Greenhouse gas monitoring and direct discharge flux accounting method for sewage treatment plant

Abstract

The invention discloses a greenhouse gas monitoring and direct discharge flux accounting method for a sewage treatment plant. The method mainly comprises the following steps: distributing points along a sewage treatment path according to a point distribution principle; installing a floating gas collecting device, and connecting the floating gas collecting device with an online detector or an air sampler; on-line or off-line measurement modes are selected according to different working conditions of the sewage treatment plant; monitoring temperature, on-site wind speed, air pressure, gas flow and pipeline pressure; the direct emission flux of the greenhouse gas is corrected and calculated on the basis of measured data and a formula provided by the invention. According to the method, the direct emission flux of the greenhouse gas of the sewage treatment plant can be flexibly and accurately calculated, the greenhouse gas emission unit is identified, and the targeted carbon reduction work of the sewage treatment plant is effectively guided.

Description

Technical Field

[0001] The invention relates to a method for greenhouse gas monitoring and direct emission flux accounting in a sewage treatment plant, and belongs to the technical field of sewage treatment. Background Art

[0002] Greenhouse gases, represented by carbon dioxide, have a significant impact on the global climate. Although sewage treatment plants alleviate water pollution, they also directly emit carbon dioxide (CO) during the sewage treatment process due to microbial degradation and metabolism. 2 ), nitrous oxide (N 2 O), methane (CH 4 ) and other greenhouse gases (GHG). Among them, most of the organic matter in the sewage treatment plant influent is of biological origin. Therefore, carbon dioxide, as a biogenic emission, is usually not included in the greenhouse gas emission inventory of sewage treatment plants. 4 With N 2 O and other strong greenhouse gases. According to statistics, sewage plants account for the world’s CH 4 The fifth largest emission source is global N 2 Therefore, reliable greenhouse gas monitoring and accounting methods are needed to clarify the actual greenhouse gas emissions in wastewater treatment plants.

[0003] The direct emission of greenhouse gases in the sewage treatment process is a typical greenhouse gas diffusion at the water-air interface. Two methods are generally used to measure its concentration, namely online monitoring and offline monitoring. Among them, the gas concentration online analysis equipment based on the optical cavity ring-down method, which has the advantages of simple operation, good stability, high sensitivity and high maturity, has been widely used in the long-term online monitoring of greenhouse gases in my country's atmospheric stations, but its investment cost is relatively high. Offline sampling monitoring includes gradient method, static box method and inverted funnel method. In practical application, the static box method is mainly used. This method collects gas by placing a floating box with a sealed top and a through bottom on the water surface, and then combines gas chromatography to detect greenhouse gases in the sample. The static box method is simple to operate and easy to carry. However, the accuracy of offline detection is easily affected by sampling conditions, environment and other factors, and it is impossible to achieve long-term continuous monitoring. At the same time, it is not suitable for water surfaces with large gas flux fluctuations in sewage treatment plants.

[0004] At present, the greenhouse gas emissions of sewage treatment plants are mainly obtained through the emission factor method provided by the United Nations Intergovernmental Panel on Climate Change (IPCC) guidelines. However, this method is intended to guide carbon emission accounting at the national scale. The emission process and the value of the emission factor are highly subjective, resulting in over-calculation, omission and miscalculation, which is not conducive to sewage treatment plants to understand their carbon emissions and then find the focus of carbon emission reduction.

[0005] In order to achieve accurate monitoring and direct emission flux accounting of greenhouse gases under complex environmental conditions in sewage treatment plants, this method designs a greenhouse gas monitoring system that combines offline monitoring methods with online monitoring. The system is easy to carry and has high accuracy, which is suitable for continuous monitoring in the specific environment of sewage treatment plants. Based on this monitoring system, a greenhouse gas direct emission flux accounting method with higher time resolution and high accuracy is provided to obtain accurate greenhouse gas emissions. Summary of the invention

[0006] Purpose of the invention: In order to promote pollution reduction and carbon reduction in the sewage treatment process, it is first necessary to clarify the carbon emissions of sewage treatment. Therefore, the present invention proposes a method for greenhouse gas monitoring and direct emission flux accounting in sewage treatment plants.

