Filtration of drinking water

Filtration of drinking water Darren Radcliffe-Oatley Swansea University, Swansea, Wales, United Kingdom

11.1

11

Introduction

Along with energy, shelter, and food, access to a source of fresh clean drinking water is essential to all life on earth. However, something as simple as a clean water source is not as readily available as one might first think. When Samuel Taylor Coleridge “water, water, everywhere, nor any drop to drink,” he did not have the 21st century’s global water problems in mind, however, he was not far from the truth. Today, the availability of water for domestic, industrial, and agricultural use is a crucial problem and according to the United Nations (2012), 783 million people, or 11% of the global population, remain without access to safe drinking water and almost 2.5 billion do not have access to adequate sanitation. The World Water Council estimates that the planet will be around 17% short of the fresh water supply needed to sustain the world population by 2020 (Charcosset, 2009). The majority of the earth’s water is contained in the oceans (B97%), while 2% is trapped in icecaps and glaciers, resulting in less than 1% being accessible as fresh water (Williams et al., 2015). This low level of available fresh water leads to problems of water scarcity. Hydrologists typically assess scarcity by looking at the population-water equation. An area is experiencing water stress when annual water supplies drop below 1700 m3 per person. When annual water supplies drop below 1000 m3 per person, the population faces water scarcity, and below 500 m3 “absolute scarcity.” Global variability in available fresh water (typically driven by rainfall) also causes localized water scarcity, however, this phenomenon is a more complicated issue than just precipitation alone and Table 11.1 highlights some of the factors concerned. Table 11.1 provides a list of countries across the globe that illustrate the spectrum of annual rainfall statistics, from the areas of highest rainfall (Columbia at 3240 mm per year) to lowest (Egypt at 51 mm per year). On first inspection, one could easily deduce that water scarcity is not a problem in places such as Columbia and Singapore and is an issue for places like Saudi Arabia and Egypt. However, other factors must be considered as quantities of rainfall alone are not the full picture. Clearly, the local population is a key component to this issue. Not only is the local population a consumer of fresh water, but also high population numbers would indicate that the location must have high levels of agriculture to feed this population and high levels of industry providing employment, both of which are high consumers of fresh water. Similarly, the local availability of land to store Fibrous Filter Media. DOI: http://dx.doi.org/10.1016/B978-0-08-100573-6.00008-3 © 2017 Elsevier Ltd. All rights reserved.

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Annual rainfall, population, and land mass of selected world countries

Table 11.1

Country name

Annual rainfall, mm

Population

Land area, sq. km

Rainfall per rea, mm per sq. km ( 3 105)

Population density, people per sq. km

Colombia Singapore St. Lucia El Salvador Haiti United Kingdom Bolivia Ethiopia United States China Canada Australia Kyrgyz Republic Israel Mongolia Saudi Arabia Egypt, Arab Rep.

3240 2497 2301 1784 1440 1220 1146 848 715 645 537 534 533 435 241 59 51

48,321,000 5,412,000 83,000 2,200,000 3,221,000 63,136,000 2,714,000 18,128,000 157,813,000 543,776,000 35,182,000 23,343,000 1,740,000 7,733,000 780,000 28,829,000 21,514,000

1,109,500 700 610 20,720 27,560 241,930 1,083,300 1,000,000 9,147,420 9,388,211 9,093,510 7,682,300 191,800 21,640 1,553,560 2,149,690 995,450

292 356,714 377,213 8610 5225 504 106 85 8 7 6 7 278 2010 16 3 5

44 7731 136 106 117 261 3 18 17 58 4 3 9 357 1 13 22

Source: Rainfall (2014) and Population (2013): Food and Agriculture Organization of the United Nations, online resource (,www.fao.org.); Land area: United Nations Department of Economic and Social Affairs/Population Division, World Population Prospects: The 2012 Revision, Volume I: Comprehensive Tables.

collected water in reservoirs or lakes is another key determinant. For example, Egypt has very little rainfall and has obvious water scarcity issues due to the shear lack of available fresh water. However, Singapore is one of the wettest regions on the planet but also has water scarcity issues and this is caused by a high consumption rate driven by high population density and high levels of industry. In addition, Singapore has a very low land mass and has practically no water storage capacity as a result. Thus, while being one of the wettest regions on the planet, Singapore has significant water scarcity issues. Water scarcity can also be a significant problem within countries where local regions may face water stress requiring water to be transported into that region. When this is the case, the development of national rather than regional water programs is essential to coordinate water management activities. When water stress is a major issue alternative water supplies must be exploited and the world’s oceans are an obvious target. Unfortunately, this water supply is contaminated with high levels of salinity (B35,000 ppm) and this must be removed prior to consumption. An alternative natural water source to the oceans is brackish water from rivers, lakes, and aquifers and this has a salinity in the region of

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5000
15,000 ppm depending on the specific source. The advantage of using brackish water is the lower levels of salt require a less intensive treatment process. The removal of salt from water is known as desalination and is now a major global industry. Desalination for water supply has grown steadily since the 1960s. Patents filed in 2010 for desalination technologies are double that of 2005, demonstrating the increasing interest and research activity in this field (Williams et al., 2015). Based on the technology employed, desalination plants are usually characterized into two main types; thermal processes (including multistage flash (MSF), multieffect distillation (MED), vapor compression distillation (VC), and freezing) and membrane filtration processes (reverse osmosis (RO), nanofiltration (NF), forward osmosis (FO), electrodialaysis (ED)), although there are other technologies such as ion exchange and hybrid processes which may also be used. Details and reviews of these technologies and methods are given elsewhere (Williams et al., 2015; Clayton, 2011; Khawaji et al., 2008; Miller, 2003; Greenlee et al., 2009). Since inception in the late 1950s, RO (often referred to as seawater reverse osmosis (SWRO)) has continually increased in application and is now the most dominant desalination technology on the planet. Currently, the annual worldwide contracted capacity of RO is 37 million m3 per day which represents B74% of the global total installed desalination capacity (Pankratz, 2015). The world’s largest desalination plant at Sorek in Israel became operational in 2013 and has the capacity to produce an output of 624,000 m3 per day, through an innovative design incorporating vertical 16v RO membrane elements together with an energy recovery system (Freyberg, 2015). Fig. 11.1 illustrates some further key facts for the desalination industry. In addition to water production from natural sources, the same filtration processes are also used for the recovery, recycle, and reuse of alternative water supplies such as industrial effluents and sewerage. These water sources often carry a significant political and social stigma that has to be considered. An example of such an integrated water system can be found in Singapore. The average water demand for Singapore is B1.82 million m3 per day and is supplied by a combination of rainwater harvesting, desalination, and water reclamation (a process known in the region as NEWater). In total there are currently two desalination plants online (Singspring, 136,000 m3 per day, online 2005: Tuaspring, 318,500 m3 per day, online 2013) with a third in planning and several NEWater plants (Bedok, 82,000 m3 per day, online 2003: Kranji, 77,000 m3 per day, online 2003: Ulu Pandan, 145,500 m3 per day, online 2009: Changi, 227,300 m3 per day, online 2010). In this case, the four operating NEWater plants supply 30% of Singapore’s water needs and the target is to expand to 50% of the total water supply by 2060 (Seah, 2015). The supply of fresh water from desalination processes does solve the problem of water scarcity. However, this simple solution does come at a cost in terms of economic and environmental impact. If one considers the idealistic case of seawater represented as 33,000 ppm sodium chloride (NaCl) then the osmotic pressure generated by this solution is 27.9 bar. Membrane processes are pressure driven and the applied pressure must be raised above the osmotic pressure in order to generate clean water. The RO process is typically operated in the region of 55
70 bar

Figure 11.1 Key facts related to the global desalination industries. (A) Capacity by region (total capacity B74.7 million m3 per day), (B) capacity by feed water source, (C) capacity by technology, and (D) capacity by size of plant. Source: All information obtained from Pankratz, T., 2015. Whither desalination? In: 2nd International conference on desalination using membrane technology (MEMDES2015), 26
29 July, Singapore.

