How to choose ammonia nitrogen wastewater treatment technology?
With the development of industrial and agricultural production and the improvement of people's living standards, the emission of nitrogen-containing compounds has increased sharply, which has become the main pollution source of the environment and has attracted attention from all walks of life. Economical and effective control of ammonia nitrogen wastewater pollution has become a major issue faced by today's environmental workers.
1. The source of ammonia nitrogen wastewater
The way that nitrogen-containing substances enter the water environment mainly includes two aspects: natural processes and human activities. The natural sources and processes of nitrogen-containing substances entering the water environment mainly include precipitation and dustfall, non-urban runoff and biological nitrogen fixation. Synthetic chemical fertilizers are the main source of nitrogen nutrients in water bodies. In recent years, with the development of the economy, more and more arbitrary discharge of nitrogen-containing pollutants has caused great harm to the environment. Nitrogen exists in various forms such as organic nitrogen, ammonia nitrogen (NH4+-N), nitrate nitrogen (NO3--N) and nitrite nitrogen (NO2--N) in wastewater, and ammonia nitrogen is the main form. one of the forms of existence.
2. The harm of ammonia nitrogen wastewater
Excessive ammonia nitrogen in the water environment can cause many harmful effects:
(1) Due to the oxidation of NH4+-N, the concentration of dissolved oxygen in the water body will decrease, resulting in the water body turning black and smelly, and the water quality decreasing, which will affect the survival of aquatic animals and plants. .
(2) Too much nitrogen content in water will lead to eutrophication of water body, which will cause a series of serious consequences.
(3) NO2--N and NO3--N in water have a great harmful effect on human and aquatic organisms. The action of NO2--N in water and amines will generate nitrosamines, and nitrosamines are "three uniform" substances.
3. Main technologies of ammonia nitrogen wastewater treatment
At present, there are many methods for ammonia nitrogen wastewater treatment at home and abroad, such as breakpoint chlorination method, chemical precipitation method, ion exchange method, stripping method and biological deamination method. These technologies can be divided into two categories: physical chemical method and biological denitrification technology.
biological nitrogen removal
The process of microbial removal of ammonia nitrogen needs to go through two stages. The stage is the nitrification process, in which nitrifying bacteria and nitrifying bacteria convert ammonia nitrogen into nitrite nitrogen and nitrate nitrogen under aerobic conditions. The second stage is the denitrification process. Nitrate nitrogen and nitrite nitrogen in sewage are reduced and converted into nitrogen by denitrifying bacteria (heterotrophic and autotrophic microorganisms are found and many types) under anaerobic or hypoxic conditions. . Common biological denitrification processes can be divided into three categories, namely multi-stage sludge system, single-stage sludge system and biofilm system.
Multistage sludge system
This process can get quite good BOD5 removal effect and denitrification effect, but its disadvantages are long process, many structures, high infrastructure cost, need for additional carbon source, high operating cost, and a certain amount of methanol remaining in the effluent.
Single stage sludge system
The form of single-stage sludge system includes pre-denitrification system, post-denitrification system and alternate working system. 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 disadvantage is that the operation and management costs are relatively high, and a computer-controlled automatic operating system must generally be configured.
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.
Physicochemical nitrogen removal
The commonly used physical and chemical methods for physicochemical nitrogen removal include breakpoint chlorination, chemical precipitation, ion exchange, and stripping.
Breakpoint Chlorination
The 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. Add hypochlorous acid HClO to the water containing ammonia. When the pH value is near neutral, the following main reactions are gradually carried out with the addition of hypochlorous acid:
NH3 + HClO →NH2Cl + H2O ①
NH2Cl + HClO → NHCl2 + H2O ②
NH2Cl + NHCl2 →N2 + 3H+ + 3Cl- ③
When the ratio of the amount of chlorine added to ammonia nitrogen (abbreviated as Cl/N) is below 5.07, the reaction of formula ① is firstly performed to generate monochloramine (NH2Cl), and the residual chlorine concentration in the water increases, and then, with the addition of hypochlorous acid As the amount increases, 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 small value is called the discontinuous point (used to be called the inflection point). The Cl/N ratio at this time is theoretically 7.6;
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.
chemical precipitation
The chemical precipitation method is to add a certain chemical agent to the water, which reacts with the soluble substances in the water to form salts that are insoluble in water, and forms 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 reactions occur:
NH4+ + PO43- + Mg2+ → MgNH4PO4↓ ④ Generate insoluble MgNH4PO4 precipitates, so as to achieve the purpose of removing ammonia nitrogen in water. 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.
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 sex 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 a selective adsorption capacity for NH4+-N, and has a high cation exchange capacity. 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). 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.
blow-off method
The stripping method is to adjust the wastewater to alkalinity, and then pass air or steam into the stripping tower, 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.
