Physical stability and biological activity of biofilms under turbulent flow and low substrate concentration (original) (raw)
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Intrinsic kinetics of biofilms formed under turbulent flow and low substrate concentrations
Bioprocess and Biosystems Engineering, 1999
Reactor operating conditions strongly affect the behaviour of biofilm systems, namely their stability and the substrate removal. In this paper, the penetration of substrate and the activity of biofilms formed by Pseudomonas fluorescens under turbulent flow and low substrate concentrations, are studied. A first order diffusion-reaction model was applied to results of biofilm accumulation in steady and non-steady-state. The substrate consumption rate of the biofilm was calculated based on the on-line determination of the biofilm accumulated on the surface. This approach is important when the residence time or the substrate concentration on the reactor is very low. Also, the mass transfer of substrate inside the biofilm was measured for every case under study and introduced in the model. The fraction of biofilm penetrated by the substrate depends on the velocity of the fluid that contacts the biological matrix: contrary to biofilms formed at higher velocities, lower velocities give raise to non completely penetrated biofilms. This fact seems to be associated to the biofilm internal structure in terms of biomass density and compactness of the matrix. They remove more substrate per reactor volume, but are less resistant from an hydrodynamic point of view. In conclusion, biofilms formed at higher velocities in turbulent flow allow a more stable reactor operation.
Biofilm formation under turbulent conditions: external mass transfer versus shear stress
This work investigates the effect of flow rate variation on the development of Escherichia coli biofilms formed in a flow cell system under turbulent conditions. Two flow rates were tested corresponding to Reynolds numbers of 4350 and 6720. The higher flow rate favored planktonic growth whereas the lower flow rate enhanced biofilm formation. Despite this, similar glucose consumption values were obtained in the whole system for both flow rates. Estimation of the external mass transfer coefficients by empirical correlations indicated that as the flow rate increased 1.5 fold, the external mass transfer coefficient increased 1.4 fold. Estimation of the shear stress in the flow cell was done by computational fluid dynamics and simulations indicated that the average shear stress increased 2.0 fold at the higher flow rate.
Biofouling, 2010
Biofilm formation is a major factor in the growth and spread of both desirable and undesirable bacteria as well as in fouling and corrosion. In order to simulate biofilm formation in industrial settings a flow cell system coupled to a recirculating tank was used to study the effect of a high (550 mg glucose l−1) and a low (150 mg glucose l−1) nutrient concentration on the relative growth of planktonic and attached biofilm cells of Escherichia coli JM109(DE3). Biofilms were obtained under turbulent flow (a Reynolds number of 6000) and the hydrodynamic conditions of the flow cell were simulated by using computational fluid dynamics. Under these conditions, the flow cell was subjected to wall shear stresses of 0.6 Pa and an average flow velocity of 0.4 m s−1 was reached. The system was validated by studying flow development on the flow cell and the applicability of chemostat model assumptions. Full development of the flow was assessed by analysis of velocity profiles and by monitoring the maximum and average wall shear stresses. The validity of the chemostat model assumptions was performed through residence time analysis and identification of biofilm forming areas. These latter results were obtained through wall shear stress analysis of the system and also by assessment of the free energy of interaction between E. coli and the surfaces. The results show that when the system was fed with a high nutrient concentration, planktonic cell growth was favored. Additionally, the results confirm that biofilms adapt their architecture in order to cope with the hydrodynamic conditions and nutrient availability. These results suggest that until a certain thickness was reached nutrient availability dictated biofilm architecture but when that critical thickness was exceeded mechanical resistance to shear stress (ie biofilm cohesion) became more important.
Biotechnology Progress, 2003
This paper presents a study about the influence of gas velocity on a methanogenic biofilm in an inverse turbulent bed reactor. Experimental results indicate a dynamic response of the growing attached biomass to the changes of hydrodynamic conditions, mainly attrition constraints. Short but intensive increases of gas velocity (U g ) are shown to induce more detachment than a high but constant gas flow rate. Hydrodynamic conditions control the composition of the growing biofilm in terms of cells and exocellular polymeric substances (EPS). The cell fraction within the biofilm (R cell ) was found to be inversely proportional to the gas velocity. The specific activity expressed in methane production rate or COD removal rate is higher in biofilms formed under high hydrodynamic constraints. The control of the hydrodynamic conditions in a biofilm reactor should make it possible to obtain a resistant and active biofilm.
