The use of calcium hydroxide in wastewater treatment

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(1) Wastewater treatment agents: Used for pH neutralization and to pre-condition acidic wastewater pipelines, thereby extending the lifespan of these pipelines and saving **significant investment costs. (2) Acid water treatment agents: capable of effectively removing anions such as phosphate, sulfate, and fluoride ions. In practical applications, lime milk is also commonly used as a phosphorus removal agent. In addition to its functions of coagulation and sedimentation as well as acid neutralization, Ca(OH)2 acts as an effective coagulant that helps to remove other pollutants as well. Under alkaline conditions, when the pH level is around 10, the phosphate content in water is at its lowest, resulting in the best phosphorus removal efficiency. The effluent can thus meet the requirement specified in the comprehensive wastewater discharge standards, namely a phosphate concentration of less than 1.0 mg/L. Fluoride-containing wastewater mainly originates from production processes such as the smelting of non-ferrous and rare metals, glass and ceramic manufacturing, the production of fertilizers and pesticides, the acid washing of stainless steel, and the cleaning of silicon-based components. However, the fluoride content in fluoride-containing wastewater varies significantly across different industries. For mixed wastewater containing a large amount of other pollutants in addition to fluorine, treatment is usually carried out without recovering the fluorine. For this type of wastewater, it is generally believed that two-stage treatment is required to reduce the residual fluoride concentration to less than 10 mg/L. Direct addition of lime in primary treatment is the standard method for reducing high concentrations of fluoride ions, with the residual fluoride concentration after treatment typically ranging from 15 to 40 mg/L. However, there are also reports indicating that when water contains large amounts of polyvalent metal ions, the same effect as that achieved by using polyvalent metal ions can be obtained; primary lime treatment can also be used to treat high-concentration fluoride-containing wastewater so that the fluoride concentration is reduced to less than 10 mg/L.