Analysis on the main methods of phosphorus contain

2022-08-02
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Analysis on the main methods of domestic treatment of phosphorus containing wastewater

excessive phosphorus, nitrogen and other elements in water will accelerate the eutrophication of water body. This phenomenon is more serious in China and has brought great harm to industry, aquaculture, agriculture and tourism. The increase in the concentration of nitrogen, phosphorus and other nutrients is the reason for the proliferation of algae, in which phosphorus is the key factor. Therefore, how to effectively reduce the concentration of phosphorus in sewage is of great significance to eliminate pollution and protect the environment

at present, there are two main types of phosphorus removal technologies for sewage at home and abroad: biological method and chemical method. Biological processes such as a/o, a2/o and uct processes are mainly suitable for the treatment of low concentration and organic phosphorus containing wastewater; Chemical method and physical-chemical method mainly include coagulation sedimentation method, crystallization method, ion exchange adsorption method, electrodialysis, reverse osmosis and other processes, which are mainly suitable for the treatment of inorganic phosphorus containing wastewater

however, some high concentration phosphorus containing wastewater often occurs in many industrial production processes. High concentration phosphorus containing wastewater is not strictly defined in the current research. It is generally considered that high concentration wastewater is defined as long as it is higher than the phosphorus content in domestic wastewater or the total phosphorus concentration is more than 100mg/l. High concentration phosphorus containing wastewater is difficult to be removed by a single biological or chemical method. Even if it can be removed, it will impose a great burden on the whole single biological or chemical treatment process, reducing the treatment effect of the whole treatment process or making it unable to operate continuously

1 source of phosphorus in water bodies

most of the phosphorus discharged into lakes comes from domestic sewage, wastewater from factories and animal husbandry, fertilizer loss from mountain forests and cultivated land, rainfall and snow. Compared with the previous items, the phosphorus content in rainfall and snow is lower. Some investigations show that the average phosphorus concentration in rainfall is lower than 0.04mg/l, and that in snowfall is lower than 0.02mg/l. Taking domestic sewage as an example, the phosphorus emission per person per day is about 1.4-3.2g, and the contribution of various detergents accounts for about 70% of them. In addition, cooking and rinsing water as well as phosphorus in feces and urine also have a considerable content. Phosphorus emission from the plant mainly comes from fertilizer, medicine, metal surface treatment, fiber dyeing and fermentation and food industry. Among the phosphorus inflows into the waters, domestic sewage accounts for 43.4% of the total, followed by 20.5%, 29.4% and 6.7%, domestic sewage 43.4%, factory and animal husbandry wastewater 20.5%, fertilizer loss 29.4% and snow precipitation 6.7%

forms of phosphorus in wastewater

phosphorus in wastewater exists in the form of orthophosphate, polyphosphate and organic phosphorus. Due to different sources of wastewater, the content of total phosphorus and various forms of phosphorus varies greatly. The total phosphorus content in typical domestic sewage is 3-15mg/L (calculated by phosphorus); In the fresh raw domestic sewage, the distribution of phosphate is roughly as follows: orthophosphate 5mg/L (calculated by phosphorus), tripolyphosphate 3mg/L (calculated by phosphorus), pyrophosphate LMG, l (calculated by phosphorus) and organic phosphorus LMG, L (calculated by phosphorus) [". Polyphosphate can be hydrolyzed to orthophosphate under acidic conditions. The pH range of most domestic sewage is 6.5-8.0 and the temperature is lo-20 ℃. Under these conditions, the hydrolysis process is very slow; however, under the action of bacterial biological enzymes in the sewage, the hydrolysis and transformation process can be greatly accelerated: many polycondensated phosphates in the domestic sewage have been converted to orthophosphate before the sewage arrives at the treatment plant. In addition, in the biochemical process of sewage During the treatment, all polyphosphate is converted to orthophosphate, and no condensed polyphosphate can survive. At the same time, under the action of bacteria, the organic phosphorus in sewage is also partially converted into orthophosphate

due to the above reasons, orthophosphate is mainly concerned in the process of wastewater phosphorus removal. Constrained by the ionization equilibrium of phosphoric acid, orthophosphate ionizes in water, and generates h3p04, h2p04-1, hp042-1 and P04 at the same time., The concentration distribution of each phosphorus containing group varies with pH value. In typical domestic sewage with pH value of 6-9, the main forms are hydrogen phosphate and dihydrogen phosphate

