The fast-rising number of desalination plants globally, now almost 16 000, quench a growing thirst for fresh water but create a “salty dilemma” on how to deal with all the chemical-laden left-over brine, authors of a recently published United Nations- (UN-) backed paper say.

In the paper, the authors estimate the fresh water output capacity of desalination plants globally at about 95-million cubic metres a day.

They explain that for every litre of fresh water output, desalination plants produce, on average, 1.5 litres of brine. These values can vary, they add, noting that it is dependent on the feedwater salinity and the desalination technology used, as well as local conditions.

In comparison, the experts highlight that plants now discharge about 142-million cubic metres a day of hypersaline brine – a 50% increase on previous assessments.

The authors – from the UN University's Canada-based Institute for Water, Environment and Health, the Wageningen University, in the Netherlands, and the Gwangju Institute of Science and Technology, in the Republic of Korea – analysed a newly-updated dataset to revise the world’s outdated statistics on desalination plants.

The authors called for improved brine management strategies to meet a fast-growing challenge, noting predictions of a dramatic rise in the number of desalination plants and, hence, the volume of brine produced worldwide.

The paper, titled ‘The state of desalination and brine production: A global outlook’, found that 55% of global brine is produced in just four countries. Of this 55%, 22% is produced in Saudi Arabia, 20.2% in the United Arab Emirates (UAE), 6.6% in Kuwait and 5.8% in Qatar.

Middle Eastern plants, which largely operate using seawater and thermal desalination technologies, typically produce four times as much brine per cubic metre of clean water, compared with plants where river water membrane processes dominate, such as in the US.

The paper says brine disposal methods are largely dictated by geography, but that the methods traditionally include direct discharge into oceans, surface water or sewers, deep well injection and brine evaporation ponds.

Desalination plants near the ocean most often discharge untreated waste brine directly back into the marine environment, the authors lament, citing major risks to ocean life and marine ecosystems as a result of increasing the seawater’s salinity and polluting the ocean with toxic chemicals, such as antiscalants and antifoulants, that are used in the desalination process.

“Brine underflows deplete dissolved oxygen in the receiving waters,” says lead author Edward Jones.

He adds that high salinity and reduced dissolved oxygen levels can have profound impacts on benthic organisms, which he says can translate into ecological effects that are observable throughout the food chain.

Meanwhile, the paper also highlights potential economic opportunities to use brine in aquaculture, to irrigate salt tolerant species, to generate electricity and by recovering the salt and metals contained in brine.

With better technology, the authors believe a large number of metals and salts in desalination plant effluent could be mined.

“There is a need to translate such research and convert an environmental problem into an economic opportunity,” says co-author Dr Manzoor Qadir, adding that this is particularly important in countries producing large volumes of brine with relatively low efficiencies, such as Saudi Arabi, the UAE, Kuwait and Qatar.

Qadir explains that, using saline drainage water offers potential commercial, social and environmental gains.

Reject brine, he adds, has been used for aquaculture, with increases of 100% in fish biomass of 300% achieved.

“It has also been successfully used to cultivate the dietary supplement Spiruline and to irrigate forage shrubs and crops, although the latter use can cause progressive land salination”.

Co-author Dr Vladimir Smakhtin further points out that, with between 1.5-billion and two-billion people currently living in areas of physical water scarcity, there is “an urgent need to make desalination technologies more affordable and extend them to low-income and lower-middle income countries.

“At the same time, though, we have to address potentially severe downsides of desalination, such as the harm of brine and chemical pollution to the marine environment and human health,” he warns.

Not all is lost, though, as the good news is that efforts have been made in recent years and, with continuing technology refinement and improving economic affordability, “we see a positive and promising outlook”, Smakhtin concludes.