Reverse Osmosis (RO) systems play a crucial role in providing purified water worldwide, from the icy conditions of Antarctica to the tropical heat of the equator. Whether installed on cruise liners, offshore platforms, or industrial plants, these systems operate under a wide range of environmental conditions. One often overlooked, yet critical, factor affecting performance is the impact of water temperature on reverse osmosis systems.
Key Effects of Water Temperature on RO Systems
Water temperature directly influences the efficiency, performance and longevity of RO systems. The impact of water temperature on reverse osmosis systems can be observed in two primary ways:
1. Water Viscosity
The viscosity of water is directly affected by temperature. In general, higher temperatures reduce water viscosity, while lower temperatures increase it. This phenomenon influences the rate at which purified water is produced, as less viscous feed water passes through RO membranes more easily, whereas more viscous water (at lower temperatures) encounters greater resistance.
Since RO systems rely on pressure to drive water through membranes, changes in viscosity can also affect the system’s pumping requirements. The table below outlines the possible effects:
| Feed Temperature | Purified Water Produced | Pressure Required |
| High | Increase ↑ | Unchanged ↔ |
| High | Unchanged ↔ | Decrease ↓ |
| Low | Decrease ↓ | Unchanged ↔ |
| Low | Unchanged ↔ | Increase ↑ |
2. Membrane Pores
The most common RO membrane elements are spiral-wound thin-film composites made from polyamides. A key distinction between RO membrane elements and other filtration types, such as, NF, UF and MF, lies in their pore sizes, which range 0.0001 to 0.001 microns.
Membrane pore sizes tend to expand at high temperatures and contract in colder conditions, alongside other structural changes such as variations in tortuosity and overall integrity. When pores expand due to higher temperatures, water permeability increases, allowing feed water to pass through the membrane more easily. While this reduces pressure requirements, it comes at the cost of lower salt rejection.
On the contrary, at lower temperatures, the pores contract, leading to finer filtration. This improves salt rejection and enhances water quality but requires higher pressure to overcome osmotic resistance.
RO membranes are designed to function optimally within a specific temperature range, typically between 1°C and 45°C. For example, DuPont’s Thin-Film Seawater membranes operate within this range. Although the manufacture sets a low limit Salt Separation Services has experience of operating RO membranes at temperatures below 0°C, Arctic and Antarctic waters. Sea water is still liquid at negative temperatures but feed and high pressure pumps impart significant energy into the water increasing the temperature above the freezing point of the purer product water. This allows for the production of drinking water in these regions with this technology. When temperature increases beyond the threshold, structural degradation can occur overtime, reducing membrane lifespan as well as potentially causing reduced permeate flow due to compaction, or possibly intrusion of the membrane composite into the permeate carrier.
| Feed Temperature | Pore Size | Salt Rejection | Pressure Required |
| High | Increase ↑ | Decrease ↓ | Decrease ↓ |
| Low | Decrease ↓ | Increase ↑ | Increase ↑ |
Optimising RO Systems for Temperature Variations
To mitigate the impact of water temperature on reverse osmosis systems, it is crucial to account for temperature fluctuations and implement proactive measures:
- Conditioning Feed Water: In colder environments, heat exchangers, calorifiers or blending techniques can warm feed water to improve efficiency. In hot environments, cooling methods may be necessary.
- Regular Monitoring: Installing temperature sensors and conducting routine system checks, can help detect performance variations and prevent potential issues.
- Membrane Selection: Choosing the most suitable membranes for specific applications and environmental conditions will ensure optimal RO system operation.
Stay Ahead with Proactive RO Management
Understanding the impact of water temperature on reverse osmosis systems allows users to make informed adjustments, ensuring optimal performance, efficiency and longevity. At Salt Separation Services, we specialise in designing and delivering customised RO solutions that reliably meet your water purification needs, no matter the environmental conditions.
Have questions? Contact Salt Separation Services today at sss@saltsep.co.uk to learn more.