Mass Transport In Biofilms Research Articles

NMR investigation of water diffusion in different biofilm structures.

Mass transfer in biofilms is determined by diffusion. Different mostly invasive approaches have been used to measure diffusion coefficients in biofilms, however, data on heterogeneous biomass under realistic conditions is still missing. To non-invasively elucidate fluid-structure interactions in complex multispecies biofilms pulsed field gradient-nuclear magnetic resonance (PFG-NMR) was applied to measure the water diffusion in five different types of biomass aggregates: one type of sludge flocs, two types of biofilm, and two types of granules. Data analysis is an important issue when measuring heterogeneous systems and is shown to significantly influence the interpretation and understanding of water diffusion. With respect to numerical reproducibility and physico-chemical interpretation, different data processing methods were explored: (bi)-exponential data analysis and the Γ distribution model. Furthermore, the diffusion coefficient distribution in relation to relaxation was studied by D-T2 maps obtained by 2D inverse Laplace transform (2D ILT). The results show that the effective diffusion coefficients for all biofilm samples ranged from 0.36 to 0.96 relative to that of water. NMR diffusion was linked to biofilm structure (e.g., biomass density, organic and inorganic matter) as observed by magnetic resonance imaging and to traditional biofilm parameters: diffusion was most restricted in granules with compact structures, and fast diffusion was found in heterotrophic biofilms with fluffy structures. The effective diffusion coefficients in the biomass were found to be broadly distributed because of internal biomass heterogeneities, such as gas bubbles, precipitates, and locally changing biofilm densities. Thus, estimations based on biofilm bulk properties in multispecies systems can be overestimated and mean diffusion coefficients might not be sufficiently informative to describe mass transport in biofilms and the near bulk.

A Fumarate Microbiosensor for Use in Biofilms

Fumarate is one of the terminal electron acceptors which can be used by microorganisms growing as biofilms. Mass transport in biofilms is dominated by diffusion, which creates a fumarate concentration gradient and could be the limiting factor for the growth of biofilms respiring on fumarate. Bulk solution measurements cannot be used to determine fumarate limitations in fumarate-respiring biofilms. Therefore, new approaches are needed for measuring fumarate concentration in biofilms. The goal of this study was to develop a microbiosensor for in situ depth profiling in biofilms. The working principle of the microbiosensor is the correlation between fumarate concentration and current consumption during fumarate reduction by G. sulfurreducens grown on a carbon microelectrode tip. The microbiosensor consists of a carbon microelectrode with G. sulfurreducens biofilm on a tip with a diameter of ∼30 μm as the working electrode and a built-in Ag/AgCl pseudo reference electrode, enclosed in a glass outer case. The microbiosensor responded in the 0–3.2 mM fumarate concentration range and had a detection limit of 258 ± 25 μM (S/N = 2). We quantified the stirring effect and the interfering effect of alternative substrates on the microbiosensor response. The utility of the fumarate microbiosensor was demonstrated by depth profile measurements of fumarate concentration in cathodic G. sulfurreducens biofilms. We found that fumarate was not a limiting factor for cathodic current generated by G. sulfurreducens biofilms.

Dynamic characterization of external and internal mass transport in heterotrophic biofilms from microsensors measurements

Knowledge of mass transport mechanisms in biofilm-based technologies such as biofilters is essential to improve bioreactors performance by preventing mass transport limitation. External and internal mass transport in biofilms was characterized in heterotrophic biofilms grown on a flat plate bioreactor. Mass transport resistance through the liquid-biofilm interphase and diffusion within biofilms were quantified by in situ measurements using microsensors with a high spatial resolution (<50 μm). Experimental conditions were selected using a mathematical procedure based on the Fisher Information Matrix to increase the reliability of experimental data and minimize confidence intervals of estimated mass transport coefficients. The sensitivity of external and internal mass transport resistances to flow conditions within the range of typical fluid velocities over biofilms (Reynolds numbers between 0.5 and 7) was assessed. Estimated external mass transfer coefficients at different liquid phase flow velocities showed discrepancies with studies considering laminar conditions in the diffusive boundary layer near the liquid-biofilm interphase. The correlation of effective diffusivity with flow velocities showed that the heterogeneous structure of biofilms defines the transport mechanisms inside biofilms. Internal mass transport was driven by diffusion through cell clusters and aggregates at Re below 2.8. Conversely, mass transport was driven by advection within pores, voids and water channels at Re above 5.6. Between both flow velocities, mass transport occurred by a combination of advection and diffusion. Effective diffusivities estimated at different biofilm densities showed a linear increase of mass transport resistance due to a porosity decrease up to biofilm densities of 50 g VSS·L−1. Mass transport was strongly limited at higher biofilm densities. Internal mass transport results were used to propose an empirical correlation to assess the effective diffusivity within biofilms considering the influence of hydrodynamics and biofilm density.

