What are the requirements for inlet water temperature for seawater industrial RO membranes, and what is the mechanism of temperature influence?
Release Time : 2026-01-05
As a core component of seawater desalination systems, the performance of industrial RO membranes is closely related to the influent temperature. Influent temperature directly affects key indicators such as water production, desalination rate, energy consumption, and membrane life by altering the thermal motion of water molecules and solutes, the physicochemical properties of the membrane material, and system operating parameters. In practical applications, the influent temperature must be precisely controlled based on the membrane material characteristics, water quality conditions, and operational objectives to achieve a highly efficient, stable, and economical seawater desalination process.
Industrial RO membranes have clearly defined applicable temperature ranges for influent. Mainstream polyamide composite membranes typically require influent temperatures controlled between 5°C and 45°C, with an optimal operating temperature of approximately 25°C. This range is based on a balance between the thermal stability of the membrane material and separation efficiency: low temperatures may cause membrane pore shrinkage and increased feed viscosity, hindering water molecule permeation; high temperatures may trigger hydrolysis, oxidation, or structural deformation of the membrane material, leading to irreversible degradation of desalination performance. For example, when the water temperature is below 5°C, the activity of water molecules decreases, the osmotic pressure increases, and the permeate flow rate decreases significantly. Conversely, when the water temperature exceeds 45°C, the amide bonds in the polyamide membrane are easily broken, the desalination layer on the membrane surface gradually loosens, the permeate flow of salt ions increases, and the desalination rate decreases accordingly.
The effect of feed water temperature on permeate flow rate shows a significant positive correlation. As the temperature increases, the thermal motion of water molecules intensifies, and their diffusion coefficient and permeation rate increase simultaneously. At the same time, the viscosity of the feed solution decreases, reducing the mass transfer resistance of water molecules on the membrane surface and within the membrane pores. This dual mechanism makes it easier for water molecules to overcome the membrane's adsorption resistance and osmotic pressure, rapidly passing through the porous structure of the reverse osmosis membrane. For example, under constant pressure conditions, for every 1°C increase in water temperature, the permeate flow rate may increase by 2% to 3%. This characteristic is particularly important in low-temperature environments: if the water temperature drops below 10°C, the permeate flow rate may decrease by 30% to 45%, severely affecting the system's treatment efficiency; while in high-temperature environments, increasing the permeate flow rate requires a trade-off between the risk of membrane performance degradation.
Contrary to the trend in permeate flow rate, the effect of influent temperature on desalination rate is generally negatively correlated. Desalination rate depends on the membrane's ability to retain salt ions, and increased temperature alters the movement of both water molecules and salt ions. On one hand, the increased permeation rate of water molecules may slightly improve the desalination rate through a dilution effect; on the other hand, the diffusion coefficient of salt ions typically increases more significantly, leading to a marked increase in their permeation rate through the membrane and consequently, an increase in salt ion concentration on the permeate side. In actual operation, the latter effect often dominates, causing the desalination rate to decrease with increasing temperature. For example, under conditions of high influent salinity, a 1°C increase in water temperature may decrease the desalination rate by 0.1% to 0.2%, posing a challenge for scenarios with stringent permeate quality requirements (such as the preparation of electronic-grade pure water).
Influent temperature also indirectly affects operational efficiency and stability by influencing system pressure and recovery rate. In constant flow operation mode, increased water temperature leads to increased permeate flow, and if the operating pressure is not adjusted, the recovery rate will increase accordingly. However, excessively high recovery rates may lead to excessive salt concentrations in the concentrate, accelerating membrane scaling or fouling and threatening the long-term stable operation of the system. Conversely, as water temperature decreases, permeate production declines. To maintain the designed recovery rate, the operating pressure needs to be increased, but excessive pressure may increase the risk of mechanical damage to the membrane. Therefore, pressure and recovery rate need to be dynamically adjusted according to water temperature changes. For example, at high temperatures, the pressure can be appropriately reduced to decrease salt permeation, and at low temperatures, the recovery rate can be reduced to increase the membrane surface flushing flow rate.
Different materials used in seawater industrial RO membranes exhibit varying temperature tolerances, requiring tailored control strategies. Polyamide membranes, due to their excellent desalination performance and chemical stability, have become the mainstream choice for seawater desalination, but their upper temperature limit is typically no more than 45°C. While cellulose acetate membranes have poorer temperature resistance (optimal operating temperature is 20°C to 30°C), they are still used in certain specific scenarios. For high-temperature environments, a "cooling + diversion" strategy can be adopted, using cooling towers or coolers to lower the water temperature below 40℃, and installing bypass diversion devices to stabilize the inlet water temperature. For low-temperature environments, a "preheating + insulation" combination is preferred, using plate heat exchangers or tubular heaters to raise the water temperature to 15℃ to 25℃, while simultaneously insulating the membrane housing and piping.
Temperature changes also alter the type and degree of membrane fouling, requiring optimized fouling control measures. At low temperatures, the feed viscosity increases, and the settling speed of colloids and suspended solids accelerates, easily forming a clogging layer on the membrane surface. This necessitates improving pretreatment precision (e.g., upgrading the security filtration precision from 5μm to 1μm) and increasing backwashing frequency. At high temperatures, increased recovery rates lead to excessive concentrations of scaling substances such as calcium, magnesium, and silicon in the concentrate, while microbial reproduction accelerates. Therefore, precise dosing of scale inhibitors is required (increasing the dosage by 20% to 30% at high temperatures) and enhanced sterilization treatment (using a combination of oxidizing and non-oxidizing bactericides). In addition, regular membrane cleaning (shortened to once every 1 to 3 months under high-temperature conditions) and selection of appropriate cleaning agents according to the type of fouling (acidic cleaning agents for scaling and alkaline cleaning agents for biological fouling) can effectively extend the service life of the membrane.