Advanced treatment of a complex pharmaceutical wastewater by Desal-5 DK nanofiltration (NF) membrane was carried out. The membrane fouling and chemical cleaning in this application were investigated. It was found that at the initial stage of NF process, the deposition of sulfate and carbonate of calcium was the main cause of membrane fouling. At the later fouling stage, besides calcium salts, complex organic foulants containing carboxyl acid, amide, and alkyl halide functional groups also deposited onto membrane surface and gradually formed a densely packed fouling layer. Accordingly, in cleaning process, the efficiency of a cleaning agent depended on its ability to break down the integrity of the compact fouling layer by reacting with the foulants. The results showed that the cleaning efficiencies of the agents increased in the sequence of NaOH (pH 11) < HCl (pH 2) < citric acid (pH 2) < EDTA (10 mM). After cleaning with EDTA for 60 min, the membrane flux recovery ratio reached 99.0%. The SEM image and element content of the membrane cleaned by EDTA were quite similar to those of the virgin membrane. (c) 2009 Elsevier B.V. All rights reserved.