Formation mechanisms of dense biological flocs in a full-scale wastewater treatment plant employing hydrocyclone and chemical phosphorus removal

Densified activated sludge (DAS) technology through hydrocyclones has emerged as a promising strategy for intensifying the treatment capacity of existing wastewater treatment plants (WWTPs). Previous studies proposed that hydrocyclones mainly served a physical selector to retain and accumulate dense biological flocs (DBFs) for enhancing sludge settleability. However, the mechanisms driving the formation of DBFs and enhancing their aggregation to resist disruptive shear within the hydrocyclone remain unclear. This study demonstrated the hydrocyclone installation in a full-scale WWTP with chemical phosphorus removal could improve sludge settleability by decreasing SVI₃₀ from 58.5 ± 1.7 to 13.9 ± 1.4 mL/g within 63 days. The installed hydrocyclone facilitated forming DBFs of tens of micrometers with hydrous aluminum and ferric oxides (HAO/HFO) as dense cores coated by biofilms. Sludge characteristic and extracellular polymeric substances (EPS) composition and structure analysis indicated that the forces inside the hydrocyclone led to the detachment of biofilms on the HAO/HFO surfaces, and re-exposed phosphates. Meanwhile, the forces induced cell disruption to release massive intracellular substances with the proportion of aliphatic/protein molecules increasing from 10.0% to 14.7%. Free arginine was selectively adsorbed to the re-exposed phosphate on HAO/HFO surfaces for biofilm reconstruction, with its concentration increasing from 0.53 to 1.29 mg/g VSS. Molecular characterization and X‑DLVO analysis indicated strong Lewis acid-base interactions and polar attractions between phosphate and arginine that enabled DBFs to resist shear and further enhanced aggregation, resulting in 45.5% ± 11.4% of DBFs being retained on a 200 µm sieve. These results advance our understanding of DBF formation and aggregation within hydrocyclones.