Effects of temperature and the amount of sludge on the physicochemical properties of the waste liquid products produced from the hydrothermal treatment (HT) of both low-rank coal and sludge were investigated. The slurryability of coal sludge slurry (CSS) prepared with HT solid and waste liquid product was also analyzed. An increase in the amount of sludge led to a decrease in the solid yield, calorific value and fixed carbon of solid products decreased, particularly the solid yield which reached 63.8 wt% in the HT of sludge at 300 degrees C. However, more than 85 wt% carbon was retained in the solid products, which is beneficial for the energy densification. The Total Organic Carbon (TOC) content increased significantly with an increase in the amount of sludge and attained a concentration of 60 g/L. The same trend was observed in the pH value of waste liquid, owing to the presence of ammonium nitrogen organics. Alkanes, phenols, and heterocyclic were the main organic compounds produced from the HT of Inner Mongolia Coal (IMC), Yunnan Coal (YNC) and sludge at 300 degrees C, respectively. CSS prepared using the HT waste liquid instead of water has a higher fixed-viscosity concentration and better stability due to the presence of dissolved organic compounds and ions. (C) 2017 Elsevier Ltd. All rights reserved.