- Design temperature of RO membranes
Most membrane manufacturers such as Dow FilmTec, Toray, and Hydranautics test and rate their membranes at 25°C. At this temperature the RO system delivers:
• Maximum salt rejection (typically 99.5%–99.7%).
• Optimum energy consumption.
• Permeate flow according to design.
- Impact of high feed water temperature
When feed water temperature rises above 25°C:
• Water viscosity decreases, leading to higher diffusion of salts and lower rejection.
• Permeate conductivity increases noticeably.
• Above 45°C, polyamide degradation occurs, and the membrane suffers permanent damage.
- Impact of low feed water temperature
During winter, feed water can drop to 18°C or even 10°C:
• Viscosity increases, reducing permeate flow by approx. 3% for every degree below 25°C.
• At 18°C, permeate flow is reduced by around 20%.
• At 10°C, the reduction can reach up to 45%.
• Higher pump pressure is required, which significantly increases energy consumption.
- Can this be compensated with a larger pump?
Yes, it is possible:
• By installing a larger capacity pump.
• Or by using a VFD (Variable Frequency Drive) to increase pump speed.
- However, this is only a temporary solution and has disadvantages:
• Much higher energy consumption.
• Higher risk of fouling and scaling due to increased flow velocities.
• Exceeding the maximum operating pressure can cause permanent membrane damage.
- The ideal solution: Heat Exchanger before the RO
The best approach is to install a Heat Exchanger directly after pre-treatment and before the RO in order to:
• Stabilize feedwater temperature at 25 °C ± 2 °C.
• Maintain the system’s design performance.
• Reduce energy consumption and extend membrane lifetime.
• Prevent problems caused by seasonal temperature variations in the feedwater.