This study examines the effects of polyester and denim microfibers (MFs) on aerobic granular sludge (AGS) over a 42-day period. Treatment performance, granulation, and microbial community changes were assessed at 0, 10, 70, 210, and 1500 MFs/L. Reactors with 70 MFs/L achieved rapid granulation and showed improved settling by day 9, while 0 and 10 MFs/L reactors showed delayed granule formation, which was likely due to limited nucleation and weaker shear conditions. Severe clogging and frequent maintenance occurred at 1500 MFs/L. Despite > 98% MF removal in all reactors, treatment performance declined at higher MF loads. Nitrogen removal dropped from 93% to 68%. Phosphate removal slightly increased in reactors with no or low microfiber loads (96–99%), declined in reactors with 70 or 210 MFs/L (92–91%, 89–88%), and dropped significantly in the reactor with1500 MFs/L (86–70%, p < 0.05). COD removal declined with increasing MF load. Paracoccus (denitrifiers) dominated low-MF reactors; Acinetobacter (associated with complex organic degradation) and Nitrospira (nitrite-oxidizing genus) were enriched at 1500 MFs/L. Performance decline likely stemmed from nutrient transport blockage and toxic leachates, highlighting the potential threat of MFs to wastewater treatment and the need for upstream MF control.

Efficient simultaneous nitrification, denitrification, and phosphorus removal typically requires either separate tanks with oxic and anoxic conditions or aerobic granular sludge (AGS) with granules larger than 0.2 mm. In this study, small granular flocs (50–100 µm) were cultivated with improved settleability, high biomass retention, strong ammonia and COD removal, late-stage phosphate removal, and enrichment of Paracoccus. The sequencing batch reactor was seeded with biomass from a municipal wastewater treatment plant and fed acetate-based synthetic wastewater for 114 days. The small granular flocs formed densified activated sludge (DAS), with biomass accumulating to approximately 12 g/L mixed liquor suspended solids. DAS developed under selection conditions commonly applied in aerobic granular sludge systems, including unaerated feeding and mixing, elevated hydrodynamic shear, and progressive reduction in settling time. The sludge volume index (SVI) improved to 30 mL/g, comparable to that of granular sludge, with a moderate SVI₃₀/SVI₅ ratio of 72%. The enrichment of Paracoccus is highly desirable in wastewater treatment. Importantly, these findings show that efficient simultaneous nutrient removal does not strictly depend on large granular structures.

Efficient simultaneous nitrification, denitrification, and phosphorus removal typically requires either separate tanks with oxic and anoxic conditions or aerobic granular sludge (AGS) with granules larger than 0.2 mm. In this study, we cultivated small granular flocs (50-100 µm) capable of simultaneous heterotrophic nitrification and aerobic denitrification (HN-AD) in a sequencing batch reactor. The bioreactor was seeded with biomass from a municipal wastewater treatment plant and fed acetate-based synthetic wastewater for 114 days. The small granular flocs formed densified activated sludge (DAS), with biomass accumulating to approximately 12 g/L mixed liquor suspended solids. DAS was cultivated under selection pressures commonly used for AGS formation, including anoxic feeding, extended anoxic mixing, elevated hydrodynamic shear, and a gradual reduction in settling time. The sludge volume index (SVI) improved to 30 mL/g, comparable to that of granular sludge, with a moderate SVI₃₀/SVI₅ ratio of 72%. The relative abundance of Paracoccus reached 72.9%, indicating the metabolic capability of these small granular flocs to carry out HN-AD. Overall, these findings show that efficient simultaneous nutrient removal does not strictly depend on large granular structures. Achieving DAS and HN-AD offers a scalable approach to intensify biological nutrient removal while reducing reliance on distinct anoxic zones.