Evaluation of FO-RO and PRO-RO designs for power generation and seawater desalination using impaired water feeds

Desalination xxx (2014) xxx–xxx

DES-12176; No of Pages 9

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Desalination

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Evaluation of FO-RO and PRO-RO designs for power generation and seawaterdesalination using impaired water feeds

Ali Altaee a,1, Adel Sharif b, Guillermo Zaragoza c, Ahmad Fauzi Ismail d

a Faculty of Engineering and Physical Science, University of the West of Scotland, Paisley PA1 2BE, UKb Qatar Energy and Environment Research Institute, The Qatar Foundation, Qatarc Ciemat, Plataforma Solar de Almería, Ctra. de Senés s/n, 04200 Tabernas, Almería, Spaind Advanced Membrane Technology Research Center (AMTEC), Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia

H I G H L I G H T S

• Different PRO-RO, RO-PRO and FO-RO design configurations were evaluated.• The salinity gradient resource was seawater and impaired water as DS and FS respectively.• Different feed concentrations from 0.2–10 g/L were used while SW concentrations was 35 g/L.• PRO-RO requires highest FO membrane area while RO-PRO requires the least FO membrane area.• FO-RO design provides the best product water quality and average FO membrane cost.

E-mail address: [email protected] (A. Altaee).1 Tel. +44 798651799.

http://dx.doi.org/10.1016/j.desal.2014.06.0220011-9164/Crown Copyright © 2014 Published by Elsevie

Please cite this article as: A. Altaee, et al., Eimpaired water feeds, Desalination (2014), h

a b s t r a c t
a r t i c l e i n f o
Article history:Received 18 May 2014Received in revised form 10 June 2014Accepted 13 June 2014Available online xxxx

Keywords:Forward osmosisPressure Retarded OsmosisSeawater desalinationReverse osmosisOsmotic energy

PRO and FO coupling with an ROmembrane process is proposed to reduce the cost of seawater desalination andthe potential for power generation. Three conceptual design configurations, PRO-RO, FO-RO and RO-PRO wereevaluated here using standard seawater concentration and impairedwater as the draw and the feed solutions re-spectively. The PRO-RO and RO-PRO designs were evaluated for power generation and seawater desalinationwhile the FO-RO design was proposed for seawater desalination only. The impact of the draw and feed solutions'flow rate and the impaired water TDS on the performance of each design was estimated using pre-developedsoftware. The simulation results showed that the performance of all designs was more sensitive to the increasein theflow rate of draw solution than to theflow rate of feed solution. Furthermore, all designs showed a decreaseinmembrane flux and recovery ratewith increasing the TDS of feedwater from 0.2 g/L to 10 g/L as a result of de-creasing the net driving force across the membrane and the concentration polarization phenomenon. The FO-ROdesign produced the lowest ROpermeate concentration followed by the PRO-RO and RO-PROdesigns respective-ly. In terms of power generation, the RO-PRO design was more efficient than the PRO-RO design. The FO-RO de-sign exhibited the lowest desalination power consumption followed by the PRO-RO and RO-PRO designsrespectively. At 10 g/L feed concentration, the net power consumption in the FO-RO was 9.4% less than that inthe PRO-ROwhichwas in turn 5.3% less than that in the RO-PROdesign. The estimated cost of the FO/PROmodulein the PRO-RO design was 2.2 and 4.3 times higher than that in the FO-RO and RO-PRO designs respectively.

Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.

1. Introduction

Energy and water sustainability is an important issue for the exis-tence of modern life in developed and semi-developed countries[1–3]. Unfortunately, the resources for water and energy supply becomescarce every day due to thepopulation increase, environmental pollutionand the rapid industrial growth. In fact, the growing concerns about thedepletion of these resources in the near future have encouraged

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valuation of FO-RO and PROttp://dx.doi.org/10.1016/j.de

scientists to find new resources for energy and water supply. Forwardosmosis (FO) and Pressure Retarded Osmosis (PRO) processes areemerging technologies which have the potential for fresh water and en-ergy supply at competitive costs [2–10]. These technologies are based onthe concept of osmotic pressure gradient applications across a semi-permeable membrane for power and freshwater supply. A proper salin-ity gradient resource, therefore, is required for the operation of FO andPRO processes. Several studies proposed that seawater and wastewatereffluent are the draw and feed solutions of the PRO process [7,9,10].Impaired water source such as brackish water, industrial wastewater,and wastewater effluent can be used as the low-salinity gradient (feed

-RO designs for power generation and seawater desalination usingsal.2014.06.022

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2 A. Altaee et al. / Desalination xxx (2014) xxx–xxx

solution) if a proper pretreatment is provided to prevent membranefouling. The typical TDS of brackish water is between 1000 mg/L and10,000 mg/L whereas domestic wastewater TDS is about 200 mg/L[13–16]. Mixing a low-salinity wastewater effluent with an average-salinity brackish or industrial wastewater is often performed to adjustthe influent TDS in a wastewater treatment plant [17–19].

