Environment Technology

Brief Analysis of Ammonia Nitrogen Wastewater Treatment Technology

2019-1-31

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With the development of industrial and agricultural production and the improvement of people's living standards, the emission of nitrogen compounds has increased dramatically, which has become the main source of environmental pollution and attracted attention from all walks of life. Economic and effective control of ammonia nitrogen wastewater pollution has become a major issue facing today's environmental workers.

1. Sources of ammonia-nitrogen wastewater
Nitrogen-containing substances enter the water environment mainly through natural processes and human activities. The natural sources and processes of nitrogen-containing substances entering the water environment mainly include precipitation and dust fall, non-urban runoff and biological nitrogen fixation. Human activities are also an important source of nitrogen in the aquatic environment, including untreated or untreated urban living and industrial wastewater, leachate and surface runoff. Synthetic chemical fertilizers are the main source of nitrogen nutrients in water. Most of the nitrogen compounds that are not used by crops are brought into groundwater and surface water by farmland drainage and surface runoff. With the development of petroleum, chemical, food and pharmaceutical industries and the continuous improvement of people's living standards, the ammonia nitrogen content in municipal domestic sewage and landfill leachate has risen sharply. In recent years, with the development of economy, more and more nitrogen-containing pollutants are discharged arbitrarily, which has caused great harm to the environment. Nitrogen exists in wastewater in the form of organic nitrogen, ammonia nitrogen (NH4 +-N), nitrate nitrogen (NO3-N) and nitrite nitrogen (NO2-N), and ammonia nitrogen is one of the main forms. Ammonia nitrogen in wastewater refers to nitrogen in the form of free ammonia and ionic ammonium. It mainly comes from the decomposition of nitrogen-containing organic matter in domestic sewage, industrial wastewater such as coking, synthetic ammonia, and farmland drainage. There are many sources of ammonia nitrogen pollution, and the discharge amount is large, and the concentration of ammonia nitrogen emission varies greatly.

2. Hazard of Ammonia Nitrogen Wastewater
Excessive ammonia nitrogen in water environment can cause many harmful effects.
(1) Because of the oxidation of NH4 +-N, the concentration of dissolved oxygen in water will decrease, which will cause the water body to become black and smelly, and the water quality will decline, which will affect the survival of aquatic animals and plants. Under favourable environmental conditions, organic nitrogen in wastewater will be converted to NH4 +-N. NH4 +-N is the most powerful form of inorganic nitrogen, which will further convert to NO2-N and NO3-N. According to the quantitative relationship of biochemical reactions, the oxidation of 1G NH4+-N to NO2-N consumes 3.43g of oxygen and 4.57g of NO3-N.

(2) Too much nitrogen in water will lead to eutrophication, which will lead to a series of serious consequences. Due to the presence of nitrogen, the number of photosynthetic microorganisms (mostly algae) increases, i.e. eutrophication of water bodies. As a result, the blockage of the filter results in a shorter operation period of the filter, which increases the cost of water treatment; impedes water movement; the final product of algae metabolism can produce compounds that cause colour and taste; and the toxins produced by blue-green algae, livestock. Damage results in death of fish, and oxygen deficiency occurs in water due to decay of algae.

(3) Nitrogen dioxide-N and nitrate-N in water are harmful to human and aquatic organisms. Long-term drinking of water containing more than 10 mg/L of NO3-N will lead to methemoglobinemia. When the content of methemoglobin in blood reaches 70 mg/L, asphyxia will occur. Nitrosamines are produced by the interaction of N O2-N and amines in water, and nitrosamines are the "three-cause" substances. The reaction of NH4 +-N with chlorine produces chloramines. The disinfection effect of chloramines is less than that of free chlorine. Therefore, when NH4 +-N exists, the water treatment plant will need a larger amount of chlorination.

Increase processing costs. In recent years, human and animal drinking water difficulties and even poisoning incidents caused by the random discharge of ammonia-containing wastewater have occurred from time to time. The Yangtze River, Huaihe River, Qiantang River and Tuojiang River basins in Sichuan Province have been reported in China. Accordingly, there have been major incidents such as the difficulty of drinking water for millions of residents caused by cyanobacteria pollution and the "involvement" of related waters. Therefore, ammonia in wastewater has been removed. Nitrogen has become one of the hotspots of environmental workers.

3. Main Technologies of Ammonia Nitrogen Wastewater Treatment
At present, there are many methods to treat ammonia-nitrogen wastewater at home and abroad, such as break point chlorination, chemical precipitation, ion exchange, stripping and biological denitrification. These technologies can be divided into physical-chemical and biological denitrification technologies.

3.1 Biological Denitrification
The removal process of ammonia nitrogen by microorganisms needs two stages. The first stage is nitrification. Nitrifying bacteria and nitrifying bacteria convert ammonia nitrogen into nitrite nitrogen and nitrate nitrogen under aerobic conditions. The second stage is denitrification process. Nitrate and nitrite in sewage are reduced to nitrogen by denitrifying bacteria (heterotrophic and autotrophic microorganisms) under the condition of no or low oxygen. In this process, organic compounds (methanol, acetic acid, glucose, etc.) are oxidized as electron donors to provide energy. Common biological denitrification processes can be divided into three categories: multistage sludge system, single-stage sludge system and biofilm system.

