Settling characteristics of fractal aggregates

Abstract

The settling velocity of aggregates found in engineered water and wastewater treatment facilities are usually assumed to follow Stokes' law. These particles, however, are typically porous and may be fractal in nature. As a result, they have very different sedimentation characteristics. This study focuses on the comparison of several techniques used to characterize the settling properties of fractal aggregates. Aggregates made of latex particles destabilized using calcium nitrate were formed in the laboratory. Settling velocity and size were then determined for each floc using latex particles ranging 0.03 mum, 0.05 mum, and 2.65 mum in diameter. Light scattering techniques were used to calculate the fractal dimension of the aggregates. Fractal dimension ranged from 1.6 to 2.3 in the experiments. A settling column was constructed to analyze actual settling velocities of flocs. Observed settling velocities of these flocs were consistently higher than those predicted using Stokes' law (which assumes an impermeable sphere).