[0007] The technical solution adopted by the present invention is a method for greenhouse gas monitoring and direct emission flux calculation of a sewage treatment plant. A greenhouse gas monitoring system based on a floating gas collection box, a portable greenhouse gas concentration analyzer or a gas chromatograph is used to measure the greenhouse gas concentration of the sewage treatment plant, and the direct emission flux of the greenhouse gas is calculated according to the formula proposed by the present invention in combination with the parameters of the sewage treatment plant. The method specifically includes the following steps:

[0008] Step 1: Before monitoring greenhouse gases in a sewage treatment plant, it is necessary to investigate the background information of the sewage treatment plant and fully understand the process flow, sewage type, and sewage pipe network of the sewage treatment plant. On this basis, monitoring points are arranged along the sewage treatment from water inlet to water outlet according to the actual situation of the sewage treatment plant.

[0009] Step 2: Use the thermometer installed on the floating gas collection box device to read the internal temperature of the collection box in real time; use the anemometer and barometer to measure the on-site wind speed and air pressure in real time; use the flow meter and pressure gauge installed on the air pipe to monitor the gas flow and pipeline pressure in real time; use a pressure-free air bag to obtain the surface gas flow of aerated grit chambers, aerobic tanks and other units containing aeration facilities.

[0010] Step 3, connect the floating gas collection box with the portable greenhouse gas concentration analyzer, obtain the concentration change of the greenhouse gas in the floating gas collection box after passing through the portable greenhouse gas concentration analyzer, calculate the gas concentration release rate by the change of greenhouse gas concentration over time, and realize the online monitoring of greenhouse gas in the sewage treatment plant; in addition, connect the air sampler to the floating gas collection box to collect the gas on the pool body, and use gas chromatography to realize the offline monitoring of greenhouse gas in the sewage treatment plant. After the floating gas collection box is stable for 1 minute, use the gas sampler and the air bag to take samples every 10 to 20 minutes, and then use gas chromatography to determine the greenhouse gas concentration in the air bag sample. The greenhouse gas release rate in the process unit is obtained by calculating the gas concentration change rate through the relationship between the sampling time and the change of greenhouse gas concentration.

[0011] Step 4: Based on the changes in greenhouse gas concentrations, weather conditions, and temperature at different monitoring points, the following greenhouse gas direct emission flux calculation formula based on the monitoring system is established:

[0012] For non-aerated units, the greenhouse gas emission flux at each point is calculated using formula (1); for aerated units, the greenhouse gas emission flux at each point is calculated using formula (2):

[0013]

[0014]

[0015] Flux: greenhouse gas diffusion flux at each point, mg·(m 2 d) -1 ;

[0016] Slop: greenhouse gas release rate, ppm·S -1 ;

[0017] F 1 :Standard temperature and pressure of gases in air from ppm to mg·m -3 Conversion factor of

[0018] α: pressure correction coefficient;

[0019] β: temperature correction coefficient;

[0020] C: greenhouse gas concentration, Vol%;

[0021] ρ: density of greenhouse gas at the monitoring temperature, kg / m 3 ;

[0022] Q: Greenhouse gas flow; m 3 / d;

[0023] F 2: Time conversion coefficient, take 86400;

[0024] V: air volume in the static box, m 3 ;

[0025] S: water surface area covered by the box, m 2 ;

[0026] T: atmospheric temperature during on-site monitoring, K;

[0027] P: atmospheric pressure when the gas sample was collected, kPa;

[0028] T 0 : air temperature under standard conditions, 273K;

[0029] P 0 : Air pressure under standard conditions, 101.325 kPa;

[0030] M: gas molar mass, g·mole -1 ;

[0031] R: gas constant, taken as 8.3144, J / (K·mol);

