Osmotic backwash of fouled FO membranes: Cleaning mechanisms and membrane surface properties after cleaning

Membrane fouling is an inevitable process in membrane filtration, and has adverse effects on membrane performance and life span. This work evaluated the performance of a simple online osmotic backwash (OsBW) process in restoring membrane flux after fouling with feed stream mimicking seawater. The traditional cellulose triacetate (CTA) forward osmosis (FO) membrane was used in the support layer facing feed solution orientation for all performed experiments. The effect of backwash permeation rate and cross-flow velocity on cleaning efficiency of the osmotic backwash process was investigated. The surface properties of the cleaned membranes were studied with a range of characterization techniques such as microscopic, spectroscopic and electrokinetic analysis. The cleaning efficiency of the osmotic backwash process was enhanced by high backwash permeation rate and increased cross-flow velocity. Water permeation weakened the attachment between the gel layer and the membrane surface and the loose gel layer components were then “swept”-off the tangential shear force generated by the backwash draw solution cross-flow velocity. Permeation rate was found to be the determinant of cleaning efficiency during osmotic backwash. Membrane surface analysis revealed that even though more than 90% of membrane flux was recovered, there were still remnants of the fouling layer on the membrane, possibly lodged in the pores of the porous support layer and they prevented complete permeate flux restoration. This implies that the active layer facing feed solution orientation is a better option to treat feed streams containing gel forming macromolecules and cleaning efficiency can possibly be improved through longer cleaning durations. Restoration of membrane surface charge was pivotal in maintaining membrane performance (rejection and water flux). The osmotic backwash process was successful in restoring membrane flux and operational flux.