The factor which has the greatest influence on the RO system design guideline is the fouling tendency of the feed water. Membrane fouling is caused by particles and colloidal materials which are present in the feed water and become concentrated at the membrane surface. The Silt Density Index (SDI) of pretreated feed water is an index of the fouling potential of particle or colloidal materials in the RO system. The concentration of the fouling materials at the membrane surface increases with increasing permeate flux, increasing