A tale of two charges: zwitterionic polyelectrolyte multilayer membranes

In this thesis, the development of selective membranes for water treatment facilities to cope with the aforementioned issues is covered. By using hollow fiber membranes, the water purification process can be simplified compared to using spiral wound membranes, a significant advantage for decentralized water treatment plants. The selectivity of ultrafiltration membranes is improved\ud by coating a dense separation layer on the membrane. For this, the simple and versatile “layer-bylayer” (LbL) technique is used. By exposing a negative substrate to a polycation in an aqueous solution, a thin layer of that polycation is adsorbed on the surface. With the modified substrate, now positively charged, the same process can be done with an aqueous solution of a polyanion.\ud This process can be repeated over and over again, steadily building a polyelectrolyte multilayer on top of the substrate. If the chosen substrate is a porous support (e.g., an ultrafiltration membrane), the formed polyelectrolyte multilayers can be used as a selective layer in water purification.\ud The versatility of the LbL technique allows for an easy control over layer properties, such as thickness, density and charge, by varying the coating conditions, the type of polyelectrolytes and the amount of layers. This versatility makes the LbL system well suited for the design of dense\ud filtration layers.\ud \ud A key property of all membranes for water treatment purposes is the life time of the membrane. In this thesis we show that an adequate membrane life time can indeed be obtained for polyelectrolyte multilayers modified membranes, if certain criteria are met (Chapter 2). First of all, the presence of ionic groups on the membrane support significantly enhances the adherence of the multilayer on the membrane when high shear forces and reversed flow are applied. Second, the capability to withstand chemical degradation by hypochlorite is superior when quaternary ammonium polycations are used in the layer. We show that when both criteria are met, backwashable hollow fiber nanofiltration membranes can be made with a life time that is comparable to commercial ultrafiltration membranes

Stable Polyelectrolyte Multilayer-Based Hollow Fiber Nanofiltration Membranes for Produced Water Treatment

Produced water (PW) constitutes a massive environmental issue due to its huge global production as well as its complexity and toxicity. Membrane technology could, however, convert this complex waste stream into an important source of water for reuse, but new and more efficient membranes are required. In particular, in the last few years, polyelectrolyte multilayers (PEMs) established themselves as a very powerful method to prepare hollow fiber-based nanofiltration (NF) membranes, and this membrane type and geometry would be ideal for PW treatment. Unfortunately, the presence of surfactants in PW can affect the stability of polyelectrolyte multilayers. In this work, we investigate the stability of polyelectrolyte multilayers toward different types of surfactant, initially on model surfaces. We find that chemically stable multilayers such as poly(diallyldimethylammonium chloride) (PDADMAC)/poly(sodium 4-styrenesulfonate) (PSS), based only on electrostatic interactions, are substantially desorbed by charged surfactants. For poly(allylamine hydrochloride) (PAH)/PSS multilayers, however, we demonstrate that chemical cross-linking by glutaraldehyde leads to surfactant stable layers. These stable PEM coatings can also be applied on hollow fiber support membranes to create hollow fiber NF membranes dedicated for PW treatment. Increased cross-linking time leads to more stable and more selective separation performance. These newly developed membranes were subsequently studied for the treatment of synthetic PW, consisting of freshly prepared oil-in-water emulsions stabilized by hexade-cyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) in the presence of a mixture of ions. For both types of produced water, the membranes show excellent oil removal (∼100%) and organics removal (TOC reduced up to ∼97%) as well as good divalent ion retentions (∼75% for Ca2+ and up to ∼80% for SO42–). Moreover, we observe a high flux recovery for both emulsions (100% for CTAB and 80% for SDS) and especially for the CTAB emulsion a very low degree of fouling. These stable PEM-based hollow fiber membranes thus allow simultaneous deoiling and removal of small organic molecules, particles, and divalent ions in a single step process while also demonstrating excellent membrane cleanability

Forward Osmosis:A Critical Review

The use of forward osmosis (FO) for water purification purposes has gained extensive attention in recent years. In this review, we first discuss the advantages, challenges and various applications of FO, as well as the challenges in selecting the proper draw solution for FO, after which we focus on transport limitations in FO processes. Despite recent advances in membrane development for FO, there is still room for improvement of its selective layer and support. For many applications spiral wound membrane will not suffice. Furthermore, a defect-free selective layer is a prerequisite for FO membranes to ensure low solute passage, while a support with low internal concentration polarization is necessary for a high water flux. Due to challenges affiliated to interfacial polymerization (IP) on non-planar geometries, we discuss alternative approaches to IP to form the selective layer. We also explain that, when provided with a defect-free selective layer with good rejection, the membrane support has a dominant influence on the performance of an FO membrane, which can be estimated by the structural parameter (S). We emphasize the necessity of finding a new method to determine S, but also that predominantly the thickness of the support is the major parameter that needs to be optimized

