Desalination: Water from Water

SECTION I

INTRODUCTION

Chapter 1

Introduction to Desalination

Jane Kucera

Nalco/an Ecolab Company

Abstract

The availability of fresh water for a growing, industrialized planet is quickly becoming scarce. Methods to attain more fresh water to meet the increasing demand include techniques such as conservation and reuse, desalination, and moving water from water-rich regions to water-poor regions. Of these techniques, the potential for desalination is most promising in that it can be applied virtually anywhere in the world. And, desalination has the potential for generating relatively large volumes of fresh water from a host of feed stocks. This chapter discusses the need for desalination and provides the framework for the detailed discussions about desalination techniques that are presented in this book.

Keywords: Desalination, Water Scarcity, Conservation, Reuse, Thermal Desalination, Reverse Osmosis

1.1 Introduction

Desalination: from the root word desalt meaning to remove salt from [1]. By convention, the term desalination is defined as the process of removing dissolved solids, such as salts and minerals, from water [2]. Other terms that are sometimes used interchangeably with desalination are desalting and desalinization, although these terms have alternate meanings; desalting is conventionally used to mean removing salt from other more valuable products such as food, pharmaceuticals, and oil, while desalinization is used to mean removing salt from soil, such as by leaching [2].

The first practical use of desalination goes back to the sixteenth and seventeenth centuries, when sailors such as Sir Richard Hawkins reported that their men generated fresh water from seawater using shipboard distillation during their voyages [3]. The early twentieth century saw the first desalination facilities developed on the Island of Cura ao and in the Arabian Peninsula [3]. The research into and application and use of desalination gained momentum in the mid-twentieth century, and over the last 30 years has witnessed exponential growth in the construction of desalination facilities.

One could ask the question, Why desalination? Desalination has become necessary for several reasons, the most compelling of which may be: 1) the increased demand for fresh water by population growth in arid climates and other geographies with limited access to high-quality, low-salinity water, and 2) the per capital increase in demand for fresh water due to industrialization and urbanization that out paces availability of high-quality water. Research and development over the last 50 years into desalination has resulted in advanced techniques that have made desalination more efficient and cost-effective. Desalination is, and will be in the future, a viable and even necessary technique for generating fresh water from water of relatively low quality. In this chapter, and in this entire book, we make the case for desalination as one of the major tools for meeting the fresh water needs of a growing planet. Thus, the title of this book, Desalination: Water from Water.

1.2 How Much Water is There?

The allocation of the world’s water is shown in Figure 1.1. More than 97%, or about 1338 million km³, of the world’s water is sea-water [3, 4]. Eighty percent of the remaining water is bound up as snow in permanent glaciers or as permafrost [4]. Hence, only 0.5% of the world’s water is readily available as low-salinity groundwater or in lakes or rivers for direct use by humans.

Figure 1.1 Allocation of the world’s water resources.

1.2.1 Global Water Availability

Some regions of the world are blessed with an abundance of fresh water. This includes areas with relatively low populations and easy access to surface waters, such as northern Russia, Scandinavia, central and southern coastal regions of South America, and northern North America (Canada, Alaska) [2, 5]. More populated areas and areas with repaid industrialization are experiencing more water stress, particularly when located in arid regions. (Water stress is typically measured by comparing the amount of water used to that which is readily available.)

There are numerous water maps available that measure current and predict future water stress. Figure 1.2 shows the Water Stress Index 2011 by Maplecroft [5] that estimates current water stress by comparing water use to the available, renewable supply for regions around the world:

Figure 1.2 Water availability estimates for 2011.

Courtesy of the Water Stress Index 2011 by Maplecroft.

"The Water Stress Index evaluates the ratio of total water use (sum of domestic, industrial, and agricultural demand) to renewable water supply, which is the available local runoff (precipitation less evaporation) as delivered through streams, rivers, and shallow groundwater. It does not include access to deep subterranean aquifers of water accumulated over centuries and millennia.

The application of the index is to provide a strategic overview of the current situation of physical water stress at global, continental, regional, and national levels. It does not take account [any] future projection, [or] water management policies, such as desalination, or the extent of water re-use" [5].

The areas of the world that are not rich in water resources and that also experience un-stable and rapid population growth and industrialization will see water stress significantly increase in the future. Figure 1.3 compares the global water stress in 1995 with that predicted for 2025 [6]. As many as 2.8 billion people will face water stress or scarcity issues by 2025; by 2050, that number could reach 4 billion people [6]. Water stressed areas will include the south central United States, Eastern Europe, and Asia, while water scarcity will be experienced in the Southwestern United States; Northern, Southern, and Eastern Africa; the Middle East; and most of Asia [2].

