Introduction

One of the biggest world challenges is the demand for clean water. As a result of factors such as population growth, prolonged drought, and stricter health-based regulations, the demand for accessible freshwater sources is increasing. In 2025, according to the UNICEF estimation, 50% of the world's population will live in water-deficient regions (UNICEF 2020). The development of nanotechnology has allowed for a host of new products and processes that address many of the current water quality problems. This includes nanoabsorbents, nanocatalysts, bioactive nanoparticles, nanostructured catalytic membranes, and enhanced nanoparticle filtration. Nanofilter developments have provided remarkable opportunities for developing environmentally friendly and cost-effective water purification processes. In addition to applications in pharmaceuticals, biotechnology, and food engineering, membrane technology is widely accepted for producing various water qualities from diverse sources such as surface water, well water, brackish water, and seawater. Lately, membrane technology has moved into the area of treating secondary and tertiary municipal wastewater and oilfield-produced water. Based on the material difference, nanofiltration (NF) membranes are generally divided into two categories, namely, organic (polymeric) and inorganic. Commercial organic NF membranes are used in many industrial applications. However, there are some disadvantages associated with these organic membranes, such as their low thermal and chemical stability and low mechanical strength.

The purpose of this paper is to review the advances in nanofilter membrane research and their advantages based on the important focus areas. The review starts by looking at membrane-based filters and characteristics, their benefits, and drawbacks. The broad scope of the study is signified by the investigation of various NF types. The application of NF membrane in a variety of water sources have investigated, and finally, NF membrane future research and development has been discussed.

Background of membrane-based filters

Compared to other separation techniques, membrane filtration has two unique features: the membranes are asymmetric, and a small-pore feed passage is implemented to reduce the overall pressure drop within the membrane, thus decreasing the tendency for plugging. Moreover, the performance of membrane systems depends on the efficient cross-flow, which is instrumental in removing filter cake accumulation to a few microns. The membrane-based separation systems are widely used in industrial water treatment and a variety of other applications due to their high efficiency, low energy consumption, and ability to scale up/down. Four main membrane processes are currently utilized in water purification according to their pore sizes, including microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), and NF. Among them, the NF process is proposed as a cost-effective method for removing impurities, sediments, and chemical effluents, as well as toxins including arsenic. Figure 1 shows how the pores for RO membranes are small enough between 0.1 and 1 nm to treat low-molecular-weight species such as aqueous inorganic solids including salts, minerals, metals, and organic molecules. MF membranes possess pore sizes of approximately 100 nm–10 µ, which reject particles, asbestos, and various cellular materials, such as red blood cells and bacteria. UF membranes with smaller pores, approximately 2–100 nm, can further filter dissolved bio-macro macromolecules, such as pyrogens, and proteins; these membranes require lower pressure (200–700 kPa). Unfortunately, they are not very effective at removing dissolved organic and inorganic solutes with a molar mass between 1 and 2 nm and a molecular weight cut-off (MWCO) around 100–2000 Da. They tend to have charged surfaces in an aqueous environment. Pore sizes for NF membranes typically range from 0.5 to 10 nm. They are often used for separating solid dust, liquid droplets, sugars, proteins, dyes, microbes, viruses, and bacteria. This process is put between reverse osmosis and ultrafiltration. NF rejects molecules whose size is less than a nanometer, equivalent to molecular weight cut-offs between 300 and 500 Da (Bohdziewicz et al. 1999). At first, the RO was used to produce drinking water in rural areas by desalinating of seawater and brackish water. Since reverse osmosis depends on salt removal and MF will relate to the particle retention and salt passage techniques, UF has been introduced to cover the gap between RO and MF (Abdel-Fatah 2018).

Fig. 1
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The classification of water purification membranes in terms of the pore size and the residual species

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The general characteristics of pressure-driven membranes are reported in Table 1. As detailed in Table 1, filtration techniques are commonly classified based on (i) the pressure required for separation, (ii) the size of the solute rejected, consequently directly correlating to pore size, and (iii) the permeability mechanism. For instance, according to Mulder's classification based on pressure, ultrafiltration (UF) operates under 0.1 to 5 bar pressure, nanofiltration (NF) utilizes pressures ranging between 3 and 20 bar, and reverse osmosis (RO) requires high operating pressures ranging from 5 to 120 bar.

Table 1 General characteristics of pressure-driven membrane
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Nanofiltration characteristics

Nanoporous membranes have been commercialized since 1980, and their properties are between reverse osmosis (RO) and ultrafiltration (UF). Membrane separation is becoming a rapidly emerging technology in many industrial applications such as the food, petroleum, chemical processing, pulp, paper, and electronic industry as a result of their high efficiency, low energy consumption, convenience for scaling up or down, and continuous flow operation capabilities. The nature of NF is complex, and micro-hydrodynamic and chemical processes occur at the membrane surface. Over the past years, there has been a nearly tenfold increase in articles citing NF, emphasizing the need for further development of this technology. While NF's applications have steadily expanded, there are still several challenges, such as membrane fouling, further treatment of concentrates, chemical resistance of membranes, insufficient rejection in water treatment, and higher operating costs compared to other membrane methods (Boussu et al. 2007).

Since NF membranes can be characterized in various ways, and this area has been researched throughout several publications in this section, we attempt to present some of the most impactful advantages and disadvantages of NF and compare them with other membrane filters.

Benefits and drawbacks of nanofiltration

RO membranes were developed to remove all dissolved salt ions and even uncharged organic solutes (Daly et al. 2020). UF membranes can reject particles with a molecular weight above 10,000 g mole for various industrial applications. However, these RO and UF membranes cannot perform well in separating solutes from the solution in the molecular weight range from 500 to 10,000 g/mol.

Benefits of nanofiltration

The use of RO and UF in water and wastewater treatment is gradually rising. However, NF is used with great growth in water and wastewater treatment as well as in other industries such as biomaterials, the pharmaceutical industry, and feed additives. From the available information, it can be deduced that membrane technology in commercial applications largely depends on the effectiveness of NF. Moreover, NF is useful in desalination; it can remove microorganisms as well as a fraction of dissolved salts, turbidity, and hardness. It leads to a significant reduction in operating pressure and provides an energy-efficient process.

