Water Treatment by Renewable Energy-Driven Membrane Distillation

CHAPTER 8

Water Treatment by Renewable Energy-Driven Membrane Distillation Mohammad Reza Rahimpour, Nooshin Moradi Kazerooni, Mahboubeh Parhoudeh Department of Chemical Engineering, Shiraz University, Shiraz, Iran

1 Introduction One of the main challenges of today’s world is massive water scarcity in many arid and semiarid areas, which is prospected to increase substantially in the next decades. Although 75% of the planet is covered by water, around 97% of this water is saline or brackish in nature, which requires treating to become accessible. To serve this purpose, various desalination techniques are developed to purify the highly saline ocean water. This treating process in addition to recovery and reuse of waste water, is an intelligent method to remedy the water shortage crisis, since the oceans are the only unceasing source of water available in the globe. Due to the fact that water treatment is an extremely energy consuming process, requiring high tonnage plants supplied by conventional fuels, only a few of the water-short regions can bear the expenses. In order to tackle this issue, the utilization of renewable energy-driven water treatment techniques have emerged among which solar energy is the most promising since accessibility to saline water usually coincides with abundance of solar resource in arid areas. Solar power is promoted for water treatment by providing thermal and electrical energy essential to drive phase-change processes and membrane processes, respectively. Additionally, geothermal energy presents a mature technology to provide the power required by water treating plants, by providing a constant stable energy (Ghaffour et al., 2015; Mathioulakis et al., 2007). Innovative renewable energy-driven systems for water treatment have arisen with the aim of mitigating energy consumption. Water treatment based on solar and geothermal membrane distillation has become the research hotspot after the massive advances in module design and membrane science. Membrane distillation is a thermal separation process which integrates the advantages of membrane-based operations with thermally-driven ones, providing an easy and robust performance. Current Trends and Future Developments on (Bio-) Membranes. https://doi.org/10.1016/B978-0-12-813545-7.00008-8 # 2019 Elsevier Inc. All rights reserved.

179

180 Chapter 8 In this context, membrane distillation technology associated with solar and geothermal power for water treatment is surveyed in this chapter. Membrane distillation principle, configurations, membrane, and modules, transport phenomena and solar collecting technologies and geothermal related operations incorporated with membrane distillation are discussed.

2 Membrane Distillation 2.1 Principles Membrane-based technologies for water treatment have gained a lot more popularity among scholars compared to conventional methods in the last two decades or so. Membrane distillation (MD) process, which was coined by Weyl (1967), is recognized as a hybrid process enjoying both thermal evaporation and membrane separation benefits in a single unit (Gude, 2015). The term MD originates from the fact that the procedure is analogous to conventional distillation, requiring energy to provide the latent heat of vaporization in order to accomplish separation under vapor-liquid equilibrium (Pangarkar et al., 2016). The driving force for MD operation is provided by partial pressure gradient that is induced by temperature gradient maintained at the two sides of a micro-porous membrane (Ghaffour et al., 2015). Separation takes place since the membrane material exhibits hydrophobic characteristics—denoting that the surface of the membrane cannot be wetted by liquid water up to a specific limiting pressure (liquid entry pressure (LEP))—which depends on solution and membrane features, leaving only water in steam form to pass through the pores of the membrane (Koschikowski et al., 2009; Qtaishat and Banat, 2013). The water vapors that pass through the membrane condense on the cooler side to provide the distillate (fresh water) (Charcosset, 2009). Fig. 1 represents a schematic of MD principles. The feed solution (seawater) is preheated up to for example 80°C at one side of the membrane while the other side is maintained at a lower temperature, for example 70°C. Since the hot (saline water feed) and cold (permeate or freshwater) streams are segregated by a membrane barrier, a very low temperature differential of 10°C is sufficient to generate the partial pressure required for producing freshwater through this process. Water vapor evaporates through the membrane and condenses on the side with lower temperature to form the distillate (Charcosset, 2009).

2.2 Configurations In the MD process, discrepancies exist among the nature of the cold (permeate) side of the membrane and the system of vapor recovery. Hence, MD can be classified mainly in five configurations which are shown in Fig. 2 and addressed in the following. It is worth mentioning that in all configurations, the hot side (feed) is in direct contact with the membrane.

Water Treatment by Renewable Energy-Driven Membrane Distillation 181

Fig. 1 Membrane distillation principles.

2.2.1 Direct contact membrane distillation Direct contact membrane distillation (DCMD) is the oldest and most widely method applied in desalination and concentration of aqueous solutions, due to the ease of handling (Alkhudhiri et al., 2012). This arrangement places both hot feed and cold permeate in direct contact with the membrane. The vapor path for diffusion is confined to the pores of the membrane, and hence decreasing the mass and heat transfer resistances (Chen and Ho, 2010; Qtaishat and Banat, 2013). The liquid permeate acts as the coolant and is recirculated to fully condense the vapor inside the membrane module, where the distillate is mixed with the coolant (Ruiz-Aguirre et al., 2015; Winter et al., 2017). The hydrophobicity of the membrane prevents the condensed vapor to permeate through the membrane matrix and maintains the liquid-vapor interface (Wang and Chung, 2015; Winter et al., 2011). Although DCMD provides high permeate flux, a significant amount of heat is dissipated by conduction through the membrane matrix owing to the continuous contact between the hot and the cold streams, hence low efficiency is exhibited (Pangarkar et al., 2016). 2.2.2 Air gap membrane distillation Air gap membrane distillation (AGMD) is another MD configuration which was the first option to be selected for pilot testing (Alsaadi et al., 2015). AGMD introduces an air-filled cavity between the membrane and the condensation surface in which case the vapor must penetrate through the membrane thickness and additionally, across the air gap before it reaches on the cold surface. This stagnant air gap acts as a thermal insulation layer between the hot feed and the cooling surface, resulting a significant reduction in heat loss by conduction through the membrane which makes AGMD the most energy efficient MD configuration (Duong et al., 2015; Summers, 2013;

Feed inlet

Feed inlet

Permeate outlet

Feed outlet Permeate inlet

Feed outlet

Permeate

Membrane

Coolant outlet

Air gap

Membrane

Membrane

Coolant outlet

Feed outlet Coolant inlet

Coolant inlet

Product

(A)

(B)

(C)

Feed inlet

Feed inlet Sweep gas outlet

Permeate

Membrane

Membrane

Product

Feed outlet Sweep gas inlet

(D)

Vacuum pump

Feed outlet

(E) Fig. 2 MD configurations: (A) DCMD, (B) AGMD, (C) LGMD, (D) SGMD, (E) VMD.

182 Chapter 8

Product Feed inlet

Water Treatment by Renewable Energy-Driven Membrane Distillation 183 Warsinger et al., 2015a). This high efficiency comes at the cost of significant increase in mass transfer resistance created by non-condensable gases in the thin layer, leading to low permeation rate (Abu-Zeid et al., 2015). Since the coolant stream is separated from the hot vapor by a condenser, internal recovery of latent heat can be achieved in AGMD. Multi-effect or multi-stage membrane modules can be developed by utilizing the integrated cooling plate in this configuration (Khalifa, 2015; Wang and Chung, 2015). 2.2.3 Liquid gap membrane distillation Liquid gap membrane distillation integrates both DCMD and AGMD modes. In this configuration, the chamber between the membrane and the condensing surface is occupied with water instead of stagnant air. The produced water is left inside the channel until it overflows from the top of the membrane module, unlike in AGMD, where the distillate leaves from the bottom (Essalhi and Khayet, 2014; Zaragoza et al., 2014). Since the gap is full of stagnant cold liquid instead of air, there is a reduction in mass transfer resistance and thus higher distillate flux compared to AGMD. It also leads to less internal heat dissipation by conduction than in DCMD (Ruiz-Aguirre et al., 2015). 2.2.4 Sweeping gas membrane distillation In sweep gas membrane distillation (SGMD), as in AGMD, a gas barrier exists between the membrane and the cold surface, resulting a reduction in heat loss and increasing the efficiency. The difference is that in this case the gas in not stationary and sweeps the membrane, which enhances mass transfer coefficient and leads to a greater permeate flux than in AGMD. This configuration also provides a greater permeate flux and evaporation efficiency than the DCMD process (El-Bourawi et al., 2006; Wang and Chung, 2015). This stripping gas is employed to collect the vapor at permeate side to condense outside the membrane module by an external condenser and produce fresh water. Condensation involves large volumes of sweep gas for a small amount of permeate and thus, incurring extra expenses (Ghalavand et al., 2015; Pangarkar et al., 2016). Due to this limitation, SGMD has received only little attention compared to other configurations. 2.2.5 Vacuum membrane distillation Vacuum membrane distillation (VMD) is among the most favorable MD configurations. In this process, the vapor is withdrawn by exerting a vacuum pressure to the permeate side of the membrane, which is kept as just lower than the saturation pressure of volatile components in the hot feed. In this case, the membrane is placed between the hot feed and a vacuum chamber. Vapor recovery takes place outside the membrane module by an external condenser (Ghalavand et al., 2015; Hassan et al., 2016). Existence of the vacuum on the permeate side allows higher partial pressure gradient and imposes additional driving force for the process which in turn leads to higher distillation production rate compared to other MD configurations (Mohamed et al., 2017; Pangarkar et al., 2016). Also, the vacuum gap results in negligible heat loss by

184 Chapter 8 conduction which is a notable advantage of VMD. However, the vacuum level must be managed attentively since the LEP may be exceeded, leading to membrane wetting (Abu-Zeid et al., 2015; Hassan et al., 2016). It should be mentioned that supplementary electrical power is demanded by the operation of the vacuum pump (Mohamed et al., 2017).

