Continuous biofilm reactor: dynamic and bifurcation analysis

Modelling of biofilm reactors has been addressed in the recent literature following different approaches for biofilm growth in different types of multiphase reactors. A few studies have also addressed the bifurcational/dynamical patterns of chemostats though the complex interaction between free and immobilized biophases in biofilm reactors has never been analyzed. Important features of biofilm reactors are represented by: i) the conflicting effects of biofilm growth and detachment, ii) the competition between immobilized and free cells for the carbon source, and iii) the inherent nonlinearity associated with the growth kinetics, which may result in a multiplicity of steady states and periodic phenomena.The present contribute is a part of a research program active at the University of Napoli about the characterization of the performance of three-phase biofilm fluidized beds. The contribute regards the modelling of Internal Loop Airlift (ILA) reactors operated with Pseudomonas sp. OX1 immobilized on a granular carrier [1]. P. sp. OX1 is able to convert phenols and features substrate-inhibited growth kinetics. The role played by biofilm detachment rate during the formation of steady amount of attached biomass has been assessed. The effect of dilution rate and substrate feeding concentration on the bifurcational pattern has also been investigated.The model is based on material balances for the substrate (phenol) and for the free and immobilized cells. The ILA reactor was assumed to behave as a CSTR with respect to the liquid phase, and as a macro-mixed reactor for the solids phase. It was assumed that the granular carrier was confined in the reactor. Mass transfer between the bulk of the liquid phase and the biofilm as well as the biofilm internal mass transport resistance were neglected. The key processes relevant to the reactor performance embodied: i) the suspended biomass growth; ii) the adhesion of suspended cells on solid surface [2]; iii) the detachment of biofilm from solid phase [3]. The model specifically aims at assessing: a) the multiplicity of steady-states and the bifurcational patterns of the system; b) the short-term dynamical response of the system under quasi-steady state operating conditions with respect to the extent of biofilm; c) the long-term dynamical response of the system. Results are analyzed in terms of maps in the phase-space of design variables aimed to identify regions characterized by no operability, single steady state and multiple steady state.. Bifurcational analysis of the steady state solutions indicates that no biofilm can be observed in the reactor at the steady regime and no substrate conversion can be obtained if the detachment coefficient exceeds a given threshold. Below this threshold multiple stable steady states can be established and the bifurcational patterns depend on values of the dilution rate and feed substrate concentration. The solution diagrams as a function of detachment coefficient suggest that, once the severity of biofilm abrasion is known, the larger the dilution rate and the substrate inlet concentration, the more reliable the operation in terms of biofilm stability. 1. Viggiani A, Olivieri G, Siani L, Di Donato A, Marzocchella A, Salatino P, Barbieri P, Galli E. 2006. J. of Biotechnology 123:464-477.2. Rijnaarts HHM, Norde W, Bouwer EJ, Lyklema J, Zehnder AJB. 1995. Colloids and surfaces B4:5-22.3. Gjaltema A, Vinke JL, van Loosdrecht MCM, Heijnen JJ. 1997. Biotechnology and Bioengineering 53:88-89