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Critical Review
The Global Rise of Zero Liquid Discharge for Wastewater Management: Drivers, Technologies, and Future Directions Tiezheng Tong, and Menachem Elimelech Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01000 • Publication Date (Web): 08 Jun 2016 Downloaded from http://pubs.acs.org on June 12, 2016
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The Global Rise of Zero Liquid Discharge for Wastewater Management: Drivers, Technologies, and Future Directions
Tiezheng Tong† and Menachem Elimelech*,†,‡ †
Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, United States ‡ Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT), Yale University
*Corresponding author: Menachem Elimelech Email:
[email protected]; Phone: +1 (203) 432-2789
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ABSTRACT
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Zero liquid discharge (ZLD) — a wastewater management strategy that eliminates liquid waste
38
and maximizes water usage efficiency — has attracted renewed interest worldwide in recent
39
years. Although implementation of ZLD reduces water pollution and augments water supply, the
40
technology is constrained by high cost and intensive energy consumption. In this critical review,
41
we discuss the drivers, incentives, technologies, and environmental impacts of ZLD. Within this
42
framework, the global applications of ZLD in the United States and emerging economies such as
43
China and India are examined. We highlight the evolution of ZLD from thermal- to membrane-
44
based processes, and analyze the advantages and limitations of existing and emerging ZLD
45
technologies. The potential environmental impacts of ZLD, notably greenhouse gas emission and
46
generation of solid waste, are discussed and the prospects of ZLD technologies and research
47
needs are highlighted.
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TOC ART
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INTRODUCTION
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Freshwater scarcity, one of the most critical global challenges of our time, poses a major threat to
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economic growth, water security, and ecosystem health 1-3. The challenge of providing adequate
55
and safe drinking water is further complicated by climate change and the pressures of economic
56
development and industrialization. The public and industrial sectors consume substantial
57
amounts of freshwater while producing vast quantities of wastewater. If inadequately treated,
58
wastewater discharge into the aquatic environment causes severe pollution that adversely
59
impacts aquatic ecosystems and public health 4.
60
Recovery and recycling of wastewater has become a growing trend in the past decade due to
61
rising water demand 3. Wastewater reuse not only minimizes the volume and environmental risk
62
of discharged wastewater, but also alleviates the pressure on ecosystems resulting from
63
freshwater withdrawal. Through reuse, wastewater is no longer considered a “pure waste” that
64
potentially harms the environment, but rather an additional resource that can be harnessed to
65
achieve water sustainability.
66
Zero liquid discharge (ZLD) is an ambitious wastewater management strategy that
67
eliminates any liquid waste leaving the plant or facility boundary, with the majority of water
68
being recovered for reuse. ZLD obviates the risk of pollution associated with wastewater
69
discharge and maximizes water usage efficiency, thereby striking a balance between exploitation
70
of freshwater resources and preservation of aquatic environments. Achieving ZLD, however, is
71
generally characterized by intensive use of energy and high cost. As a result, ZLD has long been
72
considered not viable and has been applied only in limited cases 5.
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In recent years, greater recognition of the dual challenges of water scarcity and pollution of
74
aquatic environments has revived global interest in ZLD. More stringent regulations, rising
75
expenses for wastewater disposal, and increasing value of freshwater are driving ZLD to become
76
a beneficial or even a necessary option for wastewater management. The global market for ZLD
77
is estimated to reach an annual investment of at least $100-200 million
78
from developed countries in North America and Europe to emerging economies such as China
79
and India.
6, 7
, spreading rapidly
80
Early ZLD systems were based on stand-alone thermal processes, where wastewater was
81
typically evaporated in a brine concentrator followed by a brine crystallizer or an evaporation 3
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pond. The condensed distillate water in ZLD systems is collected for reuse, while the produced
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solids are either sent to a landfill or recovered as valuable salt byproducts. Such systems, which
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have been in successful operation for 40 years and are still being built, require considerable
85
energy and capital.
86
Reverse osmosis (RO), a membrane-based technology widely applied in desalination 8, has
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been incorporated into ZLD systems to improve energy and cost efficiencies. However, RO,
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although much more energy efficient than thermal evaporation, can be applied only to feedwaters
89
with a limited salinity range. Accordingly, other salt-concentrating technologies that can treat
90
higher salinity feedwaters, such as electrodialysis (ED), forward osmosis (FO), and membrane
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distillation (MD), have emerged recently as alternative ZLD technologies to further concentrate
92
wastewater beyond RO.
93
Although ZLD holds great promise to reduce water pollution and augment water supply, its
94
viability is determined by a balance among the benefits associated with ZLD, energy
95
consumption, and capital/operation costs. Therefore, it is imperative to understand the drivers
96
and benefits that make ZLD a realistic option. Incorporating new technologies, such as emerging
97
membrane-based processes, provides opportunities to reduce the associated energy consumption
98
and costs and to expand the applicability of ZLD.
