RO membrane is the core semi-permeable filter medium of reverse osmosis water treatment systems, which can block over 98% of dissolved salts, heavy metals, bacteria and tiny organic pollutants in water under external hydraulic pressure. It breaks the natural osmosis law to separate pure water from mixed raw water. Proper operating pressure, regular flushing and standardized maintenance can effectively prevent membrane fouling and keep stable filtration efficiency for a long time. Among all common water filter materials, RO membrane has the highest precision for removing soluble inorganic impurities, making it irreplaceable in drinking water treatment and industrial water preparation.
To understand how RO membrane works, it is necessary to distinguish natural osmosis and reverse osmosis first. Natural osmosis is a spontaneous physical movement: water molecules flow from liquid with low solute concentration to liquid with high solute concentration through semi-permeable membrane until the concentration balance on both sides. This process requires no external force and will generate osmotic pressure.
Reverse osmosis is the opposite process. When external pressure higher than the inherent osmotic pressure of raw water is applied on one side of the RO membrane, water molecules will penetrate the membrane surface layer, while almost all solute particles are intercepted on the feed water side. The pure water passing through the membrane is called permeate water, and the concentrated water carrying all intercepted impurities is called concentrate water.
The mainstream roll-type RO membrane adopts a multi-layer composite structure. The surface thin separation layer determines the filtration precision, the middle support layer provides mechanical strength, and the bottom backing layer prevents structural deformation under water pressure. The surface separation layer has dense nano-scale micropores, which only allow single water molecules to pass through, and effectively block ionized mineral substances, colloids and microbial cells.
| Substance Type | Pass Through Membrane | Interception Rate |
|---|---|---|
| Water Molecules | Yes | 0% |
| Dissolved Salt Ions | No | Above 98% |
| Heavy Metal Particles | No | Above 99% |
| Bacteria & Viruses | No | Above 99.9% |
In household water treatment equipment, RO membrane is the core filter element for producing direct drinking pure water. Tap water usually contains excess calcium and magnesium ions, residual chlorine, rust colloids and trace heavy metals. After passing through RO membrane filtration, the hardness of water is greatly reduced, and harmful substances exceeding the standard can be removed, improving the taste and safety of drinking water.
Many manufacturing industries require low-salinity pure water as production raw material, and RO membrane technology is the mainstream pretreatment method. If raw water contains too many mineral ions, it will cause scale accumulation on precision equipment, affect product yield and quality, and even damage production lines.
RO membrane is widely applied in seawater desalination and brackish water treatment projects in coastal and arid regions. Natural seawater has extremely high salt content and cannot be directly used for living and production. After multi-stage RO membrane filtration, the salt concentration of water can drop to a safe drinking range, effectively alleviating regional freshwater resource shortage.
The stable operation of RO membrane is restricted by multiple water quality and operating parameters. Any abnormal index will directly reduce water production efficiency and impurity interception capacity, and accelerate membrane element fouling aging.
High turbidity raw water contains a large amount of suspended colloids, which will quickly attach to the membrane surface to form dirt layer. Too high water hardness will generate carbonate scale under constant temperature and pressure, blocking membrane micropores. Excessive organic matter and microbial content will cause biological fouling, which is difficult to clean thoroughly by conventional flushing.
Insufficient pressure cannot overcome the osmotic pressure of raw water, leading to low pure water output. Excessively high pressure will cause irreversible compression deformation of the membrane structure, permanently reducing water yield. Water temperature affects the activity of water molecules: low temperature slows down molecular penetration speed, while ultra-high temperature will damage the polymer material of the separation layer and decrease interception rate.
Recovery rate refers to the proportion of pure water separated from raw water. If the recovery rate is set too high, the solute concentration on the feed side of the membrane rises sharply, forming concentrated polarization. This phenomenon will promote scale and dirt accumulation on the membrane surface, shorten the service cycle of RO membrane significantly.
Scientific regular maintenance can prolong the usable cycle of RO membrane by more than 30% and maintain stable filtration performance without frequent replacement. Daily maintenance is divided into automatic flushing, regular chemical cleaning and long-term shutdown protection.
Every time the water purification equipment starts or stops running, the system should automatically open the concentrate water channel to flush the membrane surface with high-flow raw water. This operation can take away loose suspended impurities attached to the membrane surface in time, preventing the initial formation of dirt layer. It is recommended to set automatic flushing for every 60 to 90 minutes of continuous operation.
When the water production volume drops obviously or the desalination rate decreases, chemical cleaning is required. Acid cleaning liquid dissolves inorganic scale such as calcium carbonate and metal oxides; alkaline cleaning liquid decomposes organic pollutants and biological slime. Two kinds of cleaning agents need to be used separately according to the main fouling type of the membrane.
If the water treatment system stops running for more than three days, the RO membrane cannot be soaked in ordinary raw water. Microorganisms will multiply in stagnant water and form thick biological dirt. It is necessary to inject special protective liquid into the membrane element to isolate air and inhibit bacterial reproduction, and replace the protective liquid regularly during long-term storage.
Many water treatment filter materials exist on the market, and their filtration precision and interception targets have obvious differences. RO membrane is the only medium that can effectively remove dissolved ionic impurities, which cannot be replaced by microfiltration, ultrafiltration and nanofiltration membranes.
| Membrane Type | Main Intercepted Substances | Remove Dissolved Salts |
|---|---|---|
| Microfiltration Membrane | Suspended solids, large particles | No |
| Ultrafiltration Membrane | Colloids, bacteria, macromolecular organics | No |
| Nanofiltration Membrane | Partial high-valent salt ions | Partially |
| RO Membrane | All soluble salts, heavy metals, microbes | Almost Fully |
Ultrafiltration membrane can only filter particulate pollutants and cannot reduce water hardness or salt content, so it is often used as the pretreatment filter before RO membrane. Nanofiltration membrane has limited salt removal capacity and is mostly used for partial softening of water, while RO membrane is selected for scenes requiring high-purity desalinated water.
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