Summary of ammonia nitrogen wastewater treatment methods

There are many methods for treating ammonia nitrogen wastewater, such as breakpoint chlorination, chemical precipitation, ion exchange, stripping and biological deamination. These technologies can be divided into two categories: biological denitrification technology and physical and chemical methods.

1. Biological denitrification method

The process of microbial removal of ammonia nitrogen requires two stages. The first stage is the nitrification process, a process in which nitrosifying bacteria and nitrifying bacteria convert ammonia nitrogen into nitrite nitrogen and nitrate nitrogen under aerobic conditions. The second stage is the denitrification process. The nitrate nitrogen and nitrite nitrogen in the sewage are reduced and converted into nitrogen by denitrifying bacteria (heterotrophic and autotrophic microorganisms are found in many types) under anaerobic or low-oxygen conditions. . In this process, organic matter (methanol, acetic acid, glucose, etc.) serves as an electron donor and is oxidized to provide energy. Common biological denitrification processes can be divided into three categories, namely multi-stage sludge system, single-stage sludge system and biofilm system.

01Multi-stage sludge system

This process can achieve a very good BOD5 removal effect and denitrification effect. Its disadvantages are long process, many structures, high infrastructure costs, the need for additional carbon sources, high operating costs, and a certain amount of methanol remaining in the effluent.

02Unipolar sludge system

The single-stage sludge system includes pre-denitrification system, post-denitrification system and alternating working system. The biological denitrification process of pre-denitrification is usually called the A/O process. Compared with the traditional biological denitrification process, the A/O process has the advantages of simple process, fewer structures, low infrastructure costs, and no need for external carbon sources and effluent. High water quality and other advantages. Post-type denitrification systems generally require manual addition of carbon sources because the mixed liquid lacks organic matter, but the denitrification effect can be higher than that of the front-type denitrification system, and theoretically can approach 100% denitrification. The alternately working biological denitrification process mainly consists of two pools connected in series. By changing the direction of the water inlet and outlet, the two pools alternately operate under anoxic and aerobic conditions. This system is still an A/O system in essence, but it uses alternate working methods to avoid the backflow of the mixed liquid, so the denitrification effect is better than the general A/O process. The disadvantage is that the operation and management costs are high, and a computer-controlled automatic operating system must generally be configured.

03Biofilm system

The anoxic pool and aerobic pool in the above A/O system are changed into fixed biofilm reactors to form a biofilm denitrification system. There should be mixed liquid return in this system, but no sludge return is required. Two sludge systems adapted to denitrification and aerobic oxidation and nitrification reactions are stored in the anoxic aerobic reactor.

2. Physical and chemical nitrogen removal

Commonly used physical and chemical methods for physical and chemical nitrogen removal include breakpoint chlorination, chemical precipitation, ion exchange, stripping, liquid membrane, electrodialysis and catalytic wet oxidation.

01 Breakpoint chlorination method

The discontinuous point chlorination method is a kind of oxidation method to treat ammonia nitrogen wastewater. It is a chemical treatment method that uses ammonia in the water to react with chlorine to generate nitrogen and remove ammonia from the water. This method can also have a bactericidal effect and at the same time inorganicize some organic matter. However, residual chlorine remains in the effluent after chlorination and should be further dechlorinated.

Add hypochlorous acid HClO to water containing ammonia. When the pH value is near neutral, the following main reactions will gradually proceed with the addition of hypochlorous acid:

①NH3+HClO→NH2Cl+H2O

②NH2Cl+HClO→NHCl2+H2O

③NH2Cl+NHCl2→N2+3H++3Cl-

When the ratio of the amount of chlorine added to ammonia nitrogen (Cl/N for short) is below 5.07, the ① reaction is first carried out to generate monochloramine (NH2Cl), and the residual chlorine concentration in the water increases. Later, with the addition of hypochlorous acid As the amount increases, monochloramine reacts according to formula ② to generate dichloramine (NHCl2). At the same time, formula ③ reaction proceeds, and the N in the 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, the remaining hypochlorous acid (i.e., free residual chlorine) due to unreaction increases, and the residual chlorine in the water increases. The concentration of residual chlorine increases again, and this minimum point is called the discontinuity point (customarily called the breaking point). The Cl/N ratio at this time is theoretically calculated to be 7.6; in wastewater treatment, because chlorine reacts with 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, trichloramine is mostly generated under acidic conditions, and nitric acid is generated under alkaline conditions, reducing the denitrification efficiency.

