Oxidation ditch process transformation device and method based on double-return pre-anoxic denitrification

Abstract

The present invention relates to a device for transforming an oxidation ditch process based on pre-anoxic denitrification and nitrogen removal and a method for using the same, belonging to the field of sewage treatment. The transformation device comprises a pre-anoxic tank, an oxidation ditch, a secondary sedimentation tank and a double reflux system; by transforming the existing primary sedimentation tank into a pre-anoxic tank or newly building a pre-anoxic tank and simultaneously establishing double sludge reflux and two-stage nitrification liquid reflux to form a two-stage anoxic denitrification system, the anoxic residence time is effectively extended, and the full utilization of the raw water carbon source is realized; and by reasonably distributing the carbon source and effectively controlling the two-stage nitrification liquid reflux ratio, the nitrogen removal and phosphorus removal effects of the oxidation ditch process are improved simultaneously.

Description

Technical Field

[0001] The present invention belongs to the technical field of biological treatment of sewage, and relates to an oxidation ditch process modification device based on pre-anaerobic denitrification and denitrification, and a method for using the same. The device is a two-stage anoxic denitrification system that uses an existing primary sedimentation tank or a newly built biochemical tank as a pre-anaerobic tank and simultaneously establishes a double sludge return and a two-stage nitrification liquid return. By extending the anoxic residence time, the raw water carbon source is fully utilized to achieve a method for strengthening denitrification and phosphorus removal by the traditional oxidation ditch process. The method can expand the volume of the anoxic tank without stopping the water in the original sewage treatment facility, realize the full utilization of the raw water carbon source, reduce the amount of external carbon source added, and reduce the operating cost. It solves the problem of high total nitrogen and total phosphorus in the effluent of the conventional oxidation ditch process. Background Art

[0002] The oxidation ditch process is one of the most widely used wastewater treatment technologies. Common oxidation types include: Carrousel oxidation ditch, Orbal oxidation ditch, T-type oxidation ditch, DE-type oxidation ditch, etc. In the early days, due to the low sewage discharge standards, the oxidation ditch process was mainly aimed at removing organic matter. It is simple to operate, easy to manage, and has high shock load resistance, so it has become the mainstream process for sewage treatment. Actual operation results show that the effluent COD and NH4 + -N can all meet the standards stably, but the TN and TP indicators may be too high, especially after the release of a new round of local standards, the problem of TN and TP exceeding the standards in the effluent from sewage treatment plants has become increasingly prominent.

[0003] The sewage denitrification process is generally as follows: ammonia nitrogen (NH4 + -N) is oxidized to nitrite (NO2 - -N), which is then oxidized to nitrate nitrogen (NO3 - -N), and finally, in an anaerobic environment, denitrifying bacteria (Denitrification Bacteria) use organic matter as electron donors to reduce nitrate nitrogen to nitrogen gas (N2) to achieve wastewater denitrification. The biological phosphorus removal process relies on polyphosphate bacteria PAO (Polyphosphate Accumulating Organisms) to absorb volatile organic acids in the raw water in an anaerobic environment, fully releasing phosphates, and at the same time, excessive phosphorus absorption in an aerobic environment, enriching phosphates in the sewage into sludge and achieving phosphorus removal through sludge discharge. Based on the above theory, the main factors affecting wastewater denitrification and phosphorus removal include: anaerobic environment, carbon source, reflux ratio, temperature, sludge concentration, etc.

[0004] Based on the above analysis, the main reasons for the relatively high TN and TP in the effluent of the traditional oxidation ditch process usually lie in the following aspects: ① The oxidation ditch process pool type presents a circular channel form, without obvious anaerobic, anoxic, and aerobic zone divisions, and the reaction environment for denitrification and phosphorus removal is not sufficient; ② The return of nitrified liquid is usually controlled by a return gate, lacking accurate return metering, and the dissolved oxygen in the anoxic zone is not well controlled; ③ The C / N ratio of urban domestic sewage is generally low, and the carbon source in the raw water is not sufficient; ④ The anoxic residence time in the traditional oxidation ditch is generally 2 - 3 hours, the reaction time is generally short, and there may be problems such as uneven mixing and short circuiting, and the effective pool volume cannot be fully utilized; ⑤ The sludge return ratio is relatively large, carrying a high concentration of NO3 - -N, resulting in too high dissolved oxygen in the anaerobic zone or the competition between denitrifying bacteria and phosphorus-accumulating bacteria for electron donors, and insufficient anaerobic phosphorus release, etc.

[0005] In recent years, with the introduction of more stringent sewage discharge standards in various places, the conventional oxidation ditch process faces comprehensive upgrading and transformation. The commonly used transformation technical routes include: adding biological fillers, re-dividing the areas of the biochemical pool, increasing the depth treatment of the denitrifying filter, changing to MBR membrane treatment, adding high-efficiency strains, etc. At the same time, in order to minimize environmental pollution as much as possible, it is usually required to carry out the transformation without stopping water. According to the actual operation situation, there are various problems with the above transformations, mainly manifested in high reagent energy consumption, insufficient volume of the biochemical pool, serious accumulation of biological fillers and difficult operation, etc. For this reason, the present invention provides a process transformation device and method for the oxidation ditch process based on pre-anoxic denitrification and nitrogen removal, which can carry out the transformation without stopping water, maximize the use of the raw water carbon source for denitrification and nitrogen removal through two-stage anoxia, effectively reduce the nitrate concentration in the anaerobic zone, provide a sufficient reaction environment for the anaerobic phosphorus release of sludge, and can improve the removal rates of total nitrogen and total phosphorus through local transformation, reduce the dosage of externally added carbon source, and improve the impact resistance of the system. Summary of the Invention

[0006] The purpose of the present invention is to overcome the problems in the existing oxidation ditch process such as insufficient anoxic residence time, inaccurate control of the return ratio, and relatively high dissolved oxygen in the anoxic zone, and provide an oxidation ditch process transformation device and usage method based on double-return pre-anoxic denitrification, which has reasonable process technology, does not require water stoppage, is convenient for transformation, can maximize the use of the raw water carbon source for denitrification and nitrogen removal, thereby effectively reducing the operating cost. It has the advantages of improving the removal rates of total nitrogen and total phosphorus through local transformation, reducing the dosage of externally added carbon source, and improving the impact resistance of the system.

