Coupling computational fluid dynamics and kinetic models using a compartmental model applied to a full-scale agricultural digester

Anaerobic digestion, known for its potential to reduce climate change, has been widely studied. However, the interactions between biokinetics and hydrodynamics remain insufficiently studied, especially on a full scale. This study aims to address this gap by developing a Compartmental Model (CM) to describe the hydrodynamics and kinetic processes within a full-scale agricultural digester. The digester was divided into different numbers of compartments based on the velocity fields obtained through Computational Fluid Dynamics (CFD) simulations. The hydrodynamic component of the CM was subsequently validated by comparing the digester Residence Time Distribution (RTD) derived from both the CFD (two months of calculation) and CM approaches (15 min of calculation per configuration). The validated model was then applied to investigate the effect of mechanical agitation on RTD, offering a computationally efficient alternative to the CFD approach. Regarding the biokinetic processes, Anaerobic Digestion Model No. 1 was integrated into the CM, enabling the model variables to be determined temporally and spatially over a long-term period. The areas proximal to the reactor inlet exhibited the greatest biological activity, producing 58.9% of the specific methane (CH4) within only 26.8% of the reactor volume, thereby increasing the risk of acidification and highlighting this area as crucial for digester monitoring. Furthermore, the influence of mixing on CH4 production was explored by simulating the full-scale digester under three main hydrodynamic configurations: a single Continuous Stirred Tank Reactor (CSTR), the CM with fifteen compartments, and two CSTRs in series representing the absence of mechanical agitation. The system without mechanical mixing produced higher CH4 but exhibited an increased risk of acidification.