Best Ways to Quickly Determine Whether an Odor in RO Permeate Originates Before or After the Membrane| Insights by AQUALITEK

Introduction

When an unexpected odor appears in RO permeate, one of the first troubleshooting questions is:
Does the odor originate before the membrane, or is it caused by something happening after the membrane?

This distinction is critical because the corrective action is totally different. Feed-water-related odors may require pretreatment adjustments, while post-membrane odors indicate hygiene, storage, or distribution issues.

This Best-class article covers the fastest and most accurate methods to identify the odor source using practical tests, operator experience, and system sampling techniques.

Best Methods to Identify Whether an Odor Comes From Before or After the RO Membrane

1. Compare Feed Water, Concentrate, and Permeate Odors

This is the simplest and fastest on-site diagnostic approach.

Procedure:

•Smell feed water (influent)

•Smell concentrate (reject stream)

•Smell permeate

Interpretation:

•If the feed and concentrate smell similar, but permeate smells different:
→ The odor is after the membrane (post-treatment contamination).

•If the feed and concentrate have the same odor, and permeate has a weaker but similar odor:
→ Odor originates before the membrane (feedwater quality issue).

•If only the permeate smells bad:
→ Odor is after the membrane, often from storage tanks or distribution lines.

2. Bypass the Permeate Tank to Test Fresh Permeate

Storage tanks are a very common source of odor (algae, biofilm, stagnant water).

Steps:

•Temporarily bypass the permeate tank.

•Collect permeate directly from the RO outlet port.

Interpretation:

•If fresh permeate has no odor but tank water does:
→ Odor is after the membrane, typically tank or piping contamination.

•If both fresh permeate and tank water have odor:
→ Odor likely originates before the membrane or from system chemical issues.

3. Check the pH and Conductivity Behavior

Certain odors correlate with measurable changes.

Rules of thumb:

•Organic or chlorine-related odor with normal permeate conductivity:
→ Usually post-membrane contamination.

•High odor + abnormal conductivity spike:
→ Possible membrane integrity breach or feedwater breakthrough.

This indicator alone is not definitive, but useful when combined with other tests.

4. Inspect Post-Treatment Components

Odors often come from equipment located after the RO membrane:

Common odor sources include:

•Aging carbon filters

•Biofilm in permeate distribution pipes

•Dirty storage tanks

•Poorly disinfected UV housings

•Dead legs in piping

If any of these downstream components smell, the odor is after the membrane.

5. Evaluate Pretreatment and Chemical Additives

Some feed-side issues introduce odors that the membrane cannot fully remove.

Feed-side odor contributors:

•Chlorine (if dechlorination is incomplete)

•Hydrogen sulfide (H₂S)

•Organic compounds

•Industrial wastewater contaminants

•Biological activity in feed tanks

If these odors are detectable in influent, they likely contribute to permeate odor even after RO.

6. Perform a Membrane Integrity or Pressure Vessel Inspection

If odor appears suddenly or strongly:

•Remove end caps

•Inspect for membrane damage

•Check for O-ring failure

•Look for biological growth inside vessels

A damaged membrane may allow odorous compounds to pass through.
If integrity is compromised → odor originates before the membrane (breakthrough).

7. Conduct a Simple “Glass Test”

A classic test used by operators:

Procedure:

•Rinse 3 clean glass cups with permeate

•Fill one with feed water, one with permeate from the RO outlet, one with water from the tank

•Air them for 1 minute

•Compare smells side by side

This provides a highly intuitive smell comparison with minimal external interference.

Conclusion

Identifying whether an odor in RO permeate originates before or after the membrane is essential for quick, accurate troubleshooting. By comparing water samples, bypassing tanks, inspecting downstream components, and evaluating system chemistry, operators can pinpoint the root cause within minutes.

Following the Best practices outlined in this article will help ensure faster diagnosis, improved water quality, and reduced downtime for your RO system.