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Review Cite This: Environ. Sci. Technol. Lett. 2018, 5, 467−475
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High-Pressure Reverse Osmosis for Energy-Efficient Hypersaline Brine Desalination: Current Status, Design Considerations, and Research Needs Douglas M. Davenport, Akshay Deshmukh, Jay R. Werber, and Menachem Elimelech* Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, United States
Environ. Sci. Technol. Lett. 2018.5:467-475. Downloaded from pubs.acs.org by 191.101.54.134 on 01/19/19. For personal use only.
S Supporting Information *
ABSTRACT: Water scarcity, expected to become more widespread in the coming years, demands renewed attention to freshwater protection and management. Critical to this effort are the minimization of freshwater withdrawals and elimination of wastewater discharge, both of which can be achieved via zero liquid discharge (ZLD), an aggressive wastewater management approach. Because of the high energetic cost of thermal desalination, ZLD is particularly challenging for high-salinity wastewaters. In this review, we discuss the potential of high-pressure reverse osmosis (HPRO) (i.e., reverse osmosis operated at a hydraulic pressure greater than ∼100 bar) to efficiently desalinate hypersaline brines. We first discuss the inherent energy efficiency of membrane processes compared to that of conventional thermal processes for brine desalination. We then highlight the opportunity of HPRO to reduce energy requirements for desalination of key high-salinity industrial wastewaters. The current state of membrane materials and processes for hypersaline brine desalination is also discussed, emphasizing several process design considerations unique to HPRO. Lastly, we discuss the most pressing research needs for the development of HPRO, notably the development of membranes and modules suitable for high pressures as well as fundamental studies of compaction and transport under HPRO conditions.
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flotation, chemical precipitation, or advanced oxidation to remove oil and grease and reduce organic content.9 Produced water treated in this fashion is often discharged to the ocean at offshore locations, albeit with some environmental concerns.10 At inland sites, treated produced water can be reused in hydraulic fracturing fluid.9,11 When safe discharge or reuse is not possible, brines are commonly disposed of via deep well injection or treated using evaporation ponds prior to the disposal of solids in landfills.9,11−13 These processes, however, have serious environmental limitations. For instance, some geologic formations, such as those surrounding the Marcellus shale fields in the United States, are unsuitable for wastewater disposal via deep well injection because of the high potential to contaminate groundwater.14 Furthermore, deep well injection has been linked to groundwater contamination in other areas as well as increased seismic activity.13 Volume minimization using evaporation ponds is similarly problematic as it poses a threat to groundwater and birdlife, requires large areas of land, and is effective in only warm, arid climates.15,16 The challenges associated with brine management often result in exorbitant disposal costs, which can even lead to otherwise promising brackish water resources remaining untapped as sources for potable water.7 To avoid expensive brine disposal, an
INTRODUCTION Recent estimates suggest 1.2 billion people live in areas of physical water scarcity.1 Effective water resource management is critical to ensure the widespread availability of freshwater in the coming decades.2 Two specific concerns are the treatment and disposal of wastewater and the utilization of previously untapped water sources. Management of wastewater from industrial sources is particularly challenging as it can be highly saline, vary temporally in volume and composition, and contain complex mixtures of contaminant species.3 The beneficial reuse of industrial wastewaters mitigates unsafe disposal by eliminating wastewater discharge, while simultaneously decreasing freshwater withdrawals.4,5 A major challenge for the reuse of high-salinity wastewaters, however, is the high energetic cost of brine desalination.6 In the case of inland desalination, brine disposal costs often render the desalination of brackish groundwater economically unfeasible.7 Low-cost brine management is critically needed to enable the utilization of inland brackish groundwater and prevent the unsafe discharge of saline industrial wastewaters. Because of the energy requirements for high-salinity wastewater desalination, the preferred brine management option, when it is both safe and effective to do so, is to dispose or reuse the waste stream without extensive treatment. For example, brine solutions (also called the retentate or reject) produced from seawater reverse osmosis (SWRO) processes can be safely discharged to the ocean without extensive treatment.8 In the oil and gas industry, produced water undergoes relatively moderate treatment via dissolved air © 2018 American Chemical Society
Received: Revised: Accepted: Published: 467
May 26, 2018 June 22, 2018 June 29, 2018 June 29, 2018 DOI: 10.1021/acs.estlett.8b00274 Environ. Sci. Technol. Lett. 2018, 5, 467−475
Review
