Ammonia nitrogen wastewater treatment technology: diverse options, each with its advantages

Currently, the problem of exceeding the standard for ammonia nitrogen in wastewater is receiving attention, and related treatment technologies are emerging in large numbers. Biological denitrification, physicochemical denitrification, breakpoint chlorination, chemical precipitation, ion exchange, and stripping all have their own advantages.

　　With the development of industrial and agricultural production and the improvement of people's living standards, the emission of nitrogen-containing compounds has increased dramatically, becoming a major source of environmental pollution and attracting widespread attention. Economically and effectively controlling ammonia nitrogen wastewater pollution has become a major challenge facing environmental workers today.

　　1 Sources of Ammonia Nitrogen Wastewater
　　The pathways of nitrogen-containing substances entering the water environment mainly include natural processes and human activities. The natural sources and processes of nitrogen-containing substances entering the water environment mainly include precipitation and dust deposition, non-urban runoff, and biological nitrogen fixation. Artificially synthesized chemical fertilizers are the main source of nitrogen nutrients in water bodies, and most of the nitrogen compounds not utilized by crops are carried into groundwater and surface water by farmland drainage and surface runoff. In recent years, with economic development, the arbitrary discharge of more and more nitrogen-containing pollutants has caused great harm to the environment. Nitrogen in wastewater exists in various forms, including organic nitrogen, ammonia nitrogen (NH4+-N), nitrate nitrogen (NO3--N), and nitrite nitrogen (NO2--N), with ammonia nitrogen being one of the main forms. Ammonia nitrogen in wastewater refers to nitrogen that exists in the form of free ammonia and ammonium ions, mainly from the decomposition of nitrogen-containing organic matter in domestic sewage, coking, synthetic ammonia and other industrial wastewater, and farmland drainage. Ammonia nitrogen pollution sources are numerous, the discharge volume is large, and the discharge concentration varies greatly.

　　2 Hazards of Ammonia Nitrogen Wastewater
　　Excessive ammonia nitrogen in the water environment can cause various harmful effects:
　　(1) Due to the oxidation of NH4+-N, the dissolved oxygen concentration in the water body will decrease, causing the water body to turn black and smelly, water quality degradation, and affecting the survival of aquatic plants and animals. Under favorable environmental conditions, the organic nitrogen contained in wastewater will be converted into NH4+-N, which is a strongly reducing inorganic nitrogen form and will be further converted into NO2--N and NO3--N. According to the stoichiometric relationship of biochemical reactions, 1g NH4+-N oxidized to NO2--N consumes 3.43g of oxygen, and oxidation to NO3--N consumes 4.57g of oxygen.

　　(2) Too much nitrogen in the water will lead to eutrophication of the water body, which will in turn cause a series of serious consequences. Due to the presence of nitrogen, the number of photosynthetic microorganisms (mostly algae) increases, that is, the water body undergoes eutrophication, resulting in: clogging of filter pools, shortening the operation cycle of filter pools, thus increasing the cost of water treatment; hindering water sports; metabolic end products of algae can produce compounds that cause color and odor; due to toxins produced by blue-green algae, livestock damage, fish death; due to the decay of algae, oxygen deficiency occurs in the water body.

　　(3) NO2--N and NO3--N in water have significant harmful effects on humans and aquatic organisms. Long-term drinking water with NO3--N content exceeding 10mg/L will cause methemoglobinemia. When the methemoglobin content in the blood reaches 70mg/L, suffocation will occur. NO2--N and amines in water will react to produce nitrosamines, which are "three-carcinogenic" substances. NH4+-N and chlorine react to produce chloramine, which has a smaller disinfection effect than free chlorine. Therefore, when NH4+-N is present, water treatment plants will require a larger amount of chlorination, thus increasing treatment costs.

