Computational fluid dynamic (CFD) modelling in anaerobic digestion: General application and recent advances

Nowadays, thanks to the enhancement in computational power and software development, advanced mathematical modelling based on computational fluid dynamics (CFD) allows us to represent almost any system. Anaerobic bioreactors correspond to a complex biosystem where multiple reactions, in parallel and/or in series, take place. Besides this biological complexity, the actual digester operation increases the system's complexity given the number of transport phenomena that also occur; therefore, few real applications may be found in which mathematical models are properly harnessed. This review presents a general assessment of the CFD applications that have been applied in anaerobic digestion processes starting with the model set-up up to the post-processing of the results. In regards to the model pre-processing and setup, the generation and evaluation of the mesh and the model specifications such as multiphase flow, turbulence regimen, rheology characterization and impeller motion, are addressed. Due to the importance of the evaluation of the model's outcome, topics such as error assessment and origin, model calibration and validation are discussed. The current challenges and future perspectives of this type of mathematical modeling are also addressed, with particular emphasis on the integration of biological reactions with the conventional fluid dynamic modeling.