Many attempts have been made to control fouling in membrane bioreactors (MBRs) to sustain permeability through relaxation, backwashing and chemical cleaning. Sodium hypochlorite is one of the most useful chemical agents used to control fouling in chemically enhanced backwashing (CEB) of MBRs. However, sodium hypochlorite has the potential to adversely affect microorganisms because CEB is usually carried out within the bioreactor. Hence, this study investigated how sodium hypochlorite influences activated sludge from the viewpoint of microorganism properties and suggests the limit of sodium hypochlorite dosage. Activated sludge was evaluated using the substance removal rate, extracellular polymeric substances (EPSs), particle size distribution (PSD) and permeability. It was shown that 1 mg-NaOCl/g-MLVSS had a slight superiority in nutrient removal compared to 5 mg-NaOCl/g-MLVSS. On the other hand, the NH4-N removal ratio of I mg-NaOCl/g-MLVSS was bigger than that of 5 mg-NaOCl/g-MLVSS in nitrification for 3 h from the time of sodium hypochlorite addition. While 1 mg-NaOCl/g-MLVSS showed a similar tendency with the reference sludge in the case of EPSpolysaccharide. 5 mg-NaOCl/g-MLVSS generated EPSpolysaccharide 1.5 times as much as that of 1 mg-NaOCl/g-MLVSS. Also, according to particle size distribution, the average floc size of 5 mg-NaOCl/g-MLVSS was changed from 43.63 to 36.98 mu m. As a result, the sludge of 5 mg-NaOCl/g-MLVSS had a much higher potential for increase than the reference sludge in membrane fouling. Moreover, a microfiltration experiment confirmed that 5 mg-NaOCl/g-MLVSS had a double rate increase of resistance compared with 1 mg-NaOCl/g-MLVSS. (C) 2013 Elsevier B.V. All rights reserved.