Comparison between Forward Osmosis-Reverse Osmosis and Reverse Osmosis processes for seawater desalination

Comparison between Forward Osmosis-Reverse Osmosis and Reverse Osmosis processes for seawater desalination

Desalination 336 (2014) 50–57 Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Comparison bet...

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Desalination 336 (2014) 50–57

Contents lists available at ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

Comparison between Forward Osmosis-Reverse Osmosis and Reverse Osmosis processes for seawater desalination Ali Altaee a,1, Guillermo Zaragoza b, H. Rost van Tonningen c a b c

Faculty of Engineering and Physical Sciences, University of West of Scotland, Paisley PA1 2BE, UK CIEMAT-Plataforma Solar de Almería, Ctra. de Senés s/n, 04200 Tabernas, Almería, Spain Malmok Vision, Waterschapslaan 15, 1261JR Blaricum, The Netherlands

H I G H L I G H T S • • • • •

RO–FO and RO seawater desalination processes were compared using developed software. Total power consumption (Est) in FO–RO was higher than that in RO. Water flux in the RO process was higher than in the FO–RO process. Power consumption in FO process was 2%–4% of total power consumption in the FO–RO. The efficiency of FO–RO process was higher at higher seawater salinities.

a r t i c l e

i n f o

Article history: Received 26 October 2013 Received in revised form 12 December 2013 Accepted 2 January 2014 Available online 22 January 2014 Keywords: Forward Osmosis Reverse Osmosis Desalination FO energy consumption FO-RO process

a b s t r a c t The combination of Forward Osmosis (FO) and Reverse Osmosis (RO) was evaluated for seawater desalination. RO process was suggested for the draw solution regeneration because of its high efficiency and applicability for a wide range of ionic solution treatments. Two different salts, NaCl and MgCl2, were used as a draw solution. The performance of FO and RO regeneration processes was simulated using pre-developed software. A comparison between the RO and FO-RO processes was carried out. The simulation results showed that the total power consumption in the RO was lower than that in the FO-RO process. But, the difference in total power consumption between the RO and 0.65 mol MgCl2 FO-RO processes was insignificant. The results also showed that the power consumption in the FO process was only 2%-4% of the total power consumption in the FO-RO process. However, the difference in total power consumption between the RO process and the FO-RO process decreased with a higher seawater salinity. In the FO-RO process, the results showed that the permeate TDS was increased with increasing the concentration of draw solution. The lowest permeate TDS was achieved in the 0.65 mol MgCl2 FO-RO process and it was attributed to the high rejection rate of MgCl2 by the RO regeneration unit. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Membrane-based seawater desalination processes are one of the practical solutions for fresh water supply in arid and semi-arid areas [1–6]. A wide range of membranes were developed for the treatment of seawater and fresh water production from feed water of different salinities. Nowadays, the most popular membrane processes for saline water treatment are Reverse Osmosis (RO), Nanofiltration (NF), and Membrane Distillation (MD) [2,5.7,8]. Dual stage NF process was suggested for seawater desalination but it required a very exacting method for membrane operation [5]. Instead dual stage NF-BWRO process was proposed for seawater desalination to overcome the operating complexity in the dual stage NF process. MD has the potential to reduce

1

E-mail address: [email protected] (A. Altaee). Tel.: +44 7986517994.

0011-9164/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2014.01.002

the power consumption for seawater desalination because it does not require high pressure for membrane operation [9]. However, the low recovery rate and high thermal consumption make the MD process less attractive for seawater treatment in large desalination plants [10]. Unlike the other technologies, RO process enjoys a number of advantages which make it an attractive technology for seawater desalination because of its reliability, high water recovery rate and salt rejection rate, and its ability to treat a wide range of seawater concentrations [1,11]. At present, more than 50% of the world's desalination water is produced by RO process. Additionally, the RO membranes have found application in wastewater reuse and the production of ultra-pure water [12,13]. Although RO process has a number of advantages, the high power consumption is the process's main disadvantage. With the Energy Recovery Instrument (ERI), an average of 3.5 kWh/m3 is required for seawater desalination (seawater TDS 35,000 mg/L) [14]. Indeed, reducing power consumption in the process of reverse osmosis was the objective of many research studies [5,9,15].

