Introduction to an engineering example of treating production wastewater from the domestic nitrogen fertilizer industry using the A/O method

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Introduction to Engineering Examples of Treating Production Wastewater from the Domestic Nitrogen Fertilizer Industry Using the A/O Method 1 Introduction China is a country with a large population and also a major agricultural nation. Agricultural production relies on chemical fertilizers, which contribute about 40% to increasing agricultural yields; therefore, chemical fertilizers play a very important role in the development of the national economy. After 20 years of rapid development since the reform and opening up, China’s fertilizer industry has reached a considerable scale. Its fertilizer production ranks second only to that of the United States, making it the third largest in the world; among these, nitrogen fertilizer production is the highest in the world. The raw material route for the nitrogen fertilizer industry relies primarily on oil and coke (accounting for about 64%–67%), with a combination of oil and gas being used; natural gas accounts for only 19%–20%. Different raw material routes have different production processes, and even the same raw material route can have various production processes. As the processes differ, so do the sources of wastewater. The production processes of synthetic ammonia and major nitrogen fertilizer products, as well as the sources of wastewater, are described as follows: 1.1 Production process of synthetic ammonia and sources of wastewater: (1) In the process of producing synthetic ammonia using coal coke gasification, wastewater arises mainly from three sources: ① Desulfurization wastewater generated during the gasification step ； ②Desulfurization wastewater generated during the desulfurization process ； ③Ammonia-containing wastewater generated during the copper washing process. 　　(2) The wastewater generated in the production of synthetic ammonia via oil gasification mainly comes from the carbon black wastewater and cyanide-containing wastewater produced during the carbon removal process ； Desulfurization wastewater generated during the desulfurization process ； As well as the low-pressure shift condensate and methanation condensate generated during the removal of organic sulfur, namely ammonia-containing wastewater. 　　(3) Wastewater generated in the ammonia synthesis process using gas as a reactant includes mainly desulfurization wastewater produced during the desulfurization step, ammonia-containing wastewater generated during the copper washing step, and the condensate produced during the removal of organic sulfur – that is, ammonia synthesis wastewater. 1.2 Production processes of major nitrogen fertilizer products and sources of wastewater The wastewater generated in ammonium carbonate production is the fluorine-containing wastewater produced by the flue gas scrubber tower ； The wastewater generated in urea production is mainly the desorption liquid produced during distillation and evaporation processes, as well as the ammonia synthesis wastewater produced during vacuum evaporation processes. 　　In summary, industrial wastewater from the nitrogen fertilizer industry can be classified, based on its properties, into oxygen-containing wastewater from gas production, carbon black wastewater from oil-based gas production, self-sulfur wastewater, and ammonia-containing wastewater. Among these, wastewater from gas production and ammonia-containing wastewater have the greatest impact on the aquatic environment. Table 1: Water quality and discharge standards for gas generation wastewater
Pollutants, Water temperature (°C), Suspended solids (mg/l), Cyanides (mg/l), Sulfides (mg/L), Volatile phenols (mg/l), Ammonia nitrogen (mg/l), pH, CODcr (mg/L)
Gas generation wastewater (before treatment): 45–55, 50–500, 10–30, 0.1–10, 0.01–3, 40–100, 6–9, 30–200
Water quality requirements for the production process: <32, <50, <5, <1.0; Trace amounts –; 6–9 –
Comprehensive wastewater discharge standards GB8978-1996: Class I – 70, 0.5, 1.0, 0.5, 15, 6–9, 100; Class II – 200, 0.5, 1.0, 0.5, 50, 6–9, 150
Wastewater discharge standards for the ammonia synthesis industry GWPB4-1999 (medium scale): Class I – 100, 1.0, 1.0, 0.20, 60, 6–9, 150; Class II – 100, 1.0, 1.0, 0.20, 100, 6–9, 150

2. Process principle