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applied pressure in order to deliver sensible production rates of clean water and this high-pressure operation requires energy. A relatively simple thermodynamic calculation for the idealized seawater gives the minimum energy requirement for desalination as 0.77 kWh per m3 (2772 kJ). As an aside, the energy required for heating a ton of water by 1  C is 4180 kJ and the energy required to boil water is 2,570,500 kJ. Thus, from an energy stand point the RO process is a clear winner over thermal desalination technologies. When considering the total global desalination capacity is currently 74.7 million m3 per day, this equates to a minimum energy demand of approximately 57.5 GWh annually. In reality, desalination plants do not operate anywhere near the thermodynamic minimum energy and the current best energy demand achievable for a large scale plant is in the range of 2 kWh per m3 for the RO unit and around 4 kWh per m3 for the total process (Kurihara, 2015a). This corresponds to around 150 and 300 GWh per annum respectively and is quite a significant quantity of energy when one considers that a typical power plant generates around 300
500 MW. For this reason, water production and energy prices are effectively coupled and the cost of desalination can be significant with the water sell price from a large installation around 0.5
2.5 $ per m3. As energy for pumping is a major cost factor for desalination plants a great deal of attention has been paid to energy reduction and technologies such as pressure recovery devices are now common place. Similarly, new plants are being colocated with power plants to make use of surplus energy and to reduce transport losses as well as strategically operating desalination plants at night to make use of cheap electricity. Desalination is the process of removing salt from water and most SWRO plants typically operate with a 50% cut, that is they recover 50% of the total throughput of the plant. So for every 1 m3 of fresh water produced a total of 2 m3 of seawater is processed. The output from the desalination plant is then fresh water and a concentrated salt solution known as brine, with the brine concentration being typically twice that of the original seawater source. Disposal of this produced brine is typically achieved by sending the solution back out to sea. However, this is a major issue as the increased salt concentration is often toxic to the local marine environment and effective dispersal and mixing of this brine discharge is key to minimizing environmental damage. In order to minimize this problem several strategies have been developed and include timed discharge into tidal regions, mixing the brine with other low salinity discharges prior to disposal and the use of evaporation ponds to recover the salt as a solid. The ideal aim in this case is to operate a desalination plant under the conditions of zero liquid discharge (ZLD) and this is a goal that remains aspirational in practice. In addition the brine disposal problems, the conditioning of the seawater prior to the RO process often involves the use of chemical agents. These are used for several purposes, but are mainly for pH adjustment and sterilization (typically acids, bases, and bleaches). Furthermore, the RO process requires periodic cleaning of the membranes which uses further chemicals. Each of these chemicals not only adds cost to the process but also increases the environmental burden on discharge.

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Thus, the overall aim of any desalination operation is to install a process capable of delivering the required quantities of fresh water to an acceptable drinking water quality standard using the minimum amount of energy, reducing cost, while minimizing the environmental impact where practicable. This should deliver a low cost sustainable drinking water supply.

11.2

Types of water filter

The filtration technologies available for the production of drinking water are conventional filters and membranes. Conventional filters are typically woven cloth or fibers wrapped in a cylindrical shape to form cartridges and both are generally used for particulate removal only. A membrane is an advanced filter and is defined as a structure having lateral dimensions much greater than thickness, through which mass transfer may occur under a variety of driving forces (Koros et al., 1996). Membranes are able to separate components due to differences in physical and chemical properties between the membrane and the solutes. Transport of both solvent and solute across a membrane is caused by the action of a driving force or driving potential on the feed solution. The possibility exists to classify membrane processes based upon the nature of the driving force or driving potential (gradients in concentration, electrical potential, temperature, or pressure) and the physical state of the phase on either side of the membrane. A classification of membrane processes on this basis is presented in Table 11.2. Table 11.2

A classification of membrane processes

Membrane process

Feed phase

Permeate phase

Driving force

Microfiltration (MF) Ultrafiltration (UF) Nanofiltration (NF) Reverse osmosis (RO) Piezodialysis Gas separation Vapor permeation Pervaporation Electrodialysis (ED) Membrane electrodialysis Dialysis Diffusion dialysis Membrane contactors

Liquid Liquid Liquid Liquid Liquid Gas Gas Liquid Liquid Liquid Liquid Liquid Liquid Gas Liquid Liquid Liquid

Liquid Liquid Liquid Liquid Liquid Gas Gas Gas Liquid Liquid Liquid Liquid Liquid Liquid Gas Liquid Liquid

ΔP ΔP ΔP ΔP ΔP Δp Δp Δp ΔE ΔE Δc Δc Δc Δc/Δp Δc/Δp ΔT/Δp ΔT/Δp

Thermo-osmosis Membrane distillation

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The liquid
liquid pressure-driven processes of MF, UF, NF, and RO are the most abundant technologies used in industrial desalination processes and will be considered in further detail. MF membranes are normally used to separate suspended particles in the range of approximately 0.05
10 μm such as aggregates, bacteria, algae, and yeast at low operating (ΔP , 2 bar). The separation mechanism of MF membranes is primarily due to steric rejection (sieving). The structure of MF membranes is typically cylindrical porous (an array of fused cylindrical hollow fibers similar to honeycombs in nature), porous (a sponge-like structure) or homogeneous (a continuous layer). These structures lead to symmetric membranes, those with similar structure throughout the membrane cross-section, and the thickness is typically 10
200 μm. The resistance to mass transfer is determined by the total membrane thickness, i.e., the thinner the membrane the higher the permeation rate. UF membranes have pore dimensions ranging from 5 to 100 nm and are suitable for the separation of macromolecules (molecular weight B 104
106 Da) and colloids such as viruses, proteins, and enzymes. Initially the separation mechanisms involved in UF were thought to be predominantly steric but increasingly attention was given to charge effects, which are now considered to play a significant role. The separating layer of UF membranes is much denser than that in MF membranes and leads to a larger hydraulic resistance. As a direct result, the operating pressures are greater in UF membranes and are typically in the range 1 , ΔP , 10 bar. UF membranes are typically asymmetric in nature, with a very dense top layer of thickness 0.1
0.5 μm supported by a porous sub-layer with a thickness of 50
150 μm. In this type of membrane separation takes place at the surface of the active top layer. Often these asymmetric membranes can have additional sub layers to provide further mechanical support for the more fragile top layer. The pores change in size over the depth of the membrane with small pores in the dense top layer providing good separation characteristics and larger pores in the sub-layer reducing hydraulic resistance. These membranes offer the high selectivity of a dense membrane with the high permeation rate of a thin membrane. The materials of construction of the dense layer may be the same as that of the sub-layer, when this is the case it is known as a single phase homogeneous membrane. When the materials are different the membrane is termed a composite membrane and often the term thin film composite (TFC) is used when describing commercially available membranes of this type. The materials used are often selected to contain functional or ionizable groups in the active layer providing the membrane with an ionic charge. These charges can either be positive (formed from cationic groups such as NH41) or negative (formed from anionic groups such as COOH, SO3H, and H2PO4). The development of charge enhances the separating capabilities and helps to reduce fouling (the deposition of materials from the feed solution onto the membrane surface degrading performance). NF membranes are the most recent class of liquid phase pressure-driven membranes and first came on to the market in the early 1990s. NF membranes have properties that lie between those of UF and RO membranes. NF membranes are typically asymmetric and consist of a low resistance support layer with a functionally active porous top layer. The active layer is more dense than that of UF and this results in an increased hydraulic resistance and the

