The Global Rise of Zero Liquid Discharge for Wastewater

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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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Environmental Science & Technology

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Critical Review

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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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ACS Paragon Plus Environment


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ABSTRACT

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Zero liquid discharge (ZLD) — a wastewater management strategy that eliminates liquid waste

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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

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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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51

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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

54

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

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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

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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



82

pond. The condensed distillate water in ZLD systems is collected for reuse, while the produced

83

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

91

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.

99

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

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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.

121

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

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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

133

limits on the level of toxic metals and other harmful pollutants in wastewater discharged from

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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

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incentives for ZLD installation in U.S. power plants.

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. 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

142

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

148

ZLD overcomes the challenge of brine discharge, thereby enabling inland desalination in

149

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

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implementation.

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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

157

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

171

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

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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

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wastewater discharge pipeline for a paper mill 34, have been recently suspended or canceled as a

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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.

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India. Facing a situation similar to that in China, India is taking aggressive actions to curb

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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

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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

196

areas 39.

36, 37

CONVENTIONAL ZLD SYSTEMS

7

. In 2015, the

. As reported by Vishnu et al.

197 198

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38

, 29


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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

211

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

217

the heat transfer tubes

218

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

225

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

230

consumption in ZLD. Further, brine concentrators are able to reach salinity concentrations of

231

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

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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

11



319

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.

321

[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

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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

13



378

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

14


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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

15



82

Page 16 of 29

438

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


Page 17 of 29


467

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

17



Page 18 of 29

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


Page 19 of 29


526 527

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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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Page 24 of 29


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

Page 25 of 29

Valuable salt recovery

FIGURE 1. Drivers and benefits of zero liquid discharge (ZLD).

25


•  Compliance with environmental regulation •  No cost on WW disposal •  Augmenting water supply •  Protecting the environment


Page 26 of 29

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.

26


Page 27 of 29


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.

27



Page 28 of 29

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.

28


Page 29 of 29


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

The Global Rise of Zero Liquid Discharge for Wastewater

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