In the FO process, the draw and feed solutions are passed through asemipermeable membrane to promote freshwater transport across themembrane from the low-concentration to the high-concentration solu-tion [7,11,12]. Water flux across the FOmembrane dilutes the draw so-lution, which can be further processed either for desalination or powergeneration. For seawater desalination, a suitable membrane or thermalprocess is required for fresh water extraction and draw solution regen-eration [7,11,12,16]. If the FO process is designed only for power gener-ation, the draw solution will be pressurized before entering the PROmodule. In the PRO module, the chemical potential is transferred to ahydraulic pressure as fresh water permeates across the membrane[3–6]. The power generation takes place in the turbine system whenthe pressurized draw solution passes through. In arid and semi-aridareas, the diluted draw solution can be further treated by an ROmembrane for fresh water supply instead of discharge to the seawater[7,9,10,20,21]. In fact, recent PRO designs suggested combining the PROmodule with an ROmembrane system in order to reduce the cost of sea-water desalination [19,20]. Combining PRO with RO reduces the cost ofRO feed pretreatment as well [20]. Several PRO-RO, RO-PRO and FO-ROhybrid designs have been investigated in the literature [9,20–23].These systems are not only able to reduce the cost of seawater desalina-tion but also able to generate a useful power by the PRO process. Achilliet al. proposed a hybrid RO-PRO system for seawater desalination andpower generation [8,23]. Seawater goes first to the RO for desalinationand fresh water production. The concentrated brine from the PRO isdepressurized to a suitable level by an Energy Recovery Device (ERD)before it enters the PRO module as the draw solution while the feedsolution is an impaired wastewater effluent. Fresh water permeationacross the membrane dilutes the concentrated brine and hence it canbe discharged safely to the sea. A pressure exchanger system is used toexchange the energy stored in the diluted draw solutionwith the seawa-ter RO feed. Researchers also suggested a two stage PRO-RO system forpower generation and seawater desalination [7,10]. The PRO salinity gra-dient resource is seawater and wastewater effluent as the high and lowconcentration solutions respectively. In the PRO module, the concentra-tion of seawater decreases by the permeation flow from the feedsolution. The diluted seawater flow splits into two parts after exitingthe membrane: the first part goes to a pressure exchanger to pressurizethe seawater PRO feedwhile the second part goes to a turbine system forpower generation. In the last RO stage, the diluted seawater is pressur-ized for freshwater extraction. Kim et al. suggested four PRO-RO andRO-PRO hybrids for power generation and seawater desalination usingdifferent types of feed and draw solutions [20]. In hybrids 1 and 2, theRO process takes place before the PRO in which the concentrated RObrine is used either as a draw solution or as a feed solution dependingon its concentration. While in hybrids 3 and 4, the PRO process takesplace before the RO in which the diluted seawater or the concentratedbrackish water solution is further treated for the fresh water extraction.Elimelech et al. introduced different RO-PRO and FO-PRO schemes forseawater desalination and power generation [11]. Low-quality water(such as wastewater effluent or brackish water) and seawater are usedas the salinity gradient resource. The concentrated RO/FO brine wasapplied as a draw solution in the PRO module. Altaee et al. proposedthe PRO-RO system for power generation and seawater treatment [22].In that study, the PROmodule worked as a pretreatment for the RO pro-cess and the diluted seawater from the PROwas used as the RO feed. Thestudy showed that the PRO-RO system was able to reduce the seawaterpower consumption below that required in the conventional RO system.

In the current study, three PRO and FO design configurations, PRO-RO,RO-PRO and FO-RO, were evaluated for power generation and/or

Please cite this article as: A. Altaee, et al., Evaluation of FO-RO and PROimpaired water feeds, Desalination (2014), http://dx.doi.org/10.1016/j.de

