Treatment of high-concentration ammonia-nitrogen wastewater

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The discharge of excessive ammonia nitrogen into water bodies leads to eutrophication, reducing their aesthetic value. Moreover, the oxidized products—*azoates and sub-*azoates—can adversely affect the health of aquatic organisms and even humans. Therefore, wastewater denitrification treatment has received widespread attention. Currently, the main denitrification methods include biological nitrification-denitrification, breakpoint chlorination, air stripping, and ion exchange. Digested sludge dewatering liquid, landfill leachate, wastewater from catalyst manufacturing plants, meat processing wastewater, and synthetic ammonia production wastewater all contain extremely high concentrations of ammonia nitrogen (over 500 mg/L; in some cases, even several thousand mg/L). The aforementioned methods are limited in their application due to factors such as the biological inhibitory effect of free ammonia nitrogen or high costs. The treatment methods for high-concentration ammonia-nitrogen wastewater can be classified into physical-chemical methods, combined physicochemical and biological methods, and novel biological denitrification methods. 1 Physicochemical methods 1.1 Stripping method: A method of separation that utilizes the gas-liquid equilibrium relationship between the vapor concentration and liquid concentration of ammonia nitrogen under alkaline conditions. It is generally believed that the stripping efficiency is related to temperature, pH, and gas-liquid ratio. 　　Wang Wenbin et al. studied the removal of ammonia nitrogen from landfill leachate by the stripping method. The key factors controlling the stripping efficiency are temperature, gas-liquid ratio, and pH. At water temperatures above 25 °C, with a gas-liquid ratio maintained around 3500 and the pH of the leachate kept at around 10.5, a removal rate of over 90% can be achieved for waste leachate containing ammonia nitrogen concentrations as high as 2000–4000 mg/L. The stripping method has low efficiency in removing ammonia nitrogen at low temperatures. 　　Wang Youle et al. conducted treatment experiments using ultrasonic stripping technology on high-concentration ammonia nitrogen wastewater from fertilizer plants (such as 882 mg/L). The optimal process conditions are pH = 11, ultrasonic stripping time of 40 minutes, and a gas-to-water ratio of 1000:1. The test results show that ultrasonic irradiation of the wastewater significantly improves the efficiency of ammonia nitrogen stripping; compared with conventional stripping techniques, the removal rate of ammonia nitrogen increases by 17% to 164%, exceeding 90%, with the ammonia nitrogen level remaining below 100 mg/L after stripping. 　　To adjust the pH to alkaline at a low cost, a certain amount of calcium hydroxide must be added to the wastewater; however, this tends to cause scale formation. Meanwhile, to prevent secondary pollution caused by the stripped ammonia nitrogen, an ammonia nitrogen absorption device must be installed after the stripping tower. Izzet et al., when treating landfill leachate pre-treated with UASB (2240 mg/L), found that at a pH of 11.5, with a reaction time of 24 hours and mechanical stirring at a speed gradient of only 120 r/min, an ammonia nitrogen removal rate of 95% could be achieved. At pH=12, ammonia nitrogen was removed through aeration; the pH began to drop at hour 17, with an ammonia nitrogen removal rate of only 85%. Based on this, it is believed that the main mechanism for nitrogen removal by the stripping method should be mechanical stirring rather than air diffusion stirring. 