Phosphorus emission index has been listed as an important index leading to water eutrophication in China, with strict discharge standards. However, the methods of phosphorus removal are usually biological phosphorus removal and chemical phosphorus removal. Biological phosphorus removal is a relatively economic method of phosphorus removal. However, the phosphorus removal process can not guarantee that it can stably meet the requirements of 0.5mg/l effluent standard, so chemical phosphorus removal measures are often required to meet the requirements in order to meet the stable effluent standard. Chemical dephosphorization must use chemical dephosphorization agent for coagulation precipitation dephosphorization.
chemical phosphorus removal is completed through the process of chemical precipitation, which refers to the formation of granular and insoluble substances by adding chemicals to the sewage and mixing them with soluble salts in the sewage, such as phosphate.
the types of phosphorus removal agents are divided into the same and different types according to the difference between the coagulation and phosphorus removal effects of the agents.
I. iron salt dephosphorization agent: refers to iron series compound agent, the main dephosphorization agent represented by polymeric ferric sulfate, ferric chloride and ferrous sulfate. It is a newly emerging dephosphorization agent in the market at present, and its effect is better than other kinds of agents. Among them, the removal rate of insoluble phosphorus in sewage by polymeric ferric sulfate can reach more than 92%. And the latest enhanced phosphorus removal agent in the market.
the iron ion generated by the dissolution of iron salt in water can neutralize the negative colloidal particles in water, and can also react with phosphate to form iron phosphate precipitate. Secondly, commonly used dephosphorization agents, which are dissolved in water to produce iron oxide or iron hydroxide with adhesive effect, can adsorb and precipitate phosphate. In addition, polyferric sulfate as a high polymer dissolved in water to form polynuclear iron hydroxide has strong complexation and coagulation.
II. Aluminum salt dephosphorization agent: represented by aluminum sulfate, polyaluminum chloride and sodium aluminate, the dephosphorization effect of this kind of dephosphorization agent mostly depends on the adsorption of aluminum hydroxide. Therefore, it is inferior to iron salt in dephosphorization. And because the long-term accumulation of residual aluminum ions in the water after phosphorus removal will cause serious harm to animals and plants, it is gradually withdrawing from the market.
III. calcium salt dephosphorizing agent: calcium salt dephosphorizing agent is a dephosphorizing agent represented by lime, flake alkali, compound alkali and other alkaline agents. Calcium salt is used to remove phosphorus by reacting with phosphate to form calcium phosphate precipitation. The dosage of this kind of phosphorus removal agent is greatly affected by pH value, phosphorus formation and calcium content in water. And the amount of sludge produced by calcium salt dephosphorization is relatively large, which increases the difficulty of sludge treatment.