Over the past 40 years, the amount of fresh water available to each person in the world has decreased by 60%. More than 80 countries around the world are currently experiencing a lack of fresh water, located mainly in arid and dry areas and constituting about 60% of the total land surface of the earth.
Problem
A third of the world's population lives in countries with water stress. According to expert forecasts, by 2025 this figure will increase to two thirds.
Figure 1. Global water situation on the planet
The crisis will be triggered by the growth of the planet's population. According to UN estimates, by 2030 it will increase from 6 to 8.5 billion people. Nowadays, 2.5-3 thousand liters of water are spent annually to provide food for one person who has a diet traditional for industrialized countries. If the population increases by 2.5 billion, then an additional 2 thousand cubic meters will be needed to feed them. km of water.
In such conditions of acute shortage of fresh water, alternative technologies for replenishing water resources, including through the desalination of sea water, become especially relevant.
Water reserves
The total volume of water on Earth is approximately 1400 million cubic meters. km, of which only 2.5% (about 35 million cubic km) is fresh water. Sea water makes up about 98% of all water resources on the planet.
Table 1. Largest water reserves in the world (source: www.unep.org)
One of the most promising ways to provide fresh water is to desalinize the salty waters of the World Ocean. The feasibility of this path is confirmed by the fact that 60% of the planet's population lives in a coastal strip 65 miles wide. In addition, large areas of arid and low-water areas are adjacent to or located close to ocean shores.
Thus, ocean and sea waters can become a valuable source of water resources for industrial use. Their huge reserves are practically inexhaustible. However, at the current level of technological development, the use of desalination technologies is not economically justified everywhere.
Technologies used
Industrial desalination of seawater is carried out using one of the following methods: distillation, reverse osmosis, electrodialysis, freezing and ion exchange.
Let's take a closer look at the features of each technology.
1. MSF (Multi-Stage Flash Distillation) - multi-stage flash evaporation (distillation).
In this type of installation, the source water, before being passed through a special nozzle inside a large chamber, is pumped into the heater at a pressure at which boiling does not yet occur, i.e. the water is in a superheated state. A decrease in pressure entails the immediate conversion of part of the water into steam. The desalinated water is then passed through another nozzle into the adjacent “flash chamber”, where the flashing process continues, and so on down to the bottom of the plant.
2. MD (Membrane Distillation) - membrane distillation.
It involves heating water on one side of a hydrophobic membrane. Such a membrane allows only steam to pass through, which cools on the other side, forming fresh water, but does not allow water to pass through.
3. MED (Multi-Effect Distillation) - multi-column distillation method.
Sea water is heated in the first column, and the resulting steam is heated in subsequent columns.
4. MVC (Mechanical Vapor Compression) - mechanical vapor compression.
It involves compressing the vapor produced by the conventional distillation step before it condenses. The effect of steam compression involves heating it to a temperature above the boiling point of the water supplied to desalination (from which it was obtained). The compressed steam can then be returned to the same distillation chamber from which it was recovered and used to replace the primary steam. The cycle repeats continuously.
The use of compressed steam reduces the energy intensity of the process, but prevents the processing of large volumes of water.
5. FP (Freezing Process) - freezing method.
Sea water is cooled until the moisture crystallizes. The resulting crystals are separated and dissolved to produce fresh water.
6. RO (Reverse Osmosis) - reverse osmosis.
Involves the use of a semi-permeable membrane that allows water to pass under pressure and retains impurity molecules.
7. ED (Electrodialysis) - electrodialysis.
Two membranes are required: one allows only the cation to pass through, the other only allows the anion to pass through. A DC voltage is switched between them, which makes it possible to remove, for example, sodium and chlorine anions from sea water.
According to experts, each of the identified technologies has significant disadvantages, which include:
- significant deposits on heat transfer surfaces, membranes, etc.
- high specific energy costs
- availability of a large number of replaceable materials, components, additional consumption of chemical reagents
- environmental hazard during operation of installations
- the need for highly qualified service personnel.
In this regard, the issue of developing more efficient and environmentally friendly methods for desalinating seawater remains relevant.
