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The factors governing the formation of voids and large cavities in a wide variety of membranes made from different polymers cast from different solvents and precipitated in various nonsolvents have been studied. It has been shown that the formation of large voids in membranes can be eliminated by (1) lowering the tendency of the nonsolvent to penetrate into the casting solution or (2) increasing the viscosity of the cast solution or creating a thick gel layer on top of this cast solution. It is suggested that the formation of large finger-like cavities in membranes originates from convective flows formed within the cast (fluid) polymer solution upon its immersion in the bath of nonsolvent for final precipitation. It is also shown that the driving forces leading to the formation of these convective flows are not density gradients.

ab s t r ac tSolar-driven reverse osmosis desalination can potentially break the dependence of conventional desalination on fossil fuels, reduce operational costs, and improve environmental sustainability. The experience with solar desalination is investigated based on the analysis of 79 experimental and design systems worldwide. Our results show that photovoltaic-powered reverse osmosis is technically mature and -at unit costs as low as 2-3 US$ m -3 -economically cost-competitive with other water supply sources for small-scale systems in remote areas. Under favourable conditions, hybrid systems with additional renewable or conventional power sources perform as good as or better than photovoltaic-powered reverse osmosis. We suggest that in the short-term, solar RO desalination will gain shares in the market of small-scale desalination in remote areas. Concentrating solar power technologies have the highest potential in the medium-term for breakthrough developments in large-scale solar desalination.

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