A method for denitrifying municipal sewage with a low C / N ratio

Abstract

The present invention relates to the technical field of sewage treatment, and specifically relates to a method for nitrogen removal from urban sewage with a low C / N ratio. Monitor the flow rate, COD, total nitrogen of the total influent of urban sewage, and the nitrate nitrogen content in each biological pool. When, within 12 hours, the various parameters of sewage treatment meet the prerequisite conditions for alternating operation, stop the operation of the biological pool with the highest nitrate nitrogen content at this time and evenly distribute the influent to other pools. After a period of time, the stopped biological pool starts to receive influent. After resuming operation for 12 hours, calculate the prerequisite conditions for alternating operation. After meeting the conditions, conduct the second round of alternating operation. The method of the present invention solves the problem of increased TN in the effluent caused by the reduction of C / N in urban sewage by alternating the operation of biological pools. During the whole process, while ensuring the effluent quality, it realizes resource conservation and effectively saves production costs, meets the development needs of the era of the dual-carbon concept, and is conducive to the use in nitrogen removal from urban sewage with a low C / N ratio.

Description

Technical Field

[0001] The present invention relates to the technical field of sewage treatment, and particularly relates to a method for denitrifying urban sewage with a low C / N ratio. Background Art

[0002] Urban sewage refers to domestic sewage of urban residents, drainage from government agencies, schools, hospitals, commercial service institutions and various public facilities, as well as industrial wastewater and initial rainwater that are allowed to be discharged into the urban sewage collection system. Since the water consumption of urban residents varies with time, climate and season, the various indicators of urban sewage fluctuate frequently. Especially during holidays and summer, the influent of urban sewage treatment plants often shows characteristics such as a decrease in the influent C / N ratio and insufficient carbon source.

[0003] For denitrification based on the AAO process, first, the organic nitrogen in the influent undergoes an ammonification reaction to produce ammonia nitrogen. Nitrifying bacteria carry out a nitrification reaction under aerobic conditions, converting this part of the ammonia nitrogen and the ammonia nitrogen in the influent into nitrite nitrogen. Nitrite nitrogen is converted into nitrate nitrogen under aerobic conditions. Denitrifying bacteria carry out a denitrification reaction under anoxic conditions, converting nitrate nitrogen and carbon source into nitrogen gas and carbon dioxide, which overflow into the atmosphere. From the above reactions, it can be seen that influent carbon source, microbial content, and dissolved oxygen are the three key elements of biological denitrification. In order to meet the standard discharge of TN in the effluent when the influent carbon source is insufficient, two existing measures are adopted for control; one is to add external carbon source to the anoxic and anaerobic sections of the activated sludge system to increase the carbon-nitrogen ratio. This method has obvious effects, but there are problems such as high chemical agent costs, increased sludge production, and non-compliance with the development requirements of the era of carbon peak and carbon neutrality (dual carbon); the other is to shut down and empty some biological ponds to increase the influent load of other biological ponds, thereby achieving the treatment effect. The advantages of this measure are that the effluent water quality improves, the electricity cost and chemical agent cost decrease, and equipment maintenance can be carried out during the shutdown period. However, there are three problems. One is that when restarting operation, it is necessary to re-culture and domesticate bacteria, the operation is complex, and the labor and operation management costs increase; the second is that applying for the shutdown or idleness of environmental pollution prevention and control facilities to the environmental protection bureau for the shutdown and emptying of structures is complex; the third is that if the influent water quality changes during the shutdown period, the operation cannot be restored in time, and there is a risk of exceeding the standard for the entire activated sludge system.

[0004] Therefore, a method for denitrifying urban sewage with a low C / N ratio that can keep up with the development needs of the times, ensure the urban sewage treatment effect, realize resource conservation and effectively reduce production costs is an urgent problem to be solved at present. Summary of the Invention

[0005] The purpose of the present invention is to aim at the deficiencies of the above-mentioned existing technologies and provide a method for denitrifying urban sewage with a low C / N ratio, so as to achieve the purpose of ensuring the urban sewage treatment effect while realizing resource conservation and effectively reducing production costs.

