Hydrocyclones in wastewater treatment: evolving from particle separation to biological process regulation.

Hydrocyclones have traditionally been used for grit and particle separation, but recent studies have expanded their role toward selective sludge classification and biological process regulation. This review develops a process-oriented framework to integrate these emerging applications. Four major scenarios are examined: grit and particle separation, activated sludge fractionation and settleability enhancement, endogenous carbon release and biological nutrient removal, and sludge disintegration and reduction. As separation targets shift from dense inorganic particles toward increasingly similar-density sludge fractions, hydrocyclones evolve from particle removal toward selective sludge retention, sludge-structure regulation, and biological process enhancement. Reported studies show sludge volume index reductions of 9-66%, improvements in total nitrogen and total phosphorus removal of 6-30%, and reductions in excess sludge production of 30-35%. Beyond individual process improvements, hydrocyclones can provide synergistic benefits through improved sludge management, reduced chemical and energy demand, and in situ treatment-capacity expansion. These benefits depend on selective classification and appropriate integration with the overall treatment process rather than on separation efficiency alone. Future research should therefore move beyond unit-level performance toward integrated process optimization and system-level assessment of the material, energy, operational, and economic consequences of hydrocyclone-based wastewater treatment.