The Global Rise of Zero Liquid Discharge for Wastewater

Figure 1. Drivers and benefits of zero liquid discharge (ZLD). In practice, the incentives ..... (5, 52-54) When concentrating brines to such high sal...
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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

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and maximizes water usage efficiency — has attracted renewed interest worldwide in recent

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years. Although implementation of ZLD reduces water pollution and augments water supply, the

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technology is constrained by high cost and intensive energy consumption. In this critical review,

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we discuss the drivers, incentives, technologies, and environmental impacts of ZLD. Within this

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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-

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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

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generation of solid waste, are discussed and the prospects of ZLD technologies and research

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needs are highlighted.

48 49

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

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and safe drinking water is further complicated by climate change and the pressures of economic

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development and industrialization. The public and industrial sectors consume substantial

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amounts of freshwater while producing vast quantities of wastewater. If inadequately treated,

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wastewater discharge into the aquatic environment causes severe pollution that adversely

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impacts aquatic ecosystems and public health 4.

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Recovery and recycling of wastewater has become a growing trend in the past decade due to

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rising water demand 3. Wastewater reuse not only minimizes the volume and environmental risk

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of discharged wastewater, but also alleviates the pressure on ecosystems resulting from

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freshwater withdrawal. Through reuse, wastewater is no longer considered a “pure waste” that

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potentially harms the environment, but rather an additional resource that can be harnessed to

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achieve water sustainability.

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Zero liquid discharge (ZLD) is an ambitious wastewater management strategy that

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eliminates any liquid waste leaving the plant or facility boundary, with the majority of water

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being recovered for reuse. ZLD obviates the risk of pollution associated with wastewater

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discharge and maximizes water usage efficiency, thereby striking a balance between exploitation

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of freshwater resources and preservation of aquatic environments. Achieving ZLD, however, is

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generally characterized by intensive use of energy and high cost. As a result, ZLD has long been

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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

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expenses for wastewater disposal, and increasing value of freshwater are driving ZLD to become

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a beneficial or even a necessary option for wastewater management. The global market for ZLD

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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

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and India.

6, 7

, spreading rapidly

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Early ZLD systems were based on stand-alone thermal processes, where wastewater was

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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

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energy and capital.

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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

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with a limited salinity range. Accordingly, other salt-concentrating technologies that can treat

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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

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wastewater beyond RO.

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Although ZLD holds great promise to reduce water pollution and augment water supply, its

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viability is determined by a balance among the benefits associated with ZLD, energy

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consumption, and capital/operation costs. Therefore, it is imperative to understand the drivers

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and benefits that make ZLD a realistic option. Incorporating new technologies, such as emerging

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membrane-based processes, provides opportunities to reduce the associated energy consumption

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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

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impacts of ZLD as an important strategy for wastewater management. We highlight the evolution

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of ZLD from thermal to membrane-based processes, with a detailed analysis of the advantages

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and limitations of both existing and emerging ZLD technologies. Lastly, we discuss the

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environmental impacts of ZLD, the prospects of ZLD technologies, and research needs for

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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

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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

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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

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and geographical location. Therefore, the drivers and benefits of ZLD are discussed in this

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section in the context of its global applications. Although ZLD has been applied in places such as

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the European Union, Australia, Canada, the Middle East, and Mexico 6, 7, 9, 10, examples from the

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United States, China, and India are highlighted, as they represent the major ZLD markets with

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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

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of the Colorado River led to a regulatory mandate of ZLD for nearby power plants 6, 11. In those

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days, obtaining approval for discharge agreements for new industrial projects required several

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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

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rest were distributed across areas such as electronics, fertilizer, mining, and chemical industries.

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6, 12

. Today, power

The U.S. EPA recently completed its guidelines revising the existing regulations on 14

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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

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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

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seawater desalination, brackish water desalination requires much less energy

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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

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water or publicly owned treatment works (POTW) as well as deep-well injection

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excluded, due to potentially adverse impacts on surface water and groundwater, insufficient

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POTW capacity, geological and legal restrictions, and increasing disposal costs. As a result,

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inland desalination is still not installed at many locations where water is critically needed, such

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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.

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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

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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.

