An optimization strategy for a forward osmosis-reverse osmosis hybrid process for wastewater reuse and seawater desalination: A modeling study

An optimization strategy for a forward osmosis-reverse osmosis hybrid process for wastewater reuse and seawater desalination: A modeling study

Desalination 463 (2019) 40–49 Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal An optimizatio...

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Desalination 463 (2019) 40–49

Contents lists available at ScienceDirect

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

An optimization strategy for a forward osmosis-reverse osmosis hybrid process for wastewater reuse and seawater desalination: A modeling study

T



Jangwon Seoa, Young Mi Kimb, , Sung Ho Chaeb,c, Seung Ji Limb,c, Hosik Parkb, Joon Ha Kimb,c a

Research Institute for Solar and Sustainable Energies, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea Membrane Research Center, Advanced Materials Division, Korea Research Institute of Chemical Technology, Daejeon 34114, Republic of Korea c School of Earth Science and Environmental Engineering, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea b

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Forward osmosis Reverse osmosis Hybrid process Sensitivity analysis Desalination Wastewater reuse

The reduction of energy consumption in the reverse osmosis (RO) process is critical. The forward osmosis (FO)RO hybrid process can be suggested to reduce RO energy consumption. In this study, a numerical FO-RO process model was developed to analyze the FO-RO hybrid process. The performance of the FO-RO hybrid process was compared with the stand-alone RO process. In addition, the impacts of the control parameters for operation, and the intrinsic membrane parameters, were analyzed using sensitivity analysis. In conclusion, the FO-RO hybrid process involves less RO energy consumption than the stand-alone RO process. The FO draw flow velocity and the RO applied pressure were derived as the major factors for controlling the FO-RO process. In addition, the control parameters for operation were found to be more important than the intrinsic membrane parameters in the minimization of RO energy consumption. Subsequently, the FO elements installed in front of the RO process should be configured with a parallel connection, in order to minimize RO energy consumption. The results in this study could be used to develop guidelines for the optimal design of the FO-RO hybrid process.

1. Introduction The demand for clean water production technologies has been consistently emphasized as a way of coping with water security issues.



The water security issues can be induced by an increasing population, climate change, and geographical accessibility [1–3]. To provide sufficient fresh water, alternative water resources, such as seawater and wastewater, have been investigated. Currently, the seawater

Corresponding author. E-mail address: [email protected] (Y.M. Kim).

https://doi.org/10.1016/j.desal.2019.03.012 Received 12 December 2018; Received in revised form 16 March 2019; Accepted 22 March 2019 0011-9164/ © 2019 Published by Elsevier B.V.

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Nomenclature A B c dh D Dhd ERDeff H HPeff ICR Jw k KSpacer L pRO·Pa Q r S Sc Sh u W x

Greek symbols π η φ

Water permeability (L/m2h bar) FO solute permeability (m/s) Concentration (mg/L) Hydraulic diameter (m) Diffusion coefficient of the solute (m2/s) Hydraulic dispersion coefficient (m2/s) Energy recovery device efficiency (%) Channel height (m) High-pressure pump efficiency (%) Inner concentration ratio (−) Water flux (L/m2h) Mass transfer coefficient (m/s) Friction coefficient Channel length (m) RO applied pressure (Pa) Flow rate (m3/s) Solute rejection rate (%) Structure parameter (m) Schmidt number (−) Sherwood number (−) Flow velocity (m/s) Channel width (m) Distance from the inlet (m)

Osmotic pressure (bar) Viscosity (Pa s) Arbitrary variable adopted to perform integration

Subscripts and superscripts d.FO d.FO0 dm.FO FO f.FO f.FO0 f.RO f.RO0 fm.FO fm.RO Hybrid p.RO RO

FO draw solution Initial value of FO draw solution FO draw solution-membrane interface Forward osmosis FO feed solution Initial value of FO feed solution RO feed solution Initial value of RO feed solution FO feed solution-membrane interface RO feed solution-membrane interface FO-RO hybrid process RO permeate solution Reverse osmosis

Mathematical operators Δ

Delta

and external concentration polarization, and flow rate change inside of the membrane element were considered. In addition, the efficiency of the pump and the energy recovery device were accounted for in the calculation of RO energy consumption. To assess the feasibility of the FO-RO hybrid process, a performance comparison with the stand-alone RO process was conducted. Subsequently, major factors affecting RO energy consumption in the FO-RO hybrid process were derived and ranked using sensitivity analysis. In addition, an optimization strategy of the FO-RO hybrid process to minimize RO energy consumption was discussed. In conclusion, this study suggests an optimization strategy for the FO-RO hybrid process that considers the intrinsic membrane parameters, membrane element design, and the control parameters for operation.

