Salinity and Desalination

Salinity is the total amount of salts dissolved per unit of water; the average salinity of seawater, for example, is thirty-five grams per kilogram of water. Utilizing saline water for domestic purposes thus requires removing the dissolved materials. The desalination of saline or brackish water involves costly operations and is typically performed only in coastal or arid regions where alternative water resources are limited or nonexistent.

Dissolved Solids

Salinity is one of the most important physical properties of seawater. It is defined as the total amount of dissolved salts found in a unit of water, measured in grams of dissolved matter per kilogram of seawater. The average salinity of seawater is approximately 35 grams per kilogram, corresponding to a concentration of 35 parts of salt per thousand parts of seawater (35 parts per thousand). That is to say, evaporation of all water from one kilogram of seawater will, on average, leave 35 grams of dry solids, primarily sodium chloride and other metal salts. The main ionic components of dissolved matter in seawater and the percentages in which they are found in a typical sample are as follows: chloride ions (55.0 percent), sodium ions (30.6 percent), sulfate ions (7.7 percent), magnesium ions (3.7 percent), calcium ions (1.2 percent), and potassium ions (1.1 percent). Accordingly, one cubic kilometer of typical seawater contains approximately 75 million metric tons of sodium chloride, 11 million metric tons of magnesium chloride, five million metric tons of magnesium sulfate, four million metric tons of calcium sulfate, 2.5 million metric tons of potassium sulfate, and 344,000 metric tons of calcium carbonate. Depending on the prevailing physicochemical conditions, salinity can vary significantly, from brackish water (2 to 15 parts per thousand) in estuaries and riverine deltas to highly saline water such as in the Mediterranean Sea (38.5 parts per thousand) or Red Sea (42.5 parts per thousand). Even higher salinity levels prevail in landlocked bodies of water such as the Dead Sea. Atmospheric precipitation or freshwater discharges from inland waters (surface water or groundwater) reduce salinity, while evaporation increases salt content. Though salinity levels vary from one water body to another, the relative abundance of the main dissolved components remains almost unchanged.

The presence of dissolved salts affects the density of seawater. A salinity of 35 parts per thousand amounts to a density difference between fresh and salt waters of approximately 2 percent. Thus, some interesting phenomena occur whenever there is a confluence of fresh and salt waters. Depending on the degree of energy input by wind, currents, and tidal action, the fresh and salt waters can be thoroughly or partially mixed or remain stratified. Under quiescent conditions, the heavier saltwater will sink if all other variables (such as temperature and pressure) are the same for fresh and salt waters. Seawater can penetrate many kilometers upstream, moving along the bottom under the freshwater in many estuaries with high riverine discharge and low tidal range (such as the Mississippi River). This phenomenon is known as a “saline wedge,” the separating interface between fresh and salt waters is called the “halocline.” Mixing conditions between fresh and salt waters is essential from an ecological point of view because salinity levels affect the diversity and population of aquatic flora and fauna.

Sea salt is also a major contributor to atmospheric aerosol particles. During wave breaking, characterized by the formation of “white caps,” small droplets are carried upward by air currents into the atmosphere. When the droplets evaporate, sea salt particles of very small sizes (0.5 to 20 micrometers) are transported by the wind over the continents. About 10 percent of the total annual amount of salt generated in the oceans (1.8 billion tons) is deposited as airborne sea salt particles on the continents.

The total volume of oceanic waters is about 1.37 billion cubic kilometers. This volume constitutes approximately 96.5 percent of the total water on Earth. The other major water resources are groundwater, of which 10.53 million cubic kilometers (0.76 percent) is freshwater and 12.87 million cubic kilometers (0.93 percent) is saltwater; and the polar ice, which contains 24.02 million cubic kilometers (1.7 percent) of water. The remaining water is in lakes, rivers, marshes, soil moisture, atmosphere, and biota. Thus, Freshwater makes up only 2.5 percent of Earth’s water. Most freshwater supplies are in the polar ice (68.6 percent) and in groundwater (30.1 percent). The percentage of freshwater found in lakes is only 0.26 percent, while the amount found in rivers is even smaller (0.006 percent).

Inland Water and Salinity Measurement

Not all inland water is fresh. The amount of dissolved salts occurring in inland waters, surface or underground, depends on the composition of the soils through which the water passes. Streams and rivers flowing over rocks containing chloride and sodium compounds contribute significantly to salt generation. Therefore, some inland waters have a high salt content and are unsuitable for use unless properly pretreated. The most extreme examples of inland brine waters are the Great Salt Lake in Utah (salt concentration of about 120 parts per thousand) and the Dead Sea in the Middle East (salt concentration of about 270 parts per thousand). Salt is also found as surface crust or layers in swamps and dry lake bottoms, particularly in arid climate areas. The famous Bonneville Salt Flats in Utah and Death Valley in California reflect the accumulation of various salts over time. The water that periodically inundates these areas transports relatively small amounts of dissolved salts from the surrounding area, forming a landlocked body of water as the runoff collects and pools. When the water evaporates, those salts remain and have been slowly built up over time to produce the massive deposits of salts for which those regions are known. In coastal regions, excessive groundwater pumping can lead to seawater intrusion into an aquifer. This can have a long-term negative effect on the water resources of a region, with a subsequent devastating impact on the regional economy.

