Continuum and discrete approach in modeling biofilm development and structure: a review

The scientific community has recognized that almost 99% of the microbial life on earth is represented by biofilms. Considering the impacts of their sessile lifestyle on both natural and human activities, extensive experimental activity has been carried out to understand how biofilms grow and interact with the environment. Many mathematical models have also been developed to simulate and elucidate the main processes characterizing the biofilm growth. Two main mathematical approaches for biomass representation can be distinguished: continuum and discrete. This review is aimed at exploring the main characteristics of each approach. Continuum models can simulate the biofilm processes in a quantitative and deterministic way. However, they require a multidimensional formulation to take into account the biofilm spatial heterogeneity, which makes the models quite complicated, requiring significant computational effort. Discrete models are more recent and can represent the typical multidimensional structural heterogeneity of biofilm reflecting the experimental expectations, but they generate computational results including elements of randomness and introduce stochastic effects into the solutions

Remediation of PAH-Contaminated Soils: Experimental Analysis and Modeling of Hydrodynamics and Mass Transfer in a Soil-Slurry Bioreactor

Extended Abstract Polycyclic Aromatic Hydrocarbon (PAHs)-contaminated soils are a great environmental and public health concern nowadays. Their remediation is an important field of research as several remediation techniques have been developed with the purpose of removing PAHs from soil. However, further researches are necessary to develop environmental friendly biotechnologies that allows public and private sectors to implement efficient and adaptable treatments for contaminated soils. Aerobic soil-slurry bioreactor technology has emerged as one of these technologies with high potential as an effective and feasible treatment technic for PAH-contaminated soils. For this treatment, soil is excavated, conditioned and loaded into an aerated aqueous bioreactor. Then, mechanical and/or pneumatic mixing maintains aerobic conditions and homogeneity. Furthermore, air supply and mixing represent the most energy intensive units Although, extensive research has been done on this topic, mechanisms involved in the removal of PAHs from soil are still not completely understood. In addition to the biological processes involved, important mass transfer mechanisms need to be considered (oxygen gas-liquid mass transfer, adsorption-desorption, volatilization of PAH, etc.). In general, even for volatile PAHs, volatilization is not considered in the studies whereas, in some conditions (high aeration rate), it can be a major mechanism of "PAH removal". The soil composition and concentration in the reactor should influence strongly the fluid viscosity, which is a key parameter governing the hydrodynamics and thus the mass transfer phenomena. Therefore, the aeration and mixing optimization requires a fine understanding of how different operational parameters influence the mixing and mass transfer mechanisms involved in the removal of PAHs from soil In this study, the influence of soil content (composition and concentrations) and operating conditions (air superficial velocity, stirring rate, etc.) on the mixing (rheology, etc.) and mass transfer phenomena (gas-liquid, adsorption-desorption) is addressed. Experiments are performed in a glass standard bioreactor designed to control hydrodynamic conditions and temperature. Air is injected from the bottom through a porous glass sparger. Mechanical agitation is performed by a marine propeller connected to a motor. Hydrodynamic parameters are monitored in order to study their influence on the process and, particularly on the oxygen and PAH transfer phenomena. Rheological behavior of soil/water matrix has been measured with a capillary rheometer The oxygen transfer tests showed that for a given air superficial velocity and stirring rate, the oxygen transfer coefficient in soil/water matrix is reduced in comparison with clean water results. This decrease depends on the soil composition and was more pronounced with an increase in the soil content. Moreover, the soil/water matrix could be assimilated to a non-Newtonian fluid with shear-thinning behavior (mainly pronounced for high soil content). The impact

Numerical study of nano-biofilm stagnation flow from a nonlinear stretching/shrinking surface with variable nanofluid and bioconvection transport properties

A mathematical model is developed for stagnation point flow toward a stretching or shrinking sheet of liquid nano-biofilm containing spherical nano-particles and bioconvecting gyrotactic micro-organisms. Variable transport properties of the liquid (viscosity, thermal conductivity, nano-particle species diffusivity) and micro-organisms (species diffusivity) are considered. Buongiorno’s two-component nanoscale model is deployed and spherical nanoparticles in a dilute nanofluid considered. Using a similarity transformation, the nonlinear systems of partial differential equations is converted into nonlinear ordinary differential equations. These resulting equations are solved numerically using a central space finite difference method in the CodeBlocks Fortran platform. Graphical plots for the distribution of reduced skin friction coefficient, reduced Nusselt number, reduced Sherwood number and the reduced local density of the motile microorganisms as well as the velocity, temperature, nanoparticle volume fraction and the density of motile microorganisms are presented for the influence of wall velocity power-law index (m), viscosity parameter (c2), thermal conductivity parameter (c4), nano-particle mass diffusivity (c6), micro-organism species diffusivity (c8), thermophoresis parameter (Nt), Brownian motion parameter (Nb), Lewis number (Le), bioconvection Schmidt number (Sc), bioconvection constant (σ) and bioconvection Péclet number (Pe). Validation of the solutions via comparison related to previous simpler models is included. Further verification of the general model is conducted with the Adomian decomposition method (ADM). Extensive interpretation of the physics is included. Skin friction is elevated with viscosity parameter (c2) whereas it is suppressed with greater Lewis number and thermophoresis parameter. Temperatures are elevated with increasing thermal conductivity parameter (c4) whereas Nusselt numbers are reduced. Nano-particle volume fraction (concentration) is enhanced with increasing nano-particle mass diffusivity parameter (c6) whereas it is markedly reduced with greater Lewis number (Le) and Brownian motion parameter (Nb). With increasing stretching/shrinking velocity power-law exponent (m), skin friction is decreased whereas Nusselt number and Sherwood number are both elevated. Motile microorganism density is boosted strongly with increasing micro-organism diffusivity parameter (c8) and Brownian motion parameter (Nb) but reduced considerably with greater bioconvection Schmidt number (Sc) and bioconvection Péclet number (Pe). The simulations find applications in deposition processes in nano-bio-coating manufacturing processes

