A Next Generation System for Low-Energy Desalination

May 5, 2014 - water from desalination plants at approximately 24.5 million m3/ .... pressure exchanger from Isobaric Strategies, Inc. (Virginia Beach,...
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Experimental Results from RO-PRO: A Next Generation System for Low-Energy Desalination Andrea Achilli,† Jeri L. Prante,‡ Nathan T. Hancock,§ Eric B. Maxwell,§ and Amy E. Childress*,∥ †

Environmental Resources Engineering Department, Humboldt State University, Arcata, California 95521, United States Department of Civil and Environmental Engineering, University of Nevada, Reno, Nevada 89557, United States § Oasys Water Inc., Boston, Massachusetts 02210, United States ∥ Astani Department of Civil and Environmental Engineering, University of Southern California, Los Angeles, California 90089, United States ‡

ABSTRACT: A pilot system was designed and constructed to evaluate reverse osmosis (RO) energy reduction that can be achieved using pressure-retarded osmosis (PRO). The RO-PRO experimental system is the first known system to utilize energy from a volume of water transferred from atmospheric pressure to elevated pressure across a semipermeable membrane to prepressurize RO feedwater. In other words, the system demonstrated that pressure could be exchanged between PRO and RO subsystems. Additionally, the first experimental power density data for a RO-PRO system is now available. Average experimental power densities for the RO-PRO system ranged from 1.1 to 2.3 W/m2. This is higher than previous river-to-sea PRO pilot systems (1.5 W/m2) and closer to the goal of 5 W/m2 that would make PRO an economically feasible technology. Furthermore, isolated PRO system testing was performed to evaluate PRO element performance with higher cross-flow velocities and power densities exceeding 8 W/m2 were achieved with a 28 g/L NaCl draw solution. From this empirical data, inferences for future system performance can be drawn that indicate future ROPRO systems may reduce the specific energy requirements for desalination by ∼1 kWh/m3.

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INTRODUCTION Challenges related to global water scarcity coupled with growing concerns about the rising costs and environmental consequences of fossil fuels are driving interest and innovation in water desalination systems powered by renewable energy resources. While solar and wind are among the most well-known renewable energy resources, another nearly untapped resource exists within salinity gradients. Salinity gradient power potential is significant as it could increase current renewable energy production by 20%.1,2 One method to harvest salinity gradient power is through pressure-retarded osmosis (PRO), a process where a pressurized higher salinity (draw) solution is used to extract fresh water through a semipermeable membrane from a lower salinity solution.3,4 This increases the available mechanical (pressure− volume) work in the draw solution, which can be used to spin hydro turbines for electrical power production or can be used directly to supplement the mechanical load required for reverse osmosis (RO) desalination systems.5 RO desalination is currently the most efficient widely adopted commercial desalination technology; however, it still requires a great deal of energy to create the high pressures necessary to overcome the osmotic pressure of saline waters.6,7 RO desalination energy consumption, including pretreatment energy costs, ranges between 6 and 8 kWh/m3 without the use of energy recovery devices (ERDs).6,8 However, with efficiencies up to © 2014 American Chemical Society

97%, ERDs are now commonplace and can reduce energy consumption by as much as 60%.9 Implementing an ERD, such as a pressure exchanger (PX), can enable replacement of the high pressure RO pump with a smaller, more efficient pump,10 which can decrease energy consumption to less than 2.5 kWh/m3 in RO-PX systems.11,12 In addition to energy costs for seawater desalination, there are also environmental concerns associated with the discharge of concentrated brine through ocean outfalls.13−20 With the worldwide production of fresh drinking water from desalination plants at approximately 24.5 million m3/ d and increasing, a process that can synergistically reduce the energy demand of RO desalination systems and mitigate issues associated with discharge of RO brine to sensitive receiving environments could have wide application. Figure 1 is a diagram of a RO-PRO system whereby a volume of seawater or another high salinity feed solution (Vf) is prepressurized in the PX prior to entering the RO subsystem where desalination occurs. Two streams exit the RO subsystem: a freshwater permeate stream (Vp,RO) and a concentrated brine waste stream (Vc). The concentrated brine stream enters the Received: Revised: Accepted: Published: 6437

