Reverse Osmosis
Reverse osmosis (RO) is a pressure-driven membrane process that removes dissolved salts and most other contaminants by forcing water through a semipermeable membrane against the osmotic gradient. It is the dominant technology in modern seawater desalination and high-purity industrial water.
Principle
Osmosis is the spontaneous movement of water across a semipermeable membrane from a dilute to a concentrated solution. Reverse osmosis applies hydraulic pressure exceeding the osmotic pressure of the feed to reverse this flow, so purified permeate passes through the membrane while dissolved ions, organics and pathogens are rejected into a concentrate stream.
Seawater at ~35,000 mg/L total dissolved solids has an osmotic pressure near 27 bar; practical SWRO systems operate well above this — commonly 55 to 82 bar — to achieve useful flux and recovery.
Energy and recovery
Energy is the dominant operating cost of SWRO. Modern plants recover pressure from the concentrate stream using isobaric energy-recovery devices (ERDs), cutting specific energy consumption to roughly 3–4 kWh/m³ at plant level, down from double that in early installations. Recovery ratios are typically 40–50% for seawater and higher for brackish water.
Applications
Beyond seawater desalination, RO is central to brackish-water treatment, ultrapure water for semiconductor and pharmaceutical manufacturing, and potable and industrial water reuse, where it forms the core of full advanced treatment trains alongside UF pretreatment and UV advanced oxidation.
- [1]Reverse Osmosis Desalination: Water Sources, Technology, and Today's Challenges. Water Research (Elsevier), 2009.
- [2]Desalination and Water Purification Technologies. IDA / Elsevier reference works, 2018.