Simulation modeling for nitrogen removal and experimental estimation of mass fractions of microbial groups in single-sludge system

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 (Rm) 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 (0.35 kg NH4+-Nm-3d-1) decreased. A kinetic model that accounts for the mass fractions of Nitrosomonas, Nitrobacter, nitrate reducer and nitrite reducer (fn1, fn2, fdn1, and fdn2) in the SSNRS and an experimental approach for the estimation of the mass fractions of nitrogen-related microbial groups are also proposed. The estimated fdn1 plus fdn2 (0.65–0.83) was significantly larger than the fn1 plus fn2 (0.28–0.32); the fn1 (0.21–0.26) was larger than the fn2 (0.05–0.07); and the fdn1 (0.32–0.45) varied slightly with the fdn2 (0.33–0.38). At the lower VLR, the fdn1 plus fdn2 increased with increasing Rm; however at the higher VLR, the fdn1 plus fdn2 did not increase with increasing Rm. 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.