Modelling biofilm development: the importance of considering the link between EPS distribution, detachment mechanisms and physical properties

International audienceIn industry treatments against biofilms need to be optimized and in the wastewater treatment field, biofilm composition needs to be controlled. Therefore, describing the biochemical and physical structures of biofilms is now required to better understand the influence of operating parameters and treatment on biofilms. The present study aims to investigate how growth conditions influence EPS composition, biofilm physical properties and volume detachment using a 1D biofilm model. Two types of EPS are considered in the present model, proteins and polysaccharides. The main hypotheses are that: (i) the production of polysaccharides occurs mainly under strong nutrient limitation(s) occurs while the production of proteins is coupled to both the substrate uptake rate and the lysis process; (ii) the local biofilm porosity depends on the local biofilm composition. Both volume and surface detachment occur in biofilms and volume detachment extent depends on the biofilm local cohesion and thus on the local composition of biofilms for a given shear stress.The model is based on experimental trends and aims to represent these observations on the basis of biochemical and physical processes. Four case studies covering a wide range of contrasting growth conditions such as different COD/N ratios, applied SOLR and shear stresses are investigated. The model predicts how the biochemical and physical biofilm structures change as a result of contrasting growth conditions. More precisely simulation results are in good agreement with the main experimental observations reported in the literature, such as: (i) a strong nitrogen limitation of growth induces an important accumulation of polysaccharides leading to a more porous and homogenous biofilm, (ii) a high applied surface organic loading load allows to obtain a high biofilm thickness, (iii) a strong shear stress applied during the biofilm growth leads to a reduction of the biofilm thickness and to a consolidation of the biofilm structure. Overall, this model represents a relevant decision tool for the selection of appropriate enzymatic treatments in the context of negative biofilm control. From our results, it appears that protease based treatments should be more appropriate for biofilms developed under low COD/N ratios (about 20 gCOD/gN) whereas both glucosidases and proteases based treatments should be more appropriate for biofilms developed under high COD/N ratio (about 70 gCOD/gN). In addition, the model could be useful for other applications such as resource recovery in biofilms or granules, and help to better understand biological membrane fouling

Impact des propriétés des boues activées sur leur comportement rhéologique et sur le transfert d’oxygène dans les bioréacteurs aérés

Dans l’objectif de mieux comprendre l’impact des caractéristiques des boues activées sur leur comportement rhéologique et sur le transfert d’oxygène, un dispositif expérimental, constitué d'une colonne à fines bulles de 0,3 m3 alimentée en boues activées en continu et d'un rhéomètre capillaire, a été mis en oeuvre sur une station d’épuration à boues activées conventionnelle. La rétention gazeuse globale et le coefficient de transfert d’oxygène (kLa) ont été mesurés dans la colonne à bulles et le comportement rhéologique des boues a été déterminé en sortie de celle-ci. Parallèlement, plusieurs propriétés des boues activées ont été mesurées (MES, MVS, DCO, DCO soluble, tensioactifs, tension de surface et tension de surface dynamique). L’évolution des paramètres rhéologiques K et n du modèle d’Ostwald ont été déterminés pour différentes concentration en matières en suspension (MES) comprises entre 3,0 et 8,6 g/L. Par ailleurs, les mesures d’oxygénation ont montré une diminution du coefficient volumique de transfert de matière (kLa) avec l’augmentation de la concentration en MES. Cette diminution peut être en partie attribuée à la diminution de rétention de gaz (eG) liée à une augmentation de la viscosité apparente des boues. Finalement, les résultats expérimentaux ont permis d’estimer le taux de cisaillement moyen existant dans la colonne à bulles et de développer un modèle empirique liant le coefficient volumique de transfert d’oxygène (à 11°C, kLa11) à la vitesse superficielle de gaz (UG) et à la viscosité apparente des boues. / In order to better understand the impact of the characteristics of activated sludge on their rheological behaviour and on oxygen transfer, an experimental set-up, comprising a fine bubble column of 0,3 m3 continuously fed with activated sludge and a capillary rheometer, has been implemented in a wastewater treatment plant. The overall gas holdup and volumetric oxygen transfer coefficient (kLa) were measured in the bubble column and the rheological behaviour of the activated sludge was determined using the column liquid effluent. Simultaneously, several properties of activated sludge were measured (MLSS, MVSS, COD, soluble COD, surfactants, surface tension and dynamic surface tension). The evolution of rheological parameters K and n from the Ostwald model was determined for a MLSS concentration ranging between 3,0 and 8,6 g/L. In a ddition, aeration test showed a decrease of the volumetric mass transfer coefficient (kLa) with increasing MLSS. This reduction can be partially attributed to a lower gas holdup (εG) associated with an increase of the sludge apparent viscosity. Finally, the experimental results were used to estimate the average shear rate in the bubble column and to subsequently obtain an empirical correlation linking the volumetric oxygen transfer coefficient (at 11°C, kLa11) to the superficial gas velocity (UG) and the apparent viscosity of the sludge = 1,6 x10-5 * UG 0,8 * μapp-0,34, where the apparent viscosity (μapp) is a function of the MLSS concentration and the average shear rate in the column

