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.

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.

The occurrence of Cr (VI) in drinking water resources in low but toxicologically relevant concentrations requires the development of reliable and industrially applicable separation processes in drinking water treatment. There is little information in the literature on the removal of chromium species at concentrations below 10 µg/L. Therefore, in this study, the removal of chromium in the concentration range ≤ 10 µg/L was investigated using three separation processes, activated carbon filtration (ACF), reduction/coagulation/filtration (RCF) and low-pressure reverse osmosis (LPRO), in both laboratory- and pilot-scale tests. In ACF treatment, Cr (III) was removed by deep bed filtration over 1.5 m of anthracite at a pH of 7.5 (which was used as a prefilter prior to ACF), while Cr (VI) was removed up to 75% via ACF at a filter bed depth of 2.5 m. Fresh activated carbon (AC) exhibited the highest adsorption capacity for Cr (VI), while reactivated AC had a significantly lower capacity for Cr (VI), which was attributed to calcium and iron deposits. In technical filters, where multiple reactivated activated carbon is used, this led to a low removal rate for Cr (VI). Using the RCF process with Fe (II) dosing in a continuous flow reactor at a specific coagulant dosing ratio, high Cr (VI) removal, down to a concentration of 0.1 µg/L, was achieved within minutes. The subsequent anthracite filtration ensured the complete removal of Fe (III) and Cr (III) precipitates. The RCF process was limited by the oxygen side reaction with Fe (II), which dominated at Cr (VI) concentrations below 1 µg/L. In addition, a four-step LPRO process with concentrate recycling showed effective removal (>99%) of both Cr (III) and Cr (VI) species in raw water as well as a negligible effect of pH in the testing pH range of 5.6 to 8.3 on the Cr (VI) removal. Nevertheless, the water hardness and pH of the LPRO permeate must be increased to make it available as drinking water. The three separation processes were found to be able to meet the expected more stringent future regulations for Cr (VI) level in drinking water. The most suitable technology, however, can be selected with respect to the raw water quality/characteristics, site-specific conditions and the already existing equipment.