desalination process can provide additional fresh water successfully using the reverse osmosis (RO) process [3,4]. However, a further reduction in energy consumption in the seawater reverse osmosis (SWRO) process has been continuously demanded. To achieve this, a combination of forward osmosis (FO) and SWRO processes has been considered as a possible solution. FO is an osmotically-driven process that recovers water from dilute feed solution to concentrated draw solution through a semipermeable membrane [5]. In the FO-RO hybrid process for seawater desalination, wastewater and seawater can be utilized as the feed and draw solutions, respectively. The FO part of the FO-RO hybrid process functions as a SWRO pretreatment process, diluting seawater to reduce energy consumption and enhancing the recovery rate of the SWRO process. Moreover, by using the FO-RO hybrid process, the wastewater can be reused as fresh water [6–9]. Furthermore, the fouling reversibility of the FO process provides additional applicability [10–12]. To investigate the feasibility of the FO-RO hybrid process using wastewater and seawater, various modeling and pilot studies have been conducted using wastewater and seawater [13–25]. Studies of the FORO hybrid process have shown that the contaminants in the FO feed solutions can be removed effectively through the dual barrier process [13–15]. Subsequently, direct potable reuse of FO-RO treated water has been discussed [13,18]. In addition, en economic analysis of the FO-RO hybrid process was conducted [23,25]. Recently, Choi et al. reported the long-term stable operation of the FO-RO hybrid process using wastewater and seawater that shows better performance than standalone RO process [20]. However, the quantitative analyses of parameters affecting RO energy consumption in the FO-RO hybrid process in terms of the membrane, membrane element, and the process have yet to be conducted for the FO-RO hybrid process design. The main purpose of this study is to derive major factors for controlling and suggesting an optimization strategy for the FO-RO hybrid process in terms of RO energy consumption. In this study, the impacts of water quality, intrinsic membrane parameters, and control parameters for operation were systematically and quantitatively analyzed with regard to FO-RO hybrid RO energy consumption using a modeling approach. Wastewater and seawater were considered in the FO-RO hybrid process. In the FO and RO model, water flux, solute flux, internal

2. Method 2.1. FO, RO, and FO-RO hybrid process model development Fig. 1(a) shows a schematic diagram of the FO-RO hybrid process. The FO-RO hybrid process model is composed of two unit process models, the FO model and the RO model. Fig. 1(b) illustrates a flow chart of the FO-RO hybrid process model. In the FO process, seawater and wastewater were considered as FO draw and FO feed solutions, respectively. To evaluate the performance of the FO process, the flow velocity and concentration of the FO feed and draw solutions, the FO water flux, and the FO solute flux were calculated from the inlet to the outlet of the FO process. In addition, the internal concentration polarization (ICP) and external concentration polarization (ECP) were considered. In the RO process, FO-treated seawater (diluted FO draw) is directly used in the RO process as the RO feed solution. In the RO process, the flow velocity and concentration of the RO feed, RO water flux, RO solute flux, hydraulic pressure drop, and concentration polarization were considered. It is assumed that the values of the intrinsic membrane parameters represent the characteristics that are present after membrane compaction occurs. The hydraulic pressure of the RO brine was recovered in the energy recovery device. Sodium chloride solution was to function as FO feed, FO draw, and RO feed solutions assumed in the FO-RO process. 41

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Fig. 1. (a) Schematic diagram of FO-RO hybrid process. The FO process uses seawater and wastewater as feed and draw solutions, respectively. (b) Modeling flow chart of the FO-RO hybrid process. The calculation proceeded as per the sequence described in Fig. 1(b). In the modeling, the membrane channels were divided into n segments with equal intervals of Δx. The segments were numbered from 1 to n, from the inlet to outlet, and the segments were indicated using the index i.

flow velocity is calculated using Eq. (3):

2.1.1. The RO process model The water flux in the RO flux is defined as linearly proportional to the difference between the hydraulic pressure and the osmotic pressure of the RO feed solution [7,26]:

Jw . RO (x ) = ARO (∆pRO (x ) − ∆πRO (x ))

uf . RO (x ) = uf . RO0 −

12Kspacer η H2

∫0

x

uf . RO (φ) dφ

∫0

x

vRO (φ) dφ

(3)

where uf. RO is the RO feed flow velocity (m/s), uf. RO0 is the initial RO feed flow velocity (m/s), and vRO is the RO water flux in different unit (m/s). In this study, the RO permeate flow is considered using Eq. (4):

(1)

where Jw. RO(x) is the RO water flux (L/m2h), x is the distance from the inlet (m), ARO is the RO water permeability (L/m2h bar), ΔpRO is the effective RO applied pressure (bar), and ΔπRO is the osmotic pressure difference to be overcome (bar). A hydraulic pressure drop in the RO process was considered using Eq. (2) [27,28]. To calculate the osmotic pressure on the RO and FO processes, the Van't Hoff equation was used [29,30]:

∆pRO . Pa (x ) = ∆pRO . Pa0 −

1 H

up . RO (x ) =

1 H

∫0

x

vRO (φ) dφ

(4)

where up. RO is the RO permeate flow velocity (m/s). To consider the concentration polarization and the concentration at the membrane surface on the RO process, Eqs. (5) and (6) were used [27]:

r cfm . RO (x ) = cf . RO0 ⎜⎛1 + e−vRO (x ) H / Dhd uf . RO (x ) H ⎝

(2)

where ΔpRO. Pa is the effective RO applied pressure in different units (Pa), ΔpRO. Pa0 is the initial applied RO pressure (Pa), Kspacer is a coefficient that accounts for the hydraulic pressure drop by the spacer, η is the viscosity of the RO feed (Pa s), H is height of the membrane channel (m), and φ is an arbitrary variable adopted to perform the integration. Since the water in the RO feed is transported across the membrane, the flow velocity of the RO feed is decreased. The change in the RO feed

+

cf . RO (x ) =

rvRO (x ) uf . RO (x ) Dhd

∫0

x

x

vRO (φ) dφ

vRO (φ) dφ⎟⎞ ⎠

1 cf . RO0 uf . RO0 H − (1 − r ) uf . RO (x ) H

(

∫0

(5)

∫0

x

cf . RO (φ) vRO (φ) dφ

) (6)

42

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where cfw. RO is the RO feed concentration on the membrane surface (mg/L), cf. RO0 is the initial RO feed concentration, Dhd is the hydraulic dispersion coefficient (m2/s), and r is the solute rejection rate. The RO permeate concentration is also evaluated using the mass balance:

cp . RO (x ) = where cp.

