A method and system for urban wastewater treatment using iron salt-enhanced AOA

Abstract

This invention relates to the field of water treatment technology, and more particularly to a method and system for treating urban wastewater using iron salt-enhanced AOA (anaerobic ammonia oxidizing bacteria). This technical solution places the secondary sedimentation tank between the aerobic and anoxic zones, effectively blocking sludge flow between the two zones and maintaining the stability of the microbial community structure in the anoxic zone. This ensures that anaerobic denitrification is carried out primarily by anaerobic ammonia oxidizing bacteria, addressing operational issues caused by fluctuations in influent water quality and quantity, and offering advantages such as rapid start-up and easy maintenance. Simultaneously, the innovative placement of the chemical phosphorus removal unit in the middle section of the anoxic zone significantly reduces sludge production, enables separate discharge and resource utilization of excess sludge, and overcomes the inherent defects of insufficient carbon source and low carbon-to-nitrogen ratio in domestic wastewater. This creates favorable conditions for deep denitrification by anaerobic ammonia oxidizing bacteria, transforming the chemical phosphorus removal reagent residue from a "treatment burden" into a "denitrification resource," achieving cost reduction and efficiency improvement.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, and in particular to a method and system for treating urban wastewater using iron salt-enhanced AOA. Background Technology

[0002] With the escalation of water pollution, nitrogen and phosphorus removal from urban wastewater has become a critical objective. Currently, wastewater treatment plants in my country primarily employ modified anaerobic / aerobic / anoxic (AOA) processes, including post-denitrification, post-anaerobic ammonium oxidation, and post-short-cut denitrification-anaerobic ammonium oxidation. However, each of these processes has its limitations, making the development of novel continuous-flow AOA processes crucial for achieving efficient and stable deep nitrogen and phosphorus removal from wastewater. Furthermore, although biological phosphorus removal methods are relatively inexpensive, their effectiveness is limited, and using them alone cannot guarantee compliance with effluent total phosphorus standards; therefore, phosphorus removal processes still require further optimization.

[0003] Therefore, it is necessary to optimize existing nitrogen and phosphorus removal processes to improve wastewater purification efficiency and reduce costs. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a method and system for urban wastewater treatment using iron salts to enhance AOA.

[0005] In a first aspect, embodiments of the present invention provide a method for treating urban wastewater using iron salt-enhanced AOA, the method being applied to an urban wastewater treatment system; the method includes:

[0006] A portion of domestic sewage is sequentially introduced into an anaerobic tank, an aerobic tank, and a secondary sedimentation tank to obtain effluent from the secondary sedimentation tank and settled sludge.

[0007] The sludge settled in the secondary sedimentation tank is returned to the anaerobic tank; at the same time, another part of the domestic sewage and the effluent from the secondary sedimentation tank are introduced into the raw water denitrification tank in the anoxic zone for denitrification treatment, and the effluent from the raw water denitrification tank is obtained.

[0008] The effluent from the raw water denitrification tank is sequentially fed into a mixing tank and a flocculation sedimentation tank. The target iron salt is added to the mixing tank according to the type of nitrogen in the effluent from the raw water denitrification tank. After chemical phosphorus removal treatment, the effluent from the flocculation sedimentation tank is obtained.

[0009] The effluent from the flocculation sedimentation tank is fed into the equalization tank. Under the condition that the effective iron-nitrogen ratio is 2:1 to 5:1, it is then introduced into the iron salt enhanced denitrification tank for deep purification to obtain effluent that meets the standards.

[0010] In conjunction with the first aspect, the step of introducing a portion of domestic sewage into an anaerobic tank for anaerobic phosphorus release and denitrification to obtain effluent from the anaerobic tank includes:

[0011] The dissolved oxygen in the anaerobic tank is below 0.2-0.5 mg / L, and the oxidation-reduction potential is controlled between -100 and -200 mV.

[0012] In conjunction with the first aspect, the step of introducing another portion of domestic sewage and the effluent from the secondary sedimentation tank into the raw water denitrification tank in the anoxic zone for denitrification treatment, to obtain the effluent from the raw water denitrification tank, includes:

[0013] Domestic sewage is introduced into the anaerobic tank and the raw water denitrification tank in a specified ratio, thereby controlling the domestic sewage and the effluent from the secondary sedimentation tank to be introduced into the raw water denitrification tank in a specified ratio.

