Abstract
The internal structure of activated sludge flocs was observed under the microscope and its mathematical model was proposed. The model expresses that the volume fraction occupied by microorganisms in a floc decreases monotonously from the center to the surface of the floc. The structure of the floc was characterized by two model parameters named a floc distribution index for volume fraction occupied by microorganisms and a floc core index, respectively. These parameters were determined by measuring the size and the density of the floc. The structural difference between filamentous and pellet-like activated sludge flocs was demonstrated by comparing the numerals of the model parameters.