1 up . RO (x ) RO

(cf . RO0 uf . RO0 − cf . RO (x ) uf . RO (x ))

Jw . FO (x )

(

(

)

⎧ π ⎤ − πf . FO exp + d . FO exp ⎡− Jw . FO (x ) ⎪ ⎣ ⎦ = AFO ⎨ B Jw . FO (x ) 1 ⎪ 1 + Jw . FO (x ) exp k (x )f . FO − exp ⎡−Jw . FO (x ) k (x )d . FO + ⎣ ⎩

{ (

)

Jw . FO (x ) k (x )f . FO

(

S D

0.33 Sh (x )j = 0.04 Re (x )0.75 (turbulent flow) j Sc (x ) j

(15)

uf . FO (x ) = uf . FO0 −

1 H

∫0

ud . FO (x ) = ud . FO0 +

1 H

∫0

⎫ ⎪ ⎬ ⎤ ⎪ ⎦ ⎭ (8)

}

B Jw . FO (x )

{ ( exp

1 cd . FO0 ud . FO0 H − ud . FO (x ) H

SECFO =

(9)

SECRO =

(

1 k (x )d . FO

Jw . FO (x ) k (x )f . FO

+

S D

) ⎤⎦ − c

) − exp ⎡⎣−J

f . FO exp

(

(

Jw . FO (x ) k (x )f . FO

1 w . FO (x ) k (x ) d . FO

+

S D

)

=



x

Js (φ) dφ

(18)

)

(19)

Qf . FO0 pf . FO . Pa0 + Qd . FO0 pd . FO . Pa0 (20)

Qf . RO0 ∆pRO . Pa0 / HPeff − Qf . RO (L) ∆pRO . Pa (L) ERDeff 3600 × 1000Qp . RO (L)

(21)

RO recovery rate =

3600 × 1000Qp . RO (L)

Qp . RO (L) Qf . RO0

=

Qf . RO0 − Qf . RO (L) Qf . RO0

(23) 3

where SECFO is the SEC of the FO process (kWh/m ), SECRO is the SEC of the RO process without considering FO energy consumption (kWh/ m3), SECHybrid is the SEC of the FO-RO hybrid process considering FO energy consumption (kWh/m3), Qf. RO0 is the initial RO feed flow rate (m3/s), Qf. RO(L) is the RO brine flow rate (m3/s), Qp. RO(L) is the RO permeate flow rate (m3/s), ΔpRO. Pa(L) is the hydraulic pressure of the RO brine, HPeff is the high-pressure pump efficiency, ERDeff is the energy recovery device efficiency, Qf. FO0 is the initial FO feed flow rate (m3/s), Qd. FO0 is the initial FO draw flow rate (m3/s), Qd. FO(L)is the final FO draw flow rate (m3/s), pf. FO. Pa0 is the applied hydraulic pressure of the FO feed (Pa), pd. FO. Pa0 is the applied pressure of the FO draw solution (Pa), and 3600 × 1,000 is the unit conversion factor from J to kWh. In the ERD device, volumetric mixing is not considered. In addition, it is assumed that a flowrate of a part of RO feed that pressurized in the ERD is identical to the RO brine that used in the ERD. Also, efficiency of the high-pressure pump and the booster pump is assumed identical. In the calculation of FO energy consumption, 1 bar of pressure without hydraulic pressure drop is the assumed applied pressure for both FO feed and draw. For Section 3.2, new indicators



1 B∆cm . FO (x ) ⎧ S exp ⎡−Jw . FO (x ) ⎛ + ⎞ ⎤ − 1⎫ ⎢ ⎬ Jw . FO (x ) ⎨ D ⎠⎥ ⎝ k (x )d . FO ⎣ ⎦ ⎩ ⎭ ⎜

+ Qf . FO0 pf . FO . Pa0 + Qd . FO0 pd . FO . Pa0 (22)



(11)

J (x ) ⎞ B∆cm . FO (x ) ⎡ ⎛ Jw . FO (x ) ⎞ − 1⎤ cfm . FO (x ) = cf . FO (x ) exp ⎜⎛ w . FO ⎟ + ⎟ ⎥ ⎢exp ⎜ k (x ) k ( x ) J ( x ) f . FO w . FO f . FO ⎠ ⎝ ⎠ ⎝ ⎦ ⎣ (12) The mass transfer coefficient on the FO feed and draw sides were calculated using Eqs. (13)–(15) [29,34–36]:

d hj

∫0

)

Js (φ) dφ

3600 × 1000(Qd . FO (L) − Qd . FO 0 )

) ⎤⎦ }

1 S + ⎞⎤ cdm . FO (x ) = cd . FO (x ) exp ⎡−Jw . FO (x ) ⎛ ⎢ D ⎠⎥ ⎝ k (x )d . FO ⎣ ⎦

Sh (x )j D

(

∫0

Qf . RO0 (∆pRO . Pa0 − pd . FO . Pa0 )/ HPeff − Qf . RO (L) ∆pRO . Pa (L) ERDeff

where Δcm. FO(x) is the concentration difference across the active layer in the FO process (mg/L). The concentration at the active layer on FO draw and feed sides are expressed using Eqs. (11) and (12) [31]:

k (x )j =

(

x

SECHybrid

(10)