2 chemical treatment of phosphorus containing wastewater

the chemical precipitation method uses a variety of cations to combine with the design of the host and auxiliary equipment of the phosphoric acid experimental machine in the wastewater, taking the example of Shimadzu's Xianjin technology in Japan to produce precipitates, so that phosphorus can be effectively separated from the wastewater; Phosphorus removal by electrodialysis is a kind of membrane separation technology. It is only a method of concentrating phosphorus, and it cannot fundamentally remove phosphorus by itself; Biological method is now mostly used in urban sewage treatment plants with low phosphorus content. Compared with other methods, chemical precipitation method has the characteristics of large flexibility, high phosphorus removal efficiency and simple operation

I. phosphorus removal by calcium method

phosphorus removal by calcium method in the precipitation method, the chemical precipitants mainly include aluminum ions, iron ions and calcium ions, of which the equilibrium constant of hydroxyapatite generated by lime and phosphate is the largest, and the phosphorus removal effect is the best. Add lime to the phosphorus containing waste water, and the calcium ion reacts with the phosphate radical to form a precipitate. The reaction is as follows: 5ca2++7oh-+3h2po4-=ca5 (OH) (PO4) 3 ↓ +6 in which H2O (1) side reaction: ca2++co32-=caco3 ↓ (2) equilibrium constant of reaction (1) ks0= 9. it can be seen from the above reaction that the phosphorus removal efficiency depends on the relative concentration of anions and pH value. According to formula (1), 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, which can ensure that the quality concentration of phosphate in the effluent is 0.5mg/l. Reaction (2), i.e. calcium ion reacts with carbonate radical in wastewater to generate calcium carbonate, which is very important for phosphorus removal by calcium method. It not only affects the dosage of calcium, but also the generated calcium carbonate can be used as a weight increasing agent to help coagulate and clarify the wastewater

in the above process, the first stage reaction and sedimentation is mainly zinc removal, ph=8.5-9.0 is controlled, and polyaluminum chloride is added. The second stage reaction and sedimentation is mainly phosphorus removal by calcium method, ph=11-11.5 is controlled. The effluent is discharged or reused after neutralization, and the effluent quality meets the first-class standard

key technology:

the key technology of phosphorus removal by calcium method is to use calcium chloride or lime as reagent, use mechanical mixing reactor and high-efficiency inclined tube precipitator to control appropriate amount of reaction, mixing intensity, precipitation surface load and reaction pH

II. Two commonly used phosphorus removal substances:

1 Slag

slag is a solid waste produced in the process of iron and steel smelting. It is mainly composed of oxides such as Cao, FeO, MnO, SiO2, Fe2O3, P2O5, cr2o5, Al2O3, etc. it has many excellent characteristics, and each component contained therein can be used. The experimental study of this method is to add 200ml of simulated phosphorus containing wastewater and a certain amount of slag into several conical flasks with plugs, place them on the oscillator, oscillate at room temperature for a certain time to make the adsorption reaction reach equilibrium, and then filter them. Then, the phosphorus concentration of the clear liquid is tested, and then the adsorption capacity on the adsorbent and the phosphorus removal rate are calculated by comparing the initial concentration and equilibrium concentration of phosphorus in the solution. The results show that:

(1) with the increase of slag dosage, the phosphorus removal rate also increases, but the adsorption capacity decreases

