渗透及反渗透是一种物理现象，含有盐份的水有一种自然渗透压力，当把含盐水(原水)与纯水用微孔直径为万分之一微米的半透膜隔开时，纯水由于渗透压的作用将透过半透膜而进入原水侧。相反，要是在原水侧施加一高于其本身渗透压的压力，则原水中的分子将透过半透膜而进入纯水侧，但原水中的盐份、细微杂质、有机物等成分却不能进入纯水侧，这就是反渗透。基于此种原理，人们发明了反渗透膜和反渗透技术，并将其应用于水处理。
Osmosis and reverse osmosis is a physical phenomenon. Water containing salt has a natural osmotic pressure. When the brine (raw water) is separated from pure water by a semi permeable membrane with a micropore diameter of 1 / 10000 μ m, pure water will enter the raw water side through the semi permeable membrane due to the role of osmotic pressure. On the contrary, if a pressure higher than its own osmotic pressure is applied on the raw water side, the molecules in the raw water will enter the pure water side through the semi permeable membrane, but the components such as salts, fine impurities and organic matters in the raw water cannot enter the pure water side, which is called reverse osmosis. Based on this principle, reverse osmosis membrane and reverse osmosis technology were invented and applied to water treatment.