- The effects of shear rate in operation of reverse osmosis (RO) membrane process on biofouling phenomena were studied. RO filtration unit was continuously fed with tertiary effluents at high (445.2 s −1 ) and low (178.1 s −1 ) shear rate conditions. In addition, the two dependent parameters, organic loading rate and shear rate were compared. During desalination process of tertiary effluents, two different stages of fouling were observed. As expected, conditioning the RO membrane during the first stage of fouling was dominated by the concentration of the organic compounds in the feed water. While in contrast, the 2nd stage mainly comprised of biofilm formation, was dominated by the organic loading rate to the RO flowcell. Shear rate is shown to affect extracellular polymeric substances (EPS) chemical composition, which corresponded to changes in EPS cohesion and elasticity. Consequently, we show how these EPS physical properties affect permeate flux decline. At higher shear rate, EPS elasticity increases, probably inducing biofilm compactness and enhancing permeate flux decline with smaller amount of biofilm. EPS is known to induce permeate flux decline by increasing the hydraulic resistance to permeate flux. Here, we elucidate the strong impact of high shear rate biofouling conditions on permeate flux decline by the combination of EPS adhesion and cohesion properties.