Performance, energy and cost evaluation of a nanofiltration plant operated at elevated temperatures

The influence of temperature on cost and energy consumption during nanofiltration has been evaluated. The effect of temperature on pressure drop, fouling, concentration polarisation and changes in the membrane performance were evaluated experimentally and theoretically. Nanofiltration experiments using pure solutions and wastewater from the meat industry were performed at 25 and 55 °C with an Alfa Laval NFT-50 nanofiltration membrane. The results showed that the flux was slightly higher during wastewater concentration at 55 than at 25 °C. The difference was smaller than expected and is explained by both fouling and changes in the membrane performance due to the combined effects of pH and temperature. The concentration polarisation was found to be less at the higher temperature, as predicted by theory. However, this effect was small in comparison to the effect of temperature on fouling and the membrane performance. The influence of cleaning procedure on flux and retention was very dependent on the processing temperature due to time- and temperature-dependent changes in the membrane performance.Nanofiltration plants designed to run at 25 and 55 °C were dimensioned based on the results of the pilot study. Nanofiltration at 55 °C was found to be slightly better with respect to the investment cost. The energy required for pumping was lower at 55 °C. However, the energy required for heating is high and processing at 55 °C is only advantageous if the feed is already warm, if there is excess heat in the factory, or if the heat can be recycled.