ADVECTIVE TRANSPORT IN ACTIVATED-SLUDGE FLOCS

The liquid flow through activated sludge flocs from full-scale conventional treatment plants was evaluated on the basis of the information obtained from settling tests. In the samples of the mixed liquors studied, only a part of the activated sludge flocs was found permeable to fluid drainage. Basically, small flocs were not permeable but all the flocs in the size range from 100- 150-mu-m showed different degrees of permeability. The permeability of flocs increased with an increase in the dynamic sludge age and decreased with an increase in the shear rate measured as the mean velocity gradient in the aeration tanks. The most sensitive assumption for the estimation of the permeability of the flocs was the size of the primary particles in the aggregates, with the sphericity of the flocs being less important. It was also found that the Carman-Kozeny model reflected well the porosity-permeability relationship of most activated sludge flocs, except those that contained substantial amounts of filamentous growth. During free gravitational settling, flocs with the longest dimensions in the size range of 50 to 1000-mu-m showed a drainage rate in the range of 10(2)to 10(7-)mu-m3/s, which followed the Carman-Kozeny model with a power relationship of 4.08.