NF has numerous applications including water treatment for the production of drinking water, and treatment of different types of water like surface water, groundwater, and wastewater. It is used for softening and removing pollutants, micropollutants such as pharmaceutically active compounds, pesticides, and other relatively small organic solutes. NF can be applied for more challenging applications, such as salt fractionation. In the past decade, significant advancements have been made in water treatment and drinking water production using NF. In cases where partial water desalination is aimed, TDS plays a crucial role, or a few species are specifically reduced while the remaining species are not affected (Ghaffour et al. 2013). Especially when there are large amounts of sulfate anions, salt rejection is high. In such a case, NF has to compete with reverse osmosis. A major benefit of NF is the lower operating pressure, resulting in lower energy costs and possible savings on pump and piping investment. Ion selectivity is considered one of the features of the NF membrane. Anions' valence determines how much salinity is retained. The membrane allows monovalent anions such as chloride to access the salt, while multivalent anions (e.g., SO2) are rejected. In Fig. 2, the separation spectrum for NF membranes is shown. Various small organic molecules exhibit the ability to capture multivalent ions, eliminating organic solutes and hardness that possess a molar mass ranging from 1000 to 3000 Da (natural organic material). Due to the lower rejection of monovalent salts compared to multivalent salts, it is possible to observe the negative rejection of monovalent ions when multivalent ions or polyelectrolytes are present, which is an extreme case of charge-induced separation. For example, NF membranes have a sodium chloride rejection rate ranging from 20 to 80%. UF membranes have a salt rejection of less than 5% and a rejection of more than 90%, while RO membranes have a salt rejection of more than 90%. Na2SO4 > CaCl2 > NaCl is an order of rejection since the NF membrane rejects salts in proportion to their molecular size. Consequently, NF membranes allow for ion fractionation, an important advantage, and a reason for its rapid commercialization.

Fig. 2
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Nanofiltration and other processes' separation permeability spectrum. Reprinted from Baker (2012)

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Based on this feature, Table 2 compares nanofilters to osmotic filters in terms of the speed of compound removal.

Table 2 Nanofiltration and reverse osmosis membrane rejection values
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Another feature of the membrane is its relatively high flux and low operating pressure, which is ideal for use in the drinking water sector and wastewater collection to remove pollutants such as arsenic, pesticides, soil leachate, and dyes. The ability of NF to reduce costs has permitted many applications where the complete rejection of (mainly) ions was not required.

Another fantastic specification of NF is its high selectivity to small molecules compared with UF and RO. However, high pressures are required to operate NF, UF, and RO (Zhang 2020). NF membranes are selective primarily due to their steric hindrance and charge interactions. The narrow pore-size distribution is a promising approach for excellent solute-selective separations. It is also possible to enhance the selectivity of membranes by manipulating their surface charge, especially for charged solutes, through the Donnan effect and dielectric exclusion (Donnan 1995). However, membrane pores tend to narrow, and the selectivity of NF membranes reduces when fouling occurs over time. Since NF membranes are pressure-driven membrane types, they could perform a preferential separation for different fluids or ions. NF membrane pores have a much larger pore than reverse osmosis membrane pores, thus requiring less energy to perform separations. With the advent of nanoscale science and engineering, there have made unprecedented opportunities to develop more environmentally acceptable, cost-effective methods for water purification.

NF is distinguished by the removal of calcium and magnesium ions resulting in water softening, and no addition of sodium ions during filtration (Galanakis et al. 2012; Harman et al. 2010; Hilal et al. 2004a, b) compared to ion exchange units. NF does not require additional chemical treatment to reduce hardness. So, the water softening process is approached effectively without realizing sodium resin in water, which has been the case for 50 years (Labban et al. 2017).

The development of NF membrane technology has been partly driven by the need to create better filtration technologies for separating inorganic and organic substances from a liquid. Generally, NF advances due to better membranes made by processes such as interfacial polymerization, nanoparticles (NPs), and UV treatment (Rahimpour et al. 2008). To counter such issues, different techniques are employed to improve membrane selectivity and overcome fouling challenges.

Drawbacks of nanofiltration

Any membrane separation is subject to fouling. However, it may be more complicated in NF because the interactions leading to fouling occur at the nanoscale, making them more challenging to understand. Its adverse effects are evident: membranes require constant cleaning, limited recoveries, and feedwater losses. NF technologies are intended to eliminate dissolved ions and salt, but without appropriate pretreatment, life expectancy and operating costs of these systems their efficacy may be affected. Consequently, membrane fouling is closely linked to other issues, such as feedwater particulate matter compatibility, and fouling/scaling rates, which influence membrane life and membrane degradation due to poor source waters. Generally, if fouling were controlled completely, less cleaning would be needed and less permeate would be produced. Biofouling problem is a frequent occurrence in many membrane processes, affecting all biologically active organisms, mostly bacteria, and (in some cases) fungi. Water flux can be reduced or entirely prevented by biofilms.

Fouling still poses a great problem for membrane-based applications, including NF. Fouling reduces the cost-effectiveness and competitiveness of membrane operation. Fouling leads to several harmful effects including flux decline (drop in productivity), increase in costs due to increased energy demand, maintenance, chemicals, and frequency for cleaning, membrane degradation, and replacement, and shorter membrane lifetime. Fouling of the membrane can be controlled by taking preventive measures before the membrane unit (feed pretreatment and early fouling prediction), selecting the membrane, designing the module, adjusting operation modes, and cleaning the membrane units when the functions of membrane units fail.

NF has relatively modest separation factors; usually, 5–10 several applications of ion separation can be found. One of these applications is the separation of peptides based on charge differences (Gilron et al. 2001). Often, the pH of the solution determines the degree of separation in the latter case. Nevertheless, NF membranes have a sigmoidal rejection curve (rejection that depends on molar mass), which leads to a separation between varying compounds based on their molecular size. Here is a typical sigmoidal rejection curve in Fig. 2. Additionally, separation is dependent on hydrophobicity and charge interactions. In other words, the permeate consists of molecules with variable sizes, both below and above the membrane's claimed pore size. Both the permeate and the retentive are considered waste fractions.

Pressure-driven membrane processes, including NF, usually result in a concentrated or retentive stream. Concentrates produced during aqueous stream purification are often undesirable byproducts of the purification process and must be discharged or further treated. Since membranes only separate compounds and do not destroy or transform them, this constitutes a problem when the feed solution contains unwanted compounds or fractions that cannot be reused. Concentrate composition is similar to feed composition, but enhanced concentrations for components rejected by the membrane are present. The concentrate may also contain antiscalants (polyacrylates, polyacrylic acids, polyphosphates); sulfuric acid or hydrogen chloride can affect pH.

Generally, membrane processes are viewed from a pragmatic point of view, i.e., a feasible solution to specific problems in filtration. These factors include membrane materials, operating conditions, energy consumption, cleaning chemicals, permeate yield, and overall environmental impact. Membrane lifetime depends significantly on cleaning frequency and the overall strategy against membrane fouling in aqueous applications. The membrane degrades faster in applications requiring frequent cleaning since cleaning agents can also cause some damage to membranes.

One of the new trends in water treatment is the requirement for the completely removing of all pollutants, even at low concentrations. Indeed, this is not necessarily a customer criterion based on risks or toxicity, but a reality that must be recognized. This trend can also be seen for “suspicious” compounds such as nitrate and boron organic micropollutants, which can also be detected by analytical chemistry at concentrations as low as ng/l. Reverse osmosis membranes are frequently favored to guarantee the complete elimination of pollutants. Although this is necessary, there is a likelihood of synergistic toxicity when it is utilized alongside other chemicals like pesticides.

It has been shown that the membrane lifetime and chemical resistance of NF membranes are affected by fouling. Membrane lifetime for aqueous applications varies greatly depending on cleaning frequency and overall fouling prevention strategy.