1.2 Zeolite denitration method: This method utilizes the cations in zeolites to exchange with NH4+ in wastewater in order to achieve denitrification. Zeolites are generally used to treat wastewater with low ammonia concentrations or wastewater containing trace heavy metals. However, Jiang Jianguo et al. explored the effectiveness and feasibility of the zeolite adsorption method for removing ammonia nitrogen from landfill leachate. The results of the pilot study show that each gram of zeolite has a maximum capacity to adsorb 15.5 mg of ammonia nitrogen. When the particle size of the zeolite is between 30 and 16 mesh, the ammonia nitrogen removal rate reaches 78.5%. Moreover, with constant adsorption time, dosage, and zeolite particle size, the higher the ammonia nitrogen concentration in the influent water, the greater the adsorption rate. It is therefore feasible to use zeolite as an adsorbent for removing ammonia nitrogen from leachate. When treating anaerobically digested pig manure wastewater using the zeolite ion exchange method, Milan et al. found that among Na-Zeo, Mg-Zeo, Ca-Zeo, and K-Zeo, Na-Zeo zeolite had the best effect, followed by Ca-Zeo. Increasing the height of the ion exchange bed can improve the removal rate of ammonia nitrogen. Considering both economic factors and hydraulic conditions, a bed height of 450 px (H/D=4) with a relative flow rate of less than 7.8 BV/h is a suitable dimension. The ion exchange method is greatly affected by the concentration of suspended solids. 　　When applying the zeolite deamination method, the regeneration of the zeolite must be taken into account; common methods include the regeneration with solution and incineration. When the incineration method is used, the ammonia gas generated must be treated. 1.3 Membrane separation technology A method for removing ammonia nitrogen by utilizing the selective permeability of membranes. This method is easy to operate, features a high ammonia nitrogen recovery rate, and causes no secondary pollution. Jiang Zhanpeng et al. achieved good results in treating high-concentration ammonia nitrogen-containing inorganic wastewater using electrodialysis and polypropylene (PP) hollow fiber membrane methods. Electrodialysis can be used to treat ammonia nitrogen wastewater with concentrations of 2000–3000 mg/L, achieving a removal rate of over 85%; simultaneously, 8.9% concentrated ammonia water can be obtained. This method features a simple process flow and requires no chemicals; the amount of electricity consumed during operation is proportional to the concentration of ammonia nitrogen in the wastewater. The denitrification efficiency of the PP hollow fiber membrane method is >90%, and the concentration of recovered sulfuric acid an is around 25%. Alkali needs to be added during operation, and the amount of alkali added is proportional to the ammonia nitrogen concentration in the wastewater. 　　An emulsion film is a liquid film that exists in emulsion form and possesses selective permeability, which can be used for liquid-liquid separation. The separation process typically uses an emulsion membrane (such as a kerosene membrane) as the separation medium. The concentration difference of NH3 and diffusive transport across both sides of the oil membrane serve as driving forces, enabling NH3 to penetrate into the membrane, thereby achieving separation. The wastewater from the main discharge outlet of a hydrometallurgical plant (with 1000–1200 mg NH4+-N/L and a pH of 6–9) was treated using the liquid film method. Alkylamino polyoxyethylene ether was used as the surfactant at a concentration of 4%–6%, the pH of the wastewater was adjusted to 10–11, the emulsion-water ratio was 1:8–1:12, and the oil-in-water ratio was 0.8–1.5. With a sulfuric acid mass fraction of 10%, the removal rate of ammonia nitrogen in wastewater can exceed 97% after a single treatment. 1.4 MAP precipitation method: This method makes use of the following chemical reaction: Mg2+ + NH4+ + PO43- = MgNH4PO4. In theory, by adding phosphates and magnesium salts to wastewater containing high concentrations of ammonia nitrogen in a certain ratio, magnesium ammonium phosphate (MAP) can be formed when the concentration exceeds 2.5×10–13, thereby removing the ammonia nitrogen from the wastewater. Mu Daguang et al. used the method of adding MgCl2·6H2O and Na2HPO4·12H2O to industrial wastewater with high ammonia nitrogen concentrations to generate magnesium ammonium phosphate precipitates, thereby removing the high concentration of ammonia nitrogen from it. The results show that under the conditions of a pH of 8.91, a molar ratio of Mg2+ to NH4+ to P043- of 1.25:1:1, a reaction temperature of 25 °C, a reaction time of 20 minutes, and a precipitation time of 20 minutes, the ammonia mass concentration can be reduced from 9500 mg/L to 460 mg/L, achieving a removal rate of over 95%. Since the content of magnesium salts in most