Efficient solid-liquid phase separation of excess sludge from the mixed-liquor suspended solids in wastewater treatment is difficult to achieve without prior sludge conditioning. Several physical conditioners have been assessed as skeleton builders or filtering aids to reduce sludge compressibility; however, little is known about how the properties of the physical conditioner affect sludge compressibility and dewaterability. In this study, synthetic poly(ethylene terephthalate) (PET), polypropylene, and viscose rayon (VR) fibers were used as physical conditioners for activated sludge, and the effects of the fiber compressibility on the dewaterability of activated sludge collected from a laboratory-scale reactor were assessed. Based on expression experiments using a laboratory-scale apparatus, it was found that: (1) the rate of consolidation of sludge cakes increased after incorporation of synthetic fibers, particularly at the primary stage. (2)The porosity of the sludge cake was maintained only with fibers that required high pressures for compaction (i.e., PET and VR); thus, high dewaterability of the cakes was achieved with these fibers. It is proposed that the compressibility of the physical conditioner may be one of the more important factors for improving sludge dewaterability. (C) 2015 Elsevier B.V. All rights reserved.