How Does Desalination Turn Seawater into Drinking Water? | How It Works

What is the short answer?

Seawater is full of dissolved salts that no ordinary filter can catch. Desalination takes them out, in one of two main ways. Thermal plants evaporate pure water and condense the vapour. Membrane plants use reverse osmosis (RO): pumps push seawater so hard against a thin membrane that water squeezes through while most of the salt stays behind.

Either way, there are two outputs: fresh water, treated and piped to homes, and a saltier leftover called brine, which usually goes back to the sea.

Definition: Desalination is the removal of dissolved salts from seawater or salty groundwater to make fresh water. Reverse osmosis forces water through a semi-permeable membrane that lets water pass but holds back most dissolved salts. Brine is the salt-rich water left behind.

Why can't the student simply filter seawater and drink it?

The salt in seawater is dissolved as charged particles called ions, far too small for a sand or paper filter to stop. NOAA, the United States' ocean and atmosphere agency, says two of the most common, chloride and sodium, make up around 85% of the ocean's dissolved ions.

NOAA gives the ocean's average salinity as about 35 parts per thousand: 3.5% of the weight of seawater is dissolved salts, roughly 35 grams in every kilogram.

Drinking water is often judged by total dissolved solids (TDS). Australia's National Health and Medical Research Council (NHMRC) sets no health-based value for TDS, and notes that the World Health Organization's guidelines set none either. For taste, it rates up to 600 mg/L as good and above 1,200 mg/L as unacceptable. Reaching 600 mg/L means removing roughly 98% of the salt.

How do thermal plants turn seawater into fresh water?

Thermal desalination copies the rain: heat evaporates water, the salt stays behind, and the vapour condenses as pure distillate. To reuse their heat, plants rely on one fact: water boils at a lower temperature when the pressure is lower.

• In multi-stage flash (MSF) distillation, hot seawater passes through stages, each at a lower pressure than the last. In each, part of it "flashes" into vapour, which condenses on tubes carrying incoming seawater and warms it.
• In multi-effect distillation (MED), the vapour boiled off in one vessel heats the next, cooler vessel. Only the first vessel needs outside heat.

The World Nuclear Association says cogeneration, pairing thermal plants with power stations to use their waste heat, roughly halves the energy needed for desalination. Saudi Arabia's Ras Al Khair plant combines both families; the World Nuclear Association's 2024 page gives 727,000 m³ a day from MSF and 307,000 m³ from RO, and other sources give different totals.

How does reverse osmosis push water the wrong way through a membrane?

Osmosis happens when a semi-permeable membrane separates fresh water from salty water. Water crosses towards the saltier side until the pressure building there stops it. That pressure is the osmotic pressure. An Open University course puts it at around 26 bar for seawater with 3.5% salt.

Reverse osmosis pushes harder than that, forcing water from the salty side to the fresh side and leaving the salt behind. The World Nuclear Association gives seawater RO operating pressures of 55 to 82 bar; Sydney's plant uses 60 bar.

The course explains the margin. Some pressure is lost to resistance in membranes and pipes. And as fresh water leaves, the seawater left behind grows saltier: if half is recovered, its osmotic pressure doubles to more than 50 bar. In practice, it says, about 50% recovery is the most to expect.

What happens inside a seawater reverse osmosis plant?

Large RO plants follow the same chain of steps.

1. Intake. Sydney draws seawater through four caged pipes about 300 m offshore, slowly enough, under 0.1 m per second, for marine life to swim away.
2. Pretreatment. Filters remove algae and particles that would clog the membranes. Carlsbad, in California, uses anthracite and sand, then microfiltration. Singapore's Tuas Desalination Plant uses dissolved air flotation and ultrafiltration to reduce membrane fouling.
3. High-pressure pumps raise the water to RO pressure.
4. Membranes. Carlsbad has more than 16,000, in over 2,000 pressure vessels.
5. Energy recovery devices pass pressure from the outgoing brine to the incoming seawater.
6. Post-treatment. RO water lacks minerals, so Sydney adds them back, called remineralisation, then fluoride for dental health and chlorine for disinfection.
7. Brine outfall returns the brine to the sea.

The share of incoming seawater that becomes fresh water is the recovery ratio. At Carlsbad, roughly half the water becomes drinking water. At Sydney, about 58% returns to the ocean as concentrate, so, by subtraction, a little over 40% is recovered.

How much energy does desalination use?