seawater desalination to understand the pros and cons of each design.Seawater and impaired solution were used as a salinity gradient resource(Fig. 1). The TDS of seawater is 35,000 mg/L, while the TDS of impairedwater is between 200 mg/L and 10,000 mg/L. Pre-developed softwarewas used to estimate the performance of FO and RO membranes [12,24]. In the case of PRO coupling with RO, the system efficiency was eval-uated based on the performance of the PRO and RO processes, i.e. powerdensity, PRO water flux, desalinated water quality and RO power con-sumption. Two designs, PRO-RO and RO-PRO, were investigated here.The PRO-ROdesign uses diluted seawater as the RO feed andhence its en-ergy requirements for seawater desalination are expected to be lowerthan that in the conventional RO design in which pretreated seawaterwas the feed of the RO membrane. Unlike the PRO-RO and RO-PRO de-signs, the purpose of the FO-RO design is mainly seawater desalinationand hence its efficiency was evaluated based on the performance of theFO and RO processes. It should be noted here that the performance ofthe ROprocess in the RO-PROdesign represents the baseline performanceof the conventional RO process for desalination in which pretreated rawseawater is the RO feed. Accordingly, a comparison can bemade betweenthe conventional RO process and RO process with FO/PRO pretreatment.For the salinity gradient resource investigated here, the FO-RO designhad the advantage of being a simple design and can produce freshwater at a lower cost than the RO process. In the current study, an ROmembrane was proposed for seawater desalination and/or draw solutionregeneration because of its high performance, high recovery rates andhigh rejection rate to solutes [24]. The impact of the feed and drawsolutions' flow rates on the PRO, FO and RO processes was evaluatedusing different feed salinities. For simplicity, the composition of brackishwater and wastewater effluent was assumed to be Na and Cl while thecomposition of standard seawater, 35,000 mg/L TDS, was taken fromthe previous literature [25]. Additionally, the cost of the FO membranesystem in the PRO-RO, RO-PRO and FO-ROdesignswas estimated to iden-tify the most cost-effective design knowing that the cost of the FO mem-brane is several times higher than that of the RO. Finally, the current studyhelps in the evaluation of different design configurations that have beenproposed for combined power generation and/or seawater desalination,namely PRO-RO, FO-RO and RO-PRO, and suggesting a proper design fora certain application based on the performance and the capital cost.

2. Methodology

Three main components can be identified in the PRO-RO and RO-PRO designs; the PRO module, turbine, and RO membrane. The PROmodule is responsible for fresh water extraction from the feed andtransferring the chemical potential into hydraulic pressure, the turbinesystem converts hydraulic energy to electricity, and the RO membraneis used for seawater desalination. Slightly different to the PRO-RO, thedraw solution of the PRO module in the RO-PRO design is the concen-trated RO brine (Fig. 1). The FO-RO design is more compacted thanthe other designs and it consists of an FO system and RO system. Bothseawater and impaired solution are fed into the FO membrane forfresh water extraction and seawater dilution while the RO membraneis responsible for desalinating the diluted seawater water. It is assumedhere that the PRO and FO modules use the same FO membrane in theirdesigns. Water flux in the FO and ROmembranes, Jw, is a function of themembrane permeability and the net driving pressure across the mem-brane. In the RO process the driving force is the difference betweenthe hydraulic and osmotic pressure of the feed solution, while in theFO process it is the osmotic pressure gradient between the draw andfeed solutions. It is also assumed here that the FO process requires noor negligible hydraulic pressure for its operation [26]. In general,water flux in the RO and FOprocesses can be estimated from the follow-ing equation:

Jw ¼ Aw � ΔP−Δπð Þ: ð1Þ



Pre-treatment

ROPermeate

Brine

Seawater

ImpairedWater

TurbinePressure exchanger

Concentrated feed

Pre-treatment

RO

Permeate

Turbine

Pressure exchanger

Concentrated feed

Seawater

ImpairedWater

PRO

PRO

Diluted draw

solution

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ROPermeate

BrineConcentrated feed FO

Seawater

ImpairedWater

PRO-RO

RO-PRO System

FO-RO System

VPRO0

VPRO1=VPRO0

V0

VPRO2

V0

VPRO0

VPRO1

V0

VPRO1=VPRO0VPRO0

VPRO2

VRO

VPRO0= draw solution flow rate in (L/h)VPRO1= draw solution flow rate out/recycled (L/h)VPRO2= flow rate to the turbine (L/h)VRO= flow rate to the RO (L/h)V0= permeate flow rate (L/h)

Fig. 1. PRO-RO, RO-PRO and FO-RO designs for seawater and impaired water processing.