2.3 Advantages and Drawbacks MD technology offers numerous advantages compared to other popular water treatment methods. It is an excellent option to boost economic viability with regard to the feasibility of implementing solar-driven operating systems, which employ low-grade, environmental friendly thermal energy source. This is due to the fact that MD operates under moderate temperature conditions, usually 60–80°C, making it perfect for solar collectors to perform (Shim et al., 2015; Subramani and Jacangelo, 2015). The gentle operating conditions along with utilizing highly resistant membranes in MD, lowers the susceptibility to fouling and scaling as a major drawback of membrane-based operations which reduces maintenance costs (Amy et al., 2017; Karam et al., 2017; Mohamed et al., 2017). Another amazing characteristic of MD process is that vapor-liquid contact space is tightly compact. This is owing to the fact that MD utilizes a microporous membrane to provide a vapor-liquid interface, while in conventional distillation large vapor space is required for providing intimate vapor-liquid contact (Qtaishat and Banat, 2013). Also, the fact that the process is not driven by absolute pressure (like in reverse osmosis) and due to larger membrane pores compared to other membrane techniques, the risk of clogging is lowered, which eliminates the need of chemical pretreatment to water before entering modules (Guill
en-Burrieza et al., 2011; Ruiz-Aguirre et al., 2015). Additionally, MD yields a distillate of very high purity, which, unlike in conventional distillation, does not suffer from the entrainment of nonvolatile contaminants (Charcosset, 2009; Zwijnenberg et al., 2005). It should be mentioned that MD performance is not affected by salt concentration in the feed (Guill
en-Burrieza et al., 2011). Considering the multitude of advantages of MD process, its applications are not restricted only to the field of desalination. In food industry, MD is applied where temperature sensitive materials are involved such as diaries and juice concentration. It is also used in pharmaceutical areas such as removal of water from blood. In extraction of organic components such as alcohols from dilute aqueous solutions, in waste water treatment e.g. textile waste or nuclear waste and even in recovering crystal valuable products from effluents, MD is a proven methodology which has fascinated a lot of interests (Drioli et al., 2015; El-Bourawi et al., 2006). Despite the extensive interests of scientific community in MD technology and its application in desalination and water treatment, there are some drawbacks which have hindered the up-scaling process. The main barrier is lack of improved membrane fabrication and module design that can

Water Treatment by Renewable Energy-Driven Membrane Distillation 185 enhance permeation flux and provide low wettability (Winter et al., 2017). Another issue in the way of commercializing MD is complexity of energy consumption and economics, considering the fact that operating conditions in the MD process differs considerably from bench-scale to larger-scale due mainly to the significant variation of temperatures through the MD module itself (El-Bourawi et al., 2006; Ghaffour et al., 2015). Other challenges include pore wetting, conductive dissipation, permeation rate decay due to temperature polarization, long term performance and flux stability (El-Bourawi et al., 2006; Guill
en-Burrieza et al., 2011; Hou et al., 2015). The shortcomings in membrane material and properties can be addressed through the rapid progress in membrane technology. Additionally, development of novel integrated MD devices and other separation processes is another solution to enhance the permeate flux (Hou et al., 2015).

3 Membrane and Modules As mentioned, one of the most imperative issues impeding MD commercialization is the lack of high performance and well-structured membranes. Hence, in order to materialize MD as a feasible industrial technology, the fabrication of advanced membranes is investigated. These membranes are mainly made of polymers which can satisfy the required characteristics for an optimized MD process including: wettability resistance (high hydrophobicity), minor tendency to fouling, high porosity, satisfactory thermal stability, low thermal conductivity, ease of fabrication, etc.

3.1 Membrane Material 3.1.1 Commercial membranes used for MD Popular commercial membranes for MD process are commonly prepared employing polypropylene (PP), polytetrafluoroethylene (PTFE), polyethylene (PE) and polyvinylidene fluoride (PVDF) which have been applied in experiments on lab-scale. Among these commercial membranes, PTFE, with a surface energy of around 9–20
103 N/m, offers the best hydrophobic characteristics, while also exhibiting a respectable thermal and chemical stability during operations (Cath et al., 2004; Drioli et al., 2015; Heinzl et al., 2012; Li and Sirkar, 2005; Mericq et al., 2009; Zhao et al., 2013). However, fabrication of PTFE membranes can be accomplished only through difficult sintering, rolling or melt-extrusion methods (Alkhudhiri et al., 2012; Huang et al., 2008; Ishino et al., 1999). PP membranes, with a surface energy of 30.0
103 N/m, is also high crystalline providing low material and fabrication costs (Camacho et al., 2013; Pangarkar et al., 2016). The major drawback of this membrane is poor performance and thermal instability at severe operating conditions (Gryta, 2007). PVDF membranes, with surface energy of 30.3
103 N/m, have received the most

186 Chapter 8 attention by researchers in literature owing to satisfactory stability, high solubility and ease of fabrication and processing (Kang and Cao, 2014; Lu et al., 2013; Sukitpaneenit and Chung, 2009). These membranes used in several configurations of MD and their properties by using plate-and-frame and hollow fiber modules were investigated and reviewed extensively in literature recently (Drioli et al., 2015; Eykens et al., 2017; Pangarkar et al., 2016; Wang and Chung, 2015). It is worth mentioning that these membranes were not specifically optimized for MD technology since they were manufactured for application in ultrafiltration and microfiltration, which operates in different circumstances. 3.1.2 Recent developments With the advent of novel production techniques and in order to provide membranes that can meet the desired characteristics for specifically MD process, modified membranes have been fabricated and explored mainly with the aim of enhancing permeate flux, wetting resistance and hydrophobicity. Drastic progress in nanotechnology has opened broad horizons in the development of membranebased water treatment systems including MD. Production of nanofibers through electrospinning process has gained remarkable attention as it has exhibited interesting features in membrane technology. Electrospun membranes offer unique characteristics such as very high porosity (above 80%), great hydrophobicity (contact angle >130 degrees) and increased roughness, with interconnected pore structure which results in improved flux rate (Camacho et al., 2013; Eykens et al., 2017; Khojasteh et al., 2016; Tijing et al., 2014). The applicability of different materials that can be incorporated into nanofibers has been surveyed through experiments by many researchers. In this context, PVDF (Feng et al., 2008; Liao et al., 2013), PVDF-co-HFP (Lalia et al., 2013), PVDF-co-HFP—carbon nanotubes (Tijing et al., 2016), PVDF—hydrophobic clay (Prince et al., 2012), PVDF—SiO2 (Liao et al., 2014), PTFE/PVA (Eykens et al., 2017), Matrimid (Francis et al., 2013) and polystyrene (Ke et al., 2016; Li et al., 2014) have been employed as electro spun membrane material for MD in AGMD and DCMD configurations. Another developing area is carbon nanotubes (CNTs) involving single-wall CNTs (SWCNTs) and multi-wall CNTs (MWCNTs), which have attracted a surge of interest in lab-scale construction of innovative membranes for water desalination. They are proven to show superior durability, intrinsic hydrophobicity and porosity, considerable mechanical strength and outstanding chemical and thermal properties (Das et al., 2014; Goh et al., 2013; Lee et al., 2016; Ma et al., 2017). These characteristics along with the extremely smooth hollowed structure of the nanotubes that facilitate rapid transport of liquid and gas molecules in the channels, can fulfill the requirements leading to an increased flux (Drioli et al., 2015; Dum
ee et al., 2011; Goh et al., 2013). In order to enhance the hydrophobicity of membranes, multiple surface modification methods have been developed to hydrophobize hydrophilic membranes. Many studies have revealed that

Water Treatment by Renewable Energy-Driven Membrane Distillation 187 multi-layer approaches with gradients of hydrophobicity/hydrophilicity, in which the two sides of the membranes present opposite water wetting behaviors, can lead to high performance MD membranes (Camacho et al., 2013). One method to achieve such modified membranes is using plasma technology. Hydrophilic PES membranes were engineered via CF4 plasma to alter its contact angle from 0 degrees to over 120 degrees (Wei et al., 2012). Also, PVDF membranes in a DCMD process were modified through TiO2 coating followed by fluoro-silanization of the surface, which resulted in superhydrophobic membrane with contact angle of 130 degrees and a significant 50% increase of the LEP (Razmjou et al., 2012). Although dual layer membranes were mostly processed as hollow fiber (Edwie et al., 2012; Gryta, 2007, 2008; Su et al., 2010; Teoh et al., 2011; Wang et al., 2011), a number of studies also performed similar approaches on flat sheet membranes (Qtaishat et al., 2009). Some of these membranes, applied in various configurations of MD, their characteristics and their fabrication method along with literature references are tabulated in Table 1.

3.2 Membrane Parameters Membrane characteristics play a conclusive role in water production rate and thus the successful outcome of MD process. The relationship between the transmembrane flux and the membrane parameters is evaluated by Lawson and Lloyd (1997): Jw ∝

hr α i  ε τδ

(1)

where Jw represents the permeate flux and τ, ε, and δ are membrane tortuosity, porosity, and thickness, respectively. hrαi is the average pore size for Knudsen diffusion (when α ¼ 1), and hrαi is the average squared pore size for viscous flux (when α ¼ 2). In order to fulfill the purpose of high water flux, the thickness of the membrane should be small to increase the permeability as it dictates a significant portion of mass transfer resistance. However, it is known that heat dissipation by conduction decreases as the membrane becomes thinner, hence the interface temperature difference decreases which lowers the heat efficiency (Eykens et al., 2017; Qtaishat and Banat, 2013). Considering the contradictory effects of membrane thickness, many scholars have proposed an optimal value, which is commonly within the range of 10–60 μm for lab-scale experiments (Laganà et al., 2000; Li and Sirkar, 2016; Martı´nez and Rodrı´guez-Maroto, 2008). It must be pointed out that due to the fact that the predominant mass transfer resistance exists in the air gap for AGMD, this configuration is known to be an exception in this case (Sanmartino et al., 2016). MD membranes usually have a pore size of 100 nm to 1 μm and as the pores become larger the permeate flux increases. However, the maximum pore size is limited by the LEP which accounts for the wettability of the membrane during operation (Drioli et al., 2015;

Material

MD Configuration

Membrane Properties Thickness (μm)

CA (°)

LEP (bar)

Porosity (%)

Permeate Flux (kg/m2/h)

References

Electrospinning Nanofiber PVDF

DCMD

144

140

0.6

75

15.2

Nanofiber PTEF/PVA

VMD

156

150

1.65

80

15.8

Nanofiber polystyrene Nanofiber PDMF/ PVDF-HFP

DCMD DCMD

147 102

114 155

1.5 1.26

84 88

31 34

Essalhi and Khayet (2014) Zhou et al. (2014) Ke et al. (2016) An et al. (2017)

Carbon nano tubes (CNT) CNT

DCMD

45

113

0.55

90

12

CNT immobilized on PTEF CNT + nanofiber PVDF-co-HFP CNT + nanofiber PVDF-HFP

DCMD

–

95

–

72

69

DCMD

81

158

0.99

> 84

29.5

DCMD

96

148

69

90

32

Dum
ee et al. (2010) Bhadra et al. (2016) Tijing et al. (2016) Lee et al. (2017)