Physics sets a minimum energy for separating fresh water from salt water. In Physics Today in June 2015, Gregory Thiel put it at about 1 kWh per cubic metre for typical seawater with half recovered. He gave 3 to 4 kWh/m³ for new large RO plants, and a heat input equal to about 20 kWh/m³ for the best large thermal plants.

PUB, Singapore's national water agency, says its RO plants use about 3.5 kWh/m³. The World Nuclear Association gives the same figure for the latest plants, or 4 kWh/m³ including pumping to customers.

The San Diego County Water Authority says Carlsbad's 144 energy recovery devices, which use no electricity themselves, cut its RO energy use by 46%.

PUB says Singapore's Jurong Island plant, opened in April 2022, is about 5% more energy-efficient because it shares seawater intake and energy with a utilities complex. Israel's Sorek 2 has its own power station and steam-driven pumps; its builder, IDE, reports 10% lower energy use than similar plants.

What happens to the brine, and what are the debates?

Brine holds almost all the incoming salt in less water. The San Diego County Water Authority says Carlsbad's brine has roughly twice the salt of seawater and is diluted with seawater before discharge. Sydney's operator uses dispersion nozzles and says the concentrate returns to normal salinity within a short distance.

A United Nations University (UNU-INWEH) study, released on 14 January 2019, estimated 142 million m³ of brine a day worldwide: 1.5 litres for every litre of fresh water, on average. It found that 55% came from Saudi Arabia, the United Arab Emirates, Kuwait and Qatar.

The authors warned that brine spreading along the seabed lowers oxygen in the water. They called copper and chlorine, used against scale and fouling, a major concern.

• Energy and emissions. PUB calls desalination an energy-intensive water source. Sydney's operator says its plant is 100% powered by renewable energy.
• Cost. The San Diego County Water Authority says Carlsbad's water costs more than imported water but is drought-proof and locally controlled. In 2017, Heather Cooley of the Pacific Institute, a research group, wrote that stormwater capture, recycling and conservation in California are typically far less expensive than seawater desalination.

Where are large plants running today?

The UNU-INWEH study counted 15,906 operational plants in 177 countries, able to make about 95 million m³ a day, 48% of it in the Middle East and North Africa. A European Commission page updated in May 2026 counts over 22,000 operational plants as of 2024, with RO an estimated 70% of capacity.

• Saudi Arabia. Guinness World Records lists the Saudi Water Authority's Al Khobar plant as the largest RO desalination plant: 670,852.4 m³ a day, recorded on 25 February 2025.
• Israel. In March 2025, IDE reported that Sorek 2 was supplying water, with an annual output of 200 million m³.
• Singapore. Desalinated water is the fourth of PUB's "Four National Taps", beside local catchment, imported water and NEWater, recycled from used water. The first plant opened in September 2005; there are now five.
• Australia. Sydney's plant was built in response to the Millennium Drought and first supplied the city in 2010. Its operator says it has stayed in operation since 2019 and can meet up to 15% of Sydney's average needs.
• United States. Carlsbad, California, began commercial operation in late 2015 and can deliver more than 50 million gallons a day, enough for about 400,000 people.
• Spain. The same European Commission page says Spain holds about 60% of Europe's desalination capacity. Desalinated water's share of Barcelona's drinking water rose from 3% in 2021 to 33% in the 2024 drought.
• India. Chennai Metro Water says a 100 million-litre-a-day plant near Minjur was commissioned on 31 July 2010. DT Next, a Chennai newspaper, reported that a second Nemmeli plant, of 150 million litres a day, opened on 24 February 2024.

How does this work with real numbers?

Both examples are illustrative; only the 26 bar and the energy rates come from the sources.

What comes out of a plant that recovers 45% of its seawater?

Take an illustrative 100 m³ of seawater at 35 g/L, rounding NOAA's figure. It holds 100,000 L × 35 g = 3,500 kg of salt. At 45% recovery, 45 m³ becomes fresh water and 55 m³ leaves as brine.

Table: Illustrative mass balance at 45% recovery

This approximation ignores the little salt in the fresh water and changes in density. The brine is 63.6 ÷ 35 ≈ 1.8 times as salty as seawater, with 55 ÷ 45 ≈ 1.2 litres of brine per litre of fresh water. As osmotic pressure rises roughly in step with salt, a rough scaling estimate puts the brine's osmotic pressure at about 1.8 × 26 ≈ 47 bar, which the pumps must exceed.

How much electricity would a home or a city need?

Take an illustrative household of four using 135 litres each a day, 0.54 m³ in all, and an illustrative city of one million at the same rate.

Table: Illustrative daily electricity for desalinated water

That is 3.5 times what a plant at the physical minimum would need.