3 Main Technologies for Ammonia Nitrogen Wastewater Treatment
　　Currently, domestic and foreign ammonia nitrogen wastewater treatment methods include breakpoint chlorination, chemical precipitation, ion exchange, stripping, and biological deamination. These technologies can be divided into two categories: physicochemical methods and biological nitrogen removal technologies.
Biological Nitrogen Removal
　　The microbial removal of ammonia nitrogen process requires two stages. The first stage is the nitrification process, which is the process by which nitrosomonas and nitrobacter convert ammonia nitrogen into nitrite nitrogen and nitrate nitrogen under aerobic conditions. The second stage is the denitrification process, in which nitrate nitrogen and nitrite nitrogen in wastewater are reduced and converted into nitrogen gas by denitrifying bacteria (heterotrophic and autotrophic microorganisms have been found, and there are many types) under anaerobic or hypoxic conditions. In this process, organic matter (methanol, acetic acid, glucose, etc.) is oxidized as an electron donor to provide energy. Common biological nitrogen removal processes can be divided into three categories: multi-stage sludge systems, single-stage sludge systems, and biofilm systems.
　　Multi-stage Sludge System
　　This process can achieve quite good BOD5 removal and nitrogen removal effects. Its disadvantages are long process, many structures, high construction cost, need to add carbon source, high operating cost, and a certain amount of methanol remaining in the effluent.
　　Single-stage Sludge System
　　Single-stage sludge systems include pre-denitrification systems, post-denitrification systems, and alternating operation systems. The pre-denitrification biological nitrogen removal process is usually called A/O process. Compared with the traditional biological nitrogen removal process, the A/O process has the advantages of simple process, few structures, low construction cost, no need to add carbon source, and high effluent water quality. For the post-denitrification system, because the mixed liquor lacks organic matter, it generally needs to add carbon source artificially, but the nitrogen removal effect can be higher than the pre-denitrification system, theoretically approaching 100% nitrogen removal. The alternating biological nitrogen removal process mainly consists of two series of pools. By changing the direction of inflow and outflow, the two pools alternately operate under anoxic and aerobic conditions. This system is essentially still an A/O system, but it uses an alternating operation method to avoid the reflux of mixed liquor, so the nitrogen removal effect is better than the general A/O process. Its disadvantages are higher operation and management costs, and generally a computer-controlled automatic operating system must be configured.
　　Biofilm System
　　The above A/O system's anoxic and aerobic pools are changed to fixed biofilm reactors, forming a biofilm denitrification system. This system should have mixed liquor reflux, but no sludge reflux, and two sludge systems adapted to denitrification and aerobic oxidation and nitrification reactions are preserved in the anoxic and aerobic reactors.

Physical-chemical nitrogen removal
　　Common physical and chemical methods for physical-chemical nitrogen removal include breakpoint chlorination, chemical precipitation, ion exchange, stripping, liquid membrane, electrodialysis, and catalytic wet oxidation.
　　Breakpoint chlorination
　　Breakpoint chlorination is an oxidation method for treating ammonia nitrogen wastewater. It is a chemical treatment method that removes ammonia in water by using the reaction between ammonia and chlorine in water to generate nitrogen gas. This method can also sterilize and mineralize some organic matter, but the effluent after chlorination contains residual chlorine, and further dechlorination treatment is required.
　　Adding hypochlorous acid to ammonia-containing water HClO, when the pH value is near neutral, with the addition of hypochlorous acid, the following main reactions gradually take place:
　　NH3 + HClO →NH2Cl + H2O ①
　　NH2Cl + HClO → NHCl2 + H2O ②
　　NH2Cl + NHCl2 →N2 + 3H+ + 3Cl- ③
　　Ratio of chlorine dosage to ammonia nitrogen (referred to as Cl/N) When it is below 5.07, the reaction ① is carried out first, generating monochloramine (NH2Cl), and the residual chlorine concentration in the water increases. Afterwards, with the increase of hypochlorous acid dosage, monochloramine reacts according to the formula ② to generate dichloramine (NHCl2), and the reaction ③ is carried out at the same time, and 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 unreacted hypochlorous acid (i.e., free residual chlorine) increases, and the concentration of residual chlorine in the water increases again. This point is called the breakpoint (usually called the breakpoint). The Cl/N ratio at this time is theoretically calculated as 7.6; in wastewater treatment, because chlorine reacts with organic matter in 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 generated more under acidic conditions, and nitric acid is generated under alkaline conditions, reducing the nitrogen removal efficiency.
　　In the case of pH value of 6-7, chlorine dosage of 10mg per mg of ammonia nitrogen, and contact time of 0.5-2.0h, 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 the temperature, pH and ammonia nitrogen concentration. Sometimes 9-10mg of chlorine is needed to oxidize per mg of ammonia nitrogen. The effluent after chlorination treatment generally needs to be dechlorinated with activated carbon or SO2 before discharge to remove the residual chlorine in the water. Although the chlorination method reacts quickly and requires less equipment investment, the safe use and storage of liquid chlorine are required, and the treatment cost is also higher. If a hypochlorous acid or chlorine dioxide generator is used instead of liquid chlorine, it will be safer and the operating cost can be reduced. At present, the chlorine production capacity of domestic chlorine generators is too small and the price is expensive. Therefore, the chlorination method is generally suitable for water treatment and is not suitable for treating large-volume and high-concentration ammonia nitrogen wastewater.

