Pretreatment of waste activated sludge (WAS) results in an improved efficiency of the subsequent anaerobic biotransformation of the organic matter to volatile fatty acids. The pretreatment process has been carried out using alkaline treatment, ultrasonic treatment (20 KHz, 120 W) and different combination of these two methods: alkaline followed by ultrasonic, as well as the combining method in which ultrasonic treatment is applied to WAS samples dosed with alkaline. The hydrolysis efficiency was evaluated based on the quantity of soluble COD (SCOD) and organic nitrogen in the pretreated WAS as well as the production of total volatile fatty acids (TVFA) in the following biochemical acid potential (BAP) test. For WAS samples with described pretreatments, the released SCOD varied from 36% to 89% of the total COD (TCOD) and soluble organic nitrogen from 34% to 42%. The TVFA/TCOD ratio of the raw WAS used in this study was less than 10%. For the alkaline pretreated WAS, the TVFA/TCOD ratio increased to 30%, and the following ultrasonic treatment enhanced the ratio 66%. Further, WAS samples pretreated using simultaneous ultrasound and alkaline treatment in which ultrasonic was applied to WAS samples dosed with 40 meq/L NaOH for 14.4 sec/mL could achieve a maximum TVFA/TCOD ratio of 84% in 21 hours. Therefore, the combination of simultaneous alkaline and ultrasound pretreatment is efficient in enhancing the production of volatile acids in WAS in order to achieve recovery of volatile fatty acids from the WAS.