In the process of chemical treatment of industrial wastewater, a variety of chemicals are usually used to cooperate with the addition of coagulation sedimentation, so that various pollutants can be removed. For example, in the treatment of printing and dyeing wastewater, we usually use ferrous sulfate + lime method to treat, ferrous sulfate decolorization, chromium removal, copper removal and other metal ions, lime neutralization pH value and play the role of coagulation. Electroplating wastewater is usually pretreated with ferrous sulfate and complex alkali. So who should be the first to add ferrous sulfate and other chemicals?

1. Dosing sequence of ferrous sulfate and lime

Ferrous sulfate has strong reducibility, and can be hydrolyzed rapidly in wastewater to form ferrous hydroxide colloid and sulfuric acid. Because ferrous sulfate is acidic, its dosage will reduce the alkalinity of wastewater. In this process, lime mainly plays a role of adjusting pH value and coagulation aid. Lime and ferrous sulfate will react with pollutants in wastewater to form ferric hydroxide colloidal precipitation, and adsorb and precipitate small suspended particles, colored groups and non soluble organic matter in wastewater. If ferrous sulfate is added first, ferrous ion can be oxidized under acidic conditions, and copper and chromium in wastewater can be removed. If lime is added first, the pH value of the wastewater will be adjusted to be alkaline, and iron will be reductive under alkaline conditions, which can better promote the reduction and decolorization of the colored groups in the wastewater, reduction of hexavalent chromium and Fenton reaction. The experiment shows that adding lime to adjust the pH value of wastewater first, and then adding ferrous sulfate for flocculation, sedimentation and clarification is better.

2. Dosing sequence of ferrous sulfate and polyacrylamide

Both ferrous sulfate and polyacrylamide have flocculation effect in wastewater, but the flocculation and precipitation effect of polyacrylamide is stronger than ferrous sulfate. Usually, ferrous sulfate is added first and then polyacrylamide is added. In this way, ferrous sulfate can be hydrolyzed to form alum particles with organic pollutants. After polyacrylamide is added, these fine flocs can be quickly bonded into giant clusters and precipitated under the action of gravity to form dense sludge. If polyacrylamide is added first, ferrous sulfate will not produce much flocculation, but lead to the failure of the reagent.