A. Altaee et al. / Desalination 336 (2014) 50–57

51

Fig. 1. RO and FO–RO processes diagram.

With the emergence of Forward Osmosis technology, scientists conceived the idea that the cost of desalination could be reduced. However, the cost of desalination by FO process is affected by a number of factors such as type of FO membrane, concentration of the draw solution, type of draw solution, and the regeneration process [16,17]. Elimelech and co-workers proposed using ammonium carbon dioxide as a draw solution for seawater desalination [17,18]. The MD process, then, was used for the regeneration of the draw solution because of the lower evaporation temperature of ammonium carbon dioxide compared to water. The impact of concentration polarization on the efficiency of the FO was investigated and found to be more serious when the draw solution was facing the support layer while the feed solution is facing the membrane active layer [19]. Abdulsalam and Adel suggested a two-stage seawater desalination process using FO process in the first stage and NF process in the second stage [20]. In this case, multivalent chemical compounds such as MgCl2, Na2SO4, or MgSO4 were proposed as the draw solution due to their high rejection by the NF membranes. Shung and coworkers used magnetic nano-particles coated with hydrophilic polymers as a draw solution in the FO process. Although the magnetic nano-particles exhibited high osmotic pressure, regeneration was a problem due to the agglomeration of nano-particles [21]. Hydrogel polymers were proposed as a draw solution in the FO process because of their high osmotic pressure. Water flux across the FO membrane increased when carbon nano-particles were added but the excessive addition of carbon nano-particles resulted in a flux reduction [22]. It should be noted here that the cost of draw solution must be added to the total cost of the desalination. Regeneration is the most expensive stage in the FO process for seawater desalination regardless of the type of draw solution used and hence it will determine the overall cost of the desalinated water. Most of the previous studies were focused on the evaluation and optimization of the FO process through the membrane, while little attention was paid to the performance of the entire desalination system which includes the FO and the regeneration processes. In principle, FO only produces a concentrated solution which requires further treatment before it can be used for human applications. Fresh water is extracted from the draw solution in the regeneration process, which has been

identified as the most expensive stage in the FO desalination. The specific power consumption in the regeneration process should be added to the total power consumption of seawater desalination by FO. In the current study, RO was chosen for the regeneration of draw solution because of its high efficiency and suitability to treat different types of draw solutions. A comparison between the RO and RO-FO systems was carried out using developed RO and FO software models [23,24]. Reverse Osmosis System Analysis (ROSA6.1) was used to model the RO process. The effect of seawater TDS on the RO and FO-RO processes was evaluated. Typically, the recovery rate in RO does not exceed 50% for low salinity seawater because of the scaling problems. However, this is not an issue in FO because of the high purity of the draw solution. Thus, the recovery rate of the RO in the FO-RO process can be increased over 50%. NaCl and MgCl2 were used as draw solutions because of their high solubility in water, high osmotic pressure, and high rejection by RO membranes. 2. Methodology FO seawater desalination is a multistage process. In the first stage seawater is treated by the FO process and generates a diluted draw solution while in the second stage fresh water is extracted from the draw solution in the regeneration process. In the current study, the performance of FO process was estimated from a developed model to

Table 1 Seawater composition. SW TDS (mg/L)

32,000 35,000 36,000 38,000 40,000 45,000

Ion concentration mg/L K

Na

Mg

Ca

HCO3

Cl

SO4

SiO2

354 387 398 419 441 496

9854 10,778 11,086 11,663 12,278 13,812

1182 1293 1330 1399 1473 1657

385 421 433 456 480 539

130 142 146 154 162 182

17,742 19,406 19,960 20,999 22,105 24,868

2477 2710 2787 2932 3086 3472

0.9 1.0 1.0 1.0 1.1 1.2

52

A. Altaee et al. / Desalination 336 (2014) 50–57

70

%Re (maximum)