Colloidal mobilization and fate of trace heavy metals in semi-saturated artificial soil (OECD) irrigated with treated wastewater

The mobility of selected heavy metals in trace concentrations was investigated in a standard OECD soil irrigated with the effluent of a real municipal wastewater treatment plant. While Cd, Cu and Ni accumulation-migration patterns were mainly influenced by the mobility of colloids generated from soil organic and inorganic matter, Zn mobility was more influenced by the wastewater content of dissolved organic matter and by its salinity. Metal accumulation caused by interaction with colloids resulted in contamination peaks both in different zones of the soil column and in the leaching solution. The release of metals in the leachate was correlated to the contemporary release of silicates from kaolinite and dissolved organic matter, identified through UV absorbance and chemical oxygen demand monitoring. The hypothesized colloidal mobilization was confirmed by spectroscopic studies. The highly heterogeneous complexes of organic and inorganic molecules responsible for metal transport through soil appeared to be structured in highly stable micellar aggregates

Colloidal mobilization and fate of trace heavy metals in semi-saturated artificial soil (OECD) irrigated with treated wastewater

The mobility of selected heavy metals in trace concentrations was investigated in a standard OECD soil irrigated with the effluent of a real municipal wastewater treatment plant. While Cd, Cu and Ni accumulation-migration patterns were mainly influenced by the mobility of colloids generated from soil organic and inorganic matter, Zn mobility was more influenced by the wastewater content of dissolved organic matter and by its salinity. Metal accumulation caused by interaction with colloids resulted in contamination peaks both in different zones of the soil column and in the leaching solution. The release of metals in the leachate was correlated to the contemporary release of silicates from kaolinite and dissolved organic matter, identified through UV absorbance and chemical oxygen demand monitoring. The hypothesized colloidal mobilization was confirmed by spectroscopic studies. The highly heterogeneous complexes of organic and inorganic molecules responsible for metal transport through soil appeared to be structured in highly stable micellar aggregates

Functional potential of sewage sludge digestate microbes to degrade aliphatic hydrocarbons during bioremediation of a petroleum hydrocarbons contaminated soil

Sewage sludge digestate is a valuable organic waste which can be used as fertilizer in soil bioremediation. Sewage sludge digestate is not only a good source of nutrients but is also rich in bacteria carrying alkB genes, which are involved in aliphatic hydrocarbons metabolism. Increase of alkB genes ratio in polluted soils has been observed to improve bioremediation efficiency. In this study, for the first time, the genetic potential of indigenous microorganisms of digestate to degrade petroleum products was assessed. The objectives were to study petroleum hydrocarbons (PHCs) removal together with shifts in soil taxa and changes in the concentration of alkB genes after digestate application. Initial alkB genes concentration in contaminated soils and digestate was 1.5% and 4.5%, respectively. During soil incubation with digestate, alkB genes percentage increased up to 11.5% and after the addition of bacteria immobilized onto biochar this value increased up to 60%. Application of digestate positively affected soil respiration and bacterial density, which was concomitant with enhanced PHCs degradation. Incubation of soil amended with digestate resulted in 74% PHCs decrease in 2 months, while extra addition of bacteria immobilized onto biochar increased this value up to 95%. The use of digestate affected the microbial community profiles by increasing initial bacterial density and diversity, including taxa containing recognized PHCs degraders. This study reveals the great potential of digestate as a soil amendment which additionally improves the abundance of alkB genes in petroleum contaminated soils