Market
As of the end of 2009, there were 14,451 desalination plants in the world with a total capacity of 59.9 million cubic meters. m per day. Compared to 2008, the increase in capacity was 12.3%. In addition, 244 desalination plants (an additional 9.1 million cubic meters per day) are under construction.
In total, seawater desalination technologies are used in 150 countries around the world. The average freshwater production is about 38 million tons per year.
The market for salt water desalination technologies is growing rapidly. About 62.4% of the total industrial production of fresh water comes from the waters of the World Ocean.
Figure 2. Structure of application of fresh water production technologies depending on the type of water resources used (
Fresh water shortages are increasingly felt throughout the world, even in the United States and European countries. And in countries such as Israel or Iran, fresh water reserves are completely insufficient for the needs of the population and production. There is an opinion that humanity will eventually face the need fresh water extraction from the waters of the world's oceans.
Sea water desalination is the process of reducing the level of salts in water. In normal sea water the salt content is about 3.5 percent, and in water that is suitable for drinking, this level should not exceed 0.05 percent. Also, do not forget that after desalination it will be necessary to purify the water from calcium and harmful components, therefore, it is necessary to use water treatment plants.
Water treatment is a serious challenge in preparing ordinary fresh water for human use, and purifying desalinated water is an even more difficult task. Water treatment of seawater is complex because the level of microorganisms contained in seawater and their diversity are much higher than in fresh water. Moreover, the purification of sea water is further complicated by the fact that much more chemical compounds are dissolved in sea water than in fresh water and their concentration is much higher. All of the above suggests that sea water treatment- a process no less complex and important than fresh water purification.
There are several methods for desalination and subsequent purification of seawater. One of these methods is the distillation method.
Distillation, or distillation, is based on the fact that water is a volatile substance, and the salts dissolved in it are non-volatile. Sea water is heated to boiling point, resulting in the formation of water vapor, the resulting steam is taken and cooled, leaving ordinary water. But when using this method seawater desalination There are several problems, and the most basic problem is that when evaporating, the brine remaining in the distiller becomes more concentrated each time. This leads to failure of pipelines and the distiller itself. To solve this problem, multi-chamber distillers are used, and part of the desalinated water is discharged with a brine solution into the sea, and a new portion of water is taken in its place. Before and after the distillation process, seawater undergoes a pre-treatment process.
Another method of desalination of sea water and cleaning it from impurities is -. When using this method, water purification and desalination occurs using a membrane that is permeable to water and at the same time impermeable to salts and other impurities dissolved in sea water, using. The disadvantage of this method purification and desalination of sea water is the small amount of fresh water produced. The problem is that seawater must be supplied to the membrane under pressure so that clean water leaks through the membrane, and the salts remain on the back side of the filter. Installation by desalination and purification of sea water Usually a series of thin tubes, the walls of which are lined with cellulose acetate, seawater is fed into the tubes under sufficient pressure to force fresh water through the filter. This pressure is called osmotic pressure, it is necessary to ensure that it does not exceed permissible values, otherwise the membrane may rupture or begin to leak salts dissolved in sea water.
There are also other methods seawater desalination, for example, the freezing method. The method is based on the fact that when seawater turns into ice, the salts dissolved in it do not enter the ice.
As stated earlier, paying special attention seawater desalination process, we must not forget about purifying the fresh water already obtained. Water treatment of the resulting water for the most part does not differ from the process of filtration and purification of ordinary water. For water purification, coarse filters, fine filters, and chemical and biological water treatment filters are used.
Unfortunately, at the moment there is still no sufficiently cheap and effective method of desalination of sea water, capable of meeting humanity’s ever-increasing needs for fresh water. Methods currently used seawater desalination either ineffective, or the cost of the resulting liter of desalinated water is too high for use on an industrial scale.
Water desalination is the reduction of the amount of salts contained in natural waters. For drinking purposes, water is usually partially desalinated, reducing the salt content to a level where the taste of the water becomes suitable for drinking (less than 1000 mg/l).
