Nitrification–denitrification in a single-sludge nitrogen removal system (SSNRS; with a sufficient carbon source for denitrification) was performed. With an increase in the mixed liquor recycle ratio (R m ) from 1 to 2, the total nitrogen (TN) removal efficiency at a lower volumetric loading rate (VLR=0.21NH4+-Nm-3d-1) increased, but the TN removal efficiency at a higher VLR (0.35kg NH4+-Nm-3d-1) decreased. A kinetic model that accounts for the mass fractions of Nitrosomonas, Nitrobacter, nitrate reducer and nitrite reducer (f n1 , f n2 , f dn1 , and f dn2 ) in the SSNRS and an experimental approach for the estimation of the mass fractions of nitrogen-related microbial groups are also proposed. The estimated f dn1 plus f dn2 (0.65–0.83) was significantly larger than the f n1 plus f n2 (0.28–0.32); the f n1 (0.21–0.26) was larger than the f n2 (0.05–0.07); and the f dn1 (0.32–0.45) varied slightly with the f dn2 (0.33–0.38). At the lower VLR, the f dn1 plus f dn2 increased with increasing R m ; however at the higher VLR, the f dn1 plus f dn2 did not increase with increasing R m . By using the kinetic model, the calculated residual NH4+-N and NO2--N in the anoxic reactor and NO2--N and NO3--N in the aerobic reactor were in fairly good agreement with the experimental data; the calculated NO3--N in the anoxic reactor was over-estimated and the calculated NH4+-N in the aerobic reactor was under-estimated.