Key issues for improving the design and operation of spiral-wound membrane modules in desalination plants

Abstract

Spiral-wound membrane (SWM) modules are the most important components of reverse osmosis and nanofiltration desalination and water treatment plants; their optimum design and operation is crucial for achieving satisfactory membrane plant efficiency associated with reduced product cost and environmental impact. This paper is focused on: (a) the main SWM module design parameters, in need of optimization, which include the geometrical characteristics of retentate-side spacers and the membrane sheet dimensions (for a module of fixed total active surface area); (b) the major operating variables, i.e. the permeate flux and the cross-flow velocity at the retentate side. Membrane element problems, usually encountered in operating plants and affected by the above SWM element parameters, include membrane fouling by various species (commonly organic matter, inorganic colloids, and bio-foulants), membrane scaling due to sparingly soluble salts, increased friction losses and uneven flow distribution in the SWM channels. A brief review is presented of the fluid dynamics and mass transfer in spacer-filled narrow channels, stressing the strong interrelation of SWM design and operating parameters. Moreover, the direct effect of the main SWM design and operating parameters on the above membrane element problems is highlighted and quantified, to the possible extent; the impact of these parameters on the efficient plant performance is also outlined. Significant R and D results are summarized, including currently favoured research approaches for tackling the complicated problem of optimizing desalination SWM module and overall plant performance.

Keywords:

• Spiral-wound membrane modules
• Desalination
• Design and operating parameters
• Optimization
• Operating problems

Acknowledgments

The author wishes to acknowledge the significant contributions, summarized in this paper, by his former students and present collaborators Prof S.G. Yiantsios, Prof M. Kostoglou, Dr C. P. Koutsou and Dr D. C. Sioutopoulos. Grateful acknowledgement is also made of financial support received by the Middle East Desalination Research Center (MEDRC) for part of the work summarized herein.