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In this critical review, we discuss the drivers, incentives, technologies, and environmental
100
impacts of ZLD as an important strategy for wastewater management. We highlight the evolution
101
of ZLD from thermal to membrane-based processes, with a detailed analysis of the advantages
102
and limitations of both existing and emerging ZLD technologies. Lastly, we discuss the
103
environmental impacts of ZLD, the prospects of ZLD technologies, and research needs for
104
improving its feasibility and sustainability.
105 106
ACHIEVING ZERO: DRIVERS AND BENEFITS
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Why ZLD? Figure 1 describes the major drivers and benefits of ZLD implementation. Stricter
108
regulations for wastewater disposal are the primary driver for ZLD. More costly non-compliance
109
penalties along with increasing costs for wastewater disposal can outweigh the high expenses of
110
ZLD installation. As water scarcity intensifies globally, the capability of ZLD to recover
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wastewater to the largest extent further enhances its prospects. Increased public environmental
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awareness constitutes an additional driver, as ZLD avoids negative environmental impacts of
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wastewater discharge and reduces the corresponding public concerns.
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[FIGURE 1]
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In practice, the incentives behind ZLD implementation vary depending on its application
116
and geographical location. Therefore, the drivers and benefits of ZLD are discussed in this
117
section in the context of its global applications. Although ZLD has been applied in places such as
118
the European Union, Australia, Canada, the Middle East, and Mexico 6, 7, 9, 10, examples from the
119
United States, China, and India are highlighted, as they represent the major ZLD markets with
120
the largest served populations and economic power.
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The United States. The birth of ZLD dates back to the 1970s when the increased salinity
122
of the Colorado River led to a regulatory mandate of ZLD for nearby power plants 6, 11. In those
123
days, obtaining approval for discharge agreements for new industrial projects required several
124
years, whereas adoption of ZLD reduced this period to only a few months
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plants remain the major domain of ZLD implementation in the U.S., where feedwaters, such as
126
flue gas desulfurization (FGD) wastewater and cooling tower blowdown, are treated and
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recycled. For example, ZLD has been adopted at the Dallman Power Plant in Illinois to avoid the
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environmental impacts of boron from the FGD wastewater 13. Among the 82 ZLD plants listed in
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a survey by Mickley in 2008 11, more than 60 plants were associated with the power industry; the
130
rest were distributed across areas such as electronics, fertilizer, mining, and chemical industries.
131
6, 12
. Today, power
The U.S. EPA recently completed its guidelines revising the existing regulations on 14
132
wastewater discharge from thermal power plants
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limits on the level of toxic metals and other harmful pollutants in wastewater discharged from
134
power plants, considers zero discharge as the preferred option for pollutants in fly ash transport
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water, bottom ash transport water, and wastewater from flue gas mercury control systems
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Compliance with these tighter wastewater discharge standards provides new regulatory
137
incentives for ZLD installation in U.S. power plants.
138
. This new rule, which sets the first federal
15
.
ZLD can also be used for brine management in inland desalination plants. Compared to 16
139
seawater desalination, brackish water desalination requires much less energy
140
particularly suitable for semi-arid inland regions where seawater is inaccessible 17. However, the
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management of concentrated brines represents one of the biggest challenges for inland
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desalination. Traditional brine management practices, including direct discharge into surface
143
water or publicly owned treatment works (POTW) as well as deep-well injection
144
excluded, due to potentially adverse impacts on surface water and groundwater, insufficient
145
POTW capacity, geological and legal restrictions, and increasing disposal costs. As a result,
146
inland desalination is still not installed at many locations where water is critically needed, such
147
as Las Vegas, Phoenix, and Denver 11.
11, 18
, can be
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ZLD overcomes the challenge of brine discharge, thereby enabling inland desalination in
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water-scarce areas. So far, multiple governmental agencies and organizations, including the U.S.
150
Bureau of Reclamation and California Energy Commission, have investigated ZLD application
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to inland desalination under hypothetical scenarios in Arizona, California, Colorado, Nevada,
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and Texas 11, 19-22. These pioneering studies, however, have not resulted in full-scale ZLD inland
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desalination plants in the U.S., with cost and energy consumption providing the main barriers to
154
implementation.
155
China. Rapid economic development and urbanization have led to rising water
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consumption and rampant pollution in China. In response to this great challenge, China recently
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announced a new Action Plan to tackle water pollution, aiming to largely improve the quality of
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local water resources and ecosystems by 2020 23. This plan, enforced by the central government,
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emphasizes rigorous control of pollutant discharge and promotes water recycling and reuse,
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thereby providing regulatory support for ZLD installation.
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As in the U.S., the power industry is an important contributor to the Chinese ZLD market.
162
Although coal-fired power plants provide more than 70% of the total electricity generated in
163
China
164
owned companies are located in regions that suffer water scarcity or deficit 25. This sharp conflict
165
between energy demand and water deficiency makes ZLD one of the few sustainable solutions at
166
the energy-water nexus in China. Although no data have been revealed on the overall ZLD
167
installation in Chinese power industry, a rising trend of ZLD adoption is indicated by the recent
168
construction of the world’s first FO-based ZLD system at the Changxing coal-fired power plant
169
in Zhejiang Province 26.
