A brief discussion on the selection of wastewater treatment solutions for nitrogen fertilizer plants

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Lang Song (Guizhou Provincial Chemical and Pharmaceutical Planning and Design Institute, Guiyang, Guizhou 550002), 2008-09-17. 1 Introduction. With the rapid economic development in China and the improvement of people’s living standards, the amount of pollutants emitted has also increased sharply. Environmental pollution has become a major factor that hinders further economic and social development in China, as well as the improvement of people’s living conditions and health levels. Especially since China’s accession to the WTO, environmental issues have increasingly become one of the most important non-tariff barriers used by other countries to restrict the export of Chinese products. Therefore, reducing pollution and saving energy have become very important social issues. The wastewater from nitrogen fertilizer plants contains high levels of toxic substances such as ammonia, nitrogen, COD, oils, suspended solids, and cyanides. If discharged in excess of the allowed limits, it will pollute the surrounding environment, lead to waste of water resources, and increase production costs, among other negative consequences. 2 Main sources of wastewater from nitrogen fertilizer plants: Synthetic circulating water: compression, decarburization, synthesis, refining, freezing ； Urea cycle water: Urea system ； New compressed circulating water: newly compressed and transformed ； Desulfurization circulating water: Primary desulfurization ； Carbonation circulating water: Carbonation system ； Wastewater recirculation water: gas production, boilers. The main pollutants in the wastewater discharged from medium-sized nitrogen fertilizer plants. NH3-N levels are 2 to 4 times above the standard, COD levels are 4 to 5 times above the standard, and the pH value is slightly alkaline. The emission standards for nitrogen fertilizer plants are in accordance with the \"Emission Standards for Water Pollutants in the Ammonia Synthesis Industry\" (GB13458－2001). The main pollutant parameter is: pH 6–9 ； COD≤100mg/L ； NH3-N≤60mg/L. 3 Selection of process technology scheme: Based on the quality of wastewater from nitrogen fertilizer plants, the main tasks in wastewater treatment are to remove COD and NH3‑N. A comparison is made considering the current status of NH3‑N-containing wastewater treatment in China. 3.1 Traditional A2/O process The A2/O process is the anaerobic-anoxic-aerobic method; its three stages are separated by physical space. It is a process that enables simultaneous nitrogen and phosphorus removal, developed on the basis of the anoxic-aerobic method with nitrogen removal capabilities. The schematic diagram of this process is shown in Figure 1. This process is the simplest synchronous nitrogen and phosphorus removal process in terms of system design, with a water residence time that is generally shorter than that of other similar processes (such as the Bardenpho process). Under anaerobic, anoxic, and aerobic operating conditions, filamental bacteria cannot reproduce in large numbers, so there is no risk of sludge bulking; the SVI value is generally less than 100, resulting in good separation of the treated sludge from the water. During operation of this process, gentle stirring is required in the anaerobic and anoxic sections to prevent sludge deposition. The drawback of this process is that it requires both mixed liquor reflux and sludge reflux, resulting in high energy consumption ； There are many mechanical devices, and their maintenance is troublesome ； The ammonia nitrogen removal efficiency is unstable. 3.2 A/O process + HSBEMBM® high-efficiency microbial treatment technology: Based on successful engineering experience, the combination of the A/O process with the HSBEMBM® high-efficiency microbial treatment technology achieves a very good removal efficiency for NH3-N. This process utilizes the basic principles of the A/O process, combined with enhanced HSBEMBM® high-efficiency microorganisms, to achieve excellent results in the removal of COD and NH3-N. This technology has been successfully developed and applied in practice for the treatment of wastewater with high ammonia nitrogen levels, achieving satisfactory results, with the NH3-N concentration in the treated water remaining below 10 mg/L. HSBEMBM® High-Efficiency Microbial Environmental Protection Engineering Technology is an advanced biochemical treatment technique. The core of this technology lies in selecting and adapting microorganisms to form a microbial community composed of various highly effective degrading microorganisms, which is then introduced into a biochemical tank to achieve wastewater treatment. The HSBEMBM® high-efficiency microbial community, having been specially trained and enhanced, possesses the following characteristics: (1) The community itself is non-toxic, non-pathogenic, and does not cause secondary pollution ； (2) Removal of COD and BOD: fast speed, high efficiency ； (3) It has a unique ability to remove NH3-N and refractory organic substances ； (4) Good sludge sedimentation properties, high compactness, good stability, and