Reverse osmosis is a new membrane separation technology developed in the 1960s, which relies on reverse osmosis membranes to separate solvents and solutes in a solution under pressure. To understand the principle of reverse osmosis desalination, one must first understand the concept of "permeation". Penetration is a physical phenomenon. When two types of water containing different concentrations of salts are separated by a semi permeable membrane, it is found that the water on the side with less salt content will permeate through the membrane to the water with higher salt content, while the salt content does not permeate. In this way, the salt concentrations on both sides gradually merge until they are equal. 01 Principle of reverse osmosis desalination
However, it takes a long time to complete this process, which is also known as natural infiltration. But if a pressure is applied on the water side with high salt content, the result can also stop the above permeation, and this pressure is called osmotic pressure.
If the pressure is further increased, it can cause water to permeate in the opposite direction, leaving behind salt. Therefore, the principle of reverse osmosis desalination is to apply a pressure greater than the natural osmotic pressure in water with salt content (such as raw water), so that the permeation proceeds in the opposite direction, pressing the water molecules in the raw water to the other side of the membrane and turning them into clean water, thereby achieving the goal of removing salt from water. This is the principle of reverse osmosis desalination. 02 Classification of reverse osmosis membranes
At present, reverse osmosis membranes are mainly divided into two categories based on the chemical composition of their membrane materials: cellulose membranes and non cellulose membranes. According to the physical structure of membrane materials, they can be roughly divided into asymmetric membranes and composite membranes. Acetate cellulose membrane is widely used in cellulose membranes. The total thickness of the membrane is about 100 μ m, and the thickness of the entire skin layer is about 0.25 μ m. The skin layer is filled with micropores with a pore size of about 5-10 angstroms, which can filter out extremely fine particles. The pore size in the porous support layer is very large, about several thousand angstroms, so this asymmetric structure of the membrane is also called an asymmetric membrane. In reverse osmosis operation, the cellulose acetate membrane can only achieve the expected desalination effect when the skin layer comes into contact with high-pressure raw water, and must not be inverted. Non cellulose membranes are mainly composed of aromatic polyamides, while others include polyamide membranes, polycarbonate membranes, polysulfone membranes, polytetrafluoroethylene graft membranes, polyethyleneimine membranes, and so on. The polyamide composite film developed in recent years is supported by a layer of polyester non-woven fabric. Due to the irregularity and looseness of polyester non-woven fabric, it is not suitable as the bottom layer of salt barrier layer. Therefore, microporous engineering plastic polysulfone is cast on the surface of non-woven fabric. The pores on the surface of the maple layer are controlled at approximately 150 angstroms. The barrier layer is made of highly cross-linked aromatic polyamide with a thickness of approximately 2000 angstroms. Highly cross-linked aromatic polyamide is polymerized from benzoyl chloride and phenylenediamine. Due to the fact that this type of film is composed of three layers of different materials, it is called a composite film.
