Advances in computational fluid dynamics for anaerobic digestion: integrating heat transfer and kinetics under transient conditions

Anaerobic digestion occurs at the intersection of fluid dynamics, convective heat transfer, and the kinetics of temperature-sensitive reactions, where transient thermal influences can cause spatial variations in reaction rates that lumped models fail to capture. While previous reviews on computational fluid dynamics in anaerobic digestion have focused primarily on hydrodynamics, mixing optimization, or rheology, none have systematically synthesized the mathematical and numerical strategies for integrating heat transfer formulations with microbial kinetic models under transient conditions. This review addresses this gap by critically comparing turbulence closure strategies, energy transport approaches, and time integration schemes, relating each modeling option to its expected impact on engineering-relevant outcomes, such as thermal stability margins, across different-scale digesters. The reviewed studies indicate that thermal disturbances can produce spatially heterogeneous temperatures that alter local kinetic rates and stability margins. However, the field remains limited by the lack of experimental validation of coupled thermo-hydrodynamic–kinetic predictions, particularly for non-Newtonian slurries and heating configurations. Integrating microbial kinetics into transient energy models enhances the physical consistency of anaerobic digestion simulations. In this framework, temperature functions as an active variable that simultaneously influences reaction rates, digestate rheology, and transport properties through bidirectional coupling. This dynamic interaction cannot be fully represented by steady-state or thermally decoupled models.