On the free-settling test for estimating activated sludge floc density

The feasibility of using free-settling test for estimating activated sludge floc density was discussed in the present work. Some usually applied models or correlations, such as for the primary particle density, the drag coefficient, the correction factor for advection flow through the floc and the estimation for floc permeability, are examined. The results show that floc permeability depends strongly on the models or parameter sets applied. However, the effective floc density, and also the fractal dimension, is insensitive to the floc permeability estimation, but is strongly dependent on the drag coefficient expression employed. Possible valid experimental range is proposed.