Surface modification Tetrafluoromethane plasma on nanofiber PVDF Perfluorodecyl methacrylate plasma on polyacrilonitryl Graphene loaded nanofiber PVDF-coHFP Tetrafluoromethane plasma on polyethersulfone

AGMD

150

161

1.87

86

15

Woo et al. (2017)

VDM

–

132

4.95

80

59.42

Liu et al. (2016)

AGMD

100

> 162

1.86

>88

22.9

Woo et al. (2016)

DCMD

201

124

3.7

79

24.5

Wei et al. (2012)

188 Chapter 8

Table 1 Some examples of recent synthesized membranes for MD applications

Water Treatment by Renewable Energy-Driven Membrane Distillation 189 El-Bourawi et al., 2006). High LEP can be achieved by small pores and a highly hydrophobic material (large contact angle). It should be noted that by reducing the maximum pore size, the mean pore size decreases, resulting in lower membrane permeability (Qtaishat and Banat, 2013). Other significant factors contributing to high permeability are porosity and tortuosity of the membranes. High porosity of the media extends the surface area and increases the permeate flux for all MD configurations (El-Bourawi et al., 2006). Additionally it culminates in lower thermal conductivity due to the resistance of the gases trapped between the pores, which leads to a higher driving force and, therefore, higher flux. Nevertheless, high porosity jeopardizes the mechanical strength of the membrane, increasing the susceptibility to crack under mild pressure (Camacho et al., 2013; Eykens et al., 2017). Since the water flux is inversely proportional to tortuosity according to Eq. (1), low tortuosity is desirable. This is owing to the fact that the molecules do not diffuse straight across the thickness of the membrane but they move through tortuous paths (El-Bourawi et al., 2006).

3.3 Modules A membrane module is an apparatus that incorporates the membrane into a functional package consisting of the housing, flow channels, membrane, and its mounting. The module ensures the essential enclosed area in a specific volume. In the context of an MD process, a convenient module design should offer suitable packing density to deliver compactness of the module, with compatibility to multiple configurations. Additionally, it enhances robustness and efficiency and minimizes thermal/concentration polarization, scaling, and fouling. A wide range of modules have been developed to address process requirements; however, the studies on module design are limited in number generally for membrane processes and particularly for MD (Drioli et al., 2015; Winter et al., 2017). The most common types of module design in MD operations include: hollow fiber modules; plate and frame modules; tubular modules; and spiral wound modules. These configurations are depicted in Fig. 3 and described in the following. 3.3.1 Hollow fiber Hollow fiber type module is comprised of thousands of bundled hollow fibers bonded at both ends, which are placed inside a shell tube, as illustrated in Fig. 3A. The hot feed enters the hollow fiber and the permeate is obtained on the outer wall of the fiber (inside-outside), or the feed passes through the outer wall of the fiber and the permeate is the interior flow (outsideinside) (Alkhudhiri et al., 2012). The main advantages of this module type are excellent packing density (the ratio of membrane area to the packing volume) and low energy consumption. However, it is highly prone to fouling and difficult to clean and maintain (Alkhudhiri et al., 2012; Sharon and Reddy, 2015; Warsinger et al., 2015b).

Permeate sweep

End plate

Tubesheet Concentrate Spacer

Feed

Fiber bundle

Permeate

Retentate

Support plate with membranes on both sides Permeate

Spacer

Lumen manifold bolted to case

O-ring tubesheet-case seal Lumen distribution manifold

Feed

Shell distribution manifold End plate

(A)

Assembly bolt

(B)

Permeate flow Housing Hollow copper tube

Air gap

Membrane

Covering head

Feed flow Covering head

Hollow copper tube

Permeate channel Membrane Feed channel

(D) (C) Fig. 3 (A) Hollow fiber module, (B) plate-and-frame module, (C) tubular module, (D) spiral-wound module. From (A) Mat, N.C., Lou, Y., Lipscomb, G.G., 2014. Hollow fiber membrane modules. Curr. Opin. Chem. Eng. 4, 18–24, with permission from Elsevier; (B) Pal, P., Sikder, J., Roy, S., Giorno, L., 2009. Process intensification in lactic acid production: a review of membrane based processes. Chem. Eng. Process Process Intensif. 48, 1549–1559, with permission from Elsevier; (C) Cheng, L.-H., Lin, Y.-H., Chen, J., 2011. Enhanced air gap membrane desalination by novel finned tubular membrane modules. J. Membr. Sci. 378, 398–406, with permission from Elsevier; (D) Li, Y.-L., Tung, K.-L., Lu, M.-Y., Huang, S.-H., 2009. Mitigating the curvature effect of the spacer-filled channel in a spiral-wound membrane module. J. Membr. Sci. 329, 106–118, with permission from Elsevier.

190 Chapter 8

External case Permeate

Water Treatment by Renewable Energy-Driven Membrane Distillation 191 3.3.2 Plate and frame This configuration places the membrane and the spacers layered together between two flat sheets. Unlike in hollow fiber case, the packing density is low and also the membranes are not self-supportive. However, due to the ease of cleaning and replacing, plate-and-frame modules are widely applied by scholars on laboratory scale (Alkhudhiri et al., 2012; Karanikola et al., 2017). A typical plate-and-frame module is depicted in Fig. 3B. 3.3.3 Tubular In this case, the membrane is cast between two tubular channels (hot and cold flow channels). The feed solution flows through the center of the tubes whereas the permeate flows through the porous supporting tube into the module housing as demonstrated in Fig. 3C. The main difference between tubular and hollow fiber arises from the dimensions of the tube and that the tubular modules require a membrane support. Due to the fact that this configuration is less liable to be fouled and provides large surface area, it is the most attractive configuration in commercial fields. However, this module has a low packing density and a high operating cost (Alkhudhiri et al., 2012). 3.3.4 Spiral wound A membrane envelope is made of flat sheets rolled around a perforated central tube, where is permeate is collected while the feed solution moves across the membrane surface in axial direction as illustrated in Fig. 3D (Alkhudhiri et al., 2012; Karabelas et al., 2015). Spiral wound modules hold great membrane areas and they are regarded as relatively cost effective MD modules. They exhibit average tendency to fouling and provide high packing density (Sharon and Reddy, 2015; Warsinger et al., 2015b).

4 Transport Phenomena The transport mechanism such as mass transfer and heat transfer are coupled in membrane distillation (MD) process. A great deal of recent investigations aim to tackle the understanding of heat and mass transport phenomenon in MD from different perspectives, most of which are based on theoretical methods. Additionally, New modeling tools and computational software packages such as CFD, ASPEN, and MATLAB have been introduced to elucidate these phenomena further. A large number of publications are devoted to the heat and mass transfer in DCMD and VMD with some evolving significance discussing AGMD. The main challenge is to analyze quantitatively the thermal polarization and concentration effect in heat and mass transport phenomena (Drioli et al., 2015; Winter et al., 2011). In MD, the difference between the feed/membrane interface temperature (Tmf) and the permeate/membrane interface temperature (Tmp) is the driving force for water vapor departure through the membrane pores. The membrane/interface temperatures vary from the bulk

192 Chapter 8 temperatures because of the heat dissipation during MD process. This temperature gradient reduces the theoretical driving force (evaluated as the temperature difference between the bulk feed and the bulk permeate). This phenomenon is defined as temperature polarization. The ratio of the actual driving force to the theoretical driving force is identified as the temperature polarization coefficient (TPC), expressed mathematically in Eq. (2) (Drioli et al., 2015; Qtaishat and Banat, 2013):
 Tmf  Tmp  (2) TPC ¼
Tbf  Tbp

4.1 Heat Transfer There are two main mechanisms that cause the heat transfer in the MD operation. The first one is the heat transfer by conduction (latent heat and sensible heat) from the hot feed to the cold permeate across the membrane. The second mechanism is heat transfer by convection which occurs from the bulk flow of the feed to the boundary layer. On this account, the most significant parameters to design the MD modules include thermal conductivity, heat transfer coefficient and heat flow (Drioli et al., 2015; Elzahaby et al., 2016; Pangarkar et al., 2016). The heat transfer can be surveyed in three distinct regions: (i) heat transfer through the thermal boundary layer of the feed solution, Qf:
 Qf ¼ hf Tbf  Tmf

(3)

(ii) sum of both conductive heat transfer and heat transferred due to migration of water vapor through the membrane pores, Qm:
 Qm ¼ hm Tmf  Tmp + Jw ΔHv (4) (iii) heat transfer through the thermal boundary layer of permeate solution, Qp:
 Qp ¼ hp Tmp  Tbp

(5)

where hf and hp are the heat transfer coefficient of the feed boundary layer and permeate boundary layer, respectively. Tmf represents the temperature of membrane/feed interface and Tmp represents the membrane/permeate interface temperature. ΔHv denotes the latent heat of vaporization and hm accounts for the heat transfer coefficient of the hydrophobic membrane, which can be obtained by the thermal conductivities of the hydrophobic membrane polymer (km) and air trapped inside the membrane pores (kg) (Qtaishat and Banat, 2013). hm ¼

kg ε + km ð1  εÞ δ

(6)

Water Treatment by Renewable Energy-Driven Membrane Distillation 193

4.2 Mass Transfer The most significant mass transport mechanisms for MD process are viscous flow, Knudsen diffusion, and molecular diffusion. Therefore the mass transfer depends on the Knudsen number (kn) which is specified as the ratio of mean free path (λ) of molecules that transport to the membrane pore diameter (dp) (Abu-Zeid et al., 2015; Pangarkar et al., 2016). kn ¼

λ dp

in which the mean free path, λ, is obtained by the following equation: rffiffiffiffiffiffiffiffiffiffi 3:2μv RT λ¼ P 2πM

(7)

(8)

where μv denotes the vapor viscosity at atmospheric temperature and ambient pressure, M represents the molecular weight, R accounts for the gas constant and T and P are the temperature and the mean pressure within the pores, respectively (Pangarkar et al., 2016). In the MD process, the mass transfer is commonly determined by applying the assumption of a linear relationship between the mass flux (Jw) and the water vapor pressure difference through Bm, which is the membrane distillation coefficient (Qtaishat and Banat, 2013).
 Jw ¼ Bm pmf  pmp (9) Where pmf and pmp are the partial pressures of water at the feed and permeate sides calculated by employing Antoine equation (Eq. 10) at Tmf and Tmp, respectively.   3841 (10) P ¼ exp 23:3288  T  45

4.3 Performance Parameters The gained output ratio (GOR) and the thermal recovery ratio (TRR) are the two factors which are of essential importance in solar-driven membrane distillation processes as well as in thermal desalination processes (Sharon and Reddy, 2015; Swaminathan et al., 2016). The gained output ratio is calculated from Eq. (1) and denotes the ratio of the evaporation energy to the actual energy consumed. The higher the GOR the more efficient the desalination process is. Also, higher GOR is obtained with lower feed flow rate and this is mainly due to higher distillate production (Mohamed et al., 2017). GOR ¼

md ΔHv mh Cp ðThi  Tho Þ

(11)

194 Chapter 8 where md is the distillate flow rate (kg/h), ΔHv represents the latent heat of vaporization (J/kg), mh accounts for the feed flow rate (kg/h), Cp denotes the feed specific heat (J/kg K) and Thi and Tho are the temperatures of the feed (in K) at the module inlet and outlet. The TRR is the theoretical energy required for producing the distillate divided by the total thermal energy input. As such, the TRR can be defined as: TRR ¼

md ΔHv AI

(12)

where A represents the solar collector area (m2), and I is the global irradiation (W/m2).

