THE EFFECT OF DISSOLVED OXYGEN CONCENTRATION ON THE SETTLING PROPERTIES OF ACTIVATED SLUDGE / Effekten av syrehalt pa sedimenteringsegenkaperna hos aktivt slam
Long-term effects of dissolved (DO) concentration on the settling properties of activated sludge were studied in continuous completely-mixed pilot-plant reactors. Lower DO concentrations (0.5-2.0 mg/1) gave poorer settling properties and higher turbidities of the effluent than higher DO concentrations (2.0-5.0 mg/1). The main reasons to the deteriorated settling properties were excessive growth of filamentous bacteria and porous flocs. Alternating oxic and anoxic conditions (1-4 hours) did not effect the settling properties to a large extent. No clear relationship could be found between DO concentration and average floc diameter. The volume distribution offices in the size range 11.6-1128 urn fitted well to log-normal distribution functions. Flocs in the supernatant, after 20 minutes settling, had a diameter up to 50-60 urn. More than 80% of the number of flocs were < 2 urn, in most of the measurements. The size distribution of the flocs in the supernatant could best be fitted to power functions. The number offices in the supernatant after 20 minutes settling could relatively well be related to the turbidity.