+

(17)

cd . FO (x ) =

∆cm . FO (x ) = cdm . FO (x ) − cfm . FO (x )

1+

vFO (φ) dφ

2.1.3. Performance evaluation of the FO-RO hybrid process To calculate the specific energy consumption (SEC) of the FO and RO process and the RO recovery rate, Eqs. (20)–(23) were used:

where Js is the FO solute flux (g/m s), cdm. FO is the FO draw concentration on the membrane surface (mg/L), and cfm. FO is the FO feed concentration on the membrane surface (mg/L). Since the Van't Hoff theory was used in this study, the concentration difference across the active layer is expressed using Eq. (10) [31,32]:

=

(16)

1 cf . FO0 uf . FO0 H + uf . FO (x ) H

2

cd . FO exp ⎡−Jw . FO (x ) ⎣

x

vFO (φ) dφ

where uf. FO is the FO feed flow velocity (m/s), uf. FO0 is the initial FO feed flow velocity (m/s), vFO is the FO water flux in different units (m/ s), ud. FO is the FO draw flow velocity (m/s), ud. FO0 is the initial FO draw flow velocity (m/s), cf. FO is the FO feed concentration (mg/L), cf. FO0is the initial FO feed concentration (mg/L), cd. FO is the FO draw concentration (mg/L), and cd. FO0is the initial FO draw concentration (mg/ L).

where Jw. FO is the FO water flux (L/m2h), AFO is the FO water permeability (L/m2h bar), πd. FO is the osmotic pressure of the bulk FO draw solution (bar), πf. FO is the osmotic pressure of the bulk FO feed solution (bar), B is the FO solute permeability (m/s), D is the self-diffusion coefficient of the solute (m2/s), S is the structure parameter (m), and kf. FO and kd. FO are the mass transfer coefficient on the FO feed and draw sides (m/s), respectively. The FO solute flux is defined as linearly proportional to the concentration difference across the active layer of the FO membrane using Eq. (9) [4,31–33]:

Js (x ) = B (cdm . FO (x ) − cfm . FO (x ))

x

cf . FO (x ) =

)

)



where k is the mass transfer coefficient (m/s), Sh is the Sherwood number, Sc is the Schmidt number, and dh is the hydraulic diameter of the FO feed channel. Similar to the RO process model, the flow velocity and concentration of the FO draw and feed solutions are calculated using Eqs. (16)–(19):

(7)

2.1.2. The FO process model In the FO process, the FO mode membrane orientation is considered. Therefore, dilutive ECP and ICP on the FO draw, and concentrative ECP on the FO feed side were considered. In addition, reverse solute flux was also considered for calculating the FO water flux Eq. (8) [31,32]:

S D

(14)



is the RO permeate concentration (mg/L).

1 k (x )d . FO

dh j ⎞0.33 (laminar flow) Sh (x )j = 1.85 ⎛Re (x )j Sc (x )j L ⎠ ⎝

(j = d. FO or f . FO) (13) 43

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maximum and minimum values. In addition, the reference value of the RO SEC was calculated based on the reference value of the parameters. In the calculation of the normalized sensitivity coefficient of each parameter, random sampling with a uniform probability distribution function was used. For each parameter, one hundred normalized sensitivity coefficients were calculated. The rank of the sensitivity of each parameter was assessed by the average of the sensitivity coefficients' magnitude.

were adopted using Eqs. (24) and (25):

ICR cRO (x ) =

Δ Jw . RO (x ) =

cfHybrid . RO (x ) c fRO . RO (x )

JwHybrid . RO (x )



(24)

JwRO . RO (x )

(25)

where ICRcRO is the inner concentration ratio (ICR) of the RO feed along the RO pressure vessel between the FO-RO hybrid process and the stand-alone RO process, cf. ROHybrid is the RO feed concentration on the FO-RO hybrid process (mg/L), cf. RORO is the RO feed concentration on the stand-alone RO process (mg/L), ΔJw. RO is the RO water flux difference between the FO-RO hybrid process and the stand-alone RO process (L/m2h), Jw. ROHybrid is the RO water flux on the FO-RO hybrid process (L/m2h), and Jw. RORO is the RO water flux on the stand-alone RO process (L/m2h).

3. Results and discussion 3.1. FO and RO performance in the FO-RO hybrid process The water flux and concentration distributions in the FO-RO hybrid process are shown in Fig. 2. The concentration of the FO feed solution (i.e., wastewater) and draw solution (i.e., seawater) were 1000 and 35,000 mg/L, respectively. The flow velocity of the FO feed and draw solution was 0.05 m/s. The temperature of the FO feed, FO draw, and RO feed solutions was 300 K. The applied pressure on the RO process was 30 bar (see Table 1). Fig. 2(a) describes the water flux in the FO process with respect to distance. The distance at 0 and at 7 m represents the inlet and outlet of the process, respectively. In the FO process, 29% of the water flux drop between the initial and final water flux was calculated with an average value of 6.52 L/m2 h. The reduction of the water flux with respect to distance is caused by the reduced driving force of the FO process, and osmotic pressure difference across the active layer of the membrane, as water is transported from the FO feed to the FO draw solution (see Fig. 2(b)) [6]. Fig. 2(b) illustrates the concentration distribution in the FO process with respect to distance. The concentration of the FO draw side decreases with respect to the distance, whereas, that of the FO feed side increases. At the bulk region, 30.6% of the concentration reduction on the FO draw side and 156.0% of the concentration increase on the FO feed side were calculated. This change in concentration is caused by the FO water flux and the solute flux (see Eqs. (16) and (18)). Firstly, the FO draw and feed flow velocity were changed from 0.05 to 0.071 and 0.029 m/s, respectively (see Table 1) by the FO water flux. However, to support the dramatic increase of the FO feed concentration, the solute flux from the FO draw to the FO feed should also be addressed. In considering only the water flux, the 1724.1 mg/L of the FO feed bulk concentration at a distance of 7 m is expected with an 42% volume reduction. However, the FO feed bulk concentration at 7 m distance was