60 50 40 30 20 NaCl-1 Mol NaCl-1.2 Mol MgCl2-0.65 Mol RO

10 0 30000

32000

34000

36000

38000

40000

42000

44000

46000

Seawater TDS (mg/L) Fig. 2. Maximum recovery rate at different seawater salinities for RO and FO–RO systems.

calculate water and salt flux, Jw and Js respectively, in the FO membrane from the following equations [24]:

desalination. In RO seawater is the feed water and water flux through the membrane is calculated from the following equation:

  Jw ¼ Aw πDS−ave −π f −ave

ð1Þ

Jw ¼ Aw ðP−πÞ

  Js ¼ B C f −ave −Cp :

ð2Þ

Where P is the hydraulic feed pressure (bar) and Π is the osmotic pressure of seawater (bar). Permeate concentration in RO is calculated from the following equation:

In Eq. (1), Aw is the coefficient of membrane permeability (L/m2 h·bar), ΠDS-ave is the average osmotic pressure of draw solution to the FO (bar), and Πf-ave is the average osmotic pressure of seawater feed to the FO (bar). In Eq. (2), B is the salt permeability coefficient (kg/m2h), Cf-ave is the average concentration of seawater feed to the FO (mg/L), and Cp is the permeate concentration (mg/L). The latter parameter was calculated from the following equation [24]: BC f −in Jw þ B

ð3Þ

where Cf-in is the concentration of seawater feed to the FO (mg/L). Fig. 1 shows a schematic diagram of the RO and FO-RO processes for seawater

Am : Qp

In Eq. (5), Rj is the rejection rate of the RO membranes. It is should be noted that the concentration polarization effect in the FO membrane was considered in the simulation results. The recovery rate of the FO process was assumed to be affected by the concentration polarization in the membrane. In the FO-RO process, draw solution and seawater are pumped into the FO membrane in a countercurrent flow mode. Fresh water will

1400

8

1200

7 6

1000

Cp (mg/L)

ð5Þ

5 800

Cp-NaCl 1 Mol

4

Cp-NaCl 1.2 Mol

600

Cp-MgCl2 0.65 Mol

3

Cp-Ro

400

Es-NaCl 1 Mol

2

Es-NaCl 1.2 Mol

200

Es-MgCls 0.65 Mol

1

Es-RO

0 1500

2000

2500

3000

3500

4000

4500

Seawater TDS (mg/L) Fig. 3. Power consumption and permeate TDS at different seawater salinities (Recovery rate 32%).

0 5000

Es (kWh/m3)

Cp ¼

CP ¼ B  C fc  CP  Rj 

ð4Þ

A. Altaee et al. / Desalination 336 (2014) 50–57

cross the FO membrane from the feed to the draw solution side of the membrane in the direction of the osmotic pressure gradient. After leaving the FO membrane, the diluted draw solution is fed into the RO membrane system for fresh water extraction and osmotic agent recycling and reuse. In the FO-RO system, the recovery rate in RO and FO should be equal to maintain a steady state process. Therefore, the flow rate of the draw solution is the same of the RO concentrate (Fig. 1). The mass balance of the FO–RO process can be written as: Q sw‐in  Re þ Q ds−in ¼ Q ds−out

ð6Þ

where Qsw-in ∗ Re = Qp, and Re is the recovery rate in the RO system (the same as in the FO system, as explained above). NaCl and MgCl2 were used as draw solutions to produce an osmotic pressure equivalent to 46 bar. The specific power consumption, Es-RO, in RO is calculated from the following equation: P f  Q sw‐in 36  η  Q p

