Perspectives on the Monod model in biological wastewater treatment

Abstract

AbstractThe Monod model has been widely used in activated sludge and biofilm models to mathematically describe the kinetics of microbial growth. Mathematical models of biological systems can be distinguished according to their dimensionality as zero‐dimensional (0D), one‐dimensional (1D), two‐dimensional (2D) and three‐dimensional (3D). The Monod model is a 0D model, which does not take cell (aggregate) structure into account, yet has been used in different ways in activated sludge and biofilm modelling: In activated sludge modelling, it is used in its original sense (i.e. 0D), whilst in dimensional biofilm modelling, it is used to model intrinsic kinetics in the biofilm and thus in 1D, 2D and 3D. We believe that the latter is not the correct use of the Monod model, because both activated sludge and biofilm systems consist of similar cell aggregates that both feature concentration gradients. Diffusional mass transfer limitations are manifested by the values of kinetic parameters in the Monod model and should not be described explicitly by Fick's laws of diffusion. The 0D concept has been adopted in state‐of‐the‐art activated sludge models and more recently in 0D biofilm models. Measured kinetic parameters can be directly transferred to 0D models. The application of Fick's second law of diffusion in combination with the Monod model (i.e. the reaction–diffusion equation) therefore becomes irrelevant. In this manner, the Monod equation becomes a simple, universal approach to modelling that contains variable parameters. © 2022 Society of Chemical Industry (SCI).