New Delhi: The fast-rising number of desalination plants worldwide - now almost 16,000, with capacity concentrated in the Middle East and North Africa - quench a growing thirst for freshwater but create a salty dilemma as well - how to deal with all the chemical-laden leftover brine.
In a UN-backed study made public on Monday, experts estimate the freshwater output capacity of desalination plants at 95 million cubic meters per day - comparable to 40 per cent of the high season flow over Niagara Falls.
For every litre of freshwater output, however, desalination plants produce on average 1.5 litres of brine (though values vary dramatically, depending on the feedwater salinity and desalination technology used and local conditions).
Globally, plants now discharge 142 million cubic meters of hypersaline brine every day, a 50 per cent increase on previous assessments.
That's enough in a year (51.8 billion cubic meters) to cover Florida under 30.5 cm (one foot) of brine.
The authors, from UN University's Canadian-based Institute for Water, Environment and Health, Wageningen University in The Netherlands and the Gwangju Institute of Science and Technology in Korea analysed a newly-updated dataset - the most complete ever compiled - to revise the world's badly outdated statistics on desalination plants.
And they call 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.
Eight countries - the Maldives, Singapore, Qatar, Malta, Antigua and Barbuda, Kuwait, The Bahamas and Bahrain - can meet all of their water needs through desalination.
Six others can meet over 50 per cent of their water withdrawals through desalination: Equatorial Guinea, UAE, Seychelles, Cape Verde, Oman and Barbados.
The paper found that 55 per cent of global brine is produced in just four countries: Saudi Arabia (22 per cent), UAE (20.2 per cent), Kuwait (6.6 per cent) and Qatar (5.8 per cent).
Middle Eastern plants, which largely operate using seawater and thermal desalination technologies, typically produce four times as much brine per cubic meter of clean water as plants where river water membrane processes dominate, such as in the US.
The paper says brine disposal methods are largely dictated by geography but traditionally include direct discharge into oceans, surface water or sewers, deep well injection and brine evaporation ponds.
Desalination plants near the ocean (almost 80 per cent of brine is produced within 10 km of a coastline) most often discharge untreated waste brine directly back into the marine environment.
The authors cite major risks to ocean life and marine ecosystems posed by brine greatly raising the salinity of the receiving seawater, and by polluting the oceans with toxic chemicals used as anti-scalants and anti-foulants in the desalination process. Copper and chlorine are of major concern.
"Brine underflows deplete dissolved oxygen in the receiving waters," says lead author Edward Jones, who worked at UNU-INWEH and is now at Wageningen University, The Netherlands.
"High salinity and reduced dissolved oxygen levels can have profound impacts on benthic organisms, which can translate into ecological effects observable throughout the food chain."
The paper also highlights 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 - including magnesium and gypsum.
With better technology, a large number of metals and salts in desalination plant effluent could be mined.
These include sodium, magnesium, calcium, potassium, bromine, boron, strontium, lithium, rubidium and uranium, all used by industry, in products, and in agriculture.