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I. Disadvantages of calcium oxide in wastewater treatment 1. When used in wastewater treatment, calcium oxide absorbs water to form calcium hydroxide, as per the reaction CaO + H2O = Ca(OH)2. However, since calcium oxide does not pass through a specialized digester, the impurities present in it cannot be effectively converted into calcium hydroxide. 2. When dissolving, calcium oxide raises the water temperature, producing a large amount of fog; its odor is also quite pungent. 3. The impurities entrained when calcium oxide turns into calcium hydroxide upon contact with water increase the amount of sludge, thereby **significantly raising the treatment costs** for subsequent sludge treatment in the plant. II. Advantages of using calcium hydroxide for wastewater treatment 1. During the production process, calcium hydroxide passes through specialized digesters to remove impurities resulting from the conversion of lime into calcium hydroxide, thereby increasing its purity, activity, and quality. 2. In many industries, the flocs formed by adding aluminum-based or iron-based coagulants in wastewater are very small and dispersed. It is difficult to precipitate in the sedimentation tank. Adding calcium hydroxide at this point effectively increases the density of the flocs, allowing the flocculent particles to sink more quickly. 3. In wastewater, calcium hydroxide allows hydroxide ions to react with metal ions, forming water-insoluble precipitates; these can then be removed by adding a flocculant. Calcium hydroxide can remove anions such as phosphate, sulfate, and fluoride. In practical applications, we also usually use lime milk as a phosphorus removal agent, because in addition to its functions of coagulation and precipitation as well as acid neutralization, calcium hydroxide, as a coagulant, possesses good flocculating properties. 4. Calcium hydroxide also exhibits good removal efficiency for other pollutants. Under alkaline conditions, when the pH is around 10, the phosphate content in water can be reduced to such an extent that the effluent meets the requirement specified in the Comprehensive Wastewater Discharge Standards (GB8978-1996), 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 cleaning of stainless steel, and the cleaning of silicon-based components. However, the fluoride content in fluoride-containing wastewater varies significantly across different industries. 5. For mixed wastewater containing large amounts of various other pollutants besides fluorine, most are treated without recovering the fluorine. For this type of wastewater, it is generally believed that two treatment steps are required to reduce the residual fluoride concentration to less than 10 mg/L. Adding lime is a method to reduce high concentrations of fluoride ions; the residual fluoride concentration after calcium oxide treatment is usually 15–40 mg/L. When water contains large amounts of polyvalent metal ions, the same effects as those produced by the addition of polyvalent metal ions occur; calcium hydroxide 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. Additionally, when water contains salts such as calcium chloride and calcium sulfate, the common ions reduce the solubility of calcium fluoride; thus, through lime treatment, the fluorine concentration can be lowered to below 10 mg/L, or even below the theoretical solubility of calcium fluoride. 6. Perform PH neutralization and pre-treat acidic sewage pipelines to extend their service life. According to the Comprehensive Wastewater Discharge Standard of the People’s Republic of China, the permissible range for wastewater pH is between 6.0 and 9.0. As some chemical companies, manufacturing firms, and industrial enterprises discharge wastewater that exceeds regulatory limits into urban drainage systems, this wastewater contains highly acidic corrosive substances that cause the pipes to corrode and crack, thereby reducing the lifespan of groundwater pipelines by 5 to 10 years. The aqueous solution of calcium hydroxide is highly alkaline, and it can be used to neutralize acidic wastewater before discharge, thereby protecting pipes and preventing corrosion caused by acidic waste water.