[0007] To achieve the above object, the present invention is realized by adopting the following technical solutions:

[0008] An oxidation ditch process transformation device based on double reflux pre - anoxic denitrification. The transformation device includes a pre - anoxic tank, an oxidation ditch, a secondary sedimentation tank, a secondary sedimentation tank distribution well, a sludge return pump house, and a two - stage sludge return system and a two - stage nitrification liquid return system. Among them, the pre - anoxic tank is transformed from the original primary sedimentation tank or is a newly built structure, and the oxidation ditch, secondary sedimentation tank, secondary sedimentation tank distribution well, and sludge return pump house are original structures; the secondary sludge return pump and the secondary sludge return pipe are original facilities; the two - stage sludge return system includes a primary sludge return for returning the activated sludge discharged from the secondary sedimentation tank to the pre - anoxic tank and a secondary sludge return for returning the activated sludge discharged from the secondary sedimentation tank to the anaerobic section of the oxidation ditch; the primary sludge return is a newly built facility, which includes a primary sludge return pump, a primary sludge return pipe, and a primary sludge flowmeter; the secondary sludge return is an original structure, which includes a secondary sludge return pump and a secondary sludge return pipe; the two - stage nitrification liquid return system includes a primary nitrification liquid return for returning the nitrification liquid generated in the aerobic zone of the oxidation ditch to the pre - anoxic tank and a secondary nitrification liquid return for providing sufficient nitrification liquid to the anoxic zone of the oxidation ditch; among them, the primary nitrification liquid return is a newly built facility, including a primary nitrification liquid return pump, a primary nitrification liquid return pipe, and a primary nitrification liquid flowmeter; the secondary nitrification liquid return includes a wall - piercing pump; the pre - anoxic tank includes a reflux mixing zone and a pre - anoxic reaction zone connected to each other, and the reflux mixing zone is connected to the sewage pretreatment facility, the primary sludge return, and the primary nitrification liquid return; the oxidation ditch includes an oxidation ditch distribution well, an oxidation ditch anaerobic tank, an oxidation ditch anoxic tank, and an oxidation ditch aerobic tank connected to each other in sequence through pipelines; the pre - anoxic reaction zone is connected to the oxidation ditch distribution well through a pipeline; the oxidation ditch aerobic tank is connected to the secondary sedimentation tank distribution well; the secondary sedimentation tank is connected to the secondary sedimentation tank distribution well and the advanced treatment system through pipelines, and the advanced treatment system is composed of a flocculation sedimentation tank, a fabric filter, and an ultraviolet disinfection tank connected to each other; the secondary sedimentation tank is connected to the sludge return pump house through a pipeline; the effluent of the pre - anoxic tank is distributed through the oxidation ditch distribution well into the oxidation ditch anaerobic tank for mixing reaction with the secondary sludge return, then enters the oxidation ditch anoxic tank for mixing reaction with the secondary nitrification liquid return, and finally enters the oxidation ditch aerobic tank for reaction and then enters the secondary sedimentation tank through the secondary sedimentation tank distribution well for sedimentation and separation of mud and water; the sludge return pump house is connected to the secondary sedimentation tank sludge discharge pipe, the sludge return pump house connects the primary sludge return pump and the secondary sludge return pump, the primary sludge return pump is connected to the pre - anoxic tank reflux mixing through the primary sludge return pipe and the primary sludge flowmeter; the secondary sludge return pump is connected to the oxidation ditch distribution well through the secondary sludge return pipe; the primary nitrification liquid return pump is arranged at the end of the oxidation ditch aerobic tank, and the primary nitrification liquid return pump is connected to the pre - anoxic tank reflux mixing zone through the primary nitrification liquid return pipe and the primary nitrification liquid flowmeter; the wall - piercing pump returns the nitrification liquid in the oxidation ditch aerobic tank to the oxidation ditch anoxic tank through a pluggable oxidation ditch reflux gate.

[0009] As a further optimization of the above solution, the sewage pretreatment facility includes a filter screen for removing suspended solids and floating objects; the transformation device further includes an on-line monitoring facility, and the on-line monitoring facility includes an on-line pH meter, an ORP meter and a thermometer arranged in the pre-aerobic tank, an ORP meter and a DO detector arranged in the anoxic tank of the oxidation ditch, and a DO detector arranged in the aerobic tank of the oxidation ditch. The control range of the on-line pH meter is: 6.5 to 9.0, the control range of the ORP meter arranged in the pre-aerobic tank is less than -100 mV, and the thermometer is used to monitor the influent water temperature and the control range is: 6 to 26 °C; the control range of the ORP meter arranged in the anoxic tank of the oxidation ditch is less than -150 mV, the control range of the DO detector arranged in the anoxic tank of the oxidation ditch is less than 0.5 mg / L, and the control range of the DO detector arranged in the aerobic tank of the oxidation ditch is 2 to 4 mg / L.

[0010] As a further optimization of the above solution, an external carbon source dosing device is provided in the anoxic tank of the oxidation ditch, and the dosing point is located at the starting end of the anoxic tank.