24
, 65-84% of water-intensive thermal power plants operated by the five largest state-
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The recent boom of the coal-to-chemicals industry in China generates another promising
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niche for ZLD application. The coal-to-chemicals industry, utilizing coal rather than oil or
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natural gas to produce raw materials for chemical production, is currently under pressure to
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reduce dependence on imported energy
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amount of freshwater but are often located in water-stressed areas, such as Inner Mongolia where
175
ample coal reserves and environmentally sensitive grassland coexist. As a consequence, ZLD is
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mandatory at coal-to-chemicals plants in those areas to preserve both local water resources and
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ecosystems 28. Several ZLD facilities are already installed or in the stage of design/construction
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at Chinese coal-to-chemicals plants, with a wide range of feedwater salinities (2,000-16,000
179
mg/L of total dissolved salts, TDS) and treatment capacity (110-2,300 m3/hour) 29-32.
180
27
. Coal-to-chemicals plants consume a considerable
In addition, greater public awareness of water pollution may facilitate ZLD implementation 33
181
in China. Multiple projects, including several para-xylene (PX) chemical plants
and a
182
wastewater discharge pipeline for a paper mill 34, have been recently suspended or canceled as a
183
result of public protests. The growing influence of public concern may force industries to adopt
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ZLD as a necessary solution to gaining public acceptance.
185
India. Facing a situation similar to that in China, India is taking aggressive actions to curb
186
severe water pollution, even in the Holy River Ganga. The recent three-year target set by the
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Indian government, known as the “Clean Ganga” project, imposes stricter regulations on
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wastewater discharge and moves high-polluting industries towards ZLD
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government issued a draft policy that requires all textile plants generating more than 25 m3 of
190
wastewater effluent per day to install ZLD facilities
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dyeing plants in the city of Tirupur had already implemented ZLD by 2008, which recovered not
192
only water but also valuable salts from textile wastewater for direct reuse in the dyeing process.
193
According to a recent technical report 39, the ZLD market in India was valued at $39 million in
194
2012 and is expected to grow continuously at a rate of 7% from 2012 to 2017. In this market, the
195
textile, brewing and distilling, power, and petrochemical industries are the major application
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areas 39.
36, 37
CONVENTIONAL ZLD SYSTEMS
7
. In 2015, the
. As reported by Vishnu et al.
197 198
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Thermal ZLD Systems. Early ZLD systems were typically based on a series of thermal
200
processes (Figure 2A). In such systems, the feed wastewater undergoes a pretreatment step that
201
reduces scaling potential, and is then concentrated sequentially by two core elements ⎯ a brine
202
concentrator and a brine crystallizer (or an evaporation pond). The distillates generated by the
203
brine concentrator and crystallizer units are reused as clean product water, whereas the solids
204
produced are either stored (in evaporation ponds), further processed for landfill disposal, or
205
reused as valuable byproducts.
206
[FIGURE 2]
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Brine concentrators commonly use mechanical vapor compression (MVC) for water
208
evaporation. Although other thermal desalination technologies, such as multi-effect distillation
209
(MED) and multi-stage flash (MSF), have been extensively used in seawater desalination 40, their
210
applications in ZLD systems have not been reported in literature. In MVC, the feedwater is
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preheated by heat exchangers utilizing the sensible heat from the distillate product water, and
212
then mixed with the recirculating brine slurry at the sump of the brine concentrator. The brine
213
slurry is conveyed to the top of the concentrator and flows down inside a bundle of heat transfer
214
tubes. The flowing brine forms a thin film on the internal tube surface where water evaporation
215
occurs. Calcium sulfate seeds are often added into the recirculating brine to provide preferential
216
precipitation sites, which keep precipitating salts in suspension and prevent scale formation on
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the heat transfer tubes
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delivers the compressed vapor to the external surface of the heat transfer tubes. The superheated
219
vapor condenses, transferring its latent heat to vaporize the falling brine slurry. The condensate
220
travels down the heat transfer tubes and is collected as distillate that preheats the incoming
221
feedwater before being reused.
11, 41
. The produced water vapor flows to the vapor compressor, which
222
The formation of a falling thin film enhances the heat transfer rate, thereby reducing the
223
compression ratio and required energy of the compressor 42. The use of energy recovery devices
224
(e.g., heat exchangers) further decreases the energy consumption. Even so, MVC brine
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concentrators are still very energy-intensive and require high-grade electric energy. They
226
typically consume 20-25 kWhe/m3 of treated feedwater
227
kWhe/m3 of feedwater) reported in the literature
228
applied successfully in ZLD processes for decades 41, MVC brine concentrators set a benchmark
8
43
11, 22
, with higher values (up to 39
. As an established technology that has been
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for energy comparison with other technologies, which guides efforts to reduce energy
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consumption in ZLD. Further, brine concentrators are able to reach salinity concentrations of
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250,000 mg/L, with a water recovery of 90-98%, and produce high-quality product water (TDS
90%
298
for brackish water 48). The HERO process has been applied in multiple full-scale ZLD systems
299
worldwide
300
treatment capacity of 2300 m3/h 31.
