Effects of feed water temperature on separation performance and organic fouling of brackish water RO membranes

Data are presented from a series of laboratory experiments designed to evaluate separation performance and humic acid fouling for two RO membranes treating brackish water at temperatures of 15, 25, and 35°C. A model is proposed that considers changes only to solute and solvent properties. Model results and experimental data confirm that solute concentration polarization decreases, while water permeability and salt permeability increase with temperature. Overall, higher feed water temperature reduces energy consumption and observed salt rejection to a greater extent than that predicted by the model, which suggests that RO transport models must be developed for predicting changes in solution and membrane properties. At all temperatures, salt rejection increases after introduction of humic acid, probably due to membrane modification and sealing of thin film defects. Humic acid colloidal size decreases with increasing temperature. At higher temperatures, humic acid rejection is lower (as measured by TOC) due to changes in the membrane structure and in the fraction of dissolved TOC. Flux decline is much more severe at 15°C, but practically identical at 25 and 35°C. The mass of humic acid accumulated on the membranes is essentially the same at all temperatures, but specific cake resistance increases as temperature decreases; hence, specific cake resistance increases with increasing humic acid colloid size. These results have important implications for engineering efforts to mitigate membrane fouling and reduce energy consumption in brackish water desalination.