Fouling phenomenon is considered among the major reasons that cause significant increase of operating cost of desalination plants equipped with reverse osmosis (RO) membranes. This phenomenon is studied in the present work in the case of RO polyamide aromatic membranes using model seawater containing inorganic salts and colloidal compounds. Different solubility conditions of CaCO

3

and CaSO

4

were applied to study RO performances with and without colloid presence. During experiments

the membrane permeate fluxes were continuously monitored. Moreover

studies of chemical composition

structure

and morphology of the materials deposited on the membrane surface were conducted using energy dispersive microanalysis (EDS) X-ray diffraction and scanning electronic microscopy (SEM). Results show that in conditions of calcium carbonate oversaturation there is a reduction in the permeate flow of 11.2% due to fouling of the membrane by the precipitation of this compound. While in the same conditions of calcium sulphate oversaturation the reduction of the flow is 5%

so we can conclude that in conditions of oversaturation of both salts

calcium carbonate produces a g

reater fouling of the membrane that in its view causes greater decrease in the flow of permeate. All this based on the results of the test with both salts in oversaturated conditions. Resulting in the formation of calcite and gypsum crystals onto the membranes as XRD analyses stated. Additional presence of colloidal silica in those conditions intensifies strongly the fouling

leading until to 24.1% of permeate flux decrease.