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The potential for cavitation within reverse osmosis (RO) membranes was investigated by studying the effects of feed water degassing on RO desalination. Initial experiments, conducted using a laboratory-scale RO desalinator, and a range of NaCl feed solutions, showed a permeate flux increase of 3-5%, on average, upon feed solution degassing. These positive results led to the design and construction, with commercial support, of an industrial pilot-scale RO desalination system. Industrially prepared seawater feed was used, and operating conditions were chosen to reproduce, as closely as possible, those of industrial seawater RO installations. Upon pressure release by restricted flow, from 10-60bar to ambient pressure, cavitation was always observed when dissolved gases were present, but never with a degassed feed. Several RO membranes were found to be slightly hydrophobic, some with considerable surface heterogeneity, which supported the view that cavitation may occur within RO membranes. However, no permeate flux enhancement was observed with the pilot system, at degassing level of about 99%, for feed pressures in the range 38-55bar. In a study of osmotic distillation, it was found that the water flux, from pure water to NaCl solutions at various concentrations, correlated almost perfectly with the variation in solution vapour pressure with salt concentration. This process may be useful for the direct experimental measurement of osmotic pressures, and the determination of partial molar volumes of water in aqueous solutions. Simple asymmetric glass filter membranes were fabricated by fusing successive layers of different sized Ballotini beads. The filters were found to have predictable size-exclusion properties and good back-flushing tolerance. The simple fabrication process required only cheap, common resources. The filters proved capable of filtering a highly fouling suspension of organic waste in water, and recovered their initial flow rate upon back-flushing. Sever

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.

Degassing of nonaqueous solvents is useful for their applications in chemical synthesis and in maintaining purity (against oxidative degradation) during long term storage. In addition, degassed solvents have been found to be of value in the production of

It is demonstrated that de-gassed water is more effective at dispersing hydrophobic "dirt", such as liquid hydrocarbons or oils. This effect appears to be due to the reduction of natural cavitation, which would otherwise oppose the dispersion of hydrophobic liquid droplets into water. De-gassing of the oil enhances this effect still further, and this has led to a proposal for a novel cleaning process, based on using a combination of a de-gassed (hydrophobic) solvent followed by rinsing in de-gassed water. This method might be useful as an effective, detergent-free cleaning process. Also reported are some initial studies which suggest that the effect of "inert" dissolved gases on the electrical conductivity of water may need to be reconsidered.