Precipitation is a chemical reaction that produces a substance that is insoluble in the solution of the reactant. Literally, it means precipitation removal under the action of gravity. Suspended substances in sewage, so this is a physical process, simple and easy, good effect, is one of the important technologies of sewage treatment.

According to the properties of suspended matter, concentration and flocculation performance of polyacrylamide, precipitation can be divided into: natural precipitation, flocculation precipitation, regional precipitation. The concentration of suspended particles in regional precipitation is higher (more than 5000mg/L), and the settlement of particles is affected by other surrounding particles, and the relative positions of particles remain unchanged, forming a whole common sinking, and there is a clear mud-water interface between them and the clarified water. Regional sedimentation occurred in both secondary sedimentation tank and sludge thickening tank.

The concentration of suspended solids in the wastewater is not high, and it does not have the performance of condensation. In the precipitation process, the solid particles do not change their shape, nor do they bond with each other, and each completes the precipitation process independently. (Initial precipitation in the sand settling tank and the primary settling tank) compression precipitation occurs in the settling process of highly concentrated suspended particles. Due to the high concentration of suspended particles, the particles have been squeezed into a mass structure, contact each other and support each other. The water between the lower particles is squeezed out under the gravity action of the upper particles, so that the sludge is concentrated. There is compression precipitation in the process of polyacrylamide concentration in the sludge hopper of the secondary sedimentation tank and in the process of sludge concentration in the concentration tank. Free precipitation occurs when the concentration of suspended solids in water is not high, the precipitation process does not interfere with each other, the particles are precipitated separately, and the precipitation trajectory of the particles is in a straight line. Throughout the precipitation process, the physical properties of the particles, such as shape, size and specific gravity, do not change. The precipitation of such particles in the sedimentation tank is free precipitation.

Flocculation precipitation is a process in which particles are flocculated and precipitated in water. After adding coagulant in water, the colloids and dispersed particles of suspended matter form flocculent under the interaction of molecular force, and they collide with each other during the sedimentation process, and their size and mass continue to increase, and the sedimentation rate keeps increasing. The removal rate of suspended matter depends not only on the precipitation rate, but also on the precipitation depth. The alum formed after adding coagulant to the surface water, the organic suspended matter in domestic sewage, and the activated sludge will appear the phenomenon of flocculation and precipitation during the precipitation process.