Formation of Densified Activated Sludge with Paracoccus Enrichment for Simultaneous Nitrogen and Phosphorus Removal in a Sequencing Batch Reactor

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.