2.2. Simulation conditions and range of sensitivity analysis and optimization of FO-RO hybrid process For the simulation and sensitivity analysis, the simulation conditions and range of the sensitivity analysis of the FO-RO hybrid process were determined based on a literature review (see Table 1). The FO feed and draw were considered as wastewater and seawater, respectively [37–40]. The range of the FO and RO intrinsic membrane parameters includes both commercial- and laboratory-developed membranes to consider further improvements in the FO membrane [38,41–56]. In addition, high pressure pump (HP) and energy recovery device (ERD) efficiency were considered for evaluating the SEC of the RO process [57,58]. 2.3. Sensitivity analysis of the FO-RO hybrid process To derive the dominant parameter affecting the RO SEC of the FORO hybrid process, sensitivity analyses were employed. In the sensitivity analysis, normalized sensitivity coefficients were used to standardize differences in the order of magnitude of the parameters within the analysis range [73]. The normalized sensitivity coefficient of each parameter is calculated using Eq. (26): ∆SECHybrid

Normalized sensitivity coefficient parameteri =

∆parameteri

SECHybrid . ref parameterref

(26)

The reference value of each parameter was an average of its

Table 1 Simulation conditions and range of parameters for sensitivity analysis in the FO-RO hybrid process.

FO

RO

Parameters

Simulation value

Min.

Max.

References

Feed concentration (Wastewater), cf. FO0 Draw concentration (Seawater), cd. FO0 Feed flow velocity, uf. FO0 Draw flow velocity, udf. FO0 Water permeability, AFO Solute permeability, B Structure parameter, S Channel height, H Channel length, L Channel width, W Feed flow velocity, uf. RO0 Water permeability, ARO Solute rejection rate, r Applied pressure, ΔpRO0 Energy recovery device efficiency, ERDeff High-pressure pump efficiency, HPeff Channel height, H Channel length, L Channel width, W Friction coefficient, Kspacer Hydraulic dispersion coefficient, Dhd

1000 mg/L 35,000 mg/L 0.05 m/s 0.05 m/s 2 LMH/bar 5 × 10−7 m/s 365 × 10−6 m 6 × 10−4 m 7m 1.3 m 0.05 m/s 1.28 LMH/bar 99% 30 bar 90% 90% −4 6 × 10 m 7m 1.3 m 7 9.6 × 10−9 m2/s

150 35,000 0.01 0.01 0.32 1 × 10−8 50 × 10−6 – –

2000 45,000 0.1 0.1 5 10 × 10−7 1500 × 10−6 – –

0.01 1 – 20 – – – –

0.1 6 – 60 – – – –





[37,59–61] [39] [40,62,63] [40,62,63] [6,38,41–55,64,65] [6,38,41–55,64,65] [6,38,41–55,64,65] [27,66] [58,67] [68–71] [27] [72] [67] [72] [66] [57] [27,66] [56,67] [68–71] [27,28] [27]

44

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Fig. 2. Water flux and concentration of the FO-RO hybrid process with respect to distance. The vertical axis represents the concentration. The horizontal axis represents the distance. (a) Water flux in the FO process. (b) Concentration of the draw and feed solutions in the FO process. Red and blue lines represent FO draw and FO feed solutions, respectively. The solid line indicates concentration at the bulk. The dashed line indicates the concentration at the membrane surface, which represents the concentration at the active layer surface. Concentration polarization is considered to calculate the concentration at the membrane surface. (c) Water flux in the RO process. RO water flux decreases due to the concentrated feed solution. (d) Concentration of the feed solution in the RO process. To demonstrate that the RO model can describe the concentration polarization, Fig. S1 is provided with different simulation conditions in the supplementary material. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