3A. Altaee et al. / Desalination xxx (2014) xxx–xxx

In Eq. (1), Aw is the coefficient of membrane permeability(L/m2 h·bar), P is the feed pressure (bar) and π is the osmotic pressureon solution (bar). In this study the value of Aw was estimated to be0.79 L/m2 h·bar [24]. A Filmtech seawater membrane SW30HRLE-400i, with anAw of about 1.13 L/m2 h·bar, was applied for seawater de-salination in the RO system. The other important parameter in thefiltra-tion process is the membrane salt permeability, Js, which can beestimated from the following equation:

Js ¼ B C f−in−Cp

� �ð2Þ

where, Cf-in is the concentration of the feed solution (mg/L), B is thesalt permeability coefficient (kg/m2 h), and Cp is the permeate concen-tration (mg/L). The B factor in Eq. (3) can be estimated from the follow-ing equation:

B ¼ 1−Rjð Þ � JwRj

: ð3Þ

Membrane rejection, Rj, is the ratio of the permeate concentration tothe feed concentration; i.e. Rj= 1− (Cp ∕ Cf). The membrane rejectionrate increases with the membrane selectivity. In the PRO-RO and RO-PRO designs, the turbine system is usually positioned after the PROmodule (Fig. 1). The pressurized seawater flow is divided into twostreams after leaving the PRO module; stream one (VPRO1) returns to aPressure Exchanger (PX) to pressurize the seawater PRO feed (VPRO0)


while stream two (VPRO2) goes to the turbine for power generation. Inthe current study, 50% ROmembrane recovery rate and 95% PX efficien-cy are assumed. The specific power consumption, Es (power consump-tion per unit volume of product water), in the RO membrane isestimated from Eq. (4):

EsRO ¼ P f

ηRe: ð4Þ

Pf in Eq. (4) is the feed pressure (bar), Re is the RO recovery rate (%),and η is the pump efficiency, which is assumed 80% in this study. Powerdensity (W) in the PRO module is calculated from the water flux acrossthe FO membrane multiplied by the hydraulic pressure difference ac-cording to the following equation:

W ¼ Jw � ΔP: ð5Þ

It should bementioned here thatW is the power generated per unitmembrane area. In the current study, aΔP equals to 10 bar is applied onthe seawater side of the PROmodule. The osmotic pressure of the drawand feed solutions is estimated from the Van't Hoff equation [22]. Thepresent study will help engineers and researchers to select the properRO-FO hybrid design for a certain process application and estimatingthe cost of an FOmembrane system based on the currentmarket prices.



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FO-RO: 2g/L feedFO-RO: 10g/L feedPRO-RO: 2g/L feed

PRO-RO: 10g/L feedRO-PRO: 2g/L feedRO-PRO: 10g/L feedFO-RO: 0.2g/L feed

c

d

Fig. 2. The impact of feed flow rate on the FOmembrane performance; a) effect onmembrane flux, b) effect on osmotic pressure gradient, c) effect onmembrane %Re and d) effect on theconcentration of DS-out.

30

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Feed TDS (mg/L)

Bri

ne

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e*10

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g/L

)

FO-ROPRO-RORO-PRO

Fig. 3. Brine concentrate for discharge from different PRO and FO designs (Qf-in/Qds-in = 1).


3. Results and discussion

3.1. Impact of draw and feed solution flow rate on the design performance

The impact of feed flow rate on the performance of the FO and PROmodules is illustrated in Fig. 2. Seawater of 35 g/L concentration wasused as a draw solution in all system designs. The simulation resultsshowed a higher water flux at lower feed concentration which is dueto the higher osmotic pressure driving force across the membrane(Fig. 2a & b). It has been noticed from Fig.2a that when the feed concen-tration increased from2 g/L to 10 g/L thewaterflux decreased as a resultof decreasing the osmotic driving force across the membrane. The sim-ulation results also showed that increasing the feed solution flow ratehad a little impact on the water flux across the membrane. Forexample in the FO-RO design, there was a less than 2% increase in thewater flux when the Qf-in/Qds-in ratio (flow rate feed solution-in/flowrate draw solution-in) increased from 1 to 5 (feed TDS 2 g/L). At 10 g/Lfeed TDS, the water flux increased by 3.5% when the Qf-in/Qds-in ratio in-creased from 1 to 5. The increase in membrane flux was higher at 10 g/Lfeed TDS than at 2 g/L feed TDS due to the severe concentration polari-zation at higher feed concentration. This probably explainswhy increas-ing Qf-in/Qds-in from 1 to 5 at 0.2 g/L feed resulted only in a 0.17%increase in the water flux. In general increasing the feed flow rate de-creases the effect of concentration polarization at the membrane sur-face [24,27,28]. Fig. 2c shows the recovery rate of the FO and PROmodules at different feed concentrations. The highest recovery ratewas at 0.2 g/L feed concentration and that is probably due to firstlythe high osmotic pressure driving force across the membrane and sec-ondly the negligible concentration polarization effect. The RO-PRO sys-tem, for instance, exhibited the highest osmotic pressure driving forcebecause it uses a concentrated RO brine in the draw solution but its re-covery rate was lower than that at 0.2 g/L feed due to the adverse im-pact of the concentration polarization effect. As matter of fact, theresults here are in agreement with the previous studies which showedthe same concentration polarization effect [24]. Finally, increasing the