When the pH value is 6 to 7, the dosage of chlorine per mg of ammonia nitrogen is 10 mg, and the contact time is 0.5 to 2.0 hours, the removal rate of ammonia nitrogen is 90% to 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 of each mg of ammonia nitrogen sometimes requires a breakpoint of 9 to 10 mg of chlorine. The effluent treated by the chlorination method generally needs to be dechlorinated with activated carbon or SO2 before discharge to remove residual chlorine in the water. Although the chlorination method reacts quickly and requires little investment in equipment, the safe use and storage of liquid chlorine requires high requirements and the processing cost is also high. If hypochlorous acid or chlorine dioxide generating devices are used instead of liquid chlorine, it will be safer and the operating costs can be reduced. The current domestic chlorine generating devices produce too little chlorine and are expensive. Therefore, the chlorination method is generally suitable for treating water supply, but is not suitable for treating large amounts of high-concentration ammonia nitrogen wastewater.

02Chemical precipitation method

The chemical precipitation method is to add certain chemicals to the water to react with the soluble substances in the water to generate 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 wastewater containing NH4+, the following reaction will occur:

NH4++PO43-+Mg2+→MgNH4PO4↓④ generates MgNH4PO4 precipitate that is insoluble in water, thereby achieving the purpose of removing ammonia nitrogen from water. The common precipitants used are Mg(OH)2 and H3PO4. The suitable pH value range is 9.0 to 11, and the dosage mass ratio H3PO4/Mg(OH)2 is 1.5 to 3.5. When the ammonia nitrogen concentration in wastewater is less than 900 mg/L, the removal rate is more than 90%, and the sediment is a good compound fertilizer. Since Mg(OH)2 and H3PO4 are relatively expensive and the cost is high, it is feasible to treat high-concentration ammonia nitrogen wastewater. However, this method adds PO43- to the wastewater, which can easily cause secondary pollution.

03Ion exchange method

The essence of the ion exchange method is the exchange reaction between exchangeable ions on insoluble ionic compounds (ion exchangers) and other same-sex ions 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. 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. It is the amount of substance equivalent to 213 and 223mg per 100g (me). However, actual natural zeolites contain impure substances, so the exchange capacity of higher purity zeolites is not greater than 200m.e per 100g, generally 100 to 150m.e. As an ion exchanger, zeolite has special ion exchange characteristics. The selective exchange sequence 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 and application, the pH value of wastewater should be adjusted to 6 to 9. Heavy metals generally have no impact; alkali metals and alkaline earth metals have an impact except Mg, especially Ca, which has a greater impact 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 solutions mostly use NaOH and NaCl. Since the wastewater contains Ca2+, the ammonia removal rate of zeolite is irreversibly reduced, so supplementation and renewal should be considered.

04blow-off method

The stripping method is to adjust the wastewater to alkalinity, then introduce air or steam into the stripping tower, and strip the free ammonia in the wastewater to the atmosphere through gas-liquid contact. Introducing steam can increase the temperature of the wastewater, thereby increasing the rate of ammonia blown off at a certain pH value. When using this method to treat ammonia, it should be considered that the total amount of free ammonia emitted should meet the atmospheric emission standards of ammonia to avoid secondary pollution. Low-concentration wastewater is usually blown off with air at room temperature, while high-concentration wastewater in industries such as steelmaking, petrochemicals, fertilizers, organic chemicals and non-ferrous metal smelting is often blown off with steam.

05Liquid film method

Since Li Nianzhi discovered 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 separation and purification technology after the extraction method, especially suitable for processes such as 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 high-concentration outside of the membrane phase to the interface between the inner side of the membrane phase and the internal phase through diffusion of the membrane phase. A detachment reaction occurs with the acid in the inner phase of the membrane. 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, ammonia molecules continue to migrate to the membrane through adsorption, permeation and diffusion on the membrane surface. The inside of the membrane phase is desorbed to achieve the purpose of separating and removing ammonia nitrogen.

06Electrodialysis

Electrodialysis is a membrane separation technology that uses a voltage applied between a pair of male and female membranes to remove dissolved solids from an aqueous solution. A DC voltage is applied between the anode and cation permeable membranes in the electrodialysis chamber. When the incoming water passes through multiple pairs of anion and cation permeable membranes, ammonium ions and other ions pass through the membrane and enter the concentrated water on the other side under the influence of the applied voltage. It accumulates in concentrated water and thus separates from the incoming water.

Source: Environmentally Friendly Bee