Reducing the moisture content (Mc) of sludge contributes greatly to subsequent treatment and disposal. An ultrahigh-pressure (UHP) and thin-cake-press (TCP) dewatering device was designed to achieve high-dry sludge using technique involving the addition of little or no cationic polyacrylamide (CPAM). The relationships between the Mc of the cake and compression pressure, dewatering time, sludge quality and category, pressurization mode, pretreatment methods and the thickness of the cake were studied. The compression pressure, dewatering time, sludge quality and category, thickness of the cake and pretreatment methods substantially influence the Mc of the cake, while the pressurized mode has almost no influence on the Mc of the cake. The time of pressure drop (TOPD) reflects, to a certain extent, the dewatering performance of sludge. The Mc of municipal sludge can reach to 45% or 39.47% with 60 or 90 min of dewatering time, respectively, and 12 MPa compression pressure, achieving a cake thickness of 3.2 mm and 3.1 mm, respectively. Sludge with high organic matter content is more difficult to dehydrate, and the dewatering performance of municipal sludge is considerably worse than that of river sludge and electroplating sludge. Pretreatment methods can change the dewatering performance of sludge. Microwave and freeze-thaw pretreatments are favorable for dewatering performance, while ultrasound and magnetic field pretreatment are disadvantageous for dewatering. After microwave pretreatment, the Mc of municipal sludge can reach a minimum value of 28%. Mc has a positive correlation with the thickness of the cake; the thinner the sludge cake is, the lower the moisture content is. Thus, the thin-cake-press and microwave pretreatments are effective methods for high-dry dewatering. Sludge has good compression characteristics, and the compression ratios (C-r) of municipal activated sludge and river sludge are 79% and 59%, respectively. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.