Effect of biomass concentration on the performance and modeling of nitrogen removal for membrane bioreactors

The study investigated the effect of biomass concentration on nitrogen removal in a membrane bioreactor by model evaluation of system performance. The steady state operation of a pilot membrane bioreactor fed with domestic sewage at a sludge age of 74 days and an average biomass concentration of 27,000 mg/L was monitored. The results were evaluated by calibration of a suspended growth model designed for this purpose and compared with those of an earlier experiment on the same system operated at a sludge age of 34 days, with a markedly lower biomass concentration. The membrane bioreactor always sustained a dissolved oxygen concentration of around 2 mgO2/L which could be explained by diffusion limitation of dissoved oxygen from the bulk liquid into the floc. Nitrogen removal was controlled and limited by nitrification which occurred only partially throughout the study. The oxidized nitrogen was always fully removed by means of simultaneous nitrification denitrification reducing the level of nitrate to a very low level so that the anoxic tank in front of the membrane bioreactor was totally useless in terms of nitrogen removal. Comparison of the results of the two experimental runs indicated that increasing the biomass concentration drastically changed the system behavior from denitrification limitation to nitrification limitation due to increased constraints on the mass transfer of dissolved oxygen. The selected model could be successfully calibrated for the model parameters by means of substantially higher oxygen half saturation constants for heterotrophs (KOH) and autotrophs (KOA) determined as 2.0 mgO2/L and 2.25 mgO2/L, respectively.