Activated sludge samples collected from 20 full-scale wastewater treatment plants were investigated to determine the effects of floc size (D50) on sludge settling characteristics. Results indicated that no simple and apparent relationship was found between D50 and sludge volume indices (SVIs) because of the complicated affecting mechanism of various parameters. Effects of D50 on the Vesilind settling model parameters were evaluated. Results indicated that no apparent relationship was found between D50 and V0. However, in the case of activated sludge with higher V0 (>5 m/h), there was a power function type correlation (V0 = 67.368·D50−0.4298, R2 = 0.5304) between V0 and D50. Furthermore, the parameter k decreased with increasing D50 through the relationship of k = 11.37·D50−0.5273 (R2 = 0.5593), except for samples with D50 < 100 μm and k < 1.0 m3/kg. V0 and k sludge were negatively correlated with sludge concentration because higher sludge concentrations contributed more resistance to settling velocities and their change rates. Results obtained in this study could provide scientific information for effective separation of activated sludge.
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