[0011] The usage method of the above-mentioned oxidation ditch process transformation device based on double reflux pre-aerobic denitrification of the present invention includes the following steps:

[0012] 1) Domestic sewage enters the sewage treatment plant through the municipal pipe network, and first passes through the grille, lifting, grit removal, and sedimentation to remove suspended solids, floating objects and sediment in the water; after pretreatment, it enters the pre-aerobic tank and is mixed with the first-stage sludge reflux and the first-stage nitrification liquid reflux. Control the hydraulic retention time to be 3.5 - 5.0 h, the first-stage sludge reflux ratio to be 40% - 50%, the first-stage nitrification liquid reflux ratio to be 150% - 200%, the pH to be 6.5 - 7.5, the ORP to be -200 to -100 mV, and the temperature to be 6 - 26 °C. The sludge concentration is 3000 - 4000 mg / L. Under the action of the pre-aerobic tank agitator in the pre-aerobic tank, the sludge and the raw water are fully mixed and reacted, and the raw water carbon source is used for denitrification and nitrogen removal. The NO3 in the reflux sludge and the reflux nitrification liquid - -N is reduced to nitrogen gas; after the reaction, the COD, NH4+-N, and TN indexes of the effluent from the pre-aerobic tank are reduced to less than 60 mg / L, 15 mg / L, and 20 mg / L respectively, and the TN removal contribution rate of the first-stage anoxic denitrification can reach more than 60%; the main purpose of this stage is to remove the NO3 brought back by the first-stage sludge reflux and the first-stage nitrification liquid reflux - -N, which is an important way to solve the problem of high TN in the effluent of the oxidation ditch process; at the same time, the long-term anaerobic and anoxic environment is conducive to the conversion of macromolecular organic matter in the raw water into small-molecular organic matter, which is conducive to anaerobic phosphorus release and denitrification and nitrogen removal in the subsequent stage;

[0013] 2) The effluent from the pre-aerobic tank enters the anaerobic tank through the oxidation ditch distribution well, mixes with the secondary sludge reflux, controls the hydraulic retention time at 1.5 - 2 h, the secondary sludge reflux ratio at 50% - 60%, ORP: -300 - -150 mV, the mixed liquor sludge concentration reaches 3500 - 4500 mg / L, and the TP concentration of the mixed liquor reaches above 12 mg / L after the reflux sludge fully releases phosphorus. This stage provides favorable conditions for excessive phosphorus uptake in the aerobic zone;

[0014] 3) The effluent from the anaerobic tank of the oxidation ditch enters the anoxic tank, mixes with the second reflux nitrified liquid, controls the hydraulic retention time at 3 - 4 h, the secondary nitrified liquid reflux ratio at 150% - 200%, ORP: -150 - 0 mV, DO < 0.5 mg / L, the mixed liquor sludge concentration reaches 3500 - 4000 mg / L. When the influent BOD / TN ratio is lower than 3.0, appropriate carbon source is added according to the effluent TN concentration. The carbon source dosage is determined according to adding 6 - 8 mg / L of COD equivalent for every 1 mg / L of total nitrogen removed; when the effluent TN is stably lower than 15 mg / L, no carbon source is added;

[0015] 4) The effluent from the anoxic tank enters the aerobic tank, controls the hydraulic retention time at 9 - 15 h, the DO at the start of the oxidation ditch: 0.5 - 1.0 mg / L, the DO in the middle: 2.5 - 4 mg / L, the DO at the end: 2 - 3 mg / L, the sludge concentration: 3500 - 4000 mg / L; the main purpose of this stage is to convert NH4 + -N into NO3 - -N, while removing the residual COD and allowing the polyphosphate-accumulating organisms to fully absorb phosphorus, achieving an effluent COD lower than 40 mg / L and TP lower than 0.8 mg / L;

[0016] 5) The effluent from the oxidation ditch enters the secondary sedimentation tank for sludge-water separation. The sedimentation time of the secondary sedimentation tank is 3 - 4 h, the effluent suspended solids SS ≤ 20 mg / L, TP ≤ 1.0 mg / L, TN ≤ 12 mg / L, NH4 + -N ≤ 1.0 mg / L, COD ≤ 40 mg / L; the main purpose is to separate the sludge and water. The supernatant enters the advanced treatment system as the biochemical effluent, and the sludge is refluxed to the biochemical tank through the sludge reflux pump room to continue to play a role. The sludge in the secondary sedimentation tank enters the sludge pump room. Part of it is refluxed to the pre-aerobic tank and the anaerobic tank, and part of it is discharged as excess sludge. The daily discharge amount of excess sludge is about 3% - 4% of the influent volume.

[0017] Adopting the oxidation ditch process transformation device and method of the present invention based on double reflux pre-aerobic denitrification has the following beneficial effects:

[0018] 1) Based on the original oxidation ditch process, the present invention extends the anoxic reaction time of the traditional oxidation ditch process from 3 - 4 h to 6.5 - 8 h by adding a pre - anoxic reaction tank, making the reaction more complete and the operation more stable. It mainly solves the problem that the nitrification reaction of the traditional oxidation ditch process is complete while the denitrification reaction is insufficient, and the effluent quality can stably meet the new local standard requirements for TN.

[0019] 2) Placing the anoxic tank in front can maximize the utilization of the raw water carbon source for denitrification and nitrogen removal. The anoxic tank has a high BOD load and rich carbon sources for denitrifying bacteria, which is very conducive to the growth of denitrifying bacteria, solving the problems that the carbon source of the traditional oxidation ditch process cannot be effectively utilized, a large amount of external carbon source needs to be added, and the operation cost is high.

[0020] 3) The pre - anoxic tank is newly built or transformed from the original primary sedimentation tank, avoiding the need to re - divide the anoxic / aerobic zones or add fillers inside the oxidation ditch, and enabling the transformation without stopping the water supply. At the same time, the overall residence time of the biochemical tank is extended, ensuring that production is not reduced while meeting the stricter standards.