Cost-effectiveness of NFs

As per the water policy and regulatory reform project, the commercial adoption of NF in the water treatment sector is still limited due to its high operating costs. Since NF plants are not currently operational, cost data are obtained by analyzing real-life operating plants and calculating pilot-scale results based on experimental results. There was a strong correlation between NF cost and flux, but the recovery had a relatively small impact on the total cost of an industrial facility. Nevertheless, it is influenced by other investment parameters (Chellam et al. 1998). Plants with smaller membranes can expect the investment cost to be 20–30% of the total project cost and those with more extensive membranes may incur charges of 50%. The cost of membrane modules becomes more important at larger plants, so economies of scale are achieved at a smaller scale. The cost estimates for different NF units vary. According to a study, 42–61% of NF systems are affected by energy and materials, particularly membranes (Moslehyani et al. 2019). Changes in material cost, membrane lifetime, and energy will have a dramatic effect on the viability of NF systems (Abdel-Fatah 2018).

How the advancements have overcome the drawbacks

RO and UF have been widely used in different applications for many years, but their applications are limited and hard to extend. Energy requirements for water treatment increase with NF systems and the low-energy systems become essential. As Sombekke suggests (Sombekke and Voorhoeve 1997), green energy can reduce energy consumption. However, it is possible to reduce the energy requirements of NF by making NF more permeable. It is therefore necessary to strike a balance between optimal energy requirements and optimum operation. In recent years, NF applications have gained popularity and have replaced other membrane filtration techniques. It is possible to prepare NF membranes with various RO membrane polymers such as cellulose acetate and polyamide polymers. In addition to chemically resistant polymers membrane, membranes are now also made from ceramic materials that are heat resistant. NF can be applied to a wide range of processes due to the variety of raw materials and flexibility of preparation (Abdel-Fatah 2018). Because NF is flexible in raw material selection and easy to modify for different applications, it will soon become the most common membrane filtration technology, which demands more research attention.

Classification of nanofiltration membrane

There are several different types of NF membranes and each can be classified based on its constituent materials. There are usually two types of membranes for these processes: organic and inorganic, or organic and inorganic hybrids. The majority of organic membranes are made of polymeric materials, such as polysulfones, cellulose acetate, and polyvinylidene fluoride. Ceramic membranes (TiO2, SiO2, ZrO2, Al2O3, TiO2-SiO2, TiO2-ZrO2, Al2O3-SiC) are classified as inorganic membranes. Nowadays, inorganic nanomaterials such as graphene and carbon nanotubes are also used in their structure (Guo et al. 2018). In contrast, organic–inorganic hybrids are typically prepared by mixing a polymer material with an inorganic component metal, metal oxide, or carbon.

Nanofiltration membranes are a highly desirable filtration technique for separating organic and inorganic substances from liquid solutions. As a result of making better membranes, NF technology was advanced through techniques such as UV treatment, nanoparticle (NP) incorporation, and interfacial polymerization (IP). All these techniques contributed to membranes that have a high rejection tendency, more excellent selectivity, and overcome fouling issues.

Different interactions between surface, nanopores, and micro-hydrodynamic events occur on the membrane surface. NF is a complex process, so those factors are important. These processes combine Donnan, dielectric, and transport effects that can cause rejection through NF membranes (Yaroshchuk 2001).

The nanopores of the membrane and membrane surface present a wide variety of interfacial and micro-hydrodynamic events. Such events are important for NF since it is a complex process. The combination of dielectric, Donnan, and transport effects can cause NF membranes to reject water. Neutral solutes are transported through steric mechanisms. Donnan effects described the potential interaction and interaction between the interface and charged species. As a result of ionization at the membrane surface and its pore structure, the membrane can develop a charged nanoparticle. Based on the materials used in synthesis, this nanoparticle can be acidic, basic, or a combination of both. The surface group dissociation is influenced strongly by the pH of the solution. It is possible to detect an isoelectric point if the membrane surface chemistry is amphoteric. Additionally, a low ion exchange capacity may be observed. Due to ionizable surface groups, NF membranes may display exchange capacity.

Polymeric membranes, ceramic membranes, and thin-film composite (TFC) membranes are generally considered conventional (Facciotti et al. 2014). Due to their well-developed and outstanding performance, polymeric membranes are widely used in the seawater desalination and wastewater treatment industries (Tul Muntha et al. 2017). An exemplary polymer must resist chemical and thermal attacks, have high mechanical strength, and be easy to form sheet and hollow fibers (Bassyouni et al. 2019). The thermal and chemical stability and high mechanical strength of ceramic membranes have prompted growing interest in their application. Figure 3 illustrates that inorganic membranes include metal oxide membranes and carbon-based membranes, consisting of alumina, zirconia, titania, and their mixtures. A ceramic membrane is generally made up of a macroporous support layer and a meso- or microporous barrier layer (Vasanth and Prasad 2019).

Fig. 3
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Three common types of nanofiltration for water treatments

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Since ceramic membranes show more resistance compared to polymeric membranes, they are usually used in industries whose operating conditions are more specific, for example, high temperature, corrosive effluent, etc. (Eriksson 1988). Although various types of research are conducted to solve the problems related to the performance limitation of NF, one of the main drawbacks of polymeric membranes is still fouling. More modifications have been made to surface structure and materials to eliminate the fouling effect. For underlying issues, using materials that involve inorganic fillers in organic matrices such as mixed matrix membranes (MMMs) is a significant achievement. The technology consists mainly of conventional membranes, although recently, newer materials have been developed, including nano-structured membranes that are reactive and bio-inspired (Mohammadi and Maghsoodloorad 2013).

Polymeric NF membrane with application

Due to their excellent performance and cost-effectiveness, polymeric and organic non-woven membranes have exceeded other membranes on the global market since 1980.

Polymers’ ideal properties include resistance to thermal and chemical attacks, high mechanical strength, and the ability to be formed into flat sheet or hollow fiber structures. The phase inversion technique is typically used to fabricate polymeric membranes (Yang et al. 2019). The use of cellulose acetate, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polypropylene (PP), polyethersulfone (PES), and polysulfide (PS) is common in the polymeric membrane. However, their disadvantage is hydrophobicity, which causes membrane fouling. Membrane fouling can occur due to membrane pores clogging and sludge cake deposition. According to the mode of operation, membrane fouling increases transmembrane pressure and reduces permeate flux. Fouling is caused by (i) the desorption of sludge flocs onto membranes, (ii) the detachment of foulant due to shear forces, (iii) the adsorption of colloids or solutes within the membranes, (iv) changes in the composition of foulant during operation, and (v) the formation of cake layers on membrane surfaces. Hence, the phenomenon can be defined as the undesirable deposition and accumulation of solutes, microorganisms, cell debris, and colloids on/inside the membrane.

