Computational fluid dynamics modelling of primary sludge classification in an activated sludge process based wastewater treatment plant: Simulating the hydrodynamic behaviour and experimental verification of the classification efficiency

•Novel primary sludge classifier (hydrocyclone) was designed and tested in this study.•CFD models were developed for different influent velocity in the activated sludge process.•The simulation and experimental classification efficiencies: 27.6% and 28.2%.•The CF’s for TSS, FSS and VSS were 1.50±0.18, 1.69 ±0.27 and 1.35.±0.21.•High potential of aeration energy saving: ∼3194 kWh/year (0.07 kWh/m3).