Typical Operating Pressure of Seawater Desalination RO Systems: Range and Reasons| Insights by AQUALITEK

Introduction

Compared with conventional brackish water reverse osmosis systems, seawater desalination RO (SWRO) systems operate at exceptionally high pressures.

For many newcomers to desalination projects, a common question is:

Why does a seawater RO system require such high operating pressure, and what is the normal pressure range?

Understanding this is critical for:

•Proper system design

•Equipment selection

•Energy consumption control

•Long-term membrane protection

This article provides a clear and practical explanation from both theoretical and engineering perspectives.

1. Typical Operating Pressure Range of Seawater RO Systems

Under normal design and operating conditions, the typical operating pressure of a seawater desalination RO system is:

55–70 bar (800–1,000 psi)

Typical reference ranges

•Open-ocean seawater (≈35,000 mg/L TDS):
60–70 bar

Coastal seawater with slightly lower salinity:
55–65 bar

•Warm seawater (higher temperature):
Slightly lower pressure

•Cold seawater (lower temperature):
Slightly higher pressure

This pressure range is 5–10 times higher than that of most brackish water RO systems.

2. The Fundamental Reason: High Osmotic Pressure of Seawater

What Is Osmotic Pressure?

Osmotic pressure is the natural pressure required to prevent water from flowing across a semi-permeable membrane from low salinity to high salinity.

For seawater:

•TDS ≈ 35,000 mg/L

•Osmotic pressure ≈ 26–28 bar

Key principle of RO operation

To produce permeate, the applied pressure must:

Exceed the osmotic pressure + provide net driving force

In practice:

•Required net driving pressure: 20–30 bar

•Total operating pressure: 55–70 bar

This is the primary physical reason for the high pressure.

3. High Salt Rejection Requirements Increase Pressure Demand

Seawater RO systems typically require:

•Salt rejection ≥ 99.7%

•Boron rejection control

•Stable permeate conductivity

To achieve this:

•Higher transmembrane pressure is needed

•Membrane flux must be carefully controlled

Lower pressure would result in:

•Insufficient water production

•Elevated permeate salinity

•Failure to meet drinking or industrial standards

4. Membrane Flux Limitations and Productivity

Unlike brackish water membranes, seawater membranes:

•Have lower permeability

•Are designed for high rejection, not high flux

To achieve reasonable production capacity:

•Pressure must compensate for lower permeability

•High pressure ensures acceptable flux rates without overloading the membrane

Thus, high pressure is a design necessity, not an inefficiency.

5. System Recovery Rate Also Drives Pressure Higher

Seawater RO systems usually operate at:

•Recovery rates of 35–45%

As recovery increases:

•Salt concentration on the concentrate side rises

•Osmotic pressure increases along the membrane

•Additional pressure is required to maintain flux at the tail end

This phenomenon makes pressure staging and high pump pressure unavoidable.

6. Pressure Losses Throughout the System

Not all applied pressure reaches the membrane effectively.

Pressure losses occur due to:

•Pretreatment filters

•High-pressure piping

•Valves and fittings

•Energy recovery devices

To ensure sufficient pressure at the membrane inlet, the pump discharge pressure must be even higher, further pushing the system into the 60–70 bar range.

7. Comparison with Brackish Water RO Systems

This comparison highlights why seawater RO is fundamentally a high-pressure technology.

8. Engineering Implications of High Operating Pressure

Because of the high pressure, SWRO systems require:

•High-pressure rated pumps

•Pressure vessels rated ≥ 1,000 psi

•Duplex or super duplex stainless steel piping

•Energy recovery devices to control operating costs

•Strict safety and protection measures

High pressure defines every aspect of SWRO system design.

Conclusion

The typical operating pressure of a seawater desalination RO system ranges from 55 to 70 bar, and this pressure level is unavoidable.

It is driven by:

•High osmotic pressure of seawater

•Strict salt rejection requirements

•Low membrane permeability

•System recovery design

•Pressure losses across the system

Rather than being a drawback, this high pressure is the core enabling condition that makes modern seawater desalination possible—especially when combined with efficient energy recovery technologies.