Breaking activated sludge (AS) for organic compounds as supplemental carbon sources is beneficial for enhanced nitrogen removal from wastewater with low carbon/nitrogen ratios, but it is usually limited by the severe deterioration of microbial activity in wastewater treatment systems. In this paper, the hydrocyclone breakage method was proposed to treat AS to release carbon sources and simultaneously enhance microbial activity. The effects of hydrocyclone breakage on carbon sources release and physicochemical properties of AS were emphatically investigated through various batch experiments. The results indicated that the internal carbon sources were effectively released from AS after hydrocyclone breakage, and the soluble chemical oxygen demand (SCOD) and polysaccharides concentrations were increased by over 100% at higher inlet flow rates of the hydrocyclone. Importantly, the sludge aggregates were broken into smaller pieces which still had a good settleability and low expansibility in wastewater treatment systems. The denitrification rate of AS improved 45.5% after hydrocyclone treatment. Meanwhile, extra cellular polymeric substances (EPS) on AS surfaces and porous channels were also cleaned by shearing exfoliation and centrifugal desorption. Therefore, the hydrocyclone breakage for AS showed great advantages in releasing partial carbon sources and synchronously enhancing microbial activity of survival microorganism for advanced nitrogen removal. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.