Various methods are used to desalinate water. The chemical method of ion exchange, based on the filtration of water through special granular materials - ion exchangers, is widely used in practice for desalting waters with a total salt content of up to 2-3 g/l. Desalting water by electrodialysis is based on the fact that in an electric field, cations and anions of water move to a cathode and anode immersed in water. Electrodialyzers are separated by partitions (membranes) that are permeable to cations and anions, and desalinated water accumulates in their middle part, between the partitions.
When water is desalinated by distillation, steam is formed that is free of salts, so when it condenses, distilled water is obtained.
Desalination of water by freezing is based on the fact that when water is slowly cooled below 0°, fresh ice crystals, which freeze into aggregates, are formed before the brine freezes. With gradual heating, the brine frozen between the fresh ice will turn into a liquid state and will drain before the fresh ice crystals begin to melt. With further melting, fresh water is formed.
Before supplying desalinated water to the water supply network, it must be disinfected.
Desalination of water using nuclear desalination plants is carried out using distillation and freezing methods. Distillation is preferred. Discharging brine onto the surface of the earth or into local water bodies is unacceptable. Desalinated water is devoid of microelements, tasteless, and unsuitable for drinking, preparing drinks and food. Needs enrichment with mineral salts.
Water desalination - partial desalination of saline and sea water in order to reduce the content of dissolved salts to the extent that water becomes
suitable for household and drinking purposes. Desalination of water differs from softening, i.e. partial removal of calcium and magnesium cations from water. In connection with the development of new areas with limited fresh water resources and with the growing demand for drinking water, there is increasingly a need to use sources of saline water and even sea water (regions of Kazakhstan, the Caspian Sea, etc.).
Desalination of water is achieved by changing its aggregate (phase) state (distillation, freezing) and removing dissolved salts from it (chemical, electrochemical and ion exchange methods). Using the distillation method, used in approximately 80% of existing installations, water is desalinated at a temperature slightly above 90°, which simultaneously contributes to the disinfection of drinking water (see). There are installations that operate at low temperatures. The water obtained by distillation, devoid of salts, has an unpleasant taste and does not contain trace elements at all; therefore, the original mineralized water is added to it, but no more than is allowed by the drinking water quality standard. Under natural conditions, desalination of water by freezing it and solar desalination is possible only in certain climatic regions. When desalinating water by freezing, contamination of ice, tanks and vehicles must not be allowed.
New prospects in the field of water desalination have arisen in connection with the production of synthetic ion exchangers. These solid, water-insoluble granular organic acids and bases have the property of exchanging their constituent ions for ions contained in the desalinated water. There are ion exchange resins as cation exchangers, which extract cations from water (Ca +2, Mg +2, Na+, etc.), and anion exchangers, which extract anions from water - chlorine ion (Cl-), sulfate ion (SO -2 4) and etc.
Desalination of water using the ion exchange method is carried out by filtration through pressure filters (groundwater that does not require preliminary purification and disinfection) and non-pressure filters (water from a surface water source that is subject to preliminary purification and subsequent disinfection). Ion exchange resins should not change the organoleptic properties of water or cause the appearance of substances that may be hazardous to health (formaldehyde, blastomogenic and other unsafe substances used in the production of organic ion exchangers).
The electrochemical method of water desalination is based on the phenomenon of electrodialysis: its practical application became possible with the reduction in the cost of electricity and after the replacement of inert diaphragms with ion exchange resins (from cation- and anion-exchange resins), respectively, allowing cations and anions to pass through. The use of newly recommended ion exchange materials is permitted only with the approval of sanitary authorities.
Nuclear desalination plants. At the Geneva Conference on the Peaceful Uses of Atomic Energy in 1964, the feasibility of building combined nuclear desalination plants designed for simultaneous desalination of water and generation of electricity was pointed out. Desalination of seawater in a nuclear installation can be carried out by distillation, electrodialysis and freezing. Currently, distillation is preferred. Pilot plants built in the USA and USSR have demonstrated the efficiency and cost-effectiveness of dual-purpose plants designed to desalinate water and generate electricity. The Soviet desalination plant is designed for 150 MW of electricity and 120,000 m 3 of fresh water per day. Preliminary calculations made in the USA show that the cost of 1 m 3 of desalinated water will not exceed 7 cents for large installations and 14 cents for small ones.
