What are the methods of desalination?

As we all know, the salt content of seawater is very high, so seawater is definitely not drinkable. Therefore, people have come up with many ways to turn seawater into fresh water. The ultimate goal is to reduce the salt content of seawater. In this way, the total amount of fresh water can be greatly increased. There are many ways to desalinate seawater now. People in different countries and regions use different methods. The common methods are listed below.

Methods of desalination of seawater:

Distillation

Although distillation is an ancient method, it still dominates today due to continuous improvement and development of technology. The essence of the distillation and desalination process is the formation process of water vapor. Its principle is just like the evaporation of seawater by heat to form clouds, and the clouds form rain by cooling under certain conditions. Rain does not have a salty taste. According to equipment distillation, steam compression distillation, multi-stage flash distillation, etc.

Freezing method

Cryogenics involves freezing seawater to form ice, where the salt is separated out while the liquid fresh water turns into solid ice. Both the freezing method and the distillation method have disadvantages that are difficult to overcome. The distillation method consumes a lot of energy and produces a lot of scale in the instrument, but the amount of fresh water obtained is not much; the freezing method also consumes a lot of energy, but the fresh water obtained tastes bad and is difficult to use.

Reverse Osmosis

It is also commonly known as ultrafiltration, which is a membrane separation and desalination method that was not adopted until 1953. This method uses a semipermeable membrane that only allows solvents to pass through but not solutes to pass through to separate seawater from fresh water. Under normal circumstances, fresh water diffuses through a semipermeable membrane to the seawater side, causing the liquid level on the seawater side to gradually rise until it reaches a certain height. This process is called osmosis. At this time, the static pressure of the water column above the sea water side is called osmotic pressure. If an external pressure greater than the osmotic pressure of seawater is applied to one side of the seawater, the pure water in the seawater will reverse osmosis into fresh water. The biggest advantage of reverse osmosis is energy saving. Its energy consumption is only 1/2 of that of electrodialysis and 1/40 of that of distillation. Therefore, since 1974, developed countries such as the United States and Japan have successively shifted their development focus to anti-infiltration methods.

Reverse osmosis desalination technology is developing rapidly, and project costs and operating costs continue to decrease. The main development trends are to reduce the operating pressure of reverse osmosis membranes, increase the recovery rate of reverse osmosis systems, use cheap and efficient pretreatment technology, and enhance the system's anti-pollution capabilities.

Solar Energy Law

In the early days of human beings, solar energy was used for seawater desalination, mainly for distillation, so early solar desalination devices were generally called solar stills. Distillation Systems An example of a passive solar distillation system is the tray solar still, which has been used for nearly 150 years. Because of its simple structure and easy access to materials, it is still widely used today. At present, the research on disc solar stills mainly focuses on the selection of materials, the improvement of various thermal properties and the use of it in conjunction with various types of solar collectors. Compared with traditional power sources and heat sources, solar energy has the advantages of safety and environmental protection. Combining solar energy collection and desalination process is a sustainable desalination technology. Solar desalination technology has gradually attracted people's attention due to its advantages such as not consuming conventional energy, no pollution, and high purity of the obtained fresh water.

Low temperature multi-effect

Multiple-effect evaporation allows the heated seawater to evaporate in multiple evaporators connected in series. The steam evaporated from the previous evaporator serves as the heat source for the next evaporator and is condensed into fresh water. Among them, low-temperature multiple-effect distillation is one of the most energy-saving methods in distillation. Low-temperature multi-effect distillation technology has developed rapidly in recent years due to energy-saving factors. The scale of the equipment has been expanding and the cost has been decreasing. The main development trends are to increase the water production capacity of a single unit, use cheap materials to reduce project costs, increase operating temperature, and improve heat transfer efficiency.

Multi-stage flash

Flash evaporation refers to the phenomenon that part of seawater at a certain temperature evaporates rapidly when the pressure suddenly drops. Multi-stage flash desalination is a process in which heated seawater is evaporated in multiple flash chambers with gradually decreasing pressures, and the steam is condensed to obtain fresh water. At present, the multi-stage flash evaporation method still has the largest output, the most mature technology, high operational safety and great flexibility in the world's seawater desalination equipment. It is mainly built in conjunction with thermal power plants and is suitable for large and ultra-large desalination equipment. It is mainly used in the Gulf countries. Multi-stage flash evaporation technology is mature and reliable in operation. Its main development trends are to increase the water production capacity of a single unit, reduce unit electricity consumption, and improve heat transfer efficiency.

Electrodialysis

The technical key of this method is the development of new ion exchange membrane. Ion exchange membrane is a functional membrane with a thickness of 0.5-1.0mm, which is divided into positive ion exchange membrane (cationic membrane) and negative ion exchange membrane (anionic membrane) according to its selective permeability. The electrodialysis method is to alternately arrange cationic membranes and anionic membranes with selective permeability to form multiple independent compartments. The seawater is desalinated, while the seawater in adjacent compartments is concentrated, so that the fresh water and concentrated water can be separated. Electrodialysis can not only desalinate seawater, but also be used as a means of water treatment to contribute to wastewater reuse. In addition, this method is increasingly used in concentration, separation and purification in chemical, pharmaceutical, food and other industries.

Steam distillation

The compressed steam distillation seawater desalination technology is that the seawater is preheated, enters the evaporator and partially evaporates in the evaporator. The generated secondary steam is compressed by the compressor to increase the pressure and then introduced into the heating side of the evaporator. After condensation, the steam is discharged as product water, thus realizing the recycling of heat energy.

Dew point evaporation method

Dew point evaporation desalination technology is a new method for desalination of brackish water and seawater. It is based on the principle of carrier gas humidification and dehumidification, while recovering the heat of condensation and dehumidification. The heat transfer efficiency is controlled by the heat transfer on the mixed gas side.

Combined hydropower

Cogeneration of water and power mainly refers to the cogeneration and supply of desalinated water and electricity. Since the cost of desalination depends largely on the cost of electricity and steam consumption, cogeneration can use the steam and electricity from power plants to power desalination equipment, thereby achieving efficient energy utilization and reducing desalination costs. Most of the seawater desalination plants abroad are built together with power plants, which is the main construction model of current large-scale seawater desalination projects.

Thermal film cogeneration

Thermal membrane cogeneration mainly adopts a combination of thermal and membrane desalination (i.e. MED-RO or MSF-RO) to meet different water demands and reduce the cost of desalination. At present, the world's largest thermal membrane cogeneration desalination plant is the Fujairah Desalination Plant in the United Arab Emirates, with a daily desalination water output of 454,000 cubic meters, of which MSF produces 284,000 cubic meters per day and RO produces 170,000 cubic meters per day. Its advantages are: low investment cost and shared seawater intake. Desalinated water from RO and MED/MSF devices can be mixed in certain proportions to meet a variety of needs.