Environmental Science & Technology Letters increasing level of attention is being given to brine concentration and zero liquid discharge (ZLD) practices, in which waste is disposed of in solid form.17 A critical step in the ZLD process chain is desalination, wherein water is recovered from a saline waste stream. Desalination can be achieved by membrane-based processes, such as reverse osmosis (RO) and electrodialysis, or phasechange-based (designated here as “thermal”) processes, including multi-effect distillation (MED), multi-stage flash (MSF), and mechanical vapor compression (MVC).18 RO utilizes hydraulic pressure, in excess of solution osmotic pressure, to drive the transport of water across a semipermeable membrane while retaining most solutes.19 A combination of membrane and thermal processes are often used to achieve ZLD for saline wastewaters.20 First, RO concentrates wastewater to approximately 70000 mg L−1 total dissolved solids (TDS), which has an osmotic pressure, π, of ∼59 bar. At higher salinities, thermal technologies are used to concentrate brine streams to approximately 250000 mg L−1 (π ≈ 290 bar), the typical inlet concentration for crystallizers in ZLD.17 Thermal-based brine crystallizers then concentrate the waste stream above its solubility limit (e.g., 357000 mg L−1 for NaCl) to extract solid salts for disposal.17 Because of its superior energy efficiency, RO has displaced thermal processes in recent decades for the major applications of seawater and brackish water desalination for drinking water production.21 As such, membrane materials and processes have been optimized to treat feedwaters with salinities equal to or less than that of seawater (typically ∼35000 mg L−1; π ≈ 28 bar). Consequently, the maximum operating pressure for RO is typically around 80 bar,22 suitable for overcoming the retentate osmotic pressure of seawater treated to 50% recovery (∼70000 mg L−1; π ≈ 59 bar). Because of hydraulic-pressure limitations of RO, hypersaline brines, here defined as solutions with a TDS concentration of >70000 mg L −1, are primarily desalinated via thermal processes.17 These processes, however, are energy- and cost-intensive, which limits the implementation of brine wastewater desalination prior to disposal. In this review, we critically discuss the application of highpressure RO (HPRO), defined here as RO operating above ∼100 bar, to the treatment of hypersaline brines. We start by considering the large volumes of industrial brines that could require desalination and the energy requirements of HPRO versus other processes. We largely consider two pressure limits for HPRO: (i) 150 bar as roughly double the current operating limit and (ii) 300 bar to enable retentate concentrations of 250000 mg L−1, the inlet concentration for brine crystallizers. We then discuss the current state of HPRO and the process design requirements for its effective application. We conclude with a discussion of the research needs that must be addressed to enable viable HPRO processes.
SECmin =
RĜ (c P) + (1 − R )Ĝ (c R ) − Ĝ (c F) R
(1)
where Ĝ is the specific Gibbs free energy as a function of composition at a fixed temperature and a fixed pressure and cP and cR are the solute concentrations in the product and retentate streams, respectively. Assuming complete solute rejection, cP = 0 and cR = cF/(1 − R). Because of its salinity dependence, SECmin is high for hypersaline brines (5.3 kWh m−3 for 125000 mg L−1 at 50% recovery) as compared to those of more dilute solutions (1.1 kWh m−3 for 35000 mg L−1 at 50% recovery). In addition, the desalination energy efficiency, η, is defined as the useful specific desalination work performed by the system (i.e., SECmin) divided by its specific energy consumption (SEC): η = SECmin/SEC. Membrane-based desalination processes such as RO and electrodialysis, which do not require a phase change to separate water from dissolved solutes, can achieve energy efficiencies of >40%, particularly with multi-stage systems.24−26 In contrast, phase-change-based or thermal desalination processes, such as MED, MSF, and MVC, are inherently less efficient, with typical η values of 630 kWh m−3 for saline waters, is two orders of magnitude larger than the typical SECmin, imperfect heat recovery severely limits the energy efficiency of all thermal desalination processes.27 To directly compare membrane-based and thermal-based desalination (Figure 1A), we can quantitatively analyze the energetics of relevant processes. Typical SEC values and feed concentrations are shown in Figure 1B for conventional (largely thermal) desalination technologies,17,27 in addition to SEC calculated for HPRO. The SEC of HPRO is modeled assuming a terminal hydraulic pressure (ΔPt) that is 5 bar above the osmotic pressure of the retentate (πR).26,28,29 More detailed insights can be gained by modeling different processes for the same inlet and outlet streams. In particular, a feed solution of 70000 mg L−1 NaCl is concentrated to 250000 mg L−1 via HPRO-based, MVC-based, and hybrid HPRO−MVC desalination systems (Figure 1A). MVC is chosen as a representative thermal process because of its highly effective heat recovery. Its SEC is calculated assuming isentropic (i.e., adiabatic and reversible) vapor compression with a terminal temperature difference (ΔTt) of 5 °C above the boiling point elevation of the retentate (δR).24,30,31 In the two-stage HPRO and MVC models, water recovery in each stage is optimized to minimize SEC. In the hybrid HPRO−MVC model, the HPRO stage is limited to a maximum hydraulic pressure of 150 bar. Further details of SEC calculations are given in the Supporting Information. Panels C and D of Figure 1 show the SEC and energy efficiency of each process, which illustrates the drastic 3-fold energy savings made possible using HPRO rather than a thermal desalination process such as MVC. For example, concentration of a 70000 mg L−1 hypersaline feed to 250000 mg L−1 would require 24 kWh m−3 with two-stage MVC (15% energy efficiency) and only 7.3 kWh m−3 (47% energy efficiency) with two-stage HPRO. For context, the RO stage in a typical SWRO process can operate as low as 1.8 kWh m−3 (59% energy efficiency).25 Achieving a retentate concentration of 250000 mg L−1 with HPRO would require approximately 300 bar of hydraulic pressure, a challenging target from membrane materials and module design perspectives. For this
