Research progress on the treatment of nitrogen and phosphorus wastewater with ammonium magnesium phosphate method来源：中国给水排水 作者: 姚涛蔡伟民李龙海 阅读：97292 网友评论0条 Updated: 2010-12-24 14:21 Source: China Water & Wastewater Author: Yaotao Cai Weimin Li Longhai reading: 97 292 2010 0 comments
Abstract : The formation mechanism of magnesium ammonium phosphate in wastewater and its application status in wastewater treatment are described. The use of magnesium salt as a precipitant in wastewater treatment can simultaneously remove nitrogen and phosphorus in wastewater. This method has high removal rate, fast reaction speed, small sludge volume, and can also recover magnesium ammonium phosphate, which has certain application prospects.
Key words : Magnesium ammonium phosphate; chemical precipitation; conditioned solubility product; wastewater treatment
Magnesium ammonium phosphate (MgNH 4 PO 4 ¡¤ 6H 2 O) is commonly known as struvite, the English name is struvite (magnesium ammonium phosphate), referred to as MAP. In the wastewater treatment process, the ammonium magnesium phosphate (MAP) method is to add Mg 2+ to the wastewater containing phosphate and ammonia nitrogen to react to form insoluble magnesium ammonium phosphate precipitation, thereby removing phosphate and ammonia nitrogen in the wastewater.
Compared with the traditional activated sludge method, this method can reduce the sludge volume by 49%. Magnesium ammonium phosphate is not only widely used in the fields of chemical industry, medicine and construction, but also an efficient slow-release fertilizer.It can greatly reduce the nitrogen and phosphorus in the fertilizer from entering the nearby water body by slowly releasing nutrients to prevent eutrophication. occur. The conversion of a large amount of phosphate in wastewater to magnesium ammonium phosphate has attracted the interest of many water treatment experts.
1 Formation mechanism of magnesium ammonium phosphate in water
1.1 solubility of magnesium ammonium phosphate
Magnesium ammonium phosphate is a very complex crystalline compound. The change of pH value in water has a great influence on its formation reaction.With the change of pH value, the concentrations of NH 4 + , Mg 2+ and PO 4-3 in water constantly change. When the activity product of these three ions exceeds phosphoric acid When the solubility product constant ( K sp ) of ammonium and magnesium is used, the solution is oversaturated and then precipitates. The activity of these three ions depends on the pH of the system and the concentrations of soluble Mg, N and phosphate in the solution. In addition, ionic strength, temperature, and impurities in water will affect the activity of the three ions, and then affect the precipitation of magnesium ammonium phosphate. So far, many people have calculated the solubility product constant of magnesium ammonium phosphate through experiments and chemical equilibrium calculations (3. 89 ¡Á 10- 10 to 7. 08 ¡Á 10- 14).
1.2 Conditional solubility product of ammonium magnesium phosphate
The solubility of magnesium ammonium phosphate is not independently controlled by its solubility product constant. Adnan et al.  compared the dissolution of magnesium ammonium phosphate in pure water and sludge digestion, and found that magnesium ammonium phosphate was more soluble in digestion. Easy to dissolve. In order to accurately predict the solubility of magnesium ammonium phosphate in engineering, Stumm et al. Quoted the concept of conditional solubility product (PS), that is, an equilibrium constant that is applicable only under given test conditions (for example, at a certain pH value) . The PS of magnesium ammonium phosphate can be defined as:
1.3 Formation of magnesium ammonium phosphate
PSeq is greatly affected by pH: as the pH value increases, PO3-4 increases in water, while the concentration of Mg2 + and NH + 4 decreases sharply, PSeq of magnesium ammonium phosphate decreases rapidly, and when PS> PSeq, magnesium ammonium phosphate in water Supersaturation is reached, and when the supersaturation reaches a certain value, magnesium ammonium phosphate precipitates out. Or at the same pH value, increasing the content of any one of the three ions that make up the magnesium ammonium phosphate can also make the magnesium ammonium phosphate in the system supersaturated. The degree of supersaturation β (magnesium ammonium phosphate) is defined as follows:
Normally, β is used to distinguish crystals and precipitates. When β is greater than the critical supersaturation, the solution changes from crystalline to precipitated, and a large number of magnesium ammonium phosphate microcrystals appear in the solution. Therefore, in the wastewater treatment and chemical production, in order to make the reaction system treatment efficiency and the quality of the products produced good, generally the ammonium magnesium phosphate reaction is in the crystallization process. Adnan et al.  thought that the system was in the metastable region when the supersaturation of the reaction was between 1 and 5, and the removal rate of phosphate and the produced ammonium magnesium phosphate crystals could reach the best. Hirasawa et al. Confirmed that when the supersaturation is less than the critical supersaturation, the linear velocity of the grain growth has a linear relationship with the supersaturation. Crystals undergo two stages of nucleation and crystal growth when they are formed. The growth of magnesium ammonium phosphate crystals is complicated, not only affected by the mass transfer in the solution and the cumulative properties of the grain surface, but also greatly affected by the environment: magnesium ammonium phosphate is more likely to change in environmental factors such as rough surfaces and pipe joints Gather.
