RO Permeate Flow Drops But Rejection Fine Decision Tree

Why this matters

A reverse osmosis system whose TDS rejection is still good but whose permeate flow has fallen tells a very specific story: the membrane is rejecting fine, so it is not chemically degraded or punctured, but something is choking how much water gets across it. Low flow with good rejection is a flux problem, not a salt-passage problem, and the usual suspects are low net driving pressure (clogged prefilter, low feed pressure, high tank back-pressure), cold feed water, or early-stage fouling/scaling that adds resistance without yet wrecking rejection. Diagnosing it correctly saves the customer from a needless membrane swap (the membrane is fine) and from a slow-filling tank they will keep calling about. This tree separates pressure, temperature, and fouling causes of low permeate flow on a residential point-of-use or whole-house RO.

Symptom presentation

• Storage tank fills slowly, or permeate flow at the faucet (with tank isolated) is well below the membrane's rated production.
• TDS rejection still good (permeate TDS is a low fraction of feed TDS - rejection typically still well above 90 percent).
• Often gradual; the customer notices the kitchen RO faucet "barely trickles" or the tank no longer keeps up.

Good rejection plus low flow is the diagnostic fork. If rejection had collapsed too, you would be chasing a damaged membrane or a check-valve/feed bypass; here the membrane is intact and you are chasing driving pressure or resistance.

Quick checks

• Measure feed pressure to the membrane (psi) at the membrane inlet. Residential RO wants roughly 40-80 psi; below ~40 psi flux falls off sharply.
• Measure permeate flow directly: tank isolated, time the permeate into a graduated container (mL per minute) and compare to rated GPD.
• Measure feed water temperature. Membranes are rated at 77 F (25 C); cold water drops flux substantially - roughly 1.5-2 percent per degree F below rating.
• Check the prefilters (sediment and carbon) for clogging and the storage tank air charge (back-pressure robs net driving pressure).

Isolation tree

1. Is feed pressure to the membrane adequate (40+ psi)?

   • No, low feed pressure: flux falls with pressure. Find why - clogged prefilters, low house pressure, a failing booster pump, or a partly closed feed valve. Replace prefilters and/or add/repair a booster pump. Re-measure flow.
   • Yes, 40+ psi at the membrane: continue.

2. Are the prefilters clogged (pressure drop across them)?

   • A fouled sediment or carbon prefilter drops pressure before the membrane even though house pressure is fine. Measure pressure before and after the prefilters; a big drop means replace them. This is the single most common low-flow cause on a neglected unit.
   • Prefilters clean / recently changed: continue.

3. Is the feed water cold?

   • Cold feed (well water in winter at 45-55 F) cuts permeate flow well below rated, with rejection unaffected. This is physics, not a fault. Apply the temperature-correction factor: a unit rated at 77 F will produce far less at 50 F and the tank fills slowly all winter. Explain it; do not replace the membrane.
   • Feed water near 77 F: continue.

4. Is the storage tank back-pressure too high (low air charge or tank too full)?

   • As the tank fills, its back-pressure subtracts from net driving pressure and permeate flow tapers - normal. But an over-charged tank (air pressure set too high) chokes production immediately. Drain the tank, check the empty-tank air charge (typically ~5-7 psi for a standard 3-4 gal RO tank), and re-test fill rate with the tank isolated to separate tank back-pressure from membrane flux.
   • Tank charge correct and isolated-flow still low: continue.

5. Is the membrane fouled or scaling?

   • With good pressure, warm water, clean prefilters, correct tank charge, and rejection still acceptable, a low isolated permeate flow points to early fouling/scaling adding resistance. Hard or high-silica feed scales the membrane; biofouling/organics blind it. Check feed hardness/silica and whether a softener or antiscalant precedes the RO. Clean or replace the membrane, and add pretreatment (softening ahead of RO) to prevent recurrence.

Confirming the diagnosis

• Pressure/prefilter confirmed when the pressure drop across the prefilters is large or membrane feed pressure is below ~40 psi, and replacing prefilters / restoring pressure brings flow back up at the same rejection.
• Temperature confirmed when feed water is cold and the measured flow matches the temperature-corrected expected output for the membrane - production recovers as the water warms or with the correction factor applied.
• Tank confirmed when isolated-tank permeate flow is normal but in-service fill is slow, and correcting the air charge restores fill rate.
• Fouling/scaling confirmed when pressure, temperature, prefilters, and tank all check out yet isolated permeate flow stays low at good rejection, and cleaning or replacing the membrane (with pretreatment added) restores rated flux.

Remediation

• Replace clogged sediment/carbon prefilters; restore or boost feed pressure to spec.
• Apply the temperature-correction factor and set customer expectations for cold-season flow; no membrane change needed.
• Reset the RO tank air charge to the manufacturer's empty-tank spec.
• Soften or antiscale ahead of the RO on hard/high-silica feed; clean or replace a fouled membrane and verify pretreatment is in place to prevent rapid re-fouling.

References

1. NSF/ANSI 58, Reverse Osmosis Drinking Water Treatment Systems (rated production at standard conditions).
2. WQA Technical Reference on reverse osmosis flux, temperature correction, fouling, and scaling.
3. Membrane manufacturer data sheets (e.g., Dow/DuPont FilmTec) for temperature-correction factors and operating pressure.
4. AWWA guidance on RO pretreatment, scaling, and antiscalant use.