solution (from FO process) is utilized as the RO feed solution. On the other hand, in the stand-alone RO process, the FO draw solution (i.e., seawater) is directly used as the RO feed solution without dilution (by the FO process). Fig. 3 compares the SEC and recovery rate of the RO process on the FO-RO hybrid process and the stand-alone RO process with respect to applied pressure on the RO process. In the considered range of the RO applied pressure, the FO-RO hybrid process shows a lower SEC and a higher recovery rate for the RO process than the stand-alone RO process. In the range of 20 to 40 bar of RO applied pressure, the FO-RO hybrid process shows a notably lower RO SEC than the stand-alone RO process, compared to the RO SEC in the range of 40 to 60 bar of the RO applied pressure (see Fig. 3(a)). The RO SEC on the FO-RO hybrid process considering FO energy consumption is 2.68, 0.31, and 0.15 kWh/m3 lower than the stand-alone RO process at 30, 40, and 50 bar of RO applied pressure, respectively. On the other hand, in the range of 40 to 60 bar of the RO applied pressure, the RO SEC difference between the FO-RO hybrid process and the stand-alone RO process is decreased monotonically. The RO SEC of the FO-RO hybrid process considering FO energy consumption shows a 0.11 kWh/m3 lower RO SEC at 60 bar of RO applied pressure. The minimum RO SEC on the FO-RO hybrid process considering FO energy consumption is 24.7% lower than that of the stand-alone RO process with lower RO applied pressure, 1.37 kWh/ m3 at 30.5 bar for the FO-RO hybrid process considering FO energy consumption and 1.82 kWh/m3 at 41.4 bar for the stand-alone RO process. Since this study aimed to address RO energy consumption, and FO energy consumption is relatively low, the following analysis will focus more on factors affecting RO energy consumption. Similar to the RO SEC, the FO-RO hybrid process performs better than the stand-alone RO process in terms of the recovery rate of the RO process (see Fig. 3(b)). The RO recovery rate of both the FO-RO hybrid process and the stand-alone RO process increases as RO applied pressure increases. With a RO applied pressure of < 30 bar, the stand-alone RO process shows a poor RO recovery rate because the RO applied pressure is insufficient to overcome the osmotic pressure of the RO feed solution. Despite the higher RO recovery rate of the FO-RO hybrid process, the difference of the RO recovery rate decreases as the RO

2560.8 mg/L. Thus, the remaining portion of the FO feed concentration increase is caused by a reverse solute flux. The concentration at the membrane surface indicates the concentration on the active layer surface, considering an asymmetric membrane, which determines the osmotic pressure for the driving force of the FO process. Since the FO process was simulated considering the FO mode of the membrane orientation, the dilutive ICP, dilutive ECP, and concentrative ECP were considered to calculate the concentration at the membrane surface on the draw and feed sides, respectively (see Eqs. (8)–(12)). The impact of the concentration polarization on the FO draw side is reduced as the distance increases due to the FO water flux drop (see Fig. 2(a) and (b)). In the considered simulation condition, the use of a deeper channel height may not be suggested due to the reduction of the FO water flux induced by smaller shear stress (see Fig. S2 in the supplementary material). Fig. 2(c) shows the water flux in the RO process with respect to distance. In the RO process, 96.8% of the initial water flux drop was calculated. This drop is induced by a reduced RO driving force, an increased RO feed concentration, and a reduced RO applied pressure resulting from a hydraulic pressure drop (see Fig. 2(d), Eqs. (1) and (2)). This imbalanced water flux distribution with respect to distance may require an internally staged design (ISD) in the presence of the membrane fouling for long-term opeeration [58]. Compared to the FO process, a lesser effect of concentration polarization is calculated in the RO process in the simulation condition (see Fig. 2(b) and (d)). To demonstrate that the RO model can describe the concentration polarization, Fig. S1 is provided in the supplementary material. 3.2. Comparison of the FO-RO hybrid and stand-alone RO processes To quantify how much the FO process can contribute to the RO process for the production of potable water, the SEC and recovery rate of the RO process in the FO-RO hybrid process were evaluated and compared with those of the stand-alone RO process. The simulation conditions of the FO-RO hybrid process and the stand-alone RO process are identical except for the feed concentration for the stand-alone RO process (see Table 1). In the FO-RO hybrid process, the diluted FO draw 45

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ICRcRO(x) is > 1) is derived locally (i.e. 5 m from the RO inlet with the 50 bar of the RO applied pressure). This region can be induced by the reduced RO feed flow velocity in the FO-RO hybrid process due to the higher RO water flux at the front-end. This can also describe the negative value region of the ΔJW. RO(x) in Fig. 4(b) because the region where ICRcRO(x) is > 1 overlaps where the region ΔJW. RO(x) is less than zero. 3.3. Sensitivity analysis of the FO-RO hybrid process Based on the developed model, sensitivity analyses were conducted to identify the optimization strategy to minimize the RO SEC of the FORO hybrid process. Table 1 summarizes the parameters and the range of parameters to be analyzed. The average of the magnitude of the normalized sensitivity coefficient was used to assess parameter ranks (see Section 2.3). Table 2 shows a summary of the sensitivity analysis for the RO SEC of the FO-RO hybrid process considering FO energy

Fig. 3. Performance comparison between the FO-RO hybrid process and the stand-alone RO processes. The vertical axis in (a) and (b) represents the SEC and recovery rate of the RO process, respectively. The horizontal axis indicates applied pressure on the RO process. The solid line represents the FO-RO hybrid process, and dashed line represents the stand-alone process. (a) SEC with respect to RO applied pressure. The dashed blue line represents the SEC of the stand-alone FO process. (b) RO recovery rate with respect to RO applied pressure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