[0006] To achieve the above object, the present invention adopts the following technical solutions: A method for denitrifying urban sewage with a low C / N ratio, comprising the following steps:

[0007] S1. For N series of biological ponds, install a COD on-line monitor and a TN on-line monitor at the total inlet, install an electromagnetic flowmeter on the inlet pipe of each biological pond, and install an on-line nitrate nitrogen analyzer at the end of the aerobic pond of each biological pond;

[0008] S2. Sort the average values of the monitoring data of the on-line nitrate nitrogen analyzer (4) within 12 hours from high to low, and select the series with the highest value as the series to stop running;

[0009] S3. During the stop of operation, the total influent volume is evenly distributed to the other (N - 1) series;

[0010] S4. After the stop of operation, the measured value of the on-line nitrate nitrogen analyzer of the stopped series continues to decline. When it remains stable for more than 1 hour or starts to rise, restore the influent volume of all series, and the total influent volume is evenly distributed to all series;

[0011] S5. After 12 hours of resuming operation, calculate the prerequisite conditions for alternating operation. After meeting the conditions, perform the second round of alternating operation.

[0012] Further, in S2 for the series to stop running, stop the water inlet, carbon source dosing, and stop the operation of the external reflux pump of this series, and adjust the aeration volume according to the dissolved oxygen range at the end of the aerobic stage.

[0013] Further, in S3, the carbon source dosage of other series remains unchanged and is not adjusted according to the change of their respective influent volumes.

[0014] Further, in S4, evenly distribute the total influent volume to all series. For the series to resume water inlet, resume the operation of the external reflux pump and the dosing of carbon source agents of this series.

[0015] Further, in S5, the series that stopped running in the previous round does not stop water supply in this round.

[0016] Further, in S5, the alternating operation needs to simultaneously meet the following prerequisite conditions:

[0017] (1) The 24-hour cumulative influent volume calculated according to the inlet electromagnetic flowmeter is lower than (N - 1)Q, where Q is the designed influent flow rate of each biological pond, with the unit of m 3 / d;

[0018] (2) The 12-hour average value of the influent COD / TN is lower than 8;

[0019] (3) Calculate the average value of the monitoring data of the on-line nitrate nitrogen analyzer at the end of the aerobic tank of each biological tank within 12 hours, and this average value is higher than 10 mg / L.

[0020] Advantages of the present invention:

[0021] (1) By operating the biological tanks alternately, the problem of increased TN in the effluent caused by the reduction of the influent C / N ratio can be solved;

[0022] (2) During the period when the biological tank stops influent, for the series that does not stop influent, since the influent is evenly distributed, the influent volume increases, the influent carbon source increases, the carbon source required for the denitrification section increases, and the TN in the effluent also decreases accordingly;

[0023] (3) Compared with the traditional method of increasing the carbon source, the operation measures of alternately operating the biological tanks reduce the costs of the sewage treatment plant in aspects such as aeration power consumption, equipment shutdown power consumption, and sludge treatment power consumption, and reduce the sewage treatment cost. Description of the drawings

[0024] Figure 1 is the installation schematic diagram of each instrument of the present invention;

[0025] Figure 2 is the principle schematic diagram of the present invention;

[0026] Figure 3 is the schematic diagram of the change of nitrate nitrogen content in each series during the operation of Embodiment 1 of the present invention;

[0027] Figure 4 is the schematic diagram of the change of nitrate nitrogen content in each series during the operation of Embodiment 2 of the present invention.

[0028] Names corresponding to each mark in the figure:

[0029] 1. COD on-line monitor; 2. TN on-line monitor; 3. Electromagnetic flowmeter; 4. On-line nitrate nitrogen analyzer. Detailed implementation manners

[0030] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention.

