Modelling the Impact of Filamentous Bacteria Abundance in a Secondary Settling Tank CFD Sub-models Optimization Using Long - term Experimental Data

The objective of this work was to assess the impact of filamentous bacteria on the settling velocity and rheological behaviour of activated sludge. We then identified the relevant settling and rheological model parameters to account for the impact of filamentous bulking on the prediction of sludge mixing and transport in secondary settling tanks by a computational fluid dynamics (CFD) model. We identified the relevant settling velocity and rheology model parameters influenced by the filamentous bacteria content of activated sludge. The hindered, transient, and settling parameters of the settling velocity model proposed in our previous study were estimated using measurements from batch settling tests with a novel column setup. Additionally, the rheological measurements from experiments with a rotational viscometer were used to calibrate the HerschelBulkley rheology model including the rheology correlations with the sludge concentration obtained in our previous study. Both settling and rheological tests were performed with sludge samples collected biweekly from the Lundtofte wastewater treatment plant in a four-month measurement campaign. Quantitative fluorescent in-situ hybridisation (qFISH) analysis was carried out on the sludge samples to quantify the volume fraction of filamentous bacteria. Based on the correlations of settling and rheological model parameter values with the volume fraction of filamentous bacteria, we identify the significant impact of filamentous bacteria on the hindered settling of activated sludge. However, no significant impact on the transient and compression settling model parameters was observed. This study also finds that microbial filaments residing inside the microbial flocs can significantly alter the rheological behaviour of activated sludge. A two-dimensional, axi-symmetrical CFD was used to assess the impact of calibration scenarios for settling and rheology under low and high abundance of filamentous bacteria on the CFD predictions. Results obtained suggest that the influence of filamentous bulking on the settling and rheology of activated sludge can affect the solids distribution and transport in the SSTs.