The Potential of Membrane Contactors in the Pre-Treatment and Post-Treatment Lines of a Reverse Osmosis Desalination Plant

The flexibility of membrane contactors (MCs) is highlighted for a reverse osmosis (RO) desalination plant. MCs are applied as pre-treatment for the oxygen removal and the pH reduction of seawater, also as post-treatment for the pH increase of the RO permeate and the reduction of the RO brine volume. A decrease of the seawater pH down to neutral values, as needed when coagulation is used in the pre-treatment line of RO, together with an increase of the RO permeate pH up to 7.58, matching the target of produced water, can be obtained without the use of chemicals. Direct Contact Membrane Distillation (DCMD) and Vacuum Membrane Distillation (VMD) are investigated as function of the feed concentration (ranging from 40 g/L to 80 g/L) and temperature (40 °C-80° C). Their performance is compared at parity of operating conditions and in terms of applied driving force. Both distillation systems are able to efficiently reject salts (rejection > 99.99%), while higher distillate fluxes are obtained when a vacuum is applied at the permeate side (15 kg/mh vs. 6.6 kg/mh for the 80 g/L feed).