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In this study, a steady-state biofilm model was utilized to elucidate the aerobic granular sludge process. In order to describe the experiment data using the model, two different sizes of stabilized aerobic granules (with mean diameters of 0.6 mm and 3 mm, respectively) were investigated through the laboratory operations of sequencing batch reactors (SBRs). On the basis of the estimated parameters from experiments, the model was established to describe the biofilm functions in aerobic granules, such as substrate transfer and substrate concentration variations within the granules. In the modeling at different chemical oxygen demand (COD) concentrations (100-1000 mg/L), the biofilm functions were affected by the characteristics of aerobic granules, such as diameter and density. The larger granules modeled higher substrate fluxes (1.8-14 mg/cm(2)-d) and deeper effective film thicknesses (0.18-1.3 cm). The multi-aspect simulation results demonstrated that the substrate either approaches zero in the film (at low concentration) or penetrates the granules (at high concentration). The sum of aerobic granules in the SBRs can be considered as a mass of biofilm, where the substrate approaches zero at sufficient depth. The steady-state biofilm model could be an effective prediction method for optimizing the aerobic granular sludge process.

In this study, the characteristics partial nitrifying biomass in the Sequencing Batch Reactor (SBR) for aerobic granulation was investigated based on experiments and simulation modeling. The reactor operation was carried out at high concentration of ammonium (200~850 mg N/L) which favored Ammonia-Oxidizing Bacteria (AOB) growth over Nitrite-Oxidizing Bacteria (NOB) growth. In partial nitrifying granulation, both Free Ammonia (FA) and Free Nitrous Acid (FNA) simultaneously influenced the activity of NOB much more than that of AOB. According to the simulation results, thE Dissolved Oxygen (DO) concentration and oxygen affinity affect the growth competition and can influence the species that are predominant in the reactor. Both AOB and NOB can have growth potential under toxicants (FA and FNA) inhibition and limited oxygen condition. The AOB growth forms inner part of granule's biofilm, but the NOB growth does not engage in the formation of granules.