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typical applied pressures required are in the range 5
40 bar. The nominal molecular weight cut-off of an NF membrane is in the range 100
1000 Da, indicating that the NF membrane active layer has an approximate pore size of 1 nm. There was much debate as to the existence of defined pores in the active layer of this class of membranes, but recent work suggests that these membranes are indeed porous in nature (Oatley et al., 2013). Separation of solutes in the NF range is dependent upon the micro-hydrodynamics and interfacial events occurring at the membrane surface and inside the membrane, e.g., rejection may be attributed to a combination of both steric and charge effects. The fact that the dimensions of the NF active layer are near atomic scale lengths, coupled with limitations in current measurement technologies, has delayed a detailed knowledge of the physical structure and electrical properties of real NF membranes and has resulted in uncertainty and significant debate over the true nature of the separation mechanisms. NF membranes are typically used for the fractionation and concentration of small organics and multivalent ions from monovalent ions. The fact that NF allows the passage of significant quantities of monovalent salt provides a concentration on both sides of the membrane which lowers to osmotic pressure difference facilitating a lower operating pressure than RO. RO membranes ideally only allow the solvent (in this case water) to permeate the membrane. These membranes are denser still and so the operating pressure must be large, in the region of 55
70 bar to overcome both the hydraulic resistance and the large osmotic pressure gradient. These membranes are generally asymmetric composite and nonporous, with a membrane thickness of around 100
300 μm. The nonporous active layer indicates that flow through this class of membrane is attributed to the solution diffusion mechanism rather than pore flow. Modern RO membranes can reject all salts to levels of greater than 98.5% with relative ease (often higher depending on the operating conditions). This means that when the water product is obtained, there is a need to remineralize the water prior to human consumption. Each of the membranes described are used commercially in a variety of formats called modules. In principle there are four main module types. Flat-sheet membranes are the simplest form of membrane and essentially resemble a sheet of paper. If the membrane is asymmetric, then the active layer is usually easily identified as this will be shiny in comparison to the more dull support layer. For smallscale laboratory applications single flat sheets are typically used in frontal filtration mode (feed side flow perpendicular to the membrane surface). Often stirring is employed in order to reduce mass transfer effects at the membrane surface which causes a build-up of solute in a more concentrated layer at the membrane surface, often referred to as concentration polarization by membranologists. The thickness of this polarization layer is strongly influenced by the feed concentration, the rejection of the membrane, the permeation rate and the extent of mixing and generally leads to a reduction in membrane permeation rate and can lead to increased fouling. Thus, excessive concentration polarization is undesirable and optimization of the operating conditions for a given separation will normally consider this phenomenon. When flat-sheet membranes are used industrially they are mounted into a stack arrangement (see Fig. 11.2) in order to maximize the surface area-to-volume ratio

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Tie rod

Pressure vessel

End flange

Membrane cushion

Hydraulic disk

Connection flange

Raw water

Permeate

Concentrate

Figure 11.2 Disk stack module for flat-sheet membranes. Source: Image obtained from ,http://img.directindustry.com/images_di/photo-g/membranefilter-cartridge-reverse-osmosis-12558-7091263.jpg..

to reduce the capital cost of the pressure vessel required. The stack is efficient in terms of space, however, this configuration is requires significant effort for maintenance activities as each membrane must be individually sealed in order to maintain separation of the feed, concentrate and permeate during the process. On the positive side, if the membrane were to become damaged, then identification of which membrane is damaged is easily evaluated from a sample of the permeate on each stage. The next module type is known as the spiral wound element. This is effectively two flat-sheet membranes glued together at three edges to form an envelope, such that the active layer is on the outside of the envelope. The non-glued edge is then fixed to a tube that has a line of holes drilled, such that the membrane engulfs the holes. Generally spacers are place between the two membranes prior to fixing to the tube and also on the outside of the membrane. The whole unit is then wrapped up like a Swiss roll creating the module, see Fig. 11.3. Spiral wound modules are always used in cross-flow mode and feed flow enters one end of the module (not into the central collector tube) and travels along the membrane surface with turbulence being created by the spacer elements. Any liquids that permeate the membrane are trapped between the two membrane layers

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Brine seal Perforated central tube

on soluti Feed

Feed channel spacer Membrane

eate Perm ntrate

Conce

Permeate collection material Membrane Feed channel spacer Outer wrap

Figure 11.3 Spiral wound membrane module. Source: Image obtained from ,http://www.amtaorg.com/wp- content/uploads/ROMembrane. jpg..

inside of the envelope and is collected in the central tube as permeate. Liquids that do not permeate the membrane continue along the length of the module and exit as concentrate. Spiral wound modules have a very efficient surface area-to-volume ratio and are often assembled in series within a single pressure vessel. While these modules have space efficient, the tightly wound nature of the module element renders them prone to fouling. Thus, where there is a chance that fouling may occur from colloidal or particulate matter a pretreatment process would be wise. Tubular membranes are just this, a tube with the membrane generally coated on the inside wall with permeation from inside to out. Tubular membranes are normally used where turbulent flow is an advantage (Reynolds . 10,000), e.g., in the concentration of high solids content feed streams. The tubes themselves are typically in the range of 25
250 mm in diameter and 1
6 m in length. Tubular membranes are normally quite easy to clean and can withstand a high solids loading. However, this is at the detriment of packing density and the open internal structure also leads to a high volumetric hold up. Tubular membranes are not typically used in drinking water applications. Hollow-fiber membranes effectively consist of a bundle of fine fibers that are typically operated under lamina flow conditions. The fibers are usually in the range of 0.1
2.0 mm diameter and sealed in a flow tube. The membrane can be coated either on the inside or the outside of the fiber itself. This leads to permeation as inside-out or outside-in, respectively. When feed flow is along the inside of the fiber, mechanical strength limits the operating pressure to around 2 bar. This flow configuration is mostly used for biological separations. For RO desalination, the feed flow is along the outside of the fibers and permeation is outside-in. This leads to highly compact units with good surface area-to-volume ratio capable of highpressure operation. However, this highly compact structure is prone to fouling and if there is a chance that fouling may occur from colloidal or particulate matter a pretreatment process would be wise.