[0014] Under conditions of pH 7.07-7.29 and a carbon-to-nitrogen ratio of 1.14-2.86, the anaerobic ammonia oxidation sludge filled in the raw water denitrification tank undergoes a short-range nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation, and / or a full-range nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation.

[0015] In conjunction with the first aspect, the steps of sequentially feeding the effluent from the raw water denitrification tank into a mixing tank and a flocculation sedimentation tank, and adding a target iron salt in the mixing tank according to the type of nitrogen in the effluent from the raw water denitrification tank, followed by chemical phosphorus removal treatment to obtain the effluent from the flocculation sedimentation tank, include:

[0016] Introduce raw water effluent from the denitrification tank into the mixing tank. If the nitrogen type in the raw water denitrification tank effluent is ammonia nitrogen, add ferric salt; if the nitrogen type in the raw water denitrification tank effluent is nitrate nitrogen, or nitrate nitrogen and ammonia nitrogen, add ferrous salt.

[0017] When the mixing intensity is 1500–6000 s -1 Under the conditions of mixing and stirring, a mixed effluent is obtained;

[0018] The mixed effluent is introduced into a flocculation sedimentation tank for solid-liquid separation to obtain chemically dephosphorized effluent.

[0019] In conjunction with the first aspect, the effluent from the flocculation sedimentation tank is introduced into the equalization tank. While maintaining an effective iron-nitrogen ratio of 2:1 to 5:1, it is then further introduced into an iron salt-enhanced denitrification tank for deep purification, resulting in effluent that meets standards. This process includes:

[0020] The effluent from the flocculation sedimentation tank is introduced into the equalization tank for adjustment, resulting in effluent with a pH of 7.29-7.32 and an effective iron-nitrogen ratio of 2:1-5:1.

[0021] The effluent from the equalization tank is introduced into an iron salt enhanced denitrification tank filled with anaerobic ammonia oxidation sludge. The remaining iron salts and nitrogen in the effluent are used for denitrification to obtain effluent that meets the standards.

[0022] In conjunction with the first aspect, if the remaining iron and nitrogen in the effluent are ferrous salts, nitrate nitrogen, and ammonia nitrogen, and the effective iron-nitrogen ratio is 2:1, then the denitrification reaction is a ferrous short-range nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation.

[0023] In conjunction with the first aspect, if the remaining iron salts and nitrogen in the effluent are ferric salts and ammonia nitrogen, and the effective iron-nitrogen ratio is 3:1, then the denitrification reaction is a ferric anaerobic ammonia oxidation reaction.

[0024] In conjunction with the first aspect, if the remaining iron and nitrogen in the effluent are ferrous salts and nitrate nitrogen, and the effective iron-nitrogen ratio is 5:1, then the denitrification reaction is a ferrous full-process nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation.

[0025] In a second aspect, the present invention also provides an urban sewage treatment system, including a control unit for performing the above-described method, an AOA device, and a secondary sedimentation tank. The AOA device includes an anaerobic zone, an aerobic zone, and an anoxic zone connected in sequence, and the secondary sedimentation tank is located between the aerobic zone and the anoxic zone.

[0026] The secondary sedimentation tank is also connected to the anaerobic zone through a return pipeline to return the settled sludge from the secondary sedimentation tank to the anaerobic zone.

[0027] The inlets of the anaerobic and anoxic zones are connected to the domestic sewage pipelines, respectively.

[0028] In conjunction with the second aspect, the anaerobic zone includes several anaerobic tanks; the aerobic zone includes several aerobic tanks; and the anoxic zone includes a raw water denitrification tank, a mixing tank, a flocculation sedimentation tank, a regulating tank, and an iron salt enhanced denitrification tank connected in sequence.

[0029] The raw water denitrification tank is filled with anaerobic ammonia oxidation biofilm packing or granular sludge; the iron salt enhanced denitrification tank is equipped with anaerobic ammonia oxidation packing frame.

[0030] An automatic iron salt dosing device is connected to a mixing tank to add iron salt to the mixing tank;

[0031] The iron and phosphorus recovery device is connected to the outlet of the flocculation sedimentation tank to recover and utilize the generated iron and phosphorus precipitate.