Figure 1.3 Global water stress in 1995 and predicted for 2025.

Courtesy of Philippe Rekacezwicz (Le Monde diplomatique), February 2006.

1.2.2 Water Demand

In addition to population growth, another pressure being exerted on water supply is fact that the per capita water demand is increasing faster than the rate of population growth [7]. According to Global Water Intelligence [8], the per capital water demand has outpaced population growth by a factor of 2.

The demand for water in developed nations is relatively high. Demand in the United States is about 400 liters (105 gallons) per person per day [4]. Some Western countries that have been successful in implementing conservation and reuse measures have seen their demand for water drop to about 150 l (40 gallons) per person per day [4, 9]. However, the limited availability and access to water in some parts of the world, results in much lower consumption in such regions. For example, per capita freshwater consumption in Africa is only about 20 l (5.3 gallons) per day due to the shortage of suitable water [9]. The World Heath Organization (WHO) deems 15 to 20 l (4 – 5.3 gallons) per person per day is necessary for survival, while 50 l (13 gallons) per person per day is estimated to be needed for operation of basic infrastructure such as hospitals and schools (see Figure 1.4) [4]. The WHO estimates that by 2025, the world-wide demand for fresh water will exceed supply by 56% [9].

Figure 1.4 Global demand for water and World Health Organization basic water requirements (2010).[4, 9].

1.2.3 Additional Water Stress Due to Climate Change

While population growth and per capita increase in demand are two major water stressors, the impact of climate change on global water stress cannot be ignored. The effects of climate change actually work synergistically with population growth and increasing demand to strain water supply as population and industrialization grow, climate change accelerates, leading to more drastic climate events such as drought. A study by the National Center for Atmospheric Research (NCAR) indicates that severe drought is a real possibility for many populous countries [10]. Regions that are projected to experience considerable drought include most of Latin America, the Mediterranean regions, Southeast and Southwest Asia, Africa, the Southwest United States, and Australia [7]. Coincidentally, many of these regions will also experience increases in population and industrialization and urbanization, and the corresponding increase in per capita water demand. The United Nations forecasts that the world will have 27 cities with populations greater than 10 million by the year 2020, and all but 3, New York City, Moscow, and Paris, will be in regions under the threat of significant drought [7].

1.3 Finding More Fresh Water

For much of the world’s urbanized population, fresh water is an afterthought, a commodity that has been easy to find and always there at the tap. However, water in some parts of the world is increasingly considered a product that must to be found and developed to meet growing demand. Depending on the specific circumstances in a particular geography, one or more methods may need to be implemented to find and develop water sources to meet future water needs. Some of these methods are summarized below.

1.3.1 Move Water from Water-Rich to Water-Poor Areas

Moving water from water-rich areas to water-scarce regions, while sounding extreme, is not a new idea. Witness the diversion of water to the desert southwest United States for drinking, power, and irrigation uses. Los Angeles currently imports 85% of its water demand from outside sources: the Sierra Nevada Mountains, the Delta in Northern California, the Los Angeles Aqueduct, and the Colorado River Aqueduct [11].

However, moving water is not always palatable. Public outcry against moving water from a water-rich region can be a formidable obstacle. Consider the Columbia River in the Pacific Northwest United States. Water is Oregon’s oil, declared Oregon State Senator David Nelson in his 2007 white paper, Columbia River Diversion as a Public Revenue Source. Diversion of the Columbia River to other western states has been a topic of discussion in the State of Oregon for over 35 years. Not much has come of this discussion to date however, as water-poor areas in the region have found other sources for water, and, more to the point, Oregonians have routinely declined to give up their supply of inexpensive fresh water that also serves as their source for relatively inexpensive hydroelectric power.

Politics can also play a role in how water supplies are dispersed. In Spain, different political parties are having a tug-of-war over how to supply the south-eastern area of Spain with water. The conservative party in Spain is advocating moving water from the Ebro River (an eastern river whose delta into the Mediterranean Sea is about half way between Barcelona and Valencia) to the Community of Valencia, which lies approximately 200km from the delta. The Socialist Party in power has commissioned the Torrevieja Seawater Reverse Osmosis (SWRO) facility, the 6th largest SWRO facility in the world, which is located in Alicante, Municipality of Torrevieja, about 75 km from Valencia. Backers of the Ebro river project have denied a permit for concentrate discharge from the SWRO facility, thereby preventing the construction of the seawater intake and outfall pipelines [11]. The Valencia region has 1.5+ million people with 4 more SWRO projects under way that could encounter the same political stalemate.

