Time variation of floc properties in a settling column

Abstract

This study investigates the evolution of suspended floc size and settling velocity in an invertible column equipped with photographic and video systems. Nine experiments were performed using 50, 75, and 100 mg dm −3 of marine clay and column inversion times of 0.5, 1, and 2 h. Following inversion, the column was placed in the upright position and the suspension allowed to settle. Observations demonstrate that the initial floc population formed during inversion had median diameters of approximately 200 μm and sank at speeds of 1–4 mm s −1. As these flocs settled both median diameter and floc settling velocity decreased. After approximately 15 min, median floc diameter started to increase and by the end of the experiment had reached values similar to those of the initial floc population. Increases in floc diameter were not accompanied by increases in floc settling velocity or total solids concentration. These results indicate that flocculation took place within the settling column on 15-min time scales and that the flocs produced were less dense than those initially formed during inversion. These observations contribute to the known problems associated with settling columns and suggest that estimates of bulk sediment clearance rates made using columns be constrained to the initial minutes of sampling.