[0032] The embodiments of this invention bring the following beneficial effects: The urban sewage treatment method and system using iron salt enhanced AOA provided in this application includes: introducing a portion of domestic sewage sequentially into an anaerobic tank, an aerobic tank, and a secondary sedimentation tank to obtain secondary sedimentation tank effluent and settled sludge; returning the settled sludge from the secondary sedimentation tank to the anaerobic tank; simultaneously, introducing another portion of domestic sewage and the secondary sedimentation tank effluent into a raw water denitrification tank in anoxic zone for denitrification treatment to obtain raw water denitrification tank effluent; sequentially passing the raw water denitrification tank effluent into a mixing tank and a flocculation sedimentation tank for chemical phosphorus removal treatment to obtain flocculation sedimentation tank effluent; passing the flocculation sedimentation tank effluent into an equalization tank, and, under the condition that the effective iron-nitrogen ratio is 2:1-5:1, continuing to introduce it into an iron salt enhanced denitrification tank for deep purification to obtain compliant effluent.

[0033] The urban wastewater treatment method using iron salt-enhanced AOA provided by this invention independently sets up the chemical phosphorus removal unit in the middle two compartments of the anoxic zone. This not only effectively reduces sludge production value and achieves independent discharge and recycling of excess sludge, but also overcomes the inherent defects of domestic wastewater's lack of carbon sources and low carbon-to-nitrogen ratio, providing a water quality foundation for deep denitrification by anaerobic ammonia oxidizing bacteria. It also reduces the construction and operation costs of the water plant. In addition, the secondary sedimentation tank is set between the aerobic and anoxic zones, which can block the sludge flow between the two zones and enhance the stability of the microbial community structure in the anoxic zone. This not only solves the performance fluctuations caused by large fluctuations in raw water quality and quantity, but also makes its start-up and operation and maintenance simpler and more convenient.

[0034] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention are realized and obtained in accordance with the structures particularly pointed out in the description, claims and drawings.

[0035] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0036] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0037] Figure 1 A schematic diagram of the process for urban wastewater treatment using iron salt-enhanced AOA provided by the present invention;

[0038] Figure 2This is a schematic diagram of the urban sewage treatment system provided by the present invention. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] To facilitate understanding of this embodiment, the technical terms used in this application will be briefly introduced below. The AOA (Anaerobic / Oxic / Anoxic) process is a wastewater treatment process that combines anaerobic, aerobic, and anoxic environments, and is mainly used for efficient nitrogen and phosphorus removal.

[0041] After introducing the technical terms used in this application, the application scenarios and design concepts of the embodiments of this application will be briefly described below.

[0042] Existing AOA (Automatic Aeration) processes generally suffer from high carbon source requirements, intense competition among functional microbial communities, and insufficient system stability, making it difficult to achieve efficient and stable deep nitrogen and phosphorus removal. Furthermore, to compensate for the shortcomings of biological phosphorus removal and ensure stable effluent total phosphorus (TP) compliance, chemical phosphorus removal processes are typically required. However, chemical phosphorus removal not only significantly increases sludge production, leading to difficulties in phosphorus resource recovery and higher operating costs, but also exhibits poor synergy with biological treatment systems, further exacerbating systemic bottlenecks such as carbon source competition and sludge disposal pressure.

[0043] Based on this, embodiments of this application provide a method and system for urban wastewater treatment using iron salt-enhanced AOA.

[0044] Example 1

[0045] This application provides a method for treating urban wastewater using iron salt-enhanced AOA, which is applied to an urban wastewater treatment system; combined with Figure 1 As shown, the method includes:

[0046] S110 introduces a portion of domestic sewage sequentially into the anaerobic tank, aerobic tank, and secondary sedimentation tank to obtain effluent from the secondary sedimentation tank and settled sludge.

[0047] In the anaerobic tank, anaerobic microorganisms release phosphate by decomposing polyphosphates, while simultaneously absorbing volatile fatty acids (VFAs) from the wastewater to synthesize polyhydroxyalkanoates (PHAs). This process is accompanied by simultaneous denitrification. In the aerobic tank, oxygen is supplied through aeration, and microorganisms utilize stored PHAs as an energy source, excessively absorbing phosphates from the water and resynthesizing polyphosphates, while nitrification occurs, reducing ammonia nitrogen (PNO3) to PNO3. ) is converted into nitrate nitrogen ( The subsequent secondary sedimentation tank separates the sludge from the water. The resulting supernatant is used as the influent to the anoxic zone, while the settled sludge is partially returned to the anaerobic tank, completing the biological phosphorus removal cycle.

