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 loa ding rate of 3.6 kg COD/ (m(3) 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 a verage diameter of 3.0 mm. An isopycnic centrifugation procedure was used t o 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, respe ctively. In free-settling tests the final settling velocity of single aggre gates was examined to estimate porosity. Settling velocities up to 2.0 cm/s could be measured. Calculations based on the experimental results showed a n average granula porosity of 72% for the first run and 65% average porosit y for the second run. This paper indicates the validity of general assumpti ons in free-settling tests.