How is seawater desalinated?

Desalination of seawater is an excellent way to replenish fresh water supplies, especially in arid, desert areas where there are no aquifers.

The process involves a significant reduction of salts in the composition: if the salinity of seawater can be 35g/liter (the average value for the world ocean), for drinking water this value should not exceed 1 gram per liter.

Methods of seawater desalination

The key technologies are divided into two main groups. The first does not involve a change in the aggregate state of water (it remains liquid at all stages of treatment). The second involves the transition of the liquid to a solid or gaseous form at a certain stage.

Chemical method

Reagents are added to the water that bind salt ions and promote their precipitation. Silver and barium salts are used as reagents, and up to 5% of the total amount of desalinated water is required. The reaction releases toxic substances, so this method is practically not used.

Electrodialysis

Two electrodes in the form of electrochemically active diaphragms (with plastic or rubber casing and resin filler) are installed in a brine bath, after which direct current is passed through.

A chemical reaction occurs with the release of chlorine and oxygen into the atmosphere. Water accumulates in intermediate chambers and is drained, while the brine remains in the tank.

This method is also called ion-exchange desalination: it is used where the salinity of seawater is initially low. It is also often used for mobile units on fishing vessels, trawlers.

Ultrafiltration (reverse osmosis)

In this case, the saline solution is fed under pressure through a membrane that is permeable to water but impermeable to salt. Such membranes are made from cellulose acetate fiber and impregnated with magnesium perchlorate, which increases water permeability.

Since the pressure is significant, up to 150 kgf/cm2, the membranes are supplemented with porous bronze plates. The process can be controlled automatically or semi-automatically, with the main focus on maintaining stable water feed pressure. The yield of fresh water from saline water is up to 70%.

Freezing

In natural conditions, ice covering oceans and seas is fresh. Artificial slow freezing is carried out, which allows obtaining ice with a needle-like crystalline structure. The brine settles and does not enter the ice mass.

The obtained ice is melted, allowing water with salinity not exceeding 500-1000 mg/l to be obtained. Crystallizers (contact, vacuum, with heat exchange through the wall) are used for freezing, where contact between water and gaseous or liquid refrigerant is ensured.

Thermal desalination (distillation)

This method is often used on sea vessels to obtain fresh water from overboard saline water. In this case, seawater is heated to boiling, and the steam is condensed. This collects distillate, which is fresh water.

Distillation units include evaporators, heating elements, condensers, and distillate collectors. The evaporation process can be single-stage or multi-stage.

Up to 90% of fresh water is obtained from the primary steam in one stage. In multi-stage desalination units, where unboiled water flows from one chamber to another up to 50-60 times, the water output increases 15-20 times. However, such systems are much more complex to operate due to the significant concentration of brine in the final stages and equipment damage caused by salt deposits on pipelines.

Technologies actively used in leading desalination countries

Israel is considered a leader in this industry, where the largest desalination plants are located, providing more than 15% of drinking water needs and more than 50% of technical water needs. One of the largest local plants draws water from the Mediterranean Sea and filters it through special membranes. Then distillation is carried out, after which clean water is sent to reservoirs, and the brine is discharged into the sea.

French plants use somewhat different desalination methods: most installations operate on the principle of reverse osmosis. The evaporation technology is also popular on an industrial scale.