The effect of de-gassing on the efficiency of reverse osmosis filtration

Water permeated porous membranes, under a large hydrostatic pressure differential, are likely to create opportunities for vapour and dissolved gas cavitation, which will restrict water flow through the membrane void spaces. The formation of cavities within the voids may be facilitated by the presence of dissolved gases in water. Removal of these gases might, therefore, reduce or prevent cavitation and thereby increase the flow rate through the porous membranes used to purify water. Reverse osmosis (RO) membranes operate under high pressure differentials and will be very susceptible to cavitation reduced flow. It is well established that RO membranes have a lower water flux than expected and the reasons for this have yet to be identified. This study presents direct experimental evidence, obtained with a commercial RO unit, in support of an average increase in product flow-rate of up to 3-5%, on de-gassing the feed water, consistent with the proposed cavitation model.