The objective of this study was to determine the impact of solubilization during thermo-chemo-sonic pretreatment of waste activated sludge (WAS) on anaerobic biodegradability and cost for biogas production. The results revealed that it was possible to achieve 40-50% of solubilization of WAS when ultrasonic energy input was doubled (11,520-27,000 kJ/kg TS). The cost to achieve 30-35% of solubilization of WAS was calculated to be 0.22-0.24 USD/L, which was relatively lower than the cost of 0.53-0.8 USD/L when 40-50% of solubilisation of WAS was achieved. There was no significant difference in biodegradability (0.60-0.64 g COD/g COD) for samples with solubilization efficiency of 35-50%. Comparing energetic balance and economic assessment of samples with different solubilization percentages, the results showed that samples with 30-35% of solubilization had lower net cost (7.98-2.33 USD/Ton of sludge) and negative energy balance compared to samples with other percentages of solubilization. (C) 2016 Elsevier Ltd. All rights reserved.