**1. Introduction**

The world is now expected to become water scarce as a result of global warming, and by 2025, it is estimated that water-scarce countries will increase by more than 30% compared to 1995 [1]. The twentieth century was the era of black gold, represented by oil, but the age of water, or blue gold, is expected to emerge in the twenty-first century. Due to the global problems

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

faced by the world, such as population growth, industrialization, and climate change, a steady increase in water demand and a disparity in regional water supply are urgently needed to be resolved. Population Action International (PAI) currently has 550 million people living in water-pressure or water-starved countries, and from 2.4 billion to 3.4 billion people will live in water-starved or water-deprived countries by 2025. According to the World Meteorological Organization (WMO) report, 653 million people in 2025 and 2.43 billion in 2050 will suffer direct water shortages.

The separation membrane has a selective filtration function that selectively passes a specific component, as well as selective permeability capable of separating dissolved substances or mixed gases dissolved in a liquid [5–7]. Membrane separation technology comprehensively means various separation processes using such selective permeability of the membrane. As shown in **Figure 2**, the separation membrane used for water treatment produces clean water by allowing the water (B) to pass but not allowing the suspended material (A) to pass through. Membranes can be divided into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) depending on the pore size [8–10]. **Figure 3** shows the separation performance according to the pore size of the membrane, and **Table 1** shows the membrane

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characteristics of various process parameters [11].

**Figure 2.** Schematic of membrane filtration process.

**Figure 3.** A scheme of the membrane for water purification processes.

Various water treatment techniques have been studied to secure water resources in order to solve the water shortage phenomenon. In the water treatment field, there are water treatment processes such as wastewater and wastewater treatment to remove pollutants, water treatment for drinking water, and seawater desalination for seawater reuse (**Figure 1**).

There are also a number of related technologies, among which water treatment technologies using membranes have shown very high growth rates of 10–20% per year [2, 3]. Frost and Sullivan estimate that the world's membrane-based water treatment market will grow from \$ 5.54 billion in 2012 to \$ 1.27 billion by 2020 (CAGR of 10.2%). Major growth factors include increased demand for drinking water, reuse of sewage, increased desalination facilities based on membranes, and strengthening of environmental standards. In particular, it is expected that there will be a significant increase in the market in the Asia-Pacific region based on rapid industrialization, population growth, and demand for advanced technologies [4].

**Figure 1.** Various applications of water treatment membranes. (a) MBR process (Toray Industries, Inc.), (b) water treatment process (Yeongdeungpo water purification center), (c) desalination process (Doosan heavy industries & construction), (d) ion exchange membrane (Tokuyama America, Inc.).

The separation membrane has a selective filtration function that selectively passes a specific component, as well as selective permeability capable of separating dissolved substances or mixed gases dissolved in a liquid [5–7]. Membrane separation technology comprehensively means various separation processes using such selective permeability of the membrane. As shown in **Figure 2**, the separation membrane used for water treatment produces clean water by allowing the water (B) to pass but not allowing the suspended material (A) to pass through. Membranes can be divided into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) depending on the pore size [8–10]. **Figure 3** shows the separation performance according to the pore size of the membrane, and **Table 1** shows the membrane characteristics of various process parameters [11].

**Figure 2.** Schematic of membrane filtration process.

faced by the world, such as population growth, industrialization, and climate change, a steady increase in water demand and a disparity in regional water supply are urgently needed to be resolved. Population Action International (PAI) currently has 550 million people living in water-pressure or water-starved countries, and from 2.4 billion to 3.4 billion people will live in water-starved or water-deprived countries by 2025. According to the World Meteorological Organization (WMO) report, 653 million people in 2025 and 2.43 billion in 2050 will suffer

Various water treatment techniques have been studied to secure water resources in order to solve the water shortage phenomenon. In the water treatment field, there are water treatment processes such as wastewater and wastewater treatment to remove pollutants, water treat-

There are also a number of related technologies, among which water treatment technologies using membranes have shown very high growth rates of 10–20% per year [2, 3]. Frost and Sullivan estimate that the world's membrane-based water treatment market will grow from \$ 5.54 billion in 2012 to \$ 1.27 billion by 2020 (CAGR of 10.2%). Major growth factors include increased demand for drinking water, reuse of sewage, increased desalination facilities based on membranes, and strengthening of environmental standards. In particular, it is expected that there will be a significant increase in the market in the Asia-Pacific region based on rapid

**Figure 1.** Various applications of water treatment membranes. (a) MBR process (Toray Industries, Inc.), (b) water treatment process (Yeongdeungpo water purification center), (c) desalination process (Doosan heavy industries &

construction), (d) ion exchange membrane (Tokuyama America, Inc.).

ment for drinking water, and seawater desalination for seawater reuse (**Figure 1**).

industrialization, population growth, and demand for advanced technologies [4].

direct water shortages.

