Dissolved oxygen (DO) is entrained by mixed liquid recirculation (MLR) into the anoxic basin in the pre-denitrification biological nitrogen removal (PDBNR) process and should be removed to enhance denitrification. A laboratory-scale PDBNR reactor with an anoxic basin divided into two isometric components was built to investigate the effect of a hydrocyclone on DO removal from the MLR and the subsequent denitrification. The denitrification rate of the mixture of influent and hydrocyclone-treated nitrified wastewater increased with increasing DO and was higher than that of the wastewater in a conventional anoxic component. More total nitrogen was removed in the anoxic component due to the application of the hydrocyclone in the MLR of a laboratory-scale reactor and a full-scale anoxic/aerobic reactor.