The Salt Remainder
The world now turns seawater into drinking water at industrial scale. What gets far less attention is what comes out the other pipe: a hot, hyper-salty brine that outweighs the fresh water it leaves behind.
About 15,906 desalination plants make around 95 million m³ of fresh water a day. For every litre they deliver, they leave behind roughly 1.5 litres of brine, most of it piped back into the sea.
Jones et al., Science of the Total Environment, 2019 (UNU-INWEH)
How salty is "salty"?
Dissolved salt in milligrams per litre, on a log scale. Seawater-reverse-osmosis brine sits at about twice the strength of the sea it came from.
- Fresh-water limit1,000
- Slightly saline1,000–3,000
- Moderately saline3,000–10,000
- Highly saline10,000–35,000
- Open ocean~35,000
- Arabian Gulf*~42,000
- RO brine*~70,000
- Dead Sea*~340,000
Class thresholds: USGS. *Approximate reference values; RO brine derived from 35,000 mg/L feed at ~50% recovery.
From a rounding error to 97 million m³ a day
Installed global desalination capacity, million m³ per day. It roughly doubled in the decade to 2012 and kept climbing.
IDA / GWI DesalData inventories, as reported 2008–2018. Installed (online) capacity; reporting periods vary slightly.
Membranes beat boiling, by a mile
Energy needed per cubic metre of fresh seawater-derived water, kWh (thermal processes as electrical equivalent). Physics sets a floor near 1 kWh.
Pale = low end; dark segment = reported range. Floor at 50% recovery and SWRO: Elimelech & Phillip, Science 2011. Thermal: Al-Karaghouli & Kazmerski, RSER 2013.
One shallow sea takes most of it
Four Gulf states produce more than half the world's brine, discharged into a semi-enclosed sea that is already saltier than the open ocean.
Schematic, not to scale. Dots mark the four largest brine producers (relative sizes illustrative).
of global brine comes from Saudi Arabia, the UAE, Kuwait and Qatar, where thermal plants on salty seawater yield the most brine per litre.
of the world's desalinated water is produced in the Middle East and North Africa.
Jones et al., 2019
The cost of a glass of seawater
Desalination is now cheap enough to anchor national water supplies. The leftover brine raises salinity and temperature near outfalls and can carry treatment chemicals into the sea.
142 million m³ ÷ 2,500 m³ per pool. A derived comparison.
142 million m³ × 365. More than half the water in Lake Geneva, every year (derived).
Winning 2020 bid for a 200 million m³/yr plant. Roughly 0.15 agorot (a fraction of a US cent) per litre.
Lake Geneva holds about 89 km³ (approximate). Sorek 2 bid price: Globes, May 2020.
What you can do
Desalination is a lifeline for dry coasts. The goal isn't to stop it but to run it with less energy and cleaner outfalls.
Know where your tap water comes from
Utility water-quality reports list sources. If yours includes desalination, ask how the plant handles brine.
Save and reuse first
Fixing leaks and recycling wastewater both cut how much seawater has to be pushed through a membrane in the first place.
Back outfall standards
Diffusers, dilution zones and long-term seabed monitoring limit how much harm a salty plume does to nearby marine life.
Support brine as a resource
Research is under way to recover salts and minerals from brine and to power plants with renewables. It needs public demand to scale.