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11.3

255

Materials

There are many fabrication processes that can be used to create membranes and these use different materials, respectively. The major aims of any membrane fabrication process is to control the resultant pore size and distribution, the surface topography and roughness, and the surface chemistry. All while maintaining the integrity of the membrane itself and minimizing hydraulic resistance. The nature of the fabrication process can have a huge impact on some or all of these desirable characteristics. Some of the fabrication techniques available are sintering, stretching, etching, stamping, punching, electrospinning, the sol-gel method, and the phase inversion method. These techniques are now common and several reviews are available for further reading (Lalia et al., 2013; Hench and West, 1990). Of these techniques, the control of the pore size in order to produce membranes of the desired classification is probably the easiest way in which to categorize. For example, sintering is the process heating powdered materials to the point of coalescence to form a solid or porous mass. Thus, in this technique pores are formed between adjoining particles when fused. Therefore, control of pore size is achieved by selecting the size of the particle to be sintered and the operating conditions (mostly time and temperature). The resultant pores generated are normally only suitable for MF membranes and possibly some very open UF membranes. This fabrication technique generally uses metals or ceramics; typically aluminum or titanium or the oxides thereof, also there is a market for sintered activated carbon for removal of specific low-concentration contaminants or color. Therefore, the fabrication technique strongly influences the resultant membrane produced and is normally limited to the materials that can be used. Table 11.3 highlights some of the commonly used fabrication techniques and materials for producing membranes. The vast majority of membranes used in drinking water production are polymeric and produced via the phase inversion method. Phase inversion is a process much like crystallization or precipitation in that the basic nature of the process is to dissolve a polymer into a solvent and then add an anti-solvent causing the polymer to crystallize in a controlled manner. This is known specifically as immersion precipitation and the general steps are: G

G

G

G

Dissolve the polymer into an appropriate solvent Cast the polymer solution as a thin film over a smooth surface or non-woven support layer Immerse the cast into a water bath Allow crystallization to take place forming the resultant membrane (anti-solvent replaces the solvent in the original solution).

Careful control of the phase inversion process will allow control of the membrane characteristics and results in an asymmetric membrane. In most drinking water applications the anti-solvent is water as the polymer should have little or no solubility in the aqueous phase to ensure a stable membrane is produced. In many cases further processing is undertaken post phase inversion. This can involve chemical cross-linking to improve the stability of the active layer or to modify the surface chemistry of the active layer, i.e., the incorporation of charged moieties.

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A summary of some of the commonly used fabrication techniques and materials for producing membranes

Table 11.3

Membrane type

Typical pore size

Fabrication technique

Materials

MF

0.05
10 μm

Sintering Stretching Etching Stamping Punching Electrospinning Sol-Gel Phase inversion

UF

5
100 nm

Phase inversion Solution wet-spinning

NF

0.5
2 nm

Phase inversionInterfacial polymerizationLayerby-layer deposition

RO

,0.5 nm

Phase inversionSolution casting

Metals, effectively any Ceramics, typically TiO2, Al2O3, ZrO2, CSi Polymers Polyvinylidene fluoride (PVDF) Poly(tetrafluorethylene) (PTFE) Polypropylene (PP) Polyethylene (PE) Polyetheretherketone (PEEK) Polyethersulfone (PES) Polycarbonate Polyamide Cellulose esters Polyacrylonitrile (PAN) Polyethersulfone (PES) Polysulfone (PS) Polyethersulfone (PES) Poly(phthazine ether sulfone ketone) (PPESK) Poly(vinyl butyral) Polyvinylidene fluoride (PVDF) Polyetheretherketone (PEEK) Sulfonated Polyetheretherketone (SPEEK) Polyamides Polysulfone (PS) Polyols Polyphenols Polymethylsiloxane (PDMS) Polyimides (Lenzing P84) Cellulose acetate (CA) Aromatic polyamide Polypiperzine Polybenziimidazoline

The materials typical of drinking water applications are highlighted in italics text. The materials and fabrication techniques indicated are typical and not absolutely exclusive to each membrane type.

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Further polymer layers may be applied to the active layer, effectively leaving a series of polymer layers in what is known as layer-by-layer deposition. Again this method is used to modify the original active layer to increase stability or change the surface characteristics. Once prepared, the usual methodology would be to store the membrane in wet form to avoid drying out. Normally, this would be using an aqueous preservative such as 1% sodium metabisulfite (Na2S2O5). In some cases, some membrane fabrication processes can also involve a final thermal curing process to dry the membrane for storage purposes. When this is the case, manufacturers often cast a further preservative layer upon the active layer to protect the delicate surface. Thus, when using a virgin membrane, there is often a need to flush the membrane and permeate warm water to disperse any preservative from the membrane surface. Typical asymmetric membranes prepared by the phase inversion technique are illustrated in Figs. 8.4 and 8.5. Fig. 11.4 clearly illustrates the dense active layer of the membrane, the characteristics of which is responsible for the resulting separation process and the majority of the hydraulic resistance of the membrane. Beneath the top layer there is a region

Figure 11.4 Scanning electron micrograph (SEM) image of a single phase asymmetric membrane in cross-section.

Figure 11.5 SEM image of a thin film composite asymmetric membrane in cross-section.

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of finger-like pores that facilitate a support layer for the fragile active layer but with a more open structure to minimize hydraulic resistance to permeate flow. Finally, there is a further, more open, supporting layer that provides increased mechanical strength with minimal hydraulic resistance. Fig. 11.5 illustrates similar characteristics for a TFC membrane. In this case the support layer is manufactured from a different material and has a fibrous structure. This illustrates that the membrane structure can be quite open and when subjected to high-operational pressures will compact. This initial compaction increases the hydraulic resistance and membrane permeation rate will drop as a result. Often, this initial compaction is plastic and some manufacturers pre-compact their membranes prior to sale to ensure consistent performance. In some cases the compaction is elastic and a small degree of compaction effect is observed on each initial pressurization, i.e., the membrane will relax when the pressure is removed and then recompress when pressure is reapplied.

11.4

Applications

The major application for the production of drinking water is SWRO. The basic principle of this process are illustrated in Fig. 11.6. The basic process consists of four main sections that involve obtaining the raw seawater via a seawater intake, pretreatment of the raw seawater to produce a cleaned seawater, removal of the salts from seawater or desalination using a RO plant and then remineralization to produce potable drinking water. Seawater intake: The purpose of this section is to provide the desalination plant with a reliable and consistent source of feed seawater. There are different types of intake available, but these generally classify into two major categories; namely, the subsurface intake and the offshore open-ocean intake. The subsurface intake is taken from beneath the sea bed and is naturally filtered or pretreated by slow filtration through the sandy ocean floor. As a result, the intake from this source can contain low levels of solids, silt, oil and grease, organics, and some aquatic organisms. The well itself may be drilled vertically downwards or at an angle. In many cases, the well is vertically downwards and then extends out into the ocean bottom in the radial direction (at right angles from the base of the vertical well). Offshore open-ocean intakes are usually a vertical inlet structure with a coarse bar or screen attached that is connected to the shore via a pipeline. These inlet structures are normally a concrete or steel well (in some cases just a simple pipe) located at or above the ocean floor and always submerged below the water surface. Quite commonly there is a screen chamber or other device on shore to

Figure 11.6 Basic outline of a SWRO plant for the production of fresh drinking water.