[0048] S120, the sludge settled in the secondary sedimentation tank is returned to the anaerobic tank; at the same time, another part of the domestic sewage and the effluent from the secondary sedimentation tank are introduced into the raw water denitrification tank in the anoxic zone for denitrification treatment, and the effluent from the raw water denitrification tank is obtained.

[0049] The sludge settled in the secondary sedimentation tank is returned to the anaerobic tank, and the sludge concentration in the anaerobic and aerobic tanks is maintained through sludge return.

[0050] Simultaneously, another portion of the raw water is mixed with the effluent from the secondary sedimentation tank and introduced into the raw water denitrification tank in the anoxic zone. Anaerobic ammonia-oxidizing bacteria are used as the main functional bacteria, utilizing the carbon source (COD) in the raw water and and the water effluent from the secondary sedimentation tank This process achieves efficient simultaneous removal of ammonia nitrogen and nitrate through short-cut nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonium oxidation (PD / A) and / or full-cut nitrate dissimilatory reduction to ammonium coupled with PD / A. Compared with traditional denitrification processes, this process reduces carbon source requirements by approximately 60%, significantly lowering operating costs. Compared with short-cut nitrification-anaerobic ammonium oxidation (PD / A) processes, this process eliminates the need to regulate the competitive relationship between denitrifying bacteria and anaerobic ammonium oxidizing bacteria, resulting in a more stable microbial community, stronger resistance to water quality / quantity shocks, and simpler start-up and maintenance. Compared with short-cut nitrification-anaerobic ammonium oxidation (PN / A) processes, this process eliminates the need for inhibition and regulation of nitrite-oxidizing bacteria (NOB) in the aerobic zone, simplifying the operation and management of the aerobic section.

[0051] S130: The effluent from the raw water denitrification tank is sequentially fed into a mixing tank and a flocculation sedimentation tank. The target iron salt is added to the mixing tank according to the type of nitrogen in the effluent from the raw water denitrification tank. After chemical phosphorus removal treatment, the effluent from the flocculation sedimentation tank is obtained.

[0052] In this embodiment, the target iron salt is a ferric salt or a ferrous salt.

[0053] This invention sets up the mixing tank and flocculation sedimentation tank for chemical phosphorus removal in the middle two compartments of the anoxic zone, which is significantly different from the traditional pre-phosphorus removal, simultaneous phosphorus removal and post-phosphorus removal methods. It combines the advantages of the three methods while avoiding their disadvantages. It is not only highly economical, but also does not cause harm to the treatment system or structures. The nitrogen and phosphorus removal effect is significant.

[0054] The specific explanation is as follows:

[0055] The wastewater to be treated (i.e., a mixture of some raw water and effluent from the secondary sedimentation tank) first enters the first compartment of the anoxic zone (raw water denitrification tank). This tank is filled with anaerobic ammonia oxidation biofilm packing or granular sludge, which can fully utilize the organic matter in the domestic sewage for biological denitrification. This effectively avoids carbon source waste and significantly reduces the BOD5 and SS content in the effluent flowing into the subsequent chemical phosphorus removal treatment zone (including the mixing tank and flocculation sedimentation tank), thereby greatly reducing the sludge production in the flocculation sedimentation tank.

[0056] Subsequently, the pre-denitrified effluent enters the second compartment of the anoxic zone (mixing tank), where iron salt is added and instantaneous high-intensity mixing is performed. It then flows into the third compartment of the anoxic zone (flocculation sedimentation tank) to complete the flocculation and sedimentation process. This process design does not require additional structures, saving on the construction and operation costs of the water plant. It also achieves independence between the phosphate precipitation and biological purification processes, further reducing sludge production and facilitating the separate discharge and recycling of phosphate sludge.

[0057] After chemical phosphorus removal, the effluent enters the fourth compartment (equalization tank) of the anoxic zone for pH adjustment, and then enters the fifth compartment (iron salt enhanced denitrification tank) of the anoxic zone. This tank is equipped with an anaerobic ammonia oxidation sludge packing frame, which can continue to utilize the residual iron salts and nitrogen in the effluent for deep denitrification. This setup not only effectively alleviates the carbon source shortage problem often faced by urban wastewater treatment plants that mainly handle domestic sewage during biological denitrification, but also avoids the iron pollution problem caused by excessive dosage of chemicals during chemical phosphorus removal. Simultaneously, it reduces sludge loss, achieves deep treatment of effluent, and significantly improves the quality of the effluent.