301
29, 31, 49
22, 47, 48
. The feedwater is treated by a weakly acidic cation exchange
, including a recent project for a Chinese coal-to-liquids plant with a high
Current RO membrane modules cannot operate at very high hydraulic pressure, which 42
302
typically corresponds to a salinity of ~70,000 mg/L of the RO exit brine
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RO systems imposes a ceiling on the salinity of water that can be treated by RO in ZLD systems.
304
This salinity limit is much lower than that achieved by brine concentrators (i.e., up to 250,000
305
mg/L). Thus, a stand-alone RO system is not able to reduce the volume of concentrated brine to
306
the same extent as brine concentrators. Accordingly, RO is usually followed by a brine
307
concentrator in ZLD processes
308
salinities than RO and consume less energy than brine concentrators (as highlighted in Figure 3),
309
is of paramount importance for advancing ZLD technology.
11, 19, 20
. This restriction of
. Developing new technologies, which tolerate higher
310 311 312
BEYOND THERMAL EVAPORATORS: EMERGING MEMBRANE-BASED ZLD TECHNOLOGIES
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Three membrane-based processes ⎯ ED, FO, and MD ⎯ emerge as alternative ZLD
314
technologies to brine concentrators to further concentrate the wastewater after the RO stage. The
315
produced brine from these processes serves as a feed to the crystallizer or evaporation pond. A
316
schematic illustration of ZLD systems incorporating these technologies is shown in Figure 4.
317
Their advantages, limitations, and energy consumption, along with those of RO and MVC brine
318
concentrators, are summarized in Table 1. Some of these technologies (i.e., thermolytic FO and
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MD) are hybrids of both thermal- and membrane-based processes. While the energy input to
320
these processes is thermal, membranes are the core separation components of these technologies.
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[FIGURE 4]
322
[TABLE 1]
323
Electrodialysis. ED applies an electric potential as the driving force to remove dissolved
324
ions through ion exchange membranes. In contrast to RO membranes that reject all ions, ion
325
exchange membranes selectively permit the transport of counter ions but prevent the passage of
326
co-ions
327
passing through cation-exchange membranes, whereas anions migrate in the opposite direction
328
through anion-exchange membranes. These concurrent processes generate two streams ⎯ salt-
329
depleted diluate and concentrated brine. In a modified form of ED, electrodialysis reversal
330
(EDR), the polarity of the electrodes is reversed frequently for minimizing fouling and scaling 20,
331
thereby requiring much less pretreatment than RO
332
propensity for silica-enriched feedwaters (e.g., BWRO brines), as neutral silica is not
333
accumulated in the brine stream 20.
334
50
. As shown in Figure 4A, cations move towards the negatively charged cathode by
51
. ED and EDR also have a low scaling
Compared to RO, ED and EDR are able to concentrate feed waters to higher salinity 5, 52-54
335
(>100,000 mg/L)
336
consume 7-15 kWhe/m3 of feedwater
337
concentrators. Also, the total cost for equipment and energy by ED was estimated to be lower
338
than that of MVC
339
concentrators and RO, the salinity of ED/EDR effluent can be much higher (e.g., TDS>10,000
340
mg/L
341
energy consumption and capital cost.
53
56
. When concentrating brines to such high salinities, ED and EDR 52-55
, which is less than that required by MVC brine
. However, in contrast to the very low TDS of water produced by brine
), indicating a trade-off between the quality of the desired product water and overall
342
For ED/EDR treating concentrated feedwater in ZLD systems, low-salinity product water
343
results in a large voltage drop, high electric resistance, low current efficiency, and diluate loss,
344
further increasing the energy consumption
345
reduces the limiting current density, which increases the required membrane area and
346
capital/operation cost 51. As estimated by McGovern et al. 56, the cost of salt removal by ED is
347
higher at lower diluate salinities. As such, a stand-alone, single-stage ED/EDR system is not
57, 58
12
. Furthermore, a decrease of diluate salinity
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suitable for reaching ZLD in most cases, since one of the benefits of ZLD is the production of
349
usable water. A multi-stage configuration is a feasible solution
350
cost.
56-58
, but it increases the capital
351
As a partial desalination process, ED/EDR has been applied in combination with RO in
352
several ZLD systems. Such systems achieved the dual function of extending the salinity limit of
353
RO and reducing the energy consumption relative to brine concentrators. For example, Oren et al.
354
demonstrated a pilot RO-EDR system for brackish water desalination with a water recovery of
355
97-98% 5. In that system, EDR concentrated the RO brine to a salinity of 100,000-200,000 mg/L
356
prior to a side-loop crystallizer and wind-aided intensified evaporation. In another pilot study 53,
357
EDR effectively removed hardness to reduce the scaling potential of saline basal aquifer water,
358
thereby improving the subsequent RO recovery without chemical addition. The EDR brine could
359
reach a salinity of 125,000 mg/L and was further concentrated by a brine crystallizer to approach
360
ZLD. In both cases, the EDR effluent was further desalinated by RO or partially blended with
361
RO permeate to attain a desired product water quality 5, 53.