The average freshwater that falls daily in the United States is about 15,750 billion liters. This water feeds the various surface water bodies (lakes and rivers), recharges aquifers, evaporates into the atmosphere, or flows into the oceans. Therefore, 2.5 trillion liters of precipitated water can be used for beneficial purposes. From this amount, 1.26 trillion liters are used by industry, 530 billion liters by agriculture, and 94 billion liters by domestic and rural consumers. The average American consumes from 300 to 375 liters of water per day.

Direct measurement of seawater salinity by evaporation or chemical analysis is too complicated to be used routinely. In the past, salinity (known as “absolute salinity”) was estimated in terms of chlorinity. Chlorinity is the amount of chlorine ions plus the chlorine equivalent of bromine and iodine ions. From chlorinity, salinity was estimated by multiplying the value of chlorinity by a factor of 1.80655.

Salinity (known as “practical salinity”) is estimated indirectly by measuring the electrical conductivity of the seawater. However, because electrical conductivity is strongly affected by both salinity and temperature, the conductivity readings are properly corrected to compensate for temperature effects.

Freshwater is a precious commodity used for various domestic, rural, industrial, and agricultural purposes. Water is not distributed evenly worldwide and is subject to short-term and long-term temporal variations. In many regions, the available freshwater resources cannot meet the water demands. After the Industrial Revolution, accelerated anthropogenic pollution added to the water resource problem. In situations of limited water resources, the only alternative solutions are either cleanup and reuse of domestic and agricultural wastewater or, for islands and coastal regions, desalination of seawater. These operations are costly and require the construction of appropriate water treatment facilities. Several countries in the Middle East, including Israel and Saudi Arabia, depend heavily on water desalination for their drinking water supplies. Under emergency conditions, fresh water is obtained via portable water-purification equipment.

Freshwater can contain several impurities that have to be removed or treated before it can be used. These impurities include calcium, magnesium, iron, lead, copper, chloride, sulfate, nitrates, fluorides, sodium, different organic compounds, and suspended solids. Impurities can be hazardous to human health or give water a disagreeable taste, smell, or appearance. In addition, they can create scaling or corrosive problems in pipes or machinery that use the water. Because saltwater contains a large amount of dissolved salts, it is always subjected to desalination before use. Generally, the acceptable quality standards for drinking water are 0.5 micrograms per kilogram of total dissolved solids and 0.2s microgram per kilogram of chloride.

Water treatment for domestic water production involves many operations, such as filtration, softening, distillation, deionization, chemical disinfection, exposure to ultraviolet radiation, and reverse osmosis. The number and the type of operations required depend on the quality and properties of the water supplies. The problem of high salt content can be treated by a variety of methods. Desalination is the process by which dissolved salts are removed from the water. There are several desalination methods. Because no one method is applicable in all situations, the selection of the most appropriate desalination method is based on such variables as the amount and type of dissolved salts in water, the degree of purification of the water to be produced, and the associated costs. More than 20,000 desalination plants operate worldwide to provide freshwater to individuals in over 170 countries.

Thermal Processing

There are two general methods of desalination: thermal processing (distillation) and membrane separation. The main principle behind thermal processing is as follows: Saltwater is heated until it boils, and then the released steam condenses as it cools, forming pure water. Membrane separation is achieved by using reverse osmotic pressure so that the water passes through a membrane while the salt ions are retained by the membrane.

Distillation, the earliest desalination method, was used in steamships as early as 1884. There are several different thermal processing methods for desalination, such as thin-film multiple effect distillation (TFMED), multi-stage flash desalination (MSFD), mechanical vapor compression desalination (MVCD), and thermal vapor compression desalination (TVCD). At standard atmospheric pressure (measured at sea level), water boils at 100 degrees Celsius. However, boiling temperature decreases with decreasing pressure. Also, if the water is heated under high pressure at 100 degrees Celsius and is suddenly released into a vacuum chamber, it flashes into vapor. This technique is used in the TFMED method and MSFD method, in which the pressure is continuously reduced in sequential stages. The number of sequential stages can range from fifteen to twenty-five. The MVCD and the TVCD methods use compression to increase the pressure and thus increase the temperature of a constant volume of steam. Heating of the steam as it flows through the process facilitates the desalination process. Water obtained through thermal processing can easily have a purity of less than 0.1 micrograms per kilogram of dissolved matter. The brine wastewater resulting from the desalination of seawater has a salinity of approximately seventy grams per kilogram.

Membrane Separation

Membrane separation includes two major desalination methodspressure membrane processes (PMP) and electrodialysis reversal (EDR). In PMP, only pure water can pass through the membrane, while the majority of the dissolved material is detained by the membrane. The EDR method utilizes ion-specific membranes placed between anodic (positively charged) and cathodic (negatively charged) electrodes. Dissolved material is collected as the salt ions move under the electric current. Although the EDR method is relatively common, the most widespread desalination methods involve PMP.

