Do you know the principle of RO reverse osmosis membrane to purify water and remove salt?

Release Date:

2024-05-09

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RO reverse osmosis membrane water desalting principle is a physical separation technology based on reverse osmosis phenomenon, widely used in drinking water purification, industrial water treatment, seawater desalination and many other fields. The core of this technology is to use reverse osmosis membrane to select the difference between water molecules and dissolved salts, overcome the natural osmotic pressure by applying external pressure, and realize the directional migration of water molecules from the high salt concentration side to the low salt concentration side, so as to effectively remove salt and other impurities in water. The following details the RO reverse osmosis membrane water desalting principle and related processes.

RO reverse osmosis membrane water desalting principle

First, reverse osmosis phenomenon and osmotic pressure

In the natural state, water molecules will spontaneously permeate from the side with a low concentration of solute (that is, the low osmotic pressure side) to the side with a high concentration of solute (that is, the high osmotic pressure side), this phenomenon is called osmosis. During osmosis, the diffusion rate of water molecules through the semi-permeable membrane (which allows water molecules to pass but prevents most solute molecules) depends on the osmotic pressure difference between the two sides. The osmotic pressure is determined by the number and properties of solute particles in the solution. The higher the solute concentration, the greater the osmotic pressure.

Second, reverse osmosis principle

Reverse Osmosis (RO) is by artificially applying a pressure greater than the natural osmotic pressure, forcing water molecules from the high osmotic pressure side (high salt concentration side) to the low osmotic pressure side (low salt concentration side). In this process, water molecules pass through the reverse osmosis membrane under pressure, and the vast majority of impurities such as salt ions, organic matter, microorganisms, and heavy metal ions cannot pass through the membrane hole due to their large size or charge repulsion effect, and are effectively blocked on the side with high salt concentration.

Third, RO reverse osmosis membrane characteristics

RO reverse osmosis membrane is a key component to achieve reverse osmosis separation, with the following significant characteristics:

Microporous structure: The pore size of the reverse osmosis membrane is very small, usually around 0.0001 microns, much smaller than the diameter of water molecules (about 0.2 nanometers), but enough to prevent almost all dissolved salt ions (such as sodium ions, calcium ions, etc.) and most organic matter (such as bacteria, viruses, colloids, etc.) from passing through.

Selective permeability: The membrane material is highly selective, allowing only water molecules and some small molecules of organic matter (such as ethanol, glycerin, etc.) to pass through, and has a high retention rate for most ions, organic pollutants, microorganisms, etc.

Chemical stability: The reverse osmosis membrane needs to have good chemical stability, can work stably in a wide range of pH values, resist the erosion of chemical substances such as acid and base, oxidants, disinfectants, and ensure its long-term stable operation.

Fourth, RO reverse osmosis water purification process

Pretreatment: Raw water (such as tap water, groundwater, seawater, etc.) first goes through the pretreatment stage to remove particles, suspended matter, colloids, some organic matter and microorganisms, reduce the pollution load of reverse osmosis membrane, and protect the membrane component.

Pressurization: The pre-treated water enters the high-pressure pump and applies a pressure several times higher than the natural osmotic pressure (usually at 30-80 psi, about 2.07-5.52 bar), prompting the water molecules to reverse osmosis through the reverse osmosis membrane.

Separation: In the reverse osmosis membrane component, water molecules are driven through the membrane hole under pressure to form pure water (that is, water production), while impurities such as salt, organic matter, and microorganisms are trapped on the other side of the membrane (that is, concentrated water). At this time, the salt content of the produced water is greatly reduced to achieve the purpose of purification.

Energy recovery: In some large-scale systems, especially desalination projects, energy recovery devices are used to recover part of the pressure energy released during concentrated water discharge to reduce the energy consumption of the entire system.

Post-treatment: The produced water may also undergo post-treatment stages, such as adding minerals, adjusting pH, etc., to meet specific water quality standards or taste requirements.

Fifth, RO reverse osmosis membrane performance indicators

The main indicators to measure the performance of RO reverse osmosis membranes include:

Desalting rate: indicates the ability of the reverse osmosis membrane to remove the total dissolved solids (TDS) in water, usually above 9.jpg%, and even up to 99.9%.
Water flux: The volume of water passing through the membrane area per unit time, reflecting the permeability of the membrane. The water flux is affected by operating pressure, temperature, raw water salinity, pH value and other factors.
Membrane pollution rate: During the operation of the reverse osmosis membrane, impurities are easily adsorbed and deposited on the membrane surface and in the pores, resulting in a decrease in membrane flux. Selecting the film with good anti-pollution performance and reasonable operating conditions can delay the film pollution and prolong the service life of the film.
6. Application and Prospect

RO reverse osmosis technology because of its high efficiency, energy saving, environmental protection characteristics, has become an indispensable core technology in modern water purification, seawater desalination, waste water reuse and other fields. With the development and application of new materials and new processes, the future reverse osmosis membrane is expected to achieve higher desalination rate, lower energy consumption, stronger anti-pollution performance, and a wider range of application scenarios, and continue to provide strong support for the protection and utilization of global water resources.

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