5 Solar Collecting Technologies Among all renewable energy-powered water treatment systems, solar energy is a promising candidate to supply the heat (solar thermal) or electrical energy (solar photovoltaic) requirements to operate MD systems. Below we present a brief review of the solar desalination process with a special focus on solar technologies that could be coupled with MD.

5.1 Solar Thermal Solar collectors are widely known devices to captivate and transfer solar energy into a collection of fluid. Solar thermal desalination systems are commonly divided into two main classifications; direct and indirect systems as explained below. 5.1.1 Direct solar desalination In the direct system both evaporation and condensation take place in the same device. It may be applied effectively for small communities or household applications, where the freshwater requirement is <200 m3/day (Ma and Lu, 2011; Saleh et al., 2011). Solar stills, which come under direct desalination method, can separate freshwater by applying greenhouse technique (Manju and Sagar, 2017). Solar stills are economical, have easy operation, and do not cause emission of harmful gases that affect the world’s climate. They act as traps for solar radiations that pass through glass-covered chamber to evaporate water; water vapor then condenses to form droplets of freshwater which exit through special channels. The principle of a conventional solar still is depicted in Fig. 4 (Eltawil et al., 2009). As is clear from the figure, the solar radiation heats up the slanted panels, generating a greenhouse effect inside the chamber, which raises the temperature of the brackish water. This phenomenon causes the surface water to evaporate and then, the water vapor reaches the panels and it is there that it condenses to potable water. Using cycle, 3–4 L/day/m2 of fresh water can be obtained (Shatat et al., 2013). Due to this low production rate, it is of great importance to reduce the costs of this system. The main cost of freshwater production is mainly associated to the capital cost of

Water Treatment by Renewable Energy-Driven Membrane Distillation 195

Fig. 4 A solar still desalination unit (Ali et al., 2011). From Eltawil, M.A., Zhengming, Z., Yuan, L., 2009. A review of renewable energy technologies integrated with desalination systems. Renew. Sust. Energy Rev. 13, 2245–2262, with permission from Elsevier.

the plant construction and its maintenance and also, if the water should be transferred to somewhere else, the cost of pumping has to be taken into consideration. Therefore, one of the most convenient areas for applying solar stills is where solar energy and low-cost labor are abundant (Samee et al., 2007). Moreover, a simple and efficient way to increase the heat storage capacity of a solar still—for example to make the heat source available during night or non-sunny days—is to increase the saline water depth in the still, and therefore the heat storage capacity of the device can be increased (Gude, 2015). Since the performance of solar stills can be influenced by a large number of factors—such as ambient and brackish water temperature, slope, wind velocity, thermal insulation and salt concentration—care should be taken during installation of solar stills to avoid a drop in overall efficiency of the system (Manju and Sagar, 2017). 5.1.2 Indirect solar desalination In indirect systems, the unit plant is formed of two subsystems: desalination section and solar collecting device, usually solar collectors or solar ponds. These systems are utilized in majority of large scale applications. There are various types of collectors including evacuated tube, heat pipe, flat plate and solar concentrator which can be utilized to be coupled with thermal desalination processes such as vapor compression (VC), multi-effect evaporation (MED), multi-stage flash distillation (MSF) and membrane distillation (MD) for possible combinations of thermal desalination with solar energy (Shatat et al., 2013). These systems can be driven

196 Chapter 8 thermally—for example, MSF, MED, and MD—in which the desalination technologies require solar thermal collectors as their energy source or electrically such as reverse osmosis unit (Ali et al., 2011). Solar pond systems are large solar collectors with a high capacity of energy storage, where the solar energy trapped into the stored salty water is circulated as a heating medium in the desalination process unit (Gude, 2015). These systems are the most convenient and least expensive option for heat storage (Lu et al., 2002), which are suitable for desalination plants as waste brine from desalination can be used as the salt source for establishing density gradient (Manzoor et al., 2017). Solar ponds exhibit a unique ability to capture and store thermal energy of sun during cloudy days for months, offering it a distinct ability in comparison with other methods of solar collection. A typical solar pond has three separate zones and this system with its three convective zones is illustrated in Fig. 5. As is obvious from the figure, the zone located at the top of the pond, called upper convective zone (UCZ), contains the low density of salty water and adopts the role of absorbing and transmitting energy. The second zone, with a medium amount of saltwater and called the non-convective zone (NCZ), prevents heat from escaping to UCZ, and so it retains the temperature at a higher level for lower zones. The bottom zone is the lower convective zone (LCZ), acting as a heat storage zone with a uniform salt density (Busquets et al., 2012; Gude, 2015).

UCZ

Upper convective zone

Increasing salinity and temperature

LCZ

NCZ

Gradient zone

UCZ

NCZ

Increasing salinity and temperature

Storage zone LCZ

Fig. 5 A solar pond system along with its convective zones. From Gude, V.G., 2015. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl. Energy 137, 877–898, with permission from Elsevier.

Water Treatment by Renewable Energy-Driven Membrane Distillation 197

5.2 Solar Photovoltaic Solar photovoltaic (PV) systems, as a mature technology with life expectancy of 20–30 years, are semiconductor devices that convert sunlight into DC electricity through the transfer of electrons. The process of energy conversion mainly occurs in two stages; generation of electron-hole pair through the absorption of light in semiconductor material and afterwards, separation of electron to the negative terminal and hole to the positive terminal by the structure of the device to supply electricity (Pandey et al., 2017). In order to serve the purpose of producing a greater amount of electrical energy, solar cells may be arranged in series and multiple modules are linked together to build an array. Solar-PV systems offer several advantages including simple design, long operation life, high reliability and not producing any further pollution during energy provision (Pandey et al., 2017). Currently, there are three generations of PV cells: 1st generation: crystalline silicon (c-Si); 2nd generation: amorphous silicon thin-film (TF); and 3rd generation: nano-PV (Qtaishat and Banat, 2013). In today’s market, the most common material used for PV cells is crystalline silicon (Al-Karaghouli and Kazmerski, 2013). The overall efficiency of a PV system totally depends on temperature of the cells, in a way that the efficiency decreases with a rise in operating temperature (Qtaishat and Banat, 2013). Moreover, the core components of a solar-PV system are PV panel, charge controller, battery pack, DC/AC inverter, DC/DC converter, and DC shunt. These equipments should be added to a PV module to supply energy to a desalination plant. In order to increase this energy, tracking flat PV system can be employed and thus there is an increase in capturing solar energy due to tracking the path of sun’s movement (Abu-Khader et al., 2008). Base on how solar-PV systems connected to an electrical load, these systems are generally categorized as stand-alone and utility-interactive systems. Regarding the latter case, the system relies on the availability of sunlight, necessitating the system to be connected to the utility grid through power conditioning unit which may be a high-quality inverter. Stand-alone PV systems are autonomous systems which provide DC or AC electrical loads, making them suitable for isolated users who are far away from the electricity grid (Pandey et al., 2017).

6 MD Systems Integrated With Solar Energy Collectors Since MD modules can tolerate fluctuating and intermittent operating conditions, many scholars and researchers have been motivated to couple MD systems with solar energy collectors in order to reduce the volume of discharged brine, increase the water recovery, and overcome the limits of single units (Sanmartino et al., 2016). Moreover, a MD unit, unlike any other membrane technologies, has compatibility with the transient nature of the energy, providing a unique opportunity to be coupled with solar collectors easily.

198 Chapter 8 Some investigators have employed MD as the main stage in the desalination, achieving low separation factors (Guill
en-Burrieza et al., 2011). One point that should be taken into consideration is that MD system is most effective when applied as a second stage unit, with a recovery >90% (Mericq et al., 2010). Two possible configurations are reported for coupling of solar energy with MD; the solar-assisted MD desalination system and the solar stand-alone MD desalination system (Qtaishat and Banat, 2013). Regarding the latter case, solar-powered PV collectors are used along with battery cells and electric current inverters to supply the required energy, while diesel generators are adopted to provide energy for the solar-assisted MD desalination system, as indicated in Fig. 6. In a whole view, we can state that MD unit can be coupled with vacuum collectors, solar ponds, solar stills, flat plate collectors and parabolic troughs (Qtaishat and Banat, 2013). Hot water Solar thermal collector

Warm water

MD unit

Diesel generator

Electricity

(A) Hot water Solar thermal collector

Warm water

MD unit

Solar PV collector

DC

Batteries and inverter

AC

(B) Fig. 6 (A) Solar-assisted and (B) stand-alone MD configurations. From Qtaishat, M.R., Banat, F., 2013. Desalination by solar powered membrane distillation systems. Desalination 308, 186–197, with permission from Elsevier.

