Desalination: Turning Seawater Into Drinking Water at Scale

A Planet of Water With Little to Drink

Roughly 97.5% of Earth's water is saline, locked in oceans and brackish aquifers that no human can safely consume. Of the remaining 2.5%, most sits frozen in glaciers and ice caps. That leaves less than 1% readily available as liquid freshwater. For the 2.2 billion people who still lack safely managed drinking water services, according to the World Health Organization, desalination represents one of the most promising technological paths forward.

The concept is simple: strip salt from seawater. The execution is anything but.

Thermal Distillation: Boiling the Ocean

The oldest desalination method mimics nature's own water cycle. Thermal distillation heats seawater until it evaporates, then condenses the vapor into freshwater. Salt and impurities stay behind. Several variations dominate industrial use.

Multi-Stage Flash Distillation (MSF)

MSF plants push heated seawater through a series of low-pressure chambers. At each stage, a portion "flashes" into steam. The Jebel Ali MSF plant in Dubai processes over 2.1 million cubic meters of water per day, making it one of the largest thermal desalination facilities ever built.

Multi-Effect Distillation (MED)

MED systems use the heat released during condensation in one chamber to warm the next. This cascading reuse of thermal energy makes MED roughly 20-30% more energy-efficient than MSF. Plants typically operate between 6 and 16 effects.

Thermal Method	Energy Use (kWh/m³)	Typical Capacity	Primary Region
Multi-Stage Flash (MSF)	15–25	Large-scale municipal	Middle East
Multi-Effect Distillation (MED)	8–16	Medium to large	Middle East, North Africa
Vapor Compression (VC)	7–12	Small to medium	Remote installations

Reverse Osmosis: Pressure Does the Work

Reverse osmosis (RO) has become the dominant desalination technology worldwide. It now accounts for roughly 69% of all desalinated water production globally. Instead of heating water, RO forces seawater through semi-permeable membranes at pressures between 55 and 82 bar. Water molecules pass through. Salt ions do not.

The membranes themselves are engineering marvels. Thin-film composite polyamide layers, each about 0.2 micrometers thick, sit wound in spiral configurations inside pressure vessels. A single industrial RO element can process up to 45 cubic meters of water per day.

Pre-treatment removes suspended solids, algae, and organic matter to protect membranes
High-pressure pumps push seawater through membrane arrays at 55–82 bar
Energy recovery devices capture hydraulic energy from the brine reject stream
Post-treatment adds minerals and adjusts pH for safe drinking water standards

Energy recovery changed the economics entirely. Modern pressure exchangers recapture up to 98% of the energy in the brine stream, dropping total energy consumption to 3–4 kWh per cubic meter.

Cost Comparison Across Methods

Desalination costs have fallen dramatically since the 1990s. RO costs dropped from over $2.00 per cubic meter in 1990 to roughly $0.50–$0.80 per cubic meter at modern large-scale plants. Location, energy prices, and plant scale all influence the final number.

Factor	Reverse Osmosis	Thermal Distillation
Energy consumption	3–4 kWh/m³	8–25 kWh/m³
Capital cost (large plant)	$800M–$1.5B	$1B–$2B
Water cost per m³	$0.50–$0.80	$1.00–$2.50
Membrane replacement	Every 5–7 years	Not applicable
Feedwater flexibility	Sensitive to fouling	Handles high-salinity water well

Where Desalination Operates Today

Over 21,000 desalination plants operate in more than 170 countries. Saudi Arabia leads with roughly 15% of global capacity, followed by the United Arab Emirates, Kuwait, and Israel. The Sorek B plant in Israel, commissioned in 2023, can produce 200 million cubic meters of freshwater per year, serving roughly 1.5 million people.

Saudi Arabia: 7.5 million m³/day capacity across multiple mega-plants
Israel: desalination supplies roughly 80% of household drinking water
Singapore: NEWater and desalination together provide over 65% of water demand
Australia: Perth relies on desalination for nearly half its water supply
United States: the Carlsbad plant in California produces 190,000 m³/day

The Brine Problem and Environmental Trade-Offs

For every liter of freshwater produced by seawater RO, roughly 1.5 liters of concentrated brine flows back out. Globally, desalination plants discharge an estimated 142 million cubic meters of brine per day. This hypersaline effluent can raise local salinity, reduce dissolved oxygen, and harm benthic marine organisms near outfall points.

Mitigation strategies exist but add cost. Diffuser systems spread brine over wider areas. Some plants dilute brine with power plant cooling water before discharge. Emerging research explores extracting valuable minerals—lithium, magnesium, and potassium—from brine concentrate, potentially turning a waste stream into a revenue source.

Chemical Concerns

Pre-treatment chemicals including coagulants, antiscalants, and biocides enter the discharge stream. Advanced plants increasingly adopt ultrafiltration pre-treatment to reduce chemical use. The shift matters. Coastal ecosystems near intakes and outfalls bear the consequences of design choices made on land.

Emerging Technologies Reshaping the Field

The next generation of desalination aims to break through current energy floors. Several approaches show promise in pilot and demonstration stages.

Forward osmosis uses a draw solution to pull water across a membrane, requiring less pressure
Graphene oxide membranes could theoretically filter water 10 times faster than polyamide
Solar-powered desalination integrates photovoltaic arrays directly with RO systems
Capacitive deionization uses electrical fields to remove ions, best suited for brackish water

None of these has yet reached the scale or cost-effectiveness of conventional RO. But research funding is accelerating. The global desalination market is projected to exceed $32 billion annually by 2028.

Strategic Realities for Water-Scarce Nations

Desalination is not a silver bullet. It demands significant energy, capital, and technical expertise. Nations pursuing it must weigh the cost against alternatives: water recycling, conservation, improved irrigation efficiency, and watershed management. In practice, most water-secure countries use a portfolio approach, blending desalination with demand management and conventional supply.

Yet for island nations, arid coastal cities, and regions where climate change is shrinking freshwater supplies, desalination has shifted from luxury to necessity. The technology works. The challenge now is making it affordable, sustainable, and accessible to the communities that need it most.