Principle of biological removal of ammonia nitrogen using the A/O method: In the oxygenated stage (O stage), ammonia nitrogen in the wastewater is nitrified by nitrifying bacteria into nitrate nitrogen. A large amount of this nitrate nitrogen is recycled to the A stage. Under anaerobic conditions, facultative anaerobic denitrifying bacteria use the organic matter present in the wastewater as an electron donor and nitrate nitrogen as an electron acceptor, thereby reducing nitrate nitrogen to harmless nitrogen gas, which is released into the atmosphere, achieving the goal of nitrogen removal. 　　Nitration reaction: NH4+ + 2O2 → NO3- + 2H+ + H2O  Reverse digestion reaction: 6NO3- + 5CH3OH (organic matter) → 5CO2↑ + 7H2O + 6OH- + 3N2↑ 3 Engineering Examples 3.1 ----- Comprehensive wastewater treatment project for the group company’s wastewater treatment plant 3.1.1 Project overview  ------- The designed capacity of the wastewater treatment plant for the group company is 240,000 m3 per day. Of this, the amount of domestic wastewater is 59,000 m3/day, nitrogen-containing wastewater is 37,000 m3/day, and chemical production wastewater is 144,000 m3/day. The current actual daily water treatment capacity is 180,000 m3/day. The concentrations of the main pollutants in the influent water and the designed effluent quality parameters for this wastewater treatment project are shown in Table 2. The discharge standards for this wastewater treatment facility comply with the Class II standards of GB8978--1996. 　 Table 2: Water quality parameters of influent and effluent in Jilin sewage treatment project
Parameter | Influent concentration | Effluent concentration
CODcr/(mg/L) | 365 | 120
BOD5/(mg/L) | 156 | 30
NH3-NN/(mg/L) | 78 | 25
pH value | >7.5 | 6–9
Colority (dilution factor) | 83 | 83
SS/(mg/L) | 250 | 70

3.1.2 Process flow and brief description
(1) Process flow: file:///E:/DOCUME~1/zzjjaa/LOCALS~1/Temp/ksohtml/wps_clip_image1.png
(2) Brief description of the process flow:
The chemical wastewater is pumped into a homogenization reaction tank, where alkali is added for neutralization; thereafter, it enters a sedimentation tank for sedimentation. After the sediment sludge is concentrated, it is dewatered and transported away. The precipitated water enters a flow-stabilizing tank, then aeration-precipitation tank, and subsequently another sedimentation tank. There, it joins the domestic wastewater that has undergone aeration-precipitation treatment to enter the A/O (nitrification-denitrification) biological treatment system. Finally, the wastewater is discharged after contact disinfection. 3.1.3 Key technical and economic indicators The sewage treatment plant of Jihua Group Company has a total investment of 540 million yuan. The annual operating cost is 54 million yuan per year, of which the production cost amounts to 36 million yuan per year, administrative expenses are 13 million yuan per year, and taxes amount to 5 million yuan per year. The cost of wastewater treatment is 1.08 yuan/m3 (the direct treatment cost calculated based on production costs is 0.65 yuan/m3). 3.2 ------- Wastewater treatment project for fertilizer plants 3.2.1 Project overview 　　 ------ Fertilizer production facility with an annual output of 300,000 tons of ammonia and 520,000 tons of urea. The ammonia synthesis plant uses the Shell residue vaporization process, the Lurgi low-temperature methanol washing process, and the Caloger ammonia synthesis process, while the urea plant employs the Snam stripping process. 　　The wastewater treatment plant is one of the public facilities provided as part of this project. It includes a ash sedimentation unit, a chemical treatment unit, and a biochemical treatment unit. 3.2.2 Process Flow and Brief Description 　　 (l) Process flow: file:///E:/DOCUME~1/zzjjaa/LOCALS~1/Temp/ksohtml/wps_clip_image2.png 　　(2) Brief description of the process flow 　　The ash sedimentation unit is used to treat approximately 40 t/h of carbon black wastewater generated in the ammonia vaporization process. After passing through the ash sedimentation tank, part of the carbon black is removed from this wastewater; around 30 t/h of the treated wastewater is sent back to the residue oil vaporization section for reuse, while about 10 t/h goes to the wastewater stripping tower where NH3 and H2S are removed. Following chemical treatment to remove heavy metals such as V and VI, the wastewater is then sent to the equalization tank. 　　