2 Application in wastewater treatment
2.1 Sources of magnesium
If the hardness of the water in the area where the sewage treatment plant is located is relatively high, usually the sewage in the area will contain a large amount of magnesium ions. When the magnesium salt in the wastewater is insufficient, other magnesium salts can be added. If the sewage treatment facility is located near the sea, using seawater or salt milk as a magnesium source is a more economical and simple method. Tunay et al.  used MgCl2 to remove tannery wastewater containing a large amount of ammonia nitrogen. Magnesium oxide is widely used due to its small environmental impact, low solubility, and strong alkalinity. There is usually no need to add magnesium salt in the anaerobic digestion fluid of pig wastewater. The magnesium in the digestion fluid is sufficient to precipitate with ammonia nitrogen and orthophosphate to form ammonium magnesium phosphate.
2.2 Sources of ammonia nitrogen and phosphorus
Generally, the nitrogen-containing compounds in urban sewage produce a large amount of ammonia nitrogen during the corruption process. Some chemical tannery wastewater and old landfill leachate themselves will contain high concentrations of ammonia nitrogen, which is usually excessive relative to magnesium and phosphorus in the wastewater. of. Phosphorus in sewage is dissolved in water in the form of orthophosphate, polyphosphate and organic phosphorus. In the anaerobic dephosphorization tank of the sewage treatment plant, the polyphosphate was decomposed to form inorganic phosphorus and released back into the sewage. Jaffer et al.  made a material balance for the ammonia nitrogen, total phosphorus and magnesium salts in each treatment facility of the Slough sewage treatment plant. As shown in the figure, the dehydration of the anaerobic digestion sludge as the source of the reactants not only removes a large amount of nutrients in the water, but also prevents the return liquid pipeline from being blocked by ammonium magnesium phosphate.
2.3 Nitrogen and Phosphorus Removal by Ammonium Magnesium Phosphate
Magnesium ammonium phosphate (MAP) precipitation method has a good treatment effect on high-concentration ammonia nitrogen wastewater. Adding magnesium salt and phosphate to the wastewater can quickly remove ammonia nitrogen in wastewater. Zdybiewska et al. Found that when the pH was 8-10, the amount of ammonia nitrogen remaining in the water body was the lowest. At present, a lot of work has been done in this area in China, see Table 1. Table 1 MAP removal ammonia nitrogen test results
Ohlinger and Jaffer considered that in order to ensure a high phosphorus removal rate (> 80%), the pH in the reactor should be 8.2 to 9. E2 lisabeth et al. Used a fluidized bed to treat wastewater with a phosphate concentration of 61 mg / L, and added NaOH to achieve a phosphorus removal rate of 94% at pH 8.5 in the water. In the treatment of wastewater with a phosphate concentration of 162 mg / L, Abe achieved a phosphorus removal rate of 81% by raising the pH value to 8.52. When treating the supernatant of digested sludge, such as Battistoni, the pH of the wastewater was raised from 7.9 to 8.3 to 8.6 by adding carbon dioxide in dehydration without adding alkali, and the removal rate of phosphate reached 80%. Jaffer et al. Found that when n (Mg 2 + +): n (PO4-3) is 1.05: 1, by raising the pH in the wastewater to 9.0, the removal rate of phosphate is as high as 97%; but n ( When Mg 2 + ): n (PO4-3) <1.05, the removal rate of phosphate is only 72%.
Magnesium ammonium phosphate method has high removal rate, fast reaction speed, and can also recover magnesium ammonium phosphate. In practical applications, it is necessary to further optimize the reactant ratio and adjust the pH value economically in order to achieve stable operation and higher product recovery value.
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