[0058] In addition, iron salts commonly used for chemical phosphorus removal in wastewater include ferric chloride, ferrous chloride, and ferrous sulfate, all of which are readily available chemical products on the market. Pickling wastewater from the steel industry is also a significant source of ferrous chloride and ferrous sulfate. As long as the source is stable and the purity meets requirements, the treatment costs for nitrogen and phosphorus removal can be significantly reduced through a "waste-to-waste" approach.

[0059] S140, the effluent from the flocculation sedimentation tank is introduced into the equalization tank. Under the condition that the effective iron-nitrogen ratio is 2:1 - 5:1, it is further introduced into the iron salt enhanced denitrification tank for deep purification to obtain effluent that meets the standards.

[0060] In this embodiment, "effective iron-nitrogen ratio" refers to the molar concentration ratio of effective iron salt to effective nitrogen participating in a specified biochemical reaction.

[0061] Unlike traditional wastewater treatment plants that rely on carbon sources in domestic sewage for denitrification, this invention utilizes the metabolic diversity of anaerobic ammonia-oxidizing bacteria to achieve the simultaneous removal of multiple types of iron salts and nitrogen. This technology breaks through the limitations of traditional processes, transforming the chemical phosphorus removal agent residues from a "treatment burden" into a "denitrification resource," thereby achieving cost reduction and efficiency improvement.

[0062] The urban wastewater treatment method using iron salt-enhanced AOA provided by this invention combines biological denitrification (multiple biological reactions of anaerobic ammonia oxidation) with chemical phosphorus removal (iron salt precipitation) by setting up anoxic zones in stages and using the synergistic effect of iron salt. This solves the problems of high carbon source dependence, intense competition among bacterial communities, and high cost of chemical phosphorus removal in traditional AOA, and achieves efficient denitrification, phosphorus removal, and resource recovery of urban wastewater.

[0063] In conjunction with the first aspect, step S110 includes:

[0064] The dissolved oxygen in the anaerobic tank is below 0.2-0.5 mg / L, and the oxidation-reduction potential is controlled between -100 and -200 mV.

[0065] In conjunction with the first aspect, step S120 includes:

[0066] S121, domestic sewage is introduced into the anaerobic tank and the raw water denitrification tank in a specified ratio, thereby controlling the domestic sewage and the effluent from the secondary sedimentation tank to be introduced into the raw water denitrification tank in a specified ratio.

[0067] S122, under conditions of pH 7.07-7.29 and a carbon-to-nitrogen ratio of 1.14-2.86, causes the anaerobic ammonia oxidation sludge filled in the raw water denitrification tank to undergo a short-range nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation, and / or a full-range nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation.

[0068] Under the above conditions, the reaction of short-cut or full-cut nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation is triggered. This can adapt to different water qualities and flexibly select the denitrification path (short-cut or full-cut path) to improve denitrification efficiency and stability.

[0069] Under conditions of pH 7.27-7.29 and a carbon-to-nitrogen ratio of 1.14-2.14, the anaerobic ammonia oxidation biofilm packing or granular sludge filling the raw water denitrification tank undergoes a short-range nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation.

[0070] Under the above conditions, only the short-range dissimilatory reduction of nitrate to ammonium coupled with anaerobic ammonia oxidation reaction (short-range path) is triggered, which can optimize denitrification for wastewater with low carbon-to-nitrogen ratio.

[0071] In conjunction with the first aspect, step S130 includes:

[0072] S131, introduce raw water denitrification tank effluent into the mixing tank. If the nitrogen type in the raw water denitrification tank effluent is ammonia nitrogen, add ferric salt; if the nitrogen type in the raw water denitrification tank effluent is nitrate nitrogen, or nitrate nitrogen and ammonia nitrogen, add ferrous salt.

[0073] S132, at a mixing intensity of 1500–6000 s -1 Mixing and stirring are carried out under certain conditions to obtain mixed effluent.

[0074] S133 introduces the mixed effluent into a flocculation sedimentation tank for solid-liquid separation to obtain chemically dephosphorized effluent.

[0075] This invention ingeniously utilizes the redox properties of iron salts to flexibly select iron salts for phosphorus removal based on the type of nitrogen in the effluent from the raw water denitrification tank. In subsequent processes, anaerobic ammonia oxidation bacteria are used to simultaneously remove multiple types of iron salts and nitrogen. This technology breaks through the limitations of traditional processes, enhances the multidimensional compatibility and water quality applicability of the process, and thus achieves stability and effectiveness against changes.