[0031] The technological process of the present invention is as follows:

[0032] The total influent is shunted into each biological tank through the influent pipe. At the end of the total influent pipe, a COD on-line monitor 1 and a TN on-line monitor 2 are installed for measuring COD and TN. An electromagnetic flowmeter 3 is installed on the influent pipe of each biological tank for measuring the actual influent flow of each biological tank. The designed influent flow of each biological tank is Q, and the sum of the actual influent flows is the actual total influent flow. An on-line nitrate nitrogen analyzer 4 is installed at the end of the aerobic section of each biological tank for measuring nitrate nitrogen.

[0033] The principle of the present invention is:

[0034] When the C / N ratio of urban sewage influent decreases, the carbon source is insufficient in the denitrification reaction of biological denitrification, resulting in incomplete denitrification in the biological pool, increased nitrate nitrogen, and thus increased total nitrogen in the effluent. Stop the water inlet to the biological pool with the highest nitrate nitrogen content in the aerobic section. The high nitrate nitrogen content in the aerobic section indicates that the treatment effect of this pool is the worst, so it is treated first. After stopping the water inlet, stop the carbon source dosing and the external reflux pump at the same time. At this time, aeration is performed according to the dissolved oxygen range (2-4 mg / L in winter and 1-3 mg / L in summer), which is equivalent to suffocation.

[0035] Among them, before the water inflow to the biological pool stops, a part of the external carbon source is added to the biological pool. Therefore, during the period when the water inflow to the biological pool stops, this part of the external carbon source continues to undergo denitrification reaction in the anoxic section. At the same time, due to the extension of the hydraulic retention time of the aerobic section and the anoxic section, the nitrification reaction and denitrification reaction are more thorough, thereby improving the overall nitrogen removal efficiency.

[0036] During the period when water inflow to the biological pool was stopped, the series that did not stop water inflow had an even distribution of water, so the water volume increased, the total amount of carbon source in the water increased, the sludge load in the biological pool increased, and the effluent TN decreased accordingly.

[0037] In sewage treatment based on the AAO process, after the water inflow to the biological pool is stopped, the hydraulic retention time becomes longer. Although the hydraulic retention time is extended in a short period of time, the TN of the effluent is reduced. However, if the hydraulic retention time is too long, the external carbon source is completely consumed, and the nitrifying bacteria and denitrifying bacteria cannot proliferate due to the lack of fresh sewage, and the overall aging will lead to an increase in the TN of the effluent. Therefore, the control of the time when the water inflow to the biological pool is stopped is a difficult point. In view of this feature, the present invention controls the nitrate nitrogen in the effluent and the time when the water inflow to the biological pool is stopped, so as to obtain a suitable time when the water inflow to the biological pool is stopped.

[0038] According to the requirements of biological denitrification in the "Outdoor Drainage Design Code" (GB 50014-2006, 2016 edition): During denitrification, the ratio of BOD5 to total Kjeldahl nitrogen in sewage should be greater than 4. Traditional theory holds that sewage with BOD5 / COD>0.3 can be biodegraded, and the BOD5 / COD range of the influent of general urban sewage treatment plants is 0.3-0.8. In addition, Kjeldahl nitrogen is a general term for ammonia nitrogen and organic nitrogen that can be converted into ammonia nitrogen. TN includes Kjeldahl nitrogen, nitrate nitrogen and nitrite nitrogen. The content of nitrate nitrogen and nitrite nitrogen in the influent of general urban sewage treatment plants is relatively small and can be ignored at the time of calculation.

[0039] When BOD5 / COD is 0.5 during the process, we can get:

[0040]

[0041] In the above formula, BOD5 / COD is 0.5. Since BOD5 / TN should be greater than 4, one of the prerequisite conditions for the alternate operation in the present invention is that the 12-hour average value of the influent COD / TN is lower than 8.