wastewater is relatively low compared to phosphates and ammonia nitrogen, although the resulting magnesium ammonium phosphate can be used as agricultural fertilizer to offset some of the costs, the expense of adding magnesium salts remains a major factor limiting the implementation of this method. Seawater is inexhaustible and contains large amounts of magnesium salts. Kumashiro et al. experimentally studied the crystallization process of magnesium ammonium phosphate using seawater as a source of magnesium ions. Brine is a by-product of salt production, mainly containing MgCl2 and other inorganic compounds. The Mg2+ level of about 32 g/L is 27 times that of seawater. Lee et al. treated pig farm wastewater using the magnesium ammonium phosphate crystallization method with MgCl2, seawater, and brine as Mg2+ sources respectively. The results showed that pH was the most important control parameter; when the final pH was approximately 9.6, the reaction could be completed within 10 minutes. Due to the N/P imbalance in wastewater, compared with the other two Mg2+ sources, brine achieves the same phosphorus removal effect but has a slightly poorer nitrogen removal effect. 1.5 Chemical oxidation method A method that uses strong oxidants to directly oxidize ammonia nitrogen into nitrogen for removal. Breakpoint chlorination involves using ammonia in water to react with chlorine to produce ammonia gas for denitrification; this method also has a sterilizing effect. However, the residual chlorine generated can affect fish, so it is necessary to install equipment for removing this residual chlorine. In the presence of bromides, ozone reacts with ammonia nitrogen in a mechanism similar to breakpoint chlorination: Br－+O3+H+→HBrO+O2, NH3+HBrO→NH2Br+H2O, NH2Br+HBrO→NHBr2+H2O, NH2Br+NHBr2→N2+3Br－+3H+. Yang et al. conducted experimental studies on synthetic wastewater (with 600 mg/L of ammonia nitrogen) using a continuous aeration column with an effective volume of 32 L, to investigate the effects of Br/N, pH, and initial ammonia nitrogen concentration on the reaction, in order to determine the optimal reaction conditions for removing the maximum amount of ammonia nitrogen while generating the minimum amount of NO3-. It was found that NFR (the ratio of effluent NO3--N to influent ammonia nitrogen) has a linear correlation with Br-/N on a logarithmic scale. When Br-/N > 0.4 and the ammonia nitrogen load is between 3.6 and 4.0 kg/(m3·d), a decrease in the ammonia nitrogen load leads to a decrease in NFR. When the effluent pH is 6.0, NFR and BrO⁻–Br (toxic by-products) are at their lowest levels. BrO－-Br can be quantitatively decomposed by Na2SO3, and the amount of Na2SO3 added can be controlled by ORP. 2 Biochemical combined method: Physical-chemical methods are not limited by high ammonia nitrogen concentrations when treating such wastewater, but they are unable to reduce the ammonia nitrogen concentration to sufficiently low levels (such as below 100 mg/L). Biological denitrification, however, can be inhibited by high concentrations of free ammonia or nitrite nitrogen. In practical applications, a combined biochemical method is employed; physically and chemically treating wastewater with high concentrations of ammonia nitrogen prior to biological treatment. 　　Lu et al.’s study employed a stripping-anoxic-aerobic process to treat landfill leachate containing high concentrations of ammonia nitrogen. The results showed that when the stripping conditions were set at pH 9.5 and a stripping time of 12 hours, stripping pretreatment could remove more than 60% of the ammonia nitrogen in the wastewater; subsequent anoxic- aerobic biological treatment achieved removal rates of over 90% for both ammonia nitrogen (from 1400 mg/L to 19.4 mg/L) and COD. Horan et al. used a biological activated carbon fluidized bed to treat landfill leachate (with COD of 800–2700 mg/L and ammonia nitrogen of 220–800 mg/L). The research results show that at an ammonia nitrogen load of 0.71 kg/(m3·d), the nitrification removal rate can exceed 90%, the COD removal rate reaches 70%, and all of the BOD is removed. Fikret et al. used lime flocculation precipitation plus air stripping as pretreatment methods to improve the biodegradability of leachate, and added adsorbents (powdered activated carbon and zeolite) to the subsequent aerobic biological treatment tank; they found that the removal efficiency of COD and ammonia nitrogen increased as the concentration of the adsorbents increased within the range of 0–5 g/L. In terms of ammonia nitrogen removal, zeolite performs better than activated carbon. 　　