We purified by nanofiltration a highly concentrated (11-17% w/v) mixture of organic/inorganic salts, generated from pharmaceutical industries to separate mineral salts (sodium acetate and sodium chloride) from heteroaromatic quaternary ammonium chloride salts soluble in water. The process, using a spiral-wound nanofiltration module, consists of a diafiltration step until the minerals salts were almost completely eliminated followed by a concentration step. To optimise the process, three variables were studied: transmembrane pressure (TMP), velocity, pH. The retention of organic salts was higher than 99.5%. The higher the molecular weight, the higher the retention factor. The flux and rejection show that it is not necessary to increase transmembrane pressure to obtain the faster salt elimination. The best result is obtained with the highest velocity which reduces membrane fouling. Changing pH from 3 to 4.5 had little effect on permeate flow; however, the purification of a solution at pH 5.4 is difficult because of a very low permeate flow. Less than 2% of the ammonium salts entrapped in the membrane were lost. Under optimal experimental conditions, more than 99% of mineral salts can be eliminated.