Biofilm models are effective tools that allow the mathematical description of biofilm systems,the prediction of their removal performance as well as their conceptual exploration. Models have the advantage of being less resource and time consuming than laboratory experiments and being more flexible regarding the scenarios that can be analyzed. Due to the high complexity of biofilms, it is virtually impossible to develop a model that comprises all the phenomena occurring within the biofilm. Simplifications are considered a substantial part of the modelling process, and even the most comprehensive models developed until now make use
of assumptions and simplifications. Nevertheless, the vast majority of models fulfill their purpose and are useful to both researches and practitioners (Wanner et al. 2006). The decision on which model is better for which modelling task relies on the data available, the level of understanding of the phenomena occurring in the system and the objective of the modelling task. In the specific case of biofilms, a trade-off between microscale features and global modelling results is often present. A more detailed description of the microscale features
does not necessarily lead to more compelling modelling results, and occasionally it can compromise the identifiability and the determination of relevant parameters. However, neglecting microscale features can result in inaccurate representations of the system and thus the explanatory power of the model may be diminished. Depending on the system and its specific conditions, the modeler is presented with a dilemma: Which microscale features are worth including,
which simplifications can be afforded and which simplifications are compulsory due to the lack of information.
Two microscale features are of special interest in this work: microbial community composition and dimensionality. Three biofilm systems were used to explore the impact that these two microscale features have on the global modelling results and on the model’s explanatory power:
A biologically active Granular Activated Carbon (bGAC) filter, a Moving Bed Bioreactor (MBBR) and a Membrane Aerated Biofilm Reactor (MABR). An individual publication is dedicated to each one of these systems. These systems were used to showcase alternative modelling approaches and to illustrate the effect of choosing simple or more complex descriptions
of the microscale features of interest.
The first and second publication, P1 and P2 respectively, focus on the microbial community composition. Publication P1 deals with DOC removal from WWTP effluents in a bGAC-filter.
Within the filter, DOC is removed by simultaneous adsorption and biodegradation, therefore a suitable model should include both mechanisms. It proposes a model that integrates a traditional one-dimensional biofilm model with the ideal adsorbed solution theory that can be applied within the activated sludge model framework. A simplified microbial community composed solely composed of aerobic heterotrophic bacteria is selected. The developed model is able to describe the DOC breakthrough curves at different empty bed contact times and it also
shows the relative contribution of biodegradation and adsorption to the total DOC removal.
Publication P2 analyzes the behavior of heterotrophic bacteria in an MBBR reactor operating as a Partial Nitritation/ Anammox (PN/A) system. It discusses the growth strategies that hetrotrophic bacteria pursue when facing substrate scarcity. The effect of the yield-strategy is analyzed in two scenarios. In the first scenario a group of heterotrophic bacteria growing on endogenous COD is investigated, whereas in the second scenario, external COD is allowed into the system and a second group of heterotrophs (rate strategist) growing on the more available external COD is added. The competition between both groups over space, and electron acceptor is assessed. In both scenarios the pursuing of the yield strategy seems to be crucial for the diversity of the heterotrophic community. Effluent concentrations as well as heterotrophic produced dinitrogen gas is strongly affected by the growth-strategy that the heterotrophs selected, higher denitrification activities are observed when the yield strategy is selected.
Finally dimensionality is the microscale feature of interest in the third publication P3. An MABR reactor used for PN/A is modelled. Publication P3 compares a traditional one-dimensional model and different pseudo two-dimension models that allow the implementation of concentration gradients in the bulk liquid and the gas phase, individually and simultaneously as well as in counter or parallel flow. The results show that the one-dimensional model underestimates the effluent’s total dissolved nitrogen concentration in comparison to the prediction
delivered by the pseudo two-dimensional models. Differences in the axial gradients in the biofilm are also observed between the two evaluated modelling approaches. P3 also demonstrates that the concentration gradients in the gas phase have a more significant impact on the modelling results than the concentration gradients in the bulk liquid.
The importance of the microscale features: microbial composition and dimensionality is explored. Alongside, the implications on the global modelling results, product of the simplifications of microscale features are investigated. In the case of microbial composition, although
more information has been made available due to the new experimental techniques (molecularbiology, imagining etc.) there are still disparities between what can be determined experimentally and how this can be implemented into the existent modelling frameworks. In the case of
dimensionality, longitudinal gradients seem to be more influential than it was previously assumed and need to be taken into account to better describe MABRs. The adequate level of complexity required for a microscale feature and in general for a model should be decided
based on the modelling goals, the current understanding of the system and the available data