Qf-in/Qds-in ratio from 1 to 5 resulted in a slight decrease in the concen-tration of diluted draw solution (Fig. 2d). This is because of the higherwater fluxwhich resulted in a higher dilution of the draw solution. Gen-erally, between 0.8% and 5% of dilutionwas achieved after the first stageof FO/PRO treatment. The simulation results show that the RO-PROdesign had the highest water flux and osmotic pressure driving force.Nevertheless, the RO-PRO process utilizes more concentrated drawsolution than other designs; i.e. PRO-RO and the FO-RO. Unlike thePRO-RO and FO-RO designs, the diluted draw solution from the RO-PRO process goes to the sea instead of the RO membrane for desalina-tion. Comparing the brine waste for discharge from the PRO-RO, andFO-RO designs with the RO-PRO design shows that the latter designhas the lowest brine concentration (Fig. 3). On other words, the RO-PRO design has less environmental impact upon discharges of the RObrine to sea followed by the FO-RO and PRO-RO designs respectively.



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J w (

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FO-RO: 2g/L feedFO-RO: 10g/L feedPRO-RO: 2g/L feedPRO-RO: 10g/L feedRO-PRO:2g/L feedRO-PRO:10g/L feedFO-RO: 0.2g/L feed

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/L)


d

Fig. 4.The impact of draw solutionflowrate on the FOmembraneperformance; a) effect onmembraneflux, b) effect on osmotic pressure gradient, c) effect onmembrane %Re andd) effecton the concentration of DS-out.


Fig. 4 shows the impact of draw solution flow rate on the perfor-mance of the FO and PRO modules in the PRO-RO, RO-PRO, and FO-ROdesigns. In general, a higher water flux can be achieved by increasingthe draw solution flow rate than the feed solution flow rate (Figs. 3and 2a). Previous study showed the same results [24] and it is attributedto the increase in the net driving pressure across the FO membrane. Inthe present study, the highest membrane flux was achieved in the RO-PRO design because of the highest differential osmotic pressure acrossthe membrane (Fig. 4a and b). The membrane flux increased with in-creasing the Qf-in/Qds-in ratio from 1 to 5 due to the higher osmotic pres-sure difference that can be generated at higher draw solution flow rates(Fig. 4a and b). Generally, the draw solution bulk concentration in-creases with increasing its flow rate. Furthermore higher draw solutionflow rates decrease the concentration polarization effect at the mem-brane surface. The simulation results show that the RO-PRO designwas slightly more sensitive than the FO-RO and PRO-RO designs to theincrease in the draw solution flow rate (Fig. 4a). For example, increasingthe Qf-in/Qds-in ratio from 1 to 5 resulted in a 17% increase in the waterflux in the RO-PRO design corresponding to 16.7% and 14.1% waterflux increases in the PRO-RO and FO-RO designs respectively (feedTDS 2 g/L). It is important to be mentioned here that the relativelyhigh membrane flux at 0.2 g/L feed TDS in the FO-RO design wasbecause of the negligible concentration polarization effect. The impactof the draw solution flow rate on the system’s recovery rates is shownin Fig. 4c. Because of the higher osmotic pressure driving force acrossthe membrane, all designs showed higher recovery rates at lower feedTDS. The lower the feed concentration is the lower the concentration po-larization effect. High water flux and recovery rate is desirable in themembrane process because it reduces the membrane area required forfiltration andhence themembrane cost. Fig. 4d shows that the concentra-tion of draw solution generated from thefirst stage treatment in the PRO-RO and FO-RO designs was much lower than that from the RO-PRO de-sign. However, in the RO-PRO process the concentration of draw solution


was further diluted by the second stage PRO treatment before final dis-charge to the seawater (Fig. 3). The simulation results also showed thatthe concentration of diluted draw solution increased with increasingthe Qf-in/Qds-in ratio from 1 to 5. Evidently, this will increase the cost ofRO desalination in the PRO-RO and FO-RO designs. While in the PRO-RO design, the diluted draw solution is normally discharged to seawaterand increasing the Qf-in/Qds-in ratio will increase the negative impact ofbrine concentrate on the marine environment. Increasing the drawsolution flow rate will increase the membrane flux and flow rate but italso has detrimental effects on the design performance. For example inthe PRO-RO and FO-RO designs, the concentration of RO feed increaseswith increasing the draw solution flow rate and hence the RO desalina-tion cost will be increased. Also should be noted here that increasingthe draw solution flow rate will increase the pump power consumption.Therefore, a Qf-in/Qds-in ratio of 1 will be used for the rest of the study.