Membrane-bioreactor technology (MBR) is a novel and highly efficient wastewater treatment system that organically combines membrane separation technology with conventional biological wastewater reactors. MBR has high treatment efficiency; the treated water can be reused directly, it requires fewer devices and occupies less space, and it generates less excess sludge. The challenge lies in maintaining a high flux through the membrane and preventing it from leaking. Li Hongyan et al. used integrated membrane bioreactors to study the nitrification characteristics of wastewater with high ammonia nitrogen concentrations. The research results show that when the ammonia nitrogen concentration in the raw water is 2000 mg/L and the volumetric load of ammonia nitrogen in the influent is 2.0 kg/(m3·d), the removal rate of ammonia nitrogen can exceed 99%, and the system remains relatively stable. The specific nitrification rate of the activated sludge in the reactor remained relatively stable at around 0.36/d over a period of six months. 3 New Biological Denitrification Methods In recent years, several entirely new denitrification processes have emerged both domestically and internationally, offering new approaches for the denitrification of wastewater containing high concentrations of ammonia nitrogen. The main processes include short-cut nitrification-denitrification, aerobic denitrification, and anaerobic ammonium oxidation. 3.1 Short-term nitrification denitrification Biological nitrification denitrification is the most widely used method for nitrogen removal. Since a large amount of oxygen is required in the ammonia nitrogen oxidation process, aeration costs become the main expense for this denitrification method. Short-term nitrification-denitrification (oxidizing ammonia nitrogen to nitrite nitrogen before proceeding with denitrification) not only saves the oxygen required for ammonia oxidation but also reduces the carbon source needed for denitrification. Ruiza et al. conducted experiments using synthetic wastewater (simulating industrial wastewater containing high concentrations of ammonia nitrogen) to determine the optimal conditions for achieving nitrite accumulation. To achieve the accumulation of nitrite, pH is not a key control parameter, as nitration results in the formation of nitrite at pH values between 6.45 and 8.95; at pH 8.95, nitration is inhibited and ammonia nitrogen accumulates. When DO = 0.7 mg/L, 65% of the ammonia nitrogen can be accumulated in the form of nitrite, with an ammonia nitrogen conversion rate of over 98%. At DO1.7 mg/L, all is nitrified to *ao acids. Liu Junxin et al. conducted a comparative analysis on the effectiveness of nitritation-type and *ao-acid-type denitrification for treating high-concentration ammonia-nitrogen wastewater with a low carbon-to-nitrogen ratio. The test results show that sub-ao acid-type denitrification can significantly improve the total nitrogen removal efficiency, with the loads of ammonia nitrogen and nitrate nitrogen increasing by nearly a factor of two. Furthermore, factors such as pH and ammonia nitrogen concentration have a significant impact on the type of denitrification. 　　The pilot-scale test results of Liu Chaoxiang et al. on the treatment of coking wastewater using short-term nitrification-denitrification showed that when the concentrations of COD, ammonia nitrogen, TN, and phenol in the influent were 1201.6, 510.4, 540.1, and 110.4 mg/L respectively, the average concentrations of COD, ammonia nitrogen, TN, and phenol in the effluent were 197.1, 14.2, 181.5, and 0.4 mg/L respectively, with removal rates of 83.6%, 97.2%, 66.4%, and 99.6% respectively. Compared with conventional biological nitrogen removal processes, this process has a higher ammonia nitrogen load, and it can improve the TN removal rate under lower C/N ratios. 