(2) at the beginning, the adsorption capacity increased with the increase of time, but when the adsorption time was more than 2h, the adsorption capacity tended to be stable

(3) the adsorption capacity increases with the increase of phosphorus concentration in wastewater

(4) temperature has little effect on slag adsorption

(5) the pH value of the solution has an important influence on the adsorption effect. When the pH is 7.56, the phosphorus removal rate is the highest

therefore, when phosphorus containing wastewater is treated with slag, when the concentration of phosphorus in the waste liquid is 2-13mg/l, the amount of slag is 5g/l, the pH is 7.56, and the adsorption time is 2h, the phosphorus removal rate can reach more than 99%, and the concentration of residual liquid is also lower than the national discharge standard. Moreover, this method is safe and reliable, and will not produce secondary pollution

2. Add lime

add a large amount of lime to the phosphorus containing wastewater, adjust ph=10.5-12.5 to generate hydroxyapatite. The sediment is stable and the equilibrium constant is large. The equilibrium constant for the formation of CA10 (OH) 2 (PO4) 6 is 90, which is 3-4 times greater than that for the formation of phosphate sediment from aluminum salt and iron salt. The larger the equilibrium constant is, the more stable the precipitate will be, the better the sedimentation effect will be, the more thorough dephosphorization will be, and the solid-liquid separation effect will be good. The treatment of phosphorus containing wastewater fully meets the standard, P ≤ 0.5mg/l. Adding lime to improve the pH value of the wastewater for phosphorus removal also makes the petroleum and CODcr co precipitation in the wastewater purified, and the wastewater can be discharged up to the standard. The phosphorus containing wastewater is treated with lime, resulting in a large amount of sludge. The sludge at the bottom of the inclined tube sedimentation tank is discharged into the sludge concentration tank through the bottom tube, and the sludge is discharged once or twice a day to avoid drying and blocking the pipe. After thickening in the sludge thickener, the thick sludge at the lower layer is pumped into the plate and frame filter press for filtration to separate the solid and liquid, and the dry slag is packed and transported out

3. Mixed agent assisted chemical precipitation method

the compound precipitants used in this method are magnesium chloride and ammonium hydrogen phosphate, which produce slow effect compound fertilizer while removing phosphorus. The reaction principle is as follows:

hpo42-+mg2++nh4++6h2o=mgnh4po4 6H2O ↓ +h+

po43-+mg2++nh4+6h2o=mgnh4po4 6H2O ↓ the MgNH4PO4 6H2O generated by the reaction is large in crystal and easy to filter. For the wastewater with low phosphorus concentration, it can be discharged after one-time treatment.Standards. However, when it is difficult to meet the standard in the treatment of wastewater with high phosphorus content, it is necessary to add composite coagulants PAC (polyaluminium chloride) and PAM (polyacrylamide ammonium) to the primary treatment liquid on the basis of primary treatment. The coagulation of PAC is mainly realized by adsorption bridging and precipitation trapping. PAM is an anionic polymer flocculant. After adding the solution, PAM can disperse rapidly and evenly, so that the precipitated ions in the aqueous solution can "bridge" to form flocs and precipitate [4] The experimental results show that PAM as coagulant aid and coagulant PAC can achieve good coagulation effect. Use composite coagulation

four. The characteristics of chemical dephosphorization method

chemical dephosphorization is essentially a physical and chemical process. Its advantage is that the treatment effect. The experimental machines widely used in domestic related research include roller wear test, 4-ball friction and wear test, reciprocating friction and wear test, cut in friction and wear test, disk pin friction and wear test, etc. The results are stable, reliable, simple and flexible, The sludge will not release phosphorus again in the process of treatment and disposal, and has strong resistance to impact load. The disadvantage is that the chemical phosphorus removal method will produce a large amount of aqueous chemical sludge, which is difficult to treat. In addition, the reagent cost is high, resulting in a higher concentration of residual metal ions and an increase in effluent chromaticity

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