[0032] The greenhouse gas emissions of each process unit are calculated as follows:

[0033] E u =Flux×S u (6) E u : Greenhouse gas emissions of each process unit, mg / d;

[0034] S u : Total surface area of ​​each process unit, m 2 ;

[0035] The direct greenhouse gas emission flux of the sewage treatment plant is calculated as follows:

[0036]

[0037] E: direct greenhouse gas emission flux from sewage treatment plants, kg / d;

[0038] n: the number of process units monitored in the sewage treatment plant, units;

[0039] E uk : Greenhouse gas emissions of the kth process unit, mg / d.

[0040] Furthermore, the principles of monitoring point layout in step 1 include: ① overall monitoring of greenhouse gas emissions during sewage treatment, covering all process units through which sewage flows; ② water quality monitoring of the most important nodes in each process unit, including the front, middle and back ends of the unit, to examine the generation and transformation of greenhouse gases when sewage flows through each unit, and to grasp the regional greenhouse gas emission rules; ③ under the dual constraints of considering convenience and meeting needs, the point should be convenient for the installation and layout of floating gas collection boxes; ④ for process units with aeration and anaerobic digestion processes, the number of monitoring points in this part should be increased; ⑤ under the setting of multiple points, limited monitoring equipment is arranged on important monitoring elements to obtain key information and achieve the best monitoring efficiency; ⑥ under the constraint of economic cost investment, the number of monitoring points arranged as much as possible can determine the optimal monitoring effect that can be achieved under this condition; ⑦ according to the preliminary implementation and experiment of the monitoring plan, the monitoring plan also needs to be optimized and adjusted, and the required remaining monitoring point monitoring data is continuously supplemented according to the existing monitoring data.

[0041] Furthermore, in step 2, a fan is installed above the floating gas collection box to promote complete mixing of the gas in the box.

[0042] Furthermore, in step 2, the floating gas collection box and the pressure-free air bag should be completely submerged in the water.

[0043] Furthermore, in step 3, a drying tube is installed in front of the portable greenhouse gas concentration analyzer to reduce the influence of moisture on the online measurement.

[0044] Furthermore, in step 3, the online measurement and offline measurement modes are coupled to flexibly switch between different monitoring modes. The greenhouse gas concentration error in the two modes should be less than 10%.

[0045] Beneficial effects:

[0046] 1) The present invention proposes for the first time the principle of monitoring greenhouse gases in sewage treatment plants. Based on a comprehensive understanding of the process flow, sewage type, sewage direction, etc. of the sewage treatment plant, monitoring points are arranged along the sewage treatment process according to the principle of monitoring proposed by the present invention, which is conducive to accurately grasping the emission rules of greenhouse gases along the sewage treatment process and facilitating the accurate accounting of greenhouse gases in the sewage treatment process.

[0047] 2) This method proposes a correction monitoring method for direct greenhouse gas emission fluxes of each process unit in a sewage treatment plant. A thermometer is configured on the floating gas collection box device, and an anemometer and a barometer are used in real time to measure the temperature, wind speed and air pressure during on-site monitoring. At the same time, a flow meter and a pressure gauge are configured on the air pipe connected to the air sampler to monitor the flow rate and pipeline pressure during sampling, and to incorporate auxiliary data such as air temperature and air pressure into the emission calculation formula to achieve the correction of greenhouse gas emissions under different environmental factors. In addition, in response to the increase in gas generated by the aeration unit, this method specially adds a pressure-free air bag to correct greenhouse gas emissions. Using a pressure-free air bag, the gas flow at each point of the aeration unit can be accurately obtained, thereby achieving accurate correction of greenhouse gas emissions.