3.2. Impact of the feed TDS on the design performance

Osmotically drivenmembrane processes are sensitive to the concen-trations of draw and feed solution [24]. Preferable operating conditionoccurs at a considerable osmotic pressure difference between thedraw and feed solutions. Therefore, increasing the feed TDS has a nega-tive impact on the process performance. Fig. 5a, b and c shows the per-formance of the PRO-RO, RO-PRO and FO-RO designs at feed TDSbetween 0.2 g/L and 15 g/L. All designs showed a decrease in the mem-brane flux in correspondencewith the increase of the feed TDS (Fig. 5a).The RO-PRO exhibited the higherwater fluxbecause of the high osmoticpressure gradient across themembrane compared to the other designs;i.e. PRO-RO and FO-RO (Fig. 5b). As a result of a higher osmotic pressuregradient, the recovery rate in the RO-PRO design was also higher thanthat in the PRO-RO and FO-RO designs (Fig. 5b & c). On the otherhand, the concentration of diluted draw solution increased with in-creasing the feed TDS because of the decrease in the osmotic pressure



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Fig. 5. The impact of feed solution TDS on the performance of FO and PRO; a) effect onmembrane flux, b) effect on osmotic pressure gradient, c) effect onmembrane %Re and d) effect onthe concentration of DS-out.

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Fig. 6. Power density in PRO-RO and RO-PRO designs (Qf-in/Qds-in = 1).


gradient across the membrane (Fig. 5c & d). After the first stage mem-brane treatment, the concentration of diluted draw solution was lowerin the PRO-RO and FO-RO designs than in the RO-PRO design. However,in the RO-PRO design the concentration of draw solution decreasedafter the second membrane treatment due to dilution by fresh waterpermeates in the PRO module (Fig. 3). The simulation results alsoshowed that the concentration of draw solution going to the RO mem-brane in the PRO-RO design was higher than that in the FO-RO design.Increasing the concentration of RO feed will increase the energy re-quirements for desalination. Although the concentration of draw solu-tion from the FO-RO design was lower than that from the PRO-RO andRO-PRO designs, there are other aspects that should be consideredwhen these designs are compared such as the FO membrane area re-quired for treatment. Presently, the cost of the FO membrane is ratherhigh but it is expected to drop in the near future. It is also importanttomention here that at 12 g/L and 15 g/L feed concentrations, themem-brane flux and recovery rate declined sharply which rendered the FOand PRO processes inefficient especially in the case of the PRO-RO de-sign. At 15 g/L for instance, the membrane flux and recovery rate inthe PRO-RO design were 2.5 L/m2 h and 8.2% respectively. Therefore,the PRO-RO design requires higher draw solution concentration tomaintain the membrane productivity at elevated feed salinities. Forthe rest of the study only feed salinities between 0.2 g/L and 10 g/Lwill be investigated.

3.3. Turbine system

The power generation takes place in the turbine system which co-verts the hydraulic energy into electricity. The FO-RO design will notbe included in this section because it's for seawater desalination only.Fig. 6 shows the power density generated from the PRO-RO and RO-PRO designs. Both designs showed a decrease in the power density gen-eration with increasing the TDS of feed solution. Basically, the osmoticpressure difference across the membrane decreased with increasing


the feed TDS and hence caused a simultaneous drop in the membraneflux (Figs. 5b & 6). The simulation results showed that the PRO-RO de-sign generated less power than the RO-PRO design and this was attrib-uted to the higher flux in the RO-PRO design (Fig. 5a). At feed TDS 2 g/Land 10 g/L the power density in the RO-PRO was, respectively, 2.7 and4.3 times higher than that in the PRO-RO design. This means that theRO-PRO design ismuchmore efficient than the PRO-ROdesign at higherfeed TDS. One of the important features in the RO-PRO design is that itutilizes RO brine as a draw solution in the PRO module and hence themembranewater flux is higher than that in the PRO-RO design. Howev-er, the RO-PRO design requires more power for seawater desalinationthan the PRO-RO design as will be explained in the following section.



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Fig. 7. Permeate TDS, feed pressure and specific power consumption in the ROmembrane.


