Potential cause of aerobic granular sludge breakdown at high organic loading rates

Aerobic sludge granules are compact, strong microbial aggregates that have excellent settling ability and capability to efficiently treat high-strength and toxic wastewaters. Aerobic granules disintegrate under high organic loading rates (OLR). This study cultivated aerobic granules using acetate as the sole carbon and energy source in three identical sequencing batch reactors operated under OLR of 9-21.3 kg chemical oxygen demand (COD) m(-3) day(-1). The cultivated granules removed 94-96% of fed COD at OLR up to 9-19.5 kg COD m(-3) day(-1), and disintegrated at OLR of 21.3 kg COD m(-3) day(-1). Most tested isolates did not grow in the medium at > 3,000 mg COD l(-1); additionally, these strains lost capability for auto-aggregation and protein or polysaccharide productivity. This critical COD regime correlates strongly with the OLR range in which granules started disintegrating. Reduced protein quantity secreted by isolates was associated with the noted poor granule integrity under high OLR. This work identified a potential cause of biological nature for aerobic granules breakdown.