Dynamics of drinking water biofilm in flow/non-flow conditions
Water Research, 2007
Drinking water biofilm formation on polyvinyl chloride (PVC), cross-linked polyethylene (PEX), high density polyethylene (HDPE) and polypropylene (PP) was followed in three different reactors operating under stagnant or continuous flow regimes. After one week, a quasi-steady state was achieved where biofilm total cell numbers per unit surface area were not affected by fluctuations in the concentration of suspended cells. Metabolically active cells in biofilms were around 17-35% of the total cells and 6-18% were able to form colony units in R 2 A medium. Microbiological analysis showed that the adhesion material and reactor design did not affect significantly the biofilm growth. However, operating under continuous flow (0.8-1.9 Pa) or stagnant water had a significant effect on biofilm formation: in stagnant waters, biofilm grew to a less extent. By applying mass balances and an asymptotic biofilm formation model to data from biofilms grown on PVC and HDPE surfaces under turbulent flow, specific growth rates of bacteria in the biofilm were found to be similar for both materials (around 0.15 day À1) and much lower than the specific growth rates of suspended bacteria (around 1.8 day À1).
Desalination and Water Treatment, 2014
This work investigates the effect of shear stress and mass transfer on the development of biofilms in a flow cell that mimics industrial piping. The shear stress and maximum flow velocity were estimated by computational fluid dynamics and the external mass transfer coefficient was calculated using empirical correlations for Reynolds numbers ranging from 100 to 10,000. The effect of two flow rates on the development of Escherichia coli biofilms under turbulent flow conditions was assessed and it was observed that biofilm formation was favored at the lowest flow rate. Additionally, estimations of the shear stress and external mass transfer coefficient indicate that both parameters increase with increasing flow rates. Thus, it seems that biofilm formation was being controlled by the shear stress that promoted biofilm erosion/sloughing and not by mass transfer which would potentiate biofilm growth. Our results indicate that not only efficient pre-treatment units are required on water recirculation loops in order to reduce the effective concentration of bacteria and nutrients, but also that high flow rates are preferred at all times to reduce the buildup of bacterial biofilms. For instance, high flow rates should be used during cleaning and disinfection cycles because the increase in shear stress will promote biofilm detachment and also potentiate the effect of biocides and other cleaning agents due to the increased mass transfer from the bulk solution to the surface of the biofilm.
Influence of the nature of hydrodynamic constraints on aerobic biofilms
International Journal of Environment and Waste Management, 2011
The aim of this paper is to relate biofilm detachment to local hydrodynamics. Biofilm was submitted to erosion tests in three devices: a Couette Taylor Reactor (CTR) and two Stirred Reactors (SR) Without Baffles (WOB) and With Baffles (WB). Each device involved different hydrodynamics and different impacts on the biofilm detachment. In CTR, the detachment was significant in one specific zone on the biofilm. In SR WOB, poor detachment was observed. In SR WB, major detachment was observed only on the external face. Baffles in SR induced a transient flow and a temporal non-uniformity of shear stress improving biofilm detachment.
Water Science and Technology, 1995
The influences of reactor conditions (substrate loading rate and shear) and microbial characteristics (yield and growth rate) on the structure of biofilms is discussed. Based on research on the formation of biofilms in Biofilm Airlift Suspension (BAS) reactors a hypothesis is postulated that the ratio between biofilm surface loading and shear rate determines the biofilm structure. When shear forces are relatively high only a patchy biofilm will develop, whereas at low shear rates the biofilm becomes highly heterogeneous with many pores and protuberances. In case of a right balance smooth and stable biofilms can be obtained. A hypothesis for the evolution of biofilm structures as a function of process conditions is formulated.
Influence of flow rate variation on the development of Escherichia coli biofilms
Bioprocess and Biosystems Engineering, 2013
This work investigates the effect of flow rate variation on mass transfer and on the development of Escherichia coli biofilms on a flow cell reactor under turbulent flow conditions. Computational fluid dynamics (CFD) was used to assess the applicability of this reactor for the simulation of industrial and biomedical biofilms and the numerical results were validated by streak photography. Two flow rates of 374 and 242 L h -1 (corresponding to Reynolds numbers of 6,720 and 4,350) were tested and wall shear stresses between 0.183 and 0.511 Pa were predicted in the flow cell reactor. External mass transfer coefficients of 1.38 9 10 -5 and 9.64 9 10 -6 m s -1 were obtained for the higher and lower flow rates, respectively. Biofilm formation was favored at the lowest flow rate because shear stress effects were more important than mass transfer limitations. This flow cell reactor generates wall shear stresses that are similar to those found in some industrial and biomedical settings, thus it is likely that the results obtained on this work can be used in the development of biofilm control strategies in both scenarios.