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ENERGY EFFICIENCY OF MEMBRANE DESALINATION Desalinating highly saline waste streams is inherently energyintensive. The minimum specific energy consumption (SECmin) of desalination (i.e., the energy required by a thermodynamically reversible process per unit volume of product water) depends on the salinity of the feed stream, cF, and the water recovery ratio, R, the proportion of water recovered from the feed stream:23,24 468
DOI: 10.1021/acs.estlett.8b00274 Environ. Sci. Technol. Lett. 2018, 5, 467−475
Review
Environmental Science & Technology Letters
Figure 1. (A) Brine desalination process schematics. Three different process configurations are shown for the concentration of a 70 g L−1 NaCl solution to 250 g L−1. (B) Specific energy consumption (SEC) of various desalination processes. The typical feed concentration range and energy consumption are shown for conventional RO and thermal brine concentrators.17,27 The expected SEC calculated for high-pressure reverse osmosis (HPRO), at pressures of ≤150 and ≤300 bar, is also shown. (C) Specific energy consumption and (D) energy efficiency for HPRO and mechanical vapor compression (MVC) brine desalination processes. The energy consumption requirements of five membrane-based and thermal-based process configurations are analyzed for the desalination of 70 g L−1 NaCl at increasing retentate concentrations (i.e., water recovery). The energy efficiency, η, is the ratio of the minimum energy of desalination for a given brine salinity and recovery, SECmin, to the SEC for each process. Shown here are one- and two-stage processes for both HPRO and MVC. A hybrid HPRO−MVC process is also shown where an HPRO stage is employed at a maximum applied pressure of ≤150 bar followed by an MVC stage. The water recovery ratio of each stage is optimized to reduce specific energy consumption for each two-stage process. Further details about the calculations are provided in the Supporting Information.
regulations. In many circumstances, regulatory, financial, and environmental restrictions motivate brine concentration and volume reduction.17 The coal-to-chemicals industry in China, for example, requires ZLD for all new facilities.32 Additionally, the financial cost of brine disposal from inland desalination facilities motivates brine volume minimization.7 In addition, strict disposal regulations for landfill leachate in the United States and effluent from the textile industry in India necessitate brine volume reduction to minimize environmental impacts and health risks, despite the financial and energetic cost of brine desalination.33,34 Figure 2A shows the global desalination capacity that is currently installed or under construction for brine feedwaters with a salinity of ≥50000 mg L−1 TDS.35 At present, these brine desalination processes utilize conventional thermal separation processes (e.g., MVC, MED, or MSF) or conventional RO.35 Of the nearly 16000 desalination plants online or under construction in the GWI/IDA Desalting Inventory, only 110 treat brine feedwaters.35 In practice, the high cost of brine desalination limits hypersaline wastewater desalination before disposal. Therefore, the total volume of hypersaline wastewater present in each industry is surely much greater. In the United States, the Environmental Protection Agency cites the high financial cost of brine desalination as the reason it does not require evaporative brine concentration of flue gas desulfuriza-
reason, a hybrid process is modeled in which HPRO is limited to a maximum pressure of 150 bar, after which the retentate is further concentrated by MVC. At a retentate concentration of 250000 mg L−1, the hybrid process SEC is 12 kWh m−3, representing a 2-fold reduction compared to that of two-stage MVC. Below a retentate osmotic pressure of 150 bar, the SEC of the hybrid HPRO−MVC process is identical to the SEC of one-stage HPRO, as the optimal water recovery ratio of its first stage (HPRO) is almost equal to its final recovery. As the retentate osmotic pressure increases beyond 150 bar, the water recovery of the second stage (MVC) begins to increase, leading to an increase in SEC. It is important to note that the HPRO−MVC hybrid is a two-stage process and thus the brine flow rate entering the less efficient MVC stage is significantly lower than the initial flow rate (approximately 50% lower for the conditions modeled). Consequently, the energetic performance of the hybrid HPRO−MVC process should be assessed relative to two-stage HPRO and two-stage MVC, rather than one-stage HPRO.
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POTENTIAL IMPACT OF HPRO A variety of industrial sources generate hypersaline brine wastewaters. Table 1 shows several brines of particular concern because of their large volume, concentration, or strict disposal 469
DOI: 10.1021/acs.estlett.8b00274 Environ. Sci. Technol. Lett. 2018, 5, 467−475
Review
Environmental Science & Technology Letters Table 1. Characteristics of Significant Brine Sources, Current Practices, and Disposal Regulations brine wastewater
representative flow rate
typical TDS (mg L−1)
current disposal practice
regulations regarding disposal
oil and gas produced water
United States:11 8.7 million m3 day−1
13000−2100009
offshore: direct ocean discharge11 onshore: deep well injection (where possible), reuse for hydraulic fracturing, evaporation ponds9,11
United States: regulations vary regionally based on permits9,11 Pennsylvania: discharge monthly average TDS of