[0021] 4) Adopting a double - reflux system makes the transformation simple and convenient. Firstly, it can maintain a sufficient sludge concentration in the pre - anoxic tank to ensure the smooth progress of the denitrification reaction. Secondly, the pre - anoxic tank removes most of the TN, reducing the NO3 - -N concentration entering the anaerobic tank, avoiding the competition of denitrifying bacteria for electron donors, thus effectively strengthening the anaerobic phosphorus release effect and increasing the total phosphorus removal rate. Finally, the nitrification liquid reflux ratios of the two - stage anoxic tanks are relatively small (about 200%), while the total reflux ratio (about 400%) is greater than that of the conventional process (about 300%). While avoiding excessive dissolved oxygen brought back by the nitrification liquid reflux, the theoretical total nitrogen removal rate is increased by increasing the internal reflux ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG Figures 1-3 is a schematic diagram of a method for transforming an oxidation ditch process based on double - reflux pre - anoxic denitrification according to the present invention. Among them,

[0023] FIG Figure 1 is a schematic diagram of the transformation of an oxidation ditch process based on double - reflux pre - anoxic denitrification according to the present invention.

[0024] FIG Figure 2 is a schematic diagram of an on - line monitoring device for the transformation of an oxidation ditch based on double - reflux pre - anoxic denitrification according to the present invention.

[0025] FIG Figure 3 is a schematic diagram of the A - A cross - section of the transformation of an oxidation ditch based on double - reflux pre - anoxic denitrification according to the present invention.

[0026] In the above - mentioned drawings, the specific meanings of each reference numeral are as follows:

[0027] 1 - Pre - anoxic tank; 2 - Oxidation ditch; 3 - Sedimentation area; 1.1 - Return mixing area; 1.2 - Pre - anoxic tank reaction area; 1.3 - Pre - anoxic tank agitator; 2.1 - Oxidation ditch distribution well; 2.2 - Anaerobic zone of oxidation ditch; 2.3 - Anoxic zone of oxidation ditch; 2.4 - Aerobic zone of oxidation ditch; 2.5 - Oxidation ditch agitator; 2.6 - Wall - piercing pump; 2.7 - Secondary nitrification liquid return; 2.8 - Oxidation ditch return gate; 2.9 - Primary nitrification liquid return pump; 2.10 - Primary nitrification liquid return pipe; 2.11 - Primary nitrification liquid flowmeter; 2.12 - Primary nitrification liquid return; 2.13 - Oxidation ditch effluent area; 3.1 - Secondary sedimentation tank; 3.2 - Secondary sedimentation tank distribution well; 3.3 - Sludge return pump house; 3.4 - Secondary sludge return pump; 3.5 - Secondary sludge return pipe; 3.6 - Secondary sludge return; 3.7 - Primary sludge return pump; 3.8 - Primary sludge return pipe; 3.9 - Primary sludge flowmeter; 3.10 - Primary sludge return; 3.11 - Secondary sedimentation tank sludge discharge pipe; 4.1 - pH meter; 4.2 - ORP measuring instrument; 4.3 - Thermometer; 4.4 - DO measuring instrument. Specific implementation method

[0028] Combined with the above - mentioned drawings, a detailed description of a method for modifying the oxidation ditch process based on double - return pre - anoxic denitrification of the present invention is as follows:

[0029] An oxidation ditch process transformation device based on double reflux pre - anoxic denitrification. The transformation device includes a pre - anoxic tank 1, an oxidation ditch 2, a secondary sedimentation tank 3.1, a secondary sedimentation tank distribution well 3.2, a sludge return pump house 3.3, and a two - stage sludge return system and a two - stage nitrification liquid return system. Among them, the pre - anoxic tank 1 is transformed from the original primary sedimentation tank or is a newly built structure, and the oxidation ditch 2, the secondary sedimentation tank 3.1, the secondary sedimentation tank distribution well 3.2, and the sludge return pump house 3.3 are original structures; the secondary sludge return pump 3.4 and the secondary sludge return pipe 3.5 are original facilities; the two - stage sludge return system includes a primary sludge return 3.10 for returning the activated sludge discharged from the secondary sedimentation tank 3.1 to the pre - anoxic tank 1 and a secondary sludge return 3.6 for returning the activated sludge discharged from the secondary sedimentation tank 3.1 to the anaerobic section 2.2 of the oxidation ditch; the primary sludge return 3.10 is a transformed facility, including a primary sludge return pump 3.7, a primary sludge return pipe 3.8, and a primary sludge flowmeter 3.9; the secondary sludge return 3.6 is an original facility, including a secondary sludge return pump 3.4 and a secondary sludge return pipe 3.5; the two - stage nitrification liquid return system includes a primary nitrification liquid return 2.12 for returning the nitrification liquid generated in the aerobic zone 2.4 of the oxidation ditch to the pre - anoxic tank 1 and a secondary nitrification liquid return 2.7 for providing sufficient nitrification liquid to the anoxic zone 2.3 of the oxidation ditch; among them, the primary nitrification liquid return 2.12 is a transformed facility, including a primary nitrification liquid return pump 2.9, a primary nitrification liquid return pipe 2.10, and a primary nitrification liquid flowmeter 2.11; the secondary nitrification liquid return 2.7 includes a wall - piercing pump 2.6; the pre - anoxic tank 1 includes a reflux mixing zone 1.1 and a pre - anoxic reaction zone 1.2 connected to each other, and the reflux mixing zone is connected to the sewage pretreatment facility, the primary sludge return 3.10, and the primary nitrification liquid return 2.12; the oxidation ditch includes an oxidation ditch distribution well 2.1, an oxidation ditch anaerobic tank 2.2, an oxidation ditch anoxic tank 2.3, and an oxidation ditch aerobic tank 2.4 connected to each other in sequence through pipelines; the pre - anoxic reaction zone 1.2 is connected to the oxidation ditch distribution well 2.1 through a pipeline; the oxidation ditch aerobic tank is connected to the secondary sedimentation tank distribution well 3.2; the secondary sedimentation tank is connected to the secondary sedimentation tank distribution well 3.2 and the advanced treatment system through pipelines, and the advanced treatment system is composed of a flocculation sedimentation tank, a fabric filter tank, and an ultraviolet disinfection tank connected to each other; the secondary sedimentation tank is connected to the sludge return pump house 3.3 through a pipeline; the water outlet of the pre - anoxic tank 1 is distributed through the oxidation ditch distribution well 2.1 and enters the oxidation ditch anaerobic tank 2.2 for mixing reaction with the secondary sludge return 3.6, then enters the oxidation ditch anoxic tank 2.3 for mixing reaction with the secondary nitrification liquid return 2.7, and finally enters the oxidation ditch aerobic tank 2.4 for reaction and then enters the secondary sedimentation tank 3.1 through the secondary sedimentation tank distribution well 3.2 for sedimentation and separation of mud and water; the sludge return pump house 3.3 is connected to the secondary sedimentation tank sludge discharge pipe 3.11, the sludge return pump house 3.3 connects the primary sludge return pump 3.7 and the secondary sludge return pump 3.4, and the primary sludge return pump 3.7 is connected to the reflux mixing zone 1.1 of the pre-anoxic tank through the primary sludge reflux pipe 3.8 and the primary sludge flowmeter 3.9; the secondary sludge reflux pump 3.4 is connected to the oxidation ditch distribution well 2.1 through the secondary sludge reflux pipe 3.5; the primary nitrification liquid reflux pump 2.9 is arranged at the end of the aerobic tank 2.4 of the oxidation ditch, and the primary nitrification liquid reflux pump 2.9 is connected to the reflux mixing zone 1.1 of the pre-anoxic tank through the primary nitrification liquid reflux pipe 2.10 and the primary nitrification liquid flowmeter 2.11; the wall-piercing pump 2.6 returns the nitrification liquid in the aerobic tank 2.4 of the oxidation ditch to the anoxic tank 2.3 of the oxidation ditch through the sealable oxidation ditch reflux gate 2.8. The sewage pretreatment facility includes a filter screen for removing suspended solids and floating objects; the transformation device also includes an on-line monitoring facility, and the on-line monitoring facility includes an on-line pH meter 4.1, an ORP meter 4.2 and a thermometer 4.3 arranged in the pre-anoxic tank 1.2, an ORP meter 4.2 and a DO detector 4.4 arranged in the anoxic tank 2.3 of the oxidation ditch, and a DO detector 4.4 arranged in the aerobic tank 2.4 of the oxidation ditch. The control range of the on-line pH meter is: 6.5-9.0, the control range of the ORP meter 4.2 arranged in the pre-anoxic tank 1.2 is less than -100 mV, and the thermometer 4.3 is used to monitor the influent water temperature and the control range is: 6-26 °C; the control range of the ORP meter 4.2 arranged in the anoxic tank 2.3 of the oxidation ditch is less than -150 mV, the control range of the DO detector 4.4 arranged in the anoxic tank 2.3 of the oxidation ditch is less than 0.5 mg / L, and the control range of the DO detector 4.4 arranged in the aerobic tank 2.4 of the oxidation ditch is 2-4 mg / L. An external carbon source dosing device is arranged in the anoxic tank of the oxidation ditch, and the dosing point is located at the starting end of the anoxic tank.

[0030] The usage method of the above-mentioned oxidation ditch process transformation device based on dual reflux pre-anoxic denitrification of the present invention includes the following steps:

[0031] 1) Domestic sewage enters the sewage treatment plant through the municipal pipe network. First, it passes through the grille, lifting, grit removal, and sedimentation to remove suspended solids, floating objects, and sediment in the water; after pretreatment, it enters the pre-anoxic tank and is mixed with the primary sludge reflux and the primary nitrification liquid reflux. The hydraulic retention time is controlled at 3.5-5.0 h, the primary sludge reflux ratio is 40%-50%, the primary nitrification liquid reflux ratio is 150%-200%, the pH is 6.5-7.5, the ORP is -200 to -100 mV, and the temperature is 6-26 °C. The sludge concentration is 3000-4000 mg / L. Under the action of the pre-anoxic tank agitator in the pre-anoxic tank, the sludge and the raw water are fully mixed and reacted, and the raw water carbon source is used for denitrification and nitrogen removal. The NO3 - -N in the reflux sludge and the reflux nitrification liquid is reduced to nitrogen; after the reaction, the effluent COD and NH4 +-N and TN indexes are respectively reduced to below 60 mg / L, 15 mg / L, and 20 mg / L, and the TN removal contribution rate of the first-stage anoxic denitrification can reach over 60%; the main purpose of this stage is to remove NO3 - -N brought back by the first-stage sludge return and the first-stage nitrified liquid return, which is an important way to solve the problem of high TN in the effluent of the oxidation ditch process; at the same time, the long-term anaerobic and anoxic environment is conducive to the conversion of macromolecular organic matter in the raw water into small-molecular organic matter, which is conducive to anaerobic phosphorus release and denitrification and nitrogen removal in the latter stage;

[0032] 2) The effluent from the pre-positioned anoxic tank enters the anaerobic tank through the oxidation ditch distribution well, mixes with the second-stage sludge return, controls the hydraulic retention time at 1.5 - 2 h, the second-stage sludge return ratio at 50% - 60%, ORP: -300 - -150 mV, the mixed liquor sludge concentration reaches 3500 - 4500 mg / L, and the TP concentration of the mixed liquor reaches over 12 mg / L after the reflux sludge fully releases phosphorus. This stage provides favorable conditions for excessive phosphorus uptake in the aerobic zone;

[0033] 3) The effluent from the anaerobic tank of the oxidation ditch enters the anoxic tank, mixes with the second reflux nitrified liquid, controls the hydraulic retention time at 3 - 4 h, the second-stage nitrified liquid return ratio at 150% - 200%, ORP: -150 - 0 mV, DO < 0.5 mg / L, the mixed liquor sludge concentration reaches 3500 - 4000 mg / L. When the influent BOD / TN ratio is lower than 3.0, carbon source is appropriately added according to the effluent TN concentration, and the carbon source addition amount is determined according to adding 6 - 8 mg / L of COD equivalent for every 1 mg / L of total nitrogen removed; when the effluent TN is stably lower than 15 mg / L, no carbon source is added;