202 Desalination and Water Treatment

**Figure 3.** A scheme of the membrane for water purification processes.


osmotic pressure and thus do not require high pressure to apply pressure above osmotic pressure. Ultrafiltration is the same as reverse osmosis in mathematical modeling but fundamentally different from reverse osmosis. The reverse osmosis is largely governed by the correlation between the membrane and the dissolved salt, whereas ultrafiltration is dominated by the solute and pore size. In other words, ultrafiltration has a separation effect by the steric hindrance at the micropore inlet and the frictional resistance between the solute and the pore wall in the pore. The molecular weight cut off (MWCO) in the ultrafiltration method is an important item. The closer the slope is to infinity, the narrower the fractional molecular weight distribution which can be regarded as an excellent filter membrane. Ultrafiltration has a wide range of industrial applications in the middle of reverse osmosis and microfiltration in terms of the size of the separation object. The membrane material is the same as the material of the reverse osmosis membrane and has only a large pore size in terms of being hydrophilic [16–18].

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Nanofiltration is the process of treating hundreds to thousands of molecules with medium molecular weight, the range of treatment of reverse osmosis membranes and ultrafiltration membranes. Nanofiltration is used for separation of small solvent molecules due to deformation of reverse osmosis membrane, but even large molecules of polysaccharides such as sugar can be separated. Nanofiltration membranes usually have a fractional molecular weight of 20–70% NaCl and organic solvents of 200–500. This fractional range corresponds to a diameter of about 10 Å, or 1 nm, of the molecule. This membrane is used for seawater treatment in the pressure range of 0.4–0.7 MPa which is 1/4–1/2 of the reverse osmosis pressure. The exclusion mechanism is similar to reverse osmosis and is widely applied to the separation of salt and organic matter of appropriate molecular weight. The nanofiltration membranes can be used at a rate of 50–97.5% at the same time and are used to replace the ion exchange method in the

Reverse osmosis is a membrane separation process that separates solutes smaller than 10 Å in size of ions and molecules and was industrialized in seawater desalination and wastewater treatment in the 1970s. The membranes are composed of asymmetric cellulose acetate or aromatic polyamide which is formed as an active layer for a separating effect on the supporting layer. Recently, a composite membrane capable of removing up to 99% of dissolved salts has been developed. The composite membrane is formed of a polymer thin film having a high salt removal effect on the support layer. The support membrane is mainly composed of polysulfone having high mechanical strength and chemical resistance, and cellulose triacetate and cross-linked polyether are mainly used as the separation layer. Since the reverse osmosis membrane has almost no pores, it can be regarded as a nonporous membrane, which is permeated through the gap between micelles forming organic polymers or micelles. In the reverse osmosis method, since the dielectric constant of the organic polymer is low, the dissolved salt is not adsorbed to the membrane. In addition, in high pressure (800–1500 psig), water, which is a solvent, permeates in proportion to the osmotic pressure difference. The separation effect is increased. Since the reverse osmosis is not a separation operation according to the molecular size, deposition of organic substances such as microfiltration and ultrafiltration is less and consequently, the lifetime of the membrane is increased. Reverse osmosis membranes are being used not only for separation and removal of dissolved salts but also for separation of organic and aromatic hydrocarbons with low

water softening process [19–21].

molecular weight [22–24].