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The pilot-scale RO-PRO system that is the subject of this paper is the first reported RO-PRO system. A similar system, in which RO brine is used as draw solution and treated sewage as the impaired water source has been reported in Japan.23 However, this system does not represent a coupled RO-PRO system where the energy generated by PRO is utilized in the RO process. Instead, eight 10 in. hollow fiber PRO membrane modules are being tested alone and membrane fouling has been the major problem encountered in this system. The objectives of this investigation are to report on the feasibility of the RO-PRO system to desalinate seawater using less energy than conventional systems. The RO-PRO system was evaluated using concentrated brine from the RO subsystem as a high salinity draw solution and tap water as a low salinity feedwater in the PRO subsystem. Using tap water instead of a wastewater avoids the fouling issues that have afflicted previous systems so that the maximum potential of the RO-PRO system could be assessed. The data collected from the pilot system testing was then used to calculate the energy requirements of the RO-PRO small pilot system and also to predict requirements for a larger, optimized system.

Figure 1. Diagram of the RO-PRO system where a volume of the RO feed solution (Vf) is prepressurized in the pressure exchanger (PX) prior to entering the RO subsystem where desalination occurs. Exiting the RO subsystem are two streams: a freshwater permeate stream (Vp,RO) and a concentrated brine waste stream (Vc). The concentrated RO brine stream (Vc) is first partially depressurized using an energy recovery device (ERD) and then enters the PRO subsystem as the draw solution (Vds,en). The feed solution for the PRO subsystem (Vf,en) is a low salinity solution. Through osmosis, the pressurized draw solution extracts water from the impaired water source under isobaric conditions, resulting in a concentrated feed (Vf,ex) and diluted draw solution (Vds,ex). The energy stored in the diluted draw solution is exchanged with the seawater RO feed in the PX prior to discharge in order to recover its potential energy and increase the energy savings of the RO-PRO system.

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MATERIALS AND METHODS RO Subsystem. Three spiral-wound RO membrane modules (SW30−2540, Dow FilmTec, Midland, MI) were installed into high-pressure vessels in the small-scale pilot system. Each module had an active membrane surface area of 2.8 m2. The membrane modules were arranged in series so that the concentrated brine leaving the first module was the feed solution for the subsequent module. According to manufacturer recommendations, feed pressure and flow rate were gradually increased over a 30- to 60-s time period in order to avoid possible membrane damage. In addition, after initial wetting occurred, the elements were kept moist at all times to preserve the membranes. The manufacturer rated salt rejection for SW30-2540 modules is 99.4%. PRO Subsystem. It was not until recently that research began to focus on enhancements that would facilitate future commercial production of PRO elements.24−36 This study is the first to report on a first generation 4040 spiral-wound TFC PRO membrane module (Oasys Water, Boston, MA). The module has an active membrane surface area of approximately 4.18 m2 and was installed into a high-pressure vessel in the small-scale pilot system. According to the manufacturer, the water permeability coefficient, salt permeability coefficient, and structural parameter for the PRO membrane were 1.42 × 10−8 (m/s)/kPa, 2.41 × 10−8 m/s, and 3.10 × 10−4 m, respectively. PRO water flux was calculated by

PRO subsystem as a high salinity (draw) solution (Vds,en). The feed solution for the PRO subsystem (Vf,en) is a low salinity solution. Through osmosis, the pressurized draw solution extracts water from the impaired water source under isobaric conditions, resulting in a diluted draw solution (Vds,ex). The energy stored in the diluted draw solution is exchanged with the seawater RO feed prior to discharge in order to recover its potential energy and increase the energy savings of the RO-PRO system. The RO-PRO system has several advantages. Compared to a standard RO-PX system, RO energy consumption is further reduced with energy production by PRO. Another key advantage of this system is that the brine generated during the RO process is diluted back to seawater concentration. From a PRO perspective, simply disposing RO brine to the sea is a waste of a “good draw solution” for three reasons. First, among other readily available draw solutions, RO brine is an abundantly available, low-cost residual from existing commercial systems. Second, RO brine has a relatively high concentration to enable higher power production.21 Third, the brine entering the PRO subsystem is relatively free of foulants because it receives prior treatment by the RO pretreatment system, which eliminates additional energy expenditure that would be necessary in the river-to-sea PRO system. Thus, a PRO system that uses concentrated brine as a draw solution not only reduces feedwater pumping requirements but also minimizes the adverse environmental impact on marine ecology/habitats that can occur during seawater RO brine disposal. Fouling issues on the seawater side should also be less of a problem than they have been documented to be in river-to-sea PRO systems.22

