Multiscale Modeling of Hybrid CSTR/Biofilm Based Multi-Vessel Reactors for Anaerobic Digestion Processes

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Anaerobic digesters are used to convert agricultural waste into biogas which then can be burned to produce electricity. Most often this biological process is a 1-stage operation in which approximately 50% of the energy of the biomass is converted. Further energy can possibly be extracted in a second stage. For this secondary process a fixed film approach is more appropriate. In this project we develop a simulation tool capable of accurately modelling such a two-stage AD process. The International Water Association Anaerobic Digestion Model No.1 (ADM1) is the quasi standard and point of departure for mathematical model development of anaerobic digestion processes. For the primary step, this will be modeled by a high-dimensional chemostat type model. To model the second stage fixed film system, ADM1 can be coupled with a Wanner-Gujer-like biofilm model. The problem associated with coupling these two model frameworks is that the computational requirements balloon owing to the high dimensionality, and the models become impractical for engineering tasks that need hundreds or thousands full simulations per run. One reason for this is that such biofilm models are designed to resolve concentration fields in the biofilm, whereas for application in reactor models only the substrate fluxes between the biofilm and the aqueus phase are required. We propose to develop a dynamic, ADM1 based multi-scale model of biofilm reactors for anaerobic digestion that correctly accounts for this mass transfer between biofilm and aqueous phase but do not resolve the spatial structure of the biofilm in detail, thus gaining computational efficiency. Such a model will be useful in the design, analysis, and optimisation of a 2 stage anaerobic digester like the on under consideration here.