There are four pressure-membrane desalination processes: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). The common principle of these methods is the forced passing (under high pressure) of saline water through a membrane. The main difference among the PMP methods is the size of the particles removed from the saline water. For example, MF removes particles larger than 10 microns (1 micron equals 0.000001 meter), UF removes particles of sizes from 0.001 to 10 microns, NF removes particles greater than 0.001 micron, and RO removes particles ranging from 0.0001 to 0.001 micron. Another difference among the PMP methods is the pressure to force the water through the membrane. MF operates at a pressure of less than 10 pounds per square inch (psi), UF at a range of 15 to 75 psi, NF at 75 to 250 psi, and RO at 200 to 1,200 psi.

Reverse osmosis is an energy-consuming method that can be used very effectively, particularly for the production of water for domestic use, whenever good-quality saline water is available. Reverse osmosis can be accomplished using different design modules such as the tubular, the plate-and-frame, the spiral-wound, or the hollow fiber. In all these designs, the water allowed to pass through the membrane, the “permeate,” is collected as the product water, while the water retained by the membrane forms the so-called concentrate or reject.

There are various types of membranes used for reverse osmosis, such as the cellulose acetate group, which includes cellulose acetate (CA), cellulose acetate butyrate (CAB), and cellulose triacetate (CTA), or polyamide (PA). Production of CA membranes involves four stages: casting, evaporation, gelation, and shrinkage. During the first stage, a solution of cellulose acetate in acetone containing certain additives is cast into flat or tubular thin surfaces. The acetone evaporates, leaving a porous surface. During the gelation stage, the cast is immersed in cold water, forming a gel, while at the same time, the additives dissolve in the water. In the last stage, the film shrinks, forcing a reduction of the pore sizes. High temperatures result in smaller pore openings.

Electrodialysis is also an effective desalination process whereby ions are separated from the water by being forced through selective ion-permeable membranes under the action of an electric current. The ion-permeable membranes alternate between those allowing only the passage of cations (such as potassium, K⁺) and those allowing only the passage of anions (such as chloride, Cl^-).

Another potential methodology for desalination involves freezing the water. Since ice is theoretically free of any dissolved material, various techniques have been proposed for applying freezing for desalination. However, this method does not have any widespread applicability.

The thermal and membrane methods require some form of energy to accomplish desalination. The energy efficiency of the desalination methods is expressed either as the gained output ratio (GOR) or the performance ratio (PR). The GOR is defined as the ratio between the mass of distillate over the mass of the steam. The PR, or economy, is estimated as pounds of distillate per 1,000 British thermal units (BTUs) or kilograms of distillate per 2,326 kilojoules (kJ). Criteria for selecting a desalination method include energy consumption, process efficiency, operational and maintenance costs, auxiliary services, and growth of demand. Energy is provided mostly by electrically driven pumps, but diesel engines can also be utilized.

Significance

Domestic (potable) water is a very valuable commodity. Population growth and high demand for water for industrial and agricultural practices have created severe water shortages in many parts of the world. Additionally, increased water contamination and extreme hydrologic conditions, such as prolonged drought, can adversely affect a region's socioeconomic and human health conditions.

In addition, other applications require high-quality water. For example, high-purity water is required by pharmaceutical companies for processing drugs and medications, by hospitals for kidney dialysis, by power and other energy-intensive plants in the form of low-scaling water, and by semiconductor manufacturing to produce high-performance chips.

Since most conventional water resources are already under stress, the only alternative solution to the water-resource problem is the desalination of the vast oceanic water masses or reusing wastewater. The various existing thermal or membrane methods can effectively provide high-quality freshwater. However, the high costs associated with these methodologies limit their applicability only to communities that can afford the financial burden. Ongoing research is improving efficiency, reducing the cost of existing desalination methods, incorporating green initiatives, and developing new methodologies. What began as a small distillation process to provide fresh water to a handful of ship crew members has evolved into a complex water treatment operation that supplies freshwater to large populations.

Principal Terms

dissolved matter: the amount of normally solid materials that are completely dissolved in water

evaporation: the physical process occurring at the water-air interface where water changes its phase from liquid to vapor

filtration: the removal of particulate matter from the water by passing it through a porous medium

osmosis: a natural process whereby the solvent, usually water, in a weak solution migrates across a semipermeable membrane into a similar solution of a higher concentration, with the result being the equalization of the solution concentrations

potable water: freshwater that can be used for domestic consumption

reverse osmosis: in practice, the forced passage of seawater through a semipermeable membrane against the natural osmotic pressure to obtain pure water

sodium chloride: the main chemical compound found as dissolved material in seawater

suspended solids: the solid particles that can be found dispersed in the water column

water resources: all the surface water and groundwater that can be effectively harvested by humans for domestic, industrial, or agricultural uses

water supply: the amount of water that is actually delivered to various consumer groups

water treatment plant: a facility where water is treated by physical and chemical processes until its quality is improved to that of potable water

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