**Table 1.** Correlation of membrane features with ranges of separation [6].

Microfiltration is a membrane separation process for separating a solute having a solute size of about 0.1–10 μm. It is preferable that the membrane used at this time is about 0.01–10 μm in pore size and the pore accounts for about 80% Do. As for the material of the membrane, cellulose-type, nylon, PVC, polytetrafluoroethylene (PTFE), and various other polymer materials are suitable. In a microfiltration process, propulsion is represented by a pressure difference, where the pressure difference is typically 1–30 psig. The separation effect of this membrane is fundamentally dependent on the pore size of the membrane and the size of the substance to be separated. If the size of the substance to be separated is smaller than the pore size, it does not pass through the entire membrane but the substance to be separated is adsorbed on the membrane or is not transmitted by steric hindrance near the pore. The biggest problem of the microfiltration process is the deposition of colloidal material on the membrane surface, which reduces the flow rate by blocking the pores, which can be replaced or regenerated to restore the original state [12–15].

Ultrafiltration is a membrane separation process that separates macromolecules or colloidal particles with molecular sizes ranging from 10 to 1000 Å. The pore size ranges from 20 to 500 Å. This method uses a differential pressure as a thrust for the separation operation similar to the reverse osmosis method. The pressure differential used in ultrafiltration is usually in the range of 10–100 psig because particles with a high molecular weight have relatively low osmotic pressure and thus do not require high pressure to apply pressure above osmotic pressure. Ultrafiltration is the same as reverse osmosis in mathematical modeling but fundamentally different from reverse osmosis. The reverse osmosis is largely governed by the correlation between the membrane and the dissolved salt, whereas ultrafiltration is dominated by the solute and pore size. In other words, ultrafiltration has a separation effect by the steric hindrance at the micropore inlet and the frictional resistance between the solute and the pore wall in the pore. The molecular weight cut off (MWCO) in the ultrafiltration method is an important item. The closer the slope is to infinity, the narrower the fractional molecular weight distribution which can be regarded as an excellent filter membrane. Ultrafiltration has a wide range of industrial applications in the middle of reverse osmosis and microfiltration in terms of the size of the separation object. The membrane material is the same as the material of the reverse osmosis membrane and has only a large pore size in terms of being hydrophilic [16–18].

Nanofiltration is the process of treating hundreds to thousands of molecules with medium molecular weight, the range of treatment of reverse osmosis membranes and ultrafiltration membranes. Nanofiltration is used for separation of small solvent molecules due to deformation of reverse osmosis membrane, but even large molecules of polysaccharides such as sugar can be separated. Nanofiltration membranes usually have a fractional molecular weight of 20–70% NaCl and organic solvents of 200–500. This fractional range corresponds to a diameter of about 10 Å, or 1 nm, of the molecule. This membrane is used for seawater treatment in the pressure range of 0.4–0.7 MPa which is 1/4–1/2 of the reverse osmosis pressure. The exclusion mechanism is similar to reverse osmosis and is widely applied to the separation of salt and organic matter of appropriate molecular weight. The nanofiltration membranes can be used at a rate of 50–97.5% at the same time and are used to replace the ion exchange method in the water softening process [19–21].

Reverse osmosis is a membrane separation process that separates solutes smaller than 10 Å in size of ions and molecules and was industrialized in seawater desalination and wastewater treatment in the 1970s. The membranes are composed of asymmetric cellulose acetate or aromatic polyamide which is formed as an active layer for a separating effect on the supporting layer. Recently, a composite membrane capable of removing up to 99% of dissolved salts has been developed. The composite membrane is formed of a polymer thin film having a high salt removal effect on the support layer. The support membrane is mainly composed of polysulfone having high mechanical strength and chemical resistance, and cellulose triacetate and cross-linked polyether are mainly used as the separation layer. Since the reverse osmosis membrane has almost no pores, it can be regarded as a nonporous membrane, which is permeated through the gap between micelles forming organic polymers or micelles. In the reverse osmosis method, since the dielectric constant of the organic polymer is low, the dissolved salt is not adsorbed to the membrane. In addition, in high pressure (800–1500 psig), water, which is a solvent, permeates in proportion to the osmotic pressure difference. The separation effect is increased. Since the reverse osmosis is not a separation operation according to the molecular size, deposition of organic substances such as microfiltration and ultrafiltration is less and consequently, the lifetime of the membrane is increased. Reverse osmosis membranes are being used not only for separation and removal of dissolved salts but also for separation of organic and aromatic hydrocarbons with low molecular weight [22–24].