3.4. The RO seawater desalination

The RO membrane system is for seawater desalination and freshwater production. The PRO-RO and FO-RO designs utilized the diluteddraw solution as the RO feedwhile the RO-PRO design used raw seawa-ter as theRO feed. Therefore, theperformance of the ROmembrane in theRO-PRO designwas not affected by the TDS of impaired water feed goingto the PRO module. Fig. 7a shows the permeate TDS of the desalinatedwater in the proposed design configurations. Obviously, the highest


permeate concentration was produced by the RO-PRO design or by thedesign which has the highest RO feed concentration. Furthermore, thepermeate concentration was higher in the PRO-RO than in the FO-ROdesign because of higher RO feed TDS in the PRO-RO design (Fig. 5d). Itshould be noted here that at high RO feed concentration a dual stageRO-BWRO treatment might be required in order to reduce the productwater TDS to the desirable level [29]. The higher RO feed concentrationin the RO-PRO design required a higher feed pressure for desalinationthan in the FO-RO and PRO-RO designs (Fig. 7b). As a matter of fact thiswill increase the cost of seawater desalination by the RO membranes.

On the other hand, the concentrated RO brine from the PRO-RO andFO-RO designs returns back to the sea while in the RO-PRO design it isused as the PRO draw solution. The RO-PRO design, however, requiredthe higher RO feed pressure followed by the PRO-RO and FO-RO designsrespectively. The simulation results showed that the difference in theRO feed pressure between the RO-PRO, PRO-RO and FO-RO designs de-creased with increasing the feed salinity from 2 g/L to 10 g/L (Fig. 7b).Actually, the concentration of RO feed increased with increasing theTDS of feed solution to the FO membrane in the PRO-RO and FO-RO de-signs (Fig. 5d). The specific power consumption for RO seawater desali-nation is shown in Fig. 7c. The ROpower consumption is in the followingorder RO-PRO N PRO-RO N FO-RO design. Although FO-RO required lesspower for seawater desalination than the PRO-RO and the RO-PRO de-signs, power generation in the latter designs should be taken into ac-count when the overall system power consumption is calculated.

3.5. Power generation and desalination cost

A simple cost analysis studywas carried out here to estimate the costof the FO and PROmodules. The FOmembrane is used in both modules,i.e. PRO and FO,with an estimatedmembrane price of 104 USD/m2 [30].The FO/PRO plant capacity is 20,000 m3/day using seawater and im-paired water as the feed and the draw solutions respectively. Table 1shows the number and the cost of elements and pressure vessels inthe FO and PRO modules. The membrane area is estimated from theplant capacity divided by the membrane flux. The results show thatthe RO-PRO design required the smallest membrane area followed bythe FO-RO and PRO-RO designs respectively (Table 1). This is due tothe high membrane flux exhibited by the RO-PRO design (Fig. 5a). It isalso obvious from Table 1 that the membrane cost increased with in-creasing the feed concentration to the membrane. As explained before,membrane flux tends to decrease with increasing the feed salinity andhence more membrane area is required. Presently, the cost of the FOmembrane is almost 10 times higher than that of the RO membrane.With the current prices, the cost of the FO membrane is almost as im-portant as power consumption for desalination. For example, the costof the membrane in the PRO-RO design is 2.2 and 4.3 times higherthan that of the FO-RO and RO-PRO designs, respectively (Fig. 8). Al-though the expected lifetime of the FO membrane is longer than theRO membrane, the replacement cost is much higher than that of theRO membrane. If the cost of the FO membrane doesn't drop in thenear future, the RO-PRO design seems to be more economical than theother designs especially if the capital cost is the crucial factor that is de-termining the overall design cost. As such, the PRO-RO design is themost expensive option in terms of the FO membrane cost (Fig. 8).

Fig. 9 shows the power use or consumption and the power genera-tion in all designs. It is assumed here that the FOmembrane power con-sumption is negligible compared to the RO membrane [22]. The powerconsumption of the RO-PRO was the highest amongst all designs but itwas not affected by the impaired water feed TDS to the PRO module.Both PRO-RO and FO-RO designs required less energy for seawater de-salination than the RO-PRO design. However, the energy consumptionin the PRO-RO and FO-RO designs increased with increasing the im-paired water TDS. Consequently, the gap of power consumption differ-ence between all design tends to decrease with increasing theimpaired water TDS from 2 g/L to 10 g/L. For example, at 10 g/L PRO



Table 1The cost of the FO and PRO module for the proposed designs.

