Good characterization of raw water quality and subsequent identification of the most appropriate pre-treatment are not sufficient to ensure successful functioning of a reverse osmosis plant. The Tan Tan demineralization plant was set up in July 2003 for 20 Us production capacity. The brackish water with total dissolved salt (TDS) of 3 ppm is supplied from Khang Lahmam underground water located 70 kin from the plant. Since it started working, the main problem encountered in the Tan Tan demineralization plant was the continuous presence of a membrane biofouling problem which provokes high pressure drop values and requires an increased washing frequency, as often as twice a month. The diagnosis undertaken to remedy this anomaly has concerned all the pre-treatment steps including the raw water, quality of the applied chemical reagent types and concentrations, and the composition of the biofilm. Collected data confirm the good raw water quality specially the absence of heterotrophic bacterium (aerobic-anaerobic analyzed at 37degreesC+1degreesC and to 22degreesC+1degreesC) and low oxydation values (<3 ppm). Chlorine-raw water contact time, about 17 h, is largely sufficient to insure its efficiency. Analyses of the biofilm have shown the abundance of heterotrophic germs Pseudomonas sp. and ferobacter absence despite the reddish color of the biofilm. The hypothesis retained to interpret the origin of the biofouling is the sodium bisulfate injection point, upstream of the microfilter. The microfilter then plays the role of a bioreactor favoring a bacterial proliferation upstream of the membranes. The simple displacement of the injection point of that of sodium bisulfate will allow remedying definitively the disorder caused by frequent appearance of biofouling. The present article, through the analysis of operational data, and particularly of results and actions undertaken, focuses on the crucial role of reagent injection point in optimization of reverse osmosis plant operation.