[0034] 4) The effluent from the anoxic tank enters the aerobic tank, controls the hydraulic retention time at 9 - 15 h, the DO at the start of the oxidation ditch: 0.5 - 1.0 mg / L, the DO in the middle: 2.5 - 4 mg / L, the DO at the end: 2 - 3 mg / L, and the sludge concentration: 3500 - 4000 mg / L; the main purpose of this stage is to convert NH4 + -N into NO3 - -N, while removing the residual COD and allowing the polyphosphate-accumulating organisms to fully absorb phosphorus, achieving an effluent COD lower than 40 mg / L and TP lower than 0.8 mg / L;

[0035] 5) The effluent from the oxidation ditch enters the secondary sedimentation tank for mud-water separation. The sedimentation time of the secondary sedimentation tank is 3 - 4 h, and the effluent suspended solids SS ≤ 20 mg / L, TP ≤ 1.0 mg / L, TN ≤ 12 mg / L, NH4 +-N ≤ 1.0 mg / L, COD ≤ 40 mg / L. The main purpose is to separate the sludge from the water. The supernatant is used as the effluent from the biochemical treatment and enters the advanced treatment system. The sludge is returned to the biochemical pool through the sludge return pump room to continue to play its role. The sludge in the secondary sedimentation tank enters the sludge pump room. Part of it is returned to the pre-anoxic tank and the anaerobic tank, and part is discharged as excess sludge. The daily discharge of excess sludge is about 3% - 4% of the influent volume.

[0036] The present invention has substantial differences from the conventional retrofit process technologies, which are mainly manifested in:

[0037] (1) Compared with the conventional modified AA / O process and its variant processes, the reaction time in the pre-anoxic section of the modified AA / O process is generally controlled within 0.5 - 1.0 h, and there is only one sludge return. All or part of the sludge is returned to the pre-anoxic section to eliminate the nitrate in the returned sludge. Its main purpose is to strengthen anaerobic phosphorus release. However, the present invention contains a double return system. There is both nitrification liquid return and sludge return in the pre-anoxic tank. Its main purpose is to strengthen nitrogen removal. There are substantial differences between the pre-anoxic tank and the pre-anoxic section.

[0038] (2) The present invention has obvious differences from the conventional retrofit methods of extending the residence time in the anoxic zone. The conventional retrofit methods generally extend the anoxic residence time by designating a part of the aerobic zone as the anoxic zone, thereby compressing the aerobic residence time, which often causes insufficient nitrification and leads to a reduction in the designed water volume of the biochemical pool. Its process main body is still the AA / O process. The present invention uses an external source to expand the anoxic zone, and it is located before the anaerobic zone, forming an obvious two-stage anoxic zone. There are independent nitrification liquid return systems respectively set, which has a significant improvement and substantial differences in the total nitrogen removal rate and process principle compared with the first-stage denitrification and nitrogen removal of the conventional AA / O process.

[0039] (3) In terms of carbon source distribution, the present invention preferentially distributes the raw water carbon source to the pre-anoxic tank and then uses it for anaerobic phosphorus release. The ultra-long residence time in the pre-anoxic tank is conducive to the hydrolysis and acidification of the raw water carbon source, converting macromolecular organic matter into small molecular volatile organic acids, which is more conducive to the absorption by polyphosphate-accumulating organisms and the occurrence of anaerobic phosphorus release. Therefore, while strengthening nitrogen removal, it improves the biological phosphorus removal effect.

[0040] The following takes the retrofit and operation commissioning of a certain sewage treatment plant as an example to further illustrate the beneficial effects of the present invention. The actual operation effect of this sewage treatment plant is as follows:

[0041] The designed scale of this sewage treatment plant is 10,000 t / d, and the main body adopts the oxidation ditch process. The original process flow is: influent

[0042]

[0043]

[0044] Denitrification and nitrogen removal in the anoxic tank, and the effluent NH4 + -N is reduced to 4 - 8 mg / L, TN: 12 - 16 mg / L, COD: 40 - 60 mg / L.

[0045] (4) The effluent from the pre - anoxic tank 1 is mixed with the secondary sludge return 3.6 through the oxidation ditch distribution well 2.1 and enters the anaerobic tank 2.2 of the oxidation ditch. The hydraulic retention time is controlled at 1.96 h, the secondary sludge return ratio is 50%, ORP: - 250 to - 170 mV, the mixed - liquor sludge concentration reaches 4300 mg / L, and the TP concentration in the mixed liquor reaches more than 15 mg / L after the reflux sludge fully releases phosphorus.

[0046] (5) The effluent from the anaerobic tank 2.2 of the oxidation ditch enters the anoxic tank 2.3 and is mixed with the second nitrification liquid return 2.7. The hydraulic retention time is controlled at 3.85 h, the secondary nitrification liquid return ratio is 200%, ORP: - 80 to 0 mV, DO < 0.35 mg / L, and the mixed - liquor sludge concentration reaches 3800 mg / L. After the reaction, the effluent TN is maintained at 5 - 12 mg / L. When the effluent TN is stably higher than 13 mg / L, an external carbon source is added at a concentration of 25 - 40 mgCOD / L, and the effluent TN can be stably maintained below 12 mg / L.

[0047] (6) The effluent from the anoxic tank 2.3 of the oxidation ditch enters the aerobic tank 2.4 of the oxidation ditch. The hydraulic retention time is controlled at 14.45 h. The initial DO of the aerobic tank 2.4 of the oxidation ditch is 0.7 - 0.92 mg / L, the middle DO is 2.5 - 3.3 mg / L, the end DO is 2.5 - 3.0 mg / L, and the sludge concentration is 3800 mg / L.

