Desalination

Seawater desalination is the production of drinking water or process water from sea water by the reduction of the salt content. Desalting can be based on various processes that remove salts and minerals from the water. Partly this usually involves reusable by- products such as saline.

• 3.1 Evaporationsschläuche plastic
• 3.2 Freezing Method
• 3.3 electrodialysis

• 4.1 ionic strength
• 4.2 Bio - fuel cell

Importance

The desalination of sea water is metered for the future of great importance, since the supply of people with clean water is becoming increasingly difficult due to lack or pollution of fresh water available. Desalination of sea water is used on ships, submarines and islands. The cost to play here before the necessity of procuring salt-free water only a minor role. The starting material must have only very low contamination. In the vicinity of major shipping routes, a country must fountain which reaches the sea water level, pre-filters the sea water. In the Middle East these energy- intensive form of drinking water is widespread. In the oil- rich Gulf countries it is the main source of drinking water. Here, the drinking water is obtained by gas-or oil -fired desalination plants. Also combined gas and steam turbine power plants with attached MSF desalination plant are frequently used. The acronym MSF is multi-stage flash evaporation for multi- stage flash evaporation in meaning. The Canary Islands and the German island of Helgoland drinking water is obtained by the reverse osmosis process.

Established techniques

Solar desalination plants to desalinate sea water using solar energy. Other methods are: freezing, Evaporationsschläuche plastic, electrodialysis, vacuum distillation systems with heat pumps and equipment for the vapor recompression process.

Membrane distillation

In the method of membrane distillation, a microporous membrane is employed which allows only water vapor, liquid water but retains. On one side of the membrane is warm salt water and on the other side a colder surface. By the counter-current operation of the installation is achieved that a temperature difference exists over the entire length of the membrane. The resulting difference in water vapor pressure causes water molecules pass from the warm to the cold side of the membrane.

Reverse osmosis

In reverse osmosis ( RO Sheet (reverse osmosis ) ), the solution (sea water ) to overcome the osmotic pressure at high pressure through a semi-permeable membrane made ​​of polyamide, PTFE or pressed sulfonated copolymer with a pore diameter of 0.5 to 5 nm. This acts as a filter, allowing only certain ions and molecules through. Thus obtaining a separation of the original solution. Through the membrane filter can be salts, bacteria, viruses, withhold an oversupply of lime and toxins such as heavy metals.

For technical reasons it is necessary to supplement in connection electrolytes in the right amount, as completely demineralized water caused severe corrosion damage to pumps and pipes. Therefore, the pH value by the addition of commercially available chemicals (usually limestone) adjusted to the desired pH value of 7.

The osmotic pressure increases with increasing salt concentration, the process would thus eventually come to a halt. To counteract this, the concentrate is discharged. Since the crystallization of the salt, or minerals ( precipitation ) must be avoided in the membranes, the use of reverse osmosis up to a certain maximum concentration of the reflux is useful. Depending on the salt concentration can be expected also in optimal systems with an energy expenditure 2-4 kWh per cubic meter of drinking water due to high pressure.

The drinking water treatment plants can be equipped with additional pre-filters depending on the type of water pollution. Coarse particles can be separated as to a particle size of 20 micrometers. An additional activated carbon filter separates from organic substances such as pesticides. Also, UV irradiation can be followed, which is an additional level of security against germs. A further improvement in filter efficiency is again achieved through the use of nanotubes.

Multi-stage flash evaporation

This is a thermal process with the abbreviation " MSF " (English for multi- stage flash evaporation). Your predecessor was the multi-effect distillation.

In this method, the supplied sea water is heated to a temperature of 115 ° C with the waste heat from a power plant. The heated in the so-called Brine heater salt water evaporates in downstream expansion stages under vacuum, the water vapor condenses on as condensate within these stages with cooling liquid filled pipes down and is removed as a salt-free water. The increasingly enriched by the evaporation process with salt water is also brine ( brine ) and called in a downstream heat exchanger on the condensing temperature ( ≈ 40 ° C), the vapor of the supplied fresh water cooled. It is then used subsequently in the pipes as a coolant. The pipes themselves are continuously cleaned with sponge rubber balls of auskristallisierendem salt. Brine is fed to the last fresh salt water, and heated again by the heat of the gas turbine. Thus, the entire procedure is a closed circuit dar. The excess of the in circulation concentrating salt is recycled back into the sea.