ð7Þ

where Pf is the feed pressure (bar), and η is the pump efficiency. In the FO process the specific power consumption, Es-FO, is calculated from the following equation [25]: Es‐FO ¼ 

1 36  η  Q p

 ðP f Q sw‐in þ Pds Q ds−in Þ

ð8Þ

where Pds is the draw solution feed pressure (bar). However, in the FO-RO system the total specific power consumption, Est, is the sum of Es-RO and Es-FO as in the following equation: Est ¼ Es−RO þ Es−FO :

ð9Þ

Eqs. (7) and (9) were used to compare the total power consumption between the RO and FO-RO systems respectively. As mentioned earlier here, only two salts were evaluated in this study as draw solutions; i.e. NaCl and MgCl2, because of their: i) wide availability; ii) high osmotic pressure; iii) high rejection by RO membranes; and (iv) high solubility

in water. Different seawater salinities were evaluated in the present study. Table 1 shows the salt concentration and composition of seawater under investigation here [26]. The Filmtec RO membrane SW30HRLE-400i was used in the RO process and in the regeneration of the osmotic agent in the FO-RO process because of its high rejection rate and reasonable permeate flow rate. An HTI cellulose acetate membrane was considered for the FO system in this study [25]. It should be mentioned here that the feed and draw solution pressure in the FO process was assumed to be 1 bar and the pump efficiency, η, to be 0.8. 3. Results and discussions The maximum recovery rates of the RO and FO-RO systems at different seawater salinities are shown in Fig. 2. The highest recovery rate was achieved in the FO-RO process when 1.2 mol NaCl was used as a draw solution. However, this was achieved on the cost of higher power consumption (Fig. 3). For example, at 45,000 mg/L seawater TDS the specific power consumption was 5.04, 5.67, 5.58 and 5.45 kWh/m3 in RO, 1 mol NaCl, 1.2 mol NaCl and 0.65 mol MgCl2 FO-RO processes respectively. Considering all seawater salinities, the RO process exhibited the lowest power consumption in general, followed by 0.65 mol MgCl2, 1 mol NaCl and 1.2 mol NaCl FO-RO processes respectively (Fig. 3). The impact of seawater salinity on the permeate water TDS was also investigated. Fig. 3 shows that the 0.65 mol MgCl2 FO-RO process was able to produce the highest permeate water quality in terms of TDS concentration. This was mainly due to the higher rejection rate of MgCl2 by the RO membranes. In the RO process, the permeate TDS increased with a higher seawater salinity. Nevertheless, the permeate TDS from the RO-FO process was not affected by the seawater salinity. Logically, this was due to the high rejection rate of FO membranes to MgCl2 and NaCl, which means that the concentration of draw solution in and out of the FO membrane was almost the same for all seawater salinities (32% recovery rate) [27]. It is worth mentioning here that the recovery rate in the RO process is controlled by the scale formation and deposition on the membrane. To avoid scaling problems in the RO process, the typical recovery rate for seawater salinity 35,000 mg/L does not exceed 50%. On the contrary, the recovery rate in the FO-RO process can be increased over 50% due to the high purity of the draw solution preventing scale formation. However, one of the limiting factors to reach high recovery rates in the FO-RO process is the concentration of the draw solution. As shown in Fig. 4, it is directly related to the

70000

10 9

60000 8 50000

Jw (l/m2h)

7 6

40000

5 30000

4 3

20000

Jw-NaCl/MgCl2-Sw 35 mg/L Jw-RO-SW 35 mg/L

2

NaCl-1 Mol

1

10000

NaCl-1.2 Mol

0 10

20

30

40

50

60

0 70

%Re Fig. 4. Recovery rate and draw solution at different FO recovery rates (seawater TDS 35,000 mg/L).