Jw =

(Q f,en + Q f,ex ) Am

(1)

where Qf,en and Qf,ex are the entering and exiting feed flow rate, respectively, and Am is the membrane surface area. Power density (W) was calculated by W = Jw ΔP

(2)

where Jw is the water flux and ΔP is the pressure difference across the membrane. ΔP was calculated as ΔP = 6438

(Pds,en + Pds,ex ) 2

−

(Pf,en + Pf,ex ) 2

(3)

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Figure 2. (a) RO-alone configuration, (b) RO-PX configuration, and (c) RO-PRO configuration where Vf, Vc, and Vp,RO are the RO feed, concentrated brine, and permeate volumes, respectively; Vds,en, Vds,ex, are the entering and exiting PRO draw solution volumes, respectively; and Vf,en, Vf,ex, and Vp,PRO are the PRO feed solution entering, feed solution exiting, and permeate volumes, respectively. Red circles indicate locations where conductivity samples were taken and green squares indicate locations of flow meters.

where the subscripts ds,en and ds,ex indicate the entering and exiting draw solution, respectively, and the subscripts f,en and f,ex indicate the entering and exiting feed solution, respectively. Pressure Exchanger. An XPR (aXle Positioned Rotor) pressure exchanger from Isobaric Strategies, Inc. (Virginia Beach, VA) was used in the small-scale pilot RO-PRO system. This pressure exchanger works via a displacement method. A highpressure stream enters the pressure exchanger and turns a rotor that pressurizes a low-pressure stream on the other side. In the RO-PRO system, the high-pressure draw solution exiting the PRO subsystem was used to pressurize the low-pressure seawater stream entering the RO subsystem. In this way, the RO feed solution was pressurized before the high-pressure pump in order to decrease the work required by the pump. The pressure exchanger efficiency requires that the entering and exiting flow rates match. If the flow rates are unmatched, leaking occurs within the device until all flows are balanced. Therefore, in addition to monitoring the pressures entering and exiting the pressure exchanger, the flow rates were also monitored to evaluate internal circulation flow within the pressure exchanger. The low-pressure inlets of the pressure exchanger were monitored with dial pressure gauges, lowpressure transducers (Cole-Parmer, Vernon Hills, IL), acrylic flow meters (Cole-Palmer, Vernon Hills, IL), and flow sensors (GF Signet, El Monte, CA). The high-pressure inlet and outlet of the pressure exchanger were monitored with high-pressure transducers (Cole-Parmer, Vernon Hills, IL). RO-PRO Testing Conditions. RO-PRO testing was conducted at the United States Bureau of Reclamation’s Brackish Groundwater National Desalination Research Facility in Alamogordo, NM and the University of Nevada, Reno laboratories. Testing was performed on three configurations: RO-alone (Figure 2a), RO-PX (Figure 2b), and RO-PRO (Figure 2c). All three configurations had the same flow rate entering the seawater pump (5.0 LPM) and were tested with freshwater recoveries of 20 and 30%. Simulated seawater was used as the RO

feed solution for all configurations, and in the case of RO-PRO, filtered (1-μm) municipal tap water was used for the PRO feed solution. The concentration of the NaCl solution (simulating seawater) entering the system was approximately 35−37 g/L and the concentration entering the RO subsystem ranged from 33 to 35 g/L depending on the configuration; the reduced concentrations entering the RO subsystem were due to leaking in the PX. Stainless steel ports consisting of a ball valve and tee were used to withdraw water samples in the high-pressure sections of the RO-PRO system. Operating hydraulic pressures for the RO subsystem ranged between approximately 3500 and 4800 kPa to accomplish the desired freshwater recoveries. Pressure was established using a needle valve that decreased the pressure of the brine stream leaving the RO module to the predetermined design pressure of the PRO module. The energy from this pressure drop was lost to the environment as heat. The PRO feed solution pressure ranged between approximately 100 and 240 kPa. This was well below the draw solution pressure range between approximately 700 and 1700 kPa so there was no chance the PRO membrane envelope could be damaged (delaminated) by overpressurizing the membrane. The specific energy (SE) consumption of each configuration was calculated as