362
Forward Osmosis. Unlike hydraulic pressure-driven RO, FO utilizes an osmotic pressure 59
363
difference to drive water permeation across a semipermeable membrane
. In FO, water flows
364
from the feedwater to a concentrated draw solution with a higher osmotic pressure (Figure 4B).
365
The produced brine is sent to a brine crystallizer or an evaporation pond, whereas the draw
366
solutes are separated from the desalinated water to regenerate the concentrated draw solution.
367
Since the driving force in FO is osmotic pressure, FO can treat waters with much higher salinity
368
than RO. When using FO to concentrate feedwater beyond the salinity limit of RO, the osmotic
369
pressure of diluted draw solution will surpass the bearable pressure limit of RO. Hence, in this
370
case, draw solutes that depend on RO for regeneration (e.g., NaCl and MgSO4,
371
suitable.
60
) will not be
372
The development of thermolytic draw solutes, such as the ammonia−carbon dioxide
373
(NH3/CO2), paved the way for FO-incorporated ZLD systems. The NH3/CO2 draw solution
374
generates very high osmotic pressure-driving forces and can be regenerated by low-temperature
375
distillation
376
solution to concentrate produced water from the Marcellus shale region to an average salinity of
377
180,000 mg/L 43.
61, 62
. A recent pilot study demonstrated the application of FO with NH3/CO2 draw
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Because the thermolytic NH3/CO2 draw solution decomposes at moderate temperature
379
(approximately 60 °C at atmospheric pressure) 61, low-grade thermal energy, including industrial
380
waste heat and geothermal energy, can be utilized to regenerate the concentrated draw solution.
381
A recent study estimated that U.S. power plants produced 803 million GJ of waste heat at
382
temperatures greater than 90 °C in 2012
383
NH3/CO2 FO, could potentially produce a maximum of 1.9 billion m3 of water annually, which
384
would meet the treatment demands for boiler water makeup and FGD wastewater systems of all
385
U.S. power plants
386
such as the U.S. and China 42, 65, 66.
64
63
. This amount of heat, if utilized to power the
. Also, geothermal energy is abundantly available in major ZLD markets
387
FO operates at low pressure, resulting in foulant layers that are less compact and more
388
reversible than in hydraulic pressure-driven RO systems. Accordingly, FO has a much lower
389
fouling propensity than RO 59, which not only reduces the operation cost for fouling control but
390
also extends the applicability of ZLD to wastewaters with high fouling potential.
391
The thermolytic FO process can be used as a brine concentrator after the RO stage.
392
Compared to MVC brine concentrators, the NH3/CO2 FO can be competitive because a small
393
volume of the more volatile draw solutes (i.e., NH3 and CO2), instead of water, is vaporized to
394
regenerate the concentrated draw solution
395
smaller area footprint and also renders ZLD systems more adaptable to fluctuations in the flow
396
rate and quality of feedwater 67.
397
43
. Furthermore, the modularity of FO results in
Recently, the world’s first FO-based ZLD system was constructed at the Changxing power 26, 67
398
plant in Zhejiang Province, China
. The system treats a mixture of FGD wastewater and
399
cooling tower blowdown at 650 m3/day. The feedwater is first concentrated by RO to a
400
concentration of ~60,000 mg/L. The NH3/CO2 FO process is then used as a brine concentrator to
401
further concentrate the RO brine to above 220,000 mg/L TDS. As the last step, the FO brine is
402
fed to a crystallizer for further concentration, while a high-quality product water (TDS < 100
403
mg/L after polishing by a secondary RO) is produced for reuse as boiler makeup water 67.
404
Membrane Distillation. MD is a thermal, membrane-based desalination process, in which
405
a partial vapor pressure difference drives water vapor across a hydrophobic, microporous
406
membrane
407
the hot feedwater (typically 60-90 °C
68
. In MD, the feedwater is heated and the resultant temperature difference between 69, 70
) and colder permeate side creates a vapor pressure
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difference to drive the water vapor flux (Figure 4C). The aqueous permeate can be in direct
409
contact with the membrane (direct contact membrane distillation, DCMD). Alternatively, the
410
water vapor can be collected on a condensation surface separated from the membrane, such as in
411
air gap membrane distillation (AGMD), vacuum membrane distillation (VMD), or sweeping gas
412
membrane distillation (SGMD) 68, 71-74.
413
MD is more energy intensive than RO and ED/EDR, because water separation by MD
414
requires liquid-vapor phase transition. The theoretical minimum energy of seawater desalination
415
by single-pass DCMD with heat recovery and a feed temperature at 60 °C is 27.6 MJ/m3 of
416
product water
417
MJ/m3 of product water) 8. In practical use, DCMD was estimated to consume 143-162 MJ (40-
418
45 kWht) per m3 of product water for seawater desalination 76, and a comparable value of 80-240
419
MJ (22-67 kWht)/m3 of product water was reported for AGMD 72. However, this thermal-based
420
energy consumption cannot be directly compared with the energy consumption of electricity-
421
driven technologies (RO, ED/EDR, and MVC brine concentrators), because the efficiency of
422
electricity generation from thermal energy varies with the quality (temperature) of the thermal
423
energy. Compared to MVC brine concentrators with well-designed energy recovery devices,
424
efficient heat recovery (e.g., use of heat exchangers 75 or brine recycling 77) is critical to improve
425
the energy competitiveness of MD.
