What are the removal methods of ammonia nitrogen in sewage treatment？

1.1 Multi-stage sludge system This process can get quite good BOD5 removal effect and denitrification effect. The disadvantage is that the process is long, there are many structures, the cost of infrastructure is high, additional carbon sources are required, the operating cost is high, and a certain amount of methanol remains in the effluent, etc. .

1.2 Single-stage sludge system The form of single-stage sludge system includes pre-denitrification system, post-denitrification system and alternate working system. The biological denitrification process of pre-denitrification is usually called A/O process. Compared with the traditional biological denitrification process, the A/O process has the advantages of simple process, few structures, low capital construction cost, no need for external carbon source, effluent High water quality advantages. The post-type denitrification system generally needs to manually add a carbon source because the mixture lacks organic matter, but the effect of denitrification can be higher than that of the front-end denitrification system, which can theoretically be close to 100% denitrification. The alternately working biological denitrification process is mainly composed of two series-connected tanks. By changing the direction of inflow and outflow, the two tanks operate alternately under anoxic and aerobic conditions. The system is still an A/O system in essence, but it uses the alternate working method to avoid the backflow of the mixed liquid, so the denitrification effect is better than that of the general A/O process. The disadvantage is that the operation and management costs are relatively high, and a computer-controlled automatic operating system must generally be configured.

1.3 Biofilm system The anoxic tank and aerobic tank in the above A/O system are changed to fixed biofilm reactors, that is, a biofilm denitrification system is formed. There should be mixed liquid backflow in this system, but no sludge backflow. Two sludge systems suitable for denitrification, aerobic oxidation and nitrification are stored in anoxic aerobic reactors.

2 Physical and chemical nitrogen removal

2.1 Break point chlorination method Discontinuous point chlorination method is a kind of oxidation method to treat ammonia nitrogen wastewater. It utilizes the reaction of ammonia and chlorine in water to generate nitrogen gas to remove ammonia from water. This method can also play a bactericidal effect and at the same time make a part of the organic matter inorganic, but residual chlorine remains in the effluent after the chlorination treatment, which should be further dechlorinated. Add hypochlorous acid HClO to water containing ammonia. When the pH value is near neutral, with the addition of hypochlorous acid, the following main reactions are carried out step by step: NH3+HClO→NH2Cl+H2O①NH2Cl+HClO→NHCl2+H2O②NH2Cl+ NHCl2→N2+3H++3Cl-③When the ratio of the amount of chlorine added and ammonia nitrogen (referred to as Cl/N) is below 5.07, the reaction of formula ① is first performed to generate monochloramine (NH2Cl), and the residual chlorine concentration in the water increases, After that, with the increase of hypochlorous acid dosage, monochloramine reacts according to formula ② to generate dichloramine (NHCl2), and at the same time, formula ③ is carried out, and N in water is removed as N2. As a result, the residual chlorine concentration in the water decreases with the increase of Cl/N. When the Cl/N ratio reaches a certain value or more, the residual hypochlorous acid (ie free residual chlorine) due to unreacted increases, and the water The concentration of residual chlorine increases again, and the point of this minimum value is called the discontinuous point (used to be called the inflection point). The Cl/N ratio at this time is theoretically 7.6; in wastewater treatment, because chlorine reacts with the organic matter in the wastewater, the C1/N ratio should be higher than the theoretical value of 7.6, usually 10. In addition, when the pH is not in the neutral range, more trichloramine is generated under acidic conditions, and nitric acid is generated under alkaline conditions, which reduces the denitrification efficiency. When the pH value is 6-7, the dosage of ammonia nitrogen and chlorine per mg is 10 mg, and the contact is 0.5-2.0 h, the removal rate of ammonia nitrogen is 90%-100%. Therefore, this method is suitable for low-concentration ammonia nitrogen wastewater. The actual amount of chlorine required for treatment depends on temperature, pH and ammonia nitrogen concentration. Oxidation sometimes requires 9 to 10 mg of chlorine breakpoint per mg of ammonia nitrogen. The effluent after chlorination treatment generally needs to be dechlorinated with activated carbon or SO2 before discharge to remove residual chlorine in the water. Although the chlorination method has a rapid reaction and requires less investment in equipment, the safe use and storage of liquid chlorine requires high requirements, and the processing cost is also high. If the liquid chlorine is replaced by hypochlorous acid or chlorine dioxide generator, it will be safer and the operating cost can be reduced. At present, the chlorine production of domestic chlorine generators is too small and expensive. Therefore, the chlorination method is generally suitable for the treatment of water supply, and it is not suitable for the treatment of ammonia nitrogen wastewater with large water volume and high concentration.

