Field Studies Showing Effects of Membrane Distillation Operations On Membrane Fouling

Membrane distillation (MD) is one of the desalination technologies that has been extensively investigated in recent years. However, its industrial application has not been achieved yet and only few pilot systems are found [1-7]. There is a general lack of information on how these systems performed during the time of operation and the problems encountered. There is also a lack of data on membrane lifetime and the associated cleaning downtimes and replacement protocols. This information is relevant for the economical assesment and the final implementation of the technology [8]. The membranes used for MD purposes are perceived to be chemically stable and less likely to undergo scaling processes, due to their hydrophobic nature and the MD process itself, during which the membranes are mere supporters of the gas phase. However, distillate quality deterioration has been reported in some solar-powered MD systems, which operate under intermitent conditions. One of the few systems that have been fully described in literature is the MD-solar pilot plant in Plataforma Solar de Almeria (PSA), Spain. That pilot plant has been used to test four different MD modules from different companies including the AGMD flat sheet membrane modules developed by Scarab, AB for more than one year [2,5]. During that time, some operational issues arose including a continous worsening of the distillate quality [2]. Guillen et al. [9] showed how the current operation practices of many solar-MD systems are harmful to the membranes, exacerbating wetting processes. However, ageing processes under real operational conditions and cleaning strategies are yet to be reported.

 This work shows the findings of the autopsy performed on MD membranes obtained from field operation at the PSA facility and has two main objetives: 1) to characterize and assess the damage endured by the membranes during the long term operation at the pilot system and understand its relationship with the distillate quality worsening, and 2) to assess different cleaning strategies, analyze the effect of each of them on the membrane performance and finally select the optimal cleaning protocol for further application in MD systems.

Data regarding relevant morphological characteristics and membrane features for the MD process, such as; contact angle, pore size distribution, morphology, porosity, liquid entry pressure (LEP) and wettability, mechanical strength, etc., and their relationship with the membrane performance under MD operation are discussed and analyzed. The morphology of the fouled membranes was also investigated using imaging techniques such as SEM and EDS. Different cleaning strategies were performed and assesed at a lab-scale and, under similar propotcols, in the actual pilot system. Best cleaning procedure was applied in the pilot plant.

References

[1] J. Koschikowski, M. Wieghaus, M. Rommel, Solar thermal-driven desalination plants based on membrane distillation, Desalination (2003) 156: 295-304.

[2] E. Guillén-Burrieza, J. Blanco, G. Zaragoza, D.-. Alarcón, P. Palenzuela, M. Ibarra, et al., Experimental analysis of an air gap membrane distillation solar desalination pilot system, J.Membr.Sci. (2011) 379: 386-396.

[3] R.G. Raluy, R. Schwantes, V.J. Subiela, B. Peñate, G. Melián, J.R. Betancort, Operational experience of a solar membrane distillation demonstration plant in Pozo Izquierdo-Gran Canaria Island (Spain), Desalination (2012) 290: 1-13.

[4] F. Banat, N. Jwaied, M. Rommel, J. Koschikowski, M. Wieghaus, Desalination by a "compact SMADES" autonomous solarpowered membrane distillation unit, Desalination (2007) 217: 29-37.

[5] E. Guillén-Burrieza, G. Zaragoza, S. Miralles-Cuevas, J. Blanco, Experimental evaluation of two pilot-scale membrane distillation modules used for solar desalination, J.Membr.Sci. (2012) 409–410: 264-275.

[6] R.B. Saffarini, E.K. Summers, H.A. Arafat, J.H. Lienhard V, Economic evaluation of stand-alone solar powered membrane distillation systems, Desalination (2012) 299:  55-62.

[7] R.B. Saffarini, E.K. Summers, H.A. Arafat, J.H. Lienhard V, Technical evaluation of stand-alone solar powered membrane distillation systems, Desalination (2012) 286: 332-341.

[8] M. Khayet, Solar desalination by membrane distillation: Dispersion in energy consumption analysis and water production costs (a review), Desalination (2013) 308: 89-101.

[9] E. Guillén-Burrieza, R. Thomas, B. Mansoor, D. Johnson, N. Hilal, H. Arafat, Effect of Dry-out on the Fouling of PVDF and PTFE Membranes under Conditions Simulating Intermittent Seawater Membrane Distillation (SWMD), J.Membr.Sci. In Press, Accepted Manuscript, Available online 21 March 2013.