[0048] 3) Innovatively combine offline and online monitoring of greenhouse gases to measure greenhouse gases diffused from the pool surface of each functional area in the sewage treatment plant. Online and offline monitoring modes can be flexibly switched for different working conditions. In the online monitoring mode, the greenhouse gas in the floating gas collection box obtains concentration changes through a portable concentration analyzer, and the gas concentration change rate is calculated through parameter changes; in the offline monitoring mode, after the floating gas collection box stabilizes for 1 minute, a gas sampler and a gas bag are used to take samples every 10 to 20 minutes, and the greenhouse gas concentration in the sample is determined by gas chromatography, and the gas release rate is calculated based on the time change. By calculating the greenhouse gas release flux of different process units, the direct greenhouse gas emission flux of the sewage treatment plant can be calculated more scientifically and accurately, the greenhouse gas emission units can be identified, and the carbon reduction work of the sewage treatment plant can be effectively guided. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 Flowchart of greenhouse gas monitoring and emission flux accounting methods for wastewater treatment plants.

[0050] Figure 2 Schematic diagram of the greenhouse gas monitoring system installation in a wastewater treatment plant.

[0051] Figure 3 This is a schematic diagram of the installation of a pressure-free air bag.

[0052] Figure 4 is the greenhouse gas emissions of each process unit.

[0053] In the figure: 101 is an air bag; 102 is an air sampler; 103 is an air pipe valve; 104 is an air sampling pipe; 105 is a pressure gauge; 106 is a flow meter; 107 is a thermometer; 108 is a box valve; 109 is an air bag; 110 is a fan; 111 is a buckle; 112 is a drying tube; 113 is a portable greenhouse gas concentration analyzer; 114 is a box; 115 is a flange; 116 is a traction rope. 201 is a traction rope; 202 is a foam bottom plate; 203 is a buckle; 204 is a metal ring; 205 is a polyethylene air bag. DETAILED DESCRIPTION

[0054] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments:

[0055] like Figure 1 As shown, a method for greenhouse gas monitoring and direct emission flux accounting of a sewage treatment plant includes the following steps:

[0056] (1) First, make preparations before monitoring: Investigate the background information of the sewage treatment plant, understand the process flow, sewage type, discharge law, sewage pipe network direction and other information related to the sewage discharge from the pollution source. On this basis, according to the actual situation of the sewage treatment plant, monitor the sewage treatment route along the route. The following principles can be used for reference: ① Monitor greenhouse gas emissions during sewage treatment as a whole, covering each process unit through which the sewage flows; ② Monitor the water quality of the most important nodes in each process unit, including the front, middle and back ends of the unit, to examine the generation and transformation of greenhouse gases when sewage flows through each unit, and understand the regional greenhouse gas emission law; ③ Under the dual constraints of considering convenience and meeting needs, the point location should be convenient for the installation and layout of floating gas collection boxes; ④ For process units with aeration and anaerobic digestion processes, the number of monitoring points should be increased; ⑤ Under a certain number of point settings, the limited monitoring equipment should be arranged on important monitoring elements to obtain key information and achieve the best monitoring efficiency. ⑥ Under the constraint of a certain economic cost investment, the number of monitoring points arranged as much as possible can determine the optimal monitoring effect that can be achieved under this condition. ⑦ Based on the initial implementation and experimentation of the monitoring plan, the monitoring plan also needs to be optimized and adjusted to a certain extent. Based on the existing monitoring data, the required monitoring data of the remaining monitoring points can be continuously supplemented.

[0057] (2) Conduct actual surveys at each point to determine the rationality of the point layout.

[0058] (3) Check the air tightness of the floating gas collection box.

[0059] (4) Install the floating gas collection box before monitoring. Figure 2As shown, the floating gas collection box includes: a gas collection pipe 104 for transmitting gas; a box body 114, which is a cylindrical box body; a thermometer 107 for monitoring the temperature change inside the box body; a box body valve 108 for connecting the gas collection pipe and the box body; an air bag 109 for making the floating gas collection box float on the water surface; a ring buckle 111 for connecting a traction rope; a traction rope 116 for fixing the floating gas collection box; and a flange 115 for fixing the floating gas collection box on the air bag.

