STEADY-STATE MULTIPLICITY OF A CONTINUOUS BIOFILM REACTOR
Integration of life sciences & engineering
Biochemical Engineering (T5-1)
Keywords: biofilm, airlift, dynamic, bifurcation, detachment rate
Biofilm reactors may provide an effective tool to achieve intensification of fermentation-based processes. Indeed, this class of reactors is successfully applied in the production of amino acids, organic acids, antibiotics, chemicals as well as for carbohydrate modification and fluids bioremediation [1]. Multiphase contacting in biofilm reactors is usually carried out in fixed or fluidized beds of particles. Despite such upgrowing interest, broad areas of uncertainty still characterize design and optimization of biofilm reactors, calling for further investigation [2]. Recently, the analysis of the dynamic behaviour of the biofilm reactor has been addressed in the literature in view of the strong nonlinearity of the governing equations. Important features of biofilm reactors are represented by: i) the conflicting effects of biofilm growth and detachment [3], 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 paper addresses the characterization of the performance of three-phase biofilm fluidized beds. In particular, the study is focused on the performance of Internal Loop Airlift (ILA) reactors [4] operated with Pseudomonas sp. OX1 immobilized on a granular carrier [5]. P. sp. OX1 is able to convert phenols and features substrate-inhibited growth kinetics.
The mathematical model here proposed is capable of describing key features of the operation of a continuous ILA bioreactor such as multiplicity of steady states or onset of periodic phenomena. The model is based on material balances for the substrate (phenol) and for the free and immobilized cells (Pseudomonas sp. OX1); a simplified formulation of the biofilm detachment rate is also introduced. The ILA was assumed to behave as a CSTR with respect to the liquid phase, and as a macromixed reactor for the solids phases. 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 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. Model results are analyzed in terms of maps in the phase-space of design variables where regions characterized by no operability, single steady state, multiple steady state and periodic phenomena are identified. Model predictions are compared with experimental results obtained operating an ILA bioreactor lab-scale for phenol conversion [6].
[1] Neidleman, S.L. “BIOTECHNOLOGY. The Science and the Business”, V. Moses and R.E. Cape (Eds.) – Harwood acad. publish. – Ch.15 (1999)
[2] Qureshi et al., Microbial Cell Factories 2005; 4 (24)
[3] Nicolella et al., Journal of Biotechnology 2000; 80: 1-33
[4] Chisti M. Y., “Airlift Bioreactors”, Elsevier 1989
[5] Viggiani et al., J. of Biotechnology 2006; 123: 464-477
[6] Alfieri F. 2006 Ph. D. Thesis, Università degli Studi di Napoli Federico II, Dottorato in Scienze Biotecnologiche - XVIII ciclo
Presented Wednesday 19, 12:00 to 12:20, in session Biochemical Engineering (T5-1).
