High water affinity of extracellular polymeric substances (EPS) and high compressibility of sewage sludge solids have negative impacts on sludge dewatering. In this study, a composite conditioner, Fe2+-activated sodium persulfate (denoted as Fe2+/SPS) combined with thermal-pretreated phosphogypsum (PG), was used to improve the sludge dewaterability. The mechanism of the composite conditioning of sewage sludge was elucidated; the proteins in tightly bound EPS were transferred into the filtrate and loosely bound EPS, and the polysaccharides in loosely and tightly bound EPS were transferred into the filtrate; the bound water was released and reduced from 2.60 g/g DS (dry solid) initially to 0.81 g/g DS; specific resistance to filtration and capillary suction time were reduced by 91.6% and 88.4%, respectively. Radical quenching experiment indicated that sulfate radical (SO4 center dot-) is the dominant free radical and plays an important role in determining the oxidation-reduction potential during conditioning. Moreover, both the XRD and SEM results clearly showed that Fe2+/SPS combined with PG promoted the generation of column-shaped dihydrate gypsum in the conditioned sludge. Thus, the dihydrate gypsum crystals could act as skeleton builders, which create a more permeable and rigid lattice structure of the sludge cake. The improvement of sludge dewatering was confirmed by diaphragm filter press dewatering process, which yielded 45.7 wt% cake moisture content and 91.7% dewatering efficiency. (C) 2015 Elsevier B.V. All rights reserved.