Relation between the structure of an aerobic biofilm and transport phenomena

An aerobic biofilm was characterized using confocal scanning laser microscopy (CSLM), O2 micro-electrodes, particle tracking and microinjection of fluorescent dyes. The biofilms were found to consist of microbial clusters of cells and Extra-cellular Polymeric Substance (EPS) separated by interstitial voids. The cell clusters were ca 300 μm and the voids were ca 100 μm wide. The voids were open channels connected with the bulk fluid. Fluorescein micro-injection showed that liquid could flow through the voids, but was always stagnant in the cell clusters. Consequently, in voids both diffusion and convection may contribute to mass transfer, while in cell clusters transport is determined by diffusion only. Particle tracking with CSLM showed that flow velocity inside the biofilm was proportional to the bulk flow velocity. The importance of convective mass transport in biofilms was demonstrated by oxygen distribution measurements. At high flow velocities of the bulk liquid, the mass boundary layer followed the irregular biofilm surface. At lower velocities the mass boundary layer was parallel to the substratum. Mass transfer from voids to cell clusters increased with flow velocity, as result from vonvective mass transport from the bulk to the voids. Convective transport was insignificant at low flow velocities, but at high flow velocities it increased the total mass transport by 200-250%. The local diffusion coefficients in biofilms were measured using microinjection of fluorescent dyes and quantitative analysis of the subsequent plume formation using CSLM. The diffusion coefficient of small, non-binding molecules in cell clusters is close to that in water. Very large molecules were impeded in their diffusion through the biofilm matrix. It was calculated that the cell cluster matrix had the characteristics of a gel network with pore diameters of 80 nm.

Experimental and conceptual studies on mass transport in biofilms

It is demonstrated that the flow velocity distributions in a flat plate reactor with and without biofilm are considerably different. Flow velocity profiles perpendicular to the channel wall indicate water movement in the space occupied by the biofilm. The flow velocity does not reach zero at the biofilm surface. Water flows through the pores in the biofilm causing convective mass transport. Longitudinal profiles of the flow velocity indicate that the presence of the biofilm disturbs the flow, which increases the entry length required for fully developed viscous flow to be established. Recently it has been shown that biofilms consist of cell clusters separated by interstitial voids. This newly proposed concept of biofilm structure helps to explain these experimental observations. However, the hydrodynamics and mass transport in biofilm systems appear to be more complex than previously assumed.

Experimental and conceptual studies on mass transport in biofilms

It is demonstrated that the flow velocity distributions in a flat plate reactor with and without biofilm are considerably different. Flow velocity profiles perpendicular to the channel wall indicate water movement in the space occupied by the biofilm. The flow velocity does not reach zero at the biofilm surface. Water flows through the pores in the biofilm causing convective mass transport. Longitudinal profiles of the flow velocity indicate that the presence of the biofilm disturbs the flow, which increases the entry length required for fully developed viscous flow to be established. Recently it has been shown that biofilms consist of cell clusters separated by interstitial voids. This newly proposed concept of biofilm structure helps to explain these experimental observations. However, the hydrodynamics and mass transport in biofilm systems appear to be more complex than previously assumed.

Relation between the structure of an aerobic biofilm and transport phenomena

An aerobic biofilm was characterized using confocal scanning laser microscopy (CSLM), O2 micro-electrodes, particle tracking and microinjection of fluorescent dyes. The biofilins were found to consist of microbial clusters of cells and Extra-cellular Polymeric Substance (EPS) separated by interstitial voids. The cell clusters were ca 300 μm and the voids were ca 100 μm wide. The voids were open channels connected with the bulk fluid. Fluorescein micro-injection showed that liquid could flow through the voids, but was always stagnant in the cell clusters. Consequently, in voids both diffusion and convection may contribute to mass transfer, while in cell clusters transport is determined by diffusion only. Particle tracking with CSLM showed that flow velocity inside the biofilm was proportional to the bulk flow velocity. The importance of convective mass transport in biofilms was demonstrated by oxygen distribution measurements. At high flow velocities of the bulk liquid, the mass boundary layer followed the irregular biofilm surface. At lower velocities the mass boundary layer was parallel to the substratum. Mass transfer from voids to cell clusters increased with flow velocity, as result from convective mass transport from the bulk to the voids. Convective transport was insignificant at low flow velocities, but at high flow velocities it increased the total mass transport by 200-250%. The local diffusion coefficients in bioflims were measured using microinjection of fluorescent dyes and quantitative analysis of the subsequent plume formation using CSLM. The diffusion coefficient of small, non-binding molecules in cell clusters is close to that in water. Very large molecules were impeded in their diffusion through the biofilm matrix. It was calculated that the cell cluster matrix had the characteristics of a gel network with pore diameters of 80 nm.