The activated sludge process is widely used for wastewater treatment, and the settleability and dewaterability of activated sludge are crucial to the operation of wastewater treatment plants. In this study the surfactant-mediated settleability and dewaterability of activated sludge were investigated. It is found that dose of anionic surfactant sodium dodecyl sulfate (SDS) caused substantial release of extracellular polymer substances (EPS), especially proteins, from sludge. Laser light scattering (LLS) results reveal that the released EPS became stretched and more expanded when the SDS dosage was increased. The sludge dewaterability was significantly deteriorated at a dosage of 2.084 g/L SDS, and meanwhile the capillary suction time (CST) and specific resistance of filtration (SRF) increased by 6.3 and 5.1 times, respectively, than the raw sludge. The correlations between the EPS contents and the sludge properties, i.e., CST, SRF, moisture content of filter-cake and sludge volume index, were evaluated and the significant roles of proteins in governing the EPS properties as well as the settleability and dewaterability of sludge were further confirmed. This study elucidates how the settleability and dewaterability of activated sludge were affected by surfactants via changing the properties and structure of the released EPS from sludge. (C) 2014 Elsevier Ltd. All rights reserved.