|M.Sc Student||Eshed Lior|
|Subject||The Importance of Cross-flow on the Development of the|
Biofouling Layer on Membrane Separation Systems
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Emeritus Carlos Dosoretz|
|Professor Sima Yaron|
|Full Thesis text|
In the recent years, the use of pressure driven-membrane separation systems for the treatment of water and wastewater has increased, due to improvements in the technology and better feasibility. Most commercial systems operate in cross-flow mode in which the feed flows tangentially to the membrane surface whereas the permeate flows perpendicularly to the membrane, allowing continuous operation. The effect of permeate flux on the development of the biofilm (biofouling) layer on membrane separation systems was studied by means of a bench scale system consisting of two replicate 100 kDa MWCO-tubular ultrafiltration membrane modules, one allowing the flow of permeate and the other does not (control). The system was inoculated with Pseudomonas putida S-12 tagged with a red fluorescent protein and operated at laminar flow regime under sterile conditions with a constant fed of diluted (1:75) Luria-Bertani medium. Structural biofilm parameters, including thickness, density, coverage area and biovolume were studied by means of Field Emission-Scanning Electron Microscope and Confocal Scanning Laser Microscope and subsequently quantified by image analysis, as well as live counts and permeate flux profiles.
Biofilm development was highly enhanced in the presence of permeate flow, causing the build-up of complex three-dimensional structures along the membrane. Intensive bacterial transport towards the membrane by permeate drag was found to be the main mechanism in which cross-flow filtration contributes to the buildup of the biofouling layer, while its contribution as a mean of nutrients supplier was found to be of less importance. Fimbrial-like connectors were observed between the cells. Cell viability was found to be not essential for transport and attachment under conditions of cross-flow, since the permeate drag overcomes the effect of bacterial motility.
This research has also addressed the bacterial secretion of extracellular DNA (exDNA) and its relation to quorum sensing (QS). ExDNA was recently found to be an important component of the biofilms matrix, and its secretion is believed to be regulated by the QS system. We investigated the connection between the QS systems and exDNA secretion in wild type and QS mutants of Pseudomonas fluorescens, as well as the importance of the QS system on growth kinetics of the bacterial strains tested. We have found that there is a positive feedback between the presence of the QS and exDNA secretion, and negative feedback between the presence of the QS and growth kinetics. Therefore, we infer that there is a connection between exDNA secretion and the QS system in P. fluorescens.