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    Brief Analysis of the Main Treatment Methods of Phosphoric Acid Wastewater in Ch

    Time:2018-12-14 16:00:53 Source:

    The main treatment methods of phosphorus-containing wastewater in China at present are introduced, including precipitation, adsorption and biological methods, and various treatment methods of high-concentration phosphorus-containing wastewater are summarized, including traditional biological method, chemical method, electrolysis method newly developed in recent years, calcium method and SBR enhanced biological method, as well as flocculation-fly ash adsorption method, chemical precipitation-coagulation-flotation-activity method. Carbon adsorption method and ceramic membrane coagulation reaction method, etc. Key words: wastewater containing phosphorus, forms of phosphorus, chemical methods, calcium method, slag, lime, characteristics of chemical methods, biological method, physical and chemical method. Excessive phosphorus and nitrogen in water will accelerate eutrophication of water body. This phenomenon is more serious in China, which has brought great harm to industry, aquatic industry, agriculture and tourism. Increased concentration of nitrogen and phosphorus is the cause of algae reproduction, and phosphorus is the key factor. Therefore, how to effectively reduce the concentration of phosphorus in sewage is of great significance for eliminating pollution and protecting the environment. At present, domestic and foreign wastewater phosphorus removal technologies mainly include biological and chemical methods. Biological processes such as A/O, A2/O, UCT process are mainly suitable for the treatment of low concentration and organic phosphorus-containing wastewater; chemical and physical chemical methods mainly include coagulation and precipitation, crystallization, ion exchange adsorption, electrodialysis, reverse osmosis and other processes, mainly suitable for the treatment of inorganic phosphorus-containing wastewater. However, in many industrial processes, high concentration of phosphorus-containing wastewater often appears. There is no strict definition of high concentration phosphorus wastewater in current research. It is generally considered that as long as the phosphorus content in wastewater is higher than that in domestic wastewater or the total phosphorus concentration is more than 100 mg/L, it is called high concentration wastewater. High concentration phosphorus wastewater is difficult to be removed by a single biological or chemical method, even if it can be removed, it will cause a great burden on the whole single biological or chemical treatment process, which will reduce the treatment effect of the whole treatment process or can not run continuously. 1 Source of Phosphorus in Water Phosphorus discharged into lakes mostly comes from domestic sewage, factory and animal husbandry wastewater, fertilizer loss from forest farmland and rainfall and snow. Compared with the previous items, the phosphorus content in rainfall and snow is lower. Investigations show that the average phosphorus concentration in precipitation is lower than O.04mg/L, and that in snowfall is lower than O.02mg/L. Taking domestic sewage as an example, the daily phosphorus discharge per person is about 1.4-3.2g, and the contribution of various detergents is about 70%. In addition, cooking and washing water, as well as phosphorus in feces and urine also have considerable content. Plant phosphorus emissions mainly come from fertilizer, medicine, metal surface treatment, fiber dyeing and fermentation and food industry. Among the phosphorus inflow, domestic sewage accounted for 43.4% of the total, the others accounted for 20.5%, 29.4% and 6.7%, domestic sewage 43.4%, factory and animal husbandry wastewater 20.5%, fertilizer loss 29.4% and snowfall 6.7%. Forms of phosphorus in Wastewater Phosphorus in wastewater exists in the form of orthophosphate, polyphosphate and organic phosphorus. Because of the different sources of wastewater, the total phosphorus and various forms of phosphorus content are quite different. Typical domestic sewage contains 3-15 mg/L of total phosphorus (in terms of phosphorus); in fresh raw domestic sewage, the distribution of phosphate is roughly as follows: orthophosphate 5 mg/L (in terms of phosphorus), triphosphate 3 mg (in terms of phosphorus), pyrophosphate lmg, L (in terms of phosphorus) and organic phosphorus. For these reasons, orthophosphate is the main concern in the process of phosphorus removal from wastewater. The ionization equilibrium of phosphoric acid restricts the ionization of orthophosphate in water, while producing H3P04, H2P041, HP042 1 and P04. The concentration distribution of each phosphorous group varies with the pH value. Hydrogen phosphate and dihydrogen phosphate are the main forms in typical domestic sewage with pH 6-9. 2 Chemical Treatment of Phosphorus-Containing Wastewater Chemical precipitation method is to use a variety of cations to combine with phosphate in wastewater to form precipitation substances, so that phosphorus can be effectively separated from wastewater; electrodialysis phosphorus removal is a membrane separation technology, it is only a method of concentrating phosphorus, it can not fundamentally remove phosphorus itself; biological method is now mostly used in the situation of low phosphorus content in municipal wastewater treatment plants. Compared with other methods, chemical precipitation method has the advantages of high operating flexibility, high phosphorus removal efficiency and simple operation. I. Phosphorus Removal by Calcium Method Among the precipitation methods for phosphorus removal by calcium method, the main chemical precipitators are aluminium ion, iron ion and calcium ion. Among them, the equilibrium constant of hydroxyapatite formed by lime and phosphate is the largest and the effect of phosphorus removal is the best. When lime is added to wastewater containing phosphorus, calcium ion reacts with phosphate to form precipitation. The reactions are as follows: 5Ca2 +7OH-+3H2PO4-=Ca5 (OH) (PO4) 3+6H(1) side reaction: Ca2 +CO32-=CaCO3(2) reaction (1) The equilibrium constant KS0=10-55.9. From the above reaction, it can be seen that the phosphorus removal efficiency depends on the relative concentration of anions and pH value. Formula (1) shows that phosphate reacts with calcium ions under alkaline conditions to form calcium hydroxyphosphate, and the reaction tends to be complete with the increase of pH value. When the pH value is greater than 10, the phosphorus removal effect is better and the mass concentration of phosphate in effluent can be ensured to be less than 0.5mg/L. Reaction (2) means that calcium ions react with calcium ions. The formation of calcium carbonate by carbonate reaction in wastewater is very important for phosphorus removal by calcium method. It not only affects the dosage of calcium, but also produces calcium carbonate which can be used as a weight-increasing agent to condensate and clarify wastewater. The first-order reaction and precipitation in the above-mentioned process are mainly zinc removal, pH=8.5-9.0 controlled, polyaluminium chloride added, the second-order reaction and precipitation are mainly phosphorus removal by calcium method, and pH=11-11.5 controlled. The effluent is discharged or reused after neutralization, and the effluent quality reaches the first standard.

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