The Two Most Common Pollutants in Seawater That Pose the Greatest Threat to RO Membranes| Insights by AQUALITEK

Introduction

In seawater reverse osmosis (SWRO) desalination systems, RO membranes are the core and most vulnerable components.
Although seawater contains a wide range of dissolved salts, organics, and microorganisms, only a few pollutant categories pose the most serious and immediate threat to membrane performance and lifespan.

From decades of operational data worldwide, the two most common and most damaging pollutant types in seawater are:

1.Suspended & colloidal particulate matter

2.Biological contaminants (biofouling agents)

Understanding why these two dominate membrane fouling is essential for pretreatment design, system reliability, and operating cost control.

1. Suspended and Colloidal Particles – The Primary Physical Threat

1.1 What Are They?

Suspended and colloidal pollutants in seawater typically include:

•Fine sand and silt

•Clay minerals

•Iron and manganese oxides

•Corrosion particles

•Organic colloids

•Algae fragments and debris

These particles often range from sub-micron to several tens of microns, making them difficult to remove without advanced pretreatment.

1.2 Why They Are So Dangerous to RO Membranes

Unlike dissolved salts, particulate matter:

•Deposits directly on the membrane surface

•Blocks membrane pores and feed channels

•Increases pressure drop rapidly

Key impacts include:

•Sharp rise in differential pressure (ΔP)

•Rapid flux decline

•Uneven flow distribution

•Accelerated membrane fouling and scaling

Once compacted under high pressure, particulate fouling becomes difficult or impossible to fully remove by chemical cleaning.

1.3 Industry Evidence

Globally, high SDI (Silt Density Index) caused by particulates is the number one cause of early SWRO membrane failure, which is why SWRO pretreatment typically targets:

•SDI₁₅ ≤ 3–5

•Turbidity ≤ 1 NTU (often ≤ 0.5 NTU)

2. Biological Contaminants – The Most Complex and Persistent Threat

2.1 What Are Biological Pollutants?

Biological contaminants include:

•Bacteria

•Algae

•Plankton

•Fungi

•Microbial extracellular polymeric substances (EPS)

Even seawater that looks clear may contain millions of microorganisms per milliliter.

2.2 Why Biofouling Is Especially Harmful

Biofouling is considered the most difficult fouling type to control because:

•Microorganisms reproduce rapidly

•Biofilms protect microbes from cleaning chemicals

•EPS acts as a glue, trapping particles and nutrients

Once established, biofouling leads to:

•Continuous pressure drop increase

•Accelerated membrane degradation

•Higher cleaning frequency

•Shortened membrane life

In many SWRO plants, biofouling—not scaling—defines membrane replacement cycles.

2.3 Synergistic Fouling Effect

Biological fouling rarely acts alone. Biofilms:

•Capture suspended solids

•Promote localized scaling

•Increase organic fouling severity

This creates compound fouling, which is far more destructive than single-type fouling.

3. Why Dissolved Salts Are Not the Main Threat

Although seawater contains extremely high TDS:

•Sodium, chloride, magnesium, and sulfate are designed targets of RO membranes

•Dissolved salts do not directly foul membranes

•Scaling only occurs when solubility limits are exceeded

By contrast:
Particles and microorganisms attack the membrane physically and biologically, making them far more dangerous during normal operation.

4. Implications for Seawater Pretreatment Design

Because these two pollutant types dominate risk, SWRO pretreatment focuses on:

For Particulates:

•Coagulation–flocculation

•Multimedia filtration

•Ultrafiltration (UF)

For Biological Contaminants:

•Continuous or intermittent chlorination (followed by dechlorination)

•Biocide control strategies

•Low nutrient environments

•Frequent low-pressure flushing

5. Summary: The Two Most Dangerous Seawater Pollutants

Together, these two pollutant categories account for the majority of RO membrane failures in seawater desalination plants worldwide.

Conclusion

In seawater desalination systems, the greatest threats to RO membranes are not dissolved salts, but suspended/colloidal particles and biological contaminants.

Effective control of these two pollutant types:

•Determines membrane lifespan

•Reduces operating pressure and energy use

•Minimizes chemical cleaning frequency

•Ensures long-term system stability

For any SWRO project, pretreatment design must prioritize particulate removal and biofouling control above all else.