3.1.1 Multistage Sludge System

This process can get quite good BOD5 removal effect and denitrification effect. Its shortcomings are long process, many structures, high capital construction cost, the need for additional carbon sources, high operating costs, residual methanol in effluent, etc.

3.1.2 Single-stage Sludge System
Single-stage sludge system includes pre-denitrification system, post-denitrification system and alternating working system. Compared with the traditional biological denitrification process, the pre-denitrification biological denitrification process is usually called A/O process. A/O process has the advantages of simple process, fewer structures, low capital construction cost, no additional carbon source and high water quality. Postposition denitrification system, because of the lack of organic matter in the mixed liquor, usually requires artificial carbon source, but the effect of denitrification can be higher than that of the former, which can theoretically approach 100% denitrification. The alternating biological denitrification process is mainly composed of two series pools. By changing the direction of water inflow and effluent, the two pools operate alternately under anoxic and aerobic conditions. The system is still a A/O system in essence, but it avoids the reflux of mixed liquor by alternating working mode, so the denitrification effect is better than the general A/O process. The disadvantage is that the cost of operation and management is high, and the computer-controlled automatic operating system must be generally configured.

3.1.3 Biofilm System
The anaerobic tank and aerobic tank in the A/O system were replaced by fixed biofilm reactor to form a biofilm denitrification system. There should be mixed liquor reflux in this system, but no sludge reflux is needed. Two sludge systems suitable for denitrification, aerobic oxidation and nitrification are preserved in anaerobic aerobic reactor.

3.2 Physicochemical Nitrogen Removal
Physicochemical methods for nitrogen removal include break point chlorination, chemical precipitation, ion exchange, stripping, liquid membrane, electrodialysis and catalytic wet oxidation.

3.2.1 folding point chlorination method
Discontinuous point chlorination is one of the oxidation methods to treat ammonia nitrogen wastewater. It is a chemical treatment method to remove ammonia from water by reacting ammonia with chlorine to produce nitrogen gas. This method can also play a bactericidal role and inorganic part of the organic matter, but the chlorinated effluent leaves residual chlorine, which should be further dechlorinated.

HClO hypochlorite was added to the water containing ammonia. When the pH value was near neutral, the following main reactions were gradually carried out with the addition of hypochlorite:

NH3+HClO_NH2Cl+H2O
NH2Cl+HClO_NHCl2+H2O II
NH2Cl+NHCl2_N2+3H++ 3Cl-3

When the ratio of chlorine dosage to ammonia-nitrogen (Cl/N) is below 5.07, the first step is to react in the form of 1 to produce chloramine (NH2Cl). The residual chlorine concentration in water increases. Then, with the increase of hypochlorite dosage, monochloroamine reacts in the form of 2 to produce dichloroamine (NHCl2), while in the form of 3, N in water is removed. As a result, the residual chlorine concentration in water decreases with the increase of Cl/N. When the Cl/N ratio reaches a certain value, the residual hypochlorite (i.e. free residual chlorine) increases due to unreacted reaction, and the residual chlorine concentration in water increases again. The minimum point is called discontinuity point (commonly referred to as breaking point). The ratio of C1/N is 7.6 according to the theoretical calculation. In wastewater treatment, the ratio of C1/N is higher than the theoretical value of 7.6, usually 10, because chlorine reacts with organic matter in wastewater. In addition, when the pH is not in the neutral range, trichloroamines are formed in acidic conditions, and nitric acid is formed in alkaline conditions, which reduces the denitrification efficiency.

The removal rate of ammonia nitrogen was 90%-100% when the pH value was 6-7, the dosage of ammonia nitrogen and chlorine per mg was 10 mg and the exposure time was 0.5-2.0 H. 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. Oxygenation of ammonia nitrogen per mg sometimes requires a chlorine breakpoint of 9-10 mg. Activated carbon or SO2 are usually used to reverse chlorination of the treated effluent before discharge to remove residual chlorine in the water. Although chlorination reacts quickly and requires less investment in equipment, the safe use and storage requirements of liquid chlorine are high, and the cost of treatment is also high. If the hypochlorite or chlorine dioxide generator is used instead of liquid chlorine, it will be safer and the operation cost can be reduced. At present, the chlorine production of domestic chlorine generator is too small and expensive. Therefore, chlorination method is generally suitable for the treatment of feed water, but not suitable for the treatment of large amount of high concentration ammonia nitrogen wastewater.

3.2.2 Chemical Precipitation Method

The chemical precipitation method is to add some chemical agent into the water, react with the soluble substances in the water, and produce salt which is insoluble in the water, forming sediment which is easy to remove, thus reducing the content of soluble substances in the water. When PO43-and Mg2+ ions are added into wastewater containing NH4+, the following reactions occur:

NH4++ PO43-+Mg2+MgNH4PO4____4 forms MgNH4PO4 precipitate which is insoluble in water, thus achieving the purpose of removing ammonia nitrogen in water. The common precipitators used are Mg (OH) 2 and H3PO4. The suitable pH value ranges from 9.0 to 11, and the mass ratio of H3PO4/Mg (OH) 2 is 1.5 to 3.5. When the concentration of ammonia nitrogen in wastewater is less than 900 mg/L, the removal rate is over 90%. Sediment is a good compound fertilizer. Because Mg(OH)2 and H3PO4 are expensive and costly, it is feasible to treat high concentration ammonia-nitrogen wastewater. However, PO43-, which is added to wastewater, is liable to cause secondary pollution.

