MIHAI STEFANESCU, LAURENTIU RAZVAN DINU, COSTEL BUMBAC ULTRASONIC ALKALINE PRETREATMENT OF BIOLOGICAL ACTIVATED SLUDGE FROM WASTEWATER TREATMENT PLANTS Ultrasonic treatment has become a modern method for removal of micropollutants and refractory compounds from water/wastewater and to improve biodegradation efficiency of biological activated sludge from wastewater treatment plants, in order to produce biogas. This paper is referring to the ultrasonic pretreatment of biological activated sludge, before anaerobic fermentation phase, to increase the amount of biogas generation. The pretreatment of biological sludge is based on two steps: alkaline digestion and ultrasonication. Ten minutes of alkaline condition (pH 9.5 - 10.5) followed by 10 minutes ultrasonication (20 kHz frequency) were the general operating condition of sludge pretreatment phase. Parallel tests (with and without ultrasonic pretreatment step) were performed in order to assess the effect of ultrasonic pretreatment on biogas generation. Ultrasonication in alkaline condition of biological sludge heave led to doubling the amount of biogas generated in anaerobic fermentation phase.