Modeling nitrogen removal performance of a membrane bioreactor under dissolved oxygen dynamics

This study was focused on providing a model evaluation that could be used as an effective prediction tool for nitrogen removal performance of membrane bioreactors (MBRs) with emphasis on the effect of dissolved oxygen concentration variations in the reactor. A pilot-scale MBR fed with raw domestic sewage was operated in a predenitrification mode with a small anoxic reactor at a sludge age of 38 days. Diffusional limitation of dissolved oxygen at high biomass concentration of 16,000mgSS/L continuously sustained an anoxic mass fraction and favored simultaneous nitrification and denitrification (SNdN) in the MBR. SNdN was observed as the major process for nitrogen removal which occurred in the range of 6090% depending on variations in dissolved oxygen (DO) concentrations in the MBR. Full nitrification was observed above DO levels as low as 0.5mgO2/L. Simultaneous denitrification was the rate-limiting process under the selected operating conditions. The magnitude of SNdN occurring in the MBR fluctuated as a function of the DO profile, where the nitrate uptake increased above 30mgN/L for the low DO levels in the range of 0.30.6mgO2/L and decreased down to 1020mgN/L for DO values above 1.5mgO2/L. The specific feature of the MBR operation in terms of diffusion limitation and resulting SNdN could be interpreted by model evaluation in terms of higher values of the half saturation constants in the corresponding switching functions, reflecting the affinity for DO and nitrogen components. In fact, the appropriate calibration values for these half saturation coefficients, namely, KOH, KOA, KNH, and KNO, were determined as 1.0, 1.25, 2.0, and 2.0mg/L, respectively, much higher than the default values suggested in previous activated sludge models.