How desalination plants works

01The problem of salt water

Seawater contains roughly 35,000 parts per million of dissolved salts — about 3.5 percent by weight. The human body needs water with under 1,000 ppm to function normally. The gap between what the ocean freely offers and what we can actually drink is the entire problem desalination exists to solve.

The salts are not floating chunks. They are dissociated ions — sodium, chloride, magnesium, calcium — small enough to pass through ordinary filters. Removing them means either boiling the water away from the salt or forcing it through a barrier so fine that water molecules pass but ions do not. Both routes work. Both cost energy.

02Thermal desalination: boiling it clean

The oldest method is distillation. Heat seawater until it vaporizes, leaving the salts behind, then condense the vapor into freshwater. Multi-stage flash distillation (MSF) and multi-effect distillation (MED) are the industrial-scale versions of this idea, using staged pressure drops so that water boils at progressively lower temperatures across multiple chambers.

Thermal plants are typically built alongside power stations to use waste heat, which improves their economics. They are robust, tolerant of dirty intake water, and produce very pure output. But they are energy-hungry: 10 to 12 kWh per cubic meter of freshwater, several times what membrane methods need. Most large thermal plants are in the Middle East, where fuel is cheap and water is scarce.

03Reverse osmosis: filtering at the molecular level

Reverse osmosis (RO) is the dominant technology today. Instead of boiling, RO forces seawater under high pressure through a semi-permeable membrane. The membrane pores are on the order of 0.1 nanometers — small enough to block salt ions while letting water molecules through.

Osmosis naturally moves water from a dilute solution to a concentrated one. Reverse osmosis overrides this by applying pressure greater than the osmotic pressure of seawater, about 27 bar. In practice, plants operate at 55 to 80 bar to maintain throughput. The result is a stream of freshwater on one side and a concentrated brine on the other.

RO uses roughly 3 to 5 kWh per cubic meter — a fraction of thermal methods. This is why it has come to dominate new capacity worldwide. But it demands clean pre-treated feed water, because membranes foul easily, and replacement is expensive.

04Pre-treatment and post-treatment

Before water reaches the membranes, it must be cleaned of suspended solids, organic matter, and biological contaminants. This means screening, coagulation, sand filtration, and often cartridge filters. The goal is to protect membranes from fouling — the gradual clogging that reduces output and shortens membrane life.

After desalination, the water is almost too pure. It lacks the minerals that give drinking water its taste and stability, and it can be corrosive to pipes. Plants re-mineralize the output with calcium and magnesium, adjust the pH, and sometimes add fluoride. The product water is not just salt-free; it is engineered to be drinkable and pipe-safe.

05Energy: the dominant cost

Energy is 30 to 60 percent of the operating cost of a desalination plant. This is the central constraint on where desalination makes economic sense. A plant in Saudi Arabia burning cheap gas faces a very different cost curve than one in California buying grid electricity at peak rates.

Renewable energy is changing the picture. Solar-powered desalination plants in Australia and the Middle East pair PV arrays with RO systems to cut both cost and carbon. But the intermittency of renewables is a poor match for membranes that prefer steady pressure, and the economics still depend on location.

06Brine: the unavoidable byproduct

For every liter of freshwater a plant produces, it discharges roughly 1.5 liters of concentrated brine. This brine is saltier than the intake seawater, often by a factor of two, and it carries the chemicals used in pre-treatment.

Discharging brine back into the ocean is not free. If it is too concentrated or poorly dispersed, it sinks to the seafloor and smothers benthic organisms. Modern plants use diffusers to mix the brine rapidly with surrounding seawater, but the environmental impact remains a concern, especially in enclosed gulfs where many plants operate.