The resulting distillate is in the condensation on low-salt diet for direct use as drinking water. Furthermore, such low-salt waters are corrosive to ferrous materials, as no lime -rust protection layer can be formed. The addition of calcium bicarbonate therefore the content of carbonate hardness is increased again in the water. The calcium bicarbonate is produced by a reaction of calcium hydroxide ( lime water ) with CO2 gas. The necessary CO2 for this purpose is often obtained by combustion of natural gas with air.

Large-scale systems, such as located in Dubai power plant and seawater desalination plant Jebel Ali, can win up to 500,000 cubic meters of potable water from sea water daily. Similar amounts are also produced by the present in the region oil power plants. Powered by renewable energy desalination plants can be an economical way to produce drinking water of high quality.

Other techniques

Evaporationsschläuche plastic

The French research center CEA / Greth has developed an innovative seawater desalination plant as part of a European CRAFT project, in which the metal components have been largely replaced by polymers. This has the advantage that considerably less corroding plastics, and thus are more resistant than metals. Through the use of plastic, the process can run bar under normal conditions at 100 ° C and 1. The system can therefore be designed much simpler and more robust. The apparatus reaches a drinking water production capacity of 100 l / h Since the water is heated to 100 ° C, it is substantially sterile and contains only small amounts of salt. The basic price of a cubic meter of water by an investment amounts to € 4.

Freezing process

By cooling of sea water, ice crystals, which are free of salts form. However, the technical difficulties mainly relate to the separation of ice crystals from the mother liquor. The ice crystals must be washed from the mother liquor. Here again there is a considerable need for fresh water, which has failed on this method in practice.

Electrodialysis

The electrodialysis is economical only at very low salinities. The energy costs are in a linear relation to salinity. Therefore, the method is often worthwhile only for brackish water.

Possible future techniques

Ion power

Salt water into 4 pools. In tank 1, the salt concentration (e.g. by evaporation, solar) increases. The resulting concentrated brine in tank 1 is selective polystyrene membranes ( the pool for 2 Na - ions block - ions and the pelvis 3 Cl ), whereby in these basins, a Na - ions produced surplus - or Cl. These two pools are connected to the tank 4 via membranes. For this fourth basin, the ions diffuse to the ion balance in pools 2 and 3 The water in pool 4 thus NaCl is free. Still need other salts are removed, other ion filter can still be used. In Canada, a pilot plant was built with environmental funding. The Siemens group operates a pilot plant in Singapore.

Advantages: Very low energy consumption, provided that the evaporation takes place in pool 1 by Sun. The mineral content, with the exception of sodium and chloride will be maintained and not other minerals must be added for use as drinking water. For other purposes, additional ion filters are required.

Bio - fuel cell

Bio - fuel cells are to combine the processes of desalination and power generation and thus can solve the problem that desalination techniques used so far are energy-intensive. Research is carried out, inter alia, at the University of Queensland, Brisbane, Australia, Tsinghua University, Beijing, China, and the Oak Ridge National Laboratory, USA. A practical application is initially considered only for brackish water.

Effects on the environment

The first completely eco-friendly, floating desalination plant based on reverse osmosis is the summer of 2007 in the southern Aegean Sea off the island of Iraklia in operation and daily delivers about 70 m³ of drinking water of high quality. The energy required for operation of the system is ensured by a floating wind turbine. The system can be controlled remotely. The pilot project is a development of the Greek Aegean University and learned in the context of the award of the European Innovation Award Regional Star 2008 honorable mention.

Years ago, the desalination of sea water was expensive and energy intensive for an investigation of the environmental organization WWF. The generation of the required energy leads to an increase of greenhouse gases; very large systems would spoil coastal regions and the return of the confiscated salt as brine in the sea have in the affected coastal regions have negative consequences for fish stocks, coral and aquatic plants. Desalination could be useful in some cases, but is not generally appropriate solution to address the looming water shortage. To ensure the supply of drinking water, especially measures to make better use of existing water resources and water management would be preferable; the wastewater treatment should be considered as an alternative. These concerns are no longer relevant with the latest investment in Iraklia. Even the salt obtained can be recycled [ unintelligible]. Proponents of desalination keep the above allegations against moreover, that caused by the return of increased salt concentration was within a few meters of the sea measured technically no longer detectable. As to the objection of the necessary high energy demand, as he no longer uses the latest in a new method which is called Water Hub. It only consumes 1.5 kWh per cubic meter of reclaimed water.

Nuclear power plants for seawater desalination

1973 went in Aktau (formerly Shevchenko, USSR ), the first operating on an industrial scale breeder reactor in operation. The nuclear power plant Aktau generated 150 MW electricity and 200 MW of process heat for the desalination of sea water from the Caspian Sea.