low sludge production ； (5) Biological agents are added once only, with no need for replenishment; it features low operating costs and a low failure rate ； (6) It has strong adaptability to pH values, maintaining good treatment efficiency within the range of 6–9.5, and exhibits high resistance to shock loads. After analysis, this process is suitable for the biochemical treatment of wastewater from nitrogen fertilizer plants. The reason for this is that there are successful examples of its application in China, such as the coking wastewater treatment project at Hangzhou Iron and Steel Plant. This project came online in April 2004, and it employed an O1—A1—O2—A2 biological denitrification process. Harmful substances such as phenols and cyanides can be almost completely removed in the O1 stage, and the parameters of the final effluent meet or even exceed the first-class standards specified in GB8978－1996. The water quality of the nitrogen fertilizer plant wastewater is similar to that of the coking wastewater from Hangzhou Steel; both types of wastewater contain high levels of NH3-N (with relatively low COD), and they also contain harmful substances such as phenols and cyanides that hinder biological nitrogen removal. 3.3 Membrane treatment Membrane separation technology is a separation technique widely used for the separation, concentration, and purification of solutions and gases. It uses a selectively permeable membrane as the separation medium; the membrane surface is covered with numerous micropores. The original solution passes through one side of the membrane under certain pressure, while the solvent and small molecular solutes pass through the membrane wall to form the filtrate, whereas larger molecular solutes are retained by the membrane, thereby achieving the separation and concentration of substances. Membrane treatment is currently commonly used in water treatment, with its main applications including: desalination of brackish water and seawater to produce drinking water, pre-treatment in ion exchange systems for high-purity boiler water as well as ultra-pure water used in the manufacture of electronic components and pharmaceuticals, kidney dialysis, and the recovery and recycling of small amounts of wastewater. Different industrial sectors have varying requirements regarding the quality of water used. Membrane treatment techniques include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). Applying membrane technology to industrial wastewater treatment has been a development trend in recent years; generally, testing is required for each type of wastewater in order to develop appropriate pretreatment methods, as well as the types and configurations of membranes. The main drawback of membrane treatment is that it requires relatively high standards regarding the quality of the incoming water; otherwise, contamination may occur, which in turn affects the quality of the treated water ； Membrane treatment requires regular backwashing to prevent membrane clogging ； The lifespan of the membrane is generally short; the membrane should be replaced based on actual operating conditions ； It requires a large initial investment ； The greater the treatment capacity of the membrane, the worse the quality of the effluent water will be ； The operating cost is relatively higher compared to biochemical treatment. Therefore, membrane treatment of industrial wastewater requires pretreatment to stabilize the water quality within a narrow range before proceeding with membrane treatment. 3.4 Scheme Comparison Through comparison, Scheme 2 is the best. 3.5 Principles of the HSBEMBM® treatment process As research progresses, new phenomena have emerged in the process of biological nitrogen removal that go beyond traditional understanding. For example, nitrification is not carried out only by autotrophic bacteria; heterotrophic bacteria can also be involved in this process. Certain microorganisms are capable of denitrification under aerobic conditions. Notably, some researchers have observed a reduction in ammonia levels in anaerobic reactors in the laboratory. These findings represent advances in our knowledge of the vast unknown world of microorganisms, providing new theories and revolutionary approaches for designing biological nitrogen removal processes as well as for identifying new microbial strains. Based on the changes in free energy from chemical thermodynamics, the following fundamental reactions exist: Traditional nitrification and denitrification reactions: NH4+ + 2O2 = NO3- + H2O + 2H+; NO3- + H+ + 0.83CH3OH = 0.5N2 + 2.17H2O + 0.83CO2; NH4+ + 2O2 + 0.83CH3OH = 0.5N2 + 3.17H2O + H+ + 0.83CO2. ΔG = –895.4 kJ/mol. Short-term nitrification reactions: NH4+ + 1.5O2 = NO2- + H2O + 2H+; NH4+ + NO2- = N2 + 2H2O; NH4+ + 0.75O2 = 0.5N2 + 1.5H2O + H+. ΔG = –575.8 kJ/mol. If the nitrification process can be controlled at the nitrosation stage, 62.5% less O2 and 100% less electron donor are required. After re-evaluating the traditional biological nitrogen removal process and conducting numerous experimental studies and practical applications, it was found that the key to this process lies in enhancing the role of microorganisms – by forming communities composed of various types of nitrifying and nitrosifying bacteria, optimizing process conditions, and taking advantage of the inherent differences in microbial kinetics to achieve a dynamic balance and selective utilization of nitrifying, nitrosifying, and denitrifying bacteria. Utilizing a diverse combination of such bacteria can help reduce the amount of carbon source and alkali needed, shorten the retention time, and enable cost-effective biological nitrogen removal processes. 