Biofilms and flow · An integrative study on the distribution, morphology and composition of biofilms under the influence of secondary flows around flow obstructions

An integrative study on the distribution, morphology and composition of biofilms under the influence of secondary flows around flow obstructions

Biofilms are bacteria consortia, embedded in a self-secreted 'gel' matrix, which provides bacteria with significant advantages for their survival and proliferation. Owing to this successful strategy, biofilms may be found virtually anywhere, provided some nutrients are available. Understanding the formation and function of biofilms presents challenges to many engineered processes, as its presence may either impede, reducing process efficiency (e.g. clogging of water distributions systems, heat exchangers and desalination membranes), or it may serve a ‘positive’ purpose, for example in bioreactors used to purify wastewater or to produce bio-derived fuels and chemicals. Additionally, biofilms may cause disease transmission or trigger infections from medical implants, and thus have a large impact on human health. Due to this wide range of applications, biofilms have been extensively studied. However, the interplay between hydrodynamics, bacteria deposition and eventual biofilm characteristics remain largely illusive, particularly in complex velocity fields, such as those arising in the presence of flow obstructions. The initial deposition of bacteria is largely controlled by colloidal physico-chemical interactions and hydrodynamics. In the particular case of a permeable surface, e.g. a desalination membrane, an additional velocity component exists, which renders the surface more prone to deposition. A theoretical model that describes the hydrodynamic interaction of a particle and a permeable wall has been developed, showing the extent of the interaction and its dependence on particle size and shape, as well as the surface permeability. It is shown that the lower the permeability of the surface, the larger this force becomes, which offers some insight into why reverse osmosis membranes are sensitive to fouling. Further, the analysis was extended to account for the deformation of a bacteria cell. We show that deformation may either exacerbate deposition or delay it, depending on the characteristic length scales over which the competing forces persist. The potential impact of the developed theory on membrane technology is twofold: 1) enabling identification of sustainable operating conditions, minimizing the loss of productivity due to fouling and/or the increased energy consumption, and 2) informing better design of membrane materials and/or surface modifications by quantifying the potential for creating more substantial repulsive forces close to the membrane surface. Flow experiments were made using microfluidic channels as well as custom fabricated membrane flow-cells with an optical window that enables online observation of deposition onto a membrane surface. Operating conditions were varied, e.g. flowrate, permeation and ionic strength, affecting surface shear, drag toward the membrane, and electrostatic repulsion, respectively. Image analysis allows the extraction of a kinetic ‘rate’ constant for the deposition. Ongoing work is attempting to validate the model with the experiments. Additional flow experiments were conducted in micro-channels containing cylindrical obstructions or with a tortuous flow path. Biofilm growth was monitored using fluorescently labeled bacterial strains, and assessed as a function of position on the channel walls, as well as a general characteristic of the channel material (hydrophilic/hydrophobic). Results obtained so far have shown little apparent difference in the extent of channel coverage, for modified vs. unmodified surfaces. Visualization experiments were also conducted using physical surrogates for a biofilm, e.g. highly viscous or viscoelastic fluids, with the goal of gauging the effect of flow on the deformation of the extra-cellular polymeric (EPS) matrix, the ‘gel’ that binds the biofilm together. Experiments have shown initial indications of the way hydrodynamic heterogeneity may force the biofilm to adapt its shape. Flow obstructions, in the form of net-type spacers, are an integral part of spiral-wound membrane modules used for desalination and have been implicated as initial sites of biofilm formation, resulting in clogging of the modules. In a collaboration within the host department, silver nanoparticles were impregnated into the spacer material, and tested for biofilm growth. The membranes with the modified spacers consistently showed significantly smaller flux decline over time. A numerical model was developed to calculate the velocity field in the spacer-filled channel, and the concentration of silver released from the modified spacers. This provided a measure of the necessary silver loading that will ensure release throughout the expected lifetime of the spacer and membrane. Furthermore, the idea of ‘slippery’ spacers was conceived and a computational model was formulated and used to assess the impact of such structures on the flow and mass transfer. It was revealed that mass transfer can be improved per unit energy dissipated or, conversely, the energy consumption for pumping can be reduced without compromising mass transfer. Experimental design and fabrication of such spacers is part of a continuing project. Additionally, bio-growth will be monitored, as published studies suggest that biofilms are easy to remove from such surfaces. This constitutes an exciting future prospect. Finally, in order to evaluate full-scale impact on desalination systems, a model has been developed to account for the bio-growth, its effect on mass transfer and pressure drop in the membrane channels. The model provides a useful tool for assessing conditions leading to an increased resistance to permeation vs. channel clogging, both of which are of paramount importance in the energy efficiency of real operation. The participation in this project has proved to be monumental in my establishment as a young researcher. The research conducted provided important new knowledge on the fundamental aspects of bacterial deposition, and I have gained important exposure to experimental facilities and techniques. In addition, I had the opportunity to attended several international conferences during which research work was presented, providing me with opportunities to engage with leading scientists in the field and gain experience with presentation and scientific communication skills. I have also taught a course on membrane separation during the summer of 2012 and autumn of 2013. Most importantly, I successfully applied and re-integrated into a tenure-track position as assistant professor, which was my main career goal going into the project. This presented me with a host of new challenges, including recruitment and advising of graduate students, teaching a course and establishing a new laboratory - handling all purchases, managing expenses etc. In addition, I have succeeded in obtaining funding from two external sources. In continued to establish my collaborations, particularly within Europe. The main future extensions of this project in my newly established lab will be the following: 1) Theoretical and experimental understanding of how bacteria interact with polymer brush-coated membranes. This constitutes a theoretical extension of the framework developed as part of this project, and has far-reaching potential for making robust, fouling-resistant membranes. 2) Development of advanced spacer materials for membrane modules. Drawing on the experience gained in the course of the project, tailored structures that leverage hydrodynamic and surface interactions will be developed. 3) Continued work on the interaction of biofilms and flow; specifically, the flow-induced deformation and resulting morphology, and the relation between matrix composition, mechanical characteristics (i.e., its rheology) and the flowing environment.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз