Generation and properties of aerobic granular sludge

A sequencing batch reactor (SBR) was used to investigate the generation of different granules cultured under aerobic and alternating anaerobic/aerobic conditions. The reactor was fed with synthetic wastewater. A substrate loading rate of 3.6 kg COD/(m3 day) was applied. Granules of heterotrophic microorganisms were formed. After the first experimental period of 8 weeks the average granule diameter was 3.2 mm. In the second period, alternating anaerobic/aerobic conditions were applied to form granular sludge with an average diameter of 3.0 mm. An isopycnic centrifugation procedure was used to determine the characteristic density of the aerobic granular sludge. The average density of the granular sludge was 1.044 g/ml and 1.048 g/ml, respectively. In free-settling tests the final settling velocity of single aggregates was examined to estimate porosity. Settling velocities up to 2.0 cm/s could be measured. Calculations based on the experimental results showed an average granula porosity of 72% for the first run and 65% average porosity for the second run. This paper indicates the validity of general assumptions in free-settling tests.