The fouling can be eliminated by modifying PS and PES. Improving membrane performance can be achieved by improving the hydrophilicity of the membrane. Several methods exist to modify PES membranes chemically or physically to create more hydrophilic surfaces for increasing hydrophilicity (Sajitha et al. 2002). A membrane can be pretreated with hydrophilic components through graft polymerization, plasma treatment, and physical pre-adsorption. These are the three methods whereby hydrophilic components can be chemically attached to the membrane and subsequently incorporated through polymerization. Changing surfactants, self-assembled hydrophilic nanoparticles, and the nitrification of membranes are other possible membrane modification processes. Modifying polymers before forming membranes is another approach. Bulk modification can occur with the addition of hydrophilic additives to the membrane matrix. The sulfonation, carboxylation, and nitration techniques are examples of these methods. Also, polymer blending yields improved membrane properties (Van der Bruggen 2009).

While polymeric NF membranes can be used for liquids below 70 °C, inorganic NF membranes have a more extensive temperature range; typically, up to 120 °C, liquids vaporize at temperatures above 120 °C.

Traditionally and artificially, polyamides are macromolecules with repeated amide groups (–CO–NH–). Fibers like wool, silk, and angora can be made of natural polyamide. Phase inversion is usually used to prepare cellulose-based polymers (Sajitha et al. 2002; Simcik et al. 2016). In Table 3, the removal of NaCl and MgSO4 with different NFs at various pH and temperature levels is shown. Salts were removed in a range of 80 to near-complete elimination with the NF membrane.

Table 3 Salt removal of polymeric NF membranes for water treatment
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Composite NF Membrane with Application

Membranes based on thin-film composites (TFC) have evolved rapidly after the invention of interfacial polymerization (IP) in 1965 by Morgan (Morgan 1965). While TFC membranes have been well received in the industry, membrane scientists are still interested in making asymmetric membranes via single-step fabrication methods.

Interfacial polymerization is one of the techniques used to prepare composite nanofilter membranes. These membranes have high water permeation flux and salt rejection. Many advantages become available when IP processes are applied for membrane synthesis (Zhang et al. 2003). Firstly, thin selectively permeable barrier layers with a thickness of less than 0.1 ~ tm are highly regarded (Li et al. 2008). The second advantage of the thin-film composite membranes manufactured by the IP process is their high permeability and good selectivity. Additionally, IP techniques allow for independent optimization of the properties of the barrier layer from the supporting layer. Layers consist of aliphatic or aromatic diamine, polysulfone amide, aromatic diol, and a combination of diol and amide. The common cross-linker used is trimethyl chloride, isophthaloyl chloride, and terephthaloyl chloride (Kim et al. 2000). In high rejection, ultra-thin reverse osmosis membranes, such as piperazine and m-phenylene diamine membranes, and NF membranes, primary and secondary diamines, are most widely used. The 3,5-aminobenzoic acid, another potential membrane material known to be hydrophilic, is also more hydrophilic than m-phenylenediamine. A higher ratio of hydrophilicity to hydrophobicity in a polymer leads to a greater water exchange rate in membranes, so a polymer with hydrophilic groups would be a more suitable membrane-grade polymer. The effect of a hanging carboxylic group on the polyamide backbone on its production rate is expected to be greater (Ahmad et al. 2004).

The membrane degradation caused by contact with chlorine, one of the common disinfectants used in water and wastewater treatment, is a key limitation of commercial polyamide membranes. Previously, it has been shown that changes in the chemical nature of PA caused by chlorine exposure can reduce membrane performance and shorten membrane lifetime. A method was developed to overcome this restriction (Buch et al. 2008) using interfacial polymerization of 1,3-cyclohexane-bis (methylamine) (CHMA) in water with TMC in hexane under different conditions.

Ceramic NF membrane with application

Despite the increasing importance of NF in the wastewater treatment industry, the limitations related to chemical, thermal, and mechanical stability during membrane operation cannot be overlooked. While polymer membrane manufacturers have recently improved chemical, thermal, and physical resistance, long-term durability at industrial levels remains a concern. In contrast, ceramic membranes possess high chemical and thermal resistance, and excellent resistance to chemical stress (Weber et al. 2003), making them a more optimal choice. Because of narrow pore sizes, higher porosity, better separation characteristics, and mechanical and chemical stability, as well as lower fouling compared to polymeric membranes, ceramic membranes are considered superior (Harman et al. 2010). These membranes are composed of single/multi-channels with active layers made of titanium oxide, alumina, silica, and zirconium oxide and exhibit high porosity and surface hydroxyl groups leading to high hydrophilicity (Zhe and Chuyang 2018). Ordinary, the structure of ceramic membranes for water treatment is asymmetric walls including multiple overlaid layers, which have various pore sizes and porosity. To increase flow resistance, the membrane layer must be thinner than the support layer. The support layer may have pores from 0.5 to 15 μm and four membrane layers have pores from 50 to 1000 nm, 2–50 nm, and smaller than 2 nm for the third and fourth layers, second layer, and the first layer, respectively. These layers are also thinner than the support to minimize flow resistance.

The study conducted by Ralph researchers involved the characterization of five commercially available ceramic membranes (KNF-1 to KNF-5) in terms of their separation behavior, specifically, their permeability, retention capacity for organic matter, and salt retention. The results indicated high permeability rates and good organic matter retention while demonstrating low fouling tendencies under experimental conditions. From the industrial point of view, they are more applicable for the treatment of textile wastewater containing dye and the treatment of alkaline solutions from machines (Weber et al. 2003).

Ceramic membranes are widely used for the treatment of municipal sewage. Several characteristics, such as its resistance to high temperatures, pressures, and chemical concentration, make it more suitable. A ceramic NF membrane is more impervious to organic fouling than a polymeric NF membrane. Controlling fouling on the membrane surface is one of the most challenging problems associated with using membranes for sewage treatment to maximize water production. It can be achieved by ensuring a continuous flux and keeping filtration downtime as short as possible (Kramer et al. 2020; Yang et al. 2019). The promising performance of ceramic NF membranes, as shown in Table. 4, exemplified in their early commercialization stage, suggests great potential for water purification.

Table 4 Ceramic NF membrane for water purification
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Metal oxide and carbon base membranes are two common types of ceramic membranes. An important class of ceramic membranes includes metal oxides such as aluminum, zirconia, and titania. Metal oxide membranes from the NF class are similar to those from the RO class, but they do not have an exclusive top layer (Caso et al. 2021). Sol–gel is the most widely used method for preparing metal oxide ceramic membranes, as precursor solutions are converted to solid membranes. The fabrication of multilayered membranes and the need for expensive precursor materials indicate high manufacturing costs. Simplified synthesis methods and cheap materials will lower production costs and accelerate the development of ceramic membranes. The most widely studied inorganic membranes are alumina membranes with average pores of 2–5 nm (molecular weight coefficient of 3000–1000 Da) and are commonly utilized in NF systems. Researchers aim to improve the purification performance of an alumina membrane by conducting surface modification (Caso et al. 2021). A mixed matrix carbon molecular sieve (CMS) and the α-Al2O3 membrane are made by the vacuum-assisted impregnation method. The membranes were further evaluated using 3.5 wt% NaCl solution (seawater) at a temperature of 75 ℃; a water flux was up to 25 kg/m2h, and salt rejection ranged from 93 to 99%. Other ceramic membrane materials include zirconia (also called zirconia) and titania (also known as silicon) (Koyuncu et al. 2015).

