Membrane distillation for water desalination: how to chose an appropriate membrane?

This work focuses on vacuum membrane distillation (VMD) using hollow-fibre modules for seawater desalination. VMD is an evaporative process using porous and hydrophobic membranes, which physically separate the aqueous liquid feed from the gaseous permeate kept under vacuum. In the case of a salty water, the water passes through the membrane as a vapour without salt and is then condensed outside the module. We had previously shown that for commercially available membranes, operating conditions (velocity, temperature, pressure) and process design could be chosen to minimise energy consumption. VMD operated with these commercial membranes could clearly compete energetically with RO. However, the flux and thus the membrane area to be installed would be higher than for RO. On the basis of computations, the present study shows that for membranes 100 times more permeable than the experimental membranes, and at low temperature, DMV can compete with RO on energy consumption (less than 2 kWh/m(3)) with a same level of permeate flux (between 5 to 15 /h.m(2)). Moreover, if coupled with solar energy and operated at high temperature, the energy to be provided to the system can be maintained at a low level and the permeate flux could be enhanced to 4585 1/h.m(2). On this basis, a discontinuous process coupling VMD and solar energy could clearly compete with RO. Process feasibility is clearly linked to the membrane properties and mainly to their permeability.