3.2.3 Ion Exchange Method
The essence of ion exchange method is the exchange reaction between exchangeable ions on insoluble ion compounds (ion exchangers) and other iso-ions in wastewater. It is a special adsorption process, usually reversible chemical adsorption. Zeolite is a kind of natural ion exchange material. Its price is much lower than that of cation exchange resin. It has selective adsorption capacity for NH4+-N and high cation exchange capacity. The average cation exchange capacity of pure mordenite and clinoptilolite is equivalent to 213 and 223 mg of substance per 100 g (m.e). However, the actual natural zeolite contains impurities, so the exchange capacity of high purity zeolite is less than 200 m. E per 100 g, generally 100-150 M. E. Zeolite as ion exchanger has special ion exchange characteristics. The order of ion exchange 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 6-9. Heavy metals have little effect on the whole. Alkali metals and alkaline earth metals have effects except Mg, especially Ca has greater influence on the ion exchange capacity of zeolites than Na and K. Zeolite must be regenerated after saturated adsorption, mainly by regeneration liquid method, and combustion method is seldom used. NaOH and NaCl are mostly used in regeneration solution. Because the wastewater contains Ca2+, the removal rate of ammonia by zeolite decreases irreversibly. It is necessary to consider supplementation and renewal.

3.2.4 stripping method
The stripping process is to adjust the wastewater to alkalinity, then air or steam is injected into the stripper, and free ammonia in the wastewater is stripped to the atmosphere through gas-liquid contact. When steam is introduced, the temperature of wastewater can be raised, thus the ratio of ammonia blown off at a certain pH value can be increased. When treating ammonia with this method, it is necessary to consider that the total amount of free ammonia discharged should conform to the atmospheric emission standard of ammonia in order to avoid secondary pollution. Low concentration wastewater is usually stripped by air at room temperature, while high concentration wastewater from steelmaking, petrochemical, fertilizer, organic chemical and non-ferrous metal smelting industries is often stripped by steam.

3.2.5 Liquid Membrane Method
Since the discovery of emulsion liquid membrane by Li Nianzhi in 1986, liquid membrane method has been widely studied. Many people believe that liquid membrane separation may become the second generation of separation and purification technology after extraction, especially for the purification of low concentration metal ions and wastewater treatment. The mechanism of ammonia removal by emulsion liquid membrane process is that ammonia nitrogen NH3-N is soluble in the membrane oil phase. It migrates from the outer side of the membrane phase to the inner side of the membrane phase through diffusion and migration of the membrane phase, and reacts with the acid in the inner phase of the membrane. The NH4+ produced is insoluble in the oil phase and stable in the inner phase of the membrane. Under the impetus of the difference of ammonia concentration between the inner and outer sides of the membrane, ammonia molecules pass through the membrane continuously. Surface adsorption and osmotic diffusion migrate to the inside of the membrane phase for desorption, so as to achieve the purpose of separating and removing ammonia nitrogen.

3.2.6 Electrodialysis
Electrodialysis is a membrane separation technology, which removes dissolved solids from aqueous solution by applying a voltage between a pair of positive and negative membranes. When the influent water passes through many pairs of anionic and cationic osmosis membranes, ammonium ions and other ions enter the concentrated water on the other side through the membrane and collect in the concentrated water, so they are separated from the influent water.

3.2.7 Catalytic Wet Air Oxidation
Catalytic wet oxidation is a new technology for wastewater treatment developed in the 1980s. Under the action of certain temperature, pressure and catalyst, the organic matter and ammonia in sewage can be oxidized and decomposed into harmless substances such as CO2, N2 and H2O by air oxidation, so as to achieve the purpose of purification. The method has the characteristics of high purification efficiency (the wastewater can reach the drinking water standard after purification), simple process and less floor area. After many years of application and practice, the construction and operation cost of this wastewater treatment method is only about 60% of the conventional method, so it has strong competitiveness in technology and economy.

4 Conclusion
Various technologies and processes of ammonia nitrogen wastewater degradation at home and abroad have their own advantages and disadvantages. Because of the differences in the nature of different wastewater, there is no universal method to treat all ammonia nitrogen wastewater. Therefore, it is necessary to make a thorough and systematic study on the characteristics of wastewater in different industrial processes and the composition of wastewater, and to select and determine the treatment technology and process. At present, biological denitrification method is mainly used for the treatment of chemical wastewater and domestic wastewater with low concentration of ammonia nitrogen containing organic matter. This method has reliable technology and good treatment effect. In recent years, membrane separation technology and catalytic wet oxidation technology, which are widely used in the treatment of high concentration ammonia nitrogen wastewater, have a good application prospect.

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