Coagulation sedimentation method is the most commonly used chemical method for treating wastewater, and is widely used in many fields. The choice of coagulant directly determines the quality of coagulation, thus affecting the effect of water treatment. At present, the most commonly used coagulants are mainly inorganic coagulants such as aluminum salts and iron salts.

There are many types of coagulants, which can be roughly divided into three categories according to the action  mechanism of the coagulant: flocculants, coagulants and coagulant aids. According to the chemical properties of the coagulant, it can be divided into inorganic coagulants, organic coagulants and microbial coagulants. Currently, the most widely used are polymer coagulants, including polyaluminum chloride, polyferric sulfate, polyaluminum ferric sulfate, etc. Polymer coagulants have larger molecular weights, less dosage, and stronger electrical neutralization ability than traditional inorganic coagulants. Its multi-nuclear structure gives it a significant adsorption effect. Therefore, the research on polymer inorganic coagulants has always been a key topic in water treatment.

There are four main action mechanisms for coagulation: ① Double electric layer compression. Adding salt electrolytes to wastewater compresses the double layers, which reduces the electrostatic repulsion between molecules, shortens the distance between two colloids, and increases the attraction. When the amount of added chemical reaches a certain value, the kinetic energy of the particles can exceed the electrostatic repulsion, so that the ions will condense and settle when they collide. ② Chemical bridging. Chemical bridging refers to the formation of chemical bridges of colloid-polymer-colloid when the particles in the coagulant collide with the colloid particles through mutual bridging, thus forming floccules. ③Adsorption-electroneutralization. Adsorption-electroneutralization is the adsorption of the surface charge of the colloid particles on the asymmetric particles to destabilize them, thereby causing flocculation. ④ Net capture or sweeping type. When metal oxides or metal salts are used as flocculants, as the amount added increases, precipitates are formed. These precipitates capture and sweep pollutants in the water to achieve coagulation and sedimentation. In practical applications, these four mechanisms are generally used at the same time, but the mechanisms used for different water qualities are divided into primary and secondary ones.

Coagulants have been used to treat wastewater for a long time and have become an important part of industrial wastewater treatment. Coagulants are most commonly used to remove solid and colloidal particles in wastewater, reduce wastewater chromaticity and other indicators, and also have a certain elimination effect on heavy metal ions and microorganisms. Coagulants can form a water quality pretreatment system by themselves, or they can be combined with other treatment systems to play the role of removing toxic and harmful substances in water quality, making the greatest contribution to improving water quality.