The combination of external conditioning and Ca2+ addition prior to the reintroduction of effluent sludge into SBR sharply accelerates the formation of aerobic granules

Aerobic granular sludge (AGS) technology has been limited by its long startup time in sequencing batch reactors (SBRs). The objective of this work is to determine if a new amendment of the AGS process improves sludge granulation rate and pollutants removal efficiency. This amendment consisted in an external conditioning step during which the effluent sludge was dried, supplemented with 0.1 g CaCl2 g(-1) dried sludge (DS) and then gradually reintroduced into the SBR. Results have shown that this technique can reduce the granulation time by 17 days while ensuring the stability of AGS. Stable aerobic granules were obtained on day 20, i.e. a few days after reintroducing 1.75 g.L-1 Ca2+-conditioned sludge to the SBR for 5 consecutive days. Mixed liquor suspended solids (MLSS), sludge volume index ratio at 30 min and 5 min (SVI30/SVI5) and average granule size in the AGS reactor were maintained at 5.7 g.L-1, 1.0 and 3.5 mm, respectively. The external conditioning led to an increase in extracellular polymers (EPS) production, a decrease in moisture content, specific oxygen uptake rate (SOUR) and iron precipitation. Altogether, these impacts led to a faster aggregation of the effluent sludge compared to a control SBR. Furthermore, microbial community analyses have shown that the external conditioning process enriched specific Betaproteobacteria (e.g. Thauera genus) in aggregates and AGS. Such outcomes could be linked to the faster granulation rate and more efficient pollutants degradation.