Short-term and long term effects of nickel (Ni) (0.1-10 mg/L) on the physicochemical properties of activated sludge, including the flocculability, settleability, and dewaterability, were investigated. It was found that these properties were unaffected after short term exposure (1 day) to Ni(II) even at the level of 10 mg/L. After long term exposure (60 days) to 1 and 10 mg/L of Ni(II), however, the sludge flocculability has seriously deteriorated, while the settleability, and dewaterability became gradually better than the control. The mechanism studies revealed that long term exposure to Ni(II) resulted in the decrease of protein content in extracellular polymeric substances (EPS) and the damage to EPS structures. Although Ni(II) did not bring any adverse effect on the cell membrane, the relative hydrophobicity of activated sludge was significantly decreased. The negative effects on the flocculability and phosphorus removal performance of activated sludge could be completely eliminated by adding the chelator such as EDTA and citrate. (C) 2016 Elsevier Ltd. All rights reserved.