75
, which is much higher than that by RO with a typical recovery of 50% (3.8
426
Similar to thermolytic FO, MD is beneficial due to its ability to treat high salinity feed
427
waters that cannot be desalinated by RO, and MD’s potential to leverage low-grade thermal
428
energy. When low-grade energy is available, MD achieves both cost saving and a reduced carbon
429
footprint relative to electricity-driven desalination technologies. Furthermore, MD is modular,
430
can operate at low pressure and temperature, and has low fouling propensity 70, 72, 76, 78. However,
431
when volatile pollutants or surfactants are present in the feedwater (e.g., in coal-to-chemical 79,
432
brewery 80, and shale gas industries 42), MD suffers from membrane wetting and the passage of
433
volatile compounds into the permeate, which deteriorate product water quality and cause process
434
downtime 42, 70, 81.
435
The potential application of MD in ZLD inland desalination has been demonstrated at the 82
436
bench scale
. When applying MD to further concentrate a secondary RO brine (with TDS of
437
~17,500 mg/L), a total water recovery of >98% was obtained for a brackish groundwater in
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California
. Recently, a conceptual near-ZLD system incorporating MD with reverse
439
electrodialysis (RED) was shown to achieve both water and energy production in seawater
440
desalination 83. In that system, MD reduced the volume of simulated SWRO brine (1 M NaCl) by
441
more than 80%. The produced MD brine was then mixed with seawater in a RED stack to
442
generate electrochemical energy. To date, however, large-scale applications of MD are still
443
hindered by its technical immaturity and low single-pass, single-module water recovery 20, 75. No
444
pilot-scale applications of MD in ZLD have been reported in the literature.
445 446
ENVIRONMENTAL IMPACTS
447
Despite the main goal of ZLD to reduce water pollution and improve water sustainability,
448
application of ZLD also results in unintended environmental impacts. One risk stems from the
449
produced solid wastes. For example, solid wastes stored in evaporation ponds have raised
450
concerns about their odors, potentially negative impact on wildlife, and risk of leakage
451
Similarly, solid wastes disposed in landfills may result in leaching of chemicals into groundwater
452
84
453
prevent potential contamination from solid wastes.
22
.
. Accordingly, impervious liners and reliable monitoring systems are typically required to
454
As discussed earlier, ZLD consumes large amounts of energy, leading to significant
455
emission of greenhouse gases (GHG). Some pretreatment methods, such as acidification
456
followed by degasification, release CO2 from the feedwater into the atmosphere. For example,
457
the application of ED in concentrating RO brine increases CO2 emission via both energy
458
consumption and decarbonation for scaling control
459
emission would increase by 50% if California water supply was switched from imported water to
460
BWRO inland desalination 86. According to the U.S. Energy Information Administration 87, the
461
amount of CO2 produced by electricity generation varies depending on the fuel type. Assuming
462
939 g of CO2 per kWhe generated by bituminous coal 87, MVC brine concentrators will typically
463
produce 19-23 kg of CO2 per m3 of treated feedwater solely from electricity usage
464
(corresponding to energy consumption of 20-25 kWhe/m3
465
higher energy efficiency, such as RO, will significantly reduce the GHG emission. In addition,
466
emerging ZLD technologies that can utilize low-grade or renewable energy (e.g., waste heat,
16
85
. A life-cycle study showed that GHG
11, 22
). Incorporating technologies with
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solar energy, geothermal energy
468
systems.
18, 42, 63, 64
) enable further reduction of GHG footprint of ZLD
469 470
OUTLOOK
471
ZLD implementation is growing globally as an important wastewater management strategy to
472
reduce water pollution and augment water supply. However, high cost and intensive energy
473
consumption will remain the main barriers to ZLD adoption. As the feedwater becomes more
474
concentrated along the ZLD treatment train, its salinity increases and so does the minimum
475
energy required for desalination 8. Therefore, the energy demand of ZLD, along with its
476
associated costs, will still be higher than that of conventional wastewater treatment or disposal
477
options.
478
Future growth of the ZLD market will heavily rely on regulatory incentives that outweigh its
479
economic disadvantages. As the severe consequences of water pollution are increasingly
480
recognized and attract more public attention, stricter environmental regulations on wastewater
481
discharge are expected, which will push more high-polluting industries towards ZLD. Intensified
482
freshwater scarcity, caused by both climate change and freshwater overexploitation, will likely
483
facilitate ZLD implementation. The prolonged drought in the Southwest U.S. 88 and accelerating
484
growth of water-intensive industries (e.g., coal-fired power plants) in China
485
worldwide freshwater deficiency. In such cases, a water quota may be imposed to limit the total
486
freshwater withdrawal by high water-consuming industries 90. In this case, ZLD may be a needed
487
strategy to guarantee sustainable water supply.
