If RO Permeate Smells Like Rotten Eggs, What Contaminant Is Likely Present?| Insights by AQUALITEK

Introduction

Under normal conditions, RO permeate should be colorless, tasteless, and odorless.
If the produced water suddenly emits a rotten egg smell, this is not a minor sensory issue—it is a strong indicator of specific chemical or biological contamination.

Understanding what causes this odor and how it enters the RO system is critical to protecting membrane integrity, downstream equipment, and water safety.

This article explains what contaminant is most likely responsible, how to confirm it, and what actions should be taken.

Primary Cause: Hydrogen Sulfide (H₂S)

Why Rotten Egg Smell Points to Hydrogen Sulfide

A rotten egg odor in water is almost always caused by hydrogen sulfide (H₂S).

Key characteristics of H₂S:

•Extremely strong odor at very low concentrations

•Colorless gas dissolved in water

•Detectable by smell before it becomes measurable by many instruments

Even trace levels (below 0.1 mg/L) can produce a noticeable odor.

How Hydrogen Sulfide Enters RO Permeate

1. H₂S Present in the Feed Water

Some groundwater or surface water sources naturally contain:

•Sulfur compounds

•Sulfate-reducing bacteria (SRB)

If pretreatment is insufficient, H₂S can pass into the RO feed.

2. Biological Activity Inside the System

Under anaerobic conditions, sulfate-reducing bacteria can grow:

•In pretreatment tanks

•In carbon filters

•In RO permeate pipelines

These bacteria convert sulfate into hydrogen sulfide.

This is especially common after long shutdowns or stagnant operation.

3. Post-Membrane Contamination (Most Common)

RO membranes reject bacteria effectively, but they do not protect downstream piping.

Common post-membrane sources:

•Biofilm growth in permeate storage tanks

•Dead-end pipelines

•Poorly sanitized distribution systems

In many cases, the odor is generated after the membrane, not by membrane failure.

Why RO Membranes Do Not Remove H₂S Effectively

Hydrogen sulfide:

•Is a small, uncharged molecule

•Can partially pass through RO membranes in dissolved form

•Is not reliably rejected like salts or bacteria

Therefore, RO alone cannot be relied upon for H₂S removal.

How to Quickly Confirm the Source of the Odor

Step 1: Smell Comparison Test

•Smell RO permeate directly at membrane outlet

•Smell water at storage tank or end-use point

If odor increases downstream, contamination is post-membrane.

Step 2: Test for Hydrogen Sulfide

•Use H₂S test strips or colorimetric kits

•Measure at multiple points in the system

Step 3: Check Operating History

•Recent shutdowns?

•Stagnant pipelines?

•Lack of periodic disinfection?

These are strong indicators of biological H₂S generation.

Corrective Actions

Immediate Measures

•Stop supplying affected permeate

•Drain and flush permeate tank and pipelines

•Increase flushing frequency

Disinfection and Cleaning

•Sanitize permeate storage and distribution lines

•Remove biofilm mechanically if possible

•Apply approved disinfectants compatible with materials

Long-Term Prevention

•Avoid long stagnant periods

•Implement routine permeate line sanitation

•Ensure proper ventilation of storage tanks

•Consider post-treatment (e.g., UV or degassing)

Other Less Common Odor Causes (for Comparison)

Conclusion

If RO permeate smells like rotten eggs, hydrogen sulfide is the most likely contaminant.

In most cases, the root cause is:

•Biological activity

•Stagnation

•Post-membrane contamination

Prompt diagnosis and corrective action are essential to prevent microbial growth, corrosion, and customer complaints.

Odor is not just a sensory issue—it is an early warning signal that should never be ignored.