The Self-Cleaning Characteristic of Osmotically Driven Membrane Processes

Osmosis is the spontaneous diffusive transport of water through a selectively permeable membrane from a solution of low osmotic pressure to a solution of high osmotic pressure [1]. In osmosis, the membrane allows passage of water, but rejects almost all solute molecules and ions. From the mid 1970s, there has been growing interest in forward osmosis (FO) and pressure retarded osmosis (PRO), two osmotically driven membrane processes (ODMP) [2, 3].

In both FO and PRO, together with water, trace quantities of salts diffuse through the membrane from the feed solution into the draw solution [4]. Salts also diffuse from the draw solution into the feed solution [5] due to reverse salt diffusion that occurs because of the large difference in solute concentration between the draw solution and the feed solution [6]. Reverse salt diffusion is known to lower productivity and enhance operating costs in ODMPs [4, 5] and may also affect fouling behavior. A few recent investigations have established that FO has lower fouling propensity and/or fouling that is more reversible than in RO processes [7-10] and only a subset of these have addressed the effects of reverse salt diffusion on FO fouling [9, 10]. However, the mechanism of fouling reversibility has not been conclusively stated.

In this study, the link between reverse salt flux and fouling is systematically described and elucidated. It was hypothesized that reverse salt flux could be the cause of the low fouling propensity and high fouling reversibility of ODMPs and that reverse salt flux drives an ion exchange reaction that induces the breakup of the cross-linked foulant layer. Tailored experiments that singled out the role of reverse salt flux on membrane fouling utilizing different draw solutions with different rates of reverse salt diffusion were performed to confirm the hypothesis. It was found that higher reverse salt flux resulted in lower membrane fouling and more reversible fouling. The continuous flux of salt from the draw solution into the feed solution causes the replacement of calcium ions with sodium ions in the alginate layer. Experimental evidence for FO and PRO that supports the salt cleaning mechanism proposed in RO will be reported.

References

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[3] T.Y. Cath, A.E. Childress, M. Elimelech, Forward osmosis: Principles, applications, and recent developments, Journal of Membrane Science, 281 (2006) 70-87.

[4] N.H. Hancock, T.Y. Cath, Solute coupled diffusion in osmotically driven processes, Environmental Science and Technology, 43 (2009) 6769–6775.

[5] A. Achilli, T.Y. Cath, A.E. Childress, Selection of inorganic-based draw solutions for forward osmosis applications, journal of Membrane Science, 364 (2010) 233-241.

[6] M. Mulder, Basic principles of membrane technology, Kluver Academic Publishers, Dordrecht, The Netherlands, 1991.

[7] R.W. Holloway, A.E. Childress, K.E. Dennett, T.Y. Cath, Forward osmosis for concentration of anaerobic digester centrate, Water Research, 41 (2007) 4005-4014.

[8] B. Mi, M. Elimelech, Organic fouling of forward osmosis membranes: fouling reversibility and cleaning without chemical reagents, Journal of Membrane Science, 348 (2010) 337-345.

[9] S. Lee, C. Boo, M. Elimelech, S. Hong, Comparison of fouling behavior in forward osmosis (FO) and reverse osmosis (RO), journal of Membrane Science, 365 (2010) 34-39.

[10] S. Zou, Y. Gu, D. Xiao, C.Y. Tang, The role of physical and chemical parameters on forward osmosis membrane fouling during algae separation, journal of Membrane Science, 366 (2011) 356-362.