In this study, the application of two-step nanofiltration (NF) process was investigated in the purification of biologically treated pulp and paper wastewater for the purpose of reuse. In the first step, the effect of operating conditions such as pH, temperature, transmembrane pressure and volume reduction factor (VRF) on the membrane fouling was investigated using FM NP010 membrane. Membrane fouling was minimized by optimizing the operating conditions using Taguchi experimental design method. The smaller-the-better signal-to-noise (S/N) ratio was used to analyze the results of experiments. Flux decline caused by fouling was selected as response parameter. The best results were obtained at third level of pH (10), first level of temperature (25 degrees C), first level of transmembrane pressure (12 bar) and third level of VRF (4) which provided minimum flux decline caused by fouling (11%). Based on the analysis of variance (ANOVA), the results indicated that transmembrane pressure has significant effect on membrane fouling, while VRF shows very little effect on membrane fouling. Membrane fouling was further examined in terms of atomic force microscopy (AFM) and scanning electron microscopy (SEM) analysis. Under the optimized conditions, 91% chemical oxygen demand (COD), 92% total hardness and 98% sulphate removal were achieved using FM NP010 membrane. Also the obtained permeate is colorless and free from suspended solids. However, chloride and conductivity were partially removed. In the second step, FM NP030 membrane was used to provide better permeate quality like actual process water. Membrane fouling was also investigated in terms of flux decline, contact angle measurements and SEM analysis in this step. As a conclusion, the water produced by two step NF treatment of biologically treated pulp and paper industry meets the quality of actual process water. (c) 2010 Elsevier B.V. All rights reserved.