The principle of water purification of "polyaluminum chloride" and the related action Compression electric double layer: The structure of the micelle electric double layer determines the maximum concentration of counter ions at the surface of the colloidal particles, the greater the outward distance from the colloid particles The lower the counterion concentration, the final ion concentration in solution is equal. When an electrolyte is added to the solution to increase the ion concentration in the solution, the thickness of the diffusion layer decreases. When the two micelles are close to each other, the force of repelling each other is reduced due to the decrease of the thickness of the diffusion layer and the decrease of the zeta potential. That is, the repulsion of the high ion concentration solution in the solution is lower than that of the ion concentration. The attraction between the micelles is unaffected by the water phase composition, but due to the thinning of the diffusion layer, the distance they collide decreases and the suction between them is great. Shows that the exclusion and attraction of force by the repulsive force into the main suction (exclusion of potential energy disappeared), the particles can be quickly condensed. This mechanism can better explain the sediment phenomenon at the harbor. As the fresh water enters the sea water, the salt increases, the ion concentration increases, and the stability of the fresh water entrapping colloidal particles decreases. Therefore, clay and other colloidal particles are easily deposited in the harbor. According to this mechanism, no more excess counter ions enter the diffusion layer when the amount of electrolyte added to the solution exceeds the critical concentration at which condensation occurs. It is not possible to change the sign of the colloidal particles to re-stabilize the colloidal particles. This mechanism is by purely static phenomenon. The effect of the electrolyte on the destabilization of the colloidal particles is explained, but it does not take into account the action of other properties of the destabilization process (eg adsorption) and therefore can not account for the complexities of other destabilization phenomena such as trivalent aluminum salts with iron salts as coagulants Excessive investment, the coalescence effect but on the decline, or even re-stabilize; Another example with the rubber particles with the same number of polymer or polymer organic matter may have a good cohesion effect: the co-state should have the best cohesion, but often Coagulation effect is best when the zeta potential is greater than zero in production practice. In fact, adding coagulant in aqueous solution to destabilize the colloidal particles involves the interaction between colloidal particles and coagulants, colloidal particles and aqueous solution, coagulant and aqueous solution, which is an integrated phenomenon.