It is possible, yes – and this is already happening in several countries where fresh water from rivers, lakes and dams is scarce. Today, more than 100 nations, mainly in the Middle East and North Africa, have plants that remove sodium chloride (table salt) from salt water, leaving the liquid ready for drinking. The first desalination plant appeared in 1928, on the island of Curaçao, in the Caribbean. Pioneering equipment simply evaporated the mixture in huge distillation columns to make the water drinkable. From the 1940s onwards, however, more refined methods emerged, making it possible to install mini-mills on ships that remain at sea for a long time. Among the new techniques, the most successful is the so-called reverse osmosis, which separates the liquid by means of a porous plastic that blocks out the salts.
“In most processes, about a third of the sea water becomes drinking water, while the remaining two thirds are discarded in the form of brine, a liquid with a high concentration of salts left over from the separation”, says geologist Aldo Rebouças, from University of São Paulo (USP). The disposal of this residue is one of the great dilemmas of desalination. In soil, brine inhibits plant growth. If the mixture gets into freshwater streams, it can kill salt-sensitive aquatic life. The ideal is to dump the rest back into the sea or into brackish water ponds. Brazil, even though it is one of the richest countries in terms of fresh water, also uses desalination processes to purify groundwater in the Northeast. The initiative is controversial. “Even where the water table is more saline, the water quality of artesian wells tends to naturally improve a year after drilling at the most”, says Aldo.
A porous plastic bar the salt and makes the liquid ready to drink
1. Sea water starts to become potable water when ocean liquid is pumped through the filters in the pre-treatment phase. At this stage, coarse substances are removed, such as grains of sand that can damage the plant’s equipment, as well as viruses and bacteria that are harmful to human health.
2. After the initial purification, the salt water proceeds to the step where the salt will be effectively removed from the mixture. The most modern technique to accomplish this task is called reverse osmosis. This method is based on the use of plastic membranes with micropores that block the passage of salt, leaving the water ready to drink.
3. In addition to drinking water, the desalination process generates another by-product, brine, a liquid with a very high concentration of salts. To prevent this residue from contaminating the soil or any freshwater rivers, the solution is to return it to the sea or throw it into salt lakes, where shrimp, tilapia and other sea fish can be raised.
4. Generally, the water collected after separation can already be drunk and goes on to be distributed. But in some cases, the liquid even receives a chemical treatment to reduce acidity. Another inconvenience is that desalination does not only remove salt from seawater, but also minerals such as calcium, potassium and magnesium, essential compounds to strengthen teeth and prevent cavities, for example.
SEPARATION BATTALION
In the reverse osmosis phase, the water coming from the sea is divided into hundreds of metallic cylinders, each with plastic membranes that separate the drinkable liquid from the salt. To facilitate the process, a hydraulic pump increases the pressure of the saline mixture, forcing seawater against the separating membranes.
PIONEER EVAPORATION
A different method was born in the 1920s
Distillation, the oldest method of making seawater drinkable, is based on a very simple principle: inside huge distillation columns, the salty liquid is heated until the water begins to evaporate, separating from the salt. At the top of the container, the steam is collected and then cooled down, turning it into drinkable water. One of the advantages is that heating virtually eliminates the risk of water contamination by microorganisms. The biggest problem is that the energy expenditure to heat the boilers is much higher than in processes such as reverse osmosis.
PURIFYING PORES
Inside, each cylinder is hollow, having only one pipe covered by the membranes in the central part. Sea water fills the contents of the structures and is launched against the micropores of the membrane, which only lets unsalted water pass into the pipe in the center of the cylinder. The drinkable liquid is collected and sent to distribution
At the bookshop:
An Introduction to Desalination, OK Buros, United Nations, 1987
On the Internet: