How does feed water temperature primarily affect the permeate flux in an RO system, assuming constant pressure?

Feed water temperature primarily affects the permeate flux in an RO system by changing the viscosity of the water. Permeate flux refers to the rate at which pure water passes through the RO membrane, typically measured in liters per square meter per hour (LMH) or gallons per square foot per day (GFD). As temperature increases, the viscosity of water decreases. Viscosity is a measure of a fluid's resistance to flow. Lower viscosity means that water can flow more easily through the membrane pores. Therefore, at a constant pressure, a warmer feed water temperature will result in a higher permeate flux because water experiences less resistance as it passes through the membrane. Conversely, a colder feed water temperature will increase the viscosity of water, making it more difficult to pass through the membrane, and thus reducing the permeate flux. This relationship is generally linear within typical RO operating temperature ranges. For example, a feed water temperature increase from 20°C to 30°C could result in a significant increase in permeate flux, assuming the pressure remains constant. This effect must be accounted for in RO system design and operation. Normalizing permeate flux data to a standard temperature (usually 25°C) is common practice to compare membrane performance accurately over time, regardless of temperature fluctuations.