(∑ SE =

Q ·ΔP η

Q p,RO

) (4)

where Q and ΔP are the flow rate and pressure difference across each pump, η is the overall pump efficiency, and the Qp,RO is the RO permeate flow rate. The overall pump efficiency was selected to be 80% as is typical of high-pressure pumps used in RO.37 Each configuration required a different number of diaphragm variablespeed pumps (Hydra-Cell, Wanner Engineering, Inc., Minneapolis, MN). The RO-alone configuration (Figure 2a) required two pumps (seawater pump and RO pump) while the RO-PX (Figure 6439

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Table 1. SE (kWh/m3) for Three Configurations with Empirical Results and Pump Efficiencies of 80%

2b) and RO-PRO (Figure 2c) configurations required three pumps (seawater pump, RO pump, and booster pump). High-Flow-Rate PRO Test System. A high-flow-rate test system was also designed to evaluate PRO performance of the membrane element with higher flow rates (cross-flow velocities). The system included a high-pressure draw solution pump (CRNE1−5, Grundfos, Denmark) and feed pump (CRNE1−27, Grundfos, Denmark). Differential pressure measurements were recorded manually with ANSI-certified analog pressure gauges (Aschcroft, Stratford, CT). Water flux was measured in the conventional manner by monitoring the differential weight as a function of both time of feed and draw solution tanks set on balances (3636-02, Salter Brecknell, Fairmont, MN). The draw solution consisted of 28 g/L of food grade NaCl (Culinoxx 999, Morton, Chicago, IL) and the feed solution consisted of RO permeate from a drinking water source. Each solution was contained in a 950-L (250-gal) tank and was in equilibrium with ambient temperature (∼22 °C). Each PRO element was subjected to 1 h of applied hydraulic pressure at 100 psig prior to testing. Steady-state operation was rapidly achieved with these elements based on draw solution and feed inlet and outlet flow rates and pressures as well as measured water fluxtypically within minutes at the conditions evaluated. During testing, each test condition was maintained for roughly 20 min before recording data to ensure steady state.

SERO SERO‑PX SERO‑PROa RO-PRO with 2nd PXb

20% recovery

30% recovery

6.51 3.73 6.11 3.08

5.25 3.38 4.75 2.64

a

for PRO draw solution of 1,034 kPa (150 psi). bPX efficiency was assumed to be 93%.9

Average PRO power density results are shown in Figure 3a and b. The average experimental power densities for the small-scale

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Figure 3. Average power densities with increasing hydraulic pressure difference for (a) 20% RO recovery and (b) 30% RO recovery.

RESULTS AND DISCUSSION RO-PRO Experimental Results. The baseline specific energy values of the RO subsystem alone (SERO) were 6.51 and 5.25 kWh/m3, for 20 and 30% recoveries, respectively. As expected, SERO was lower for the higher recovery tested. From the literature, SERO passes through a minimum at 50% recovery;38 thus, operation of a full-scale RO subsystem at a recovery of approximately 50% would be desired. However, due to the RO membranes sizing limitations of the pilot RO subsystem, reaching 50% recovery was not possible. A pressure exchanger was then added to the RO subsystem in order to recover the energy from the high-pressure brine solution. The RO-PX configuration is the traditional configuration where the high pressure of the brine solution is transferred by the pressure exchanger into the RO feed solution (Figure 2b). The specific energy values of the RO-PX system (SERO‑PX) were 3.73 and 3.38 kWh/m3 for 20 and 30% recoveries, respectively. As expected, because of the PX, compared to SERO, SERO‑PX is lower and the SERO‑PX still decreases as recovery increases from 20 to 30%.38 This is because the PX efficiency is obviously less than 100%, which means that for this specific system, the minimum SERO‑PX would be between 30 and 50% recovery. A PRO subsystem was then added to the RO-PX system in order to utilize the concentrated brine of the RO subsystem as the draw solution for the PRO subsystem. The specific energy results of the RO-PRO pilot system (SERO‑PRO) are presented in Table 1 alongside the SERO and SERO‑PX results. The specific energies of the RO-PRO system (SERO‑PRO) were 6.11 and 4.75 kWh/m3. These values were higher than the SERO‑PX results simply due to the pressure drop that occurs on the RO brine prior to entering the PRO subsystem. This energy (currently wasted) could be recovered by employing a second PX to save approximately 3.03 kWh/m3 and 2.11 kWh/m3 for 20 and 30% recoveries, respectively. This would bring the SERO‑PRO consumption to a minimum of approximately 3.08 kWh/m3 and 2.64 kWh/m3 for 20 and 30% recoveries, respectively.

