A submerged nanofiltration membrane bioreactor (NF MBR) using cellulose acetate membranes was operated for 240 days to examine the performance of the NF membrane in domestic wastewater treatment. For the first 130 days, dissolved organic carbon (DOC) concentration in the permeates ranged from 0.5 to 2.0 mg/L. The rejection rates of monovalent and divalent ions by the NF membranes varied between 40 and 60% and between 70 and 90%, respectively, for the initial 80 days. The permeate flux increased gradually with decreasing transmembrane pressure (TMP) during the operation. The results of MW distribution and fractionation of dissolved organic matter in the waters demonstrated the increase in membrane pore size and decrease in membrane hydrophobicity over operating time. From the results, the degradation of each water-quality parameter over operation time might be attributed to the qualitative deterioration of the NF membrane caused by the hydrolysis of cellulose acetate, which leads to an increase in pore size and porosity, and decrease in surface charge and hydrophobicity of cellulose acetate membranes. (c) 2006 Elsevier B.V. All rights reserved.