TARGiM: Transport of Antibiotic Resistance Genes in Membranebioreactors (MBR)

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Mitigating the spread of antibiotic resistance genes (ARGs) will be one of the major future challenges of international wastewater treatment. Membrane bioreactors (MBR) have been established in wastewater treatment for the removal of bacterial cells. However, ARGs do not only appear inside bacterial cells, but also in the form of free DNA, possibly released by MBR aeration. Transport and retention of these free ARGs in membrane bioreactors are not yet fully understood. We hypothesize that the presence of membrane biofouling layers significantly alters the retention characteristics of MBRs for these free ARGs. While biofouling might reduce the general filtration performance, thicker biofilm layers and pore blocking increase the likelihood of ARG sorption, residence time and biodegradation, thus increasing the removal of free ARGs. Another hypothesis is that increased shear stress leads to the compression and hence higher density of the biofouling layer and consequently to improved ARG retention. To verify these hypotheses, a MBR fouling simulator platform will be developed, that allows integration and interpretation of ARG removal experiments under controlled and reproducible operational conditions. At its core, the platform consists of a shear cell in which realistic flow conditions and shear gradients corresponding to those in a commercial MBR are realized by means of an adjustable stirrer. The mass transfer coefficients will be calculated for different membrane and fouling layer characteristics and integrated into the MBR fouling simulator platform. Experiments will be carried out with different, commercially available and previously characterized, porous membranes. ARGs will be added to the feed in the form of free plasmids. To obtain ARG log removal rates, plasmids will be quantified before and after membrane passage through an improved multi-target ddPCR assay that simultaneously allows determining plasmid integrity.To elucidate the effects of membrane fouling on ARG removal, a model waste water community will be inoculated into the MBR shear cells. We suppose that the predominant type of membrane fouling (pore blocking, cake filtration) can be actively manipulated by altering physico-chemical membrane properties (e.g. hydrophobicity, pore size and surface roughness) and shear stress applications. The physical structure of the fouling layers will be characterized at various stages of ageing using an optical coherence tomography (OCT) system. This will allow creating and testing representative, reproducible biofouling layers of varying, defined properties for ARG removal.Finally, results will be used to determine and correlate operational parameters that are crucial for improving the MBR’s ARG removal efficiency. To validate the insights gained from the MBR fouling simulator platform, membranes fouled in a real MBR setup will be characterized, their mass transfer predicted and their ARG removal efficiency tested.