Low Water Flow or High TDS? A Troubleshooting Guide for Commercial RO Membranes

1. Membrane Fouling or Scaling — The Gradual Decline

What it looks like: Permeate flow drops steadily over days or weeks, even though feed pressure hasn't changed. Differential pressure (ΔP) across the membrane stages is rising. Water quality may also decline gradually as salt rejection suffers.

What's happening: Particles, organics, biofilms, or inorganic scale deposits are building up on the membrane surface. Fouling is most severe in lead-end elements, while scaling tends to hit the tail elements where salts are most concentrated.

How to check: Compare normalized permeate flow against historical data — if it's down more than 15% at stable temperature and pressure, fouling or scaling is likely. Rising ΔP across stages is another strong indicator.

What to do: Run a Clean-in-Place (CIP) procedure with appropriate chemistry. If flow recovers, you've confirmed the diagnosis. If not...

2. Leaking O-Ring — The Localized Quality Problem

What it looks like: Permeate conductivity spikes suddenly, but permeate flow rate stays normal. The problem is often isolated to one or two pressure vessels rather than the entire system.

What's happening: An O-ring at the permeate tube interconnector, adapter, or end plug has failed — either damaged during installation, chemically degraded, or shifted out of position. Untreated feed water is bypassing the membrane and mixing with product water.

How to check: Use probing — sample permeate conductivity from each element individually within a pressure vessel. If one element shows abnormally high conductivity while neighbors are fine, you've found an O-ring leak.

What to do: Inspect and replace the damaged O-ring. This is one of the cheapest fixes on the list — don't skip it before assuming the membrane itself is dead.

3. Back Pressure Damage — The Sudden Failure

What it looks like: Salt rejection drops sharply — sometimes from 99.5% down to below 95%— and permeate flow increases (sometimes by 30–50%). Chemical cleaning does nothing to restore performance.

What's happening: Permeate pressure exceeded concentrate pressure by more than 0.05 MPa (about 7 psi) at some point. This reversed flow caused delamination of the membrane's polyamide active layer from the support layer. The damage is often irreversible and typically hits the last element in the last stage first.

How to check: If you can autopsy a suspect element, look for parallel creases on the membrane surface (spacing about 2–3 mm) or "fish scale" peeling of the polyamide layer. On-site, probing will show permeate conductivity rising steadily from the feed end to the concentrate end of the damaged vessel.

What to do: There is no repair for back pressure damage. If product water quality no longer meets spec, replacement is the only option.

4. TDS Creep or Membrane Aging — The Slow Quality Erosion

What it looks like: Permeate TDS is higher than usual, but flow rate may be normal or only slightly reduced. The problem is most noticeable after the system has been idle — the first 30–60 seconds of production show elevated TDS.

What's happening: There are actually two distinct scenarios here:

• TDS creep:When the system sits idle, salts diffuse across the membrane from the concentrate side to the permeate side. This is normal behavior — flush the system for a minute and TDS should drop back to normal.
• Membrane degradation:If TDS remains high even after flushing, the membrane may be oxidized, chemically attacked, or simply at the end of its service life. Commercial RO membranes typically last 2–7 years depending on feed water and maintenance, but some can go longer with excellent care.
• How to check:Calculate rejection rate: (Feed TDS − Permeate TDS) ÷ Feed TDS × 100. If rejection has fallen below 80–85% and cleaning doesn't restore it, the membrane is likely degraded.
• What to do:If it's TDS creep, a simple flush solves it. If rejection is permanently low, replacement is the answer.

The Bottom Line: When to Call It

Here's the diagnostic cheat sheet:

One rule to remember: If you've run a proper CIP and performance still hasn't recovered — flow is still low, TDS is still high, or both — the membrane has likely reached the end of its useful life. Irreversible fouling, structural damage, or chemical degradation can't be cleaned away.

At that point, continuing to chase the problem with more cleanings, higher pressure, or filter changes is just throwing money at a dead membrane. It's time to replace the element.

Need help diagnosing a specific issue? Our technical team can review your operating data and help you pinpoint the root cause — before you spend another dollar on the wrong fix.