Bio-sludge, which contains large quantities of water, biomass and extracellular polymeric substances (EPS), is difficult to be dewatered as a "super-compactable" sludge [Water Res. 35 (5) (2001) 1358]. Reported water content in bio-sludge was commonly about 80-90% wt after dewatering process. The EPS and the form of water in sludge influences the structure of sludge. Adding cationic fluctuations can change the form of water in sludge and increase the velocity of dewatering process, but has little influence on the final water content. According to Bien's work [Water Sci. Technol. 36 (4) (1997) 287], a combination of ultrasound or other method could agglomerate the sludge, improve the activities of biomass [Ultrasonics 38 (2000) 711; Ultrasonics 40 (2002) 25], enhance anaerobic process [Water Sci. Technol. 36 (11) (1997) 155; Water Sci. Technol. 36 (11) (1997) 121; Water Res. 35 (8) (2001) 2003; Water Sci. Technol. 36 (12) (1997) 117] and decrease over 10% final water content of sludge. The mechanisms of ultrasonic influence on sludge are not very clear, but the application of ultrasound to industrial process is relatively easy and possible.