A modeling perspective on biofilm formation in granular activated carbon filters – local promotion of autotrophic microorganisms due to the effects of adsorption processes on the availability of organic substrates

Various studies demonstrated that biofilm formation occurs in granular activated carbon (GAC) filters for (waste-)water treatment. However, little is known about how transient adsorptive interactions between organic solutes and the GAC within the filter bed influence biofilm development on the macroscale. This study proposes a numerical approach to simulate biofilm development in a GAC filter bed. For this purpose, a model approach for simulations at the single grain scale was extended to additionally account for spatial gradients along the filter bed length. The model was successfully tested with operational data from pilot-scale GAC filters. The subsequently simulated scenarios aimed at conceptually identifying key interactions between the GAC and biofilm formation, including spatial gradients in its composition. The simulation results showed that both heterotrophic and autotrophic microorganisms grew in GAC filters under typical operating conditions. The heterotrophs grew closer to the filter influent, consistent with the system's plug-flow-like behavior. Adsorption of organic solutes onto the GAC resulted in a stricter longitudinal separation of the two general types of microorganisms in the filter bed compared to a non-adsorbing reference filter bed by decreasing the downstream concentrations of organic substrate. Considering explicit backwash events further consolidated this separation for the GAC case. Together with the periodic adsorptive retention and release of organic solutes in the upper filter bed section, depending on the current biological activity, backwash events created even more favorable conditions for autotrophic growth in intermediate regions of the filter bed. Overall, the simulation results showed that autotrophic activity was locally enhanced by adsorptive effects of the GAC and that its extent was directly influenced by the simulated backwash regime. Considering the link between autotrophic activity and co-metabolic biotransformation of organic micropollutants discussed in literature, the results further highlight the potential biological contributions to the overall removal of certain micropollutants in GAC filters, but also the necessity to adequately represent longitudinal gradients and biofilm thickness control mechanisms in mathematical models.