However, desalination of water and the use of desalinated water in the national economy is not only a technical problem, but also a hygienic one. Desalinated water is devoid of microelements necessary for the human body, tasteless, and unsuitable for drinking, preparing drinks and food. It needs to be enriched with mineral salts; it is necessary to determine which mineral elements and in what quantities should be added to desalinated water intended for drinking and cooking, and what amount of mineral elements is permissible in desalinated water. It is known that in sea water the amount of mineral salts reaches 35,000 parts per million (ppm); in brackish water of steppes and deserts - from 2,000 to 4,000 ppm; in irrigation drainage water - 4000 ppm and above. The WHO expert committee in 1964 made recommendations on the content of mineral residue in desalinated water: from 500 to 1000 parts per 1 million.
When desalinating water at nuclear installations, discharging the brine remaining after evaporation of bitterly salty water onto the surface of the earth or discharging it into local water bodies is unacceptable. In hot climates, ponds lined with plastic film can be installed to evaporate wastewater to dry sludge at atmosphere. At installations located on the coast, waste after distillation of water is discharged into the sea.
The European Investment Bank has announced the signing of a €142 million financing contract for the design and construction of a seawater desalination plant using reverse osmosis technology. The Israeli technology has long been used all over the world, but in Israel itself there was not enough money to build new desalination plants.
Desalination plant will satisfy 20% of the country's needs
The contract signed with Sorek Desalination will significantly increase the availability of water resources in a region where water has always been scarce. According to the publication, the desalination plant should satisfy 20% of the country's needs for fresh water for domestic use. It is expected that the production capacity of the new plant will be 150 million cubic meters of water per year. Finance Minister Steinitz said that the desalination plant in Sorek is one of the largest in the world and will significantly overcome the consequences of the country's water crisis.
Mixing different waters will improve water quality
The expansion of desalination technologies will have a direct impact on people's daily lives: mixing desalinated water with fresh drinking water from the national water supply will improve the quality of water supplied to consumers by reducing its hardness and the concentration of salts, nitrates and boron. This will ultimately lead to a noticeable reduction in water withdrawal and thus protect against the intrusion of salt water into the aquifers. In Israel, in 2007 and 2009, the European Investment Bank supported the construction and expansion of the Hadera desalination plant with loans totaling €130 million. In the Mediterranean region as a whole, the bank has invested more than 1.050 billion euros in the development of the water sector.
Membrane water treatment plants for seawater desalination
Reverse osmosis water treatment plants are designed for desalting water with a salinity of up to 45 g/l. The salt content of desalinated water meets the requirements of the World Health Organization and does not exceed 0.5 mg/l.
The basic version of a water treatment installation includes:
- mechanical cleaning unit mechanical filter with a filtration rating of 10 microns;
- the membrane desalination unit provides deep cleaning and reduction of the total salt content of water;
- ultraviolet sterilizer to prevent secondary biological damage to water and its deep disinfection before serving to the consumer.
The above scheme can be used when supplying water from a coastal well; in the case of supplying water from the open sea, the installation is additionally equipped with a pre-treatment unit based on the following:
a) reagent treatment;
b) based on micro- or ultrafiltration.
UPO pre-cleaning unit based on reagent treatment
It includes a homogenizing reactor and a settling tank, in which a thin-layer module is mounted, which significantly increases the settling efficiency, a self-washing mesh filter that retains mechanical particles up to 20 microns in size, and mechanical filters with a filtration rating of 10 microns.
UPO pre-cleaning unit based on micro- or ultrafiltration
Membrane filtration ensures complete removal of coarse and fine, suspended, colloidal particles and other insoluble impurities
Israel has commissioned one of the world's largest desalination plants, capable of supplying nearly 20% of the country's drinking water needs, local television reported.
The plant in the coastal city of Hadera will annually produce 127 million cubic meters of water using membrane purification, as much as the other two Israeli plants combined. In the coming years, it is planned to build two more desalination plants in a country suffering from a shortage of traditional water resources.
