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Item type:Publication, Access status: with files , Development and performance of Layer-by-Layer modified hollow fiber membranes as capillary nanofiltration(2025); The need for alternative water sources due to water stress or changes in raw water qualities for use as drinking water often requires more advanced treatment techniques than currently in use. Thus, there is a demand for resource and energy efficient processes, with high process stability and reliability. Within this work the potential of Layer-by-Layer (LbL) modified hollow fiber ultrafiltration (UF) membranes was examined as one option in terms of ion rejection, rejection of dissolved organics, molecular weight cut-off (MWCO), fouling, and mechanical and process stability. Furthermore, a model was derived to differentiate between predominant polyelectrolyte (PE) build up during modification in terms of pore vs layer dominating PE multilayer formation. The results also identified the PE layering location as another crucial factor during membrane modification. Filtration in lab scale as well as pilot scale results in water works achieved high rejection rates for divalent ions (> 90 % for sulfate and magnesium, and > 80 % for calcium). However, some external parameters were investigated which influenced the resulting ion rejection. As shown in lab scale, high ionic strength in the feed solutions led to swelling of the PE structure, resulting in a change of separation characteristics. Nevertheless, swelling was highly dependent on the present type of ions and respective concentration. Model supported results clearly identified concentration polarization (CP) through the laminar boundary layer as dominating factor for the removal efficiency for sulfate as a model substance for divalent ions. Besides crossflow velocity and resulting laminar boundary layer thickness, model results showed a severe influence of CP dependent on membrane length. This underscores the importance of additional experiments beyond the lab to pilot scale on industrial length modules. Further investigations in lab scale showed that the modification of the membranes resulted in an MWCO in the lower range of NF membranes, providing high removal rates of dissolved organic substances. SAC254 removal rates of > 90% and TOC removal of > 80 % could be achieved even in Dead End operation for solutions containing natural organic matter (NOM). Fouling and removal rates were dependent on the NOM composition and their molecular weight distribution. Hereby, the solution containing a higher share of larger molecules reached higher rejection, while impact of fouling was lower. It is attributed to the deposition of foulants on top of the membrane surface instead of internally within the modified membrane structure. Membranes could successfully withstand the stress of hydraulic backwash (hydr. BW), though it was limited to a maximum hydr. BW flux of 50 L/(m² h). Combined with the good chemical stability, it would allow the implementation of a regular CEB, which was highly efficient for the removal of NOM foulants off the membrane surface. Overall, the LbL modification of hollow fiber UF membranes was successful on lab scale and on industrial scale membranes and could be operated successfully for several months in two different waterworks. The modified membranes combined exceptionally high NOM removal rates with the possibility for high divalent ion rejection and the stability of regular mechanical and chemical cleaning. Thus, LbL modified hollow fiber UF membranes did show a great potential for treatment of waters with high contents of dissolved organics and particle loads.Publicationtype: Doctoral Thesis124 74 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Combining real-time fluorescence spectroscopy and flow cytometry to reveal new insights in DOC and cell characterization of drinking water(Frontiers Media S.A., 2022-08-30); ; ; ; Graphical AbstractPublicationtype: Journal Article150 366 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Efficient drinking water treatment with increasing sulfate levels(2022); ; ; Publicationtype: Journal Article11 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Theoretical evaluation of polyelectrolyte layering during layer-by-layer coating of ultrafiltration hollow fiber membranes(Multidisciplinary Digital Publishing Institute (MDPI), 2021-02-02); ; ; ; Layer-by-layer (LbL) modification of porous membranes for water filtration has become an active research field in the past few years. Different mechanisms regarding polyelectrolyte film growth, swelling and smoothing, transport through these films, etc., have been studied. Although there are conjectures, it is not yet fully understood where the polyelectrolyte layering takes place when modifying porous membranes, either within the pores or on top of the porous material. This study presents a theoretical approach to investigate the dominant layer buildup regime between pore-dominated vs. layer-dominated growth of polyelectrolytes on porous membranes without mechanically interfering or damaging the membrane material. For this, fouling mechanism processes are used as an analogy. The presented approach gives a new insight into layering conformation and might be helpful to investigate the interaction between the membrane surface and the PE film. Moreover, the MgSO4 rejection behavior of two types of modified membranes was investigated: one with an initial pore-dominated layer growth followed by a layer-dominated film growth; the other one with a completely layer-dominated film growth. The data confirm that a rejection for MgSO4 could only be achieved in the regime of layer-dominated film growth. Additionally, when layer-dominated film growth prevails from the early stages of the coating process, permeability values are higher at similar MgSO4 rejection rates compared to an initial pore-dominated and then layer-dominated film growth. Accordingly, the interaction between the membrane pore size and molecular weight of the polyelectrolytes in the coating solutions plays an important role during LbL coating.Publicationtype: Journal ArticleScopus© Citations 8 211 342 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Characterization and performance of LbL-coated multibore membranes : zeta potential, MWCO, permeability and sulfate rejection(Multidisciplinary Digital Publishing Institute (MDPI), 2020-12-10); ; ; The characterization of membranes is suitable to investigate changes in the membrane properties caused by Layer-by-Layer (LbL) modification. Besides permeability, rejection, and molecular-weight cut-off (MWCO), which give information about the modification of the separation behaviour of the membrane, the zeta potential is capable of describing the surface charge of the membrane and its variation impacted by the properties of the polyelectrolyte multilayers (PEM). In this study, a new method for zeta potential measurement of hollow fibre membranes with several capillaries was developed and further investigations on the LbL modification of such membranes were performed. The results showed that an LbL coating with 8 DL PDADMAC/PSS led to a significant increase in the membrane charge of more than 20 mV. The coating with a different number of polyelectrolyte (PE) layers showed a zig-zag behaviour, comparable to data from flat sheet studies. However, in contrast to most flat sheet membranes, the charge curve assumes a totally negative trajectory at neutral pH. Further experiments on the MWCO of the LbL-modified membrane showed a reduction in the pore diameter from approx. 20 nm to less than 2 nm, reaching the range of nanofiltration membranes. With information on both the zeta potential and the MWCO, it was found that the rejection mechanism in LbL-modified multibore membranes is a complex interplay between the sieving effect due to reduction in the pore diameter and the repulsion effect of the charged membrane.Publicationtype: Journal ArticleScopus© Citations 10 214 260