Bifurcation and stability analysis of an anaerobic digestion model

Abstract

This paper presents the dynamic behaviour of the anaerobic digestion process, based on a simplified model. The hydraulic, biological and physicochemical processes such as those which underpin anaerobic digestion have more than one stable stationary solution and they compete with each other. Further, the attractive domains of the stable solutions vary with the key parameters. Thus, some initial transient process moving toward one stable solution could suddenly move towards another solution, at which a so-call catastrophe takes places (e.g. washout). The paper systematically analyses the stationary solutions with their associated stability, which provides insight and guidance for anaerobic digestion reactor design, operation and control.

Maintenance affects the stability of a two-tiered microbial ‘food chain’?

“…The current ADM1 consensus model (IWA Task Group for Mathematical Modelling of Anaerobic Digestion Processes, 2002) has a staggering 32 dynamic state variables and successfully captures the necessary dynamics of the process. However, this model is far too complex to permit mathematically analysis of its nonlinear dynamics (Shen et al, 2007). In order to make such studies possible, we have therefore reduced the model to its very backbone, a two-tiered microbial 'food chain' with feedback inhibition, which encapsulates the essence of methanogenic degradation processes, see Figure 1.…”

“…This paradigm shows some resemblance to the classical concept of a food chain, the difference being that it is not the organisms themselves but rather their products that serve as a food source for the organisms at the next trophic level (Dolfing and Prins, 1996). Surprisingly little is known about the stability of these 'food chains' (Shen et al, 2007). We therefore seek to investigate the stability of these microbial 'food chains' at a fundamental level and contrast the findings to what is known about the stability of canonical food chains.…”

Steady state multiplicity of two-step biological conversion systems with general kinetics

“…As a rule, these kinetic expressions all consider substrate limitation, but they differ in the number and type of inhibition terms considered -from no inhibition terms to all inhibition terms in (5) and (6). Anaerobic digestion processes are usually described with substrate inhibition of the second reaction as the only inhibition effect (see [3]; [14]), which is described by the term in d 2 . other parameter values and to other processes satisfying the stoichiometry and general kinetics given by (1) and (3)- (6), respectively.…”

“…The steady states in which only intermediate is formed, corresponding to suppression of the second reaction in (1), are obtained from (13)- (14).…”

“…The stability characteristics of an anaerobic wastewater treatment process have been studied by Hess and Bernard [8] resulting in a criterion to detect the conditions of process destabilisation, characterized by VFA accumulation. A two-step anaerobic digestion model with VFA as an intermediate, extended with gasliquid transfer and pH effects has been analysed by Shen et al [14]. In the recent search for new sources of renewable energy, controlling anaerobic digestion towards the production of hydrogen, another process intermediate, is gaining a lot of attention [6].…”