Upgrading waste material flow analysis with process models: The case of anaerobic digestion

Abstract

Material flow analysis (MFA) has been shown to be a valuable tool for goal-oriented decision making of waste management systems. When used to evaluate or compare waste management scenarios, it relies on applying transfer coefficients to model how the flows of goods and substances are transformed through waste treatment processes. Although transfer coefficients provide a useful simplification, their applicability to represent complex processes such as anaerobic digestion (AD) is limited. In this study, three levels of model complexity were compared to upgrade an MFA: a material-specific empirical transfer coefficient method using the biochemical methane potential, a kinetic method using a first-order model, and a process-based method using the Anaerobic Digestion Model No. 1 (ADM1). The three models were integrated in an MFA and compared based on their prediction of methane production and nitrogen recovery for different waste compositions, reactor operating conditions and simulation time steps. The results showed that the simpler models were sufficient to predict the methane production, but that only complex models such as ADM1 could predict behaviour regarding the loss of nitrogen in the liquid digestate. The results also showed the advantages of integrating the model in transient state for nitrogen modelling. An MFA upgraded with a process model such as ADM1 allows to better implement the analysis on both the levels of goods and substances and can be useful for the case of waste treatment processes other than AD.