What Is Coagulation, And What Types of Coagulation for Water Treatment?

What is Coagulation?

In the coagulation treatment of wastewater, sometimes the use of a single flocculant cannot achieve good coagulation effect, and it is often necessary to add certain auxiliary agents to improve the coagulation effect. This auxiliary agent is called a coagulant. Commonly used coagulants include chlorine, lime, activated silicic acid, bone glue, sodium alginate, activated carbon and various clays.

Some coagulants themselves do not have a coagulation effect, but play a role in assisting the flocculants to produce coagulation effects by adjusting and improving coagulation conditions. Some coagulants participate in the formation of flocs, improve the structure of the flocs, and turn the small and loose flocs produced by inorganic flocculants into thick and dense alum flowers.

Types of commonly used coagulants

There are many types of coagulants, but according to their role in the coagulation process, they can be roughly divided into the following two categories:

1. Agents to adjust or improve coagulation conditions

The coagulation process should be carried out within a certain pH range. If the pH value of the raw water cannot meet this requirement, the pH value of the raw water should be adjusted. Such coagulants include acids and alkalis. When the pH value of the raw water is low and the alkalinity is insufficient, making it difficult to hydrolyze the flocculant, CaO, Ca(OH)2, Na2CO3, NaHCO3 and other alkaline substances (commonly used lime) can be added; when the pH value is high, then Sulfuric acid or CO2 is commonly used to lower the pH value of raw water.

For wastewater with a large content of dissolved organic matter, oxidants such as Cl2 can be used to destroy the organic matter and improve the removal effect of dissolved organic matter. In addition, when ferrous salt is used as a flocculant, chlorine gas can be used to oxidize ferrous iron (Fe2+) into high-valent iron (Fe3+) to improve the coagulation effect.

The above alkaline agents, sulfuric acid, CO2, chlorine, etc. do not themselves have a coagulation effect, but only serve to assist coagulation.

2. A coagulant that increases the particle size, density and firmness of alum flowers.

The result of coagulation requires the generation of alum flowers with large particle size, high density and firmness, which is conducive to precipitation and not easy to break. In order to obtain this kind of result, combined with the characteristics of water quality, it is sometimes necessary to add certain substances or chemicals to the water. For example, in low-turbid wastewater containing light impurities that are not suitable for sedimentation, adding coarse particles such as silica, activated carbon, and clay or returning partially precipitated sludge can aggravate and increase alum blooms; when using aluminum salt When iron salts are used as flocculants and can only produce small and loose flocs, polymer coagulants such as polyacrylamide, activated silicic acid and bone glue can be added to use their strong adsorption and bridging effects to make small and loose flocs. The flocs become thick and dense.