The biochemical treatment unit processes wastewater that has been treated in the chemical unit, CO2 scrubbing water from the ammonia synthesis plant, process condensate from the urea production plant, domestic sewage, and polluted rainwater from the tank area. These five types of wastewater flow into the biochemical treatment unit, which employs an A/O process. 3.3 ------ Wastewater Treatment Project for the Nitrogen Fertilizer Plant of the Chemical Group 3.3.1 Project Overview The plant currently has a synthetic ammonia production facility with an annual capacity of 25,000 tons. The existing wastewater treatment facilities are only capable of carrying out sedimentation treatment on the wastewater before it is discharged; as a result, certain pollutants such as cyanides, ammonia nitrogen, and sulfides do not meet the required discharge standards. 　　This process uses the A/O treatment technology. The designed water volume is 1200 m3/d. 　　Design water quality for feedwater: CODcr ≤ 230 mg/l, pH: 7–8, SS ≤ 600 mg/l, cyanides ≤ 12.0 mg/l, ammonia nitrogen ≤ 460 mg/l, volatile phenols ≤ 0.50 mg/l, sulfides ≤ 5.58 mg/l. Water quality standards after treatment: in compliance with the secondary discharge standards specified in Table 1 of the **“Emission Standards for Water Pollutants in the Synthetic Ammonia Industry” (GWPB44-1999)**. See Table Depth for details. 3.3.2 Process Flow and Brief Description 　　(1) Process Flow 　　Production wastewater and some domestic sewage (as required by the process, production wastewater must be mixed with some domestic sewage for combined treatment) are combined and then sent to a regulating tank to balance the water volume and improve water quality. The mixed wastewater is pumped up, chemicals are added to it, and then it enters a primary sedimentation tank where some of the suspended solids and most of the cyanide are removed, thereby reducing the load on subsequent treatment processes and minimizing the inhibitory and toxic effects of cyanide on microorganisms. The effluent from the sedimentation tank and the recycled mixture enter the anoxic tank, where denitrification occurs; NO3‑N is reduced to N2 and released into the air, while the wastewater proceeds to the aerobic tank. In the aerobic tank, degradation and nitrification reactions take place; most of the BOD is degraded, and NH3-N is converted into NO3-N. Most of the water from the aerobic tank is recycled, while the remaining portion goes to the secondary sedimentation tank. After passing through this tank, the water enters a storage tank, from where it is pumped, via pumps, into the water feature and fountain systems in the factory’s office and residential areas. This not only enhances the aesthetic appearance of the factory area but also helps save water resources. The sludge from the secondary sedimentation tank and that from the primary sedimentation tank are both sent to the sludge thickening tank. After thickening, they are transferred to a drying facility for drying and dehydration, while the supernatant is returned to the regulation tank for further treatment. 3.3.3 Key technical and economic indicators The total investment in the project is 1.35 million yuan, with the cost per ton of water treated being 0.68 yuan. This plan recommends that the factory reuse wastewater for production or in water features and fountains. The reused wastewater is considered at a rate of 20%, with a cost of 0.8 yuan per cubic meter; thus, the economic benefit per ton of water reused is: 1 × 20% × 0.8 = 0.16 yuan/m3 of water. 4 Conclusion The A/O biological nitrogen removal wastewater treatment technology was developed in the early 1990s; it is capable of effectively oxidizing and decomposing the COD components as well as ammonia nitrogen pollutants present in chemical wastewater, thereby ensuring that all pollutant levels in the wastewater meet the required standards before discharge. However, over the past decade, this highly effective wastewater treatment technology has not been widely utilized. The A/O biological nitrogen removal wastewater treatment method is only used in a few large enterprises such as Jihua Company and Jiujiang Large Fertilizer Plant; it needs to be promoted and applied in smaller and medium-sized fertilizer plants, particularly in the nitrogen fertilizer industry (synthetic ammonia, urea).