3.6 HSBEMBM® High-Efficiency Microbial Treatment Process 3.6.1 Reasons for Choosing the HSBEMBM® High-Efficiency Microbial Treatment Process The fundamental principle of biological treatment is to use microorganisms to break down organic matter; only bacteria with strong adaptability and high decomposition capabilities can fully leverage the advantages of biological treatment. The HSBEMBM® high-efficiency microbial environmental protection engineering technology has been used in recent years to treat more than a dozen types of difficult-to-treat industrial wastewater, with successful results ； Meanwhile, dozens of engineering projects are either under construction or have already been completed. Based on the pilot and laboratory tests conducted by Panzhihua Coal Chemical Company, this technology was successfully implemented on an industrial scale at Hangzhou Iron and Steel Co.’s coking plant in 2002. Accurate process parameters were obtained, and an effective process route was developed, which can serve as a reference for the biological treatment of wastewater. 3.6.2 Removal of ammonia nitrogen In the treatment of ammonia synthesis wastewater, the removal of ammonia nitrogen is of particular importance. Based on the actual performance of this technology when applied to wastewater of similar types, it can be concluded that the HSBEMBM® high-efficiency microbial environmental protection technology is highly effective in the nitrification and denitrification processes involved in the removal of ammonia nitrogen. The results from practical applications confirm this; after treatment, the NH3-N concentration in the wastewater can be reduced to below 10 mg/L, or even lower. The effluent meets the water quality requirements for circulating water makeup. 3.6.3 Wastewater treatment process flow The process flow (see Figure 3) is as follows: The ammonia-nitrogen-containing wastewater and other wastewater discharged from various workshops first enter a regulating tank for homogenization and volume balance; if necessary, phosphates are added to supply the nutrient P ； After homogenization, the wastewater enters the primary aeration tank, which is an aerobic aeration tank; its main purpose is to remove the toxic and harmful substances in the wastewater that inhibit nitrogen-removing bacteria. Since the amount of excess sludge generated by high-efficiency microorganisms is much lower than that of ordinary microorganisms, in order to reduce the load on the subsequent sludge treatment systems while ensuring effective wastewater treatment, high-efficiency microorganisms are added in the primary aeration tank ； The water from the primary aeration tank flows naturally to the primary sedimentation tank for sludge-water separation, with the sludge being returned to the primary aeration tank. The effluent from the primary sedimentation tank flows by gravity to the biochemical stage, namely the carbon and nitrogen removal units, which consist of an aeration tank, an aerobic tank, an anoxic tank (multi-functional tank), and a secondary sedimentation tank. The biochemical treatment process for this unit is designed to address the high concentrations of organic matter and NH3-N in wastewater, based on the results of treatment operations for similar types of wastewater. High-efficiency microbial agents are added to the tank; by adjusting their living environment, it is possible to take advantage of their different properties in anaerobic and aerobic conditions, thereby enabling effective treatment. This approach facilitates the nitration and denitrification processes for nitrogen removal, as well as carbon removal. Based on previous successful experiences, submersible mixing is used in the anoxic zone, aeration is employed in the aerobic zone, and both submersible mixing and aeration are used in the anoxic zone to maintain complete mixing and suspension of the mixture within each structure. An internal circulation system is installed between the anoxic tank and the aerobic tank to ensure effective nitrogen removal ； The sludge settled in the secondary sedimentation tank is returned to the aerated tank. 4 Conclusion Through the application of the A/O process combined with the HSBEMBM® high-efficiency microbial treatment technology, substances such as COD, SS, NH3-N, and cyanides were brought to levels that meet the standards for recycled water use. As a result, water used in synthesis, urea production, carbonization, gas generation, boiler operations, desulfurization systems, as well as for cooling and washing purposes, can be circulated in a completely closed loop, achieving zero wastewater discharge from the plant. This technology will be the direction for the development of many nitrogen fertilizer plants.