Water Treatment by Renewable Energy-Driven Membrane Distillation 199

6.1 MD Integrated With Vacuum or Flat Plate Collectors The pioneers of coupling MD systems with vacuum and flat plate collectors are Hogan et al. (1991) in Australia and Thomas (1997) in Japan in 1990s, where they used 3 m2 flat plate solar collectors and 12 m2 field of vacuum tube collectors, respectively. In order to overcome the freshwater shortage of in China, a solar-heated hollow fiber vacuum MD (VMD) was utilized for potable water production from underground water. The experiment results of this system demonstrated that the temperature polarization influence on the permeate flux of the VMD becomes lesser as the feed temperature rises (Wang et al., 2009). To provide freshwater at terminal lakes, a DCMD coupled to a salt-gradient solar pond (SGSP) was investigated, proposing that the success of such project mainly depends on the fact that SGSP should be constructed inside the terminal lake and so, there is little or no net increase in surface area (Sua´rez et al., 2010). One approach to address the drawbacks of VMD, which require high energy to heat the feedwater, is the combination of VMD and solar collectors, resulting in an interesting solution by presenting high fluxes (Mericq et al., 2011). In more recent studies conducted into this issue, a numerical analysis of the hollow fiber multistage VMD was carried out in order to enhance productivity and specific heat energy consumption of VMD, reporting that the multi-stage VMD with 20 stages offer the highest productivity (Lee and Kim, 2014). Moreover, a new concept of implementing an aspirator as the vacuum generator for a VMD was presented to eliminate many of the concerns associated with the conventional design of using a condenser and a vacuum pump, such as incomplete vapor condensation, continuity of vacuum pump operation, and the risk of water vapor damage to the pump (Hassan et al., 2016). An introduction of a hybrid system that integrates a VMD with pressure-retarded osmosis (PRO) was provided to treat shale gas drilling flow-back fluid (Lee et al., 2015). The operation of the multi-stage VMD systems which can achieve performance comparable to MSF was probed for the same operating conditions, demonstrating that they can operate at lower temperatures without the need for a steam generator, allowing the use of low temperature heat resources (Summers, 2013). An evaluation of the performance of a multi-VMD module without energy recovery was performed by adding a heat exchanger to recover the waste heat included in the discharge brine, resulting in a GOR of less than one (Shim et al., 2014). The performance of a multi-stage VMD was evaluated for desalination, brine concentration, and produced water reclamation applications. The results presented that the increased boiling point elevation of the feed stream resulted in lower fluxes, larger heating requirements and lower GOR values at high salinities. Furthermore, this system is efficient as it uses reasonable membrane areas, and can be applied for a wide range of feed salinities (Chung et al., 2016). The feasibility of combining a low-temperature DCMD driven by a SGSP was examined to determine the experimental fresh water production rates. The experiment results demonstrated that the coupled DCMD-SGSP system treats approximately six times the water flow treated

200 Chapter 8 UCZ NCZ P F

T EC

LCZ Solar pond

T EC Water chiller

P T EC

Heat exchanger Feed side

F T EC

Feed reservoir

Distillate side DCMD module

P T EC

Heat exchanger

F

Distillate reservoir

Water treated

T EC

Balance

Fig. 7 The diagram of a coupled DCMD-SGSP system. The symbols P, EC, F, and T represent pumps, electrical conductivity probes, flow meters, and temperature sensors, respectively. From Sua´rez, F., Ruskowitz, J.A., Tyler, S.W., Childress, A.E., 2015. Renewable water: direct contact membrane distillation coupled with solar ponds. Appl. Energy 158, 532–539, with permission from Elsevier.

by a similar system that consisted of an air–gap MD unit driven by a SGSP (Sua´rez et al., 2015). The diagram of this system is shown in Fig. 7. A sustainable water desalination (SWD) was developed through the combination of DCMD with solar ponds by using the surface of the solar pond as heat sink for the permeate water and introducing floating cooling pipes that are acting as wave suppressors to reduce the surface mixing in solar pond (Rahaoui et al., 2017). A comparison between two coupling configurations—a module membrane in series with SGSP and a hollow fiber module immersed in the SGSP—of VMD hollow fiber module with SGSP was drawn, revealing that the immersed module production presents more than one and a half times that of the separated module, therefore immersing the module in the solar pond improves the performance of the hollow fiber module (Abu-Khader et al., 2008).

6.2 MD Integrated With Parabolic Trough Collectors and Solar Stills Parabolic troughs are linear collectors with a parabolic cross-section and reflective surface which concentrate sunlight onto a receiver tube located along the trough’s focal line, heating the heat transfer fluid in the tube (Qiblawey and Banat, 2008). Parabolic trough collector (PTC) can be installed appropriately to track the sun’s movement, providing much higher temperature to receiver tube than flat plate or evacuated-tube collectors. The MEDESOL (Seawater Desalination by Innovative Solar-Powered Membrane Distillation System) was one of the first projects to couple parabolic trough with MD unit, and was supported by European Commission. In this project, an assessment of the solar multi-stage

Water Treatment by Renewable Energy-Driven Membrane Distillation 201 MD concept was completed to develop a high-efficiency and cost-effective system for seawater desalination. The heat source for the feedwater used in the system was a compound parabolic solar concentrator, developed for the specific concentration ratio to achieve the specific needed range of temperatures (Ga´lvez et al., 2009). An AGMD module with a total membrane surface area per module of 2.8 m2 was coupled with a static solar collector field (compound parabolic collector type) to be energy-efficient at MD working temperatures and cover water demands of small settlements which proved to be suitable for coupling with transient solar thermal energy (Guill
en et al., 2011). A schematic view of the mentioned solar MD desalination experimental prototype is depicted in Fig. 8. In addition, the results obtained from experiment of MD modules coupled with a solar collector field indicated that the multi-stage concept for MD can noticeably reduce heat consumption and the use of higher feed flow rates can improve MD modules’ performance (Guill
en et al., 2011). There are few studies examining the feasibility of coupling solar still with MD module. In an experiment conducted on this topic, saline solution was pumped into the solar still, where it was heated by solar energy in the outdoor experiments and by heating tapes in the indoor experiment and then, the saline was pumped from the solar still through the lumen side of a tubular MD module, whereas the cooling water and permeate flowed counter currently on the shell side. The distilled water was produced from both the solar still and the MD unit, and the flux of the MD module was four times higher than the obtained flux of the solar still. Moreover, fluxes of water from both the solar still and the MD module reached their maximum values 2–3 h after the solar irradiation peak (Banat et al., 2002).

7 Geothermal Energy Technology Geothermal energy originates from interior heat generation of Earth, caused by magma, as well as decay of the naturally occurring radioactive isotopes of uranium, thorium, and potassium (Tripathi et al., 2016). The high temperature hydrothermal reservoir of the Earth, to a depth of 10 km, is estimated to be 1.3
1027 J, which is theoretically sufficient for worldwide consumption for some million years. Although the temperature of the Earth increases with an average of 30°C per kilometers of depth, a significant number of geothermal energy exploitation, are explored in areas with higher gradient, to achieve a shallower and therefore, more economical drilling (Goosen et al., 2010). Energy from the Earth is commonly extracted by ground heat exchangers, made of extremely durable materials that allow efficient transmission of heat (Al-Karaghouli and Kazmerski, 2013). Ground zones can be identified as surface, shallow, and deep, with geothermal energy sources being distinguished in terms of their measured temperatures as low (<100°C), medium (100–150°C) and high temperature (>150°C), respectively (Garcia-Rodriguez, 2002). Geothermal energy presents a mature technology to provide the power required by water treatment plants (Mathioulakis et al., 2007). Contrary to solar power, which is intermittent,

202 Chapter 8 Fig. 8 A schematic diagram of the solar MD desalination experimental prototype coupled with a solar field. From Guill
en-Burrieza, E., Blanco, J., Zaragoza, G., Alarco´n, D.-C., Palenzuela, P., Ibarra, M., Gernjak, W., 2011. Experimental analysis of an air gap membrane distillation solar desalination pilot system. J. Membr. Sci. 379, 386–396, with permission from Elsevier.

Water Treatment by Renewable Energy-Driven Membrane Distillation 203 a constant stable energy can be provided by geothermal sources, eliminating the need for intricate capturing devices and costly energy storage systems (Ghaffour et al., 2015). While high temperature geothermal fluids can be applied to run electricity-driven desalination operations, the most promising option is the direct use of geothermal fluid with medium or high temperature as a stream power in thermal-driven desalination plant (Kiaghadi et al., 2017). Since the enthalpy water energy is low, geothermal power is not reasonable for conventional desalination methods (Sarbatly and Chiam, 2013). With the advent and progress of membrane distillation technology, the application of direct geothermal brine with temperature up to 60°C has gained attention (Goosen et al., 2010).

7.1 MD Systems Integrated With Geothermal Energy MD units are well suited to be coupled with geothermal fluids; however, geothermal energy has rarely been examined in MD (Sarbatly and Chiam, 2013). Technical feasibility of assembly constituted of fluidized bed crystallizer (to reduce hardness) and a cell of AGMD driven by geothermal energy has been demonstrated in lab-scale (Bouguecha and Dhahbi, 2003). However, MD recovery fraction was unable to reach a high value using only a sensible heat from a geothermal well (Bouguecha and Dhahbi, 2003). To address this issue, two options can be envisaged. One option is to take advantage of solar energy potential, since the coupled solar plan collectors and geothermal energy leads to considerable increase of MD efficiency. The second option is to integrate several membrane processes, such as MD integrated with RO, with RO using softening and warm brine of MD. Another research was conducted on AGMD and DCMD configurations and it was demonstrated that AGMD is more adapted to desalination by geothermal resources and energy consumption is lower (El Amali et al., 2004). Also, evaluation of geothermal energy application in water desalination was carried out for a VMD configuration (Sarbatly and Chiam, 2013). The temperature of the geothermal water, which was from a reservoir located at Ranau, Sabah, Malaysia, was in the range of 56–62°C at corresponding reservoir depth. Optimum operating conditions were sought considering the lowest specific energy consumption when distilled water was fed into the VMD system. The water production costs were reported to be 0.50 and 1.22 $/m3 for VMD desalination plant with geothermal energy and without geothermal energy, respectively. This investigation revealed that geothermal energy could boost the efficiency of the process by saving about 95% of the total energy consumption.

8 Conclusion and Future Trends Concerns over scarcity of drinkable water and ever-increasing energy demand have urged the global community to tap renewable energy-driven water treatment technologies, among which solar power is the most promising candidate for contribution to water availability. This is due to the fact that solar desalination systems are most suitable to be applied in dry regions.

