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1 Introduction

      1.1 What Is Desalination?

Desalination is a process of separating and removing unwanted dissolved salts and minerals from feedwater sources such as seawater, brackish water, or wastewater.

      The aim of desalination process is to produce a stream of fresh water (potable) with high quality (purity) according to the standards of World Health Organization (WHO), which state that the accepted maximum limit of total dissolved salts (TDS) in fresh water is 500 ppm (500 mg/l); if we take seawater (called feed or saline) as an example with a salinity of 35 000 ppm, feed can be desalted through two main industrial processes: (i) thermal desalination technologies where feed phase changes through evaporation and condensation processes and (ii) membrane desalination technologies where separation of salts achieved through using semipermeable membrane without feed phase change. In either process, potable water is produced, and salty water (called brine or concentrate) is rejected from desalination system; the rejected brine salinity varies in concentration based on technology, and it can reach up to 90 000 ppm.

      Today, desalination industry plays a vital role in society development and economic growth, and worldwide freshwater consumption rate is approximately doubled every 20 years, where the availability of natural water sources is depleted. According to UN report and in the light of global population growth, statistics showed that one‐third of world's population lives under a state of insufficient potable water resources and in communities that suffer from scarcity and water stress. As prehuman body, water is essentially needed for building tissues, blood circulation, and maintaining stable blood temperature, and based on human weight it is recommended that a human drinks 8–14 glasses of water every day. Today desalination industries tend to provide safe drinking water to achieve and maintain sustainable human life and minimize negative environmental impacts. As per industry, there are eight major water‐consuming industrial sectors: power generation, food, pharmaceutical, mining, oil, petrochemical, electronics, and paper.

      There are several types of “well‐proven” industrial desalination technologies that have been used in the last seven decades. Note that the major parameters that affect desalination technology performance are feed type and its thermal‐physical characteristics, in addition to the required desalted water quality (purity).

Figure 1.1 shows the classification of desalination systems based on separation processes. Note that such processes can be operated by conventional energy type such as fossil fuel (thermal and electrical) or by renewable energy types such as solar, wind, and geothermal (Chapter 9).

      Statistics by IDA for year 2015 shows that there are 18 426 desalination plants worldwide, producing 86.8 × 10⁶ m³/d and serving more than 300 × 10⁶ in 150 countries per day, and such production can provide potable water for municipal, industrial, and agriculture sectors. Today Gulf countries produce around 57% of world desalination capacity [2].

Figure 1.1 Desalination processes based on separation type.

1.2 Aims of Desalination Processes

      Several desalination technologies have been invented, developed, and employed during the last seven decades, and the potentials behind such industrial developments are:

      1 Satisfy the global increase in freshwater demand for drinking, and in industrial and agriculture sectors, in addition to hygiene requirements.

      2 Compensate the capacity of limited natural freshwater resources specially in arid and remote areas.

      3 Reduce the values of elevated intensive energy cost of freshwater production.

      4 Reduce the potential levels of emissions and minimize negative environmental impacts on human and climate in terms of greenhouse effect and acid rain.

      5 Provide efficient large size desalination system with the ability of integration with other conventional or renewable energy types.

      6 Achieve water quality based on WHO rules and regulations or other specific industrial standards.

      7 Manage wastewater processes that are associated with municipal and industrial sectors.

1.3 Desalination Processes

There are four main industrial desalination processes that exist today to separate freshwater from feedwater, and separation processes can be achieved via:

      1 Thermal processes

      2 Membrane processes

      3 Chemical processes

      4 Adsorption processes

Figure 1.2 shows the basic principles of general industrial desalination processes.

Figure 1.2 Basic principles of desalination processes.

      Feed can be in the form of seawater with high salinity that can reach a value of 45 000 ppm, or it can be in the form of brackish water or even wastewater that has been taken from industry or municipal.

      The example of disposal fluid from desalination system can be in the following forms:

      1 Brine fluid (salty water): Which have very high value of salinity that can reach 90 000 ppm in case of thermal desalination system.

      2 Concentrate: Which is a fluid with very high value of salinity in case of membrane desalination system.

      3 Waste: Which is a product in the form of industrial concentrated fluids.

      4 Thick liquor: Product that is produced under specific purposes and applications.

The products from desalination system can be in the following forms:

      1 (a) Drinkable water with salinity less than 500 ppm.

      2 (b) Fresh water.

      3 (c) Fluid with specific required salinity or concentration.

      Choosing the proper desalination system (technology) is always a challenge for engineers, because each desalination technology can provide different levels of performance under different circumstances (design and operational conditions). To ensure the performance level for any industrial desalination system, it must be examined in terms of 4E's – energy analysis, exergy analysis, economic analysis, and environmental analysis, in addition to sustainability tendency. Student should remember that selecting

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