Despite all the blue on the world's map, only 2.5 percent of our water is fresh. And less than 1 percent is fit for human consumption. 

Around one in four people already live in regions where water resources are insufficient for part of the year, or indeed all year round, according to the United Nations. At the same time, an increasing incidence of drought and growing desertification are depleting traditional water sources such as lakes and rivers, making it ever more difficult to quench the thirst of a growing world population. 

A United Nations University (UNU) study puts the number of desalination plants across the world at 16,000. While that might sound like a whole lot of potential solutions, turning our salty seas into drinking water is not without harmful environmental impacts. 

First up is the issue of energy. Desalination requires a lot of it, and the primary source is currently fossil fuels, which contributes to global warming, potentially trapping our future in a vicious circle. 

There are two main techniques to separate water from salt: Thermal desalination and reverse osmosis. In thermal desalination, water is heated up until it evaporates and condenses, which requires large amounts of energy. This is how most plants in the Arabian Peninsula work, fed by waste heat from fossil-fueled power plants.

Reverse osmosis, in turn, uses energy to push water through a filter membrane that allows water molecules, but not salt and other substances, to pass. Currently this is the prefered option, due to its much greater energy efficiency.

Researchers are now developing solar thermal desalination plants in different countries, including Spain, Israel and Colombia. 

But Guillermo Zaragoza, a senior researcher at the energy department of the Spanish Centre for Energy, Environment and Technology Research, says the variability of solar power limits a wider use, especially for reverse osmosis plants.

"Reverse osmosis is designed to operate continuously," Zaragoza said. "It would have difficulties to work with a fluctuating energy source."

Brine threatens wildlife

Another issue is the hypersalinated slush generated during the treatment process. For every liter of fresh water produced, 1.5 liters of this so-called brine, which also contains traces of chemicals, are dumped back into the sea. It then sinks to the ground, where it can kill fish eggs or damage marine wildlife, such as seagrasses. That in turn affects the whole food chain. 

"It can have cascading effects on the ecosystem," Jens Berggren, the Stockholm International Water Institute communications director, told DW, adding that the low oxygen levels of hypersalinated water can also hinder marine breeding. 

During the last decade, brine has contributed to a decrease in biodiversity and changes in the phytoplankton community, according to a paper recently published by the The Economist Group's World Ocean Initiative, which aims to foster conversation to help protect the ocean. 

The Persian Gulf in particular suffers from brine discharge, due to its semi-closed position and the inefficient, old thermal desalination plants in the region.

Spain, however, which is home to more than half of Europe's desalination plants, has not suffered the same degree of alarming impacts.

"The Mediterranean Sea is much larger and allows to disperse and dilute the brine better," Zaragoza said, but stressed the need for greater research, especially on the impact on endemic species, such as the Posidonia Oceanica seagrass.

He is particularly concerned about the accumulation of chemicals that end up back in the sea as a result of in the desalination and cleaning processes. 

Berggren too, who says some chemicals, such as antifouling products — which prevent algae and bacteria from growing — are particularily detrimental because they're designed to kill marine life.

Moving forward

The European Life Dreamer project, a pilot-scale desalination plant in Girona, Spain, is working towards more efficient desalination techniques that reduce energy use and minimize the dumping of brine and chemicals.

"To reduce the energy used per cubic meter of desalinated water, we could use waste heat from the plant itself," Carlos Bayona Gonzalez, technical expert of the project, told DW. 

"Recovered phosphorus could be used as a fertilizer for soils," he added. "The idea behind is to create a circular economy."

According to the UNU paper, giving brine a second life also opens up a wide world of opportunities and benefits. The salty slush could be used in aquaculture, to irrigate salt-tolerant species or to generate electricity.  

It could even be a source of coveted minerals and metals, as seawater is rich in potassium, magnesium and lithium, among others. 

"The traditional extraction of these materials is increasingly expensive and conflicting from an environmental and geopolitical point of view," Zaragoza said. "But they're abundant in the sea."

While it's still too expensive at the moment, in the future, with the right technology, brine could be less of a burden and more of a treasure. 

However, more sustainable desalination plants shouldn't distract us from the importance of taking care of the globe's fresh water sources, Berggren said.