RO stands for Reverse Osmosis in English, which means reverse osmosis in Chinese. The general flow pattern 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 so-called reverse osmosis principle: due to the pore size of the RO membrane being one millionth of a human hair (0.0001 microns), it is generally invisible to the naked eye, while bacteria and viruses are 5000 times larger. Therefore, only water molecules and some mineral ions can pass through (the ions passing through do not lose their orientation), while other impurities and heavy metals are discharged from the wastewater pipe. All seawater desalination processes, as well as astronaut wastewater recovery and treatment, use this method, hence the RO membrane is also known as the high-tech "artificial kidney" in vitro. At present, both domestic and international medical, military and civilian fields adopt top-level RO membranes for polymer filtration.

RO reverse osmosis membranes have pore sizes as small as nanometers (1 nanometer=10 * -9 meters). Under certain pressure, water molecules can pass through the RO membrane, while impurities such as inorganic salts, heavy metal ions, organic matter, colloids, bacteria, viruses, etc. in the source water cannot pass through the RO membrane. This strictly distinguishes between permeable pure water and impermeable concentrated water.