Fractal analysis of pore distributions in alum coagulation and activated sludge flocs

The information on floc porosity is essential for estimation of the permeability of the aggregate. The average porosity of alum and activated sludge flocs determined in this study on thin sections of the aggregates was similar, varying in the range from 8 to 9%. Similar porosity of the two different types of aggregates suggested that the permeability of these flocs could also be similar. However, the experimental observations of floc settling rates and floc shape factors did not support this expectation and led to a conclusion that the permeability of an aggregate cannot be estimated based on the average geometric porosity of a floc only. When the size distributions of pores on flocs' sections were analyzed using the concept of fractal geometry, different characteristic values for alum coagulation and activated sludge flocs were found. Larger pore size found in activated sludge flocs allows for more flow through these flocs. Therefore, the size of characteristic pores rather than the average total porosity determines the permeability of an aggregate.