MIHAI STEFANESCU, GHEORGHE NECHIFOR, COSTEL BUMBAC, IOANA IONESCU, OLGA TIRON IMPROVEMENT OF ACTIVE BIOLOGICAL SLUDGE QUALITY FOR ANAEROBIC DIGESTION PHASE IN THE WASTEWATER TREATMENT PLANT BY ULTRASONIC PRETREATMENT This paper emphasized the influence of the main operating parameters of ultrasonic biological sludge pretreatment onto disintegration (DD-COD) and solubilizing degree of organic matter content. The optimal ultrasonic reaction time, energy and amplitude of ultrasonic waves and pH of the sludge (non-modified pH and pH 8.5) were established taking into account their influence on the solubility level of COD, an evolution of particles size. Correlations between disintegration degree and dimensional analysis were performed. Ultrasonic reactor operating to 20 kHz ultrasound frequency was a close system having cooling water jacket. Experimental tests in different operating conditions (reaction time 5 – 60 min., ultrasound energy 2000 – 9000 kJ, pH = 6.5/8.5) demonstrated that alkaline ultrasonic treatment (pH = 8.5, initial homogeneous COD = 72600 mg O2/L) at medium - high ultrasound energy levels led to highest disintegration degree – DD 12% (comparing with pH = 6.5 – DD ~ 5%). Still, the process should be managed carefully because of possibility to have important variation of DD during the ultrasonic treatment.