Fouling and scaling are the most serious problems in membrane processes. In sea/brackish water applications, pretreatment of RO feed water is the key step in designing the plants to avoid membrane fouling and scaling. Recent developments in pretreatment processes are more adapted to raw water quality. But, in some cases, raw water quantity/quality varies during seasons and is also influenced by the environment. Thus, pretreatment design becomes complicated and should cope with the raw water quality changes. The success of such operation requires qualified operators who will be able to adapt with different situations. Surface and brackish water sources are mostly facing these problems. In this paper, performance evaluation carried out for a brackish water reverse osmosis (BWRO) plant located in the west of Algeria is presented. This plant showed poor performance after a few months of operation. The operating pressure and pressure drop increased significantly without an increase in the production capacity and the permeate conductivity decreased surprisingly. Frequent shutdowns of the plant were observed due to severe membrane fouling. To identify the causes for the Poor performance, different investigations were carried out. Membrane autopsy was performed and chemical analyses of foulants on the membrane surface by scanning electron microscopy were carried out to identify the matters responsible for fouling. The results showed that the quality of raw water changed widely due to drying of some wells and decrease of the water level in other wells. RO membranes were fouled by inorganic matters mainly colloidal/particulate silica and fine particles of clay present in raw water. Thus, the pretreatment scheme was thoroughly reviewed to find out why suspended solids were not removed by the sand and cartridge filters even though SDI was always less than 1. The problem was resolved by injecting a coagulant before the sand filters.