Settling and coagulating behaviour of fractal aggregates

Sedimentation of fractal aggregates (200-1000 μm) and their coagulation with suspended small particles (1.5 μm) were investigated through both theoretical analysis and experimental measurement. The settling velocities of the aggregates were nearly 3 times faster than calculated using Stokes' law. Attachments of small particles on the aggregates were found to be 1 order of magnitude higher than predicted by a curvilinear collision model and 2 orders of magnitude lower than predicted by the rectilinear model. It is suggested that the internal flow through large pores within fractal aggregates likely contributed to the faster settling velocities and enhanced coagulation between the aggregates and suspended particles. The predictions for the interior flow rates and settling velocities of aggregates can be largely improved using a new fractal structure model in which aggregates are built directly from large clusters instead of the primary particles.