89
exemplify a
488
Due to the unrivaled energy efficiency of RO, expanding the salinity range of RO is of
489
paramount importance in ZLD systems. A robust RO system with higher resistance to hydraulic
490
pressure and fouling/scaling will effectively improve the energy efficiency and economic
491
feasibility of ZLD. At the core of such systems are fouling mitigation technologies, such as
492
fouling- and scaling-resistant membranes, which will reduce the operation cost through less
493
extensive pretreatment and cleaning needs
494
reuse 93. Major progress has been made to develop RO membranes with resistance to organic and
495
biological fouling
94-96
91, 92
and enhance the quality of the product water for
, but more remains to be done to test their performance in ZLD systems
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496
with various feedwater composition and very high concentration factors. Membranes with low
497
propensity to inorganic scaling (e.g., gypsum and silica scaling) are particularly desirable.
498
We have reviewed three membrane-based technologies ⎯ ED/EDR, thermolytic FO, and
499
MD ⎯ as three emerging ZLD technologies to further concentrate the feedwater after the RO
500
stage. However, compared to the technical maturity of RO and MVC brine concentrators, these
501
technologies are less established. More pilot or field studies are desirable to validate their large-
502
scale performance and viability in pursuing ZLD. Especially, their energy consumption and cost
503
need to be further evaluated to make a direct comparison with MVC brine concentrators. For MD
504
and thermolytic FO, their capability of harnessing low-grade energy will significantly reduce the
505
prime energy demand, operation cost, and GHG footprint of ZLD.
506
Resource recovery may provide an additional economic incentive for ZLD. Beneficial
507
components in the feedwater (e.g., valuable salts, nutrients, critical metals and elements) can
508
precipitate or be largely enriched when the feedwater is concentrated. For example, a proprietary
509
technology has been used for sequential salt recovery while achieving ZLD
510
technology involves multiple mineral precipitation and crystallization steps, producing useful
511
salts such as gypsum-magnesium hydroxide, magnesium hydroxide, and precipitated calcium
512
carbonate
513
carbonate 11) can partially compensate for the operation cost of ZLD. Further, the emerging ZLD
514
technologies reviewed in this article can recover various nutrients from wastewater
515
harvesting critical metals and elements from ZLD desalination systems has been recently
516
proposed 99.
11, 97
11, 97
. This
. The economic values of these byproducts (e.g., $350/ton of precipitated calcium 98
, and
517
In addition, the environmental impacts of ZLD need to be better understood. A life-cycle
518
assessment analysis of the energy demand and GHG emission will provide additional insights
519
into the cost-benefit balancing of ZLD. Along with advances in improving the energy and cost
520
efficiencies of ZLD technologies, particularly by incorporating membrane-based processes, ZLD
521
may become more feasible and sustainable in the future.
522 523
ACKNOWLEDGEMENT
524
We acknowledge the support received from the National Science Foundation through the
525
Engineering Research Center for Nanotechnology-Enabled Water Treatment (ERC-1449500). 18
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90. Jiang, Y. L.; Chen, Y. S.; Younos, T.; Huang, H. Q.; He, J. P., Urban water resources quota management: The core strategy for water demand management in China. Ambio 2010, 39, (7), 467-475. 91. Fritzmann, C.; Lowenberg, J.; Wintgens, T.; Melin, T., State-of-the-art of reverse osmosis desalination. Desalination 2007, 216, (1-3), 1-76. 92. Greenlee, L. F.; Lawler, D. F.; Freeman, B. D.; Marrot, B.; Moulin, P., Reverse osmosis desalination: Water sources, technology, and today's challenges. Water Res 2009, 43, (9), 23172348. 93. Lee, S.; Cho, J.; Elimelech, M., Influence of colloidal fouling and feed water recovery on salt rejection of RO and NF membranes. Desalination 2004, 160, (1), 1-12. 94. Perreault, F.; Tousley, M. E.; Elimelech, M., Thin-film composite polyamide membranes functionalized with biocidal graphene oxide nanosheets. Environ Sci Tech Let 2014, 1, (1), 7176. 95. Rana, D.; Matsuura, T., Surface modifications for antifouling membranes. Chem Rev 2010, 110, (4), 2448-2471. 96. Ye, G.; Lee, J. H.; Perreault, F.; Elimelech, M., Controlled architecture of dual-functional block copolymer brushes on thin-film composite membranes for integrated "defending" and "attacking" strategies against biofouling. Acs Applied Materials & Interfaces 2015, 7, (41), 23069-23079. 97. Neilly, A.; Jegatheesan, V.; Shu, L., Evaluating the potential for zero discharge from reverse osmosis desalination using integrated processes - A review. Desalin Water Treat 2009, 11, (1-3), 58-65. 98. Xie, M.; Shon, H. K.; Gray, S. R.; Elimelech, M., Membrane-based processes for wastewater nutrient recovery: Technology, challenges, and future directio. Water Res 2016, 89, 210-221. 99. Diallo, M. S.; Kotte, M. R.; Cho, M., Mining critical metals and elements from seawater: Opportunities and challenges. Environ Sci Technol 2015, 49, (16), 9390-9399.
