Introduction to Desalination. Louis Theodore
Читать онлайн книгу.
organizations within governments about water-related issues. These debates/conflicts may even take place between multiple countries, not just the individuals within those countries. Commonly, these types of conflicts are solved by establishing a system known as Riparian Water Rights. This is a system used to allocate water among different countries or jurisdictions within countries that are in control of the land through which the water travels. These types of rights are considered common law amongst many countries in the world such as the United States, Canada, Australia, and Great Britain.
Even though many of the largest countries in the world have used this system to manage water conflicts for years, there are many other countries where these systems are relatively new. A prime example of this is conflict between Egypt and Ethiopia. For centuries, these two countries have been in constant conflict over who owns the developmental rights for the water in the River Nile. It wasn’t until 2015 that these two counties set aside their differences and signed an agreement ensuring shared and equitable access to the river (Lufkin 2017).
Even with compromises such as the one between Egypt and Ethiopia, some individuals, such as Ismail Serageldin (former Vice President of the World Bank), see water becoming the next so called “Oil War.” Ismail Serageldin famously said, “The wars of this century were fought over oil, the wars of the next century will be fought over water” (Barnaby 2009). However, when one sees countries coming together to solve their water resources conflicts, forming compromises like Egypt and Ethiopia, the potential for water wars seem less likely, and the arguments by Barnaby (2009), Fröhlich (2012), and McCrary (2018) appear to have more validity. These latter authors believe that physical conflict over water will not happen because established systems for international trade and international agreements leading to mutual benefits ease these conflicts, thus preventing the scale of these problems from becoming large enough to cause military style confrontations. Certainly, the expansion of available water supplies through the expansion of desalination technologies will contribute to the reduction in multinational conflicts over water and ensure more adequate water supplies for all peoples.
1.5 History of Desalination
Three prime candidate desalination processes have emerged and are briefly reviewed in the sections to follow.
1.5.1 Evaporation Processes
The oldest and best developed process for saline water conversion is the evaporation (or what some define as distillation) method. While there are many desalination technologies in use or being developed today, desalination began using evaporative processes. These evaporative desalination techniques were recognized over 2,000 years ago when Aristotle wrote in 320 B.C., “saltwater, when it turns into vapor, becomes sweet and the vapor does not form saltwater again when it condenses.” Evaporation remains the major method today for commercial production of fresh water from seawater. In principle, evaporation is the simplest method. Seawater is boiled in an evaporator (Flynn et al. 2019 ; Theodore 2014; Theodore et al. 2017) by passing hot steam through a steam chest where the steam condenses on the inside of the tubes of the chest and is usually returned to the boiler. The vapors rising from the seawater feed are cooled in a condenser and thus converted into pure liquid water which is collected in a storage vessel. The resulting concentrated brine solution is continuously or intermittently withdrawn from the evaporator. The most advanced commercial evaporation processes are the following:
1 Multiple-Effect Evaporation.
2 Flash Evaporation.
3 Vapor Compression Evaporation.
Details of these processes are provided in Chapter 10.
1.5.2 Membrane Processes
Electrodialysis was the membrane separation process employed for desalination a century ago. Here, the ions forming the salt are removed from the saltwater by electric forces and concentrated in separate compartments. However, the higher the salinity of the raw water, the more electric power is needed for this process. Hence, this process was applied primarily in the treatment of moderately brackish waters containing 1,000 to 2,000 mg/L of dissolved salts. In recent years, reverse osmosis (RO) (Theodore and Ricci 2010) has displaced electrodialysis as the primary membrane separation desalination process, leaving the latter as the choice for medical kidney applications.
An RO system consists of an intake, a pre-treatment component, a high-pressure pump, a membrane apparatus, remineralization, and pH adjustment components, as well as a disinfection step. Generally, a pressure of about 1.7 to 6.9 MPa is required to overcome the osmotic pressure of saltwater.
This advanced separation technique may be used whenever low molecular weight solutes such as inorganic salts or small organic molecules (e.g. glucose) are to be separated from a solvent (usually water). In normal (as opposed to reverse) osmosis, water flows from a less concentrated salt solution to a more concentrated salt solution as a result of an innate concentration driving force (thermodynamically referred to as the chemical potential). As a result of the migration of water through the membrane, an “osmotic pressure” is created on the side of the membrane to which the water flows. In RO, the membrane is permeable to the solvent or water and relatively impermeable to the solute or salt. In order to make water pass through an RO membrane in the desired direction (i.e. away from a concentrated salt solution), a pressure must be applied that is higher than the osmotic pressure. The membranes used for RO processes are characterized by a high degree of semi-permeability, high water fluxes, good mechanical strength, chemical stability, and relatively low operating and capital costs. Early RO membranes were composed of cellulose acetate, but restrictions on process stream pressure, temperature, and organic solute rejection spurred the development of noncellulosic and composite material membranes (“sandwiches”). These membranes may be configured into a variety of geometries for system operation, including: plate and frame, tubular, spiral wound (composite), and hollow fiber.
1.5.3 Crystallization Processes
Crystallization processes were also employed over 2,000 years ago. Today, these processes are important mass transfer operations that are often employed in the preparation of a pure product. In the process, a crystal usually separates out as a substance of specific composition from a solution of varying composition. Any impurities in the liquid (often referred to as the mother liquor) are carried in the crystalline product only to the extent that they adhere to the surface or are occluded (retained) within the crystals that may have grown together during or after the crystallization operation. The separation of a solid from a solution onto a crystal occurs only if there is a state of imbalance involving a mass driving force; namely, a decrease in chemical potential (or concentration) between the bulk of the liquid solution and the crystal interface. This effectively means that the solution must be supersaturated. There are several different ways that crystallization can occur. The four most often encountered in practice are (Theodore 2014):
1 Cooling.
2 Evaporation.
3 Cooling and evaporation (also referred to as adiabatic evaporation).
4 A salting out process.
Process 1 is the most commonly employed, provided the solubility of the component being crystallized decreases with decreasing temperature.
World-wide development of desalination techniques in the last half century has been driven out of necessity due both to water scarcity and population growth. The private sector has primarily led the investment in research and development since water has begun to be seen not as a commodity, but as a product to be sold at a profit. This development by the private sector has led to a significant drop in the cost of water generated using these desalination techniques.
1.6 Summary Observations
The following summary observations can be made related to water, seawater desalination, and water treatment in general.
1 Water demand at both