Osmosis-assisted cleaning of organic-fouled seawater RO membranes

Fouling of various kinds continues to limit membrane-based desalination and water treatment. Fouling is commonly countered through chemical cleaning of membrane elements, resulting in process downtime, membrane degradation and increased operation costs. Recent studies suggest that reversing permeate flux through RO membranes by dosing a slug of high salinity feedwater may offer an effective, chemical-free cleaning method. Herein, seawater RO membranes were fouled using alginic acid as a model seawater foulant and cleaned by osmotic backwashing with draw solutions of different salt concentration and chemistries. Flux recovery by osmotic backwashing was comparable with chemical cleaning (using caustic and a chelating agent); both recovered more flux than physical cleaning (rinsing with DI water). In particular, results illustrate the importance of combining chemical and physical mechanisms, the former contributing to ‘loosening’ of the foulant layer, and the latter facilitating its removal through fluid shear, enhanced by the presence of an osmotic backflow. Hence, osmotic backwashing may offer the potential for in-line, ‘low-chemical’ RO membrane cleaning, which would minimize discharge of cleaning chemicals to the environment and their impacts on RO membranes. Numerical simulations of the osmotic backwash cycle illustrate important time-scales and mass transfer limitations governing osmotic backwashing, through which operational insight may be obtained. The model offers a possible theoretical approach for optimization of RO membrane cleaning by osmotic backwashing.

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► Removal of Alginate with osmotic backwashing is comparable to chemical cleaning.
► Substantially higher cleaning efficiency achieved using a mono-valent draw solution.
► Chemical ‘loosening’ of foulant layer is essential for backwashing effectiveness.
► Optimization of contact time, draw strength and chemical composition is required.