Water desalination for urban supply in the Kerkennah Islands started in 1983 when a reverse osmosis plant was built with a production capacity of 3300 m(3)/d.The membranes are spiral-wound cellulosic acetate. The brackish water feed is obtained from deep wells with a TDS of 3.6 g/l and which contains an important quantity of iron (0.7 mg/l). The Kerkennah desalination plant suitably ensured its task during its first years of operation. However, during the last few years, a remarkable reduction of plant productivity was noted; this reduction is caused by aging of the equipment, especially the acetate membranes and high frequency of their replacement. This is explained by the instability of the pH, necessary to avoid the phenomenon of hydrolysis, and also by the inefficiency of membrane conservation after production. Moreover, the presence of a significant iron concentration in the raw water blocked the improvement of productivity of the aerobic pre-treatment. Indeed, colloidal particles such as iron hydroxide (Fe(OH),) are the cause of the increase in SDI. To keep the Kerkennah plant in good repair, a technical program was designed. It is based primarily on a new process using the technique of anaerobic operation and the use of polyamide RO spiral membranes with proven high productivity and efficiency. Compared with acetate modules, the polyamide membranes require only half the feed pressure necessary for the production of osmosis water, thus an important profit of electric power. In addition, the quality of water produced is largely better than that produced by acetate. The anaerobic process consists of brackish water passage directly from artesian wells drilling to RO membranes without any contact with oxygen. The advantage of this process is to maintain brackish water balance, to save its stability and to avoid the risks of chemical reactions with oxygen in the air. Line 3 of the plant was equipped with polyamide modules in May 2001 and line 4 in July 2001. Thus, the plant has two lines which operate with acetate membranes and two others lines with polyamide. After 2 years of operation with the new process the following performances are obtained: (1) reduction of the pressure of attack necessary to the production of osmosis water from 300 in to 160 in; (2) electricity energy saving of 46%; (3) reduction in the salinity of water produced with less than 100 mg/l; (4) economy of chemicals consumption; (5) use of the same membranes and chemicals products for all desalination plants managed by Sonede.