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Although coal-fired power plants provide more than 70% of the total electricity generated in

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China

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owned companies are located in regions that suffer water scarcity or deficit 25. This sharp conflict

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between energy demand and water deficiency makes ZLD one of the few sustainable solutions at

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the energy-water nexus in China. Although no data have been revealed on the overall ZLD

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installation in Chinese power industry, a rising trend of ZLD adoption is indicated by the recent

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construction of the world’s first FO-based ZLD system at the Changxing coal-fired power plant

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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

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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

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mg/L of total dissolved salts, TDS) and treatment capacity (110-2,300 m3/hour) 29-32.

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27

. Coal-to-chemicals plants consume a considerable

In addition, greater public awareness of water pollution may facilitate ZLD implementation 33

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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

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only water but also valuable salts from textile wastewater for direct reuse in the dyeing process.

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According to a recent technical report 39, the ZLD market in India was valued at $39 million in

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2012 and is expected to grow continuously at a rate of 7% from 2012 to 2017. In this market, the

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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.

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Thermal ZLD Systems. Early ZLD systems were typically based on a series of thermal

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processes (Figure 2A). In such systems, the feed wastewater undergoes a pretreatment step that

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reduces scaling potential, and is then concentrated sequentially by two core elements ⎯ a brine

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concentrator and a brine crystallizer (or an evaporation pond). The distillates generated by the

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brine concentrator and crystallizer units are reused as clean product water, whereas the solids

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produced are either stored (in evaporation ponds), further processed for landfill disposal, or

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reused as valuable byproducts.

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[FIGURE 2]

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Brine concentrators commonly use mechanical vapor compression (MVC) for water

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evaporation. Although other thermal desalination technologies, such as multi-effect distillation

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(MED) and multi-stage flash (MSF), have been extensively used in seawater desalination 40, their

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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

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then mixed with the recirculating brine slurry at the sump of the brine concentrator. The brine

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slurry is conveyed to the top of the concentrator and flows down inside a bundle of heat transfer

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tubes. The flowing brine forms a thin film on the internal tube surface where water evaporation

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occurs. Calcium sulfate seeds are often added into the recirculating brine to provide preferential

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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

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vapor condenses, transferring its latent heat to vaporize the falling brine slurry. The condensate

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travels down the heat transfer tubes and is collected as distillate that preheats the incoming

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feedwater before being reused.

11, 41

. The produced water vapor flows to the vapor compressor, which

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The formation of a falling thin film enhances the heat transfer rate, thereby reducing the

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compression ratio and required energy of the compressor 42. The use of energy recovery devices

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(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

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typically consume 20-25 kWhe/m3 of treated feedwater

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kWhe/m3 of feedwater) reported in the literature

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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%

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for brackish water 48). The HERO process has been applied in multiple full-scale ZLD systems

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worldwide

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treatment capacity of 2300 m3/h 31.

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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

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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.

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This salinity limit is much lower than that achieved by brine concentrators (i.e., up to 250,000

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mg/L). Thus, a stand-alone RO system is not able to reduce the volume of concentrated brine to

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the same extent as brine concentrators. Accordingly, RO is usually followed by a brine

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concentrator in ZLD processes

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salinities than RO and consume less energy than brine concentrators (as highlighted in Figure 3),

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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

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produced brine from these processes serves as a feed to the crystallizer or evaporation pond. A

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schematic illustration of ZLD systems incorporating these technologies is shown in Figure 4.

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Their advantages, limitations, and energy consumption, along with those of RO and MVC brine

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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

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these processes is thermal, membranes are the core separation components of these technologies.

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[FIGURE 4]

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[TABLE 1]

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Electrodialysis. ED applies an electric potential as the driving force to remove dissolved

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ions through ion exchange membranes. In contrast to RO membranes that reject all ions, ion

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exchange membranes selectively permit the transport of counter ions but prevent the passage of

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co-ions

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passing through cation-exchange membranes, whereas anions migrate in the opposite direction

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through anion-exchange membranes. These concurrent processes generate two streams ⎯ salt-

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depleted diluate and concentrated brine. In a modified form of ED, electrodialysis reversal

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(EDR), the polarity of the electrodes is reversed frequently for minimizing fouling and scaling 20,

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thereby requiring much less pretreatment than RO

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propensity for silica-enriched feedwaters (e.g., BWRO brines), as neutral silica is not

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accumulated in the brine stream 20.