While moving fresh water makes sense in some cases, public and political pressures, as well as technical issues, such as moving water long distances, particularly when elevation changes are involved, will not make moving water supplies feasible or even possible to meet the requirements of all regions in need of fresh water.

1.3.2 Conservation and Reuse

Conservation is a term that has been used for decades to mean more efficient usage and savings of a resource, in this case, water. The twenty-first century equivalent terms for conservation are sustainability and green. Regardless of which term is used, the need to conserve through more efficient usage, recycling, and reuse has become popular in today’s culture. While these techniques are oft times the first choice of populations located in arid areas or far from an ocean as a means of finding more fresh water, all populations can benefit from these techniques.

For example, consider the City of Los Angeles, California, an arid, coastal city that receives only about 40cm (15.7 in) of rain a year. Los Angeles is one large metropolitan area that has selected conservation to supply a portion of its future water needs. The Los Angeles area has a current population of about 4 million people and is expected to grow to at least 10 million inhabitants by 2020; water demand is expect to rise by 123 million m³ per year [7, 12]. The Los Angeles Department of Water and Power (LADWP) describes the future of the city’s water philosophy: Conservation will continue to be a foundation of LADWP water resource management policy, and will be implemented to the fullest extent concurrent with further consideration of alternative water supplies [13].

Additionally, the LADWP has developed a new Recycled Water Master Plan which relies heavily on recycling highly-treated wastewater as a cost-effective solution to meet some of the future demands of the city [14]. The Edward C. Little Water Recycling Facility (ELWRF) located in the City of El Segundo, Los Angeles County, CA (commonly referred to as West Basin), is a model for water conservation, recycling, and reuse. The facility, funded in 1992 following the severe drought in California in the late 1980’s and early 1990’s, today produces about 114,000 m³/d of recycled water (41.5 m³ per year) at a current investment of $500 million [15]. Five grades of water, known as designer water, are produced by the facility to match the needs of local industry (water type listed roughly from lowest to highest quality):

1. Ternary wastewater (known as Title 22 Water) for general industrial and irrigation uses, such as irrigating golf courses,

2. Nitrified water for use in industrial cooling towers,

3. Softened reverse osmosis water for ground water recharge,

4. Reverse osmosis water for low-pressure boiler feed water at local refineries, and

5. Ultra-pure reverse osmosis water for high-pressure boiler feed water at local refineries.

The goal of the LADWP Recycled Water Master Plan is to recycle a total of about 62 million m³ of water per year by 2019 at an estimated cost of $715 million to $1 billion [11, 13]. The ELWRF (West Basin) has already achieved 2/3 of that water recycling goal.

Conservation and recycling wastewater, using West Basin as the example in Southern California, will require treatment, such as desalination, to produce water that is suitable for reuse. Conservation and recycling has the potential to slow the rate at which new, future supplies of fresh water may need to be developed, but will not, by itself, meet the total need for fresh water.

1.3.3 Develop New Sources of Fresh water

Developing sources of fresh water other than traditional sources, such as lakes, rivers, or relatively shallow wells, is another method for meeting the demand for more fresh water. The most logical new sources for developing new fresh water supplies are seawater and deep wells or saline aquifers.

Seawater is the traditional source water when of thinks of desalination. Seawater represents the feed water source for the majority of desalination facilities in the world (58.85%), and an explosion in growth of large seawater facilities is responsible for the steep increase in desalination capacity since 2003 [16]. The majority of these facilities were developed in the Arabian Gulf region, Algeria, Australia, and Spain; for sea-bounded and generally arid areas such as the Gulf Coast and Australia, turning to the sea for water is only natural.

Seawater supply is only suitable as a source for coastal areas; inland areas would need to rely on sources such as saline aquifers for new water supply. Figure 1.5 shows a United States Geologic Survey map of US saline aquifers; the map was generated in the early 1960’s, and has not been updated since its initial publication. Note that most current activity involving saline aquifers centers on using them as storage for greenhouse gases, primarily carbon dioxide, rather than as sources for fresh water [17]. This is presumably due to the need to treat the water to generate fresh water from the saline brines as opposed to the relative ease of injecting greenhouse gases, a process that does not require treatment, into the aquifers.