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remove fines and small debris. The choice of which type of inlet to use is not straightforward and depends on a number of site specific factors such as plant size, ocean depth, the geology of the ocean floor, impact of surrounds (i.e., wastewater and storm water outfalls, shipping, port activities, etc.) and the ease of installation. Both open and subsurface intakes offer different advantages and disadvantages in terms of capital, operating and maintenance, construction complexity, environmental impact, and subsequent source water pretreatment requirements. Thus, the selection of the most appropriate intake option should consider the full life cycle and cost-benefit analysis for the specific location. Consideration should also be given for the concentrate disposal required as this is normally integral to the intake section as a parallel brine outlet. At present, openocean intakes are the most commonly used technology for all sizes of desalination plant and are particularly beneficial for larger installations. Pretreatment: A good quality pretreatment process is instrumental to the successful operation of a SWRO plant. The raw seawater contains several materials that are responsible for fouling the RO membranes. These include inorganic suspended solids, sand, oil, clays, marine organisms and microorganisms, and dissolved organic and inorganic matter. Thus, in order to prevent fouling from these contaminants, pretreatment of the raw seawater is essential. The pretreatment technologies employed to prevent membrane fouling can also significantly extend the lifetime of the RO membranes and are commonly grouped into two categories, conventional and nonconventional. Both of these treatment strategies are currently applied in SWRO plants around the world and the pretreatment technology applied is highly site specific and depends on the site legacy and seawater type. Conventional pretreatment normally includes filtration, disinfection, coagulation/flocculation, and pH adjustment in several different arrangements. Solids with high settling velocities are easily removed from water by gravity settling or filtration (often with sand filters), but most nonsettling solids, organic matter, immiscible liquids, and sparingly soluble salts need to be reduced by a chemical treatment. Seawater contains a vast array of microorganisms such as bacteria, algae, fungi, and viruses, which can cause serious biological fouling of the RO membranes. There are various methods to prevent and control biological fouling but the most widely used is chlorination or use of another biocide agent. However, due to the risk of oxidation of the membrane polymer, the use of oxidants must be monitored carefully to keep the chlorine concentration below 0.1 mg/L of free chlorine residual. In many cases, chlorination is often followed by dechlorination using sulfites prior to the membranes to eliminate the risk of damage. Coagulation is a process for combining small particles into larger aggregates by neutralizing the electrical charges on the surface of the particles. Commonly used coagulants include alum, ferric salts, lime, and polyelectrolytes. Coagulation has been shown to successfully improve water quality. For example, when using potassium polymer ferrate (VI) as a coagulant and preoxidant there is an increase in algae and microbial removal to more than 98% (Maa et al., 2007). Acidity regulation or pH adjustment is an efficient way to control calcium scaling. Calcium salts found naturally in seawater have low solubility, e.g., calcium sulfate solubility is B0.2 wt% (2 g/L) in the temperature range 10
40  C. Thus, when these salts are concentrated at the membrane surface, they can precipitate

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causing scaling. The solubility is highly dependent on the pH as the salt capable of forming a precipitate is in natural equilibrium with the ionic constituents which are perfectly soluble. Thus, increasing proton concentration by reducing the pH causes the salt to remain in ionic form and remain soluble, see Eq. 8.1 as an example for calcium carbonate. 1 Ca21 1 HCO2 3 2H 1 CaCO3

(11.1)

Thus, by adding H1 as acid, the equilibrium can be shifted to the left side to keep calcium carbonate dissolved. Adjustment chemicals to lower the pH include carbon dioxide, sulfuric acid, and hydrochloric acid. Carbon dioxide should not be used for pH adjustment of lime addition systems due to scaling problem associated with lime pretreatment. Note that the pH is always modified after coagulants have been added and the pH must be returned to a neutral state for the final product water. The magnitude of calcium scaling can vary by source water content, Table 11.4 illustrates the typical dissolved contents of several ocean waters. Note that the major component by far is common salt (NaCl) and calcium ions exist at about the 400 mg/L concentration. Also, Table 11.4 is a simple summary of some of the major ionic constituents found in seawater. Seawater by nature is a very complex media and has an immeasurable number of constituents and varies in concentration from ocean to ocean. For example, seawater concentration is typically accepted to be around 35,000 ppm. However, in estuarine waters this may be as low as 20,000 ppm and in the Red Sea, seawater concentration has been measured

Seawater compositions from various global locations (mg/L or PPM) Table 11.4

Component

Typical seawater

Eastern Mediterranean

Arabian Gulf at Kuwait

Red Sea at Jeddah

Chloride (Cl2) Sodium (Na1) Sulfate ðSO22 4 Þ Magnesium (Mg21) Calcium (Ca21) Potassium (K1) Bicarbonate ðHCO2 3Þ Strontium (Sr21) Bromide (Br2) Borate ðBO32 3 Þ Fluoride (F2) Silicate ðSiO22 3 Þ Iodide (I2) Others Total dissolved solids (TDS)

18,980 10,556 2649 1262 400 380 140 13 65 26 1 1 ,1 1 34,483

21,200 11,800 2950 1403 423 463

155 72

2
38,600

23,000 15,850 3200 1765 500 460 142
80

1.5

45,000

22,219 14,255 3078 742 225 210 146
72




41,000

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at 42,000 ppm. The generic salt make-up of seawater is generally considered to be a cocktail of major constituents, trace constituents, nutrients, dissolved gasses, and organics. The major constituents are illustrated in order of abundance in Table 11.4 and account for around 99% of the total dissolved salts. Some other minor salts worthy of note are bromine (Br2) and strontium (Sr21), although almost all of the naturally occurring 92 elements may be found in seawater following rigorous analysis. Thus, exact modeling of such a complex fluid is inherently difficult and many bench scale or pilot scale studies often use a simplified mixture of ions to represent seawater. Kester et al. (1967) have provided a formula for the make-up of a representative artificial seawater. The nonconventional approach to seawater pretreatment is using a battery of membrane technologies in sequence. Microfiltration (MF) followed by Ultrafiltration (UF) and Nanofiltration (NF) has gradually gained acceptance as the preferred pretreatment operation in recent years (Pearce, 2007). In this process sequence the MF membranes typically remove zooplankton, algae, bacteria, turbidity, and general particulates. The UF membranes generally remove macromolecules, colloids and act as the sterile barrier removing viruses. The NF membranes generally remove dissolved organics and multivalent ions (calcium removal). Thus, the combination of the technologies makes an effective pretreatment prior to the RO membranes for desalination. Commercially, the modules employed for this activity include immersed plate, pressure-driven capillary, pressure-driven spiral wound, and immersed hollow-fiber membranes. Prihasto et al. (2009) have reviewed the available literature on pretreatment technologies and have concluded that although conventional pretreatment systems in some RO plants can provide feed water for the RO systems with SDI less than 4.0 and even less than 3.0, the pretreatment system was not easy to control. Experience from the SWRO plants in the Middle East where there were periodic irregular changes of the seawater quality the conventional pretreatment showed instability of SDI value, high rate of chemical consumption, frequent backwashing which leads to high rate of water consumption, and produces unsteady feed water quality and quantity. All of these factors add cost and more importantly down-time for maintenance to occur. Thus, with membranes being supplied in ever increasing quantities and the unit cost becoming cheaper, the capital investment for membranes is becoming lower. Membrane life span is also increasing and replacement time is typically 5
10 years and with diligent operating practices this time frame could be even longer. Thus, with the increased consistency of pretreatment water quality and operating costs becoming more competitive, the future looks set to see the membrane pretreatment process prevailing as the most cost effective and robust solution. Reverse osmosis (RO): Osmosis is the movement of water molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on both sides of the membrane. The natural process was first described in 1748 by French Scientist Jean Antoine Nollet, who noted that water spontaneously diffused through a pig bladder membrane into alcohol. During this process water moves under a driving force of osmotic potential.