DS out Concentration (mg/L)

Es‐RO ¼

53

54

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concentration of the feed solution in the regeneration unit. Increasing the concentration of draw solution also results in a higher permeate TDS (Fig. 3). Finally, the simulation results showed that the higher membrane flux rate, Jw, was achieved in the RO process (Fig. 4). Generally, membrane flux in the RO process was almost 20% higher than that in the FO-RO process (Fig. 4).

The energy requirements for seawater desalination in the RO process and in the regeneration unit of the FO-RO process were estimated using Eq. (7) (Fig. 5). It should be mentioned here that Fig. 5 compares only the specific power consumption of the RO regeneration unit in the FO-RO process with that of the RO process. For the range of seawater salinities investigated in this study, Es decreased with increasing the

12

10

Es (kWh/m3)

NaCl-1.2 Mol NaCl-1 Mol MgCl2-0.65 Mol RO

A

8

6

4

2

0 0

10

20

30

40

50

60

70

%Re 12 NaCl-1.2 Mol NaCl-1 Mol MgCl2-0.65 Mol RO

B

Es (kWh/m3)

10 8 6 4 2 0 0

10

20

30

40

50

60

70

%Re 12 NaCl-1.2 Mol NaCl-1 Mol MgCl2-0.65 Mol RO

C

Es (kWh/m3)

10

8

6

4

2

0 0

10

20

30

40

50

60

70

%Re Fig. 5. Recovery rate of RO and FO-RO systems at different seawater salinities; (a) 32,000 mg/L, (b) 38,000 mg/L, (c) 45,000 mg/L.

A. Altaee et al. / Desalination 336 (2014) 50–57

55

10.0 FO only FO-RO-1.2 Mol NaCl FO-RO-1 Mol NaCl FO-RO-0.65 Mol MgCl2 %Es FO-1.2 Mol NaCl %Es FO-1 Mol NaCl %Es FO-0.65 Mol MgCl2

Est (kWh/m3)

10 8 6

9.0 8.0 7.0 6.0 5.0 4.0

4

%Es-FO

12

3.0 2.0

2

1.0 0 0

10

20

30

%Re

40

50

60

0.0 70

Fig. 6. Es in RO and FO–RO systems at different recovery rates (seawater TDS 32,000 mg/L).

RO/FO-RO process recovery rate which is in agreement with the results reported in previous studies [28]. At 32,000 mg/L seawater TDS, the specific power consumption in the RO process was lower than that in the FO-RO process (Fig. 5a). As far as the FO-RO process is concerned, the lowest power consumption was achieved in 0.65 mol MgCl2 draw solution followed by 1 mol and 1.2 mol NaCl draw solutions respectively. This was due to the fact that the concentration of feed solution going to the RO regeneration unit was the highest in the case of the 1.2 mol NaCl draw solution. The simulation results also showed that the Es difference between the RO and FO-RO processes decreased by increasing the recovery rate. This observation was noticed in all seawater salinities and it was attributed to the higher flow rate required in the RO process compared to the FO-RO process. In the RO process, the feed flow rate, as recommended by the membrane manufacturing companies, increases with increasing the Silt Density Index (SDI) of the feed solution to alleviate membrane fouling problems. The feed SDI values were 1 and 3 for the FO-RO and RO processes respectively due to the difference in the feed solution quality. According to Eq. (7), Es increases with increasing the feed flow rate and hence it was higher in the RO process when surface water was considered (assume conventional pretreatment; SDI ~ 3). As can be seen in Fig. 5, the recovery rate in the FO-RO process could reach higher values than in the RO process. Practically, the recovery rate of the RO process cannot be increased over 50% due to the fouling and scaling problems. However, in the FO-RO process a higher recovery rate can be achieved due to the high purity of the feed solution. Interestingly, at 35% to 45% recovery rates the difference in power consumption between the RO and the 0.65 mol MgCl2 FO-RO process was negligible for the high salinities (Fig. 5b and c). In general, the simulation results indicate that the FO-RO process seems to be more efficient at high seawater salinities because the difference in the Es between the RO and the FO-RO process becomes insignificant. Accordingly, the FO-RO process is more suitable for the treatment of high salinity waters. The type and concentration of the draw solution can also significantly affect the performance of the FO-RO process. For example the concentration of feed solution to the regeneration unit increases with the concentration of draw solution hence the cost of desalination will be increased (Figs. 4 and 5). Fig. 6 shows the total power consumption, Est, in the FO-RO process. The simulation results show that the highest total power consumption was in the case of the 1.2 mol NaCl draw solution. However, the specific power consumption in the FO process alone was only a small percentage of the total specific power consumption; Est. As a matter of fact, the power consumption in the FO process was only 2% to 4% of the total power consumption in the FO-RO process (Fig. 6). The simulation results showed that the power consumption in the FO process was