[0042] Combined with the above specifications and production practice, when the influent flow rate is less than (N - 1)Q (where Q is the designed influent flow rate of each biological pond, with the unit of m 3 / d) and the 12-hour average value of the influent COD / TN is lower than 8, the effluent TN of the urban sewage with a low C / N ratio increases significantly at this time and needs to be treated. According to the provisions of the "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB18918 - 2002), the TN content in the effluent should be less than 15 mg / L. Combined with production practice, when the average value of the nitrate nitrogen content in each biological pond is higher than 10 mg / L, the alternate operation of each biological pond is required.

[0043] Example 1

[0044] For a sewage treatment plant with a pre-anoxic AAO process, the designed influent volume is 300,000 tons / d, with a total of 4 series, and the actual influent volume of each series is 70,000 - 80,000 tons / d; an electromagnetic flowmeter 3 is installed on the influent pipe of each biological pond, a COD on-line monitor 1 and a TN on-line monitor 2 are installed at the total influent port, and an on-line nitrate nitrogen analyzer 4 is installed at the end of the aerobic zone of each biological pond. Each biological pond is divided into a pre-anoxic zone, an anaerobic zone, an anoxic zone and an aerobic zone. Among them, the hydraulic retention time of the pre-anoxic zone is 1.45 h, the hydraulic retention time of the anaerobic zone is 1.45 h, the hydraulic retention time of the anoxic zone is 5.5 h, and the hydraulic retention time of the aerobic zone is 10.0 h.

[0045] On the first day of influent fluctuation, the total influent flow rate is 224,264 tons / d, and the influent volume is evenly distributed among the 4 series. The 12-hour average value of the influent COD / TN is 5.19, and the 12-hour average values of the on-line nitrate nitrogen analyzers at the end of the aerobic zones of the biological ponds in series 1, 2, 3, and 4 all exceed 10 mg / L, being 13.56 mg / L, 12.29 mg / L, 11.34 mg / L, and 12.24 mg / L respectively.

[0046] Select series 1 as the first series to stop influent; stop the influent and carbon source dosing of series 1, and stop the operation of the external reflux pump of series 1; the aeration volume of series 1 is regulated according to the dissolved oxygen range. Since the monitoring time is in winter, the dissolved oxygen range is adjusted to 2 - 4 mg / L at this time.

[0047] After series 1 stops influent, the influent volume is evenly distributed among series 2, 3, and 4; during the period when series 1 stops influent, the dosing amount of the carbon source agent in series 2, 3, and 4 remains unchanged.

[0048] After 12 hours, water inlet to Series 1 began, and the total water inlet volume was evenly distributed among the four series of 1, 2, 3, and 4. The external reflux pump operation and carbon source chemical dosing of Series 1 were resumed.

[0049] After continuing to operate for 12 hours, the total influent flow rate (cumulative influent volume in 24 hours) was 219,160 tons / d, and the 12-hour average value of the influent COD / TN was 5.64. The 12-hour average values of the on-line nitrate nitrogen analyzers at the end of the aerobic tanks in the biological tanks of Series 1, 2, 3, and 4 were all higher than 10 mg / L. Select the series with the highest 12-hour average value of the on-line nitrate nitrogen analyzer among Series 2, 3, and 4 for the second round, and so on. The specific data during the operation is as follows (the on-line nitrate nitrogen analyzer outputs data continuously once a minute, and each data in the following table is the hourly average value):

[0050] Table 1 Variation table of nitrate nitrogen content in each series

[0051]

[0052]

[0053] It can be seen from the above table that by stopping the operation of the biological tanks with relatively high nitrate nitrogen content in each biological tank, the hydraulic retention time of this biological tank was extended, the denitrification efficiency of denitrifying bacteria was improved, the nitrate nitrogen content was reduced, and the TN content was less than 15 mg / L (the ammonia nitrogen and nitrite nitrogen contents were almost 0), which could meet the discharge standards (Grade A standard in the "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB18918 - 2002)). At the same time, in the above table, the stop operation time of each tank was basically 12 hours, which was determined by the biological tank process and the hydraulic retention time.

