A pilot study for wastewater treatment in Exir pharmaceutical Co. (Borojerd, Iran) was conducted using a RO system with the capacity of 14.38 m(3)/d. A Filmtec TW30HP-4641 RO element (polyamide, thin-film composite) was used in this study. The pilot plant consists of two spiral-wound RO elements. The RO train was configured in series. Trial runs were conducted at different operating conditions including pressures, temperature and concentration. The pilot results showed that flux of water containing nitrate, nitrite, phosphate and sulfite was about 58 l/m(2) h. Taguchi method was employed for flux optimization. Analysis of the experiments indicated that the temperature of feed solution and transmembrane pressure have the most contribution in water flux. The flux was improved to 69 l/m(2) h by setting the control factors according to the Taguchi method. The technique showed that concentration of feed solution has the highest contribution in rejection of a solution containing nitrate, nitrite, sulfite and phosphate. After setting the control factor according to the Taguchi method rejection was enhanced to 99.9% for this case study. (c) 2006 Elsevier B.V. All rights reserved.