Sludge disintegration can simultaneously improve biological nutrient removal and reduce excess sludge. A novel sludge disintegration method is proposed wherein a hydrocyclone is used for internal release of carbon source. The effect of hydrocyclone disintegration on mixed liquor recirculation was studied in a side-stream device of an anoxic/aerobic (A/O) wastewater treatment plant (WWTP). The mechanisms of hydrocyclone disintegration were comprehensively investigated, and its energy consumption was compared with those of other disintegration methods. The sludge disintegration degree (DD) reached 6.66-12.25% in the hydrocyclone processed mixed liquor recirculation, leading to a significant increase in the concentration of soluble chemical oxygen demand (SCOD), protein and polysaccharide. However, the sludge size distribution and the stable structure for microorganism aggregation and cell attachment changes only slightly. The sludge disintegration process using the hydrocyclone involves sheared shedding, rotation desorption and centrifugation lysis which are mainly induced by disordered turbulence. The biological degradation experiments with continuous modes revealed significant improvements in microbial activity in disintegrated mixed liquor recirculation. Similarly, the averaged denitrification rate of disintegrated mixed liquor recirculation increased by 14.43% in batch experiments. The continuous operation indicated that the observed biomass yield and efficiency of excess sludge reduction of hydrocyclone-improved A/O process were 0.34 kg VSS/kg COD and 30.61%, respectively. Compared with other disintegration methods, hydrocyclone disintegration exhibited less intensity but lower energy consumption, which indicated that this method shows promises in advanced nitrogen removal and excess sludge reduction. (C) 2016 Elsevier B.V. All rights reserved.