On the basis of experiments carried out using an image analysis system and static zone settlement, the alum sludge floc structure and the floc packing status with varied polymer dosages were examined in greater detail to reveal the link with the settling behaviour manifested in the "CML30 method". The results of image analysis lead to the findings of the floc size with a pattern in initially great increase with low polymer dose and finally a plateau being reached at higher amount of polymer addition. Meanwhile, a polymer dosage in the range of 2.0-20.0 mg/l (for sludge SS = 4,595 mg/l) can result in the increase of the floc fractal dimension (D-F) up to an average value of 1.72 (raw sludge D-F being 1.06), indicating greater compactness of the floc solids. These floc features control the settling behaviour of the "CML30 method", especially in small polymer dosages. However, in large polymer doses, the controlling factor is derived from the floc 'packing' status (referring to the inter-floc contact manner). This 'packing' status may associate with viscosity of settling medium (supernatant) and the excess polymer in bulk solution. All these affect the sludge floc networked structure in overdosing range. (C) 2003 Elsevier B.V. All rights reserved.