Multiple Approaches to the Buildup of Asymmetric Polyelectrolyte Multilayer Membranes for Efficient Water Purification

The versatility of polyelectrolyte multilayer (PEM) coatings is very promising for their use as separation layers in nanofiltration applications. These membranes can for example be suited for the removal of micropollutants, such as medicines and pesticides, from water. The selectivity of PEM coatings can be further improved by so-called asymmetric coating. In this approach, the pores of the support membrane are filled with an open PEM layer to maintain a good water permeability, and subsequently a thin, dense layer is coated on top to determine the separation properties. Coating a dense top layer can be achieved in different ways. In this work, we systematically investigate the effectiveness of these different types of top layers. We show that coating a top layer at lower ionic strength than the bottom layer leads to a higher permeability and MgSO4 retention, compared with the reference, bottom-type layer coated with the same total number of layers. Also, other salt retentions can be improved with this approach. However, micropollutant retentions are hardly affected. Coating a top layer with a polyelectrolyte pair that forms denser layers at equal ionic strength, in contrast, leads to a significant change in separation properties with much higher MgSO4 and micropollutant retentions and improved water permeability compared with the reference layer. The concept of membrane optimization via asymmetric coating is thus most effective when using different polyelectrolyte pairs on top of each other. Moreover, we show that this approach allows us to selectively cross-link the top layer for further enhancement of the micropollutant retention, while water permeability is not much reduced. Asymmetric PEM coatings are therefore a promising method to optimize PEM membranes for micropollutant removal and other separation processes

Navigating the balance between nanofiltration and oxidation to remove organic micropollutants from wastewater treatment plant effluent

The removal of organic micropollutants (OMPs) from wastewater treatment plant effluent is becoming more important due to the adverse effects of these compounds on the environment. To overcome the limitations of currently available technologies, this study proposes a combination of hollow fiber membrane filtration with advanced oxidation to remove OMPs. The possible synergy between these processes was investigated. The nanofiltration membrane ensures the removal of organic matter and thus an improvement of transmittance, after which oxidation with UV/H2O2 of the permeate can remove OMPs more effectively and at a significantly lower energy consumption than without a membrane. Six membranes were evaluated with a pure water permeability between 6.7 and 106 Lm−2 h−1 bar−1 and a MgSO4 retention ranging from 0.93 to 0. The molecular weight cut-off (MWCO) varied from 250 Da to more than 10 kDa. The measured MWCO can depend strongly on the applied flux. The UV-transmittance of NF permeate of treated wastewater was investigated experimentally to be between 97% and 50%. These values were used for an estimation of the specific energy consumption (SEC) for the membrane and the oxidation step, resulting in a combined SEC of 0.17–0.18 kWhm−3 for 70 or 80% removal of OMPs, respectively. Remarkably, this lowest SEC was not found for the combination with the most dense membrane, but for a slightly more open membrane. The reported SEC is comparable to the total energy consumption required for ozonation and adsorption, while producing clean water with a double barrier, high transmittance and 70–80% removal of OMPs.</p

Evaluation of Membrane Integrity Monitoring Methods for Hollow Fiber Nanofiltration Membranes:Applicability in Gray Water Reclamation Systems

Source-separated gray water reclamation using nanofiltration as an advanced post-treatment option has received substantial interest in meeting the growing water demand. During reclamation, membrane integrity is crucial to ensure the water’s safety. This study evaluated several chemical and novel microbial indicators as indirect membrane integrity-monitoring methods for hollow fiber nanofiltration membranes in reclamation schemes. Under normal conditions, high retention of divalent ions and organic matter and near-complete removal of Escherichia coli (E. coli) were observed. Limited removal of the antibiotic gene (ARG) tetO was observed due to low feed concentrations and a higher detection limit (LOD). While 16S rRNA and ARG sul1 were not limited by their LODs, lower removals were observed, most likely due to free-floating DNA passing through the membranes. A broken fiber in a pilot-scale module reduced organic matter and microorganism removal substantially, while flux and ion rejection remained similar. Predictions made using the observed results and a previously proposed model allowed for the evaluation of the selected methods in upscaled reclamation systems. Based on these results, it was concluded that microorganisms could be employed as indicators in indirect membrane integrity-monitoring methods in large-scale reclamation schemes, while UV254nm absorbance (used in organic matter determination) could be a viable solution in pilot-scale systems.</p