The main objective of this study was to evaluate the toxicity of 4-chlorophenol (4-CP) to aerobic granular sludge in the process of treating ammonia rich wastewater. In the short-term exposure of 4-CP of 5 and 10 mg/L, ammonia nitrogen removal efficiencies in the batch reactors decreased to 87.18 +/- 2.81 and 41.16 +/- 3.55%, which were remarkably lower than that of control experiment (99.83 +/- 0.54%). Correspondingly, the respirometric activities of heterotrophic and autotrophic bacteria of aerobic granular sludge were significantly inhibited in the presence of 4-CP. Moreover, the main components of extracellular polymeric substances (EPS) including polysaccharides and proteins increased from 18.74 +/- 0.29 and 22.57 +/- 0.34 mg/g SS to 27.79 +/- 0.51 and 24.69 +/- 0.38 mg/g SS, respectively, indicating that the presence of 4-CP played an important role on the EPS production. Three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy further showed that the intensities of EPS samples were obviously quenched with the increased of 4-CP concentrations. To be more detailed, synchronous fluorescence spectra indicated that the interaction between EPS and 4-CP was mainly caused by tryptophan residues. The mechanism of fluorescence quenching belongs to static quenching with a formation constant (K-A) of 0.07 x 10(4) L/mol, implying the strong formation of EPS and 4-CF complex. The results could provide reliable and accurate information to determine the potential toxicity of 4-CF on the performance of aerobic granular sludge system. (C) 2015 Elsevier B.V. All rights reserved.