Long-term exposure to Ni(II) exhibited adverse effects on the sludge flocculability. The content of PN in EPS decreased after long-term exposure to Ni(II). EEM and FTIR showed the change of structures and functional groups in EPS. The hydrophobicity of sludge decreased with the increasing Ni(II) level. The deleterious effect of Ni(II) could be mitigated by adding the EDTA or citrate.