[0054] During the alternate operation, the costs of the sewage treatment plant in terms of aeration power consumption, equipment shutdown power consumption, sludge treatment power consumption, etc. were reduced. During the shutdown of Series 1, the relevant data is as follows in the table (the electricity price per unit is 0.730 yuan / kWh):

[0055] Table 2 Economic benefit analysis table during the shutdown of Series 1

[0056]

[0057] It can be seen that the sewage treated by the method of the present invention can not only meet the discharge standards, but also reduce the cost of sewage treatment.

[0058] Example 2

[0059] A sewage treatment plant adopting the AAO process has a designed influent flow rate of 150,000 tons per day, with a total of three series, and the designed influent flow rate for each series is 50,000 tons per day. Electromagnetic flow meters 3 are installed on the influent pipes of each biological pond, a COD on-line monitor 1 and a TN on-line monitor 2 are installed at the total influent port, and on-line nitrate nitrogen analyzers 4 are installed at the ends of the aerobic zones of each biological pond. Each biological pond is divided into an anaerobic zone, an anoxic zone and an aerobic zone. Among them, the hydraulic retention time of the anaerobic zone is 2.2 hours, the hydraulic retention time of the anoxic zone is 6.5 hours, and the hydraulic retention time of the aerobic zone is 10.3 hours.

[0060] On the first day of influent fluctuation, the total influent flow rate (cumulative influent volume in 24 hours) is 90,052 tons per day. The influent volume is evenly distributed among the 3 series. The 12-hour average value of the influent COD / TN is 4.53. The 12-hour average values of the on-line nitrate nitrogen analyzers at the ends of the aerobic zones of the biological ponds in series 1, 2, and 3 all exceed 10 mg / L, being 13.64 mg / L, 13.12 mg / L, and 14.38 mg / L respectively.

[0061] Select series 3 as the first series to stop influent; stop the influent and carbon source dosing of series 3, and stop the operation of the external reflux pumps of series 3; the aeration volume of series 3 is regulated according to the dissolved oxygen range. Since the monitoring time is in autumn, the dissolved oxygen range is adjusted to 2 - 4 mg / L at this time.

[0062] After series 3 stops influent, the influent volume is evenly distributed between series 1 and 2; during the period when series 3 stops influent, the dosing amounts of the carbon source agents in series 1 and 2 remain unchanged.

[0063] After 8 hours, the nitrate nitrogen at the end of the aerobic zone of series 3 begins to rise continuously.

[0064] After 9 hours, series 3 starts to take in influent, and the total influent volume is evenly distributed among the three series of 1, 2, and 3. Resume the operation of the external reflux pumps and the dosing of the carbon source agents in series 3.

[0065] After 21 hours (continuing to operate for 12 hours), the total influent flow rate is 84,236 tons per day. The 12-hour average value of the influent COD / TN is 3.89. The 12-hour average values of the on-line nitrate nitrogen analyzers at the ends of the aerobic zones of the biological ponds in series 1, 2, and 3 are all higher than 10 mg / L. Select the series with the highest 12-hour average value of the on-line nitrate nitrogen analyzer among series 2, 3, and 4 for the second round, and so on. The specific data during the operation are as follows: (The on-line nitrate nitrogen analyzer outputs data continuously once a minute, and each data in the following table is the hourly average value)

[0066] Table 3 Variation table of nitrate nitrogen content in each series

[0067]

[0068]

[0069] After three rounds of alternating operation, the water inflow gradually recovered. The daily water inflow was 130,281 tons per day, and the 12-hour average value of the influent COD / TN was 9.01. The alternating operation of the biological tank in the next round was stopped.