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Fibrous Filter Media

Osmotic potential is the energy potential generated between the two different zones separated by the membrane as a result of the concentration in each zone. The natural system wants to achieve thermodynamic equilibrium and thus water is drawn from the dilute zone to the concentrated zone. When the two zones are closed, the influx of water to the concentrated zone will create a pressure and this is known as the osmotic pressure. RO, as the name suggests, is the reverse of this process. Pressure is applied to a solution of given concentration to squeeze water out via a semipermeable membrane. In this case, the applied pressure must be greater than the osmotic pressure or there will be no net movement of water. Osmotic pressure is directly proportional the concentration of the dissolved solute and may be calculated from the simple van’t Hoff formula at low concentrations, see Eq. (8.2). π 5 cRT

(11.2)

where c is the concentration, R is the universal gas constant, and T is temperature. Van’t Hoff was awarded the 1901 Nobel prize for his work on understanding osmosis, but it was not until the 1950s, as a result of research funded by the Office of Saline Water in the USA, that practical RO membranes capable of discriminating against small ions were developed. The typical modern RO water treatment plant uses spiral wound elements. As these are under pressure, several modules may be placed into the same filtration housing to reduce the number of pressure vessels used. The spiral elements are simply connected together as illustrated in Fig. 11.7. In this case the membrane housing contains a total of nine spiral wound elements. As each element is effectively concentrating the feed water, the solution concentration is increasing along the length of the total module. By the same token, the osmotic pressure is increasing along the length of the module and the membrane flux will reduce as a result. Also, due to frictional losses, there will be a natural pressure drop along the feed side of the module such that the pressure is lower at the outlet than the inlet. Thus, there will be a double effect reducing the driving force from element 1 to element 9, i.e., increased osmotic pressure due to concentration and reduction in applied pressure due to head loss. As fouling is dependent on concentration and flux, the concentration of foulants and flux will be highest in the earlier elements, thus any fouling will be more prevalent in element 1 and some operators treat element almost as a sacrificial element that should be replaced periodically. The normal operation of an RO plant is to take a cut of between 40% and 60%, i.e., 40
60% of the feed water entering will be harvested as clean water and the remaining solution will exit as brine. The level of cut taken by a plant is normally

Figure 11.7 Diagram of a 9 module spiral wound arrangement in an RO pressure housing.

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Figure 11.8 Single and two stage RO plant operation. Solid line single stage, dashed line two stage.

site specific and is the result of a combination of factors such as source water quality, pretreatment quality and local practices. In many cases a two stage RO process is employed, see Fig. 11.8. If a single stage RO process were to be employed then the flux obtained from the final membrane elements would be significantly lower than that of the first element. However, with a two stage process the feed pressure is reduced in the first stage such that the driving force across the two stages is similar and a similar flux rate is then obtained from all membrane elements. This makes flow balancing and other operational considerations more simplistic and has the added benefit of requiring less pumping power, thus reducing energy consumption. In principle, the plant could be operated with many RO stages, each with increased pressure progressively. However, in reality the increased capital required for the additional pumps and ancillary equipment does not justify more than a two stage operation. In practice, there is a host of different configurations that can be employed for the RO operation, such as single pass, two-pass, split modules, blocked modules, and multiple passes. However, only the basic single and two stage plants have been outlined for simplicity of the discussion. As the operational pressure for RO is high, generation of this pressure is one of the main contributors to the operational costs. For this reason, pressure recovery devices are now common place to recover as much of the pressure energy from the desalination process as possible. There are two major types of pressure recovery technology, the energy recovery turbine and the pressure exchanger. In principle, both technologies perform the same task, however, the nature of how they achieve this task is different and as a result the energy losses are different. In general terms, the energy recovery turbine capital investment is slightly lower than the pressure exchanger but the operating costs are slightly higher. Thus, in areas where energy prices are low, the energy recovery turbine is the better option and when energy costs are high the pressure exchanger is the better option. The pressure exchanger is also the better option for larger installations due to the lower operational costs. Fig. 11.9 illustrates the energy recovery technologies in practical operation.

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Figure 11.9 Basic configuration of an RO plant using energy recovery. Top—the energy recovery turbine and bottom—the pressure exchanger.

A major concern for RO systems remains to be the cost dictated by both energy consumption and membrane replacement costs. Installing an energy recovery system can reduce energy consumption from 6
8 to 4
5 kWh/m3 (Khawaji et al., 2008). Modern large desalination plants can achieve an energy consumption of approximately 2 kWh/m3 for the RO plant and 4 kWh/m3 for the total process (Kurihara, 2015b). The RO process is intrinsically the same depending on the source water used for the operation. However, the operational aspects and parameters do vary accordingly. The typical RO profiles expected from brackish water, seawater, and wastewater reclamation are shown in Table 11.5. Clearly the feed salinity has a profound impact on the feed pressure required for the operation. For example, SWRO uses a feed pressure in the range of 70 bar as opposed to brackish RO which uses only 15 bar. This is a direct result of the increased osmotic pressure caused by the increased concentration of the seawater and also has a profound impact on the costs. Table 11.6 highlights the typical costs for desalination. The absolute cost for a desalination plant is in the region of $1500
2500/m3/ day for an SWRO plant, of $500
1000/m3/day for a BWRO plant, and of $1000
1500/m3/day for an WWRO plant. In most instances, research and development effort is normally invested into generating improved membranes to increase

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Typical RO process parameters for different source water inputs

Table 11.5

Application

Seawater RO

Brackish RO

Wastewater reclamation

Typical feed salinity (ppm) Representative osmotic pressure (bar) System recovery (%) Number of stages Membrane permeability (LMH/bar) Salt rejection (%) Average flux rate (LMH) Feed pressure (bar)

32,000
45,000 44

1200
10,000 6

800
2000 3

40
50 1 1
1.5

70
85 2 5
8

75
85 2
3 5
8

99.75
99.85 13.6
17.0 55
70

99.0
99.7 22
27 10
15

99.0
99.7 17
20 8
12

Table 11.6 Typical costs as % breakdown for a desalination plant of 150,000 m3/d capacity Component of cost

Seawater RO

Brackish RO

Wastewater reclamation

38.39 19.36 4.20 12.94 18.00 7.12

24.62 25.32 4.25 10.95 28.84 6.02

36.49 20.35 2.95 11.70 22.07 6.43

100

100

100

9 55 6 18 12

21 45 6 16 12

16 38 15 19 12

100

100

100

Capital costs Raw water and pretreatment RO equipment Electrical and instrumentation Equipment installation Site work Project management and commissioning

Operational costs Labor Electricity Membrane replacements Chemicals Parts and maintenance

the RO plant efficiency. However, Table 11.6 illustrates that the membranes only account for approximately 20% of the overall cost of an RO plant. Based on the cost data, one could argue that at least as much effort should be placed in reducing the costs of the raw water and pretreatment stages of the process.

