May 10, 2021

Desalination plants generate so much brine as to cover Spain | Science

Desalination plants generate so much brine as to cover Spain | Science


A report from the United Nations University (UNU) has estimated the number of desalination plants on the planet. The work shows the amount of drinking water they generate and that relieves the thirst of millions of people. But it also reveals the millions of cubic meters (m3) of hypersaline waters (brine) that they produce in the process. Although most plants are found in the West, only four countries of the Persian Gulf concentrate more than half of the brine, which they throw directly into the sea.

The study starts from a reality: 40% of the population suffers water shortage. The problem will be aggravated in the future by two growing trends. On the one hand, there is the expected increase in population and, on the other, anthropogenic factors such as pollution and climate change will further reduce the availability of water from conventional sources. That is why we will have to look for it where there is more: in the sea.

The review of the UNU scientists estimates that there are 15,906 operational desalination plants (the figure includes those that are under construction). This number supposes almost to triple the existing ones at the beginning of the century. Added, they have a theoretical capacity to generate drinking water (the real is impossible to calculate) of about 95 million cubic meters per day (m3/ day), about 34,000 million a year. 70% of the plants are in rich countries and 0.1% in the 100 poorest. Among the first, the USA stands out with 10% or Spain with 5.7% (more than half of the desalination plants in Europe). But the greatest concentration is in the Persian Gulf: Saudi Arabia, the Arab Emirates, Kuwait and Qatar account for a third of the facilities.

The work, published in Science of the Total Environment, also determines the uses that are given to desalinated water, the sources of origin or the technologies used to make it drinkable. Most of the water (62.3%) is destined for direct human consumption and another third for industrial use, since many processes require pure water or with a very low concentration of salts. Almost two thirds of the water is obtained from the sea and another quarter of indoor brackish water. The dominant technology today is reverse osmosis, in which water is separated from salts by the use of membranes. Almost 70% of the water is obtained in this way. The other main technologies, with a quarter of the water produced, are thermal, which use evaporation and condensation processes.

Saudi Arabia, Kuwait, the United Arab Emirates and Qatar produce 55% of the brine but only one third of the desalinated water

The biggest problem of the desalination plants is, according to this new work, the brine. Until now it was accepted that, in global terms, the conversion ratio was close to 0.50. That is, to produce one liter of drinking water, another brine was generated. The logic is simple: if the average salt concentration in seawater is 30/35 grams per liter and you take all the salt out of it, you have another salt with 60/70 grams of salt.

However, the reality that reveals this study is very different. Every day, in the world, desalination plants generate 141.5 million m3 brine. It is 50% more than what was believed. At year they are about 51,700 million m3, enough to cover the extension of Spain with a thin layer of hypersaline water. So to produce a liter of drinking water, you have to generate 1.5 liters of brine.

The geographical distribution of responsibility is very unequal: 55% of all this brine is produced in Saudi Arabia, the Arab Emirates, Kuwait and Qatar. However, the plants of these ancient desert countries only produce a third of the water. It is true that the oil states need desalinated water to cover up to 100% of their needs, but this work also reveals its enormous inefficiency to achieve it.

"These countries need to review the strategies for managing the brine," the assistant director of the Institute for Water, Environment and Health of UNU and co-author of the study, Manzoor Qadir. In particular, he believes it is urgent to devote greater efforts in research to achieve, on the one hand, "reduce the volume of brine generated, for example, increasing the efficiency of the desalination process" and, on the other hand, "treat and / or use the brine in a way that is economically viable and respectful of environmental fear, "he adds.

The margin for improvement is enormous. The Gulf countries were the first to install desalination plants more than half a century ago. Supported by cheap oil, they opted for thermal technologies, which use energy to evaporate and condense water. But today they have been overtaken by other systems. According to experts of the sector, to obtain a cubic meter of desalinated water they need 10 times more energy than a desalination plant by reverse osmosis in Spain. Although the origin of the water also intervenes in the conversion ratio (the marinas have a higher concentration of salts than the brackish or river ones) and not only the technology, the plants in the Middle East have conversion ratios that rarely exceed 0.25, half its equivalent in Europe and up to a tenth of that of some of the plants for industrial use.

Although the study does not enter into the environmental impact of so much brine, Qadir recalls how different studies have indicated that "the increase in salinity and temperature due to the spillage of the brine can cause a decrease in dissolved oxygen content, which is known as hypoxia. " The big problem is that, while indoor desalination plants have to comply with a very demanding regulation to reduce this brine, in which they are nourished by sea water, the only viable option for now is to return the water to the sea with the extra salt with the risk involved for the environment.

"In Spain the most affected species are the Posidonia Oceanica, the Cymodea nodosa wave Zostera noltii, which are aquatic grasslands, some of them endemic to the Mediterranean and protected, "says the researcher of the Institute of Environmental Hydraulics from the University of Cantabria, Íñigo Losada, not related to the study. "There are also algae or mollusks that can be affected, but in general, although they can also affect fish, they usually leave the area of ​​the landfill when conditions are not right," he adds. It remains to be seen whether, although these effects are local, the large amount of desalination and brine they produce will eventually become a global problem.

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