Figure 1.5 United States Geologic Service map of aquifers in the United States, cir. 1965.

Seawater and other saline sources present an opportunity to meet the growing water needs of the world. Table 1.1 lists generally-held classifications of water as a function of salinity (note that saline aquifers are generally considered to be at least moderately brackish). Of these classifications, even the higher-salinity fresh water would require treatment for potable or industrial use to reduce the concentration of dissolved minerals. Thus, desalination techniques are once again necessary to generate high-quality water from water that is, without treatment, not suitable for direct use.

Table 1.1 Classification of sources waters as a function of total dissolved solids (TDS).

* World Health Organization [8].

1.4 Desalination: Water from Water

1.4.1 Drivers for Desalination

One can conclude from the discussions in this chapter that new sources of fresh water must be developed to meet the growth in the demand. Apart from moving water from location to location, reuse of wastewater and use of alternate sources of water will require treatment to yield water that is suitable for potable or industrial use. And, since wastewaters and alternate source waters are generally high is dissolved solids, desalination technologies will most certainly be required as part of the treatment scheme. Thus, the driver for desalination is clear: future demand for high quality water will require desalination of water sources that are lower in quality (higher in dissolved solids) than are commonly utilized today (and which may not be available tomorrow).

Desalination of various water sources to provide a supply of fresh, usable water has been growing almost exponentially since 1965, when global commissioned desalination capacity was less than 2 million m³/d [18]. By 2011, the global commissioned desalination capacity was over 71 million m³/d [16]. New, on-line capacity for the year 2010 was about 6.2 m³/d, and new, on-line capacity has increased year over year since 1995, with steep increases in new capacity since 2003, as demonstrated in Figure 1.6a; Figure 1.6b shows the cumulative on-line capacity since 1980 [16].

Figure 1.6 Growth of new, on-line desalination capacity.

Courtesy of Global Water Intelligence.

1.4.2 Feed Water Sources for Desalination

Feed water sources for desalination are varied. As previously discussed, feed sources can range from seawater and saline aquifers to wastewater for recycle and reuse. While seawater represents the feed water source for the majority of desalination facilities, the use of other feed water sources, such as brackish water, saline aquifers, and wastewater, has been growing steadily since 2000 [16]. Figure 1.7 shows the growth in annual new contracted capacity by feed water type, and Figure 1.8 shows total worldwide installed capacity by feed water type through 2010 [16, 18].

Figure 1.7 Annual new contracted desalination capacity by feed water type [16].

Courtesy of Global Water Intelligence.

Figure 1.8 Total, global installed capacity by feed water source as of 2010 [16].

Courtesy of Global Water Intelligence.

Although the alternative sources appear to be limited in number, composition of specific examples of the various make-up source classifications can differ greatly depending on their hydrologic origin. Table 1.2 demonstrates some of the variability in well and surface waters, with a standard seawater and a sample grey water source included for comparison (the well, river, and grey waters shown in Table 1.2 either are currently being used as feed water sources for desalination facilities or have been considered for use as source water for such facilities) [3].

Table 1.2 Sample water composition of seawater, well water, surface water, and grey water sources.

Despite variations in quality among the various feed water sources, they all share the characteristic of being relatively high in salinity of total dissolved solids (TDS). High salinity (and, in some cases, other constituents) makes the water unsuitable for potable and industrial use. Therefore, demineralization or desalination treatment to reduce the concentration of TDS must be part of the treatment system employed if these sources are to be used to supplement or replace existing fresh water supplies.

1.4.3 Current Users of Desalinated Water

The primary user for desalinated water is the municipal sector; nearly two-thirds of desalinated water is used for potable application. Industrial and power users together accept another third of the worldwide desalination capacity [18]. Although potable applications account for nearly twice the total volume of desalinated water used than industrial applications, the number of industrial facilities (including power) out number municipal facilities by almost 2 to 1 (8,715 to 4,415 respectively), indicating that the size of industrial desalination facilities are considerably smaller than municipal facilities [18]. The remaining 6% of desalinated water is used for irrigation, tourism, military, and other applications [18A].

Figure 1.9 provides a breakdown of users of desalinated water for 2010 and 2011 [18, 18A]. These data shows little change over the year, but do show the trend toward more industrial users relative to municipal users.

Figure 1.9 Total, 2010 and 2011 global installed capacity by type of user [18, 19].

Courtesy of Global Water Intelligence.

1.4.4 Overview of Desalination Technologies

The world-wide installed desalination capacity in 2011 was about 74.8 million m³/d [18A]. Figure 1.10 shows the relative breakdown of installed capacity of various desalination technologies for 2010 and 2011 [18, 19].

Figure 1.10 Global installed desalination capacity by technology for 2010 and 2011 [18, 19].

Courtesy of Global Water Intelligence.

Membranes are currently outpacing traditional thermal technologies in total installed desalination capacity. Prior to 1980, membrane technologies made up less than a third of the global desalination capacity, while today, membranes account for just under 2/3 of the total installed desalination capacity [14]. As shown in Figure 1.10, installed capacity of reverse osmosis (RO) grew from 60% of total installed capacity in 2010 to 63% in 2011, an increase of 5% over the year, while installed capacity of multi-stage flash (MSF) evaporation decreased by over 14% for the year; installed capacity of multi-effect distillation (MED) remained steady at 8%. Membrane technologies such as RO offer the advantage of smaller infrastructure, and RO total treated water cost is becoming competitive with traditional thermal processes [19].

Membrane-based systems are popular in rising markets such as Algeria, Spain, and Australia, while thermal processes are still strong in traditional markets such as the Middle East, where energy costs are lower. Saudi Arabia accounts for 34.8% of the total installed thermal capacity in the world at 5.9 m³/d, followed closely by UAE at 5.8 m³/d, and then by Kuwait, Qatar, Libya, Bahrain and Oman (0.5 to 2.1 m³/d) [18]. By contrast, the United States is ranked 9th in total installed thermal capacity (~0.35 m³/d), but 1st in total installed membrane capacity at 7.5 m³/d (second is Saudi Arabia at just under 5 m³/d) [18]. Figure 1.11 shows the fraction of total global desalination capacity by region since 2003 [16]. Since 2003, growth in desalination technologies is still led by the Middle East (Saudi Arabia and UAE). However, 3 – 7 in rankings of total growth in desalination capacity since 2003 are Spain, United States, China, Algeria, Australia, Israel, and India, respectively [18].

Figure 1.11 Fraction of total global desalination capacity by region since 2003 [16].

Courtesy of Global Water Intelligence.

1.4.5 History of Desalination Technologies

Desalination has grown substantially since the mid 1960s. In 1952, there were only about 225 desalination facilities world-wide with a total capacity of about 100,000 m³/d. (either [2] or Evans 1969 (roadmap)) Today, there are over 15,000 desalination facilities globally with a total global capacity of over 65 million m³/d [18].

While there are many desalination technologies in use or being developed today, desalination began using thermal processes. Membrane-based processes, such as RO, helped to further promote desalination over the last 30 years. The history of these pioneering technologies is outlined below.

1.4.5.1 History of Thermal Desalination

Thermal desalination techniques were recognized as early as 320 B.C. when Aristotle wrote, ‘saltwater, when it turns into vapor, becomes sweet and the vapor does not form saltwater again when it condenses.’ Shipboard distillation beginning in the sixteenth century is the first practical use of distillation to generate fresh water from seawater [20]. In 1843, Rillieux successfully patented, built, and sold multi-effect evaporators [20].

The number of thermal desalination installations has grown rapidly over the last 100 years. However, while reverse osmosis and other membrane technologies were revolutionary in development, the development of thermal desalination technologies over the last 40 years has been more evolutionary than revolutionary [21].

Multi-effect distillation (MED) was the first thermal desalination technology employed [20, 21]. The first units were designed with submerged tube evaporators the exhibited low heat transfer rates and high scaling rates. Vertical- and horizontal-tube evaporators (also known as falling-film evaporators) used in modern MED facilities provide higher heat transfer coefficients and lower specific heat transfer surface area requirements than their older counterparts. Drawbacks of the current MED technology are the complexity and production capacities [21]. Also, the relatively low, maximum-brine temperature of MED (~65°C) due to scale-forming issues, is another limitation of MED. However, the use of membrane pretreatment such as nanofiltration (NF) prior to MED to remove the calcium that contributes to calcium-sulfate scale in MED units has been considered as a way of allowing higher temperature operation of MED and, thereby enhance the use of MED for desalination [21].