Microfiltration is a membrane separation process for separating a solute having a solute size of about 0.1–10 μm. It is preferable that the membrane used at this time is about 0.01–10 μm in pore size and the pore accounts for about 80% Do. As for the material of the membrane, cellulose-type, nylon, PVC, polytetrafluoroethylene (PTFE), and various other polymer materials are suitable. In a microfiltration process, propulsion is represented by a pressure difference, where the pressure difference is typically 1–30 psig. The separation effect of this membrane is fundamentally dependent on the pore size of the membrane and the size of the substance to be separated. If the size of the substance to be separated is smaller than the pore size, it does not pass through the entire membrane but the substance to be separated is adsorbed on the membrane or is not transmitted by steric hindrance near the pore. The biggest problem of the microfiltration process is the deposition of colloidal material on the membrane surface, which reduces the flow rate by blocking the pores, which can be replaced

**Microfiltration Ultrafiltration Nanofiltration Reverse osmosis**

exclusion

Porous asymmetric Finely porous asymmetric/ composite

Darcy's law Darcy's law Fick's law Fick's law

0.1–10 0.01–0.1 0.001–0.01 <0.001

anions

1–30 3–80 70–220 800–1200

500–10,000 100–2000 20–200 10–100

HMWC, mono-, di-, and oligosaccharides, polyvalent

CA, PA, TFC CA, PA, PS, TFC

Solution/diffusion + exclusion

Nonporous asymmetric/

HMWC, LMWC, sodium chloride, glucose, amino acids,

composite

proteins

Sieving Sieving Sieving + solution/diffusion +

MWCO (Da) >100,000 >2000–100,000 300–1000 100–200

CA, CE, PA, PAN, TFC, PS, PVDF

Macromolecules, proteins, polysaccharides, viruses

**Table 1.** Correlation of membrane features with ranges of separation [6].

Mechanism or separation

Materials CA, CE, PAN,

204 Desalination and Water Treatment

Structure Porous

Rejects Particulates,

Law governing transfer

Pore size range (μm)

Operating pressure (psi)

Fluxes (L/m2 h) PC, PE, POF, PP, PS, PTFE, PVDF

isotropic

clay, bacteria

Ultrafiltration is a membrane separation process that separates macromolecules or colloidal particles with molecular sizes ranging from 10 to 1000 Å. The pore size ranges from 20 to 500 Å. This method uses a differential pressure as a thrust for the separation operation similar to the reverse osmosis method. The pressure differential used in ultrafiltration is usually in the range of 10–100 psig because particles with a high molecular weight have relatively low

or regenerated to restore the original state [12–15].

Treatment and the reusing of wastewater are mainly based on the activated sludge process. In the activated sludge process, the amount of generated sludge is large, the treatment cost is high, and it is vulnerable to impacts such as biological oxygen demand (BOD) overload and toxicity, and problems such as sludge bulking occur. However, the membrane bioreactor (MBR) does not need to regulate the amount of microorganisms in the reactor and does not cause the sludge expansion phenomenon. In addition, it has excellent durability against load generated in the operation such as impact, toxicity, and organic load. It is expected that the technology of the MBR process will increase gradually because of the advantages of this separation membrane process [25, 26].

publication of papers, technical trends and recent technology trends, And to analyze the trend

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In this chapter, we show all patents and papers for water treatment membranes. In the past 20 years (1995–2014), we have divided membranes for wastewater treatment, separation membranes for water treatment, and seawater desalination membrane trends. In evaluating the competitiveness of patent technology, patents filed after 2015 are analyzed only as valid patents before 2015 except for the fact that they were undisclosed. The patent database of WIPS was used for the analysis of the patent. The patent data were searched through the keyword search and the secondary classification was performed using the library method for noise and pattern removal. Finally, a final classification was carried out by experts in each field. The patent activity index (PAI), the patent intensity index (PII), the patent market-power index (PII), the patent market power index (PMI), and patent citation index (PCI) were the categories. These terms can be defined as follows. Patent activity is defined as the absolute number of patent applications based on the number of public/patent publications issued by the Patent Office. Patent concentration refers to providing information about the technological innovation activities that a country concentrates on relative to other countries. The patent market power refers to the use of patents as an indicator of the patentability of patents when the number of family patents is large, because patents are applied only when they are in commercial profit or for technology competition in the relevant country. Finally, patent impact is the measure of the impact of the patent on future patents, and the US patent with patent