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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

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(>100,000 mg/L)

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consume 7-15 kWhe/m3 of feedwater

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concentrators. Also, the total cost for equipment and energy by ED was estimated to be lower

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than that of MVC

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concentrators and RO, the salinity of ED/EDR effluent can be much higher (e.g., TDS>10,000

340

mg/L

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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

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For ED/EDR treating concentrated feedwater in ZLD systems, low-salinity product water

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results in a large voltage drop, high electric resistance, low current efficiency, and diluate loss,

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further increasing the energy consumption

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reduces the limiting current density, which increases the required membrane area and

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capital/operation cost 51. As estimated by McGovern et al. 56, the cost of salt removal by ED is

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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

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usable water. A multi-stage configuration is a feasible solution

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cost.

56-58

, but it increases the capital

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As a partial desalination process, ED/EDR has been applied in combination with RO in

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several ZLD systems. Such systems achieved the dual function of extending the salinity limit of

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RO and reducing the energy consumption relative to brine concentrators. For example, Oren et al.

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demonstrated a pilot RO-EDR system for brackish water desalination with a water recovery of

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97-98% 5. In that system, EDR concentrated the RO brine to a salinity of 100,000-200,000 mg/L

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prior to a side-loop crystallizer and wind-aided intensified evaporation. In another pilot study 53,

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EDR effectively removed hardness to reduce the scaling potential of saline basal aquifer water,

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thereby improving the subsequent RO recovery without chemical addition. The EDR brine could

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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

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RO permeate to attain a desired product water quality 5, 53.

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Forward Osmosis. Unlike hydraulic pressure-driven RO, FO utilizes an osmotic pressure 59

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difference to drive water permeation across a semipermeable membrane

. In FO, water flows

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from the feedwater to a concentrated draw solution with a higher osmotic pressure (Figure 4B).

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The produced brine is sent to a brine crystallizer or an evaporation pond, whereas the draw

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solutes are separated from the desalinated water to regenerate the concentrated draw solution.

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Since the driving force in FO is osmotic pressure, FO can treat waters with much higher salinity

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than RO. When using FO to concentrate feedwater beyond the salinity limit of RO, the osmotic

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pressure of diluted draw solution will surpass the bearable pressure limit of RO. Hence, in this

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case, draw solutes that depend on RO for regeneration (e.g., NaCl and MgSO4,

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suitable.

60

) will not be

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The development of thermolytic draw solutes, such as the ammonia−carbon dioxide

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(NH3/CO2), paved the way for FO-incorporated ZLD systems. The NH3/CO2 draw solution

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generates very high osmotic pressure-driving forces and can be regenerated by low-temperature

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distillation

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solution to concentrate produced water from the Marcellus shale region to an average salinity of

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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

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(approximately 60 °C at atmospheric pressure) 61, low-grade thermal energy, including industrial

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waste heat and geothermal energy, can be utilized to regenerate the concentrated draw solution.

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A recent study estimated that U.S. power plants produced 803 million GJ of waste heat at

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temperatures greater than 90 °C in 2012

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NH3/CO2 FO, could potentially produce a maximum of 1.9 billion m3 of water annually, which

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would meet the treatment demands for boiler water makeup and FGD wastewater systems of all

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U.S. power plants

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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

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FO operates at low pressure, resulting in foulant layers that are less compact and more

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reversible than in hydraulic pressure-driven RO systems. Accordingly, FO has a much lower

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fouling propensity than RO 59, which not only reduces the operation cost for fouling control but

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also extends the applicability of ZLD to wastewaters with high fouling potential.

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The thermolytic FO process can be used as a brine concentrator after the RO stage.

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Compared to MVC brine concentrators, the NH3/CO2 FO can be competitive because a small

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volume of the more volatile draw solutes (i.e., NH3 and CO2), instead of water, is vaporized to

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regenerate the concentrated draw solution

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smaller area footprint and also renders ZLD systems more adaptable to fluctuations in the flow

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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

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fed to a crystallizer for further concentration, while a high-quality product water (TDS < 100

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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

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membrane

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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

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contact with the membrane (direct contact membrane distillation, DCMD). Alternatively, the

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water vapor can be collected on a condensation surface separated from the membrane, such as in

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air gap membrane distillation (AGMD), vacuum membrane distillation (VMD), or sweeping gas

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membrane distillation (SGMD) 68, 71-74.

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MD is more energy intensive than RO and ED/EDR, because water separation by MD

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requires liquid-vapor phase transition. The theoretical minimum energy of seawater desalination

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by single-pass DCMD with heat recovery and a feed temperature at 60 °C is 27.6 MJ/m3 of

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product water

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MJ/m3 of product water) 8. In practical use, DCMD was estimated to consume 143-162 MJ (40-

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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