Due to the early issues with MED (e.g., scaling and low heat transfer rates), multi-stage flash (MSF) distillation technology was developed in the late 1950s and early 1960s as an alternative. Flashing distillation was first commercially employed by Westinghouse in Kuwait in 1957 [20]. That same year, the MSF distillation patent was issued and in 1959/60, the first commercial MSF facilities were installed in Kuwait (19 stages, 4550 m³/d) and the Channel Islands (40 stages, 2775 m³/d) [22, 23]. In 1973, the standard MSF units, that produce 27,277 m³/d and consist of 24 stages, were developed [20].

Recent developments in the thermal desalination technology have focused on scale and corrosion control techniques and on the increase in distiller production capacity [21]. Early, pre 1980, MSF units were primarily constructed using carbon steel for the shell and the internals [24]. Corrosion of the metal due to seawater resulted in the use of thicker materials of construction, which made the units larger and heavier. Units built after 1980 use stainless and duplex stainless steel to reduce corrosion, allowing for lighter and smaller MSF units. Future advances in the technology will most likely focus on improvements in thermodynamics and material selection [24].

1.4.5.2 History of Membrane Desalination

While the earliest recognition of thermal desalination was a few hundred years B.C., the earliest recorded documentation of semipermeable membranes was in 1748, when the phenomenon of osmosis was observed by Abbe Nollet.[25] Osmotic phenomenon was also studied in the 1850’s, and then in the 1940’s, when Dr. Gerald Hassler began investigation of the osmotic properties of cellophane [26]. Modern RO technology truly began in the late 1950’s when C.E. Reid and E.J. Breton at the University of Florida and Sidney Loeb and Srinivasa Sourirajan at the University of California at Los Angeles (UCLA) independently demonstrated RO using polymeric membranes.

The United States was the early leader in desalination research in the 1960s and 70s due in most part to government funding. The Saline Water Conversion Act of 1952 established the Office of Saline Water (OSW) in 1955, which later became the Office of Water Research and Technology (OWRT) in 1974. It was under such governmental programs that Loeb and Sourirajan developed the first commercially-viable reverse osmosis membrane while at the UCLA [26]. In 1965, the tubular, cellulose acetate membrane developed at UCLA became the membrane used in the first commercial reverse osmosis facility located in Coalinga, California [27].

Government funding also lead to the development in 1965 of the solution/diffusion membrane transport model by Harry Lonsdale, U. Merten, and Robert Riley at the General Atomic Division of General Dynamics, Corp [28]. This model has become the basis of research and development of new membrane materials since that time.

It was under similarly-funded governmental programs that John Cadotte, while at North Star Research, prepared the first interracial polyamide membrane that soon after became the basis of the FilmTec FT30 membrane (now part of Dow Water and Process Solutions) [29]. The original FT30 membrane chemistry is the basis of the majority of reverse osmosis membranes in use today [22].

The OWRT was abolished in 1982 and government funding of desalination research in the United States dropped considerably. By that time, however, much of the foundation for reverse osmosis, membrane-based desalination had been laid. Since then, incremental membrane improvements have been made in the areas of flux, rejection, and operating pressure requirements, as shown in Table 1.3 [23]. However, no major breakthroughs in terms of higher membrane selectivity with high water flux and chlorine tolerance have occurred in 30+ years since the revolutionary early developments. Research is continuing, however, and the development of nanotechnology and nanocomposite membranes circ. 2005 has raised hopes that reverse osmosis membranes with higher selectivity, water flux, and chlorine tolerance may be on the horizon, all of which will reduce costs and improve efficiency associated with membrane desalination [22].

Table 1.3 Advances in brackish water membrane performance [23].

1.4.5.3 Developments in Desalination Since 1980

Since 1980, the world-wide development of desalination techniques has been driven out of necessity due to water scarcity and population growth. The private sector has led the investment in research and development as they began to see water not as a commodity, but as a product to be sold at a profit [3]. This development by the private-sector has lead to a significant drop in cost of water generated through desalination techniques. An example of such is the 80% reduction in price of reverse osmosis membrane elements over the last 15–20 years, while incremental improvements in flux, selectivity, and operating pressure were realized (Tables 1.3 and 1.4 [21, 29]. In 1991, the cost to produce water at the SWRO Santa Barbara desalination facility was about $8/kgal; in 2007, the estimated cost had dropped to about $3.40/kgal [2, 24, 30]. Figure 1.12 shows the general cost range of desalinated water (variability in cost is due to factors such as size of plant and degree of pretreatment employed) [19].

