The reverse osmosis system is exposed to many problems that affect its efficiency. The major problem is fouling membrane which is frequently related to the interaction between the quality of feed water and the pretreatment process. This paper treats two problems of RO plant in Djerba Island, the sulfuric water and the fouling membrane. For the first problem, the anaerobic process showed better performance than the aerobic process. Furthermore, the aerobic process of non-sulfuric water was affected after 6 years of operation with a fouling phenomenon at R.O membrane. This problem was correlated with the increase of the operating pressure, pressure drop and permeates conductivity. To identify causes of these problems, different investigations are carried out. Membrane autopsy, chemical analyses of feed water and follow-up of the operation parameters are performed. The membrane autopsy was achieved by SEM/EDS. The results show that the film is composed mostly with organic matter, silica, metallic matter and aluminum phosphate. This result is correlated with the phosphonate based antiscalant composition. Therefore, antiscalant selection and dosage appear to be critical for membrane fouling development. (C) 2011 Elsevier B.V. All rights reserved.