204 Chapter 8 Among water treatment methods, MD exhibits massive potential to be coupled with solar collectors, considering the remarkable advantages such as moderate operating conditions, 100% (theoretical) rejection of non-volatiles and compact vapor space. These advantageous have also made MD the perfect candidate to integrate with geothermal energy, which can provide a predictable power, and hence eliminates energy converters and storage systems required in solar collectors. This chapter first dealt with the potentials and status of MD in current water treatment sector, covering the fundamental aspects and key issues including theoretical modeling, MD membranes and modules. In the next level, various solar collecting and geothermal technologies were discussed and the most significant efforts in integrating MD with solar and geothermal technologies were reviewed. Future research directions should focus on eliminating the drawbacks of MD process which are barriers to its commercialization. The fabrication of new membranes particularly for MD applications is urgently required to overcome low permeate flux. With the recent advances in this field and emerge of revolutionary generation of membranes, fabrication of membranes with excellent thermal stability, high fouling resistance and high hydrophobicity, made from low cost materials should be undertaken. Additionally, engineering membrane modules with adequate membrane arrangement and improved channel design is a challenging issue in availability and cost of MD operation. Although utilization of renewable energies (solar and geothermal) have substantially reduced energy consumption, which is the main barrier in up-scaling, parallel investigations should be carried out on development of low-cost and durable solar collectors and PV materials and optimization techniques to achieve the best schemes of renewable-energy driven MD processes. Also, innovative coupling of MD with other thermal water treatment processes such as MSF, MED, RO, etc. can be considered to minimize production costs.

List of Acronyms AC AGMD CF4 CNT DC DCMD GOR HFP LCZ LEP LGMD MD MED

alternating current air gap membrane distillation tetrafluoromethane carbon nano tube direct current direct contact membrane distillation gained output ratio hexafluoropropylene lower convective zone liquid entry pressure liquid gap membrane distillation membrane distillation multi-effect evaporation

Water Treatment by Renewable Energy-Driven Membrane Distillation 205 MSF MWCNT NCZ PE PES PP PRO PTC PTFE PV PVA PVDF SGMD SGSP SWCNT SWD TF TPC TRR UCZ VC VMD

multi-stage flash multi-wall carbon nano tube non-convective zone polyethylene polyethersulfone polypropylene pressure-retarded osmosis parabolic trough collector polytetrafluoroethylene photovoltaic poly vinyl alcohol polyvinilidenefluoride sweep gas membrane distillation salt-gradient solar pond single-wall carbon nanotube sustainable water desalination thin-film temperature polarization coefficient thermal recovery ratio upper convective zone vapor compression vacuum membrane distillation

List of Symbols A Bm Cp dp hf hm hp I Jw kg km kn M md mh P pmf pmp Qf Qm Qp R rα T

solar collector area (m2) membrane distillation coefficient (s/m) specific heat (J/kg/K) membrane pore diameter (nm) heat transfer coefficient in the feed boundary layer (W/m2/K) heat transfer coefficient through the membrane pores (W/m2/K) heat transfer coefficient in the thermal permeate boundary layer (W/m2/K) global irradiation (W/m2) mass flux (kg/s/m2) thermal conductivity of the air trapped inside the membrane pores (W/m/K) thermal conductivity of the hydrophobic membrane polymer (W/m/K) Knudsen number molecular weight (kg/mol) distillate flow rate (kg/h) feed flow rate (kg/h) pressure (Pa) partial pressure of water at the feed side (Pa) partial pressure of water at the permeate side (Pa) heat transfer in the feed boundary layer (W/m2) heat transfer through the membrane pores (W/m2) heat transfer in the thermal permeate boundary layer (W/m2) gas constant (J/mol//K) average pore size (nm) temperature (K)

206 Chapter 8 Tbf Tbp Thi Tho Tmf Tmp δ ε λ μv τ ΔHv

bulk feed temperature (K) bulk permeate temperature (K) inlet feed temperature (K) outlet feed temperature (K) feed/membrane interface temperature (K) permeate/membrane interface temperature (K) membrane thickness (μm) membrane porosity (%) mean free path (nm) viscosity of vapor (kg/m/s) membrane tortuosity latent heat of vaporization (kJ/mol)

References Abu-Khader, M.M., Badran, O.O., Abdallah, S., 2008. Evaluating multi-axes sun-tracking system at different modes of operation in Jordan. Renew. Sust. Energy Rev. 12, 864–873. Abu-Zeid, M.A.E.-R., Zhang, Y., Dong, H., Zhang, L., Chen, H.-L., Hou, L., 2015. A comprehensive review of vacuum membrane distillation technique. Desalination 356, 1–14. Ali, M.T., Fath, H.E., Armstrong, P.R., 2011. A comprehensive techno-economical review of indirect solar desalination. Renew. Sust. Energy Rev. 15, 4187–4199. Al-Karaghouli, A., Kazmerski, L.L., 2013. Energy consumption and water production cost of conventional and renewable-energy-powered desalination processes. Renew. Sust. Energy Rev. 24, 343–356. Alkhudhiri, A., Darwish, N., Hilal, N., 2012. Membrane distillation: a comprehensive review. Desalination 287, 2–18. Alsaadi, A.S., Francis, L., Maab, H., Amy, G.L., Ghaffour, N., 2015. Evaluation of air gap membrane distillation process running under sub-atmospheric conditions: experimental and simulation studies. J. Membr. Sci. 489, 73–80. Amy, G., Ghaffour, N., Li, Z., Francis, L., Linares, R.V., Missimer, T., Lattemann, S., 2017. Membrane-based seawater desalination: present and future prospects. Desalination 401, 16–21. An, A.K., Guo, J., Lee, E.-J., Jeong, S., Zhao, Y., Wang, Z., Leiknes, T., 2017. PDMS/PVDF hybrid electrospun membrane with superhydrophobic property and drop impact dynamics for dyeing wastewater treatment using membrane distillation. J. Membr. Sci. 525, 57–67. Banat, F., Jumah, R., Garaibeh, M., 2002. Exploitation of solar energy collected by solar stills for desalination by membrane distillation. Renew. Energy 25, 293–305. Bhadra, M., Roy, S., Mitra, S., 2016. Flux enhancement in direct contact membrane distillation by implementing carbon nanotube immobilized PTFE membrane. Sep. Purif. Technol. 161, 136–143. Bouguecha, S., Dhahbi, M., 2003. Fluidised bed crystalliser and air gap membrane distillation as a solution to geothermal water desalination. Desalination 152, 237–244. Busquets, E., Kumar, V., Motta, J., Chacon, R., Lu, H., 2012. Thermal analysis and measurement of a solar pond prototype to study the non-convective zone salt gradient stability. Sol. Energy 86, 1366–1377. Camacho, L.M., Dum
ee, L., Zhang, J., Li, J.-d., Duke, M., Gomez, J., Gray, S., 2013. Advances in membrane distillation for water desalination and purification applications. Water 5, 94–196. Cath, T.Y., Adams, V.D., Childress, A.E., 2004. Experimental study of desalination using direct contact membrane distillation: a new approach to flux enhancement. J. Membr. Sci. 228, 5–16. Charcosset, C., 2009. A review of membrane processes and renewable energies for desalination. Desalination 245, 214–231. Chen, T.-C., Ho, C.-D., 2010. Immediate assisted solar direct contact membrane distillation in saline water desalination. J. Membr. Sci. 358, 122–130.

Water Treatment by Renewable Energy-Driven Membrane Distillation 207 Chung, H.W., Swaminathan, J., Warsinger, D.M., 2016. Multistage vacuum membrane distillation (MSVMD) systems for high salinity applications. J. Membr. Sci. 497, 128–141. Das, R., Ali, M.E., Hamid, S.B.A., Ramakrishna, S., Chowdhury, Z.Z., 2014. Carbon nanotube membranes for water purification: a bright future in water desalination. Desalination 336, 97–109. Drioli, E., Ali, A., Macedonio, F., 2015. Membrane distillation: recent developments and perspectives. Desalination 356, 56–84. Dum
ee, L.F., Sears, K., Sch€utz, J., Finn, N., Huynh, C., Hawkins, S., Duke, M., Gray, S., 2010. Characterization and evaluation of carbon nanotube Bucky-Paper membranes for direct contact membrane distillation. J. Membr. Sci. 351, 36–43. Dum
ee, L., Campbell, J.L., Sears, K., Sch€utz, J., Finn, N., Duke, M., Gray, S., 2011. The impact of hydrophobic coating on the performance of carbon nanotube bucky-paper membranes in membrane distillation. Desalination 283, 64–67. Duong, H.C., Chivas, A.R., Nelemans, B., Duke, M., Gray, S., Cath, T.Y., Nghiem, L.D., 2015. Treatment of RO brine from CSG produced water by spiral-wound air gap membrane distillation—a pilot study. Desalination 366, 121–129. Edwie, F., Teoh, M.M., Chung, T.-S., 2012. Effects of additives on dual-layer hydrophobic–hydrophilic PVDF hollow fiber membranes for membrane distillation and continuous performance. Chem. Eng. Sci. 68, 567–578. El Amali, A., Bouguecha, S., Maalej, M., 2004. Experimental study of air gap and direct contact membrane distillation configurations: application to geothermal and seawater desalination. Desalination 168, 357. El-Bourawi, M., Ding, Z., Ma, R., Khayet, M., 2006. A framework for better understanding membrane distillation separation process. J. Membr. Sci. 285, 4–29. Eltawil, M.A., Zhengming, Z., Yuan, L., 2009. A review of renewable energy technologies integrated with desalination systems. Renew. Sust. Energy Rev. 13, 2245–2262. Elzahaby, A.M., Kabeel, A., Bassuoni, M., Elbar, A.R.A., 2016. Direct contact membrane water distillation assisted with solar energy. Energy Convers. Manage. 110, 397–406. Essalhi, M., Khayet, M., 2014. Application of a porous composite hydrophobic/hydrophilic membrane in desalination by air gap and liquid gap membrane distillation: a comparative study. Sep. Purif. Technol. 133, 176–186. Eykens, L., De Sitter, K., Dotremont, C., Pinoy, L., Van der Bruggen, B., 2017. Membrane synthesis for membrane distillation: a review. Sep. Purif. Technol. 182, 36–51 Feng, C., Khulbe, K., Matsuura, T., Gopal, R., Kaur, S., Ramakrishna, S., Khayet, M., 2008. Production of drinking water from saline water by air-gap membrane distillation using polyvinylidene fluoride nanofiber membrane. J. Membr. Sci. 311, 1–6. Francis, L., Maab, H., AlSaadi, A., Nunes, S., Ghaffour, N., Amy, G.L., 2013. Fabrication of electrospun nanofibrous membranes for membrane distillation application. Desalin. Water Treat. 51, 1337–1343. Ga´lvez, J.B., Garcı´a-Rodrı´guez, L., Martı´n-Mateos, I., 2009. Seawater desalination by an innovative solar-powered membrane distillation system: the MEDESOL project. Desalination 246, 567–576. Garcia-Rodriguez, L., 2002. Seawater desalination driven by renewable energies: a review. Desalination 143, 103–113. Ghaffour, N., Bundschuh, J., Mahmoudi, H., Goosen, M.F., 2015. Renewable energy-driven desalination technologies: a comprehensive review on challenges and potential applications of integrated systems. Desalination 356, 94–114. Ghalavand, Y., Hatamipour, M.S., Rahimi, A., 2015. A review on energy consumption of desalination processes. Desalin. Water Treat. 54, 1526–1541. Goh, P.S., Ismail, A.F., Ng, B.C., 2013. Carbon nanotubes for desalination: performance evaluation and current hurdles. Desalination 308, 2–14. Goosen, M., Mahmoudi, H., Ghaffour, N., 2010. Water desalination using geothermal energy. Energies 3, 1423–1442. Gryta, M., 2007. Influence of polypropylene membrane surface porosity on the performance of membrane distillation process. J. Membr. Sci. 287, 67–78.

