Reverse osmosis (RO) is a highly effective method for removing salt from water. The process targets and eliminates dissolved inorganic solids, primarily common salt (sodium chloride). Salt, minerals, and metals are collectively measured as Total Dissolved Solids (TDS). RO systems consistently achieve high rates of purification for both large-scale municipal applications and small home units.
The Mechanism of Salt Removal
The salt removal process modifies the natural phenomenon of osmosis. Osmosis naturally moves water across a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration to achieve equilibrium.
Reverse osmosis applies external pressure to the water on the side with the higher salt concentration, reversing the natural osmotic flow. This hydrostatic pressure must overcome the water’s natural osmotic pressure. The applied force pushes water molecules through a specialized, semi-permeable membrane.
The membrane is engineered with microscopic pores, often around 0.0001 microns in size, allowing water molecules (\(\text{H}_2\text{O}\)) to pass through. Larger dissolved salt ions, such as sodium (\(\text{Na}^+\)) and chloride (\(\text{Cl}^-\)), are physically blocked. These rejected salt ions remain on the pressurized side and are flushed away in a separate concentrate stream, leaving purified water on the opposite side.
Measuring Reverse Osmosis Performance
RO system performance is measured using the salt rejection rate, which quantifies the percentage of dissolved salts removed from the feed water. A properly functioning system typically achieves a salt rejection rate between 95% and 99% for dissolved salts, including sodium chloride. This rate is calculated by comparing the Total Dissolved Solids (TDS) concentration in the incoming water to the TDS concentration in the purified water, often measured using electrical conductivity.
Although the technology is highly efficient, 100% salt removal is not realistically achievable. Several variables influence the actual performance and salt rejection rate. These factors include the operating pressure of the system, the temperature of the input water, and the initial concentration of TDS in the feed water.
Higher operating pressure generally leads to better performance, while colder water temperatures can slightly reduce efficiency. The concentration of salt in the source water directly impacts the required pressure. Regular monitoring of the treated water’s TDS level ensures correct operation and indicates when the membrane needs replacement.
Applications in Desalination
Reverse osmosis is the leading technology used worldwide for desalination, the process of converting saline water into fresh, potable water. This application is divided into two scales: large-scale industrial plants and small-scale consumer systems.
Large-Scale SWRO Plants
Large-scale operations, known as Sea Water Reverse Osmosis (SWRO) plants, are massive facilities providing municipal water to coastal cities and arid regions. SWRO plants manage extremely high salt concentrations, requiring very high operating pressures, often ranging from 600 to 1,200 pounds per square inch (psi). A salt rejection rate greater than 99.7% is necessary for these industrial systems to meet international drinking water standards. These facilities often incorporate energy recovery devices to manage the significant power requirements for pumping at high pressures.
Consumer-Grade Systems
Consumer-grade RO systems are smaller, point-of-use units typically installed under a kitchen sink. These home systems operate at much lower pressures, usually between 30 and 100 psi, as they treat tap or well water with lower initial salt concentrations than seawater. Residential units are designed for convenience and lower throughput, offering homeowners purified drinking water.