- The effects of polysaccharides composition on the interactions between EPS and RO membrane, including fouling, were studied in the presence of Ca 2 + . EPS originated from Pseudomonas aeruginosa PAO1 biofilms of the wild type and its Δ psl isogenic mutant was used for RO membranes fouling and QCM-D adsorption experiments. For the wild type strain EPS, bridging of alginates by Ca 2 + led to an increase in both adsorption and rigidity of the adsorbed layer. However, no change was detected for the Δ psl EPS adsorption and the obtained layer showed reduced rigidity, likely due to the interference of Ca 2 + with interactions between the polysaccharides composing the EPS. In agreement with the QCM-D results, once Ca 2 + was added, an increased in RO fouling was observed with the wild type EPS, while reduced fouling was found for the Δ psl mutant EPS. The presence of an ATR-FTIR peak characterized glycosidic linkage only for the fouling layer formed by the EPS of the Δ psl mutant in the presence of Ca 2 + , suggested that Ca 2 + alters the scaffold matrix of this EPS. The results of this study showed that polysaccharide composition in the EPS matrix significantly influences the interaction with Ca 2 + as well as membrane fouling.