Researchers have increasingly turned to ordered mesoporous materials to address water pollution and water shortages (Xu et al. 2019). It is possible to fabricate carbon nanotubes (CNT) for use in water desalination and purification, either independently or as part of other materials. The study of Sadia et al. investigated a highly electrochemically active membrane made of carbon nanotubes, with significant applications in biological and chemical wastewater treatment. CNTs possess a few advantages that allow them to commercialize, including high cost and low selectiveness for specific ions (arsenates, arsenic, and sodium). One of the most outstanding features of graphene is its high permeability and selective properties, making it unique among other two-dimensional carbon allotropes (Teow and Mohammad 2019). It is easy to produce nanoporous graphene either electrochemically or by growth on support from different chemical reactions (Ahmad et al. 2004).

Mixed matrix NF membranes with application

Inorganic fillers are often used to create mixed matrix membranes (MMMs), composed of organic materials with inorganic fillers (Rozaini et al. 2019). MMMs have gained considerable attention due to their numerous advantages such as high packing density, exceptional selectivity, and low cost of manufacturing. By combining the advantages of polymer materials, such as low cost and excellent selectivity, with the long-term stability, high mechanical strength, and regeneration capacity offered by ceramic materials, MMMs can be designed with superior performance for various applications (Zhu et al. 2019). One type of MMMs is an inorganic nanoparticle blended with a polymeric membrane, which is prepared through dispersible cross-linking, interfacial polymerization, or dip coating. The use of titanium, zeolite, silica, alumina, etc., as fillers is being investigated.

Experimental results suggest that extra inorganic nanoparticles alter the polymeric structure and vary the transport of molecules through membrane pores. Ayaz et al. (2019) argue that small inorganic nanoparticles could enhance the purification efficiency of organic membranes. Titania is one of the most widely used antifouling coating materials due to its photocatalytic properties. According to research, researchers also found that adding zeolite and silica nanoparticles to the surface increases the impact on water flux, surface roughness, and contact angle. MMMs possess the attributes of polymeric and ceramic membranes, but their analysis is a challenge as the interfacial region among multiple constituents may exhibit undesired structures and mutual solubility issues. Before developing a manufacturing apparatus for large-scale production, it is necessary to conduct studies on MMMs with a larger surface area (Fathizadeh et al. 2011; Huang et al. 2013).

Cellulose-based NF membranes with application

Reid and others 1955 developed and commercialized cellulose-based membranes with acetone as a solvent. A phase inversion method to synthesize the CA membrane consists of a complex step (Holloway et al. 2015). However, some studies have been published to improve CA membranes through better separation performances and comparable costs. Researchers concluded that the dispersing of silver nanoparticles on the CA membrane surface would make it more biologically stable while maintaining its permeability and salt rejection. Aluminum oxide has been used to improve the compliance strength of CA membranes greatly during the past few years. Thin-film composite membranes have dominated the market because of their superior permeability and rejections compared to CA membranes. Despite this, CA membranes remain popular because of their exceptional chlorine resistance based on numerous variables including polymer type, synthesis procedure, and the pH of the feed solution (Chou et al. 2005).

The cellulose acetate membrane exhibits good permeability, and excellent rejection characteristics are susceptible to hydrolysis, limited pH resistance, and chlorine tolerance, and are impervious to fouling. In addition, polyvinylidene fluoride )PVDF(, polystyrene (PS), and polyacrylonitrile (PAN) can be combined into different types to increase their abilities. Their permeability, flexibility, and strength are excellent. The PP polymer is limited in blending capacities and is readily oxidized (Li et al. 2011; Arthanareeswaran and Kumar 2010).

TFC membranes are made of polyurea, polyamide, and polyurea-amide (Pendergast and Hoek 2011; Lau et al. 2012). Various materials can be used as support for this membrane, such as PES, PS, and PAN. For TFC membranes, there is an important film structure, which through the composition of polymers, surfactants, and additives, different structures can be generated (Lau et al. 2012). Membranes with TFCs are used to get high efficiency and selectivity in desalination processes. Also, these membranes are used to desalt brackish water, recover wastewater, soften the water, and remove organic compounds. There are asymmetrical structures in TFC membranes. Optimizers can therefore select support and selective layers independently. Interfacial polymerization or coating is generally the way they are fabricated. Then they are cross-linked to alter their properties, such as dielectric bonding, thin-film structure stability, selectivity, and permeability (Kadhom and Deng 2019).

Application of nanofiltration membrane in water treatment

Only 3% of the earth's water is freshwater and others are saline. Due to inadequate rainfall in some places, water has become scarce. Consequently, the significance of groundwater resources has magnified in such circumstances. Many regions in the world depend on seawater both for drinking and industrial use. With the recent development of nanotechnology, many issues related to water quality could be resolved by using nanoparticles, NF, or other related products (Adesina 2004). Water desalination is the most sensational innovation in the development area of water purification. In addition, in every sector of life, waste generation is inevitable and it is an integral part of the value chain. The important issue explored at many centers is utilizing unusable water with potable water with specific nanoparticles, which can be more effective, inexpensive, and rapier than others. In addition, one of the important applications of NF is wastewater treatment in the industry. By using NF, contaminants are removed and the water is purified, which leads to a significant reduction in cost, time, and labor (Savage and Diallo 2005).

Due to excellent contaminant removal, ongoing improvement in price, and reduced energy consumption, membranes have become widely accepted and popular. Many researchers have applied NF in environmental applications for groundwater treatment, surface water reclamation, and sewerage treatment. Research has shown that NF is increasingly used for new and increasingly important applications such as removing arsenic, persistent organic pollutants, pharmaceutically active compounds, and removing salts and small organics (Mohammad et al 2015). According to Fig. 4, various water sources have been shown that could be changed to potable water with different membrane steps.

Fig. 4
figure 4

Membrane steps to obtain potable water and their sources

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It is typical for NF membrane systems to have three separate subsystems: pretreatment, membrane processing, and post-treatment. Desalination of saline water, surface water, and groundwater is the primary use of NF membranes. Surface water often changes in composition or chemistry with seasonal changes or dilution from rain. As a result, NF is a reliable surface water treatment method, although the main objective is to remove organics rather than to soften the water (Van der Bruggen et al. 2003; Hilal et al. 2004a, b).

Therefore, in this section, we examine the application of nanofilters in various water resources from groundwater to surface water and further explore the problems and limitations ahead.