applied pressure increases, 28.2, 21.8, and 20.1% at 30, 40, and 50 bar of the RO applied pressure, respectively. To identify the characteristics of the decreasing performance gap between the FO-RO hybrid process and the stand-alone RO process in the higher RO applied pressure range (i.e., 40 to 60 bar), the ICR of the RO feed solution along the RO pressure vessel (ICRcRO(x)) and the RO water flux difference (ΔJW. RO(x)) between the FO-RO hybrid process and the stand-alone RO process with respect to distance and RO applied pressure are evaluated (see Fig. 4(a) and (b), and Eqs. (24) and (25)). According to Fig. 4(a), ICRcRO(x) increases as the distance increases. This implies that the RO feed of the FO-RO hybrid process is concentrated more quickly than that of the stand-alone RO process. Furthermore, in the cases in which the RO applied pressure is sufficiently high, such as 35 bar, the ICRcRO(x) approaches 1 as distance increases. Once ICRcRO(x) approaches 1, it is difficult to expect a higher RO water flux in the FORO hybrid process compared to the stand-alone RO process. The left right arrows in Fig. 4(a) indicate the distance that ICRcRO(x) < 1 from the distance 0 at the given RO applied pressure. Fig. 4(b) illustrates the RO water flux difference between the FO-RO hybrid process and the stand-alone RO process, with ΔJW. RO(x), approaches zero as ICRcRO(x) approaches 1. The left right arrows in Fig. 4(b) indicate the distance that ΔJW. RO(x) > 0 from the distance 0 at the given RO applied pressure. Consequently, the decreasing performance gap between the FO-RO hybrid and the stand-alone RO processes in a higher-pressure range (i.e., 40 to 60 bar) is induced by the reduced distance shown by ΔJW. (x) > 0 (which is indicated by the left right arrows in Fig. 4(b)). In RO Fig. 4(a) the higher RO applied pressure range (the region where

Fig. 4. (a) The ICR of the RO feed along the RO pressure vessel and (b) RO water flux difference between the FO-RO hybrid and the stand-alone RO processes with respect to the distance and RO applied pressure. The vertical and horizontal axes indicate the RO applied pressure and distance, respectively. In (a), the colour bar represents the RO feed concentration ratio between the FORO hybrid and the stand-alone RO processes. Values higher than 1.0 are colored the same as for 1.0. The ICR ranges from 0.69 to 1.01. In (b), the colour bar represents the RO water flux difference between the FO-RO hybrid and the stand-alone RO processes. RO water flux difference ranges from −0.88 to 10.09 L/m2 h. 46

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Table 2 Sensitivity analysis of FO-RO hybrid and stand-alone RO processes. Sensitivity of the stand-alone RO process was calculated as follows: (1) RO applied pressure, 0.4591, (2) RO feed flow velocity, 0.0207, and (3) RO water permeability, 0.0052. * Italic bold parameters indicate control parameters for operation.

RO FO

RO FO RO

Parameters

FO-RO hybrid process rank

FO-RO hybrid process sensitivity

Categories

Applied pressure, ΔpRO0 Draw concentration, cd. FO0 Draw flow velocity, ud. FO0 Structure parameter, S Water permeability, AFO Feed flow velocity, uf. FO0 Feed flow velocity, uf. RO0 Feed concentration, cf. FO0 Solute permeability, B Water permeability, ARO

1 2 3 4 5 6 7 8 9 10

0.6202 0.3276 0.0864 0.0435 0.0258 0.0155 0.0124 0.0116 0.0058 0.0028

Control parameter for operation Water quality Control parameter for operation Intrinsic membrane parameter Intrinsic membrane parameter Control parameter for operation Control parameter for operation Water quality Intrinsic membrane parameter Intrinsic membrane parameter

RO SEC (kWh/m3)

1.8

consumption. The RO applied pressure is the most sensitive to the RO SEC on the FO-RO hybrid among the considered conditions. The FO draw (which becomes the RO feed after the FO process) concentration is ranked second. This result stems from the nature of the RO water flux (see Eq. (1)). With a major drop in sensitivity, the FO draw flow velocity, and FO structure parameter are ranked third and fourth, with values of 0.0864 and 0.0435, respectively. In addition, FO water permeability and FO feed are ranked fifth and sixth, respectively. Considering the magnitude of the sensitivity, the control parameters for operation (RO applied pressure and FO draw flow velocity) affect more than the intrinsic membrane parameters to the RO SEC on the FO-RO hybrid process. In addition, among the intrinsic membrane parameters, the sensitivity analysis implies that minimizing the ICP on the FO draw side and the maximizing FO water permeability is critical. As shown in Fig. 5, the RO SEC on FO-RO hybrid process considering FO energy consumption increases as the FO draw flow velocity increases. On the other hand, RO SEC decreases as FO feed flow velocity increases. This tendency can be analyzed as follows: (1) resistance time of the FO draw on the FO process should be increased to maximize the dilution of the FO draw; (2) resistance time of the FO feed on the FO process should be decreased to maximize the FO water flux by supplying fresh FO feed; and (3) sufficient flow velocity of the FO feed is required to minimize ECP in the low velocity range on the FO feed side (as described in Section 3.1).

1.7 1.6 1.5 1.4 0.01

RO SEC vs FO draw flow velocity RO SEC vs FO feed flow velocity RO SEC reference

0.04

0.07

0.10

Flow velocity (m/s) Fig. 5. The SEC of the RO process on FO-RO hybrid process considering FO energy consumption with respect to FO draw and FO feed flow velocity. The solid line represents RO SEC with respect to FO draw flow velocity. The dashed line represents RO SEC with respect to FO feed velocity. The solid red line indicates the reference value of RO SEC. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3.4. Optimization strategy for the FO-RO hybrid process To find the optimal FO-RO hybrid process design minimizing the RO SEC, the impacts of RO applied pressure and FO draw flow velocity that ranked first and third were analyzed (see Table 2). Fig. 6 shows that the RO SEC on the FO-RO hybrid process decreases as the FO draw flow velocity decreases (see Fig. 5). On the other hand, the optimal RO applied pressure range was derived with respect to the FO draw flow velocity. The solid black line in Fig. 6 indicates the optimal combination of the RO applied pressure and the FO draw flow velocity that minimizes the RO SEC on the FO-RO hybrid process. In cases where the FO draw flow velocity is < 0.016 m/s, < 20 bar of the RO applied pressure is recommended to minimize the RO SEC on the FO-RO hybrid process. In cases in which the FO draw flow velocity is greater than the 0.016 m/s, the RO applied pressure should be chosen along the solid black line in Fig. 6. Overall, both the RO applied pressure and the minimum RO SEC on the FO-RO hybrid process increase as the FO draw flow velocity increases (also see Section 3.3).

