Seawater is the world’s most abundant water source \u2014 96.5% of Earth’s water is in the oceans. The catch is the salt. Seawater contains roughly 35,000 ppm of dissolved salts (primarily sodium chloride), compared to the WHO’s guideline of 500 ppm for safe drinking water. Seawater desalination is the process that bridges this gap, and understanding how it works is increasingly relevant as freshwater scarcity intensifies globally.<\/p>\n
Before understanding how desalination works, it helps to understand why it’s difficult. Water naturally moves through semi-permeable membranes from areas of low salt concentration to high salt concentration (osmosis). To reverse this \u2014 to push water from saltwater to freshwater \u2014 you have to apply pressure greater than the osmotic pressure of seawater.<\/p>\n
Seawater’s osmotic pressure is approximately 27 bars (400 PSI). Seawater reverse osmosis (SWRO) systems typically operate at 55\u201385 bars (800\u20131,200 PSI) to overcome this osmotic pressure and drive water through the membrane at practical flow rates. This high pressure is why seawater desalination is more energy-intensive than treating brackish groundwater (which operates at 10\u201340 bars).<\/p>\n
Raw seawater enters through intake structures \u2014 either open ocean intakes or subsurface\/beach well intakes. Open intakes draw from the surface or mid-depth and require more aggressive screening. Subsurface intakes (horizontal collector wells, beach galleries) use the seabed itself as a natural filter, drawing cleaner water with lower turbidity and biological load.<\/p>\n
Initial screening removes large debris (fish, seaweed, plastics) through bar screens and drum screens. This protects downstream equipment and membranes from physical damage.<\/p>\n
Pre-treatment is arguably the most critical stage for membrane protection and long-term system performance. Seawater contains suspended solids, organic matter, algae, bacteria, and scaling minerals that will foul or destroy RO membranes without proper pre-treatment. Options include:<\/p>\n
In favorable coastal conditions (low turbidity, low biological activity), ultrafiltration (UF) membranes are increasingly replacing conventional pre-treatment. UF produces more consistent pre-treated water quality and is less sensitive to seasonal source water variation.<\/p>\n
Pre-treated seawater is pressurized to 55\u201385 bar by high-pressure pumps. Energy consumption at this stage is the largest single operating cost in SWRO \u2014 typically 50\u201360% of total power used. High-pressure pump selection (centrifugal pumps with precision engineering for this pressure range) significantly affects long-term reliability and efficiency.<\/p>\n
Pressurized seawater passes along the surface of spiral-wound polyamide thin-film composite (TFC) membranes arranged in pressure vessels. Water molecules are forced through the membrane; dissolved ions and molecular contaminants are rejected. The membrane achieves 99\u201399.8% salt rejection in seawater applications.<\/p>\n
The output is two streams:<\/p>\n
Recovery rates in SWRO typically run 35\u201345% \u2014 meaning 35\u201345% of the intake water becomes permeate, and 55\u201365% becomes concentrate. This is physically limited by the osmotic pressure of the concentrate stream: as water is removed and concentrate becomes more saline, the osmotic pressure increases until further recovery becomes uneconomical.<\/p>\n
The high-pressure concentrate still carries significant hydraulic energy. Modern SWRO systems capture this energy using pressure exchangers (isobaric energy recovery devices) that transfer pressure from the concentrate stream to incoming feed water. Energy Recovery Inc.’s PX Pressure Exchanger operates at 94\u201398% efficiency and can reduce overall plant energy consumption by up to 60% compared to early-generation SWRO plants.<\/p>\n
Energy recovery devices are now standard in commercial SWRO plants. Without them, specific energy consumption runs 4\u20136 kWh\/m\u00b3 of permeate. With modern ERDs, this drops to 2\u20133 kWh\/m\u00b3.<\/p>\n
Permeate water from SWRO membranes is purified but not yet potable. Post-treatment adjusts it for distribution:<\/p>\n
Responsible brine disposal is one of the most significant environmental considerations in seawater desalination. High-salinity discharge can damage marine ecosystems if concentrated brine settles on the seabed near sensitive habitats. Mitigation approaches include:<\/p>\n
Today, roughly 22,000 seawater desalination plants operate globally, producing over 100 million cubic meters per day of fresh water. The largest plants \u2014 Israel’s Sorek facility, Saudi Arabia’s NEOM plants \u2014 produce hundreds of millions of gallons per day. Compact SWRO systems for marine vessels and remote communities produce as little as 200\u2013500 gallons per day.<\/p>\n