What Are the Special Requirements for Flushing and Pressurization Before Starting a Seawater Desalination System?| Insights by AQUALITEK

Introduction

Compared with brackish water RO systems, seawater desalination (SWRO) systems operate at much higher pressures and face far greater fouling and scaling risks. As a result, the flushing and pressurization procedures before startup are critical and must be performed with stricter control.

Failure to follow proper startup practices can lead to:

•Irreversible membrane compaction

•Mechanical damage to membrane elements

•Sudden fouling or scaling

•High-pressure pump overload

This article outlines the special requirements and best practices for flushing and pressurization before starting a seawater desalination system.

Why Flushing and Pressurization Are More Critical in SWRO Systems

Seawater RO systems differ fundamentally from conventional RO in several ways:

•Operating pressure typically reaches 55–70 bar (800–1,000 psi)

•Feedwater contains high salinity, suspended solids, organics, and microorganisms

•Membrane elements are more sensitive to pressure shock and fouling

Therefore, startup must focus on protecting membranes, stabilizing hydraulics, and ensuring clean operating conditions before applying full pressure.

Special Requirements for Flushing Before Startup

1. Thorough Low-Pressure Flushing Is Mandatory

Before pressurization, the system must be flushed at low pressure and low flow velocity to:

•Remove preservatives from new membranes

•Flush out debris, air, oil, and installation residues

•Remove biological growth in idle systems

Key requirements:

•No backpressure on concentrate line

•Permeate valves fully open

•Pressure typically below 3 bar

2. Use Qualified Water for Flushing

Flushing water quality must meet specific criteria:

•SDI₁₅ ≤ system design limit

•Turbidity < 1 NTU

•No free chlorine or oxidants

•Temperature within membrane limits

⚠️ Using raw seawater without proper pretreatment can cause instant fouling during startup.

3. Flush Without Producing Product Water Initially

During initial flushing:

•Permeate should be discharged to drain

•Product tanks must remain isolated

This prevents:

•Contamination of storage tanks

•Residual chemicals entering downstream systems

4. Extended Flushing Time for New or Idle Systems

Typical flushing duration:

•30–60 minutes for normal restarts

•1–2 hours or longer for new installations or long shutdowns

Flushing should continue until:

•No visible air bubbles

•Stable pressure readings

•Conductivity trends stabilize

Special Requirements for Pressurization Before Operation

1. Pressurization Must Be Gradual and Stepwise

Sudden pressurization is one of the most common causes of membrane damage.

Best practice:

•Increase pressure slowly over 5–10 minutes

•Monitor differential pressure and flow continuously

•Avoid pressure spikes or oscillations

This protects membranes from:

•Mechanical shock

•Element telescoping

•Seal damage

2. Maintain Adequate Crossflow During Pressurization

During pressure ramp-up:

•Concentrate flow must be sufficient

•Concentrate valve should not be closed too early

This ensures:

•Uniform flow distribution

•Prevention of localized concentration polarization

•Reduced fouling risk

3. Avoid High Recovery During Initial Pressurization

Initial startup recovery should be lower than design recovery.

Typical recommendation:

•Start at 50–70% of design recovery

•Gradually increase after stabilization

High recovery during startup can cause:

•Rapid scaling

•Uneven membrane loading

•Premature fouling

4. Closely Monitor Key Parameters

Critical parameters to watch during pressurization:

•Feed pressure rise rate

•Differential pressure across membranes

•Permeate conductivity trend

•High-pressure pump current

Any abnormal trend indicates that pressurization should be slowed or stopped immediately.

Additional Best Practices for SWRO Startup

•Ensure complete air removal from pressure vessels

•Verify correct valve positions before startup

•Confirm energy recovery device readiness

•Synchronize high-pressure pump and ERD operation

•Follow membrane manufacturer startup guidelines strictly

Common Startup Mistakes to Avoid

Conclusion

Flushing and pressurization before starting a seawater desalination system are not routine steps but critical protective procedures. Due to high operating pressures and complex feedwater quality, SWRO systems require extended low-pressure flushing, careful air removal, and slow, controlled pressurization.

Following best practices during startup significantly extends membrane life, reduces fouling risk, and ensures long-term system stability.