Fig. 6. SEC of the RO process on the FO-RO hybrid process with respect to RO applied pressure and FO draw solution flow velocity. The vertical axis indicates applied pressure on the RO process, and the horizontal axis represents the flow velocity of draw solution on the FO process. Colors indicate the specific energy consumption of the RO process. A SEC of the RO process that is > 2 kWh/m3 is depicted in red. Optimal RO applied pressure minimizing specific energy consumption of the RO process with a given FO draw solution flow velocity is represented as a solid black line. In the supplementary material, RO recovery rate on the FO-RO hybrid process with respect to RO applied pressure and FO draw solution flow velocity is provided in Fig. S3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3.5. Discussion A sensitivity analysis was conducted to derive the dominant parameter that is affecting the RO SEC on the FO-RO hybrid process. According to the sensitivity analysis, control parameters for operation affect the RO SEC on the FO-RO hybrid more than the intrinsic membrane parameters (see Table 2). Moreover, a slow FO draw flow velocity 47

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Fig. 7. Conceptual design of the FO process to minimize RO SEC on FO-RO hybrid process. The vertical axis indicates the RO SEC on the FO-RO hybrid process. The horizontal axis represents the number of parallel connections of the FO process for the FO-RO hybrid process.

and a fast FO feed velocity are recommended (see Figs. 5 and 6). To apply this idea to the FO-RO hybrid process design, the FO elements installed in front of the RO process should be configured with a parallel connection to minimize RO energy consumption. Fig. 7 shows the conceptual idea that the parallel connection of the FO elements can reduce RO SEC on the FO-RO hybrid process. Considering a fixed seawater intake flow rate to meet the required water production on the RO process, FO draw (seawater) flow velocity can be reduced by the parallel connection. In addition, to increase flow velocity and maintain a sufficient flow rate of the FO feed (wastewater), increased intake of the wastewater can be recommended. In addition, an FO element capable of the high flow rate change of the FO draw and feed is required. However, despite the reduction of the operational cost on the RO process by minimizing the RO SEC, the capital cost of the FO process can be increased. In addition, to maintain a sufficient FO feed flow rate, the pretreatment cost and energy consumption of wastewater before sending the FO that is due should be carefully considered in the cost analysis. Moreover, the effect of a slow FO draw flow velocity on the FO membrane's fouling should also be considered. In the RO process, the initial RO water flux is increased due to the reduced RO feed concentration by the FO process. Therefore, an imbalance of the RO water flux from the inlet and the outlet of the RO process is induced. For the long-term operation of the RO process with the imbalance of the RO water flux, the adoption of an ISD can be used to manage membrane fouling [58]. Moreover, applying the secondstage RO process can be considered for the FO-RO hybrid process to increase water production. A lower RO brine concentration from the FO-RO hybrid process can be utilized in the second-stage RO process. On the other hand, the first stage of the RO process applied to the FORO hybrid process can be recommended to reduce the environmental impacts by the RO brine.





less RO applied pressure is required in the FO-RO hybrid process compared to the stand-alone RO process. However, the performance gap between the FO-RO hybrid and stand-alone RO processes decreases in the higher RO applied pressure range. The faster concentration of RO feed on the FO-RO hybrid process reduces the performance gap in the higher RO applied pressure region. In the FO-RO hybrid process, the RO SEC is affected by the control parameters for operation being more sensitive than the intrinsic membrane parameters. The RO applied pressure and the FO draw flow velocity are derived as the major factors from the sensitivity analysis. In addition, the FO process, with a slower FO draw flow velocity and a faster FO feed flow velocity, is recommended to maximize the dilution of the FO draw and minimize the concentration polarization phenomena of the FO feed in the FO process. The importance of the membrane structure parameter is also emphasized rather than the FO water permeability and the FO solute permeability in the FO process. To minimize the RO SEC on the FO-RO hybrid process, the parallel connection of the FO elements can be suggested to maximize the dilution of the FO draw that becomes the RO feed. Using the parallel connection of the FO elements, the FO draw flow velocity can be reduced. To explore the suggested idea, further research should be conducted, such as the FO element design, the RO process design for the FO-RO hybrid process, and a cost analysis of the FO-RO hybrid considering capital and operational costs.

Acknowledgement This Research has been performed as a project No KK1923-10 (Development of key membranes for high efficiency seawater desalination) and supported by the Korea Research Institute of Chemical Technology (KRICT).

4. Conclusion Appendix A. Supplementary data The feasibility of the FO process to the RO-based hybrid process has been investigated in various studies. This study investigated the FO-RO hybrid process for desalination using numerical modeling. Beginning with the development of the FO-RO hybrid process model, performance comparison between the FO-RO hybrid process and the stand-alone RO process, sensitivity analysis of the FO-RO hybrid process, and an optimal design strategy for the FO-RO hybrid process were discussed. The main conclusions of this study are summarized as follows:

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