[0048] (7) The effluent from the oxidation ditch 2 enters the secondary sedimentation tank 3.1 for sludge - water separation. The sedimentation time of the secondary sedimentation tank 3.1 is 3.96 h. The effluent suspended solids SS ≤ 12.8 mg / L, TP ≤ 0.86 mg / L, TN ≤ 12 mg / L, NH4 + -N ≤ 0.26 mg / L, COD ≤ 32 mg / L. The sludge from the secondary sedimentation tank 3.1 enters the sludge pump house 3.3, 50% is refluxed to the pre - anoxic tank 1, 50% is refluxed to the anaerobic tank 2.2 of the oxidation ditch, and the remaining sludge discharge is about 310 m 3 , with a sludge concentration of 7600 mg / L, equivalent to removing about 11.78 t / d of 80% of the remaining sludge per day.

[0049] After 11 months of stable operation, the actual influent and effluent water quality indicators are as follows:

[0050]

[0051] (8) After transformation, the TN and TP indicators of the effluent are significantly lower than those of the original oxidation ditch process. When the influent temperature reaches as low as 6°C, all the effluent indicators can stably meet the Class A standards of the Comprehensive Wastewater Discharge Standard for the Yellow River Basin in Shaanxi Province (DB61 / 224-2018). The actual operating energy consumption and the cost of carbon source chemicals decrease by 12% and 60% respectively.

[0052] The above actual case is for the convenience of those skilled in the art to further understand the present invention. Those skilled in the art can obviously easily apply it to the technical transformation of similar oxidation ditches and AA / O processes. Therefore, the present invention is not limited to the application in this single project, but has significant general applicability and scientific rationality. Other technicians in the art should be within the protection scope of the present invention for the improvements and modifications made to similar cases according to the inspiration of the present invention.

Claims

1. An oxidation ditch process transformation device based on double reflux pre-anoxic denitrification, characterized in that: The retrofit device uses the existing primary sedimentation tank as a pre-anoxic tank and establishes a two-stage anoxic denitrification system with dual sludge recirculation and two-stage nitrification liquid recirculation. By extending the anoxic residence time and making full use of the raw water carbon source, it realizes enhanced nitrogen and phosphorus removal in the traditional oxidation ditch process. It includes a pre-anoxic tank, an oxidation ditch, a secondary sedimentation tank, a secondary sedimentation tank water distribution well, a sludge recirculation pump house, and a two-stage sludge recirculation system and a two-stage nitrification liquid recirculation system. Among them, the pre-anoxic tank is retrofitted; the oxidation ditch, secondary sedimentation tank, secondary sedimentation tank water distribution well, and sludge recirculation pump house are original structures; the secondary sludge recirculation pump and the secondary sludge recirculation pipe are original facilities; the two-stage sludge recirculation system includes a primary sludge recirculation for returning the activated sludge discharged from the secondary sedimentation tank to the pre-anoxic tank and a secondary sludge recirculation for returning the activated sludge discharged from the secondary sedimentation tank to the anaerobic section of the oxidation ditch; the primary sludge recirculation is a retrofit facility, including a primary sludge recirculation pump, a primary sludge recirculation pipe, and a primary sludge flowmeter; the secondary sludge recirculation is an original facility, including a secondary sludge recirculation pump and a secondary sludge recirculation pipe; the two-stage nitrification liquid recirculation system includes a primary nitrification liquid recirculation for returning the nitrification liquid generated in the aerobic zone of the oxidation ditch to the pre-anoxic tank and a secondary nitrification liquid recirculation for providing sufficient nitrification liquid to the anoxic zone of the oxidation ditch; among them, the primary nitrification liquid recirculation is a retrofit facility, including a primary nitrification liquid recirculation pump, a primary nitrification liquid recirculation pipe, and a primary nitrification liquid flowmeter; the secondary nitrification liquid recirculation includes a wall-piercing pump; the pre-anoxic tank includes a reflux mixing zone and a pre-anoxic reaction zone connected to each other, and the reflux mixing zone is connected to the sewage pretreatment facility, the primary sludge recirculation, and the primary nitrification liquid recirculation; the oxidation ditch includes an oxidation ditch water distribution well, an oxidation ditch anaerobic tank, an oxidation ditch anoxic tank, and an oxidation ditch aerobic tank connected in sequence through pipes; the pre-anoxic reaction zone is connected to the oxidation ditch water distribution well through a pipe; the oxidation ditch aerobic tank is connected to the secondary sedimentation tank water distribution well; the secondary sedimentation tank is connected to the secondary sedimentation tank water distribution well and the advanced treatment system through pipes, and the advanced treatment system is composed of a flocculation sedimentation tank, a fabric filter, and an ultraviolet disinfection tank connected to each other; the secondary sedimentation tank is connected to the sludge recirculation pump house through a pipe; the effluent of the pre-anoxic tank is distributed through the oxidation ditch water distribution well and enters the oxidation ditch anaerobic tank for mixing reaction with the secondary sludge recirculation, then enters the oxidation ditch anoxic tank for mixing reaction with the secondary nitrification liquid recirculation, and finally enters the oxidation ditch aerobic tank for reaction and then enters the secondary sedimentation tank through the secondary sedimentation tank water distribution well for sedimentation and sludge-water separation; the sludge recirculation pump house is connected to the secondary sedimentation tank sludge discharge pipe, the sludge recirculation pump house is connected to the primary sludge recirculation pump and the secondary sludge recirculation pump, the primary sludge recirculation pump is connected to the reflux mixing zone of the pre-anoxic tank through the primary sludge recirculation pipe and the primary sludge flowmeter; the secondary sludge recirculation pump is connected to the oxidation ditch water distribution well through the secondary sludge recirculation pipe; the primary nitrification liquid recirculation pump is arranged at the end of the oxidation ditch aerobic tank, and the primary nitrification liquid recirculation pump is connected to the reflux mixing zone of the pre-anoxic tank through the primary nitrification liquid recirculation pipe and the primary nitrification liquid flowmeter; the wall-piercing pump returns the nitrification liquid in the oxidation ditch aerobic tank to the oxidation ditch anoxic tank through a pluggable oxidation ditch reflux gate.

