Modeling of anaerobic digestion of sludge

Anaerobic digestion has been successfully employed for more than one hundred years to stabilize sewage sludge. The main aim of sludge digestion is the degradation and destruction of organic matter, with consequent sludge stabilization and pathogen reduction. One of the advantages of the fermentative system over aerobic treatment is the production of biogas (i.e. an available source of energy), which can be exploited to satisfy part of the energy requirements of the treatment plant or, depending on the plant potentiality, recovered for other uses. Nevertheless, inadequate knowledge of the principles governing the process often prevents satisfactory digester management, and thus there is an evident need for reliable kinetic models as a tool to be applied in order to improve process performance and optimize the control strategies. This paper reviews the current state of the art in anaerobic digestion modelling, identifying several key areas and crucial aspects that need to be investigated to enhance process modelling and to provide a basis for efficient and reliable design and control strategies. Models are grouped according to the degree of substrate characterization and thus of complexity (in terms of number of equations and parameters) and reported in the chronological order which reflects their evolution. To make the model comparison easier to use, a representation in matrix form (according to the methodology proposed by the International Water Association (IWA) Task Group) is also given. The potentialities and limits of available models are discussed with a view to defining both fundamental research needs and critical aspects for their practical application.