Nanofiltration at elevated temperatures: Influence of the combined effects of temperature, salts, pH and cleaning

Sammanfattning: The influence of temperature, salts, pH, and various pre-treatments and cleaning on the nanofiltration (NF) performance of thin-film polyamide (PA) composite membranes has been investigated. The change in membrane performance was correlated to membrane swelling or/and electroviscous friction. Aspects of costs and energy demand while using NF at elevated temperatures were also evaluated. The increased water permeability and decreased retention due to membrane swelling in the presence of salt was found to be strongly dependent on the temperature. The increased swelling/relaxation was explained by higher polymer flexibility at an increased temperature, while at a lower temperature the structure could appear to be frozen. Both mono- and divalent salts were found to swell the membranes. Swelling increased with increasing KCl concentration, which resulted in an increase in water permeability and a decrease in retention. Increasing the pH in the range 5-9 in the presence of KCl increased the membrane swelling. In the absence of KCl there was no indication of membrane swelling when increasing the pH. Membrane swelling and electroviscous friction counteract each other; electroviscous friction was found to be important at low KCl concentrations and swelling at high ones. The influence of acidic and alkaline cleaning during NF with 20 mM KCl at pH 6 was evaluated. It was found that aromatic PA NF/RO membranes were less influenced by cleaning than the semi-aromatic PA ones. For aromatic PA membranes, the most stable NF conditions for elevated temperatures were achieved after a final alkaline cleaning step, compared with a final acidic cleaning step for semi-aromatic membranes. Due to hysteresis, the retention can increase, decrease or remain unaffected at increased temperature. However, for a stable membrane the retention always decreased at increased temperature. The influence of acidic and alkaline cleaning on membrane performance was evaluated during NF of acidic wastewater, and it was found that a higher initial flux is not necessarily better for long-term performance. Due to an increase in fouling with increasing temperature the flux and retention during NF at 25 and 55°C were found to be approximately the same. Based on these findings, the difference in energy consumption during the treatment of wastewater in a spiral wound membrane module configuration was evaluated. It was found that it is advantageous to use NF at an elevated temperature if the feed is already warm, or if the feed can be heated by waste heat.