208 Chapter 8 Gryta, M., 2008. Fouling in direct contact membrane distillation process. J. Membr. Sci. 325, 383–394. Gude, V.G., 2015. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl. Energy 137, 877–898. Guill
en, E., Blanco, J., Alarco´n, D., Zaragoza, G., Palenzuela, P., Ibarra, M., 2011. Comparative evaluation of two membrane distillation modules. Desalin. Water Treat. 31, 226–234. Guill
en-Burrieza, E., Blanco, J., Zaragoza, G., Alarco´n, D.-C., Palenzuela, P., Ibarra, M., Gernjak, W., 2011. Experimental analysis of an air gap membrane distillation solar desalination pilot system. J. Membr. Sci. 379, 386–396. Hassan, M.I., Brimmo, A.T., Swaminathan, J., Lienhard, V.J.H., Arafat, H.A., 2016. A new vacuum membrane distillation system using an aspirator: concept modeling and optimization. Desalin. Water Treat. 57, 12915–12928. Heinzl, W., B€uttner, S., Lange, G., 2012. Industrialized modules for MED desalination with polymer surfaces. Desalin. Water Treat. 42, 177–180. Hogan, P., Fane, A., Morrison, G., 1991. Desalination by solar heated membrane distillation. Desalination 81, 81–90. Hou, D., Dai, G., Fan, H., Huang, H., Wang, J., 2015. An ultrasonic assisted direct contact membrane distillation hybrid process for desalination. J. Membr. Sci. 476, 59–67. Huang, L.-T., Hsu, P.-S., Kuo, C.-Y., Chen, S.-C., Lai, J.-Y., 2008. Pore size control of PTFE membranes by stretch operation with asymmetric heating system. Desalination 233, 64–72. Ishino, T., Nabata, N., Maeoka, T., 1999. Process of making a porous PTFE membrane. Google Patents. Kang, G.-D., Cao, Y.-M., 2014. Application and modification of poly (vinylidene fluoride) (PVDF) membranes—a review. J. Membr. Sci. 463, 145–165. Karabelas, A., Kostoglou, M., Koutsou, C., 2015. Modeling of spiral wound membrane desalination modules and plants—review and research priorities. Desalination 356, 165–186. Karam, A.M., Alsaadi, A.S., Ghaffour, N., Laleg-Kirati, T.M., 2017. Analysis of direct contact membrane distillation based on a lumped-parameter dynamic predictive model. Desalination 402, 50–61. Karanikola, V., Corral, A.F., Jiang, H., Sa´ez, A.E., Ela, W.P., Arnold, R.G., 2017. Effects of membrane structure and operational variables on membrane distillation performance. J. Membr. Sci. 524, 87–96. Ke, H., Feldman, E., Guzman, P., Cole, J., Wei, Q., Chu, B., Alkhudhiri, A., Alrasheed, R., Hsiao, B.S., 2016. Electrospun polystyrene nanofibrous membranes for direct contact membrane distillation. J. Membr. Sci. 515, 86–97. Khalifa, A.E., 2015. Water and air gap membrane distillation for water desalination—an experimental comparative study. Sep. Purif. Technol. 141, 276–284. Khojasteh, D., Kazerooni, M., Salarian, S., Kamali, R., 2016. Droplet impact on superhydrophobic surfaces: a review of recent developments. J. Ind. Eng. Chem. 42, 1–14. Kiaghadi, A., Sobel, R.S., Rifai, H.S., 2017. Modeling geothermal energy efficiency from abandoned oil and gas wells to desalinate produced water. Desalination 414, 51–62. Koschikowski, J., Wieghaus, M., Rommel, M., Ortin, V.S., Suarez, B.P., Rodrı´guez, J.R.B., 2009. Experimental investigations on solar driven stand-alone membrane distillation systems for remote areas. Desalination 248, 125–131. Laganà, F., Barbieri, G., Drioli, E., 2000. Direct contact membrane distillation: modelling and concentration experiments. J. Membr. Sci. 166, 1–11. Lalia, B.S., Guillen-Burrieza, E., Arafat, H.A., Hashaikeh, R., 2013. Fabrication and characterization of polyvinylidenefluoride-co-hexafluoropropylene (PVDF-HFP) electrospun membranes for direct contact membrane distillation. J. Membr. Sci. 428, 104–115. Lawson, K.W., Lloyd, D.R., 1997. Membrane distillation. J. Membr. Sci. 124, 1–25. Lee, J.-G., Kim, W.-S., 2014. Numerical study on multi-stage vacuum membrane distillation with economic evaluation. Desalination 339, 54–67. Lee, J.-G., Kim, Y.-D., Shim, S.-M., Im, B.-G., Kim, W.-S., 2015. Numerical study of a hybrid multi-stage vacuum membrane distillation and pressure-retarded osmosis system. Desalination 363, 82–91.

Water Treatment by Renewable Energy-Driven Membrane Distillation 209 Lee, J., Jeong, S., Liu, Z., 2016. Progress and challenges of carbon nanotube membrane in water treatment. Crit. Rev. Environ. Sci. Technol. 46, 999–1046. Lee, J.-G., Lee, E.-J., Jeong, S., Guo, J., An, A.K., Guo, H., Kim, J., Leiknes, T., Ghaffour, N., 2017. Theoretical modeling and experimental validation of transport and separation properties of carbon nanotube electrospun membrane distillation. J. Membr. Sci. 526, 395–408. Li, B., Sirkar, K.K., 2005. Novel membrane and device for vacuum membrane distillation-based desalination process. J. Membr. Sci. 257, 60–75. Li, L., Sirkar, K.K., 2016. Influence of microporous membrane properties on the desalination performance in direct contact membrane distillation. J. Membr. Sci. 513, 280–293. Li, X., Wang, C., Yang, Y., Wang, X., Zhu, M., Hsiao, B.S., 2014. Dual-biomimetic superhydrophobic electrospun polystyrene nanofibrous membranes for membrane distillation. ACS Appl. Mater. Interfaces 6, 2423–2430. Liao, Y., Wang, R., Fane, A.G., 2013. Engineering superhydrophobic surface on poly (vinylidene fluoride) nanofiber membranes for direct contact membrane distillation. J. Membr. Sci. 440, 77–87. Liao, Y., Loh, C.-H., Wang, R., Fane, A.G., 2014. Electrospun superhydrophobic membranes with unique structures for membrane distillation. ACS Appl. Mater. Interfaces 6, 16035–16048. Liu, L., Shen, F., Chen, X., Luo, J., Su, Y., Wu, H., Wan, Y., 2016. A novel plasma-induced surface hydrophobization strategy for membrane distillation: etching, dipping and grafting. J. Membr. Sci. 499, 544–554. Lu, H., Walton, J.C., Hein, H., 2002. Thermal desalination using MEMS and salinity-gradient solar pond technology. NASA STI/Recon Technical Report N 3. Lu, J., Xiao, T., Qin, H., Chen, S., 2013. Effects of diluent on pvdf microporous membranes structure via thermally induced phase separation. Technol. Water Treat. 39, 33–36. Ma, Q., Lu, H., 2011. Wind energy technologies integrated with desalination systems: review and state-of-the-art. Desalination 277, 274–280. Ma, L., Dong, X., Chen, M., Zhu, L., Wang, C., Yang, F., Dong, Y., 2017. Fabrication and water treatment application of carbon nanotubes (CNTs)-based composite membranes: a review. Membranes 7, 16. Manju, S., Sagar, N., 2017. Renewable energy integrated desalination: a sustainable solution to overcome future fresh-water scarcity in India. Renew. Sust. Energy Rev. 73, 594–609. Manzoor, K., Khan, S.J., Jamal, Y., Shahzad, M.A., 2017. Heat extraction and brine management from salinity gradient solar pond and membrane distillation. Chem. Eng. Res. Des. 118, 226–237. Martı´nez, L., Rodrı´guez-Maroto, J., 2008. Membrane thickness reduction effects on direct contact membrane distillation performance. J. Membr. Sci. 312, 143–156. Mathioulakis, E., Belessiotis, V., Delyannis, E., 2007. Desalination by using alternative energy: review and state-ofthe-art. Desalination 203, 346–365. Mericq, J.-P., Laborie, S., Cabassud, C., 2009. Vacuum membrane distillation for an integrated seawater desalination process. Desalin. Water Treat. 9, 287–296. Mericq, J.-P., Laborie, S., Cabassud, C., 2010. Vacuum membrane distillation of seawater reverse osmosis brines. Water Res. 44, 5260–5273. Mericq, J.-P., Laborie, S., Cabassud, C., 2011. Evaluation of systems coupling vacuum membrane distillation and solar energy for seawater desalination. Chem. Eng. J. 166, 596–606. Mohamed, E.S., Boutikos, P., Mathioulakis, E., Belessiotis, V., 2017. Experimental evaluation of the performance and energy efficiency of a vacuum multi-effect membrane distillation system. Desalination 408, 70–80. Pandey, A., Rahim, N., Hasanuzzaman, M., Pant, P., Tyagi, V., 2017. Solar photovoltaics (PV): a sustainable solution to solve energy crisis. In: Green Technologies and Environmental Sustainability. Springer, Cham, pp. 157–178. Pangarkar, B., Deshmukh, S., Sapkal, V., Sapkal, R., 2016. Review of membrane distillation process for water purification. Desalin. Water Treat. 57, 2959–2981. Prince, J., Singh, G., Rana, D., Matsuura, T., Anbharasi, V., Shanmugasundaram, T., 2012. Preparation and characterization of highly hydrophobic poly (vinylidene fluoride)—clay nanocomposite nanofiber membranes (PVDF–clay NNMs) for desalination using direct contact membrane distillation. J. Membr. Sci. 397, 80–86.