Figure 1.12 Cost range of desalinated water [19].

Courtesy of Global Water Intelligence.

Table 1.4 Decline in membrane cost relative to 1980 [31].

There have been other desalination techniques developed over the years, some more commercially successful than others, and none more commercially successful than traditional thermal and reverse osmosis desalination processes. Table 1.5 lists a selection of desalination technologies and their current status.

Table 1.5 Sample desalination technologies, technologies covered in this volume are noted in italics.

1.4.6 The Future of Desalination

Desalination today is still a capital and energy-intensive proposition. Methods to reduce costs are necessary to make the desalinated water more affordable. To that end, developments to increase the efficiency and thereby reduce costs of desalination are needed. Some areas of current development include:

Energy: renewable energy to drive desalination projects, such as wind and solar, are being considered to reduce the energy footprint and costs of desalination (today’s contribution of renewable energy sources to desalination processes is only about 0.05%) [41]. Table 1.6 shows some of the renewable energy sources (RES) currently considered to power desalination processes.

Table 1.6 Renewable energy sources (RES) used to replace traditional fossil-fuel based energy sources to drive traditional desalination processes. These RES play a relatively small role in desalination today [41], but given the concerns regarding fossil fuels (e.g., availability, carbon footprint (see Table 1.7)), these sources show potential for future application in powering desalination. Solar energy is covered in this volume.

Table 1.7 Current airborne emissions per cubic meter of water generated by various desalination technologies when powered by fossil fuels and/or waste heat [41].

Materials: materials of construction for thermal processes that resist corrosion and reduce the size and weight (and costs) of units need to be developed. New membrane materials that resist attack by chlorine and show high selectivity and high flux are needed.

Chemicals: antiscalants for both thermal and membrane – based desalination processes to increase the degree of water recovery per given unit size.

Current desalination techniques can also have a significant impact on the environment. Some of these issues are described below [41]. Before desalination can become sustainable, these environmental issues must be addressed. The two most pressing issues are concentrate disposal and airborne emissions:

Concentrate disposal—very high salinity wastewater is generated through thermal and reverse osmosis membrane desalination. Total dissolved solids can be as high 100,000 ppm in the wastewater from the desalination of seawater. Furthermore, concentrate can be highly turbid and be at elevated temperatures (thermal desalination plants), and can contain chemical additives such as polymers/coagulants, acid, biocides, corrosion inhibitors, and cleaners. The issue becomes how to dispose of the wastewater in an environmentally-safe manner that is also cost effective. Seawater desalination facilities currently discharge to the ocean, which some argue is damaging to local flora and fauna via increases in sea-water temperature, salinity, and turbidity, which may be harmful to marine life and cause them to migrate away and at the same time enhancing the populations of algae and nematodes. [45, 46] Inland brackish water facilities use other disposal methods, including discharge to surface water (45%), discharge to sewer (27%), deep well injection (16%), land application (8%), and discharge to evaporation ponds (4%) [46]. Each of these disposal methods has its own environmental concerns that are directly related to the high salinity and other components of the water for discharge.

Carbon footprint and airborne emissions—Table 1.7 lists airborne emissions for fossil fuel-powered desalination technologies. The carbon dioxide emissions from thermal desalination processes are a full order of magnitude higher than that for RO, when powered by fossil fuels. And, energy to power the desalination facilities has the largest impact on the carbon footprint of the process [47]. The emissions for desalination technologies can be reduced significantly when these processes are powered by waste heat or renewable energy sources such as solar radiation rather than by fossil fuels [41].

Future demand for fresh water will likely result in an estimated $40–$50 billion expenditure on global desalination projects over the next ten years [41]. It is apparent that RO will be the primary mode of desalination for the foreseeable future, [48], as Figure 1.13 [16] demonstrates the relative growth of RO to traditional thermal processes. Thermal processes, while in decline, still have a foothold, primarily in the Gulf Region. (16)

Figure 1.13 Growth of installed membrane and thermal desalination capacity.

Courtesy of Global Water Intelligence.

For desalination to be a sustainable method to develop sources of fresh water in the future, desalination technologies need to be efficient, cost effective, and have low environmental impact. Indeed, concerted efforts on three fronts have resulted in significant advances in desalination [49], each of which is developed in this book:

development of innovative new technologies,

improvements in performance and design of conventional technologies,

the marriage of desalination technologies with renewable energy sources (RES).

