Why Must a Seawater Desalination System Be Flushed with Low-Pressure Fresh Water Before Slowly Increasing Pressure at Startup?| Insights by AQUALITEK

Introduction

Starting up a seawater reverse osmosis (SWRO) desalination system is a high-risk operational phase.
Improper startup procedures can cause irreversible membrane damage, severe fouling, and mechanical failure—sometimes within minutes.

For this reason, industry-standard operating procedures require that SWRO systems be flushed with low-pressure fresh water first, and only then slowly pressurized.

This article explains why this step is mandatory, what risks it mitigates, and how it protects both membranes and high-pressure equipment.

1. The Core Reason: Preventing Osmotic Shock to the Membrane

1.1 What Is Osmotic Shock?

Osmotic shock occurs when a membrane experiences a sudden and large osmotic pressure difference across its surface.

•Seawater osmotic pressure: ≈25–30 bar

•Fresh or low-salinity water: ≈0–2 bar

If high-pressure seawater is applied suddenly:

•Water flux becomes highly uneven

•Membrane layers experience extreme stress

•Delamination and compaction may occur

Low-pressure freshwater flushing equalizes osmotic conditions before pressurization.

2. Removing Stagnant High-Salinity Brine from the System

2.1 What Happens During Shutdown?

During shutdown:

•Seawater remains stagnant in membranes and pressure vessels

•Salt concentration increases locally

•Bioactivity may start

•Oxygen and corrosion risk increase

If the system is pressurized directly:

•Highly concentrated brine is instantly compressed

•Local scaling and fouling accelerate

•Membrane surface damage risk rises sharply

Low-pressure flushing:

•Displaces stagnant seawater

•Reduces salinity inside the membrane

•Restores safe starting conditions

3. Preventing Instant Scaling and Fouling

3.1 Scaling Risk at Startup

At startup:

•Flow is unstable

•Recovery is uncontrolled

•Local concentration polarization is extreme

Direct pressurization with seawater can cause:

•Calcium carbonate scaling

•Sulfate scaling

•Immediate particulate deposition

Low-pressure freshwater flushing:

•Lowers ionic strength

•Keeps salts below saturation

•Prevents early-stage irreversible fouling

4. Protecting the High-Pressure Pump and ERD

4.1 Mechanical Protection

Sudden pressurization can lead to:

•Pressure surges

•Hydraulic shock

•Cavitation risk

•ERD instability

Gradual pressurization allows:

•Smooth hydraulic transition

•Stable ERD engagement

•Reduced mechanical stress on seals and bearings

5. Avoiding Membrane Compaction and Structural Damage

SWRO membranes are designed for high pressure—but not for sudden pressure spikes.

Direct startup risks:

•Permanent membrane compaction

•Reduced permeability

•Long-term flux loss

Slow pressure ramping:

•Allows membrane structure to adapt

•Preserves designed water flux

•Extends membrane lifespan

6. Industry-Standard Startup Logic for SWRO Systems

A typical safe startup sequence includes:

1.Low-pressure freshwater flushing

Pressure: < 3 bar

Purpose: displacement & equilibration

2.Seawater introduction at low pressure

No permeate production initially

3.Gradual pressure increase

Stepwise or ramped

Controlled concentrate valve adjustment

4.Stabilization before reaching design pressure

This sequence is recommended by all major SWRO membrane manufacturers.

7. What Happens If This Step Is Skipped?

Skipping low-pressure flushing may result in:

•Immediate membrane fouling

•Sudden rise in differential pressure

•Rapid flux decline

•Shortened membrane life

•Increased cleaning frequency

•Unexpected early membrane replacement

In severe cases:
👉 Membranes can be permanently damaged on the very first startup.

Conclusion

Flushing a seawater desalination system with low-pressure fresh water before slowly increasing pressure is not a precaution—it is a fundamental requirement.

In summary, this practice:

•Prevents osmotic shock

•Removes stagnant high-salinity seawater

•Avoids instant scaling and fouling

•Protects membranes, pumps, and ERDs

•Ensures stable, long-term system performance

A correct startup procedure is one of the most cost-effective ways to protect a seawater RO system.