Modeling interactions between the adsorption of dissolved organic carbon and biological activity on granular activated carbon

Several studies observed that biological removal of organic substances in biofilters during advanced wastewater treatment depends on the type of filter bed material. Even after long operation, granular activated carbon (GAC) filters showed superior removal of organic substances compared to biofilters with non-adsorbing filter bed materials, suggesting that enhanced biological activity may be the main driver for this observation. In this study, a one-dimensional (1D) continuum multispecies biofilm model coupled with multisolute adsorption of dissolved organic carbon (DOC) was developed to simulate a single biologically active GAC grain. Simulations with an impermeable, non-adsorbing grain were carried out for comparison. The results of this conceptual study demonstrated that adsorptive removal of biodegradable DOC did not influence biofilm development or its final microbial community composition (differences in volume fractions of active biofilm components were < 1.5 % with a total particulate volume fraction of 25 % in the biofilm). Even desorption of previously adsorbed biodegradable DOC, triggered by corresponding concentration gradients between the grain and the biofilm, had no significant effect, although up to 20 % of it was released in the simulated scenarios. This desorption was driven either by increasing biological activity during biofilm development or by sudden concentration changes in the bulk and was also influenced by the biofilm thickness. Overall, the biofilm thickness proved to be a decisive influencing factor. With increasing biofilm thickness, the models predicted increased biological activity and a more diverse microbial community composition, independent of adsorption or desorption processes. The simulation results therefore suggest that the increased biological activity observed in GAC systems does not result from adsorptive interactions between the GAC and biodegradable DOC. Other influencing factors such as protection against detachment, which leads to thicker biofilms, might play a more important role.