Groundwater

Minerals such as calcium, magnesium, carbonate/alkalinity, and silica can be found in most groundwater sources. If the concentrations of ions exceed mineral solubility limits, there are chances of precipitation. After the snowfall, precipitation results in the deposition of scales, which can cause issues. Membrane technologies are effective in solving this problem (Kinsela et al. 2012). One of the important parameters regarding groundwater contamination is arsenic (AS) which has increased worldwide attention. Arsenic is found in both organic and inorganic forms in natural water resources. Exposure to arsenic causes cancer over the long term. According to the World Health Organization, arsenic levels in water must be below 0.01 mg/L. The deployment of membranes for removing arsenic has emerged as a prevalent method, and it has proven successful. Several pilot and large-scale studies have been published in the literature (Koyuncu et al. 2015). Various studies have been conducted regarding AS removal for NF efficiencies. However, most investigations were conducted on a laboratory scale or employed synthetic water contaminated with arsenic as a model solution. Several variables were used to determine whether the membranes had identical selectivity for both naturally contaminated groundwater and artificially contaminated water. Pal et al., have developed a sustainable treatment scheme (Fig. 5) using continued treatment. This approach integrates a largely fouling-free and highly selective NF membrane module with an up-stream pre-oxidation unit and a down-stream stabilization unit. The output result of the membrane-integrated hybrid system was more than 98% removal of arsenic from water that contained 190 mg/L of total suspended solids (TSS) (Pal et al. 2014).

Fig. 5
figure 5

A schematic of an integrated membrane-based water treatment process for removing arsenic from water (Pal et al. 2014)

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Studies have been done on water ground softening as well. Galanakis et al. studied the NF of BGW by applying a commercial poly piperazine membrane (NF99). In some specific conditions such as high hardness retention coefficients of 70–76%, with flux of 15–47 L/m2L, and pressure of 6–10 bars, they found that salinity removal was low (44–66%). Thus, this application is restricted to processing salinities less than 1100 NaCl/L (Galanakis et al. 2012).

Chakrabortty et al. studied the elimination of fluoride from groundwater in some parts of India under varying operating conditions. As a result of the membrane-based technology, the outcomes were positive. Another experiment was conducted on removing radium, uranium, carbonate complex, and other similar compounds from groundwater by RO (AD4040F) and NF (HL4040F) in parallel. Their results indicated that constant desalination with the positive rejection of all major isotopes is superior to conventional assay methods (Chakrabortty et al. 2013).

In studies conducted by Schaep and colleagues, 20–75% of multivalent ions could be removed using different NF membranes. Based on the comparison between the NF membranes and activated carbon adsorption, it was revealed that both processes were effective. However, NF membranes offered some benefits in terms of investment costs (Schaep et al. 1998; Van der Bruggen et al. 1998; Hilal et al. 2004a, b).

One large-scale NF drinking water treatment study by Radjenović and colleagues indicates that the full-scale NF treatment is more effective in rejecting pharmaceutical residues. The analytical method included 31 compounds belonging to different therapeutic groups and various physical and chemical properties, 12 of which were frequently detected in groundwater wells. A total of five sampling campaigns detected analgesics and anti-inflammatory pharmaceutical residues in groundwater used as feedwater, including b-blockers sotalol and metoprolol, antiepileptic drugs carbamazepine, ketoprofen, diclofenac, acetaminophen, and propyphenazone, the antibiotic sulfamethoxazole, a lipid regulator gemfibrozil, and diuretic hydrochlorothiazide. Rejection rates of almost every pharmaceutical examined exceeded 85% for both NF and RO, showing excellent overall performance. Acetaminophen membrane retention was (43.3–27%) reduced, gemfibrozil (50–70%) reduced, and mefenamic acid (30–50%) decreased. The brine stream from NF processes contains several pharmaceutical residues at concentrations of several hundred milligrams per liter, meaning its disposal in a nearby river may have detrimental effects on the environment (Radjenović et al. 2008).

Surface water

Notably, due to the expense and non-availability of potable water, industries and municipal entities are increasingly turning to groundwater as a primary source. Fang et al. (2013) indicate that surface water should be treated to achieve the required water quality. Surface water is typically contaminated with bacteria, algal algae, and organic matter, which create an unpleasant odor and bad taste. In the case of surface water, the greatest focus of NF is on softening and removing persistent organic pollutants. Fang et al. developed novel composite hollow fiber membranes for water softening through low operating pressure. Under 2 bar operating pressure, the filter removed 96.7% MgCl2 and 80.6% MgSo4. The filter was developed as a thin-film selective layer that combined Donnan exclusion and steric hindrance separation mechanisms (Fang et al. 2014).

Removing natural organic matter (NOM) and disinfection byproducts (DBPs) from chlorinated water has also been studied extensively. Many water utilities are striving to remove NOM from their water. Studies have shown that NF alone can reduce TOC to as low as 3.5 mg/L. Also, in research applying ceramic NF membranes with an MWCO of 450 Da (NF450) and 1000 Da (NF1000), pressure and conductivity reduced DBPs in natural water. Under operating conditions, the NF450 membrane demonstrated improved DOC removal and reduced the production of DPBs (Martínez‐Huitle and Brillas 2008).

Currently, contamination from xenobiotics is a primary environmental concern exhibiting a drastic increase. Among them, glyphosate-based herbicides are most contaminated in rivers and superficial water. With an NF, Saitúa et al. (2012) examined the effect of bacteria on synthetic and river water contaminated with glyphosate. Over 80% of the sample’s contaminants were removed.

Because of its high potential to solve water shortages and water quality problems, NF technology attracted more attention from an environmental point of view. In dry areas and remote areas where electricity is not available, mixing renewable energy with NF technology is an increasingly common trend (Sanches et al. 2012).

Seawater

The NF membranes used in seawater desalination are typically between 200 and 2000 Da. With NF membranes, about 98% of divalent ions and 30–85% of monovalent ions are rejected from the water. Because NF membranes have greater permeability than RO membranes, they are better at leaving divalent ions. In Fig. 6, we can see a flow diagram of a dual-step natural network. Some researchers question whether dual-step NF can be used to desalinate seawater more cost-effectively than RO. A two-step nanofiltration (NF) process is employed, wherein the feed permeates through the initial NF membrane before undergoing additional processing in the second NF membrane, ultimately yielding potable water. Concentrates may be optionally introduced into the NF feed tank. The dual-stage NF system requires a proportionate energy input for effective seawater desalination, primarily determined by the ratio of salinity to flux (AlTaee and Sharif 2011). Diminished permeating salinity levels necessitate increased energy demand.

Fig. 6
figure 6

The desalination process in two stages. Reprinted by AlTaee and Sharif (2011)

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Harvesting rainwater

It is preferable for irrigation purposes to use rainwater harvested from the ground. Alternatively, rainwater is increasingly being harvested for use as drinking water and other indoor services. Rainwater can be filtered using NF technology to meet the requirements for drinking water. Alternatively, households can implement small-scale water treatment systems for the onsite production of potable water. End-users can purchase several filtered water types including UV, granulated activated carbon, and membrane filtration sourced from suppliers (Koyuncu et al. 2015).