pilot RO-PRO system ranged from 1.1 to 2.3 W/m2. As can be seen by the relatively large error bars, one challenge with the small-scale pilot RO-PRO system was achieving consistency between tests. This is likely a result of wetting issues across the length of the membrane.39 According to the manufacturer, the PRO membrane element requires a high-flow-rate flush of the support side of the membrane that is located in the inner part of the membrane envelop to flush out entrained air and ensure full fluid contact within the membrane envelope. Due to the original sizing and flow rate limitations of the small-scale pilot RO-PRO system, the desired wet-out conditions were not consistently attainable. Isolated PRO System Testing. During RO-PRO testing, it was seen that the exiting feed solution salt concentration increased as draw solution pressure increased (data not shown), indicating that the salt permeability varied as operational conditions changed. Therefore, isolated PRO subsystem testing was performed independent of the RO-PRO testing to evaluate PRO subsystem performance. The draw solution hydraulic pressure was varied and entering feed pressure and concentrations were known (left side of Table 2). Readings were taken once the system reached steady state (after approximately 10 to 15 min) for each hydraulic pressure change. Exiting concentrations and the permeate flow rate were then determined (right side of Table 2) and water flux, salt flux, and power density were calculated (Table 3). The information from Table 2 was used to back-calculate the PRO module salt and water permeability coefficients using a model-based PRO previously developed.5 The PRO model accounts for the effects of water flux, reverse salt flux, concentration polarization, and pressure drops on the flow rates, concentrations, and pressures as the feed and draw solutions pass through the PRO membrane module. The calculated water permeability coefficient (A) decreases and the calculated salt permeability coefficient (B) increases with increasing draw solution pressure (Figure 4). Although the 6440

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Table 2. Operating Conditions for PRO-Alone Testing at Increasing Draw Solution Pressures PRODS,en pressure (kPa)

PROF,en pressure (kPa)

PRODS,en concentration (g/L)

PROF,en concentration (g/L)

PRODS,ex concentration (g/L)

PROF,ex concentration (g/L)

PRO permeate flow rate (LPM)

208 345 483 689 1034 1379 1724

103 103 124 138 193 262 400

35.1 35.1 34.8 34.7 34.5 34.1 33.4

0.0 0.0 0.0 0.0 0.0 0.0 0.0

27.4 28.5 28.7 29.6 30.9 31.7 32.4

0.6 0.7 0.8 0.9 1.2 1.4 1.6

1.2 1.0 0.8 0.5 0.3 0.0 −0.2

Focused testing was also conducted with higher feed and draw solution flow rates to demonstrate the performance of the PRO element with reduced external concentration polarization effects. A comparison of empirical element power density measurements for different feed and draw solution cross-flow velocities are shown in Figure 5. Increasing the cross-flow velocity of either

Table 3. Average Water Flux (Jw), Salt Flux (Js), and Power Density (W) for PRO-Alone Testing at Increasing Draw Solution Pressures PRODS,en pressure (kPa)

PROF,en pressure (kPa)

Jw (L/m2h)

Js (L/m2h)

W (W/m2)

208 345 483 689 1034 1379

103 103 124 138 193 262

17.22 14.35 11.48 7.18 3.59 0.00

31.09 39.62 45.22 56.20 79.77 101.20

0.50 0.96 1.15 1.10 0.84 0.00

Figure 5. Measured PRO element power density for various cross-flow velocities expressed in cm/s. Data series are represented by the effective cross-flow velocity of the feed solution and the effective cross-flow velocity of the draw solution (feed/draw solution).

