Evaluation of mass transfer coefficients within biofilms

Abstract

The knowledge of mass transfer rates in microbial biofilms is often the limiting step on biofilm modelling, and therefore on the prediction of the bebaviour of the biofilm system (bioreactor, a biofilm in a water dustribution system or in a heat exchanger). A biofilm is a complex gelatinous matrix containing microorganims. The characteristícs of this pellicle, in terms of thickness, porosity, density, and therefore the physical structure, depend upon the hydrodynamic and substrate conditions under which it is formed, as well as on its age, and type of microorganisms present. Thus, the internal diffusion in biofilms is strongly dependent on biofilm structure and must be determined for every case under study. ln spite of that, some authors assume that the diffusion coefficient of the substances within the microbial film are the same as in water or is a fixed perceritage of the latter (Characklis & Marshall, 1990). Others calculated the diffusion through the application of diffusion reaction models or developed methods for measuring the mass transfer within biofllms, based on artificial biofilms (formed by centrifugation, sedimentation or filtartion) or inactivated biofilms (by UV light or chemical inactivation). ln recent years NMR ( Lens et al., 1996) and an electrochemical method ( L'Hostis et al., 1995), had been used to measre the mass transfer in biofilms. ln this work the determination of mass transfer coefficients within biofilms by mass transfer measurements will be presented. The technique is non-invasive and can be used along the biofilm formation. lt is based on the formation of a biofilm on a permeable membrane, and on the diffusion of an inart substance through the system biofilm-membrane. lt is a reproducible technique and it has already been tested in different conditions, namely in: i) biofilms formed by Pseudomonas fluorescens both in laminar and turbulent flow, witb high and low substrate concentrations as well as in the absence and in the presence of abiotic particles; ii) in biofilms formed under anerobic conditions in laminar flow similar to those present in bioreactors. The values obtained for the mass transfer coefficients is steady-stated biofilms range between 0.8 X 1 Q-o m/s and 1.8 x 10-o m/s. This indicates similar mass transfer conditions in all cases although the biofilm thickness and structure varied from case to case: some biofilms were very loose and fluffy (with thicknesses of the order of several millimeters), others were more dense and thinner (thicknesses between 100 -800 micrometers).