Wastewater

In water pollution caused by human activities, such contaminated water is typically deemed as waste. There are several types of wastewater sources, including domestic, industrial, commercial, and agricultural activities, surface runoff, and sewer inflows. The contamination of water resulting from human activity is often regarded as waste. Various industries such as the chemical, food, and textile industries, as well as leather manufacturers, utilize NF membranes for wastewater treatment. The NF membrane effectively blocks the entry of dye species into its pores due to its negative surface charge, which creates an electrostatic repulsion. In a study, loose NF membranes were shown to be useful as anion-conductive membranes in integrated treatments to recover resources from highly saline wastewater. A loose NF is used in electrodialysis (ED) for the anion-conductive membrane in a novel way to separate dye and salts. The method of electrodialysis found here offers potential implications for the sustainable treatment of textile wastewater in Fig. 7. Based mainly on the synergistic effect of steric and Donnan repulsion, the NF membrane with a molecular weight cut-off of 678 Da retained dyes with small molecular weights (627 Da). On the other hand, the NF membrane had a remarkable ability to transport anions (i.e., Cl) as its loose surface structure created additional nanochannels for ion transfer. A single-step dye/NaCl fractionation performed excellent separation with the anion-conductive loose NF membrane-based ED process. Results show 98.9% desalination efficiency and 99.4% dye recovery with the loose NF-based ED process. The NF membrane and its negative surface charge acted as a barrier by resisting dye species’ entry into its pores by electrostatic repulsion (Ye et al. 2020).

Fig. 7
figure 7

A facile electrodialysis based on loose nanofiltering to enhance dye/salt fractionation. Reprinted from Ye et al. (2020)

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In another study, a loose NF membrane, which was made by an electrospray interfacial polymerization (EIP) method was used to treat dye wastewater. Through this technique, a thin layer of poly(piperazine-amide) is controlled in thickness and cross-linking degree; it can lead to changes in pore size and water permeance, and dye/salt rejection ratio. EIP membranes fabricated under efficient fabrication conditions (20.2 LMH/bar), high dye rejection (99.5%), and low salt rejection (6.3% for NaCl) were shown to possess excellent pure water permeance (20.2 LMH/bar). In addition to improving antifouling properties, it was also seen that the EIP membrane displayed enhanced flux recovery rates of 87.1% and decreased irreversible fouling rates of 12.9% after fouling with bovine serum albumin (BSA). This suitable antifouling property is due to the relatively low protein adsorption, low roughness (Ra), high hydrophilicity, zeta potential, and superb smoothness of the surfaces (Kang et al. 2020).

A novel graphene oxide (GO)-coated composite membrane shows great potential for continuously removing chemically contaminated water with a combination of NF and catalysis. Stacked GO nanosheets with plentiful nanopores incorporated into polymeric membranes are potentially suited for water purification but maintaining the high permeability of the membrane continues to be a challenge. The delimitation of the GO layer from the membrane surface is another bottleneck affecting the efficiency of the material. A new catalytic membrane with enlarged interlayer space was immobilized on the surface of a cross-linked GO composite layer to solve these problems. After 6 h of filtration in a cross-flow model, the modified GO nanosheets-coated substrate had good separation robustness despite the most potent lateral shear force. This membrane removed Congo red and Basic blue extremely efficiently (99% and 96%) due to its GO composite layer coated with a surface coating (Zhong et al. 2020).

High water recovery can be achieved by reticulating NF concentrate into the membrane bioreactor for textile wastewater treatment. The high water recovery and its suitability for reuse should bring considerable benefits. Onsite textile wastewater treatment was conducted using a pilot-scale NF membrane bioreactor system. At first, the hydraulic retention time of membrane bioreactors was tested, then different water recovery rates were examined for membrane bioreactor-nanofiltration systems. It was found that the sludge activity was stable; however, the recirculation of NF concentrate influenced the pollutants removal performance of the membrane bioreactor. These results generated a net income of 1.24 million US$ per year, and the payback period of 3.11 years made the process more economically feasible (Li et al. 2020).

Chlorides and sulfates were removed from the rejected water with a nanofilter in steel products using the precipitation method of di-isopropyl amine, isopropyl amine, and ethylamine. A solvent-based precipitation technique demonstrated that a miscible organic solvent such as DIPA/IPA/EA effectively reduces chloride and sulfate concentrations of NF-rejected water by precipitating salts. Wastewater can be reused in the system or safely discharged into the environment. The efficacy of the process was 98% and 77%, respectively, and the solvent was recovered for further use by simple distillation (Sinha et al. 2020).

A simple method was designed to remove heavy metals with positive ions in contaminated water. In this method, polyamide (PA) NF membranes with a positive charge are produced through the linked of polyethyleneimine (PEI) amino groups to the surface of the PA membrane. The characteristics of the PEI-modified PA membrane, including its surface morphology, zeta potential, and hydrophobicity, are affected by variables such as molecular weight, PEI concentration, and exposure time to the membrane. Further, the nature of the membrane influenced its hydraulic flow and metal-ion rejection. A model solution with 5 mg/L of Cu2+ and a PEI membrane requiring 70,000 MW of energy had a rejection of > 90% under pressure. There was a minimal flux reduction in comparison to blank water. Over repeated filtration cycles, the Cu2+ rejection and membrane flux were sustained and humic acid alone did not adversely affect the rejection. As the PEI-modified PA membrane was different, electrostatic repulsion and adsorption acted the rejection of Cu2+ (Wang et al. 2020).

Oilfield-produced water

Because of oil and gas production, co-produced water is a major source of oily wastewater. Gas production produces a tremendous amount of produced water, the most significant product or waste stream associated with it. Excess amount of produced water leads to challenges in operation and, even shutoff the wells (Hajilary et al. 2015a, b). The need for new technologies will be increased due to an increased focus on water conservation and environmental regulations in the industry (Shams et al. 2007). When surface water is released, not only do suspend solids in the wastewater constitute the major constituent, but they also contribute to the COD of the water. Water can be handled in numerous ways including disposal, reinjection, and treatment. Most oil and gas companies re-inject produced water back into the oil and gas formation as a process of extraction (Li et al. 2005). This water could be used as a drinking water source by being treated. By doing so, these oil and gas fields will become more economically viable. In oil exploration, a common technique worldwide is to inject produced water near or beneath the oil deposit to boost oil yield and well productivity.

An acceptable water specification for injection must resemble natural water in oil-bearing strata as much as possible, or at least not block the petroleum deposit's porous geological formation. Moreover, the age of oil fields has a direct impact on the water content of oil well. Water production increases with time while the reservoirs gradually deplete, requiring an ever-increasing amount of water to enhance production. Some older oilfields have 90% produced water as the liquid brought to the surface. It contains not only free oil at the top, which should be removed via hydro-cyclones, but also hardness that tends to block the oil-containing stratum. Furthermore, the water extracted from oil wells could contain harmful elements such as arsenic, mercury, and heavy metals, which may cause difficulties in its disposal. With the stringent environmental regulations implemented globally, the demand for treating produced water is progressively rising (Veil 2002; Lee et al. 2001).