Design conf. Feed TDS (g/L) Memb. area (m2) No. elem. No. vessel Elem. cost (USD) Vessel cost (USD)

PRO-RO 2000 91,878 5568 2784 9,572,016 3,421,7594000 104,690 6345 3172 10,906,807 3,898,9146000 120,598 7309 3654 12,564,136 4,491,3688000 141,483 8575 4287 14,739,929 5,269,160

10,000 170,068 10,307 5154 17,717,996 6,333,746FO-RO 2000 53,694 3254 1627 5,593,955 1,999,701

4000 58,357 3537 1768 6,079,705 2,173,3446000 63,468 3847 1923 6,612,200 2,363,6988000 69,156 4191 2096 7,204,828 2,575,548

10,000 75,689 4587 2294 7,885,393 2,818,833RO-PRO 2000 34,407 2085 1043 3,584,566 1,281,394

4000 35,446 2148 1074 3,692,819 1,320,0926000 36,630 2220 1110 3,816,184 1,364,1918000 37,965 2301 1150 3,955,270 1,413,911

10,000 39,438 2390 1195 4,108,764 1,468,782


feed TDS the difference of power consumption between the PRO-ROand RO-PRO was 12.4% while the difference of power consumption be-tween the FO-RO and RO-PRO was 26.5% compared to 26.5% and 44.8%respectively at 2 g/L PRO feed TDS (Fig. 9). On the other hand, Fig. 9shows that the power generation in the RO-PRO was higher than thatin the PRO-RO because of the lower membrane flux in the PRO-RO de-sign (Fig. 5a). The profile of power generation in both designs shows adrop in the amount of power generation at higher PRO feed concentra-tion. This is due to the lowermembrane flux in all designs at higher PROfeed concentrations. The net power usewhich is the difference betweenthe power use and power generation, is illustrated in Fig. 10. For exam-ple, in the RO-PRO design, the net power consumption was almost 4.5%less than the power consumption. The RO-PRO design had the highestnet power consumption amongst all designs (Fig. 9). Actually at 10 g/Lfeed concentration, the difference in the net power consumption be-tween the FO-RO and RO-PRO designs was 13.3% while the differencein the net power consumption between the PRO-RO and RO-PRO de-signs was 5.3%. At the same feed concentration the power saving inthe PRO-RO and RO-PRO designs was 2.8% and 8.9% respectively(Fig. 10). The higher power saving in the RO-PRO design is due to thehigher power generation in the PRO process. Unlike other designs, thepower consumption and the net power consumption are the same inthe FO-RO design because the system was designed for desalinationonly (Fig. 9). Furthermore, the power saving which is the ratio ofpower generation to power use is plotted in Fig. 10. A higher power sav-ing can be achieved in the RO-PROdesign than in the PRO-ROdesign be-cause of the higher membrane flux in the latter design.

In general, the FO-RO design is the most efficient design in terms ofpower consumption followed by the PRO-RO design and the RO-PRO

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Fig. 8. Element and pressure vessel cost of the FO and PRO module.


design respectively. In terms of FO membrane cost, the RO-PRO designis themost cost-effective followed by the FO-RO design and PRO-RO de-sign respectively. However, the RO-PRO design becomes more compet-itive to the FO-RO if the energy prices drop down.On theother hand, theFO-RO design becomesmore economical if the cost of the FOmembranedrops down in the near future. The simulation results also show that thecost of RO desalination can be reduced when the seawater RO feed ispretreated either by the FO or PRO process. Additionally, the brine dis-charge to seawater from the RO-PRO design is less concentrated thanthat from the FO-RO and PRO-RO designs. This point should be consid-ered as an advantage for the RO-PRO design especially when theguidelines of brine discharge and disposal is very strict. In terms of de-salinated water quality, the FO-RO design produces the highest brinequality compared to other designs (Fig. 7a). This issue is important ifthe desalinated water is for drinking purposes or high quality productwater is required. The aforementioned points should be carefully con-sidered in the selection of a proper design for the PRO and FO plantsfor power generation and/or seawater desalination using seawaterand impaired water as a salinity gradient.

4. Conclusion

Three design configurations were evaluated for power generationand/or seawater desalination using seawater and impaired water as asalinity gradient resource. The results showed that the performance ofPRO-RO, FO-RO, and RO-PRO designs vary with the feed and the drawsolution flow rates, FO feed solution TDS, and RO feed TDS. The key fac-tors that have been considered here to evaluate the performance of eachdesign are net power consumption, FOmembrane capital cost, RO prod-uctwater quality and concentration of RO brine discharge to the seawa-ter. The RO-PROdesign has the lowest FOmembrane capital cost andRO

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Fig. 10. The net power use and power saving in PRO-RO, FO-RO, and RO-PRO designs.


brine concentration but its RO power consumption is very high. The FO-RO design exhibits the lowest RO power consumption and average FOmembrane capital cost while the PRO-RO design requires very high FOmembrane capital cost and average RO power consumption. At thepresent FO membrane cost, the FO-RO and RO-PRO designs have thebest performance. Decreasing the FO membrane cost will tip the scalesin favour of the FO-RO design while decreasing the energy cost will tipthe scales in favour of the RO-PRO design.

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Documents

Evaluation of FO-RO and PRO-RO designs for power generation and seawater desalination using impaired water feeds