1. Introduction

When the weather heats up, reverse osmosis (RO) membranes can become a bit "temperamental." With the same operating parameters, you may notice fluctuations in water output, pressure drop, and salt rejection as soon as summer arrives. Rising temperatures directly affect water viscosity and membrane permeability, while also promoting microbial growth and faster fouling. If not addressed proactively, you could face water quality issues, reduced permeate flow, increased pressure drop, membrane clogging, and even unplanned shutdowns. Here are four key practical tips for operating RO membranes during hot weather – we hope you find them helpful.

2. Four Key Points

2.1 Higher temperature and higher permeate flow – not necessarily good

A common rule: for every 1°C increase in water temperature, the permeate flow of an RO membrane increases by about 2–3%. 

This might seem like a "bonus," but it carries hidden risks. Higher flow means more water passing through the membrane area, which also speeds up the migration of colloids, organic matter, and microorganisms toward the membrane surface. This worsens concentration polarization and significantly accelerates membrane fouling. Worse, prolonged high-temperature operation can accelerate membrane aging and shorten its useful life. Therefore, summer operation should not solely pursue high water output. Instead, actively record the feed water temperature, adjust operating pressure or permeate flow accordingly, and keep the flux within a reasonable range. If the feed water temperature exceeds 35°C, consider using cooling equipment or appropriately reducing the system recovery rate to reduce the load on each membrane element.

2.2 Hot weather accelerates microbial growth

High temperatures are a "golden period" for microbial growth. Bacteria and algae multiply rapidly in water temperatures between 25°C and 40°C. If the pretreatment disinfection is insufficient, biofouling is almost inevitable. Early signs include a slow increase in the first-stage pressure drop, followed by a gradual decline in permeate flow. In severe cases, you may notice a fishy smell when opening the pressure vessel, and a slimy biofilm on the membrane surface. 

To address this, strengthen disinfection in the pretreatment stage, for example, by shock-dosing non-oxidizing biocides (compatibility confirmed) to control the total bacteria count. Closely monitor pressure drop changes. If the pressure drop rises faster than usual, even if salt rejection hasn't yet changed significantly, consider shortening the chemical cleaning cycle to prevent biofilm from deeply penetrating the membrane pores.

2.3 Keep daily data records – more reliable than intuition

Sound data support is essential for membrane system operation in hot weather. It is recommended to record key parameters at least once a day: feed water temperature, permeate flow, salt rejection, and pressure drop per stage, building a continuous data log. A single day's data may not tell much, but comparing it with historical data can reveal early anomalies. For example, if salt rejection remains stable but the first-stage pressure drop has been rising slightly for several days, it may indicate that foulants are accumulating on the membrane surface. 

If you have a SCADA system, try creating a temperature-performance curve to visually see the relationship between water temperature and permeate flow/pressure drop. Even without such a system, manual records are still very useful. Early detection and timely intervention are far better than dealing with obvious performance deterioration.

2.4 Chemical cleaning frequency and formulas need adjustment in summer

High temperatures enhance both the adhesion of foulants and chemical reaction rates, so the cleaning protocol used in spring/autumn may no longer be sufficient. Generally, if the pressure drop rises significantly faster than in cooler seasons, consider shortening the chemical cleaning cycle by 10–20% to prevent excessive accumulation and compaction of foulants. Pay special attention to temperature control during cleaning: the cleaning solution temperature should not exceed 30°C. While higher temperatures help remove organic matter, they also increase the risk of membrane oxidation, especially for polyamide composite membranes, which have limited tolerance to high-temperature alkaline solutions. When using alkaline cleaning agents, strictly control the concentration and circulation time, and do not exceed recommended soak times. After cleaning, rinse thoroughly to avoid residual chemicals. 

A well-adjusted cleaning strategy is key to extending membrane life and ensuring stable summer operation.

3. Conclusion

High temperatures do challenge RO membrane systems, but these challenges are manageable. By adjusting operation strategies in advance – balancing flow control, microbial prevention, data monitoring, and cleaning maintenance – your system can smoothly get through the summer. If you encounter complex or uncertain issues, you can always contact the membrane supplier or a professional service provider – don't try to solve everything alone. Careful daily observation and standard operating procedures are the most reliable guarantees of stable operation.

——Have you ever experienced RO membrane "strike" due to hot weather? Feel free to leave a comment and share your story.