2. The transformation device of an oxidation ditch process based on double reflux pre-anoxic denitrification according to claim 1, characterized in that: The sewage pretreatment facility includes a filter screen for removing suspended solids and floating objects; the retrofit device also includes an on-line monitoring facility, which includes an on-line pH meter, an ORP meter and a thermometer installed in the pre-aerobic tank, an ORP meter and a DO detector installed in the anoxic tank of the oxidation ditch, and a DO detector installed in the aerobic tank of the oxidation ditch. The control range of the on-line pH meter is: 6.5 - 9.0, the control range of the ORP meter installed in the pre-aerobic tank is less than -100 mV, and the thermometer is used to monitor the influent water temperature with a control range of: 6 - 26 °C; the control range of the ORP meter installed in the anoxic tank of the oxidation ditch is less than -150 mV, the control range of the DO detector installed in the anoxic tank of the oxidation ditch is less than 0.5 mg / L, and the control range of the DO detector installed in the aerobic tank of the oxidation ditch is 2 - 4 mg / L.

3. The transformation device of the oxidation ditch process based on double reflux pre-anoxic denitrification according to claim 2, characterized in that: An external carbon source dosing device is installed in the anoxic tank of the oxidation ditch, and the dosing point is located at the starting end of the anoxic tank.

4. The usage method of a transformation device for an oxidation ditch process based on double-return pre-anoxic denitrification according to claim 3, characterized in that, The usage method includes the following steps: 1) Domestic sewage enters the sewage treatment plant through the municipal pipe network, and first passes through the grille, lifting, grit removal, and sedimentation to remove suspended solids, floating objects and sediment in the water; After pre-treatment, it enters the pre-aerobic anoxic tank and is mixed with the first-stage sludge reflux and the first-stage nitrification liquid reflux. The hydraulic retention time is controlled at 3.5 - 5.0 h, the first-stage sludge reflux ratio is 40% - 50%, the first-stage nitrification liquid reflux ratio is 150% - 200%, pH is 6.5 - 7.5, ORP is -200 to -100 mV, and the temperature is 6 - 26 °C. The sludge concentration is 3000 - 4000 mg / L. Under the action of the pre-aerobic anoxic tank agitator in the pre-aerobic anoxic tank, the sludge is fully mixed and reacted with the raw water, and the raw water carbon source is used for denitrification and nitrogen removal. NO3 - -N in the reflux sludge and the reflux nitrification liquid is reduced to nitrogen gas; after the reaction, the effluent COD, NH4 + -N and TN indexes in the pre-aerobic anoxic tank are respectively reduced to below 60 mg / L, 15 mg / L, and 20 mg / L, and the TN removal contribution rate of the first-stage anoxic denitrification can reach more than 60%; 2) The effluent from the pre-aerobic tank enters the anaerobic tank through the oxidation ditch distribution well, and is mixed with the secondary sludge reflux. The hydraulic retention time is controlled at 1.5 - 2 h, the secondary sludge reflux ratio is 50% - 60%, ORP: -300 - -150 mV, the mixed liquor sludge concentration reaches 3500 - 4500 mg / L, and the TP concentration of the mixed liquor reaches more than 12 mg / L after the return sludge fully releases phosphorus; 3) The effluent from the anaerobic tank of the oxidation ditch enters the anoxic tank and is mixed with the secondary nitrification liquid reflux. The hydraulic retention time is controlled at 3 - 4 h, the secondary nitrification liquid reflux ratio is 150% - 200%, ORP: -150 - 0 mV, DO < 0.5 mg / L, the mixed liquor sludge concentration reaches 3500 - 4000 mg / L. A carbon source dosing device is installed in this area. When the influent BOD / TN ratio is lower than 3.0, the carbon source is appropriately dosed according to the effluent TN concentration. The carbon source dosing amount is determined according to 6 - 8 mg / L of COD equivalent for every 1 mg / L of total nitrogen removed; when the effluent TN is stably lower than 15 mg / L, no carbon source is dosed; 4) The effluent from the anoxic tank enters the aerobic tank, and the hydraulic retention time is controlled at 9 - 15 h. The DO at the starting end of the oxidation ditch is 0.5 - 1.0 mg / L, the DO in the middle is 2.5 - 4 mg / L, and the DO at the end is 2 - 3 mg / L. The sludge concentration is 3500 - 4000 mg / L. The main purpose of this stage is to convert NH4 + -N into NO3 - -N, while removing the residual COD and enabling the polyphosphate-accumulating bacteria to fully absorb phosphorus, achieving an effluent COD of less than 40 mg / L and a TP of less than 0.8 mg / L; 5) The effluent from the oxidation ditch enters the secondary sedimentation tank for sludge-water separation. The sedimentation time in the secondary sedimentation tank is 3 - 4 h. The suspended solids (SS) in the effluent are ≤ 20 mg / L, total phosphorus (TP) ≤ 1.0 mg / L, total nitrogen (TN) ≤ 12 mg / L, ammonium nitrogen (NH₄ + -N) ≤ 1.0 mg / L, and chemical oxygen demand (COD) ≤ 40 mg / L. The main purpose is to separate the sludge from the water. The supernatant is used as the biochemical effluent and enters the advanced treatment system. The sludge is returned to the biochemical tank through the sludge return pump house to continue to play its role. The sludge in the secondary sedimentation tank enters the sludge pump house. Part of it is returned to the pre-anoxic tank and anaerobic tank, and part is discharged as excess sludge. The daily discharge amount of excess sludge is about 3% - 4% of the influent volume.

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