[0076] In conjunction with the first aspect, step S140 includes:

[0077] S141, the effluent from the flocculation sedimentation tank is introduced into the equalization tank for adjustment to obtain effluent from the equalization tank with a pH value of 7.29-7.32 and an effective iron-nitrogen ratio of 2:1-5:1;

[0078] S142 introduces the effluent from the equalization tank into an iron salt enhanced denitrification tank filled with anaerobic ammonia oxidation sludge. The remaining iron salts and nitrogen in the effluent are used for denitrification to obtain effluent that meets the standards.

[0079] Specifically, the reactions of iron salts and nitrogen in S142 include the following three types:

[0080] In conjunction with the first aspect, if the remaining iron and nitrogen in the effluent are ferrous salts, nitrate nitrogen, and ammonia nitrogen, and the effective iron-nitrogen ratio is 2:1, then the denitrification reaction is a ferrous short-range nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation.

[0081] The ferrous short-range nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation reaction of anaerobic ammonia oxidizing bacteria is divided into two steps: (1) using ferrous salt in the raw water, the nitrate nitrogen in the raw water is converted into nitrite nitrogen through the short-range nitrate dissimilatory reduction to ammonium reaction; (2) the nitrite nitrogen generated and the ammonia nitrogen in the raw water are converted into nitrogen gas through the anaerobic ammonia oxidation reaction.

[0082] In this case, the effective iron-nitrogen ratio is 2:1, which means that the molar concentration ratio of ferrous salt to nitrate nitrogen is 2:1.

[0083] In conjunction with the first aspect, if the remaining iron salts and nitrogen in the effluent are ferric salts and ammonia nitrogen, and the effective iron-nitrogen ratio is 3:1, then the denitrification reaction is a ferric anaerobic ammonia oxidation reaction.

[0084] The ferric anaerobic ammonia oxidation reaction of anaerobic ammonia oxidizing bacteria refers to the conversion of ammonia nitrogen into nitrogen gas using ferric salts in the raw water.

[0085] In this case, the effective iron-nitrogen ratio is 3:1, which means that the molar concentration ratio of ferric salt to ammonia nitrogen is 3:1.

[0086] In conjunction with the first aspect, if the remaining iron and nitrogen in the effluent are ferrous salts and nitrate nitrogen, and the effective iron-nitrogen ratio is 5:1, then the denitrification reaction is a ferrous full-process nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation.

[0087] The ferrous nitrate dissimilatory reduction to ammonium coupled anaerobic ammonia oxidation reaction of anaerobic ammonia oxidizing bacteria is divided into two steps: (1) using ferrous salt in the raw water, the nitrate nitrogen in the raw water is converted into nitrite nitrogen through the full-process nitrate dissimilatory reduction to ammonium reaction, and some of the nitrite nitrogen is further converted into ammonia nitrogen; (2) the generated nitrite nitrogen and ammonia nitrogen are converted into nitrogen gas through the anaerobic ammonia oxidation reaction.

[0088] In this case, the effective iron-nitrogen ratio is 5:1, which means that the molar concentration ratio of ferrous salt to nitrate nitrogen is 5:1.

[0089] It should be noted that in S142, when the effective iron-nitrogen ratio is in the range of 2:1 to 5:1, the above three types of reactions may occur individually (as listed) or simultaneously (in other cases).

[0090] Unlike traditional wastewater treatment plants that rely on carbon sources in domestic sewage for denitrification, this technical solution not only effectively solves the limitation of biological denitrification caused by carbon source shortage, but also avoids the iron pollution problem caused by excessive addition of chemicals during chemical phosphorus removal, thereby achieving deep treatment of effluent and significantly improving the quality of the effluent.

[0091] In conjunction with the first aspect, in step S110, the effluent from the anaerobic tank is introduced into the aerobic tank for aerobic phosphorus uptake and nitrification to obtain the effluent from the aerobic tank, whose ammonia nitrogen concentration is less than 0.5 mg / L.

[0092] In conjunction with the first aspect, the step of returning the sludge settled in the secondary sedimentation tank to the anaerobic tank in step S120 includes: controlling the sludge concentration in the anaerobic tank and the aerobic tank to 3000-5000 mg / L through the return of the sludge settled in the secondary sedimentation tank.

