A desalination plant gets the salt out of seawater one of two ways. It boils the water and catches the vapour, or it forces the water through a membrane fine enough to hold the salt back while the water passes through. Either method takes seawater as salty as 45 grams per litre and turns it into water fit to drink, at about half a gram per litre.
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Which method a plant uses mostly comes down to how salty its water runs, and how cheap its energy is.
Boiling still wins where oil is cheap and water is very salty
Thermal desalination is the brute-force method: heat the seawater until it turns to vapour, then cool that vapour back into liquid, leaving the salt behind. Its advantage is that the energy it needs stays the same however salty the water is. So it handles the Persian Gulf’s unusually salty water, at 45 grams per litre, as easily as it handles ordinary seawater. The cost is steep: evaporating and pumping the water can burn through as much as 80 megawatt-hours for every megalitre produced.
That cost is why the method survives mainly in the Middle East, where plants run on waste heat from neighbouring power stations and on oil cheap enough to burn rather than buy as electricity. In the United States, where electricity runs about $0.065 a kilowatt-hour, the same process would cost roughly $5,200 per megalitre, just over half a cent a litre. That price suits drinking water and bathing. It does not suit a farmer irrigating a field or a factory running a process line.
Saudi Arabia’s Shoaiba plant on the Red Sea, which pairs power generation with desalination, produces 880 megalitres a day, the largest output of any thermal-desalination plant on Earth. Even so, the market-research firm Lux forecast that the technology would not spread much past the Middle East, because that is the one place where cheap oil and very salty water happen to coincide.
Reverse osmosis fights osmosis rather than copying it
Left alone, a membrane between fresh water and salt water lets water molecules pass while holding salt ions back, and osmosis pulls the fresh water across to dilute the salty side. A reverse-osmosis plant pumps the salt water hard enough to force water molecules back the other way instead, leaving the salt trapped against the membrane.
The saving is real. Reverse osmosis has a comparatively low energy cost. But the pressure a plant needs rises with the water’s salinity, so the saving shrinks as the feedwater gets saltier. Brackish groundwater, which carries only three to five grams of salt per litre, needs at least 3.5 megawatt-hours per megalitre. Seawater, being saltier, needs more. That trade-off makes brackish groundwater its best match, though plants increasingly use it on seawater too. Outside the Persian Gulf, reverse osmosis is now the usual method.
The membranes have limits. They cannot fully filter out boron, arsenic, lithium, or some pharmaceutical compounds, all of which can pass through alongside the water. They also clog: a fouled membrane has to be scraped and treated with bleach or it stops filtering properly, and each cleaning shortens how long it lasts. Plants pretreat the incoming water to slow that fouling, a step that takes a lot of land to run. IEEE Spectrum reported that a plant at Carlsbad, California, was due to open in 2012, drawing more than 189 megalitres a day from the Pacific. That would make it the biggest reverse-osmosis plant in the country.
The salt has to go somewhere
Both methods leave behind the same unwanted byproduct: brine, seawater with the fresh water taken out and the salt concentrated into what remains. Around 16,000 desalination plants now operate worldwide. Together they discharge roughly 5 billion cubic feet of brine a day, against 3 billion cubic feet of fresh water produced, according to a 2019 study from the United Nations Institute for Water, Environment and Health. That brine volume runs about 50% higher than earlier estimates had it.
Pumped straight back into the sea, that brine can kill fish and the smaller creatures the food chain depends on. Near outfalls, fish accumulate toxic chemicals, and creatures living on the seabed cope with less oxygen and more salt than they would find elsewhere. Manzoor Qadir, who wrote the UN study, argued that the brine is a resource rather than only a hazard. Some of it already irrigates “forage shrubs and crops,” he said, and feeds ponds that grow the dietary supplement spirulina.
Engineers at MIT have tested a more direct fix. John Lienhard, a professor of mechanical engineering there, said it is “much better to recover resources from the brine” than simply release it. His team ran an electric current through brine to split water molecules apart, producing sodium hydroxide and hydrochloric acid, two chemicals a desalination plant already buys to treat its incoming seawater and clean its filters. Making them on site, said co-author Amit Kumar, “could be a big advantage” for a plant that currently pays for both. Kumar put the underlying problem plainly: brine is “a resource waste,” and pumping it back out to sea costs energy on top of the waste itself.
The obstacle so far is money. The MIT engineers named the cost of electricity and equipment as the challenge. MIT’s team was still working out whether that cost pencils out at the scale a real plant would need. The industry is not waiting on the answer. The analyst firm Technavio forecast in 2018 that global desalination capacity would keep growing by close to 9% a year through at least 2022, brine problem or not.
