Desalinated water starts as seawater from the ocean, salty groundwater from underground aquifers, or in some cases, treated wastewater from cities. The source depends on geography: coastal plants pull directly from the ocean, while inland facilities tap into naturally salty underground water reserves. Globally, more than 21,000 desalination plants process these salty sources into roughly 99 million cubic meters of freshwater every day.
Seawater: The Most Recognized Source
Ocean water is what most people picture when they think of desalination, and for good reason. The Middle East alone uses about 70 percent of the world’s desalination capacity, almost entirely from seawater. Countries like Saudi Arabia, Kuwait, the United Arab Emirates, and Qatar depend heavily on turning the ocean into drinking water because they have vast coastlines but almost no freshwater rivers or lakes.
Seawater is the saltiest source desalination plants handle. It typically contains more than 35,000 milligrams per liter of dissolved solids, mostly sodium chloride along with magnesium, calcium, and dozens of other minerals. That heavy salt load means ocean desalination requires the most energy of any source type, around 3.5 to 4.5 kilowatt-hours per cubic meter of freshwater produced. For comparison, treating conventional freshwater from a river or lake uses roughly 0.2 to 0.4 kilowatt-hours per cubic meter.
Brackish Groundwater: The Inland Alternative
Not all desalinated water comes from the sea. More than 95 percent of desalination facilities in the United States are actually inland, and most of them process brackish groundwater rather than ocean water. Brackish water sits in underground aquifers and contains dissolved solids in the range of 1,000 to 10,000 milligrams per liter. That’s noticeably salty (too salty to drink or irrigate with) but far less concentrated than the open ocean.
Florida is the biggest user of brackish groundwater desalination in the U.S., home to nearly half of the country’s 314 municipal desalination facilities. California and Texas follow. These states sit on top of large brackish aquifers that are too salty for direct use but much easier and cheaper to desalinate than seawater, since the lower salt content means less energy is needed to push water through the membranes.
Brackish aquifers form over thousands of years as minerals dissolve into groundwater from surrounding rock, or as ancient seawater gets trapped underground. In coastal areas, saltwater from the ocean can also seep into freshwater aquifers, creating brackish zones that expand as freshwater is pumped out faster than it’s replenished.
Treated Wastewater: A Growing Third Source
Some advanced systems now use treated municipal wastewater as a starting source. After a city’s sewage goes through conventional treatment, the resulting water still contains too many contaminants and dissolved solids for direct reuse. Running it through desalination membranes produces high-purity water that meets drinking standards. This approach effectively recycles water that would otherwise be discharged into rivers or the ocean.
One newer design combines wastewater reuse and seawater desalination in a single system. Treated wastewater dilutes incoming seawater before it reaches the main desalination membranes, which lowers the salt concentration the plant has to work against. This reduces energy costs and membrane wear while also recycling wastewater in the process. The concept is still gaining traction, but it reflects a shift toward viewing used water as a resource rather than something to dispose of.
How Plants Actually Pull Water In
Coastal desalination plants use two main approaches to collect seawater. Open-ocean intakes are large pipes that draw water directly from the sea, sometimes from considerable depth or distance offshore. They’re the only practical option for very large plants, but they come with complications. Seaweed can clog them seasonally, jellyfish swarms can foul the pretreatment equipment, and marine organisms get pulled in, which raises environmental permitting concerns.
The alternative is subsurface intakes: wells or galleries built into the beach or seabed. These draw seawater that has already filtered through sand and sediment, which naturally removes algae, sediment, and many organic compounds before the water ever reaches the plant. Subsurface intakes consistently produce cleaner source water, reducing the amount of pretreatment needed. Their limitation is capacity. A single beach well can’t supply enough water for the largest modern plants, so some facilities use networks of wells or combine intake types.
What Happens Before Desalination Begins
Regardless of the source, raw water goes through several preparation steps before the actual salt removal. For plants using reverse osmosis (the dominant technology worldwide), the sequence typically includes adjusting the water’s acidity, adding chemicals that cause fine particles to clump together, letting those clumps settle out, filtering through layers of sand or other granular media, and finally passing through cartridge filters that catch anything remaining. Each step protects the delicate membranes that do the real work of separating salt from water.
Thermal desalination plants, more common in the Middle East, take a different approach. In multi-stage flash distillation, seawater is heated to around 110 to 120 degrees Celsius, then pumped through a series of chambers at progressively lower pressures. At each stage, some of the hot water “flashes” into steam because the pressure is too low for it to stay liquid at that temperature. The steam is collected and condensed into freshwater. Multiple-effect distillation works similarly but uses steam to directly heat the seawater across a series of evaporators. Both methods essentially mimic the natural water cycle (evaporation followed by condensation) at industrial speed.
Why the Source Matters for Your Tap Water
The original source determines the cost, energy use, and environmental footprint of the freshwater you eventually drink. Seawater desalination is the most energy-intensive option, but it’s essentially unlimited in supply for coastal regions. Brackish groundwater is cheaper to process, but aquifers can be depleted if overused. Wastewater reuse conserves both the source water and the energy, but faces public perception challenges.
Every desalination plant also produces a concentrated byproduct called brine. Globally, plants generate more than 150 million cubic meters of brine per day, which is actually more than the freshwater they produce. For ocean-sourced plants, brine is typically discharged back to the sea. For inland brackish plants, disposing of brine is more complicated and expensive, often requiring deep-well injection or evaporation ponds. The saltier the source water, the more brine the process creates per unit of freshwater.