What Substances Must Be Critically Controlled During Seawater RO Pretreatment?| Insights by AQUALITEK

Introduction

In seawater reverse osmosis (SWRO) systems, pretreatment is not about improving water quality for users—it is about protecting the RO membrane system.

Unlike brackish water, seawater contains a complex mixture of:

•Suspended solids

•Colloids

•Organic matter

•Microorganisms

•Oxidants and metals

If these substances are not properly controlled, RO membranes will foul, scale, or degrade rapidly, regardless of membrane brand or system pressure.

This article outlines the most critical substances that seawater pretreatment must remove or control, and explains why each one is dangerous to SWRO systems.

1. Suspended Solids (SS) and Particulate Matter

Why They Are Critical

Suspended solids include:

•Sand

•Silt

•Clay

•Rust particles

•Fine debris

Even at low concentrations, they:

•Cause rapid surface fouling

•Increase differential pressure

•Block membrane feed channels

Control Target

•SDI₁₅ ≤ 3–5

•Turbidity ≤ 1 NTU (typically ≤ 0.5 NTU for SWRO)

Typical Removal Methods

•Coagulation + flocculation

•Media filtration

•Ultrafiltration (UF)

•Cartridge (security) filtration

2. Colloidal Particles

Why They Are Dangerous

Colloids (0.001–1 μm) are:

•Electrically charged

•Stable in suspension

•Difficult to settle or filter

They cause:

•Dense, irreversible fouling layers

•Poor cleanability

•Rapid flux decline

Examples

•Iron hydroxides

•Aluminum flocs

•Silica colloids

•Organic-metal complexes

Control Strategy

•Proper coagulation chemistry

•Sufficient flocculation time

•Fine filtration or UF membranes

3. Natural Organic Matter (NOM)

Why It Must Be Controlled

Seawater contains:

•Humic substances

•Fulvic acids

•Dissolved organic carbon (DOC)

These substances:

•Adsorb strongly onto membrane surfaces

•Promote biofouling

•React with disinfectants to form DBPs

Control Indicators

•TOC

•UV₂₅₄ absorbance

Removal Methods

•Optimized coagulation

•Activated carbon (in some designs)

•UF with enhanced coagulation

4. Microorganisms and Biological Activity

Why Biofouling Is the Most Severe Risk

Seawater is biologically active and contains:

•Bacteria

•Algae

•Biofilm-forming microorganisms

Biofouling leads to:

•Rapid pressure drop increase

•Irreversible membrane damage

•Frequent chemical cleaning

Key Control Approach

•Prevention, not elimination

•Reduce nutrients and attachment sites

Typical Measures

•Coagulation to remove biomass

•Physical filtration

•Periodic shock dosing (non-oxidizing biocides)

5. Oxidants (Free Chlorine, Chloramines, Ozone)

Why Oxidants Are Extremely Dangerous

Most RO membranes are polyamide-based and:

•Are irreversibly damaged by oxidants

•Lose salt rejection permanently

Acceptable Limit

•Free chlorine: < 0.01 mg/L at RO inlet

Control Methods

•Sodium metabisulfite (SMBS) dosing

•Activated carbon dechlorination

•ORP monitoring

6. Iron, Manganese, and Metal Ions

Why Metals Cause Serious Fouling

Iron and manganese:

•Oxidize and precipitate

•Form dense, sticky fouling layers

•Are difficult to remove by CIP

Sources

•Seawater intake corrosion

•Chemical dosing impurities

•Coagulant carryover

Control Target

•Total iron typically < 0.05 mg/L

7. Oil, Grease, and Hydrocarbons

Why They Are High-Risk Contaminants

In coastal and industrial areas, seawater may contain:

•Petroleum hydrocarbons

•Lubricants

•Organic solvents

These substances:

•Strongly adhere to membrane surfaces

•Cause severe organic fouling

•Are difficult to clean completely

Control Measures

•Proper intake location

•Oil-water separation

•Emergency shutdown protocols

8. Silt Density Index (SDI) – A Key Composite Indicator

SDI does not represent a single substance, but reflects:

•Particulates

•Colloids

•Organic matter

It is one of the most important pretreatment performance indicators for SWRO.

Typical design requirement:

•SDI₁₅ ≤ 3–5

Conclusion

Seawater pretreatment is not optional fine-tuning—it is the foundation of RO system reliability.

The most critical substances that must be removed or controlled include:

•Suspended solids and colloids

•Natural organic matter

•Microorganisms

•Oxidants

•Metals and hydrocarbons

Effective pretreatment:

•Extends membrane life

•Reduces chemical cleaning frequency

•Stabilizes energy consumption

•Protects high-value RO assets

In SWRO systems, pretreatment quality determines overall plant success.