Nanofiltration membrane ultrafiltration membrane

According to the accuracy of the filtration pore size and the ability to remove dissolved ions, water purification membranes are classified into four categories: microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. The main filtration methods for drinking water in daily life are only ultrafiltration, nanofiltration, and reverse osmosis.

1. Ultrafiltration membrane (UF)

Hollow fiber ultrafiltration membrane fibers: 0.5-2.0mm, inner diameter 0.3-1.4mm, with micropores covering the hollow fiber tube wall. The filtration accuracy is 0.5-0.01 microns, and the pore size can intercept substances with molecular weights ranging from several thousand to several hundred thousand. The raw water flows under pressure outside or inside the membrane, forming external pressure and internal pressure, respectively. Ultrafiltration is a dynamic filtration process in which trapped substances can be removed as the concentration decreases, without clogging the membrane surface, and can operate continuously for a long time. Ultrafiltration membrane is one of the earliest developed polymer separation membranes. It can remove impurities larger than 0.01 microns, such as bacteria, colloids, and organic matter.

2. Nanofiltration membrane (NF)

Nanofiltration concept: Nanofiltration is a membrane separation technology that falls between ultrafiltration and reverse osmosis. Its molecular weight cut-off ranges from 80 to 1000 and the pore size is 1 nanometer, hence it is called nanofiltration. Nanofiltration technology is a membrane separation technology separated from RO technology, which is a continuation and development branch of ultra-low pressure RO technology. For a long time in the past, nanofiltration membranes were known as Low Pressure Reverse Osmosis (LPRO) membranes. Nowadays, nanofiltration technology has been separated from reverse osmosis technology and has become an independent separation technology between ultrafiltration and reverse osmosis technology. It has been widely applied in many fields such as drinking water preparation, pharmaceutical water preparation, food industry, environmental protection, etc., and has become an important branch of membrane separation technology.

Principle: Nanofiltration membrane, like ultrafiltration and reverse osmosis membrane separation processes, is also a membrane separation process driven by pressure difference and is an irreversible process. The separation mechanism can be described using charge models (space charge model and fixed charge model), pore models, and recently proposed electrostatic repulsion and steric hindrance models.

Advantages: Compared with other membrane separation processes, one advantage of nanofiltration is that it can intercept small molecular weight organic matter and high valence ions that pass through the ultrafiltration membrane, as well as dialyze some of the inorganic salts and minerals intercepted by the reverse osmosis membrane - that is, it can synchronize "concentration" and desalination. It can maximize the removal of all pollutants in water while retaining the appropriate amount of mineral requirements for the human body.

3. Reverse osmosis membrane (RO)

RO stands for Reverse Osmosis Membrane in English, which means reverse osmosis in Chinese. Generally, the flow of water is from low concentration to high concentration. Once water is pressurized, it will flow from high concentration to low concentration, which is the principle of reverse osmosis. Due to the pore size of RO membrane being five millionths of a human hair (0.0001 micrometers), it is generally invisible to the naked eye. Bacteria and viruses are 5000 times larger than it. Therefore, only water molecules can pass through, and any other impurities, ions, and heavy metals are discharged from the wastewater pipe, resulting in purified water. All seawater desalination processes and astronaut wastewater recycling treatment use this method.