Modeling Approach Shows the Growth Difference of Filaments and Floc Formers Inside the Activated Sludge Flocs

Kinetic selection (KST) and substrate diffusion limitation (SDL) are two important factors that affect the competition between filaments and floc formers in activated sludge system. The combination of both effects has been previously used to explain our contradictory experimental results (Lou and de los Reyes, 2008), from which a conceptual qualitative framework was developed with three regions (bulking, intermediate and non-bulking regions) suggested. However this framework has not been confirmed and the range of the substrate concentration for each region was not determined. To approve the framework quantitatively, a basic model integrating KST and SDL was developed in this study for simulating the growth rates of filaments and floc formers inside the activated sludge flocs. The kinetic parameters measured in our previous experiment were used to incorporate into the model. The model was further extended to verify the results from our previous experiments that were operated in the sequencing batch reactors (SBRs) at different influent substrate concentrations (100 mg/L, 300 mg/L and 600 mg/L). The effect of differences in decay rates of both organism types, and oxygen limitation were also tested in this study to evaluate their effects on the growth rates coming KST and SDL. The basic model simulation results showed that the model was successful for confirming the proposed conceptual framework with three regions determined by the substrate concentration. The results of the extended model were consistent with the experimental data that bulking occurred at large floc while non-bulking happened at small flocs. The sensitivity analysis indicated that kinetic parameters have greater effect than diffusion parameters on the simulation results, and the growth of floc formers were more sensitive to the parameters than filaments.