2.2 Chemical precipitation method The chemical precipitation method is to add a certain chemical agent to the water and react with the soluble substances in the water to form salts that are insoluble in water, forming a sediment that is easy to remove, thereby reducing the content of soluble substances in the water. When PO43- and Mg2+ ions are added to the wastewater containing NH4+, the following reaction will occur: NH4++PO43-+Mg2+→MgNH4PO4↓④Insoluble MgNH4PO4 precipitates are formed, so as to achieve the purpose of removing ammonia nitrogen in water. The common precipitants used are Mg(OH)2 and H3PO4, the suitable pH range is 9.0-11, and the mass ratio of H3PO4/Mg(OH)2 is 1.5-3.5. When the ammonia nitrogen concentration in the wastewater is less than 900mg/L, the removal rate is above 90%, and the sediment is a good compound fertilizer. Because the price of Mg(OH)2 and H3PO4 is relatively expensive and the cost is high, it is feasible to treat high-concentration ammonia nitrogen wastewater, but this method adds PO43- to the wastewater, which is easy to cause secondary pollution.

2.3 Ion exchange method The essence of ion exchange method is the exchange reaction of exchangeable ions on insoluble ionic compounds (ion exchangers) with other ions of the same nature in wastewater. It is a special adsorption process, usually reversible chemical adsorption. Zeolite is a natural ion exchange material, its price is much lower than that of cation exchange resin, and it has selective adsorption capacity for NH4+-N, and has high cation exchange capacity. The cation exchange capacity of pure mordenite and clinoptilolite is average. As amounts (m.e) equivalent to 213 and 223 mg of substance per 100 g. However, the actual natural zeolite contains impure substances, so the exchange capacity of zeolite with higher purity is not more than 200m.e per 100g, generally 100-150m.e. As an ion exchanger, zeolite has special ion exchange characteristics. The selection exchange order of ions is: Cs(Ⅰ)>Rb(Ⅰ)>K(Ⅰ)>NH4+>Sr(Ⅰ)>Na(Ⅰ)>Ca( Ⅱ)>Fe(Ⅲ)>Al(Ⅲ)>Mg(Ⅱ)>Li(Ⅰ). In engineering design applications, the pH value of wastewater should be adjusted to 6-9, and heavy metals generally have no effect; alkali metals and alkaline earth metals have influences except Mg, especially Ca on the ion exchange capacity of zeolite than Na and K. big. Zeolite must be regenerated after adsorption and saturation. The regeneration liquid method is mainly used, and the combustion method is rarely used. The regeneration solution mostly uses NaOH and NaCl. Because the wastewater contains Ca2+, the removal rate of zeolite to ammonia is irreversibly reduced, and supplementation and renewal should be considered.

2.4 Stripping method The stripping method is to adjust the wastewater to be alkaline, and then pass air or steam into the stripper, and strip the free ammonia in the wastewater to the atmosphere through gas-liquid contact. The introduction of steam can increase the temperature of the wastewater, thereby increasing the ratio of ammonia stripped off at a certain pH value. When using this method to treat ammonia, it should be considered that the total amount of free ammonia discharged should meet the atmospheric emission standard of ammonia, so as to avoid secondary pollution. Low-concentration wastewater is usually blown off with air at room temperature, while high-concentration wastewater from steelmaking, petrochemical, fertilizer, organic chemical non-ferrous metal smelting and other industries is usually blown off with steam.

2.5 Liquid film method Since Li Nianzhi discovered the emulsion liquid film in 1986, the liquid film method has been widely studied. Many people believe that the liquid membrane separation method may become the second generation of separation and purification technology after the extraction method, which is especially suitable for the purification of low-concentration metal ions and wastewater treatment. The mechanism of ammonia nitrogen removal by emulsion membrane method is: ammonia nitrogen NH3-N is easily soluble in the oil phase of the membrane phase, it migrates from the outer side of the membrane phase with high concentration, and reaches the interface between the inner side of the membrane phase and the inner phase through the diffusion and migration of the membrane phase. The dissociation reaction occurs with the acid in the inner phase of the membrane, and the generated NH4+ is insoluble in the oil phase and is stable in the inner phase of the membrane. Driven by the difference in ammonia concentration between the inside and outside of the membrane, the ammonia molecules continuously migrate to the surface of the membrane through adsorption, osmosis and diffusion. The inner side of the membrane phase is desorbed, so as to achieve the purpose of separating and removing ammonia nitrogen. 2.6 Electrodialysis Electrodialysis is a membrane separation technique that removes dissolved solids in an aqueous solution using a voltage applied between pairs of negative and positive membranes. A DC voltage is applied between the anion and cation permeable membranes in the electrodialysis chamber. When the influent water passes through multiple pairs of anion and cation permeation membranes, ammonium ions and other ions, under the influence of the applied voltage, pass through the membrane and enter the concentrated water on the other side and enter the concentrated water on the other side. The concentrated water is collected and thus separated from the feed water.