Flocculant is a kind of substance that can reduce or eliminate the precipitation stability and polymerization stability of dispersed particles in water, so that dispersed particles can condense and flocculate into aggregates.
According to the chemical composition, flocculants can be divided into inorganic flocculants and organic flocculants.

1, inorganic flocculant
The traditional inorganic flocculants are low molecular weight aluminum salts and iron salts, aluminum salts are mainly aluminum sulfate (AL2(SO4)3∙18H2O), AL2(SO4)3∙K2SO4∙24H2O), sodium aluminate (NaALO3), sodium aluminate (AL2(SO4)3∙K2SO4∙24H2O). The main iron salts are ferric chloride (FeCL3∙6H20), ferrous sulfate (FeSO4∙6H20) and ferric sulfate (Fe2(SO4)3∙2H20).
Generally speaking, inorganic flocculants have the characteristics of easy access to raw materials, simple preparation, cheap price and moderate treatment effect, so they are widely used in water treatment.

1) Aluminum sulfate
Aluminum sulfate on the market has two forms of solid and liquid, solid and according to the content of insoluble matter is divided into refined and crude two, China's folk often used for drinking water purification of solid products alum, is aluminum sulfate and potassium sulfate double salt, but in industrial water and wastewater treatment applications are not much.

The pH range applicable to aluminum sulfate is related to the hardness of raw water, the appropriate pH value is 5 ~ 6.6 for the treatment of soft water, 6.6 ~ 7.2 for the treatment of medium hard water, and 7.2 ~ 7.8 for the treatment of high hard water. The applicable water temperature range of aluminum sulfate is 20oC ~ 40oC, and the coagulation effect is very poor when it is lower than 10oC. Aluminum sulfate is less corrosive and easy to use, but the hydrolysis reaction is slow, and a certain amount of alkali needs to be consumed.

2) Ferric chloride

Ferric chloride is another commonly used inorganic low molecular coagulant, the product can be solid dark brown crystals, but also a higher concentration of liquid. It is easy to dissolve in water, large and heavy alum, good precipitation performance, wide range of adaptation to temperature, water quality and pH, etc.

 The applicable pH value range of ferric chloride is 9 ~ 11, the formed floc density is large, easy to precipitate, and the effect is still very good at low temperature or high turbidity. Solid ferric chloride has strong water absorption, strong corrosion, easy to corrode equipment, high corrosion requirements for dissolution and dosing equipment, pungent odor, poor operating conditions.

 The action mechanism of ferric chloride is to use various hydroxyl iron ions generated by step-by-step hydrolysis of trivalent iron ions to achieve flocculation of impurity particles in water, and the formation of hydroxyl iron ions requires the use of a large number of hydroxyl groups in water, so a large amount of alkali will be consumed during the use of the process, when the raw water alkalinity is not enough, it is necessary to supplement lime and other alkali sources.

 Ferrous sulfate is commonly known as green alum, the formation of flocculation is fast and stable, the precipitation time is short, suitable for high alkalinity, turbidity, but the color is not easy to clean, corrosion is also strong.