Evaluating the viability of double-skin thin film composite membranes in forward osmosis processes

Internal concentration polarization (ICP) and membrane fouling are two major factors impairing the performance of forward osmosis membranes. Their relative severity differs in the two operating modes of forward osmosis. The PRO mode (where the skin layer is facing the draw solution) is less susceptible to ICP, and as a result, produces a higher water flux than the FO mode (where the skin layer is facing the feed solution). Membrane fouling is severe in the PRO mode, resulting in the decline of membrane performance with time. Membrane cleaning is also more difficult in the PRO mode. There have been suggestions to add another skin layer to alleviate membrane fouling. The effects of the sandwiched membrane design on normal operations other than fouling abatement have yet to be systematically examined. In this study, the forward osmosis performance of double-skin membranes was evaluated and compared with the single-skin membranes by both theoretical calculations and experimental measurements. The results from a series of membranes with different transport properties suggested that the double-skinned membranes are not superior to the single-skinned membranes in any aspect other than their low fouling properties. Even for forward osmosis applications with serious fouling, the single-skinned membranes operating in the FO mode can still be a better alternative.