Reverse Osmosis (RO) is a widely used and highly effective water purification technology designed to remove dissolved salts, contaminants, and impurities from water. It is commonly applied in residential, commercial, and industrial water treatment systems where high-quality purified water is required. The RO process is valued for its ability to produce consistent, reliable, and safe water for drinking and process applications.
Reverse Osmosis Mechanism
The reverse osmosis mechanism works by applying external pressure to feed water, natural equilibrium forcing it to flow through a semi-permeable membrane.
This pressure overcomes the natural osmotic pressure, allowing purified water (called permeate) to pass through the membrane while rejecting dissolved salts, organic compounds, microorganisms, and other impurities. The rejected contaminants are carried away in a separate stream known as concentrate or reject water.
This process purifies the water, often reducing total dissolved solids content by 99%.
Semi-Permeable Membrane
The semi-permeable membrane is the core component of an RO system. It is designed to allow water molecules to pass through while blocking larger ions, salts, and contaminants. RO membranes are typically made from thin-film composite materials that provide high salt rejection and durability. Feed water is delivered under pressure to the membranes, where reverse osmosis takes place. Water permeates the minute pores of the membrane and is delivered as purified product water.
The performance and lifespan of the membrane are critical to the overall efficiency of the RO system.
Rejection and Recovery
Two key performance indicators of an RO system are rejection and recovery.
• Rejection refers to the percentage of dissolved salts and contaminants removed from the feed water by the membrane. High-quality RO systems typically achieve salt rejection rates of 95–99%.
• Recovery refers to the percentage of feed water converted into permeate. Higher recovery improves water efficiency but must be carefully managed to avoid membrane fouling and scaling.
RO Pre-Treatment and Post-Treatment
RO pre-treatment is critical for protecting the membrane and maintaining system performance.
Pre-treatment processes
• Multimedia filtration to remove suspended solids
• Activated carbon filtration to eliminate chlorine and organic compounds
• Water softening to reduce hardness.
• In some cases, antiscalant dosing and micron cartridge filters are also used.
Post-treatment improves the quality and usability of RO water. This may include UV disinfection to control microbial contamination, mineralization to restore essential minerals, or pH adjustment for improved taste and corrosion control.
Factors Affecting RO Membrane Performance
Several factors influence the performance and longevity of RO membranes:
• Pressure: Adequate operating pressure is essential to drive water through the membrane. Low pressure reduces permeate flow, while excessive pressure can damage the membrane. The fouling causes the membrane to become less porous, thus decreasing the amount of product water.
• Hydrolysis: Prolonged exposure to extreme pH levels or high temperatures can cause chemical degradation of the membrane material. In general, happens when the water temperature is high and the pH is below 2.5 and above 7.
• Bacteria: Microbial growth on the membrane surface can lead to biofouling, reducing performance and increasing pressure drop.
• Temperature: Higher temperatures increase water permeability but may shorten membrane life if operating limits are exceeded.
• Surface Coating or Fouling: Accumulation of suspended solids, organic matter, or scale on the membrane surface reduces efficiency and requires proper pretreatment.
Feed Water Requirements
To ensure optimal RO system performance, feed water quality must meet specific parameters. Important feed water requirements include controlled turbidity, low suspended solids, appropriate pH range, limited iron and manganese levels, and reduced chlorine content. Pretreatment processes such as multimedia filtration, activated carbon filtration, and softening are often used to protect the RO membrane and enhance system reliability.
Reverse Osmosis systems provide an advanced and dependable solution for producing high-purity water. By understanding the RO mechanism, membrane characteristics, rejection and recovery rates, performance-influencing factors, and feed water requirements, users can ensure efficient operation and long-term system performance. RO technology remains one of the most trusted methods for modern water purification needs.
Author – Gimhara Nemindhi
