Porosity and structure of alum coagulation and activated sludge flocs

Knowledge of floc porosity and structure is the basis for the study of floc permeability. This study determined the porosity and structure of alum coagulation and activated sludge flocs. The porosity of these flocs can be expressed in geometric and mass terms. Yet mass porosity can be 10 times higher than geometric porosity due to an underestimation of floc density in the settling test. Given the more reliable data expressed by geometric porosity, an experimental method for studying the geometric porosity of aggregates was developed.; The average geometric porosity was 9% for alum flocs and 8% for activated sludge flocs. Similar average geometric porosity and size of the primary particles in these two aggregates suggested similar permeability of these flocs, while the settling behaviour of these flocs suggested that activated sludge flocs were more permeable than alum flocs. This suggested that the floc permeability couldn't be estimated based on the average porosity of the floc.; Interpretation of the geometric porosity data using the concepts of fractal geometry enabled three pore populations---small, medium and large---to be identified within alum and activated sludge flocs. Small pores had a cross-sectional area smaller than 3 mum2. The medium pores of alum flocs were smaller than 10 mum2, and the medium pores of activated sludge flocs were smaller than 20 mum2. Activated sludge flocs are more permeable than alum flocs because the pores forming flow channels---medium and large---are bigger in activated sludge flocs than the corresponding pores in alum flocs.; Based on the size of pores, permeability and structural models were proposed for the flocs studied. In the activated sludge floc model flocculi were two times larger than the flocculi in alum flocs. The size of the flocculi determines the size of pores facilitating the internal flow; therefore, it should be incorporated in the floc permeability models.; This study makes two recommendations: (1) developing floc permeability models that do not require floc porosity estimate; (2) re-calculation of floc mass transfer and drag force predictions using an improved estimate of floc permeability.