Impact of Winter Seawater Temperature Drop on RO Desalination Water Production and Operating Pressure| Insights by AQUALITEK

Introduction

Seawater reverse osmosis (SWRO) desalination systems are widely used to produce freshwater in coastal and island regions. However, seasonal variations—especially the significant drop in seawater temperature during winter—can strongly impact system performance.

A decrease in seawater temperature directly affects membrane permeability, water viscosity, osmotic pressure, and hydraulic conditions. As a result, water production rate declines while operating pressure and energy consumption rise. Understanding these effects is crucial for system design, operation optimization, and energy efficiency management.

This article provides a comprehensive analysis of how winter seawater temperature drop impacts water production and operating pressure, explores the underlying physical principles, and offers effective operational strategies.

1. Why Seawater Temperature Matters in RO Desalination Systems

RO membrane performance is highly temperature-dependent. Temperature influences:

•Water viscosity

•Diffusion coefficient of water molecules

•Membrane permeability

•Osmotic pressure behavior

In general, higher temperature improves water permeability, leading to higher flux and lower pressure demand. Conversely, lower temperature significantly reduces membrane flux, resulting in reduced water production and higher operating pressure.

2. Effect of Winter Temperature Drop on Water Production

2.1 Decrease in Membrane Flux

As temperature drops, water viscosity increases, reducing water molecule mobility inside membrane pores. This directly leads to lower membrane flux.

Typical performance rule:

For every 1°C decrease, permeate flow decreases by 2–3%.

Example:

•Summer seawater temperature: 25°C

•Winter seawater temperature: 15°C

•Temperature drop: 10°C

•Expected production decline: 20–30%

2.2 Reduced System Water Output

At constant operating pressure:

•Lower temperature → Lower flux → Reduced daily water production

If pressure is not adjusted, total plant capacity can drop significantly, which may lead to supply shortages, especially in municipal desalination projects.

2.3 Lower Recovery Efficiency

Cold seawater also slightly increases osmotic pressure, limiting achievable recovery rate and requiring conservative operation to protect membrane integrity.

3. Effect of Winter Temperature Drop on Operating Pressure

3.1 Increased Hydraulic Resistance

Colder water becomes more viscous, increasing flow resistance across membrane modules and pipelines, resulting in higher feed pressure demand.

3.2 Pressure Compensation Requirement

To maintain the same production rate during winter, operators must increase high-pressure pump output.

Typical pressure compensation:

At 15°C, production capacity drops to only 62% of rated output at 25°C.

6. Engineering Design Strategies for Winter Operation

6.1 Oversized System Design

Design systems based on worst-case winter temperatures, ensuring:

•Adequate membrane area

•Higher pump pressure margin

6.2 Variable Frequency Drive (VFD) Control

VFD pumps allow:

•Dynamic pressure adjustment

•Improved energy efficiency

•Reduced mechanical stress

6.3 High-Permeability Membrane Selection

New-generation SWRO membranes provide:

•Higher flux at low temperatures

•Lower operating pressure

•Better winter performance

6.4 Optimized Pretreatment

Good pretreatment reduces fouling, which is especially critical in winter when low temperatures worsen fouling recovery.

7. Operational Optimization Strategies in Winter

7.1 Adjust Operating Pressure Gradually

Avoid sudden pressure increases to protect membranes and piping systems.

7.2 Optimize Recovery Rate

Operate at slightly lower recovery to maintain system stability and reduce fouling risk.

7.3 Increase Chemical Cleaning Frequency (If Necessary)

Cold water may promote biofouling and scaling. Timely CIP ensures stable performance.

7.4 Energy Recovery Device (ERD) Optimization

Proper ERD tuning helps offset increased energy consumption caused by higher pressure demand.

8. Practical Industry Case Example

A 20,000 m³/day SWRO plant:

•Summer seawater temperature: 27°C

•Winter seawater temperature: 14°C

Results:

•Water production drops by ~25%

•Operating pressure increases from 58 bar → 67 bar

•Energy consumption rises from 3.1 → 3.9 kWh/m³

This clearly demonstrates the strong influence of temperature on desalination economics.

9. Future Technologies for Low-Temperature Desalination

•Ultra-high permeability RO membranes

•Graphene-based membranes

•Advanced pressure exchanger ERDs

•AI-based predictive control systems

These technologies aim to minimize winter performance penalties and improve overall system stability.

Conclusion

The drop in seawater temperature during winter significantly affects SWRO system performance, causing:

•Reduced water production

•Increased operating pressure

•Higher energy consumption

•Lower system efficiency

Through proper system design, membrane selection, energy recovery optimization, and intelligent control strategies, desalination plants can effectively mitigate winter performance challenges and maintain stable, energy-efficient operation year-round.