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eu
sa W W
ZLD
y/r er
ov
No
Benefits
ec
dis
rr
po
Drivers
se
• Stricter regulations on wastewater (WW) disposal • Intensified freshwater scarcity • High cost of WW disposal • WW disposal options may not be feasible • Public environmental awareness
ate W
l
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Valuable salt recovery
FIGURE 1. Drivers and benefits of zero liquid discharge (ZLD).
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• Compliance with environmental regulation • No cost on WW disposal • Augmenting water supply • Protecting the environment
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Water reuse
(A) Wastewater
Wastewater
Filtration pH adjustment Deaeration Anti-scalant
Pretreatment
(B)
Solid disposal/ recovery
Brine concentrator
Brine crystallizer
or
Water reuse Evaporation pond
Softening Filtration Ion exchange pH adjustment Deaeration
Pretreatment
or Solid disposal/ recovery
RO Brine
RO
Brine concentrator
Brine crystallizer
FIGURE 2. Schematic illustration of (A) thermal and (B) RO-incorporated ZLD systems. Incorporation of RO, an energy-efficient technology, into ZLD reduces the volume of wastewater entering the brine concentrator, which consumes much higher energy per volume of treated water than RO.
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FIGURE 3. Specific energy consumption by RO, brine concentrator, and brine crystallizer. Although RO is energy efficient, its limited salinity range (typically with an upper concentration of ~70,000 mg/L) provides opportunities for other technologies to be applied in ZLD systems. The specific energies shown in the figure are in kWhe per cubic meter of feedwater.
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Reverse Osmosis Feedwater
Permeate Water reuse
RO Brine Thermal energy (e.g., low-grade heat)
Electricity + + + + + +
_ _ +
_ _
_ +
_
+
_
+
_ +
+
_ +
_
+ +
_ +
+
Brine
_ _ _ _ _ _
HOT
Diluted draw solution Brine Brine Brine Diluate Diluate
Draw solutes regeneration
Concentrated draw solution
Water reuse or back to RO Thermal energy (e.g., low-grade heat)
Back to RO
(A) ED/EDR-incorporated system
Brine
Cold water Permeate for water reuse
(C) MD-incorporated system
(B) FO-incorporated system
FIGURE 4. Schematic illustration of emerging membrane-based ZLD technologies in which (A) ED/EDR, (B) FO, or (C) MD is incorporated. ED/EDR uses an array of cation-exchange (green) and anion-exchange (orange) membranes that selectively reject anions and cations, respectively; FO employs a semipermeable membrane that allows water to pass through but ideally rejects all salts; MD employs a porous hydrophobic membrane that allows passage of water vapor through the membrane (as indicated by the blue curved arrows) but not liquid or salt. The produced brine is further concentrated by brine crystallizers or evaporation ponds to achieve ZLD.
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TABLE 1. Advantages, limitations, and energy consumption of different salt concentrating technologies used in ZLD operations Technology
Advantages
Limitations
Energy consumption
Energy efficient Modular Technical maturity
Limited salinity range (upper concentration ~75,000 mg/L) High fouling propensity
Seawater: 2-6 kWhe/m3 of product water 8, 16; Brackish water: 1.5-2.5 kWhe/m3 of product water 16
ED/EDR
High salinity limit (upper concentration > 100,000 mg/L) Low fouling propensity (especially for silica-enriched feedwater) Modular
High energy consumption and cost when treating high salinity feedwater with high-quality water product Incapability of removing noncharged contaminants Using only prime energy
7-15 kWhe/m3 of feedwater (with feed salinity > 15,000 mg/L) 52-55
FO (with NH3/CO2 thermolytic draw solution)
High salinity limit (upper concentration > 200,000 ppm) Utilization of low-grade heat Low fouling propensity Modular
Low water flux at very high feed salinities Reverse solute flux (NH3 may contaminate product water) Emerging technology with limited field performance data
21 kWhe/m3 of feedwater (with feed salinity of 73,000 mg/L and recovery of 64% in average) 43
MD
High salinity limit (upper concentration > 200,000 ppm) Utilization of low-grade heat Low fouling propensity Modular
Low water flux and water recovery Potential of membrane wetting Post-treatment is needed if volatile pollutants are present Emerging technology with limited field performance data
40-45 kWht/m3 of product water 76 22-67 kWht/m3 of product water 72
RO
MVC brine concentrator
Technical maturity High energy consumption High salinity limit (upper High capital and O&M costs concentration > 200,000 ppm) Operating at high temperature Using only prime energy ACS Paragon Plus Environment Not modular
20-25 kWhe/m3 of feedwater 11, 22 28-39 kWhe/m3 of feedwater 43