[0070] As can be seen from the above table, by shutting down the biological tanks with relatively high nitrate nitrogen content in each biological tank, the hydraulic retention time of this biological tank was extended, the denitrification efficiency of denitrifying bacteria was improved, the nitrate nitrogen content was reduced, and in some series, the nitrate nitrogen could even be reduced by 4 - 5 mg / L, and the TN content was less than 15 mg / L (the ammonia nitrogen and nitrite nitrogen contents were almost 0), meeting the discharge standards (Grade A Standard of "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB18918 - 2002)).

[0071] During the alternating operation, the costs of the sewage treatment plant in aspects such as aeration power consumption, equipment shutdown power consumption, and sludge treatment power consumption were reduced. During the shutdown period of Series 3, the relevant data is as follows in the table (the unit price of electricity is 0.698 yuan per degree):

[0072] Table 4 Economic Benefit Analysis Table during the Shutdown Period of Series 3

[0073]

[0074] It can be seen that in the process of treating urban sewage with a low C / N ratio, the sewage treated by the method of the present invention can not only meet the discharge standards, but also reduce the cost of sewage treatment, which is beneficial to environmental protection and resource conservation, and meets the development needs of the times.

[0075] The present invention is not limited to the above - mentioned best implementation mode. Anyone can obtain various other forms of products under the inspiration of the present invention. However, no matter what changes are made in its shape or structure, as long as it has the same or similar technical solutions as this application, it falls within the protection scope of the present invention.

Claims

1. A method for denitrifying low C / N ratio municipal sewage, characterized in that, It includes the following steps: S1. For N series of biological ponds, install a COD on-line monitor (1) and a TN on-line monitor (2) at the total inlet, install an electromagnetic flowmeter (3) on the inlet pipe of each biological pond, and install an on-line nitrate nitrogen analyzer (4) at the end of the aerobic pond of each biological pond; S2. Sort the average values of the monitoring data of the on-line nitrate nitrogen analyzer (4) within 12 hours from high to low, and select the series with the highest value as the series to stop running; S3. During the period of stopping operation, the total influent volume is evenly distributed to the other (N - 1) series; S4. After stopping operation, the measured value of the on-line nitrate nitrogen analyzer of the stopped series continues to decline. When it remains stable for more than 1 hour or starts to rise, restore the influent volume of all series, and evenly distribute the total influent volume to all series; S5. After 12 hours of resuming operation, calculate the prerequisite conditions for alternate operation. After meeting the conditions, conduct the second round of alternate operation; The alternate operation needs to meet the following prerequisite conditions simultaneously: (1) The cumulative water inflow calculated by the influent electromagnetic flowmeter (3) within 24 hours is lower than (N - 1)Q, where Q is the designed influent flow rate of each biological tank, with the unit of m 3 / d; (2) The 12-hour average value of the influent COD / TN is less than 8; (3) Calculate the average value of the monitoring data of the on-line nitrate nitrogen analyzer (4) at the end of the aerobic pond of each biological pond within 12 hours, and this average value is higher than 10 mg / L.

2. The method for denitrifying low C / N ratio urban sewage according to claim 1, wherein: In S2, for the series to stop running, stop the influent, carbon source dosing, and stop the operation of the external reflux pump of this series, and adjust the aeration volume according to the dissolved oxygen range at the aerobic end.

3. A method for denitrifying low C / N ratio municipal sewage according to claim 1, characterized in that: In S3, the carbon source dosing amount of other series remains unchanged and is not adjusted according to the change of their respective influent volumes.

4. A method for denitrifying low C / N ratio municipal wastewater according to claim 1, characterized in that: In S4, evenly distribute the total influent volume to all series. For the series to resume influent, resume the operation of the external reflux pump and carbon source dosing of this series.

5. A method for denitrifying low C / N ratio municipal wastewater according to claim 1, characterized in that: In S5, for the series that stopped running in the previous round, do not stop the water supply in this round.

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