The effect of dissolved oxygen concentration on the structure, size and size distribution of activated sludge flocs

The variations in activated sludge flee structure, size and size distributi on were studied for different dissolved oxygen (DO) concentrations in pilot scale completely mixed reactors. The size distribution by volume for Aocs larger than about 10 mu m fitted well to log-normal distribution functions. No clear relationship between DO concentration and average flee diameter c ould be found; there was only a trend towards larger flocs at higher DO con centrations. Lower DO concentrations (0.5-2.0 mg/l) produced sludge with po orer settling properties and higher turbidities of the effluent than higher DO concentrations (2.0-5.0 mg/l). The main reasons to the deteriorated set tling properties were excessive growth of filamentous bacteria and the form ation of porous flocs. Alternating aerobic and anaerobic conditions (1-4 h) did not affect the settling properties to a large extent. The turbidity in creased significantly during the anaerobic period and decreased during the aerobic period. The supernatant was also analysed with a particle analyser. During alternating aerobic/anaerobic conditions, the proportion of smaller flocs (2-20 mu m) increased gradually during the anaerobic periods. Direct ly after that the oxygen supply was turned on, the number of small flocs de creased. In most of the measurements, more than 80% of the number of flocs in the supernatant were smaller than 2 mu m. The size distribution of small Aocs could best be fitted to power functions. The number of flocs in the s upernatant could relatively well be related to the turbidity. (C) 1998 Else vier Science Ltd. All rights reserved.