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Abstract:The paper presents the investigations aimed at the determination of the effect of time and wavelength of ultrasound fi eld on the value of capillary suction time (CST), sludge thickening and dry matter of the excess sludge subjected to the process of stabilization.The investigations were carried out on the excess sludge which comes from communal waste treatment plant. The sludge was exposed to ultrasound fi eld, using ultrasound generator with power of 1500 W, frequency of 20 kHz and amplitude 39.42 μm (which corresponded to the amplitude of 100%). Sonication of the sludge was carried out for different amplitudes and sonication times. The non-conditioned sludge and the sludge initially conditioned with ultrasound fi eld were subjected to the process of stabilization in laboratory fl asks (V = 0.5 dm 3 ) for the period of 10 days. On each day, sludge thickening and dewatering capacities were determined.The sludge subjected to the effect of ultrasound fi eld exhibited elevated levels of CST. However, the sonication time had positive effect on the increase in the degree of thickening for each of the amplitudes studied. Also, the process of stabilization positively affected fi nal thickening and dewatering of the sludge.