Long-term aerobic granular sludge stability through anaerobic slow feeding, fixed feast-famine period ratio, and fixed SRT

The deterioration of the structural integrity of aerobic granular sludge (AGS) in long-term operation is a major limitation of the AGS biotechnology. The present study explored AGS long-term stability at semi-pilot-scale using the combined strategy of long anaerobic slow feeding, 1:3 fixed ratio of feast-famine period within each cycle, and selective discharge of mature granules. Biomass characteristics, extracellular polymeric substances (EPS), dissolved oxygen and removal profiles were monitored. Results indicate that the 1:3 ratio of feast-famine period controlled the EPS content at a suitable level to allow for granular sludge stability. The system exhibited high proteins (PN) content of EPS, resulting in high PN/polysaccharides (PS) ratios: 0.01–19 and 2.05–13.15 for loosely-bound EPS (LB-EPS) and tightly-bound EPS (TB-EPS), respectively. Selective withdrawal of mature granules from the bottom of the reactor resulted in a good mix of new and old granules in the system. The strategy allowed for system stability as the reactor was operated for over 240 d without any granule disintegration. High removals of COD (98.3 ± 0.8 %), NH₃-N (98.5 %), and TN (89±6 %) were achieved. Phosphate-P removal was in the range 50–100 %. EPS producers (Thauera, Flavobacterium and Meganema) and slow growing bacteria (Acinetobacter and Simplicispira) in the system contributed to the AG stability.