This is the world's largest water desalination plant using the membrane method. In addition, this is a new word in energy saving, which allows reducing the cost of water, says Tidar Golan, deputy director of the operating company.
There are larger desalination plants in Saudi Arabia, but they use seawater heating technology. At Hadera, water is passed through membranes that trap salts and impurities so that the desalinated water must be mineralized before it is sent to the consumer.
Seawater desalination technologies
Desalination of seawater is one of the most serious problems today, the solution of which will avoid many problems in the future. In view of the global and constant reduction in fresh water reserves in the world, the task of seawater desalination, purification, and water purification becomes, if not the most important, then, in any case, one of the most significant.
Understanding the importance of the ability to obtain drinking water from sea water for our entire planet, the Agbor Engineering company, along with water treatment and wastewater treatment technologies, is also developing its activities in the field of seawater desalination.
To date, the most common desalination methods are:
- Thermal desalination is a very energy-intensive process.
- Desalination using semi-permeable membranes is an energy-efficient process.
Peculiarities
The high salinity content of sea waters determines the use of marine membranes and special materials. One of the main problems is corrosion of equipment and pipelines, therefore pipelines made of fiberglass, polymer materials, and high-quality corrosion-resistant steels of duplex or austenitic type are used.
Also, due to high mineralization, it is necessary to create high pressure for osmosis to work, 50-80 bar. Electricity consumption for high-pressure pumps can be reduced by using energy recovery devices.
Our developments coincide with the recommendations of membrane element manufacturers regarding such technological parameters as recovery, specific flow, etc. We also take into account the material of pipeline and shut-off valves, pumps, and instruments. Production facilities allow installation of pipelines made of high-quality steels, such as 254 SMO, etc.
It is important that the control system is developed with the direct participation of technologists, which ensures a detailed study of the control of transient processes, such as starting and stopping the system, taking into account the presence of an energy recovery system.
Desalination of water is the removal of dissolved salts from water in order to make it suitable for drinking or for performing certain technical tasks.
Water with a content of soluble salts of no more than 1 g/l is suitable for drinking water supply. Therefore, a practical task in water desalination is to reduce its excess salinity. This is achieved in various ways:
- evaporation, including:
conventional distillation,
multi-stage flash distillation,
low pressure distillation,
thermocompression distillation,
- freezing,
including through gas hydrates,
- ion exchange,
- electrodialysis,
- reverse osmosis,
- direct osmosis,
- hydrodynamic separation.
In the research stage:
- an electrochemical method in which a special microcircuit divides the flow of salt water into two flows with increased and decreased salt content.
Desalination of water for industrial and domestic needs is carried out at desalination plants. Depending on the method used, energy consumption per cubic meter ranges from 0.7 kWh to 20 kWh.
Sources: www.vodainfo.com, vladbmt.ru, evroplast.in.ua, www.agbor.ru, ru.cyclopaedia.net
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Planet Earth has huge reserves of water, but most of it is part of the world's oceans and is salty. The quality of sea water does not allow it to be used in its pure form for industrial agricultural and especially for food purposes. Seawater contains more than 50 elements of the periodic system in dissolved form. The concentration of each element individually is extremely insignificant, but together they determine the indicator due to which sea water is called salty. , suitable for food purposes, must contain no more than 0.002 g/ml salts. To achieve such a concentration, a large number of methods have been developed, the main goal of which is to remove salts from sea water and purify it. The main task of the developers is to find a method that would have low energy consumption and the most complete purification, after which the water could be used by the population.
Desalination methods
Today, there are desalination methods such as distillation, reverse osmosis, ionization and electrodialysis that can be used on an industrial scale.
- The most popular way is conventional or multi-stage distillation, which uses the property of boiling and vaporization at high temperatures. More than half of desalinated water is obtained this way.
- Membrane distillation, a method in which water is heated on one side of a membrane, which allows only steam to pass through and forms fresh water from it.
- Reverse osmosis method relatively cheap, since one dollar invested allows you to get 16 tons of fresh water. By applying pressure to seawater and forcing it through the smallest filters, you can obtain fresh water with a low salt content. The performance of the membrane and the degree of desalination depend on many factors: the amount of salt content in the feedstock, salt composition, temperature and pressure.
