Calcium hydroxide has the following five specific roles in wastewater treatment: 1. Environmental protection neutralizer: calcium hydroxide solution is less corrosive and alkaline than sodium hydroxide, but its aqueous solution is alkaline, which can neutralize acidic wastewater well, and can also reconcile the pH while allowing metal ions to precipitate. Such as aluminum products factory wastewater, neutralize the acidity, adjust the PH to 8-9, aluminum can be generated insoluble matter and precipitation. 2. Trapping agent: Hydroxide in calcium hydroxide aqueous solution can capture heavy metal ions in wastewater very well. Make it form an insoluble precipitate, and then add flocculant precipitation to remove. 3. Removal of fluoride ions: Fluoride ions and many metal ions are soluble, which determines that aluminum flocculants (polyaluminum chloride, industrial aluminum sulfate, etc.) and iron flocculants (polyferric chloride, polyferric sulfate, ferrous sulfate) have no effect. Only calcium and fluoride ions combine to form an insoluble and water-soluble precipitate. Only when the precipitate is formed, flocculant is added to separate the precipitate (calcium fluoride) from the water. Such as some chemical fluoride wastewater treatment, and glass products factory wastewater treatment. 4. Increase the weight: the flocculants generated by aluminum or iron flocculants in wastewater of many industries are very small and very loose. It is difficult to sink in the sedimentation tank, so calcium hydroxide is added to increase the specific gravity of floc and accelerate the sinking of flocculent. Such as some automobile factory paint spraying wastewater, coating plant wastewater, etc. 5. Chemical conditioner: The special flocculate structure and high hydrophilicity of the residual activated sludge in some industries make the water contained in it difficult to be removed, and the use of calcium hydroxide as a chemical conditioner can change its characteristics, improve the dewatering of the sludge, sterilization and promote the hydrolysis of organic matter, thus reducing the difficulty of operation. Such as petrochemical wastewater, amylating plant wastewater. It should be noted that in order to ensure the good use of calcium hydroxide in sewage treatment, the dissolved concentration must be controlled (it is recommended not to be higher than 15%). Because the solubility ratio is too high, calcium hydroxide will form a saturated solution, in which the hydroxide and calcium ions cannot be dissociated, resulting in waste, and these insoluble calcium hydroxide will also increase the amount of later processing.