## 2 Application of Membranes in Contamination Situations  
Potential emerging pollutants, such as hazardous chemicals, toxic metals, bio-waste, etc., pose a serious threat to health, hygiene, and ecology by polluting the environment. These pollutants from various sources, including industrial wastewater, mainly from the pharmaceutical, food, and metal processing industries, can contaminate water and disrupt aquatic ecosystems. The discharged wastewater at the source requires clear identification, separation and disposal, otherwise, it can pose serious problems for water quality and ecology in general. Conventional water treatment methods, such as adsorption, bio-oxidation, coagulation, sedimentation, and filtration as well as hybrid methods, such as chlorination and UV irradiation, have been widely described in the literature, although most of these approaches are insufficient for effective wastewater treatment [1]. On the other hand, water treatment by membrane-based separation processes is quite expensive and energy-intensive compared to other conventional treatment technologies. However, membrane treatment processes have several clear advantages, such as the production of high-quality water with a high recovery rate of valuable chemicals/metals and  
Figure 1: An example of a decision-making path necessary for the correct selection of the technique and type of membrane. low maintenance costs [2; 3]. The most commonly used methods are microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), and forward osmosis (FO), and some hybrid technologies such as membrane bioreactors (MBR) and photocatalytic membrane reactors (PMR). Membrane processes depend on the type of membranes that are made of various materials, including polymers, ceramics, zeolites, etc., with specific filtration properties. The effectiveness of membranes depends on the surface charge, pore size, membrane morphology, and hydrophobic/hydrophilic properties. Polymer membranes can be used in various filtration methods, such as MF, UF, NF, RO, and FO depending on pore size and morphology as well as specific separation needs (see Figure 2). The above filtration methods use various separation techniques, e.g., solution diffusion or molecular diffusion or the size exclusive principle [4]. The membranes used for MF have larger pore sizes (0.1-5 \(\upmu\)m) than the membranes used for UF, which are typically used to separate contamination with a particle size in the range of 0.1-10 \(\upmu\)m. On the other hand, UF membranes with pore sizes from 0.01 to 0.1 \(\upmu\)m can be used to separate colloidal particles, macromolecules, biopolymers, and viruses, whose sizes are usually in the range from 0.01 to 0.2 \(\upmu\)m, and the process uses the principle of size exclusion. Commercially, UF is widely used for wastewater treatment, recovery of surfactants from industrial line washing, and in food processing and protein separation, etc. UF membranes are made of cellulose derivatives, inorganic materials, such as TiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), ZrO, as well as from common polymers, such as poly(acrylonitrile) (PAN), poly(sulfonamide) (PSA), poly(ether sulfone) (PES), poly(vinylidene fluoride) (PVDF), etc. [5; 6]. NF membranes enable the separation of particles in the size range of 0.001-0.01 \(\upmu\)m, which include most organic compounds, biomacromolecules, and various metal salts (except divalent salts). The capacity of NF is between RO and UF [7]. RO membranes are non-porous, made of solid polymers with voids, free-volume channels, or pore sizes ranging from \(\sim\)0.0001 to 0.001 \(\upmu\)m [8]. RO membranes separate low molecular weight inorganic components, including metal ions. The most common applications of RO are the treatment of wastewater from pulp and paper mills to produce drinking water [9]. Figure 2: Dependence of filtration methods on the type of pollutants.