We hung the activated sludge floes on an elastic nylon stick and then subjected it to a uniform water How and measured its displacement. The hydrodynamic drag force exerted on the floc was subsequently estimated, both for cationic flocculated floes and for flocculated and then frozen/thawed flocs. A confocal laser scanning microscope (CLSM) was employed to probe the interior structure of flocs. Polyelectrolyte flocculation leads to a compact global structure, and hence high drag force exerted on the floc by water. The corresponding C(D)Omega value at Re = 12-27 for floes ranges from 1.58 to 3.61. Fast freezing. would little affect the hydrodynamic drag force. Slow freezing, in contrast, considerably consolidated the floc structure and hence presented impermeable sphere-like behavior of the slowly frozen/thawed floes. (C) 2003 Elsevier Inc. All rights reserved.