In this study, we took advantage of the conventionally used physical backwashing and chemical cleaning methods to develop an on-line maintenance cleaning method using the NaOH solution (0.01 mol/L, pH around 12) as the backwashing agent in a membrane bioreactor (MBR) operated at two imposed fluxes (6.5 and 13 L/(m(2).h) (LMH)). The results indicate that NaOH backwashing had little adverse effect on biological nutrient removal, and the pH in the MBR reactor remained at stable and acceptable levels (6.6-7.8) because of the alkali consumption by the nitrification process and the buffering role of the mixed liquor. Importantly, the NaOH backwashing significantly slowed the increased rates of total fouling (50% and 69% for 6.5 and 13 LMH, respectively) and irreversible fouling (40% and 50% for 6.5 and 13 LMH, respectively) compared with the pure water backwashing. Off-line backwashing tests indicate that NaOH backwashing enhanced the detachment of biopolymers from the surface of fouled membranes due to the co-occurrence of hydraulic actions and chemical alterations. The characterizations by scanning electron microscopy (SEM) indicate that both backwashing methods increased the size of membrane pores. Nevertheless, the Fourier transform-infrared spectrometry (FTIR) of membranes indicates that the long-term exposure to the NaOH solution did not change the chemical structure of membranes. This study suggests that on-line NaOH backwashing could not only maintain membrane permeability but also simultaneously supply alkali to bioreactors to ultimately facilitate the operation of MBRs. (C) 2014 Elsevier B.V. All rights reserved.