The number of patent applications has been evaluated in the 10 countries, including Korea, the USA, Japan, China, and Europe, which are the major developing countries, for patent technology competitiveness. A total of 4629 patent applications were searched. 1144 patents were related to separation membranes for wastewater treatment, 734 patents related to water treatment membranes, 668 patents related to seawater desalination membranes, and 2083 patents related to membranes for ion exchange processes. **Figure 4** shows the patent application

The thesis has been published in the past 20 years (1997–2016), and it has been evaluated for technical competitiveness. 13,506 papers are related to the separation membrane for wastewater treatment, 7958 papers are related to the separation membrane for water treatment, 9524 papers related to separation membrane for desalination, and 16,254 papers related to separation membrane for an ion exchange process. **Figure 5** shows the trend of publications by technology in the past 20 years. The database used to analyze the technological competitiveness of the paper is the Scopus paper retrieval system, which collects information by category and country and uses the Bibliometric Activity Index (BAI), Bibliometric Citation Index (BCI), and Bibliometric Intensity Index (BII). These terms can be defined as follows. The thesis activity is the number of absolute dissertations, which shows the corresponding country for the technology divided by the total number of countries. The impact of a paper is defined as an index that provides information that can be compared with other countries in terms of the quality of the paper. Finally, the thesis density can be defined as an index that provides information on the

of technology development of water treatment membranes by summarized graphs.

**2. Analysis of patents and articles on water treatment membranes**

information for the patents is targeted [40–43].

trends by technology in the last 20 years.

In most of the domestic large and medium-sized water purification facilities, using river water or lake water as a water source, problems occur periodically. However, the conventional water treatment methods such as coagulation, sedimentation, filtration, and disinfection processes are inferior in taste and odor. There is a limit in effectively controlling harmful organic substances and the like. In order to overcome the limitations of this conventional treatment method, microfiltration or ultrafiltration using a membrane has been shown to be a breakthrough technology that can replace the existing rapid sand filtration system because it completely removes turbidity and pathogenic microorganisms. In addition, it can easily combine with the existing unit-altitude water treatment process such as ozone-activated carbon to optimize the process configuration suitable for the characteristics of raw water and water quality, and it is more compact and easier to maintain than the existing water treatment method [27, 28].

Techniques for converting seawater to fresh water include conventional coagulation, coagulation, sedimentation, single- or two-phase granular filtration, dissolved air flotation, and lowpressure membrane filtration techniques using microfiltration or ultrafiltration membranes. Conventional pretreatment processes are generally used in seawater desalination, but it is difficult to completely remove float or colloidal particles, which makes it difficult to supply water in a stable manner. However, when the membrane is pretreated, it has the advantage of reducing the cost required in the desalination process, such as increasing the permeation flow rate, reducing the washing cycle, reducing the use of washing chemicals, reducing energy consumption, and reducing maintenance costs [29, 30].

A membrane electrode assembly (MEA), which is one of the most important components among the separation membranes used in the ion exchange process, includes a proton exchange membrane fuel cell (PEMFC). The membrane consists of two electrodes, cathode and anode, which determine the performance of the fuel cell [31–33]. Currently, the most widely used hydrogen ion exchange membranes are produced by DuPont's nafion® as well as Dow Chemical, 3 M, and others. The perfluorinated proton exchange membrane has been applied to most commercial fuel cell devices due to its high chemical/mechanical stability and excellent hydrogen ion transfer ability. However, since the production process requires high temperature/high pressure conditions, the production cost has increased. The limitation is that it has a pollution problem, and there is a problem that performance is reduced at high temperature due to low glass transition temperature [34, 35]. Therefore, research on the production of a hydrocarbon-based proton exchange membrane having a relatively high manufacturing cost and high thermal stability has been actively pursued as an alternative thereto [36–39].

In this review, the water treatment membranes are divided into wastewater treatment membranes, water treatment membranes, seawater desalination membranes, and ion exchange membrane separators. Through analysis of domestic and foreign patent information and publication of papers, technical trends and recent technology trends, And to analyze the trend of technology development of water treatment membranes by summarized graphs.
