Fresh water from the sea - The use of vacuum reduces energy requirements during desalination

Fresh water from the sea - The use of vacuum reduces energy requirements during desalination

Water is an increasingly scarce resource as the global population increases, and tapping into the inexhaustible reserves of the ocean is the most obvious next step. Potable fresh water is extracted from sea water in desalination plants, and vacuum plays a crucial role in the process.

Vacuum processes increase efficiency

There are two fundamentally different options for removing salt from sea water: with or without evaporation, the technical term for which is "phase transition". When water evaporates, i.e. passes from the liquid phase to the gaseous phase, all substances that have a higher boiling point than water remain. Salt is one of those substances. When the vapour is distilled again, i.e. returns to the liquid phase, the result is pure water.

This principle is used in many sea water desalination plants. To increase efficiency, it is often applied in a vacuum; this is because sea water must be heated to a temperature of approximately 115 °C at atmospheric pressure in order to evaporate. However, in a vacuum, this can be achieved at significantly lower temperatures. With a rough vacuum of 42.4 mbar, this is even possible at just 30 degrees Celsius. "Vacuum makes it possible to use the applied thermal energy efficiently," explained Dr Markus Spinnler, from the Verein Deutsche Meerwasserentsalzung (German Sea Water Desalination Association).

Small and robust

There is a huge variety of methods that do not use phase transition. Membrane processes are often used, such as what is known as "reverse osmosis". In this case, salty or polluted water is pressed through a membrane that only permits water molecules to pass through and retains unwanted particles, including salt ions.

Membrane distillation is a sort of hybrid that uses both thermal and mechanical means to separate water and salt. On the one hand, applying a vacuum in this process reduces the evaporating temperature. On the other hand, it is used to remove gases that have molecules so large they could clog the pores of the membrane. "Membrane distillation makes smaller, very robust plants possible," explained Dr Markus Spinnler. "Vacuum increases their performance and economic efficiency."

Desalination provides 300 million people with water

Many regions suffer from water scarcity, while the number of areas affected and level of severity are both constantly increasing. This is partially due to climate change, which is resulting in many areas becoming much drier. In addition, the economy and population are growing globally, and per capita water use is increasing along with economic development. Furthermore, the increase in population is often greatest in locations where natural sources of fresh water are particularly scarce: around the Persian Gulf, in the arid zones of Africa, North America and Central Asia, or on the northern coast of the Mediterranean.

According to information from the International Desalination Association (IDA), 18,426 desalination plants with an overall capacity of approximately 87 million cubic metres were in operation worldwide in June 2015, supplying approximately 300 million people with water. In Israel, approximately 40 percent of water demand is already covered by desalinated sea water, representing the highest percentage of any country. Other than those in Israel, the largest plants are located in the Gulf States, Australia and California. All forecasts predict strong growth for sea water desalination in the medium and long term.

Busch has been supplying vacuum technology for sea water desalination plants all over the world for over 30 years.

Even London needs sea water

There is even a shortage of rainwater in areas of England, which overall has high levels of rainfall. A desalination plant began operation at the mouth of the Thames in 2010 to ensure water supply to the metropolis of London. It can create 150 million litres of fresh water per day from the outgoing sea water at low tide. London's water needs are greater than the sources of fresh water that can be tapped in the region. Growth of the population and economy has for years been increasing the amount of water required.

Other large cities such as Beijing, New York and Los Angeles are facing a similar situation. They draw their water from regions far away using an elaborate system of canals or are increasingly turning to sea water desalination.

In addition to use of water resources in large cities, agriculture worldwide is contributing to the aggravation of water shortages. The natural freshwater reservoirs are subject to such excessive demands that natural regeneration is hardly possible. The need to save water and tap into new sources is therefore growing.

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