Chemical precipitation
　　Chemical precipitation is a method of adding a certain chemical reagent to water to react with soluble substances in water to form salts that are difficult to dissolve in water, forming sediment that is easy to remove, thereby reducing the content of soluble substances in water. When PO43- and Mg2+ ions are added to wastewater containing NH4+, the following reaction occurs:
　　NH4+ + PO43- + Mg2+ → MgNH4PO4↓ ④ Insoluble MgNH4PO4 precipitate is generated, thereby achieving the purpose of removing ammonia nitrogen in water. The commonly used precipitants are Mg(OH)2 and H3PO4, and 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 wastewater is less than 900mg/L, the removal rate is above 90%, and the precipitate 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 can cause secondary pollution.

Ion exchange
　　The essence of ion exchange is insoluble ionic compounds (ion exchangers) The exchange reaction between exchangeable ions and other homotypic ions in wastewater is a special adsorption process, which is 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, with high cation exchange capacity. The average cation exchange capacity of pure mordenite and clinoptilolite is 213 and 223 mg of substance amount (m.e) per 100g. However, natural zeolite contains impurities, so the exchange capacity of high-purity zeolite is not more than 200 m.e per 100g, generally 100-150 m.e. Zeolite, as an ion exchanger, has special ion exchange characteristics, and the selective 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 little effect; among alkali metals and alkaline earth metals, except for Mg, they all have an effect, especially Ca has a greater effect on the ion exchange capacity of zeolite than Na and K. After zeolite adsorption is saturated, it must be regenerated, mainly using regeneration liquid method, and combustion method is rarely used. The regenerant is mostly NaOH and NaCl. Due to the presence of Ca2+ in the wastewater, the ammonia removal rate of zeolite is irreversibly reduced, and supplementation and renewal should be considered.

Stripping
　　Stripping is a method in which wastewater is adjusted to alkaline, and then air or steam is introduced into the stripping tower. Through gas-liquid contact, free ammonia in wastewater is stripped into the atmosphere. Introducing steam can increase the wastewater temperature, thereby improving a certain The ratio of ammonia stripped at a given pH value. When treating ammonia with this method, the total amount of free ammonia emitted should comply with the atmospheric emission standards for ammonia to avoid secondary pollution. Low-concentration wastewater is usually stripped with air at room temperature, while high-concentration wastewater from industries such as steelmaking, petrochemicals, fertilizers, organic chemicals, and non-ferrous metal smelting often uses steam for stripping.

Liquid Membrane Method
　　Many believe that liquid membrane separation may become the second-generation separation and purification technology after extraction, especially suitable for the purification of low-concentration metal ions and wastewater treatment processes. The mechanism of removing ammonia nitrogen by emulsion liquid membrane is: Ammonia nitrogen NH3-N is easily soluble in the oil phase of the membrane. It migrates from the high-concentration outer side of the membrane phase through diffusion to the inner side of the membrane phase and the interface with the inner phase, where it undergoes a stripping reaction with the acid in the inner phase. The generated NH4+ is insoluble in the oil phase and remains stable in the inner phase. Driven by the difference in ammonia concentration on both sides of the membrane, ammonia molecules continuously adsorb, permeate, and diffuse to the inner side of the membrane phase for desorption, thus achieving the purpose of separating and removing ammonia nitrogen.

Electrodialysis
　　Electrodialysis is a membrane separation technology that uses the voltage applied between the anion and cation exchange membranes to remove dissolved solids from aqueous solutions. A direct current voltage is applied between the anion and cation exchange membranes in the electrodialysis cell. When the influent passes through multiple pairs of anion and cation exchange membranes, ammonium ions and other ions, under the influence of the applied voltage, pass through the membrane into the concentrate on the other side and concentrate there, thus being separated from the influent.

Catalytic Wet Oxidation
　　Catalytic wet oxidation is a new wastewater treatment technology developed internationally in the 1980s. Under certain temperature, pressure, and catalyst conditions, air oxidation can oxidize and decompose organic matter and ammonia in wastewater into harmless substances such as CO2, N2, and H2O, achieving purification. This method has the characteristics of high purification efficiency (wastewater can reach drinking water standards after purification), simple process, and small land occupation. After years of application and practice, the construction and operation costs of this wastewater treatment method are only about 60% of conventional methods, so it has strong competitiveness in both technology and economy.