Desalination and Reverse Osmosis: How Salt Water Becomes Drinking Water

Desalination is the process of removing salt from seawater or salty groundwater to make fresh water. Reverse osmosis is the most widely used way of doing it: pumps push the salty water at high pressure against a membrane that lets water through and holds the salt back.

So desalination and reverse osmosis are not two rival things. Desalination is the goal, and reverse osmosis is one method of reaching it; the other main method uses heat. This guide explains both, what they cost in energy, what happens to the leftover brine, and why the subject matters so much in Gaza.

How salty is salt water?

Salinity is measured as the amount of dissolved salt in the water, in parts per million (ppm). The US Geological Survey (USGS) classes water with less than 1,000 ppm as fresh. Ocean water contains about 35,000 ppm.

In between is brackish water: water that is too salty to drink but much less salty than the sea, often found in coastal aquifers where seawater has seeped in. The USGS calls water between 1,000 and 10,000 ppm slightly to moderately saline. The distinction matters because the less salt there is to remove, the less pressure and energy the job takes.

What is reverse osmosis?

Osmosis is something water does naturally. If fresh water and salty water are separated by a membrane that lets water through but not salt, water moves toward the salty side.

Reverse osmosis forces it to go the other way. In the US Department of Energy's description, the process uses high-energy pumps to pressurize water on one side of a membrane, so that fresh water passes through and a highly concentrated salty brine is left behind. The USGS puts it more simply: the salt molecules are too large to pass through the membrane, and the smaller water molecules are not.

Two streams come out of a reverse osmosis plant. One is the fresh water. The other is the brine, which carries away the salt.

The saltier the water going in, the more pressure is needed. The Department of Energy notes a limit, too: water that is much saltier than seawater typically cannot be treated with reverse osmosis.

Reverse osmosis vs thermal desalination

The older way to desalinate is to copy the rain. Thermal desalination, or distillation, heats salt water until it turns to steam, leaving the salt behind, and then cools the steam back into fresh water. The USGS notes that the simplest version is boiling seawater in a pan and catching the steam.

The difference between the two methods comes down to energy. Turning water into steam and back takes a great deal of heat. The Department of Energy states that even the most efficient thermal desalination processes typically use more energy than alternatives like reverse osmosis.

Thermal plants still have their uses. They can handle water that is too salty for membranes, they can produce very pure water for industry, and some of them recover waste heat to cut their energy use. But for drinking water, membranes dominate; according to the Department of Energy, more than two-thirds of desalination facilities in the United States are brackish-water reverse osmosis plants.

Energy and brine: the two costs of desalination

Reverse osmosis uses less energy than distillation, but it is still energy-intensive, because the pumps must hold the water at high pressure all the time the plant is working. A desalination plant without reliable electricity or fuel does not produce water.

The second cost is brine. A 2019 study led by the United Nations University's Institute for Water, Environment and Health counted 15,906 operating desalination plants worldwide, producing about 95 million cubic meters of fresh water a day, 48% of it in the Middle East and North Africa. The same plants produced about 142 million cubic meters of brine a day. On average, that is roughly one and a half liters of brine for every liter of fresh water.

Brine has to go somewhere, usually back into the sea. The study's authors describe its disposal as costly and associated with negative environmental impacts, and call for better brine management.

Plants also need a steady supply of treatment chemicals and spare parts. Where those cannot get in, output falls, as the figures from Gaza below show.

Why Gaza depends on desalination

Gaza's natural source of water is the coastal aquifer beneath it, and that water has been undrinkable for years. In 2017, UN OCHA reported that more than 95% of the water extracted from the aquifer was unfit for human consumption, because of long-standing over-extraction and the infiltration of sewage and seawater. Chloride and nitrate levels in most areas were up to eight times the limits WHO recommends for drinking water.

That left two ways to get drinking water: remove the salt from the brackish groundwater, or remove it from the sea. OCHA noted in 2017 that over 80% of the desalinated water available in Gaza came from private plants treating groundwater. A larger seawater desalination plant, built by UNICEF with European Union funding, was inaugurated at Deir al Balah on 19 January 2017, with a first-phase capacity of 6,000 cubic meters a day.

Gaza's desalination plants in 2026

The same technology is now working under severe constraints. In its water and sanitation update of 29 May 2026, UNICEF reported that seawater desalination output had fallen from 20,000 cubic meters a day in March to 16,000, because of shortages of chemicals and spare parts. It also reported that 82% of families were water insecure and that up to 70% of people could not collect even 6 liters per person a day for drinking and cooking.

Power is the other weak point. On 25 March 2026, damage to high-voltage cables knocked the Southern Gaza Seawater Desalination Plant off the grid and onto back-up diesel generators, and UNICEF said its output dropped to less than 20% of full capacity. UNICEF reported repairing the line within days and restoring about 80% of the plant's production.

Small plants carry much of the load. In the first half of 2026, UNICEF says, 36 private desalination plants it supported supplied around 8,630 cubic meters of drinking water a day for delivery by truck.

Our Gaza water aid guide explains how much water people are getting and what water aid involves, from trucking to repairs. Unsafe water also drives illness, which adds to the pressure on Gaza's hospitals.

How this connects to giving

Fight for Palestine does not build or operate desalination plants. We raise funds for emergency aid delivered through local partner teams, including clean water for displaced families. If you want to help, the water aid guide sets out what to look for in a water appeal, and you can give through our donate page.