- In addition to the effect of extracellular polysaccharides on biofilms viscoelastic properties and their related integrity and cohesion, their presence in biofilms of RO membranes can affect biofilm porosity and associated electrolytes diffusion. In order to elucidate how polysaccharides can affect both biofilm viscoelasticity and back diffusion of electrolytes on RO membranes, three bacterial strains of Pseudomonas aeruginosa PAO1, differentially expressing the Psl polysaccharide, were used as biofouling agents. During the biofouling stage, similar permeate flux decline was observed for all strains, while elevation in salt passage was lower for the Psl-deletion mutant biofilm. After the biofilm washing stage, as shear rate was elevated, permeate flux recovery was significantly higher and a decrease in salt passage was evident, in the Psl-deletion mutant biofilm. In contrast, in Psl containing biofilms, an elevation in salt passage was evident after the washing stage. Salt passage results, CLSM analysis and the intrinsic mass transfer coefficient kd, provided us with an overall range of biofouling layer’s porosity between 0.25 and 0.35. Lastly, the higher amounts of biomass of Psl containing biofilms persisted on the RO membranes after the washing stage, corroborated with both EPS viscoelastic properties measured by QCM-D and biofilm elasticity measured by AFM. This study highlights mechanistically, the EPS effects on biofouling of RO membranes, mediated by biofilm mechanical properties as well as ion transport.