Where can the student go deeper in World 101?

This page follows seawater from an intake pipe to a kitchen tap. The World 101 subject Freshwater Systems covers the wider picture: where the world's fresh water comes from, how it is shared and managed, and why some places run short.

Three related subjects take these ideas further. Oceans and Marine Systems looks at the seas that supply the intake water and receive the brine. Chemical Engineering studies separation processes like the membranes, pumps and evaporators inside a plant. Energy Systems and the Power Grid explains where a plant's electricity comes from and how large, steady loads fit into a grid.

Sources

• NOAA Ocean Service, "Why is the ocean salty?", updated 23 September 2026.
• NHMRC, Australian Drinking Water Guidelines, total dissolved solids (endorsed 2011), read 9 November 2026.
• World Nuclear Association, "Nuclear Desalination", updated 2 May 2024.
• The Open University, OpenLearn Create, reverse osmosis transcript, last modified 19 June 2020.
• G. P. Thiel, "Salty solutions", Physics Today, June 2015.
• Sydney Desalination Plant, "How desalination works", "Our history" (since 2010; in operation since 2019) and home page, read 9 November 2026.
• San Diego County Water Authority, "Seawater Desalination" (operating from late 2015), read 9 November 2026.
• PUB, Singapore, "Our Water Story" and "Desalinated Water" (first plant, 2005; Jurong Island, 2022), read 9 November 2026.
• IDE Water Technologies, Sorek 2 release, 18 March 2025.
• Guinness World Records, "Largest reverse osmosis water desalination plant", 25 February 2025.
• UNU-INWEH, news release on the global brine study, 14 January 2019 (EurekAlert copy).
• European Commission, EU Blue Economy Observatory, "Desalination", updated 21 May 2026 (2024 count; Barcelona, 2021 and 2024).
• H. Cooley, Pacific Institute, "Why go for desal when California has cheaper options?", June 2017.
• Chennai Metro Water, "Water supply system" (Minjur, 31 July 2010), read 9 November 2026; DT Next, Nemmeli opening, 24 February 2024.

Common errors and misconceptions

• Misconception: A fine enough filter can make seawater drinkable. Correct: Dissolved ions pass through ordinary filters; evaporation or a high-pressure membrane must separate them.
• Misconception: Reverse osmosis lets osmosis pull water through the membrane. Correct: It works against osmosis, with pumps pushing above the osmotic pressure, at 55 to 82 bar for seawater.
• Misconception: A plant turns all its seawater into drinking water. Correct: Only the recovery ratio's share does, roughly half at Carlsbad; the rest leaves as brine.
• Misconception: Water is ready to drink as soon as it leaves the membranes. Correct: It lacks minerals, so plants such as Sydney's remineralise and disinfect it.
• Misconception: Better engineering could make desalination need almost no energy. Correct: Physics sets a minimum of about 1 kWh/m³ for typical seawater at half recovery.

Key takeaways

• Seawater holds about 35 parts per thousand of dissolved salts, and a plant must remove roughly 98% of them for good-tasting water.
• Thermal plants evaporate and condense water in MSF stages or MED vessels, often using waste heat from power stations.
• Reverse osmosis pumps seawater well above its osmotic pressure of around 26 bar, so water crosses a membrane and salt stays behind.
• An RO plant runs intake, pretreatment, high-pressure pumps, membranes, energy recovery, post-treatment and a brine outfall.
• The physical minimum is about 1 kWh per cubic metre; modern RO plants use about 3.5, helped by energy recovery devices.
• Brine's effects on the sea, energy use and cost are debated, with operators and researchers such as UNU-INWEH stating different views.

Test yourself

Why can't a filter remove salt from seawater?

The salt is dissolved as ions such as sodium and chloride, far too small for sand or paper filters to stop.

What does the recovery ratio measure?

The share of the incoming seawater that becomes fresh water; the rest leaves as brine.

Why must RO pressure be well above the osmotic pressure of seawater?

Some pressure is lost to flow resistance, and the remaining water grows saltier, raising its osmotic pressure.

In the 45% recovery example, why is the brine about 63.6 g/L?

Almost all 3,500 kg of salt stays in 55 m³ of brine, and 3,500 ÷ 55 ≈ 63.6.

What do energy recovery devices do?

They pass pressure from the outgoing brine to incoming seawater; at Carlsbad, they cut RO energy use by 46%.

How does multi-effect distillation save heat?

Vapour boiled off in one vessel heats the next, cooler vessel, so only the first needs outside heat.