3.2 Anaerobic ammonium oxidation (ANAMMOX) and complete autotrophic nitrogen removal (CANON) Anaerobic ammonium oxidation refers to the process in which ammonia is directly oxidized to nitrogen gas under anaerobic conditions, using nitrite as an electron acceptor. The biochemical reaction equation for ANAMMOX is: NH4++NO2－→N2↑+2H2O. ANAMMOX bacteria are obligate anaerobic autotrophs, making them highly suitable for treating ammonia-containing wastewater with NO2－ and a low C/N ratio. Compared with traditional processes, the denitrification method based on anaerobic ammonium oxidation features a simple process flow, does not require an external organic carbon source, prevents secondary pollution, and holds great application prospects. There are mainly two applications of anaerobic ammonium oxidation: the CANON process and its combination with moderate-temperature nitritation (SHARON) to form the SHARON-ANAMMOX combined process. The CANON process is a method that, under oxygen-limited conditions, utilizes completely autotrophic microorganisms to simultaneously remove ammonia nitrogen and nitrite. In terms of its reaction mechanism, it represents a combination of the SHARON and ANAMMOX processes, which occur in the same reactor. Meng and others found that at the leachate treatment plant at Shenzhen Xiaping Solid Waste Landfill, the dissolved oxygen level was maintained at around 1 mg/L, while the ammonia nitrogen content in the incoming water was 90%. Research by Sliekers et al. shows that both the ANAMMOX and CANON processes can operate efficiently in air-lift reactors, achieving very high nitrogen conversion rates. By maintaining the dissolved oxygen at around 0.5 mg/L, the nitrogen removal rate in the air-stirred reactor for the ANAMMOX process reaches 8.9 kgN/(m3·d), while that for the CANON process is 1.5 kgN/(m3·d). 3.3 Aerobic denitrification Traditional denitrification theory holds that denitrifying bacteria are facultative anaerobes; their respiratory chain uses oxygen as the terminal electron acceptor under aerobic conditions, and *AO acid as the terminal electron acceptor under anaerobic conditions. Therefore, for denitrification to occur, an anaerobic environment is necessary. In recent years, aerobic denitrification has been continuously discovered and reported, gradually attracting people's attention. Some aerobic denitrifying bacteria have been isolated, some of which are capable of performing both aerobic denitrification and heterotrophic nitrification (such as Tpantotropha.LMD82.5 isolated and screened by Robertson et al.). This allows for true simultaneous nitrification and denitrification in the same reactor, simplifying the process flow and saving energy. 　　Jia Jianhui et al. used a sequencing batch reactor to treat ammonia-nitrogen wastewater. The experimental results confirmed the existence of aerobic denitrification. The nitrogen removal capacity of aerobic denitrification decreased as the dissolved oxygen concentration in the mixed liquor increased. When the dissolved oxygen concentration was 0.5 mg/L, the total nitrogen removal rate could reach 66.0%. 　　Continuous dynamic test studies by Zhao Zongsheng and others have shown that for high-concentration ammonia nitrogen leachate, the total nitrogen removal rate achieved by conventional activated sludge in aerobic denitrification processes can exceed 10%. The nitration reaction rate decreases as the dissolved oxygen concentration drops ； The denitrification reaction rate increases as the dissolved oxygen concentration decreases. Kinetic analysis of nitrification and denitrification shows that synchronous nitrification-denitrification occurs when the dissolved oxygen level is around 0.14 mg/L, resulting in equal rates of nitrification and denitrification. Its rate is 4.7 mg/(L·h), and the nitrification reaction KN = 0.37 mg/L ； The KD for denitrification is 0.48 mg/L. 　　During denitrification, N2O is produced, which is a greenhouse gas that causes additional pollution. The mechanisms underlying this process have not been thoroughly studied, and many of the related technologies are still at the laboratory stage; further research is needed before they can be effectively applied in practical engineering applications. In addition, processes such as full-autotrophic denitrification and simultaneous nitrification-denitrification are still in the experimental research stage, but they all hold great potential for application.