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What Happens to Reverse Osmosis Membranes in Hot Weather?
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Release Date:
2026-06-29
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I. Introduction
As summer arrives, many reverse osmosis (RO) membrane systems start to "act up" – some plants notice a sudden increase in permeate flow, others see slight fluctuations in salt rejection, and still others observe a gradual rise in differential pressure. On the surface, these changes may look like system malfunctions, but in fact, a considerable portion of them are related to rising ambient temperatures.
When temperature increases, RO systems undergo changes on three levels: first, the properties of water itself change – the most direct effect being viscosity variation, followed by changes in the solubility of sparingly soluble salts; second, the membrane material responds to temperature; and third, system operating data "appear different" due to temperature effects. In other words, the parameter variations seen in summer do not necessarily indicate performance degradation – they may simply be normal responses to temperature.
Only by understanding the underlying causes of these changes can we accurately determine which fluctuations are within normal ranges and which have exceeded reasonable limits, requiring timely intervention and adjustment.
II. Effect of Temperature on Water: Lower Viscosity, Higher Flux
As temperature rises, the viscosity of water decreases noticeably, water molecules become more mobile, and the resistance through the membrane surface diminishes. Generally, for every 1°C increase in water temperature, permeate flux increases by about 2%–3%.
Under the same operating pressure, RO membrane permeate flux typically increases, so a rise in system output during summer is very common.
However, this is not entirely "free production gain." As flux increases, the transport of salts, colloids, organic matter, and other foulants toward the membrane surface also accelerates, exacerbating concentration polarization and raising the risk of fouling. At the same time, temperature changes may alter the dissolution equilibrium of some sparingly soluble salts, increasing the tendency for scaling.
III. Effect of Temperature on the Membrane Itself: Material and Performance Changes
RO membranes are not completely static materials. As temperature rises, the polyamide separation layer on the membrane surface undergoes certain thermal responses, such as slight swelling, changes in hydration state, and alterations in surface charge distribution. These changes may further affect the mass transport characteristics of the membrane.
Typically, with increasing temperature, the diffusion rate of salts through the membrane also accelerates, so the system's salt rejection may show a slight decline. Within a reasonable temperature range, this change is usually reversible; when the operating temperature returns to normal, membrane performance gradually recovers as well. However, if the membrane is exposed to excessively high temperatures for prolonged periods, or exceeds the maximum allowable operating temperature specified by the membrane element, structural damage to the membrane material may occur, leading to irreversible performance loss.
IV. Effect of Temperature on Operating Parameters: Temperature Compensation of Data
In the management of RO system operations, temperature not only affects actual performance but also influences how we interpret data. The most typical example is permeate flow: since higher temperatures naturally enhance flux, production data from different seasons cannot be directly compared. They usually need to be normalized to standard conditions at 25°C – this is what is commonly referred to as temperature correction or temperature compensation.
The same permeate flow rate might simply be due to high water temperature and low viscosity in summer, whereas maintaining the same flow in winter would typically indicate higher applied pressure or better membrane condition. If temperature factors are ignored, operators might mistake normal seasonal variations for membrane performance improvement or deterioration, and may also overlook real fouling, scaling, or membrane damage issues.
V. Conclusion
In hot weather, RO membrane systems do undergo a series of changes: the flow characteristics of water change, the membrane material responds, and operating parameters appear differently. But these changes do not necessarily mean system abnormalities – many of them are regular, predictable, and manageable.
For operations management, the key is not to "panic at every change," but to understand exactly what temperature has altered. Only by understanding the underlying mechanisms and making judgments based on temperature-corrected data can we avoid overreacting to normal fluctuations and also more promptly identify issues that truly need attention.
If you would like to know specific measures for coping with high temperatures, you may refer to our previously published article "4 Key Points for RO Membrane Operation in Hot Weather."
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