decreased linearly with increasing the recovery rate of FO process. The percentage of power consumption in the FO was in the following order; 0.65 mol MgCl2 N 1 mol NaCl N 1.2 mol NaCl draw solution (Fig. 6). The effect of recovery rate on the permeate TDS was evaluated for a number of seawater salinities varying from 32,000 mg/L to 45,000 mg/L (Fig. 7). The permeate TDS decreased with increasing the recovery rate of the RO/FO-RO process due to the higher dilution factor. In the FO-RO process, the permeate TDS increased with increasing the concentration of draw solution from 0.65 mol MgCl2 to 1.2 mol NaCl. As a matter of fact the lowest permeate TDS was achieved in the 0.65 mol MgCl2 FORO process. This was due to the higher rejection of MgCl2 by the RO membrane compared to NaCl and/or seawater. Unlike the RO process, the simulation results also showed that the permeate TDS in the FORO process was not significantly affected by the increase of seawater salinity (Fig. 7a, b, c). As mentioned before, for a given recovery rate, the high rejection rate of the FO membrane maintained the concentration of draw solution in and out of the FO membrane almost the same for all feed salinities. Unlike the FO-RO process, the permeate TDS in the RO process increased with increasing seawater salinity (Fig. 7). The permeate TDS in RO process was lower than that in the 1.2 mol and 1 mol NaCl FO-RO process but higher than that in the 0.65 mol MgCl2 FO-RO process (Fig. 7). As the seawater salinity increased from 32,000 mg/L to 45,000 mg/L, the difference between the permeate TDS in the RO and 0.65 mol MgCl2 FO-RO process increased as well. Compared to the RO process, the simulation results show that the efficiency of the FO-RO process increased with increasing the seawater salinity (Figs. 5 and 7). The type and concentration of the draw solution should also be carefully selected in order to achieve a competitive FO-RO desalination process. Finally the concentration of Mg (mg/L) in the permeate solution was investigated at different RO recovery rates (Fig. 8). The concentration of Mg in permeate solution is particularly important in the FO-RO system when MgCl2 was used as a draw solution. High Mg in the permeate water has a negative impact on human health. The results show that the permeate concentration of Mg decreased with increasing the system recovery rate. However, there was not a significant difference in the concentration of Mg when seawater TDS increased from 35,000 mg/L to 38,000 mg/L due to the high rejection rate by the RO membranes. 4. Conclusions The current study evaluated the feasibility of FO-RO process for seawater desalination and compared to the RO desalination process. Two software packages were applied to simulate the performance of FO

56

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2500

NaCl-1.2 Mol NaCl-1 Mol MgCl2-0.65 Mol RO

A

Cp (mg/L)

2000

1500

1000

500

0 0

10

20

30

40

50

60

70

%Re 2500

NaCl-1.2 Mol NaCl-1 Mol MgCl2-0.65 Mol RO

B

Cp (mg/L)

2000

1500

1000

500

0 0

10

20

30

40

50

60

70

%Re 2500

NaCl-1.2 Mol NaCl-1 Mol MgCl2-0.65 Mol RO

C

Cp (mg/L)

