RO TDS Creep Above Membrane Spec Decision Tree

Why this matters

A residential reverse osmosis system that started at 12 ppm TDS at the faucet and is now 60 ppm is either at end-of-membrane life, suffering an automatic-shutoff valve failure, leaking past a check valve, or being misread by a customer who never calibrated the meter. The decision tree separates these in a defined order. Wrong call: customer pays for a membrane that was good. Worse call: a system feeding undertreated water to a sensitive use (aquarium, dialysis-assist, infant formula) when the actual cause was a valve bleed past undetected for months.

Symptom presentation

RO faucet TDS reads higher than commissioning value, often more than 4 to 5 times higher. Storage tank TDS is similar to faucet TDS. Customer reports off-flavor or visible film on cookware. Sometimes the tank does not fill as fast as it used to.

Quick checks

1. Calibrate or compare the TDS meter against a known standard solution. Cheap pen meters drift; a check against a 342 ppm NaCl standard or a verified bottled water reading resets the reference.
2. Test raw feed TDS. Membrane rejection is reported as a percentage; a feed of 400 ppm with a permeate of 20 ppm is 95 percent rejection. A feed of 800 ppm (after a city softener change or a well-water shift) with the same membrane gives 40 ppm permeate at the same rejection - the membrane is not the problem.
3. Compare RO tank pressure to feed pressure. The automatic shutoff valve (ASV) closes feed flow when tank pressure reaches roughly two-thirds of feed pressure. A failing ASV runs the system in continuous reject-mode, and TDS climbs as the reject path narrows.
4. Inspect the check valve between membrane and tank. A leaking check valve back-feeds tank water past the membrane and ruins rejection.
5. Check membrane age. NSF/ANSI 58 rates membranes for daily output and rejection over a defined run. Standard residential membrane life is 2 to 3 years on city water and 12 to 18 months on hard well water.

Isolation tree

Step 1: Read feed TDS, post-prefilter TDS, post-membrane (line direct from membrane permeate elbow) TDS, and faucet TDS. If post-membrane is low and faucet is high, the rise is happening downstream of the membrane - tank back-feed or post-carbon issue. If post-membrane is also high, the membrane is the issue.

Step 2: With the membrane permeate sample, calculate rejection: (1 minus permeate / feed) times 100. A new TFC membrane runs 95 to 99 percent. A membrane reporting 90 percent is partially fouled or aged. Below 85 percent indicates end of life or membrane chemical damage (chlorine exposure on a TFC).

Step 3: If membrane rejection is acceptable but faucet TDS is high, isolate the tank. Drain the tank, refill from membrane production only (close the manual faucet output during fill), then sample faucet. If TDS is now low, the tank water was contaminated by prior pre-leak; clean and recheck. If TDS is still high, the check valve is back-feeding tank into permeate or the post-carbon polish stage is leaching.

Step 4: Confirm the ASV. Close the tank valve, run the faucet to drain the line, watch the drain line. The drain should pulse (reject flowing) when the faucet is open. Close the faucet. The drain should slow and stop within a few minutes as the ASV closes. A drain that runs continuously with the faucet off is an ASV failure - the membrane is running continuous reject and the customer is paying for it on the water bill.

Step 5: Look for chlorine damage on the membrane. A TFC (thin-film composite) membrane fed unfiltered municipal water with 1 ppm free chlorine for a few months loses rejection permanently. Confirm the pre-carbon filter has been changed on schedule; if not, the membrane is likely chlorine-damaged regardless of age.

Confirming diagnosis

Membrane end-of-life confirms by direct-from-membrane rejection below OEM minimum. ASV failure confirms by continuous drain flow with the faucet closed. Check valve failure confirms by back-pressure test at the permeate port. Chlorine damage confirms by membrane history (pre-carbon overdue) and by below-spec rejection paired with the right history. TDS meter error confirms by recalibration against a known standard.

Remediation

For end-of-membrane life: replace with the OEM-specified membrane (Filmtec, GE, or Hydranautics depending on the system). Flush per the install instructions; do not skip the initial flush or the carbon dust will foul the new membrane fast.

For ASV failure: replace the ASV. These are inexpensive parts; replacement is the durable fix.

For check valve failure: replace the check at the permeate elbow.

For chlorine damage: replace the membrane and replace the pre-carbon filter. Move the customer to a 6-month pre-carbon change schedule (versus the 12-month default on heavily chlorinated water).

For tank contamination from prior leaks: drain, sanitize per the OEM cleaning procedure with food-grade hydrogen peroxide, refill, retest.

After any change, document feed TDS, permeate TDS, faucet TDS, and the calculated rejection on the work order. Re-test at 30 days to confirm.

References

1. NSF/ANSI 58 Reverse Osmosis Drinking Water Treatment Systems, sections on rejection rating and daily output.
2. NSF/ANSI 53 Drinking Water Treatment Units, including the pre-filter health-claim test methods.
3. Filmtec Reverse Osmosis Membranes Technical Manual, including TFC chlorine tolerance and end-of-life criteria.
4. EPA Safe Drinking Water Act Secondary Drinking Water Regulations on TDS at 500 ppm aesthetic limit.
5. Water Quality Association Technical Bulletin on residential RO membrane performance.