection test. A healthy RO membrane rejects roughly 90–98% of dissolved solids: measure your feed water TDS and your finished water TDS, then divide. If your feed is 300 ppm and the product reads under about 30 ppm, the membrane is fine. Once rejection slips below roughly 85% — product creeping past 45 ppm on that same feed — the membrane is failing and it's time to replace it.

Reviewed by the ClearTap editorial team. We publish plain specs, model compatibility and NSF/EPA-based standards so you can judge for yourself — no lab-test theatre and no upsell. We do not run a water lab; our guidance is built from published specifications and NSF/EPA standards, not invented tests. General information about water quality only, not medical or drinking-water advice: for legal or health decisions about your water, test it with a certified laboratory.

Rejection rate: the only honest verdict

Everything a reverse osmosis membrane does can be reduced to one figure: what fraction of dissolved solids it refuses to pass. You calculate it with two meter readings and a bit of arithmetic.

Rejection % = (1 − product TDS ÷ feed TDS) × 100.

Take a sample from the incoming line — the feed — and read its TDS. Then read the water coming out of the RO faucet. A new membrane on 300 ppm feed might deliver 6–15 ppm at the tap, which pencils out to 95–98% rejection. As the membrane ages, that product number climbs. The membrane isn't "a little tired" at 90%; it's telling you precisely how much of its job it has stopped doing.

Feed TDS	Product TDS	Rejection	Verdict
300 ppm	6–15 ppm	95–98%	New / healthy
300 ppm	15–30 ppm	90–95%	Normal aging, keep watching
300 ppm	30–45 ppm	85–90%	On the way out, plan a swap
300 ppm	45+ ppm	Below 85%	Failing — replace now

The percentages are what matter, not the raw ppm, because your feed number is personal. Someone on 500 ppm well water and someone on 150 ppm city water will never share a product target — but both should be seeing 90-plus percent rejection from a good membrane.

The symptoms that send you to the meter

You won't usually run the test out of nowhere; something tips you off first. Three complaints show up again and again:

Taste comes back. Water that used to taste flat and clean starts tasting like the tap again. Dissolved solids are getting through.
The tank refills slowly or never tops off. Production has dropped. This can be a fouled membrane or a clogged pre-filter starving it — the meter tells you which.
More water going down the drain. A degrading membrane can throw off the drain ratio, so the system runs longer for less product.

Any of these is your cue to grab the meter and calculate rejection. The symptom raises suspicion; the number convicts.

Why membranes die early — and it's almost always chlorine

A thin-film composite membrane, the standard type in home systems, should give you two to five years. When one dies at eighteen months, the cause is rarely the membrane itself. It's chlorine that slipped past exhausted carbon pre-filters and oxidized the film. Chlorine is death to this material, which is the entire reason a carbon block sits directly upstream. Let that carbon exhaust, keep running the system, and you're feeding oxidizer straight onto the membrane. The second most common killer is neglected sediment upstream, which fouls the membrane surface and drops both flow and rejection.

Field note: Before you condemn a membrane on a bad reading, rule out the auto-shutoff and the flow restrictor. If the system never builds proper pressure — a failing shutoff valve, a waterlogged tank bladder, or a clogged restrictor — the membrane can't reject properly even when the membrane itself is fine, and you'll see a falsely high product TDS. A cheap membrane swap that doesn't fix the number means the problem was never the membrane. Check tank air pressure and shutoff behavior first, then trust the rejection math.

What this test can and can't prove

We don't lab-assay membranes, and the rejection method above isn't a substitute for one — it measures dissolved solids by conductivity, which is a superb gauge of membrane health and a poor gauge of specific contaminants that don't carry much charge. A strong rejection number tells you the membrane is doing its physical job; it does not certify the water against every possible concern. Keep the two questions separate: the meter answers "is my membrane working," and a certified laboratory answers "what's actually in my water."

Common mistakes

Judging the membrane by taste alone. Taste is the alarm, not the diagnosis. Do the arithmetic before you buy a part.
Comparing your product ppm to someone else's. Rejection percentage is the portable metric. Raw ppm depends entirely on your feed water.
Replacing the membrane while ignoring the carbon that killed it. A fresh membrane behind exhausted carbon dies the same way the last one did. Fix the cause.
Mistaking a pressure or tank fault for a dead membrane. Low system pressure fakes the same symptoms. Confirm tank air pressure and shutoff first.
Measuring product from a nearly empty tank. Sample after the tank has refilled and stabilized, not mid-cycle, or the reading skews.

FAQ

What TDS reading means my membrane is bad?

There's no single ppm that fits everyone, because it depends on your feed. Convert to rejection: divide product by feed, subtract from one. Below about 85% rejection means the membrane is failing, whatever the raw numbers happen to be.

How long should an RO membrane last?

Two to five years is typical for a home thin-film membrane, provided the carbon pre-filters are kept fresh so chlorine never reaches it. Neglected pre-filters or a chlorine-heavy supply can cut that to a year or less.

My product TDS is high but the membrane looks new — why?

Often it isn't the membrane at all. A waterlogged tank, a failing auto-shutoff, or a clogged flow restrictor keeps the system from building the pressure the membrane needs to reject properly. Check those before spending on a replacement.

Does a higher feed TDS wear the membrane out faster?

Higher dissolved solids make the membrane work harder and can shorten life somewhat, but chlorine exposure and sediment fouling are far bigger factors. A well-protected membrane on hard feed water still lasts years.

Related:

General information about water filtration, not medical advice. We do not run a lab; our figures come from manufacturer specifications and NSF/EPA standards. For any health or legal decision, consult a professional and have your water tested by a certified laboratory. Prices and specifications vary by model, region and water chemistry.