stream can have a substantial effect on element power density. As the support layer of commercial PRO membranes becomes thinner, thus lowering internal concentration polarization, the effects of external concentration polarization on the support (i.e., feed) side of the membrane begin to play a more substantial role in determining overall membrane performance.42 This also shows that the effects of reverse salt flux along with concentration polarization phenomena should be taken into account during process performance modeling.26,43,44 A specific PRO energy yield analysis on the data from the PRO element power density for various cross-flow velocities was also performed. For this evaluation, the energy data were normalized to the draw solution flow rate. The draw solution flow rate is the limiting stream in the RO-PRO system as it is a finite quantity produced by the RO subsystem, as opposed to the river-to-sea PRO configuration where a virtually unlimited quantity of seawater is available. A comparison of specific energies calculated for different feed and draw solution cross-flow velocities is shown in Figure 6. In the first set (Figure 6a), the specific energy is calculated considering only the energy generated by the permeate flow, normalized to the draw solution flow rate. In the second set (Figure 6b), the specific energy is calculated considering also the energy required to overcome the pressure losses in the membrane module by both the feed and draw solution. By comparing the two data sets, it is apparent that when the energy to overcome the pressure losses within the membrane module is included in the calculation, for each flow condition, the overall specific energy produced decreases.

Figure 4. Calculated PRO water permeability coefficients (A) and salt permeability coefficients (B) from experiments using variable draw solution pressure and constant seawater draw solution concentration (35 g/L NaCl).

water and salt permeability coefficients are innate characteristics of the membrane, and thus, not expected to change with operating conditions, nonconstant water and salt permeability coefficients have been observed in other investigations of osmotic membrane processes and have been attributed to membrane deformation due to applied pressure.40,41 Decreasing water permeability coefficients and increasing salt permeability coefficients account for the decrease in PRO permeate flow and increase in exiting feed solution concentration with increasing draw solution pressure seen in Table 2. In theory, PRO membranes have a peak power density at a specific hydraulic pressure difference (equal to approximately half the osmotic pressure difference).21 However, if either the water permeability decreases or the salt permeability increases as the draw solution pressure increases, the peak power density decreases. As shown in Achilli et al. (2009),21 the effective water flux decreases as salt permeability increases, because salt passage and concentration polarization reduce the driving force across the membrane. This further supports the goal of PRO membrane manufacturing to have a maximum water permeability and minimum salt permeability in order to achieve the largest power density. 6441

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densities could result in significantly lower requirements for membrane surface area, and hence land area required for a PRO facility. Results from this investigation can be used to provide insight into land area comparisons for alternative energy facilities. For example, the Ashkelon desalination facility in Israel covers approximately 25 acres and has a capacity of approximately 56 MW of power.48 If it is considered to couple this facility with either PRO, solar, or wind power, results from the current investigation suggest that a PRO facility of the same size (25 acres) would provide a 25% power reduction. This assumes that the PRO membranes have the same productivity as existing RO membranes.5 If Ashkelon were instead coupled with a solar farm, the solar farm would require approximately 100 acres for the same power reduction.49 Similarly, if Ashkelon were coupled with a wind farm, the wind farm would require approximately 1110 acres for the same power reduction.50 Thus, the estimated land use for a PRO facility is 25% of the required area for a solar farm and 3% of the required area for a wind farm. In considering pressure-retarded osmosis (PRO) as one component of an alternative energy portfolio that will reduce dependence on fossil fuel combustion, comparisons with leading renewable energy technologies are necessary; additional comparisons to consider for full-scale pilot RO-PRO facilities include capital costs, operating costs, and availability of the renewable energy source.

Figure 6. Calculated PRO element specific energy for various cross-flow velocities expressed in cm/s. (a) Specific energy is calculated considering only the energy generated by the permeate flow, normalized to the draw solution flow rate. (b) Specific energy is calculated by subtracting the energy required to overcome the pressure losses in the feed and draw solutions from the energy generated by the permeate flow, normalized to the draw solution flow rate. Data series are represented by the effective cross-flow velocity of the feed solution and the effective cross-flow velocity of the draw solution (feed/draw solution).

