A Review of Kinetic Model Evaluation for Anaerobic Digestion Process

Variability of process conditions, diversity of bio-sourced feedstock as well as the slow and unstable nature of the process make anaerobic digestion (AD) technology, a subject of recent research despite several centuries of its practice. Model is one of the tools exploited not only to predict the system’s performance but also for improved production. The search for appropriate models is currently a major priority for optimizing the process and the literature is flooded on this subject. This paper reviewed the prominent ones among them from mathematical, experimental and statistical background. It was found that despite the practicality of the exponential model when complemented by the Monod’s model, in describing AD process, the mathematical models failed to describe cell behavior at stationary phase. Knowledge of cell behavior in this phase is significantly relevant especially when high retention times and ‘digestate’ valorization are desired. Hence, the emergence of Gompertz (statistical) models which tends to describe cell growth rate during stationary phase. However, the complexity of the Gompertz models and the inadequacy of mathematical model beyond growth phase led to development of experimental models which do not focus on the growth rate but on the kinetics of substrate degradation or product formation. Specifically, first-order kinetic models, model based on transfer function, and cone model emanating from different types of substrates and combinations in co-digestion have been formulated. Evaluation of these models is critical and requires both the coefficient of determination of the fit (r2) and the root mean squares errors (RMSE). Based on gas potential generated, it is found that anaerobic digestion processes are more of waste management/treatment strategy than energy production irrespective of the substrate combo as energy generation is relatively less than that of the conventional energy.