1.5 Desalination: Water from Water Outline

The objective of this volume is to present the case for desalination, describe conventional and innovative new desalination technologies, touch on RES for desalination, and conclude with a discussion of future directions.

Section I of this book discusses the need for finding new sources of fresh water to meet the ever growing demand. The case for desalination as a method of preparing fresh water from salty or impaired water, that has proven to be successful, is developed.

In addition to conventional desalination technologies (thermal and RO), there are many new technologies under development, as listed in Table 1.5. Current and several of the more promising desalination technologies under development are discussed in Sections II through IV of this book:

Section II: this section covers traditional thermal desalination technologies, including MED, and MSF. These technologies are very mature, but do have limitations that may be overcome with through future development of new materials to improve corrosion resistance and heat transfer, and through the development of antiscalants.

Section III: this section describes several membrane-based technologies, including RO, continuous electrodeionization, and membrane distillation, as well as some membrane-based desalination technologies that have only recently been commercialized, namely forward osmosis. Significant research is on-going to surmount the limitations of relatively mature membrane technologies such as RO, while also developing less commercialized technologies such as membrane distillation.

Section IV: this section details non-traditional desalination technologies. Technologies covered in this section include freezing-melting desalination processes, capacitive deionization, and ion exchange. These technologies may be limited for desalination applications, and some require additional development to become viable for commercial or industrial desalination.

The future need for renewable energy sources (RES) to power desalination may be as great as the need for desalination itself, if the issues associated with fossil fuels become even more acute. A common RES, solar energy, is described in Section V as an alternative to fossil fuels to power desalination. The feasibility of solar energy is relatively high as an RES to power desalination today, due to the availability, and shows great future potential, particularly in arid areas where sunshine is plentiful, but fresh water is not.

The book concludes with a discussion about the future prospects for desalination in Section VI. This final section discusses future water sources for desalination, including traditional sea-water sources, and other, more impaired sources, such as industrial wastewater. Future water demand for desalination water, including traditional municipal users and emerging water users, such as oil field hydrofracking, is profiled. Finally, research needs to develop additional desalination technologies that are efficient and cost effective are presented along with some of the more promising desalination techniques to come out of research and development.

Abbreviations

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SECTION II

TRADITIONAL THERMAL PROCESS

Chapter 2

Thermal Desalination Processes

Joachim Gebel

Abstract

This chapter starts with an extract of fundamentals of engineering science such as thermodynamics and heat transfer. Building on this, mass- and energy balances for single-effect and multiple-effect distillation processes are introduced. A complete set of design equations for MED, MSF and mechanically as well as thermally driven vapor compression plants is presented. In order to be able to compare the different processes in terms of energy demand, the so-called Gained Output Ratio as a performance indicator is introduced and discussed. Based on a vivid description of history of thermal seawater desalination, future prospects and challenges for thermal desalination technologies are discussed at the end of the chapter.

Keywords: Mass- and Energy Balances, Single-Stage Evaporation, Multiple-Effect Distillation (MED), Multi-Stage-Flash - Evaporation (MSF), Multiple-Effect Distillation with Thermally Driven Vapour Compression (TVC), Single-Stage Evaporation with Mechanically Driven Vapour Compression (MVC), Gained Output Ratio (GOR), Performance Ratio, Primary Energy Consumption, Historical Review

2.1 Thermodynamic Fundamentals

2.1.1 First and Second Rule of Thermodynamics

It is mandatory to know the first and second rule of thermodynamics for the energy-related balancing and design of seawater desalination plants, in particular of thermal plants. The first rule is concerned with the conservation of energy, whereas the second rule makes a statement on the direction in which the process runs.

The first rule may be explained with the help of a simple system. Let us examine system A as represented in Figure 2.1 This is a closed system, in other words there are no mass flows passing across the borders of the system. Furthermore the system is insulated, i.e. there are no heat flows passing across the system borders. Within the system there is a homogeneous fluid, for example water. At a particular time t, heat is introduced from outside into the system. If the temperature of the fluid is measured, a steady increase in temperature will be observed until the heat flow is switched off. The temperature increase is equal to an increase in the internal energy of the system.

Figure 2.1 First rule of thermodynamics (Closed system).

Now system B should be examined. This system is also closed and insulated. However, instead of the introduction of heat a stirrer is placed in the fluid. At a particular time t, the stirrer is switched on. What happens? How does the system react to this introduction of energy? The mechanical