A sensitivity and model reduction analysis of one-dimensional secondary settling tank models under wet-weather flow and sludge bulking conditions

In this study, we provide the sensitivity and reduction analysis of the current, most advanced one-dimensional (1-D) secondary settling tank (SST) model, the Bürger-Diehl model, under non-ideal flow and settling conditions. Parameter subsets suitable for the Bürger-Diehl model calibration are identified. For example, under the wet-weather condition, all model parameters, except fn and Dc,0, are influential to SST performance. When filamentous bulking occurs, the outputs of the Bürger-Diehl model are most sensitive to the hindered settling parameters, v0 and rh. In terms of parameter interactions, strong interactions are found among parameters in predicting Ce. However, for Cu, SI and Fluxop under the bulking condition, the Bürger-Diehl model is almost additive with negligible parameter interactions. Moreover, the sensitivity of the Bürger-Diehl model outputs to parameters is highly impacted by the imposed simulation conditions, thus resulting in different parameter subsets for model calibration. For example, under the wet-weather condition, the compression settling parameters can be as important as the hindered settling parameters, while bulking of the sludge greatly increases the influence of the hindered settling parameters (v0 and rh), and decreases the influence of the compression settling parameters. In terms of model reduction, reliable reduction of the Bürger-Diehl model can be achieved based on GSA results. For example, under the bulking condition, the Bürger-Diehl model can be reduced to the hindered-dispersion model regardless to model outputs. The reliability of the Bürger-Diehl model reduction can be evaluated based on uncertainty, and unreliable reduction can negatively impact the decision making in SST design and control.