Effect Of A Spacer On Concentration Polarization During Forward Osmosis Process

As a phenomenon inherently associated with membrane separations, concentration polarization has long been identified as a major problem that deteriorates the real performance of its water flux. Especially for forward osmosis (FO) process, which uses the asymmetric cellulose triacetate membrane as the separation, as a result, there will occurred not only external concentration polarization (ECP) at the surface of the active layer, but also internal concentration polarization (ICP) at the interface of support layer. Based on such condition, it’ s very urgent to find a simple and effective method to mitigate the concentration polarization (especially the ICP).In this study, by placing the metal mesh spacer in the specific location (feed or draw channel) of traditional flat membrane module, the turbulence and mass transfer effect of the solution which increased by the metal mesh spacer could finally play the role of mitigating concentration polarization in FO process. Besides, under the condition of metal mesh spacer existed in the membrane module, the relationship between osmotic pressure difference as well as pore size of the metal mesh spacer and the concentration polarization was also investigated. By such investigation, a suitable operation condition which can best exert the mitigation effect of the metal mesh spacer on concentration polarization could be eventually determined.By a series of batch-scale experiments, it was found that the metal mesh spacer which located in the specific area of the feed or draw channel can really mitigate the concentration polarization during the process of FO. When operated in FO mode and utilize0.5M and2M NaCl as the feed and draw solution, metal mesh spacer (pore size:1×1mm) located in the feed channel and far from the membrane as well as in the draw channel and contacted with the membrane can both increase the performance of water flux, the water flux can be increased by5.4%and9%separately when compared with the blank group. Besides, when putting the metal mesh spacer in both the feed and draw channel, the water flux can then be increased by14.4%; When operated in PRO mode, similarly, metal mesh spacer that located in the feed channel and far from the membrane as well as in the draw channel and contact with the membrane can mitigate the concentration polarization, the water flux can be increased by1.5%and2.1%separately. When put the metal mesh spacer in both the feed and draw channel, there will be4.1%water flux increase margin. Besides, in the investigation of the relationship between osmotic pressure difference and concentration polarization, by verifying the concentration of draw solution into1M,2M and4M while making the concentration of feed solution0.5M, it was found that high osmotic pressure difference gain much higher water flux, and much improvement of concentration polarization could be arrived by placing the metal mesh spacer in the channel. For example, when utilize0.5M as the feed solution and1M,2M,4M NaCl as the draw solution, in FO mode, the water flux could be improved by2.94%,9.01%and12.5%respectively; in PRO mode, the water flux could be improved by1.41%,2.19%and8.41%respectively. It was also found that the pore size of the metal mesh spacer (0.4x0.4mm,1.0x1.0mm,3.5×3.5mm) will have some effects on the water flux performance. Too small pore size of the metal mesh spacer will have a negative effect on the real performance of the FO process.