This paper improves the energy efficiency for sludge treatment using ultrasound. The sludge organic content, solid concentration, ultrasonic intensity, and time were optimized. A new index, kWh/kg SCOD-increase, was used to evaluate the energy efficiency that covers both sludge characteristics and lysis effectiveness. The results showed that the energy efficiency of sludge sonication was in the range of 102-347 kWh/kg SCOD-increase. Higher VSJTS ratio gave higher energy efficiency. Optimal solid concentration (TS) was found to be 20 g/L; low TS (5.0 g/L) consumed 137% more energy; while higher TS might assimilate the sound energy and decrease the efficiency. The cavitation threshold for sludge was 2030 W/cm(2), below which no Sludge lysis was observed. The lowest effective sound intensity (31.4 W/cm(2)) was best and saved 20-35% energy compared to higher intensities. The initial first minutes of sonication were most effective: hence intensive-short treatment was preferred. (C) 2008 Elsevier Ltd. All rights reserved.