- Use of dialysis, in which water passes through a chamber with electrodes, causes cations and anions to be distributed on the corresponding electrodes. The advantage of electrodialysis is that the process uses chemically and thermally resistant membranes, which makes it possible to carry out desalination at high temperatures.
- Gas hydrate method is based on the ability of carbon gases at a certain pressure and temperature to create, with the participation of water, clathrate-type compounds. Frozen salt water is treated with a hydrate-forming gas, and crystals form. After separating them from the brine, the crystals are washed and melted, obtaining pure crystals.
- evaporation (distillation), including: freezing;
- conventional distillation;
- multi-stage flash distillation;
- low pressure distillation (vacuum distillation);
- hydrodynamic separation (separation).
For desalination in the southern regions, solar desalination plants are used, in which the marine mode is heated and evaporated. There is also a completely opposite method, in which they simply freeze sea water, or rather freeze and separate fresh water, since it freezes faster than sea water.
Industrial desalination
There is a shortage of clean drinking water in more than 80 countries around the world. This crisis is provoked by the growth of industrial production, population growth, deteriorating environmental conditions throughout the world and planetary climate changes. The world community is on the verge of an acute shortage of fresh water. In such a situation, the issue of finding alternative technologies for replenishing fresh water supplies becomes especially acute. The most optimal way to desalinize the waters of the world's oceans is considered. Scientists see the feasibility of this path in the fact that a large number of the population lives in the coastal zone, having free access to an almost free resource.
Desalination plants are being built in many countries where there is a shortage of drinking water, for example in Kuwait, Saudi Arabia, Israel, the United Arab Emirates, the USA (California). The most powerful desalination plants are located in the Middle East; for example, in Saudi Arabia there are seven such plants and each of them can produce up to 400,000 cubic meters of fresh water per day. The production market is constantly expanding. Countries such as Australia, Spain and Algeria are developing large-scale government support programs to stimulate industrial production of fresh water.
Russia lags significantly behind in this matter; our desalination industry market is not developed. The climatic and geographical location of the country makes it possible not to strive to become the leader of states that invest huge amounts of money in water desalination. But nature always leaves the last word for itself and makes its verdict. The presence of such problem areas as the Stavropol region, the Volgograd region, the Caspian region and the Orenburg steppes does not make it possible to forget about the shortage of fresh water.
Alternative options
- Antarctica gives hope. While scientists are puzzling over new industrial methods of desalinating sea water, another part of the bright heads turned towards Antarctica. There are projects based on the idea of transporting ice blocks with fresh water directly to the Mediterranean Sea. Calculations show that transporting an ice floe the size of a football field can be carried out in at least a year, since a higher speed of the accompanying caravan is not technically possible. There are other projects that involve crushing relic icebergs and delivering them in crushed form in holds.
- Water regeneration. For areas that are located far from the sea coast and where there are no other sources of fresh water, it is quite difficult to find alternative options. Here people rely only on water restoration. Collecting wastewater and surface water and returning it to circulation can be an ideal option for obtaining water. This method is used for land irrigation. Collecting rainwater, targeted capture and subsequent storage in underground storage facilities, allows us to solve the problem, even if only a small part of it.
Ship desalination plants
Solving the problem of seawater desalination on a global scale requires agreement and mutual understanding of scientists, businessmen and politicians from different countries. Smaller problems, such as ship desalination plants, are being solved today at the level of industrial enterprises involved in mechanical engineering. Marine purifiers and desalination units with membrane filters are the most ideal solution for equipping a sea vessel to obtain fresh water during a long voyage. The need for such installations is growing every day, and not only because the number of ships, yachts and submarines has increased. Such installations are also used in coastal areas, in areas where there is increased salinity of water at the mouth of a river or in a lake.
Household desalinators – distillers
Household desalination plants are used for purification and desalination of water in domestic conditions, in laboratories, car dealerships, medical institutions and beauty salons. Household distillers work on the principle of the water cycle in nature: heating, conversion to steam, evaporation and cooling. This method allows you to get soft and clean water.
Water shortage: myth or reality?
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