To meet updated standards, agencies hold stringent environmental regulations to control treatment technologies. For example, the European standard for onshore petroleum on-ground effluent must have a total concentration of less than 5 mg/L and a suspended solids concentration of less than 10 mg/L (Hammond and Stapleton 2001). To overcome the challenges caused by strict regulations and to minimize additional costs, membrane filtration can be used. A new class of methods for processing produced water is membranes that are made from a variety of materials with different processing which present broad physical–chemical advantages (Hajilary et al. 2015a, b). Nanofiltration is an effective method that can eliminate particles, and emulsify and disperse oil with minimal energy consumption and high throughput. Some drawbacks of these methods include the loss of volatile compounds that typically dissolve at room temperature and the fact that they contain radioactive byproducts that require pretreatment (Ciarapica and Giacchetta 2003). To prevent membranes from fouling during operations, more attention should be given to providing expedient and cost-effective antifouling membranes (Shams et al. 2007).

According to Shams et al. (2007), ceramic membranes are economically attractive when used to treat produced waters that contain elevated levels of oil and medium-sized particles.

Yuliwati and Ismail (2011) examined the effectiveness of adsorption and NF for separating total petroleum hydrocarbons (TPH) from wastewater from an oil refinery in Bushehr, Iran. The main polymer used for synthesizing polyaniline membranes was polyaniline and its mineral was clinoptilolite zeolite. The permeation flux during the NF process was 190.96 kg/s m2 and the TPH rejection was 99.77%. Based on gas chromatography and mass spectrometry, TPH removal efficiencies were 92.3% in batch and 87.4% in continuous systems while the conditions were optimized. Moreover, the hybrid absorption and NF process gave 99.83% removal efficiency (Esmaeili and Saremnia 2018).

Three produced water, two chemical NF membranes, and one reverse osmosis membrane were tested in Colorado, USA. Based on their results, multiple surface characterization methods have proven to help measure membrane fouling. To obtain high-quality water from NF, the requirements for the intended application must be mentioned (Mondal and Wickramasinghe 2008).

According to Cabrera et al., the NF membrane was used in recycled water from a Canadian oil sand mine to reduce the ion concentration, TSS, and TOC levels. This unit was the first of its kind and was tested nearly two years to evaluate membrane performance in actual process water conditions.

The analysis of more than 20 ions indicates that divalent cations are the most susceptible due to the differences in hydrated ionic sizes and electrostatic phenomena. TOC rejections were 75–90% and almost 100% TSS rejections. It has been shown that this technology is likely to gain significant water quality improvements in oil sand mines while reducing river water intake and river pollution (Cabrera et al. 2021).

In a study conducted by Zhao et al. to evaluate the onsite NF efficiency, the NF module utilized in the experiment was continuously unfolded and examined. Regarding dry weight, the results suggest that 86.3% was derived from both anionic polyacrylamide and crude oil, which collectively accounted for a majority (86.3%). In addition, dissolved organics with a high molecular weight tended to accumulate on the membrane surface. The main inorganic species within the fouling layer were multivalent elements, such as Mg, Ca, Al, Fe, and Si. In the fouling layer, Si occupied the highest proportion of inorganic elements and existed in the form of SiO2. The autopsy results indicated that organic and inorganic fouling contributed to the decline in the flux (Zhao et al. 2019).

NF in future

Humanities have faced major problems related to water resources for many years. In this study, a notable shift was observed with the progression of membrane production. Specifically, the exceptional selectivity of NF membranes has facilitated their effective deployment thus far for targeted separation species. It is important to select membranes with suitable selectivity depending on the application of interests to achieve the greatest separation efficiency. Due to this capability, NF membranes can offer new applications in various areas, especially water and wastewater treatment, brackish water desalination, pretreatment of seawater in desalination, and separation of small organics from other species in the pharmaceutical, biotechnology, and food industries. There will be more exciting applications explored in the future.

Some membranes are not optimized for energy consumption and fouling; however, many advances have been made lately. The research is focused more on the fabrication of new NF membrane materials, design, calculation, and finding new operation modes to decrease energy requirements for treating water more effectively. Additional aspects of NF membrane that should be examined by future studies include controlling fouling, capacity, cost, selectivity, and environmental effects. The NF membrane has been widely used in many different fields, but few studies have dealt with its operations on a large scale. Studies on block copolymers, biomimetic membranes, and patterned membranes indicate that these materials have high permeability rates and selectivity. It may be hard to commercialize these NF membranes, but their efficiency and cost may improve in the future. The development of high-temperature-tolerant ceramic membranes is one of the major challenges NF membranes face in the future (Abetz et al. 2006). With the continued development of membrane technology, nanofiltration become more powerful. Molecular modeling and material science are the most powerful tools that can be extremely helpful in moving this forward. Although the filters have demonstrated high-efficiency levels nearing 100%, alternative methods for enhanced salt reduction are currently under investigation. As oxidizing agents and abrasive chemicals become more widespread, NF membranes will be able to remove chlorine more effectively from the system. The fabrication and modification of membranes have been used in several studies to produce membranes with significantly reduced fouling tendency.

Conclusion

Given the growing demand for clean water, expedited access to these sources has become an increasingly pressing concern. It is noteworthy that the provision of clean water is not only critical for human survival but also plays a crucial role in various industries such as petrochemicals, electronics, pharmaceuticals, and food. There has been a nearly tenfold increase in articles citing NF over the past few years, emphasizing its necessity for further development. Because NF membranes belong to the pressure-driven liquid membrane process family, they have achieved considerable success in water purification. Pore sizes for NF membranes typically range from 0.5 to 10 nm. It is often used to separate solid dust, liquid droplets, sugars, proteins, dyes, bacteria, and microbes. This process is between reverse osmosis and ultrafiltration and rejects molecules whose size is less than a nanometer, equivalent to molecular weight cut-offs between 300 and 500 Da. Based on the material type, NF membranes are generally divided into two categories, namely, organic (polymeric) and inorganic. Commercial organic NF membranes are used in many industrial applications. The first part of this review focuses on developing a comprehensive understanding of the properties, characteristics, and performance of all types of NF membranes, the benefits and drawbacks of nanofiltration, and their cost-effectiveness of them. Membrane materials technologies have recently improved to enhance permeability and selectivity. Due to the improved performance of NF membranes and the use of membrane modules, concentration polarization and fouling problems have decreased. In the following, the applications of NF membranes are discussed, including the treatment of surface water, groundwater, seawater, and harvesting rainwater. The NF membrane processes can remove many specific contaminants effectively. However, to obtain more benefits from NF membranes, technical and scientific problems need to be solved. While some of these NF membranes still require more evaluations, they are likely to become commercialized soon for use in water purification and wastewater treatment. Nanofiltration still has a long way to go concerning understanding, materials, and process control. Despite the drawbacks discussed, NF is widely used in industry, and the properties of NF membranes enable novel separations that are difficult or expensive with other separation techniques. Additionally, NF is still underdeveloped for industrial applications due to these drawbacks. Further development of NF will be closely connected to the research objectives mentioned previously. Therefore, it is predictable that a lot will be accomplished in the coming decade.