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

The variations in activated sludge floc structure, size and size distribution were studied for different dissolved oxygen (DO) concentrations in pilot scale completely mixed reactors. The size distribution by volume for flocs larger than about 10 μm fitted well to log–normal distribution functions. No clear relationship between DO concentration and average floc diameter could be found; there was only a trend towards larger flocs at higher DO concentrations. Lower DO concentrations (0.5–2.0 mg/l) produced sludge with poorer settling properties and higher turbidities of the effluent than higher DO concentrations (2.0–5.0 mg/l). The main reasons to the deteriorated settling properties were excessive growth of filamentous bacteria and the formation of porous flocs. Alternating aerobic and anaerobic conditions (1–4 h) did not affect the settling properties to a large extent. The turbidity increased 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 μm) increased gradually during the anaerobic periods. Directly after that the oxygen supply was turned on, the number of small flocs decreased. In most of the measurements, more than 80% of the number of flocs in the supernatant were smaller than 2 μm. The size distribution of small flocs could best be fitted to power functions. The number of flocs in the supernatant could relatively well be related to the turbidity.