RO is the abbreviation of reverse osmosis membrane in English, which means reverse osmosis in Chinese. Generally speaking, water flows 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: because the pore size of RO membrane is five millionths (0.000 1 micron) of hair, it is generally invisible to the naked eye. Bacteria and viruses are 5000 times as many as it. Therefore, only water molecules and some mineral ions beneficial to human body can pass through, and other impurities and heavy metals are discharged from the waste water pipe. This method is used in all seawater desalination processes and astronaut wastewater recovery and treatment, so RO membrane is also called high-tech artificial kidney in vitro.