RO Produces Water But TDS Rejection Drops Slowly Over Months Decision Tree

Why this matters

A reverse-osmosis system whose rejection percentage slips gradually over months, rather than suddenly, is telling you something different from an acute TDS spike: slow decline points to progressive membrane fouling or scaling, neglected prefilter changes letting chlorine creep through, or a feed-water shift that the system was never sized for. Replacing the membrane without addressing the cause of the slow decline just resets the clock until the same fouling repeats. This tree separates scaling, organic/colloidal fouling, chlorine attack, biofouling, and feed-water change so the membrane replacement comes paired with the right pretreatment fix.

Symptom presentation

Commissioning rejection was strong (95 percent or better) and has drifted down over weeks to months to the low 90s or 80s. Permeate flow may also have dropped, or the system runs longer to fill the tank. The customer notices a gradual return of taste, spotting, or scale. There was no single triggering event, just a slow slide.

Quick checks

1. Measure feed TDS, permeate TDS, and calculate rejection: (1 minus permeate/feed) times 100. Track it against the commissioning value.
2. Check feed water for changes: a softener that ran out of salt, a new well condition, or a municipal source change raises feed hardness or TDS and lowers apparent rejection.
3. Confirm prefilter and carbon change history. Overdue carbon lets chlorine reach a TFC membrane and slowly destroy rejection.
4. Note permeate flow trend. Declining flow with declining rejection leans toward fouling/scaling; stable flow with falling rejection leans toward membrane chemical damage.
5. Inspect for upstream hardness. Hard, unsoftened feed scales the membrane over time, especially at high recovery.

Isolation tree

Step 1: Recalculate rejection with current feed TDS. If feed TDS climbed (softener failure, well shift), the membrane may be performing the same percentage rejection but on dirtier water, so permeate rises. Correct the feed and re-evaluate before condemning the membrane.

Step 2: If feed is stable, look at the prefilter/carbon history. A TFC membrane fed chlorinated water with an exhausted carbon prefilter loses rejection slowly and permanently as chlorine oxidizes the membrane. Overdue carbon plus slow decline strongly implicates chlorine damage.

Step 3: If carbon is current, assess fouling versus scaling by the flow trend. Falling permeate flow alongside falling rejection indicates the membrane surface is loading up: scale (hardness, often at the concentrate end) or colloidal/organic fouling. Hard feed and high recovery point to scaling; turbid or organic-rich feed points to fouling.

Step 4: If decline tracks with a stagnant or low-use system, suspect biofouling: a membrane that sits idle grows a biofilm that degrades rejection and flow. Confirm by usage pattern and by a sanitizing flush response.

Step 5: If feed, prefilter, fouling, and biofouling are all ruled out, the membrane has reached normal end of life. TFC membranes degrade gradually; a multi-year-old membrane on hard well water at end of its expected run simply needs replacement.

Confirming diagnosis

A feed-water change confirms when restoring the softener or correcting the source brings permeate back without a new membrane. Chlorine damage confirms by an overdue carbon prefilter plus permanently depressed rejection that a sanitize does not recover. Scaling confirms by hard high-recovery feed and rejection that partially recovers after a low-pH membrane clean. Organic/colloidal fouling confirms by rejection/flow recovery after a high-pH or surfactant clean. Biofouling confirms by recovery after sanitizing and by a low-use history. End of life confirms when a new membrane restores commissioning rejection and no upstream cause remains.

Remediation

For feed change: restore softening or add appropriate pretreatment and re-baseline. For chlorine damage: replace the membrane and put the carbon prefilter on a 6-month change schedule on chlorinated water. For scaling: lower the system recovery, add antiscalant or softening pretreatment, clean the membrane at low pH if early, replace if advanced. For fouling: clean per the membrane maker's CIP procedure (high pH/surfactant for organics), add sediment pretreatment. For biofouling: sanitize the system and encourage regular use or a recirculation feature. For end of life: replace with the OEM-spec membrane and flush per the install instructions.

Log feed TDS, permeate TDS, rejection, and flow at each visit so the slow trend is visible and the cause is read from the curve.

References

1. NSF/ANSI 58 Reverse Osmosis Drinking Water Treatment Systems, on rejection rating and daily production.
2. NSF/ANSI 42 and 53 Drinking Water Treatment Units, on prefilter chlorine reduction.
3. Membrane manufacturer technical manuals (DuPont FilmTec, Hydranautics) on chlorine tolerance, cleaning, and fouling.
4. EPA Secondary Drinking Water Regulations on TDS aesthetic limit of 500 ppm.
5. WQA Technical Bulletin on residential RO performance and membrane fouling.