The effect of dissolved oxygen concentration on the structure, size and size distribution of activated sludge
Journal article, 1999

The variation 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 mm 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 reason to the deteriorated settling properties were excessive growth of filamentous bacteria and the formation of porous flocs. Alternating oxic and anoxic conditions (1-4 hours) did not effect the settling properties to a large extent. The turbidity increased dramatically during the anoxic period and decreased during the oxic period. The supernatant was also analysed with a particle analyser. During alternating oxic/anoxic conditions, the proportion of smaller flocs (2-20 mm) increased gradually during the anoxic periods. Directly after that the oxygen supply was turned on, the number of small flocs decreased. The flocs in the supernatant had after 20-60 minutes settling normally a diameter up to 70-80 mm. In most of the measurements, more than 80% of the flocs were smaller than 2 mm. 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.

dissolved oxygen concentration


settling properties

floc size distribution

activated sludge

filamentous microorganisms



Britt-Marie Wilen

Department of Sanitary Engineering

Water Research

Vol. 33 2 391-400

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Other Environmental Engineering

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