About one third of the industrial activated sludge (AS) plants worldwide suffer from bad settling sludge, often caused by filamentous bulking phenomena. The present study investigated the effectiveness of a sludge granulation/densification strategy, based only on a metabolic selection mechanism, to eliminate sludge bulking in a sequencing batch reactor (SBR) treating real industrial wastewater. The wastewater originated from a tank truck cleaning company transporting chocolate and beer. The proposed strategy involved the introduction of a slow unaerated/anaerobic feeding step in the SBR operation. On lab-scale, the new feeding strategy resulted in (1) excellent settling with a sludge volume index (SVI) decreasing from more than 300 mL·g−1–100 mL·g−1 and lower, (2) the elimination of sludge bulking genera and (3) the significant enrichment of glycogen-accumulating organisms (GAO), mainly Defluviicoccus and Candidatus Competibacter, and this in less than 80 days. The feeding strategy was then applied to the full-scale installation, yielding similar results: a stable average SVI of 37 mL·g−1 was reached after approximately 150 days. Full granulation was however not reached, which warrants further optimization. The present study shows that the proposed strategy can easily be applied in existing SBR systems to solve the problem of sludge bulking.

For a successful granulation process in activated sludge systems, the stimulation of slow growing organisms such as glycogen accumulating microorganisms (GAOs) is a key factor. Here we show that the introduction of an anaerobic feast followed by an aerobic famine phase successfully transforms bulking sludge, caused by the abundance of genus Kouleothrix, to a hybrid floccular-granular sludge. Two sequencing batch reactors (SBRs) were operated for 228 days treating the same industrial wastewater derived from the cleaning of trucks transporting liquid food (the cargo consists of approximately. 70% chocolate and 30% beer). By respectively applying a fast and slow feeding in two parallel SBRs, different degrees of substrate build-up were achieved in the two reactors during the feast phase. The F/M ratio over the feeding time was 1.41 ± 0.48 and 0.57 ± 0.16 kg COD·(kg VSS*d)−1 for the fast-fed and the slow-fed SBR SBR_fastF and SBR_slowF respectively. Our results demonstrate that substrate build-up during the anaerobic selection step is not necessary to obtain well settling granular-like sludge.

This study quantifies the hydraulic performance of a pilot-scale ultrafiltration system integrated into a full-scale industrial aerobic granular sludge (AGS) plant. The treatment plant consisted of parallel AGS reactors, Bio1 and Bio2, with similar initial granular sludge properties. During the 3-month filtration test, a chemical oxygen demand (COD) overloading episode took place, affecting the settling properties, morphology, and microbial community composition in both reactors. The impact on Bio2 was more severe than on Bio1, with higher maximal sludge volume index values, a complete loss of granulation, and the excessive appearance of filamentous bacteria extending from the flocs. The membrane filtration properties of both sludges, with these different sludge qualities, were compared. The permeability in Bio1 varied between 190.8 ± 23.3 and 158.9 ± 19.2 L·m−2·h−1·bar−1, which was 50% higher than in Bio2 (89.9 ± 5.8 L·m−2·h−1·bar−1). A lab-scale filtration experiment using a flux-step protocol showed that the fouling rate remained below 0.5 mbar·min−1 for Bio1, but increased in Bio2, resulting in a critical flux of 30–35 L·m−2·h−1. The membrane resistance due to pore blocking was three times higher in Bio2 than in Bio1. This study shows the positive impact of granular biomass on the long-term membrane filtration properties and stresses the importance of granular sludge stability during reactor operation.