The high water content and complex components of municipal waste sludge (or sludge) lead to increased difficulty and cost of handling sludge. Coal sludge slurry (CSS) technology blends sludge into coal water slurry to produce a new slurry fuel that can be combusted or gasified as a substitute for petroleum. Environmental problems caused by sludge can then be solved and combustible matter in sludge can be used. The poor slurryability of raw sludge, which resulted from its high water holding capacity, significantly increases the viscosity of CSS. In this study, sludge was pretreated by ultrasonic energy and then mixed with coal to prepare CSS. After ultrasonic pretreatment, sludge flocs were significantly disrupted and scattered, and their particle size greatly decreased. Thus, the water trapped by flocs was released. When the ultrasonicated sludge was used to prepare CSS, the released water acted as a lubricant to decrease friction and interaction among coal particles. When the specific energy input of ultrasonic increased from 0 to 30 with 75 kJ/g dry sludge at 190 W, the characteristic viscosity of CSS decreased by 27.54% and 41.04%, respectively. Ultrasonic significantly improved the slurryability of sludge, and thus, could enhance sludge disposal scale to a high level. (C) 2014 Elsevier B.V. All rights reserved.