Despite the progress in chemical engineering, efficient dewatering of ultra-fine coal (<150 mu m) from slurries is still one of the biggest challenges in coal-mines. This process consists of two consecutive steps: suspension flocculation and sludge filtration. Unfortunately a sort of paradox appears in this situation. On the one hand large flocs are desired in order to fast and efficiently separate solid particles from the suspension, on the other hand large and porous flocs have worse dewaterability characteristics than small and tight ones. In this paper solution to this problem is proposed. In order to create large and tight aggregates a dual-polymer system was used using unmodified and sonicated flocculant. The flocculation process was investigated on a special apparatus for on-line particle size distribution (PSD) measurements consisting of the laser particle sizer, baffled mixing tank with turbine mixer and peristaltic pump. Moreover, pressure filtration tests were made. Presented results proved that ultrasonically conditioned polymers could be an attractive alternative for standard dual-polymer systems. The application of polymer blends resulted in accelerated aggregation kinetics, whereas sequential addition allowed the system to obtain slightly larger aggregates. In high overdose region a synergistic effect was found the size of aggregates was larger than in case of unmodified polymer optimal dosage both for sequential addition and polymer blend cases. Finally, enhanced dewaterability characteristics of received sludge were obtained. (C) 2010 Elsevier B.V. All rights reserved.