It appears that doubling of flow rates (shown by “×” symbol) substantially increases power density (Figure 5) without significant loss in energy produced (Figure 6b), but higher flow rates than that may not be beneficial (Figure 6b). Flow rates beyond that result in a minor increase in average power density (Figure 5) and significant decrease in energy produced (Figure 6b). In particular, the negative yield in specific energy in Figure 6b indicates that the energy required for pumping the two fluids outweighs the benefits of harvesting pressure−volume work contributed by the PRO process. Thus, hydraulic pressure drop within the membrane module must be carefully considered when operating PRO systems because conditions that maximize power density (high cross-flow rate) may not maximize energy output, and in general, module designs that minimize pressure drop should be considered. Considerations for RO-PRO Scale-Up. According to previous model results,5 operating a RO-PRO pilot system at 50% RO recovery would result in an SERO‑PRO of approximately 1.13 kWh/m3 − thus reducing the specific energy requirements for desalination by ∼1 kWh/m3.38 Therefore, a full-scale ROPRO system should operate with the RO subsystem at approximately 50% recovery and with 80% efficiency pump systems in order to realize substantial energy savings from the system. For systems that utilize wastewater, or another impaired source with a relatively high fouling potential as the feedwater, the burden of pretreating this water (typically estimated around 0.2 kWh/m3)45 would have to be considered. With regards to the PRO module, the fact that FO/PRO membrane technology has improved in recent years by an order of magnitude (in terms of water and salt permeabilities) is very important. Previous modeling efforts to simulate PRO performance and RO-PRO performance proved that (at the time) current generation membranes needed increased water permeability and decreased salt permeability to enable the technology to be competitive.5 The improvements made by osmotic membrane manufacturers reflect the manufacturers’ response to this and the growing commercial interest in this technology. The membrane power densities achieved in this investigation are higher than that reported for the river-to-sea PRO pilot system in Korea (1.5 W/m2)46 and can exceed 5 W/m2, a goal set for making PRO an economically feasible technology.47 Furthermore, improvements made to increase PRO membrane power

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AUTHOR INFORMATION

Corresponding Author

*Phone: (213) 740-6304; fax: (213) 744-1426; e-mail: amyec@ usc.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support of the U.S. Department of the Interior − Bureau of Reclamation desalination and water purification research and development program, federal opportunity announcement No. R10SF80251.

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REFERENCES

(1) Annual Energy Review (AER); U.S. Energy Information Administration, 2012. (2) Aaberg, R. J. Osmotic power. A new and powerful renewable energy source? Refocus 2003, 4 (6), 48−50. (3) Childress, A. E.; Achilli, A., Pressure-retarded osmosis. In Encyclopedia of Membrane Science and Technology; Hoek, E. M. V., Tarabara, V. V., Eds.; Wiley, 2013. (4) Achilli, A.; Childress, A. E. Pressure retarded osmosis: From the vision of Sidney Loeb to the first prototype installationReview. Desalination 2010, 261 (3), 205−211. (5) Prante, J. L.; Ruskowitz, J. A.; Childress, A. E.; Achilli, A. RO-PRO desalination: An integrated low-energy approach to seawater desalination. Appl. Energy 2014, 120, 104−114. (6) Khawaji, A. D.; Kutubkhanah, I. K.; Wie, J.-M. Advances in seawater desalination technologies. Desalination 2008, 221 (1−3), 47−69. (7) Semiat, R. Energy Issues in Desalination Processes. Environ. Sci. Technol. 2008, 42, 8193−8201. (8) Fritzmann, C.; Lowenberg, J.; Wintgens, T.; Melin, T. State-of-theart of reverse osmosis desalination. Desalination 2007, 216, 1−76. (9) Stover, R. L. Seawater reverse osmosis with isobaric energy recovery devices. Desalination 2007, 203 (1−3), 168−175. (10) MacHarg, J. P. Retro-fitting existing SWRO systems with a new energy recovery device. Desalination 2002, 153, 253−264. (11) Seacord, T. F.; Coker, S. D.; MacHarg, J. Affordable desalination collaboration 2005 results. Desal. Water Reuse 2005, 10−22.

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dx.doi.org/10.1021/es405556s | Environ. Sci. Technol. 2014, 48, 6437−6443