[0093] Secondly, this application provides an urban sewage treatment system, combined with Figure 2As shown, it includes a control unit (not shown in the figure) for performing the above method, an AOA device and a secondary sedimentation tank. The AOA device includes an anaerobic zone, an aerobic zone and an anoxic zone connected in sequence. The secondary sedimentation tank is located between the aerobic zone and the anoxic zone.

[0094] The secondary sedimentation tank is also connected to the anaerobic zone through a return pipeline to return the settled sludge from the secondary sedimentation tank to the anaerobic zone.

[0095] The inlets of the anaerobic and anoxic zones are connected to the domestic sewage pipelines, respectively.

[0096] In conjunction with the second aspect, the anaerobic zone includes several anaerobic tanks, the aerobic zone includes several aerobic tanks, and the anoxic zone includes a raw water denitrification tank, a mixing tank, a flocculation sedimentation tank, an equalization tank, and an iron salt enhanced denitrification tank connected in sequence.

[0097] The raw water denitrification tank is filled with anaerobic ammonia oxidation biofilm packing or granular sludge; the iron salt enhanced denitrification tank is equipped with anaerobic ammonia oxidation packing frame.

[0098] An automatic iron salt dosing device is connected to a mixing tank to add iron salt to the mixing tank;

[0099] The iron and phosphorus recovery device is connected to the outlet of the flocculation sedimentation tank to recover and utilize the generated iron and phosphorus precipitate.

[0100] This invention places the secondary sedimentation tank between the aerobic and anoxic zones, a significant departure from the conventional practice of placing it at the end of the process. The main advantages of this arrangement include: the dissolved oxygen content of the sludge returned from the secondary sedimentation tank to the anaerobic zone is significantly lower than that in the aerobic zone, thus preserving the anaerobic environment and maintaining efficient anaerobic phosphorus release and denitrification; simultaneously, the sludge concentration in the secondary sedimentation tank is higher than in the aerobic zone, effectively reducing the energy consumption required for sludge return. Furthermore, the low dissolved oxygen characteristics of the effluent from the secondary sedimentation tank when entering the anoxic zone do not disrupt the anoxic environment; the secondary sedimentation tank also blocks the mutual flow of sludge between the aerobic and anoxic zones, preventing bacterial competition due to sludge mixing, thereby helping to maintain the biological activity, abundance, and stability of the functional bacteria (i.e., anaerobic ammonia-oxidizing bacteria) and their microbial system within the anoxic zone.

[0101] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working process of the system and apparatus described above can be referred to the corresponding process in the foregoing method embodiments, and will not be repeated here.

[0102] Furthermore, in the description of the embodiments of the present invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in the present invention based on the specific circumstances.

[0103] If the aforementioned functions are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this invention, essentially, or the part that contributes to the prior art, or a portion of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0104] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0105] Finally, it should be noted that the above embodiments are merely specific implementations of the present invention, used to illustrate the technical solutions of the present invention, and not to limit it. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any person skilled in the art can still modify or easily conceive of changes to the technical solutions described in the foregoing embodiments within the technical scope disclosed in the present invention, or make equivalent substitutions for some of the technical features; and these modifications, changes, or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for treating urban wastewater using iron salt-enhanced AOA, characterized in that, The method is applied to an urban wastewater treatment system; the method includes: A portion of domestic sewage is sequentially introduced into an anaerobic tank, an aerobic tank, and a secondary sedimentation tank to obtain effluent from the secondary sedimentation tank and settled sludge. The sludge settled in the secondary sedimentation tank is returned to the anaerobic tank; simultaneously, another portion of the domestic sewage and the effluent from the secondary sedimentation tank are introduced into the raw water denitrification tank in the anoxic zone at a specified ratio for denitrification treatment. Under the conditions of pH 7.07-7.29 and C / N ratio 1.14-2.86, the anaerobic ammonia oxidation sludge filled in the raw water denitrification tank undergoes a short-range nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation reaction and / or a full-range nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation reaction, to obtain the effluent from the raw water denitrification tank; The effluent from the raw water denitrification tank is sequentially fed into a mixing tank and a flocculation sedimentation tank. According to the type of nitrogen in the effluent from the raw water denitrification tank, a target iron salt is added to the mixing tank. After chemical phosphorus removal treatment, the effluent from the flocculation sedimentation tank is obtained. The effluent from the flocculation sedimentation tank is fed into the equalization tank. Under the condition that the effective iron-nitrogen ratio is 2:1 to 5:1, it is further introduced into the iron salt enhanced denitrification tank for deep purification to obtain effluent that meets the standards.

