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Molecular biology --- Monera --- Bacterial genetics --- Bacteriële genetica --- Génétique bactérienne --- ESCHERICHIA COLI --- Genetics --- SALMONELLA TYPHIMURIUM --- genetics --- microbial --- genetics. --- microbial. --- Escherichia coli --- Salmonella typhimurium --- Genetics. --- Microbial.

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579.842.11 --- Escherichia --- Theses --- Sciences and engineering --- biological sciences --- biology --- molecular --- microbiology --- physical sciences --- pure sciences --- chemistry --- biochemistry --- 579.842.11 Escherichia --- molecular. --- microbiology. --- biochemistry. --- Biological sciences --- Biology --- Microbiology. --- Molecular. --- Physical sciences --- Pure sciences --- Chemistry --- Biochemistry.

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A membrane bioreactor (MBR) combines the biological degradation process by activated sludge with a direct solid–liquid separation by membrane filtration. Over the past decade, MBRs became an attractive option for the treatment and reuse of industrial and municipal wastewaters as it has many advantages over conventional wastewater treatment systems. However, one of the major obstacles impeding the faster and larger development of this technology is membrane fouling. Due to the inherent interaction between the activated sludge and the membrane, all kind of particles adsorb in or on the membrane surface. Especially the microbes, which have the ability to attach to almost any substrate, to divide on it and produce a slimy matrix and thus form a biofilm or “biofouling” layer, remain problematic. There are mainly two reasons why the biofouling is still considered as a “black box”: not well-adapted methodologies and the use of too simple or too complex systems without an interdisciplinary approach. In this doctoral thesis, these two problems were investigated in an attempt to understand what is really occurring on the membrane surface. As membranes are completely submerged in the activated sludge biomass, it is nearly impossible to see what is happening on the membrane surface. Moreover, not only the membrane surface is of main interest, but also what is occurring inside the membrane pores. Therefore an online visualization tool was developed in order to follow up the biofouling formation over time (chapter 4). This system not only allows for a nondestructive membrane visualization, but also offers the possibility to measure bacterial cell densities or to characterize the bacterial community after membrane autopsy. In order to measure the latter, well established DNA extraction and quantification methods are required. Therefore these two techniques have been optimized in chapter 2 and 3 of this dissertation. The inadequate use of the right model system is another biofouling related issue. Either to simple experimental set-ups, only using one species, or to complex and versatile activated sludge communities are applied. For this reason, three model systems with increasing complexity were used in this dissertation: a monospecies approach, using Pseudomonas aeruginosa as a filtration feed, a duospecies approach where both P. aeruginosa and Escherichia coli were used as a feed, and finally an activated sludge feed spiked with P. aeruginosa cells. Different biofouling related parameters, such as bacterial attachment, biofilm formation, production of exopolymeric substances, interaction between species and resistance against chemical agents, were measured on three types of microfiltration membranes (chapter 5, 6 and 7).One main conclusion could be stated from all the performed experiments: biofouling is strongly species dependent. Independent of which kind of membrane or experimental design was used, it was clear that when a good biofilm pioneer, like P. aeruginosa, was applied, nearly all the membrane were fouled in the same way.Also whenanother species, like E. coli, was present, P. aeruginosa was still dominant, notwithstanding the two species interacted with each other. In contrast, when a more diverse but better adapted community was used, such as activated sludge spiked with P. aeruginosa, interesting changes occurred. Activated sludge communities were found to attach significantly better to membranes to which they were brought into contact before. It is therefore suggested that acclimatization of species can play an important role in biofilm development. The strong species dependent effects were also observed in experiments where bleach and citric acid were evaluated for their use as chemical cleaning agents of membranes (chapter 7). The biocide resistance was found to be directly related to the complexity of the biofouling community.In summary, this dissertation provides new fundamental insights on how bacteria behave on membranes and on how they interact with other species. Keeping those in mind, certain concepts of existing anti-fouling strategies should be rethought.

Bioréacteur --- Bioreactors --- Membrane de filtration --- Filter membranes --- Interactions biologiques --- biological interaction --- Pseudomonas aeruginosa --- Escherichia coli --- Encrassement --- Fouling --- Academic collection --- Theses