Since ferrous sulfate heptahydrate is mainly composed of ferrous ion (Fe2 +), in the coagulation process, part of ferrous ion is oxidized to ferric ion (polymeric ferric sulfate) to adsorb organic matter. If the dosage is not well controlled, the excess iron ion will settle in the wastewater, resulting in the increase of water color and yellow water quality, That is what we often call ferrous sulfate heptahydrate secondary pollution phenomenon.

When ferrous sulfate heptahydrate is used for coagulation treatment of printing and dyeing wastewater, if the dosage of ferrous sulfate heptahydrate is excessive, the residual iron ions will enter the anaerobic tank and react with sulfide dyes in the wastewater, Ferrous sulfide is a kind of fine granular black organic matter. In the process of oxidation and aeration, iron oxide is generated due to the biochemical effect of aeration, which makes the water black.

When Fenton process is used to treat ferrous sulfate heptahydrate, if the dosage of ferrous sulfate heptahydrate is not controlled properly, the COD removal rate of wastewater will decrease, the COD of effluent will increase, and the biochemical treatment system of wastewater will be affected. Ferrous sulfate and polymerized ferric sulfate are strong acidic compounds, which can reduce the pH value of water and the removal rate of sulfide;

Therefore, when ferrous sulfate heptahydrate is used for coagulation treatment of wastewater or Fenton reagent is used for treatment of wastewater, the dosage of ferrous sulfate heptahydrate should be strictly matched through experiments to prevent improper dosing from leading to poor coagulation effect.

Ferrous sulfate heptahydrate has excellent coagulation and decolorization performance. In various kinds of wastewater, through a series of reactions, such as compressing the electric layer of polluted organic matter or suspended matter, adsorption electric neutralization, polymerization coagulation, net precipitation (adsorption, bridging, cross-linking), the pollutants form alum precipitation. And destroy the chromogenic group in the wastewater, make its decolorization effect become good. Many enterprises also use ferrous sulfate heptahydrate to remove phosphorus and cod.