2. The method according to claim 1, characterized in that, The process of introducing a portion of domestic sewage into an anaerobic tank for anaerobic phosphorus release and denitrification to obtain effluent from the anaerobic tank includes: The dissolved oxygen in the anaerobic tank is below 0.2-0.5 mg / L, and the oxidation-reduction potential is controlled between -100 and -200 mV.

3. The method according to claim 1, characterized in that, The steps include: sequentially feeding the effluent from the raw water denitrification tank into a mixing tank and a flocculation sedimentation tank; adding a target iron salt to the mixing tank according to the type of nitrogen in the effluent from the raw water denitrification tank; and obtaining the effluent from the flocculation sedimentation tank after chemical phosphorus removal treatment. The raw water denitrification tank effluent is introduced into the mixing tank. If the nitrogen type in the raw water denitrification tank effluent is ammonia nitrogen, ferric salt is added; if the nitrogen type in the raw water denitrification tank effluent is nitrate nitrogen, or nitrate nitrogen and ammonia nitrogen, ferrous salt is added. When the mixing intensity is 1500–6000 s -1 Under the conditions of mixing and stirring, a mixed effluent is obtained; The mixed effluent is introduced into the flocculation sedimentation tank for solid-liquid separation to obtain chemically dephosphorized effluent.

4. The method according to claim 1, characterized in that, The effluent from the flocculation sedimentation tank is introduced into an equalization tank. While maintaining an effective iron-nitrogen ratio of 2:1 to 5:1, it is further introduced into an iron salt-enhanced denitrification tank for deep purification to obtain compliant effluent. This process includes: The effluent from the flocculation sedimentation tank is introduced into an equalization tank for adjustment to obtain effluent with a pH of 7.29-7.32 and an effective iron-nitrogen ratio of 2:1-5:

1. The effluent from the equalization tank is introduced into an iron salt enhanced denitrification tank filled with anaerobic ammonia oxidation sludge. The remaining iron salts and nitrogen in the effluent are used for denitrification to obtain effluent that meets the standards.

5. The method according to claim 4, characterized in that, If the remaining iron and nitrogen in the effluent are ferrous salts, nitrate nitrogen, and ammonia nitrogen, and the effective iron-nitrogen ratio is 2:1, then the denitrification reaction is a ferrous short-range nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation.

6. The method according to claim 4, characterized in that, If the remaining iron salts and nitrogen in the effluent are ferric salts and ammonia nitrogen, and the effective iron-nitrogen ratio is 3:1, then the denitrification reaction is a ferric anaerobic ammonia oxidation reaction.

7. The method according to claim 4, characterized in that, If the remaining iron and nitrogen in the effluent are ferrous salts and nitrate nitrogen, and the effective iron-nitrogen ratio is 5:1, then the denitrification reaction is a ferrous full-process nitrate dissimilatory reduction to ammonium coupled with anaerobic ammonia oxidation.

8. A city wastewater treatment system, characterized in that, The device includes a control unit for performing the method as described in any one of claims 1-7, an AOA device, and a secondary sedimentation tank, wherein the AOA device includes an anaerobic zone, an aerobic zone, and an anoxic zone connected in sequence, and the secondary sedimentation tank is located between the aerobic zone and the anoxic zone. The secondary sedimentation tank is also connected to the anaerobic zone via a return pipeline to return the settled sludge from the secondary sedimentation tank to the anaerobic zone. The inlet ends of the anaerobic zone and the anoxic zone are respectively connected to the domestic sewage pipeline; the anoxic zone includes a raw water denitrification tank, a mixing tank, a flocculation sedimentation tank, an equalization tank and an iron salt enhanced denitrification tank connected in sequence. The raw water denitrification tank is filled with anaerobic ammonia oxidation biofilm packing or granular sludge; the iron salt enhanced denitrification tank is equipped with an anaerobic ammonia oxidation packing frame.

9. The system according to claim 8, characterized in that, The anaerobic zone includes several anaerobic tanks; the aerobic zone includes several aerobic tanks. An automatic iron salt dosing device is connected to the mixing tank to add iron salt to the mixing tank; The iron and phosphorus recovery device is connected to the outlet end of the flocculation sedimentation tank to recover and utilize the generated iron and phosphorus precipitate.

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