Aerobic granulation

Heterotrophic aerobic granular sludge was cultivated in a sequencing batch airlift reactor (SBAR). A steady state operation, at a COD loading of 2.3 kg COD/(m³· d), was run for 50 days after which the experiment was terminated. The granules obtained were smooth, 1.0 mm in diameter and had a density of 48 g/l. Selection of dense granules occurred by applying a short settling time. Slow settling s ludge was washed out with the effluent and only the granules with good settling properties settled fast enough (v_min>16.2 m/h) to be retained in the reactor. Nitrification and almost complete denitrification occurred in the fully aerobic SBAR without any optimization of the process conditions. Compared to a sequencing batch bubble column (SBBC) much more dense granules with a smaller diameter were obtained in the SBAR at the same substrate loading rate and the same mixing intensity. Compared to granules formed in a, continuous fed, biofilm airlift suspension (BAS) reactor much more dense granules were obtained in the SBAR as well. The diameter of the granules in the BAS reactor was smaller than in the SBAR. Possible reasons for the better quality of the sludge granules in the SBAR are discussed.