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Fibrous Filter Media

Posttreatment: Desalinated water is ultra-pure and has no or very little dissolved content. If one consumed a reasonable quantity of this water then there would be a severe osmotic shock to the body that could in principle be fatal. Also, demineralized water does not taste very good at all. During posttreatment, the water must be stabilized or remineralized prior to distribution to reduce the corrosive nature of ultra-pure water. Stabilization is commonly achieved by adding chemical constituents such as calcium and magnesium carbonate along with pH adjustment. In addition, a certain degree of remineralization is essential to make the water palatable and to meet the required health standards of the local region. Generally, the selection of the remineralization process is determined by regulatory water and quality standards which vary from region to region. As a general overview, any drinking water provision should conform to a water management framework and a target quality standard that is periodically monitored for compliance. An outline example of such a framework is provided by the WHO (2011). Drinking water typically contributes only a small proportion to the recommended daily intake of essential elements, with most of the intake occurring through food. Thus, the purpose of remineralization is not to provide nutrients in most cases. However, in some areas the local authorities artificially add some materials or essential nutrients to the water supply for health reasons. For example, fluoride would be missing from desalinated water unless added prior to distribution, which may be considered by countries in which sugar consumption is high. Similarly, depending on the source water supply, contaminants must be removed from the water prior to distribution. For most contaminants that can produce toxicity, there is often a concentration below which no adverse effect will occur. For chemicals that give rise to such toxic effects, a tolerable daily intake (TDI) is established and a guideline value (GV) advised by the local regulators. There are some naturally occurring materials that have health significance that are important to consider for desalinated waters, namely arsenic, barium, boron, chromium, fluoride, selenium, and uranium. Boron is a particularly interesting case for desalination plants and many studies have been conducted to determine the optimal boron control methodologies. The human body contains approximately 0.7 ppm of boron, an element that is not considered as a dietary requirement. Daily intake from food and other sources is approximately 2 mg. At a daily intake of over 5 g of boric acid the human body is clearly negatively influenced, causing nausea, vomiting, diarrhea and blood clotting and amounts over 20 g are life threatening. Similarly, boron concentrations in water are extremely important for farming purposes as many species of plant are highly sensitive to boron levels. Boron naturally exists in seawater at 4
5.5 mg/L as borate (H3BO2) or boric acid (H3BO3). RO membranes are highly efficient at removing salts, i.e., the borates, but struggle with the boric acid and residual boron levels of around 1.15 mg/L are possible. This is a particular problem in high salinity and high temperature seawaters such as that of the Persian Gulf, the Red Sea, the Eastern Mediterranean, and the Caribbean. Therefore, a specific boron removal process is required to achieve the 0.5 mg/L required by the WHO guideline and these have been comprehensively reviewed (Hilal et al., 2011). There are many alternative posttreatment practices aimed to introduce hardness to desalinated water. Among the methods

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are: blending with source water, direct dosage of chemicals, limestone, dolomite and magnesium oxide dissolution, ion exchange, and a novel micronized limestone dissolution process. Each of these processes are self-explanatory and have been comprehensively reviewed elsewhere (Shemer et al., 2015). Fig. 11.10 shows a complete desalination process train for the Ashkelon SWRO desalination plant. At the time of construction, this was the largest desalination plant in the world providing 330,000 m3/day of fresh water for the local region. The plant was constructed on a build, operate, and transfer agreement (BOT) by a number of companies and after the 25-year period of agreement the plant will be transferred to the government of Israel. The total project cost was estimated as $212 million and was funded as a mixture of equity and loans/credit provided by lenders. The plant includes seawater pumping, raw water pretreatment, membrane desalination, permeate water remineralization treatment, and brine disposal. The plant is connected to the electrical grid, but a dedicated cogeneration power station has also been installed. 56 MW of the 80 MW produced by the power station will be used by the desalination process. The use of RO technology and advanced recovery system to reduce operating costs has achieved a very competitive water price of $0.53/m3. About 42% of this price covers energy costs, variable operation and maintenance costs, membranes, and chemicals costs. 58% covers capital expenditure and fixed costs. Further information for the Ashkelon plant is found in SauvetGoichon (2007), Dreizin (2006), Gorenflo et al. (2007), Molina et al. (2011). Another very interesting example of the industrial application of RO desalination processes is found in Singapore. The integrated water network in Singapore is virtually a closed loop where all water sources are recovered and recycled where possible. An outline of this integrated water network is shown in Fig. 11.11. Fresh water is introduced to the network by traditional rain collection and processing along with seawater desalination (SWRO) to produce fresh water supplies. In addition, used wastewaters are collected, processed, and reissued as fresh water supplies (NEWater). The NEWater treatment processes are very similar to traditional wastewater reclamation plants. However, the Singaporeans are continually investing in water research and are gradually upgrading all of their water-based technologies to modern, more efficient technologies where possible. For example, recent progress has been made in the waste-treatment processes by introducing membrane bioreactors. This technology has many advantages over the traditional methods including smaller footprint, increased process reliability and performance, however, this does come at an increased energy cost. The integration of this technology is illustrated in Fig. 11.12.

11.5

Future trends

Some might argue that the state of RO desalination has consistently been improving over the past 30 years and that most of the major improvements that could be made

Figure 11.10 Schematic of the plant layout for the Ashkelon desalination plant in Israel. Source: Image taken from Sauvet-Goichon, B., 2007. Ashkelon desalination plant
a successful challenge. Desalination, 203, 75
81.

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Figure 11.11 Illustration of the closed loop water cycle for fresh water supply in Singapore.

