Renáta Berta, Orsolya Adamcsik, Ildikó Galambos, Nikoletta Kovács, Gábor Maász, Zita Zrínyi, Etelka Tombácz: What are we drinking? • Microplastic problems, current situation
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7. Technologies to produce water free of micropollutants and microplastics for different raw waters (drinking water, well water)
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7.3. Membrane separation processes for the removal of potential emerging pollutants (PEPs).

7.3. Membrane separation processes for the removal of potential emerging pollutants (PEPs).

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Potential emerging pollutants (PEPs), such as hazardous chemicals, toxic metals, biowaste, etc. pose a serious threat to human health, hygiene and ecology by contaminating the environment and water resources. Membrane-based separation processes are more effective than conventional water treatment processes that provide clean water. Microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), and forward osmosis (FO), as well as hybrid technologies, are discussed [225], citing published results over the last decade. It is a very useful compilation that includes much data on micropollutant removal. Microplastics are not mentioned but may be carriers of micropollutants, so it is worth summarizing the main findings of the article.

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The United States Environmental Protection Agency (USEPA) has classified PEPs (Figure 12 [226] shows their sources) as hazardous substances, but there are no regulatory requirements for them. Conventional water treatment methods (e.g. adsorption, biooxidation, coagulation, sedimentation, and filtration, and their hybrid combinations such as chlorination and UV irradiation) have been studied extensively, but most of them are not suitable for effective treatment. PEPs are medical wastes. Their types include: phthalates, psychostimulants, disinfectants, diuretics, β-blockers, antipyretics and analgesics, antiepileptics, antibiotics; biological, diagnostic agents, nutrients, cosmetics, fragrances, sunscreens; dioctyl phthalates; polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), organic dyes, and trace metal contaminants. Concentrations of PEPs in wastewater ranged from 0.007 to 56.63 g/L.
 

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Figure 12. Typical sources and occurrence of potential emerging pollutants (PEPs: pharmaceuticals, pesticides, industrial compounds, etc.) in wastewater treatment plants, surface and groundwater, and drinking/tap water [226]
 

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Various membrane separation processes are used to remove PEP: specific filtration properties depend on surface charge, pore size, membrane morphology, and hydrophobic/hydrophilic properties. They are available in different modules, such as MF, UF, NF, RO and FO, which contain different types of membranes depending on the specific separation requirements, as shown in Figure 13.
 

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Figure 13. Specific, size-based separation needs (left) and applicable filtering procedures (right) [227]
 

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In recent years, ceramic or zeolite composite membranes have been developed and are used as high-performance RO and NF membranes and successfully marketed in PEP separations. MF is suitable for the removal of colloidal particles, dyes, organics, and other high molecular weight soluble PEPs, while UF is well suited for the separation of smaller solutes from water to be purified.

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NF membranes have a much narrower pore system than UF membranes and therefore offer many advantages, for example, in the treatment of divalent salts and textile effluents. Recently, there has been growing interest in the use of hybrid RO /NF membranes for the removal of PEP from wastewater and industrial effluents.

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The FO method uses osmotic gradients artificially created by drawing the highly concentrated solution through the membrane. Unlike other membrane separation methods, FO is less sensitive to impurities, which is why the process is more commonly used in wastewater treatment, in the food industry to concentrate biomacromolecules, to recover precious metals, and to remove toxic metals.

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Conventional water treatment plants today face difficulties in removing PEPs, posing a threat to safe water supplies. A recent review [228] discusses the available advanced membrane-based treatment methods such as reverse osmosis, nanofiltration, ultrafiltration, forward osmosis, and membrane distillation to effectively remove them.

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Several studies have proposed combined methods (hybrid technologies) for cases where the RO membrane had severe fouling, using coagulation and disk filtration processes to reduce membrane fouling.

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Hybrid technologies: The integration of the FO system with electrochemical oxidation (electrochemical oxidation - ECO) is interesting, referred to as the FOwECO method; a synergistic effect in antibiotic removal was observed with this hybrid method.

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Membrane bioreactors (MBR): Apart from the lack of membrane modules and aeration stages, MBR treatment resembles conventional activated sludge processes where biodegradation and separation occur simultaneously.

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Photocatalytic membranes/reactors (PM/PMR): These new trends offer synergistic benefits in removing PEPs but may be more effective when combined with membrane separation. In such a configuration, suspended photocatalysts can mineralize organic material, minimize fouling caused by organic contaminants, and improve membrane separation efficiency. TiO2 is used suspended or immobilized on a support material (e.g. glass, steel). Self-cleaning membranes, various types of metal oxide (e.g. TiO2, ZnO, CuMn2O4, WO3) and nanocarbon composites (e.g. GO, rGO, rGO-g-C3N4) are also used. The most commonly appliedmaterial for the fabrication of photocatalytic membrane reactors is titanium dioxide [229].
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