2000

1500

1000

500

0

0

10

20

30

40

50

60

70

%Re Fig. 7. Permeate concentration vs recovery rate at different seawater salinities; (a) 32,000 mg/L, (b) 38,000 mg/L, (c) 45,000 mg/L.

and RO processes. The following key points were concluded from the following study: 1. The total power consumption, Est, for seawater desalination was higher in the FO-RO process than in the RO process. However, the difference in Est between the FO-RO and RO processes decreased with increasing the seawater salinity. 2. The maximum recovery rate in the FO-RO can be increased with increasing the concentration of the draw solution but will be at the

expense of higher power consumption. In general, higher recovery rates can be reached in the FO-RO process than in the RO process. 3. The membrane water flux in the RO process was higher than in the FO-RO process. A higher water flux can be achieved with higher recovery rates. 4. The permeate TDS increased with increasing the concentration of the draw solution. However, the lowest permeate TDS was achieved in the 0.65 mol MgCl2 FO-RO process followed by the RO process, 1 mol NaCl FO-RO and 1.2 mol NaCl FO-RO processes respectively.

A. Altaee et al. / Desalination 336 (2014) 50–57

57

160 150 140

Mg (mg/L)

130 120 110 100 90 80

SW 35 g/L

70

SW 38 g/L

60 20

25

30

35 40 Recovery rate (%)

45

50

55

Fig. 8. Mg concentration in the permeate solution at different recovery rate (FO–RO system, draw solution 0.65 M MgCl2).

5. The power consumption in the FO process alone was only a small percentage (2% to 4%) of the total power consumption in the FO-RO process. Therefore, most of the power consumption in the FO-RO process was realized in the RO regeneration unit. 6. Although increasing the concentration of draw solution allows reaching a higher recovery rate, the cost of desalination and power consumption tends to increase with increasing the draw solution concentration. 7. The simulation results showed that the efficiency of the FO-RO process was higher at higher seawater salinities. Depending on the type and concentration of the draw solution, the FO-RO process can be highly competitive to the RO process, especially at high feed salinities. Therefore, the selection of draw solution is crucial in the FO-RO process. 8. It should be noted here that the simulation results represent the performance of static systems in which the power consumption in the RO generation unit was higher than that in the RO system. As matter of fact these results did not take into account fouling problems in the RO system which highly affects its performance and power consumption. Acknowledgement The University of West of Scotland would like to thank the Carnegie Trust fund which helped to accomplish this study. References [1] Baltasar Peñate, Lourdes García-Rodríguez, Current trends and future prospects in the design of seawater reverse osmosis desalination technology, Desalination 284 (2012) 1–8. [2] N. Misdan, W.J. Lau, A.F. Ismail, Seawater Reverse Osmosis (SWRO) desalination by thin-film composite membrane — current development, challenges and future prospects, Desalination 287 (2012) 228–237. [3] Sulaiman Al-Obaidani, Efrem Curcio, Francesca Macedonio, Gianluca Di Profio, Hilal Al-Hinai, Enrico Drioli, Potential of membrane distillation in seawater desalination: thermal efficiency, sensitivity study and cost estimation, J. Membr. Sci. 323 (2008) 85–98. [4] Samer Adham, Altaf Hussain, Joel Minier Matar, Raul Dores, Arnold Janson, Application of Membrane Distillation for desalting brines from thermal desalination plants, Desalination 314 (2013) 101–108. [5] Ali AlTaee, Adel O. Sharif, Alternative design to dual stage NF seawater desalination using high rejection brackish water membranes, Desalination 273 (2011) 391–397. [6] Majed M. Alhazmy, Multi stage flash desalination plant with brine–feed mixing and cooling, Energy 36 (2011) 5225–5232. [7] Sundarrajan Subramanian, Ramakrishna Seeram, New directions in nanofiltration applications — are nanofibers the right materials as membranes in desalination? Desalination 308 (2013) 198–208.

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