Figure 11.12 Traditional and modern technologies in NEWater production process.

have already been realized. However, there are a number of areas by which the technology can develop and continue to improve. These areas include improved membranes, improved fouling control, energy efficiency, and hybrid processes. There has been a wealth of development activities related to improving membranes for increased flux and higher rejection with the requirement for less input of energy. There are two promising technologies on the horizon in this area; the fabrication of membranes to incorporate carbon in the form of nanotubes or graphene and biomimetic membranes. Studies have demonstrated that membranes fabricated with or incorporating carbon nanotubes have much lower hydrophobicity and can achieve high water fluxes at reasonably low pressure (Liu et al., 2013). Similarly, recent reports indicate that the water permeability of nanoporous graphene membranes is several orders of magnitude higher than conventional RO membranes and that these membranes may have a valuable role to play for water purification (Cohen-Tanugi and Grossman, 2012). Biomimetic membranes are membranes that have been fabricated to incorporate learnings from nature. Based on their unique combination of

270

Fibrous Filter Media

offering high water permeability and high solute rejection, aquaporin proteins have attracted considerable interest as functional building blocks of biomimetic membranes for water desalination and reuse (Tang et al., 2013). Fouling is a long standing issue with membrane processes and reduces process performance over time and in some cases significantly reduces the life-span of the membranes. Fouling is affected by three main factors; the feed water composition, the membrane properties and the operating conditions which affect the hydrodynamics within the membrane module. These factors have been discussed in some detail in the literature (Tang et al., 2011) and efforts are now moving toward fouling control and mitigation rather than total prevention. One of the most effective fouling control strategies is the feed pretreatment before the membrane unit, i.e., remove as much foulant as is sensibly possible. Other workers have focused on understanding the nature of the fouling mechanisms in order to produce a working strategy to control fouling (Mohammad et al., 2015). Another interesting advancement in this field is the development of monitoring systems to measure fouling in real time desalination plants. There are currently devices being tested that are capable of measuring the real time extent of fouling using electrical impedance spectroscopy (Chilcott et al., 2013) or fluid dynamic gauging (Lewis et al., 2012). Both methods use a “canary” device that sits in a parallel stream to the main process stream and provides information on the state of the membrane elements. Energy efficiency is one of the main goals for desalination processes. Currently, the best available technology for the RO process requires around 2 kWh/m3 to produce fresh water from seawater. When compared to the thermodynamic optimum of around 0.77 kWh/m3, this represents a process efficiency of around 38%. There are obvious incremental gains in energy efficiency that can be made by improving pressure recovery systems and optimizing energy integration. There are some efforts to increase the pressure of RO operations to 100
200 bar to increase recovery and use pumps in a more optimal efficiency range. Similarly, there are efforts to produce low pressure RO membranes that can be used for low energy operation. The real future challenge is to solve the membrane process using a transformative approach. Several workers are looking at membranes in a different format of FO and pressure retarded osmosis (PRO). FO is the terminology used by the membrane sector for simple osmosis. In this process, a concentrated solution or draw solution is used to “pull” water from a source of lower concentration through a semipermeable membrane using the osmotic pressure driving force. The pulled water must then be recovered by separation from the original draw solution, see Shaffer et al. (2015) for more details. In principle, this process works. However, for the process to be energy neutral in comparison to RO, then recovery of the water from the draw solution needs to use less energy per m3 recovery than RO. This seems some way off at the moment and somewhat unrealistic. Even if a novel draw solution could be found that is energy efficient, the low driving force of this process (only osmotic pressure difference) requires an enormous area of membrane in order to compete with RO in terms of capital costs required for equivalent flux. There are obvious challenges to overcome for this technology, but given that there may well be niche applications where the factors converge to make this technology cost-effective, there could be a future. PRO is a

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process where a pressurized higher salinity (draw) solution is used to extract fresh water through a semipermeable membrane from a lower salinity solution. This increases the available pressure used to spin hydro turbine machines for electrical power production or can be used directly to supplement the mechanical load required for RO desalination systems. A recent PRO pilot system operating at 50% RO recovery recorded a specific energy requirement of 1.13 kWh/m3, thus reducing the energy requirements for desalination by B1 kWh/m3 (Achilli et al., 2014). While the study needs to be verified, this suggests that PRO could indeed have a bright future. In essence, PRO is a hybrid technology process in that this is ROPRO as the technology needs to function in parallel with an RO plant. There are other exciting hybrid processes on the horizon that may also help improve the desalination process. Membrane distillation (MD) is a promising technology for desalting highly saline waters. Thus, used in conjunction with RO, this hybrid RO-MD process could lead to greater water recovery than RO alone. MD is a thermallydriven separation (microfiltration) process, in which only vapor molecules are able to pass through a porous hydrophobic membrane. This separation process is driven by the vapor pressure difference existing between the porous hydrophobic membrane surfaces. Using MD has many attractive features, such as low operating temperature and low hydrostatic pressure. Therefore, MD is expected to be a cost-effective process for desalination (Alkhudhiri et al., 2012). Another technology that is beginning to gather interest is freeze separation. The freeze-melt process can purify and concentrate liquids by the fact that during the freezing process the water expels ions and dissolved solutes from the ice crystals as they form. The simplest natural example is that sea-ice has a much lower salt content than the surrounding seawater. Freeze processes offer a very high separation factor, with much lower energy than a thermal process, they are insensitive to biological fouling, there is no chemical requirements for pretreatment and due to the low temperature the process can utilize inexpensive materials with little corrosion or creep. Thus far, the technology has only been demonstrated at the pilot scale for desalination purposes, but this study has shown that improved water recovery is possible and ejection of a highly concentrated brine is possible (Williams et al., 2015; Ahmad and Williams, 2011). Thus, there is certainly a need to evaluate the economics of such a hybrid Freeze-RO process.

11.6

Conclusion

Fresh clean water is a basic human necessity and as the population of the planet continues to grow and the effects of global warming take hold, there is an ever increasing demand for this vital resource. Fortunately, the world is a big place and there is plenty of water out there, but it costs energy (or money) to convert the natural saline water sources into a reliable and robust drinking water supply. Variations in water scarcity mean that the problem of drinking water supply is, or can be, a localized problem and solutions have to be made at the local level. Technologies

272

Fibrous Filter Media

that utilize fibrous membranes are available that can not only be used to desalinate natural seawater and brackish water supplies, but can also be used to recover and recycle water from domestic and industrial wastewaters. The real challenge is to deliver these essential water supplies at the correct rate and quality standard, while reducing the economic cost as much as possible. The current cost of product water from seawater desalination processes is around 4 kWh/m3 produced fresh water and the goal is to progressively improve the efficiency of the technologies used and to develop new technologies in order to reduce the required energy demand and cost in order to deliver a sustainable water supply.

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2348. Hench, L.L., West, J.K., 1990. The sol-gel process. Chem. Rev. 90, 33
72. Hilal, N., Kim, G.J., Somerfield, C., 2011. Boron removal from saline water: A comprehensive review. Desalination. 273, 23
25. Kester, D.R., Duedall, I.W., Connors, D.N., Pytkowicz, R.M., 1967. Preparation of artificial seawater. Limnol. Oceanogr. 12, 176
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Koros, W.J., Ma, Y.H., Shimidzu, T., 1996. Terminology for membranes and membrane processes (IUPAC Recommendation 1996). J. Membr. Sci. 120 (2), 149
159. Kurihara, M., 2015a. Innovative desalination technology ‘Mega-ton water system’, large scale desalination system for 21st century key technology with low energy and low environmental impact, 2nd International Conference on Desalination Using Membrane Technology (MEMDES2015), 26
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