210 Chapter 8 Qiblawey, H.M., Banat, F., 2008. Solar thermal desalination technologies. Desalination 220, 633–644. Qtaishat, M.R., Banat, F., 2013. Desalination by solar powered membrane distillation systems. Desalination 308, 186–197. Qtaishat, M., Khayet, M., Matsuura, T., 2009. Novel porous composite hydrophobic/hydrophilic polysulfone membranes for desalination by direct contact membrane distillation. J. Membr. Sci. 341, 139–148. Rahaoui, K., Ding, L.C., Tan, L.P., Mediouri, W., Mahmoudi, F., Nakoa, K., Akbarzadeh, A., 2017. Sustainable membrane distillation coupled with solar pond. Energy Procedia 110, 414–419. Razmjou, A., Arifin, E., Dong, G., Mansouri, J., Chen, V., 2012. Superhydrophobic modification of TiO2 nanocomposite PVDF membranes for applications in membrane distillation. J. Membr. Sci. 415, 850–863. Ruiz-Aguirre, A., Alarco´n-Padilla, D.-C., Zaragoza, G., 2015. Productivity analysis of two spiral-wound membrane distillation prototypes coupled with solar energy. Desalin. Water Treat. 55, 2777–2785. Saleh, A., Qudeiri, J., Al-Nimr, M., 2011. Performance investigation of a salt gradient solar pond coupled with desalination facility near the Dead Sea. Energy 36, 922–931. Samee, M.A., Mirza, U.K., Majeed, T., Ahmad, N., 2007. Design and performance of a simple single basin solar still. Renew. Sust. Energy Rev. 11, 543–549. Sanmartino, J.A., Khayet, M., Garcı´a-Payo, M., Singh, R., Hankins, N., 2016. Desalination by membrane distillation. In: Emerging Membrane Technology for Sustainable Water Treatment. Elsevier, Amsterdam, pp. 77–109. Sarbatly, R., Chiam, C.-K., 2013. Evaluation of geothermal energy in desalination by vacuum membrane distillation. Appl. Energy 112, 737–746. Sharon, H., Reddy, K., 2015. A review of solar energy driven desalination technologies. Renew. Sust. Energy Rev. 41, 1080–1118. Shatat, M., Worall, M., Riffat, S., 2013. Opportunities for solar water desalination worldwide: review. Sust. Cities Soc. 9, 67–80. Shim, S., Lee, J., Kim, W., 2014. Performance simulation of a multi-VMD desalination process including the recycle flow. Desalination 338, 39–48. Shim, W.G., He, K., Gray, S., Moon, I.S., 2015. Solar energy assisted direct contact membrane distillation (DCMD) process for seawater desalination. Sep. Purif. Technol. 143, 94–104. Su, M., Teoh, M.M., Wang, K.Y., Su, J., Chung, T.-S., 2010. Effect of inner-layer thermal conductivity on flux enhancement of dual-layer hollow fiber membranes in direct contact membrane distillation. J. Membr. Sci. 364, 278–289. Sua´rez, F., Tyler, S.W., Childress, A.E., 2010. A theoretical study of a direct contact membrane distillation system coupled to a salt-gradient solar pond for terminal lakes reclamation. Water Res. 44, 4601–4615. Sua´rez, F., Ruskowitz, J.A., Tyler, S.W., Childress, A.E., 2015. Renewable water: direct contact membrane distillation coupled with solar ponds. Appl. Energy 158, 532–539. Subramani, A., Jacangelo, J.G., 2015. Emerging desalination technologies for water treatment: a critical review. Water Res. 75, 164–187. Sukitpaneenit, P., Chung, T.-S., 2009. Molecular elucidation of morphology and mechanical properties of PVDF hollow fiber membranes from aspects of phase inversion, crystallization and rheology. J. Membr. Sci. 340, 192–205. Summers, E., 2013. Cycle performance of multi-stage vacuum membrane distillation (MS-VMD) systems. Proceedings of the 2013 IDA World on Desalination and Water Reuse for Water Treatment is held at Tianjin, China. Swaminathan, J., Chung, H.W., Warsinger, D.M., 2016. Simple method for balancing direct contact membrane distillation. Desalination 383, 53–59. Teoh, M.M., Chung, T.-S., Yeo, Y.S., 2011. Dual-layer PVDF/PTFE composite hollow fibers with a thin macrovoidfree selective layer for water production via membrane distillation. Chem. Eng. J. 171, 684–691. Thomas, K.E., 1997. Overview of Villag’, Cale, Renewable Energy we red Desalination. Tijing, L.D., Choi, J.-S., Lee, S., Kim, S.-H., Shon, H.K., 2014. Recent progress of membrane distillation using electrospun nanofibrous membrane. J. Membr. Sci. 453, 435–462.

Water Treatment by Renewable Energy-Driven Membrane Distillation 211 Tijing, L.D., Woo, Y.C., Shim, W.-G., He, T., Choi, J.-S., Kim, S.-H., Shon, H.K., 2016. Superhydrophobic nanofiber membrane containing carbon nanotubes for high-performance direct contact membrane distillation. J. Membr. Sci. 502, 158–170. Tripathi, L., Mishra, A., Dubey, A.K., Tripathi, C., Baredar, P., 2016. Renewable energy: an overview on its contribution in current energy scenario of India. Renew. Sust. Energy Rev. 60, 226–233. Wang, P., Chung, T.-S., 2015. Recent advances in membrane distillation processes: membrane development, configuration design and application exploring. J. Membr. Sci. 474, 39–56. Wang, X., Zhang, L., Yang, H., Chen, H., 2009. Feasibility research of potable water production via solar-heated hollow fiber membrane distillation system. Desalination 247, 403–411. Wang, P., Teoh, M.M., Chung, T.-S., 2011. Morphological architecture of dual-layer hollow fiber for membrane distillation with higher desalination performance. Water Res. 45, 5489–5500. Warsinger, D.E., Swaminathan, J., Maswadeh, L.A., 2015a. Superhydrophobic condenser surfaces for air gap membrane distillation. J. Membr. Sci. 492, 578–587. Warsinger, D.M., Swaminathan, J., Guillen-Burrieza, E., Arafat, H.A., 2015b. Scaling and fouling in membrane distillation for desalination applications: a review. Desalination 356, 294–313. Wei, X., Zhao, B., Li, X.-M., Wang, Z., He, B.-Q., He, T., Jiang, B., 2012. CF 4 plasma surface modification of asymmetric hydrophilic polyethersulfone membranes for direct contact membrane distillation. J. Membr. Sci. 407, 164–175. Weyl, P.K., 1967. Recovery of demineralized water from saline waters. Google Patents. Winter, D., Koschikowski, J., Wieghaus, M., 2011. Desalination using membrane distillation: experimental studies on full scale spiral wound modules. J. Membr. Sci. 375, 104–112. Winter, D., Koschikowski, J., Gross, F., Maucher, D., D€ uver, D., Jositz, M., Mann, T., Hagedorn, A., 2017. Comparative analysis of full-scale membrane distillation contactors-methods and modules. J. Membr. Sci. 524, 758–771. Woo, Y.C., Tijing, L.D., Shim, W.-G., Choi, J.-S., Kim, S.-H., He, T., Drioli, E., Shon, H.K., 2016. Water desalination using graphene-enhanced electrospun nanofiber membrane via air gap membrane distillation. J. Membr. Sci. 520, 99–110. Woo, Y.C., Chen, Y., Tijing, L.D., Phuntsho, S., He, T., Choi, J.-S., Kim, S.-H., Shon, H.K., 2017. CF 4 plasmamodified omniphobic electrospun nanofiber membrane for produced water brine treatment by membrane distillation. J. Membr. Sci. 529, 234–242. Zaragoza, G., Ruiz-Aguirre, A., Guill
en-Burrieza, E., 2014. Efficiency in the use of solar thermal energy of small membrane desalination systems for decentralized water production. Appl. Energy 130, 491–499. Zhao, K., Heinzl, W., Wenzel, M., B€uttner, S., Bollen, F., Lange, G., Heinzl, S., Sarda, N., 2013. Experimental study of the memsys vacuum-multi-effect-membrane-distillation (V-MEMD) module. Desalination 323, 150–160. Zhou, T., Yao, Y., Xiang, R., Wu, Y., 2014. Formation and characterization of polytetrafluoroethylene nanofiber membranes for vacuum membrane distillation. J. Membr. Sci. 453, 402–408. Zwijnenberg, H., Koops, G., Wessling, M., 2005. Solar driven membrane pervaporation for desalination processes. J. Membr. Sci. 250, 235–246.

Further Reading Cheng, L.-H., Lin, Y.-H., Chen, J., 2011. Enhanced air gap membrane desalination by novel finned tubular membrane modules. J. Membr. Sci. 378, 398–406. Li, Y.-L., Tung, K.-L., Lu, M.-Y., Huang, S.-H., 2009. Mitigating the curvature effect of the spacer-filled channel in a spiral-wound membrane module. J. Membr. Sci. 329, 106–118. Mat, N.C., Lou, Y., Lipscomb, G.G., 2014. Hollow fiber membrane modules. Curr. Opin. Chem. Eng. 4, 18–24. Pal, P., Sikder, J., Roy, S., Giorno, L., 2009. Process intensification in lactic acid production: a review of membrane based processes. Chem. Eng. Process. Process. Intensif. 48, 1549–1559.