Gross Dimensions of Activated Sludge Flocs with Reference to Bulking

Diameters of flocs and total length of associated filamentous microorganisms per floc were measured on flocs from four activated sludge treatment plants. Densities ranged from 0.02 to 0.2 mm and total length of filaments from 0 to 2.0 mm. Population of the flocs in a given sample was extremely heterogeneous with respect to these characteristics. Floc diameter was not consistently related to sludge volume index (SVI). The more filamentous sample from a given plant invariably had the higher SVI; at two plants, total length per floc was related to SVI by a constant factor. Total biological surface area ranged from 20 to over 100 sq cm/cu cm mixed liquor. Filamentous organisms contributed little to surface area except in a bulking sludge.