[0060] (5) A fan 110 is installed above the floating gas collection box to promote complete mixing of the gas in the box; a drying tube 112 is installed in front of the portable greenhouse gas concentration analyzer to reduce the impact of moisture on online measurement; a gas pipe valve 103 is installed on the gas pipe to open and close the gas pipe; a pressure gauge 105 is installed on the gas pipe to monitor the internal pressure of the floating gas collection box; a flow meter 106 is installed on the gas pipe to monitor the flow change when collecting gas. By installing the above-mentioned devices, it is helpful to improve the accuracy of monitoring and realize the correction of greenhouse gas emission accounting results.

[0061] (6) For the online monitoring mode, the valve of the floating gas collection box at the relative position on the side surface and the portable greenhouse gas concentration analyzer are connected by a gas pipe as shown in the figure. Figure 2 The connection enables the gas in the floating gas collection box to circulate between the floating gas collection box and the portable greenhouse gas concentration analyzer 113.

[0062] (7) For the offline monitoring mode, the upper end of the floating gas collection box is connected to the gas sampler by a gas pipe. Figure 2 The air bag 101 is used to collect the gas in the floating gas collection box; the air sampler 102 is used to collect the gas in the floating gas collection box. The gas in the floating gas collection box is collected in the air bag through the air sampler.

[0063] (8) If the unit facility is an open structure, place the floating gas collection box in step (4) on the surface of the unit cell body so that the gas collection box is completely immersed; if the unit facility is a closed structure, place the floating gas collection box in step (4) at the unit gas collection facility or at the ventilation port.

[0064] (9) The temperature inside the floating gas collection box is read in real time by the thermometer installed on the floating gas collection box device; the wind speed and air pressure on site are measured in real time by the anemometer and barometer; and the gas flow and pipeline pressure are monitored in real time by the flow meter and pressure gauge installed on the gas pipe.

[0065] (10) Aeration unit is added as follows Figure 3The pressure-free air bag shown is used to correct the greenhouse gas accounting results. The pressure-free air bag mainly includes: a traction rope 201, which is used to fix the pressure-free air bag; a foam base plate 202, which allows the pressure-free air bag to float; a buckle 203, which is used to connect the traction rope; a metal ring 204, which fixes the pressure-free air bag on the foam base plate on all sides; a polyethylene air bag 205, which is used to collect gas. The pressure-free air bag should be slowly placed into the corresponding point of the aeration unit and completely immersed. The timing starts from the time when the pressure-free air bag is immersed in the pool surface to the end of the timing when the pressure-free air bag is filled with gas. The gas flow rate at the corresponding point of the aeration unit can be accurately obtained by using the volume and filling time of the pressure-free air bag. The greenhouse gas emission accounting results of the aeration unit are corrected based on the measured gas flow rate.

[0066] (11) For the online monitoring mode, the greenhouse gas in the floating gas collection box can be measured by a portable greenhouse gas concentration analyzer to obtain the concentration change, and the gas concentration release rate can be calculated based on the change of greenhouse gas concentration over time.

[0067] (12) For the offline monitoring mode, after the floating gas collection box is stabilized for 1 min, samples are taken using a gas sampler and an air bag every 10 to 20 min. The greenhouse gas concentration in the air bag samples is then determined by gas chromatography. The rate of change of gas concentration is calculated based on the relationship between sampling time and greenhouse gas concentration.

[0068] (13) Select online or offline monitoring working mode according to needs.

[0069] (14) The area of ​​each process unit is divided into grid units according to the point distribution. Reasonable grid division can effectively realize the accurate calculation of the direct greenhouse gas emission flux.

[0070] (15) The direct greenhouse gas emission flux is calculated by combining the greenhouse gas concentration, weather conditions, temperature and other parameters monitored at different points in step (9), and then the direct greenhouse gas emission flux at each point is multiplied by the corresponding grid area of ​​each point. The total daily greenhouse gas emissions of the sewage treatment plant during the sewage treatment process can be obtained by formula (1)-formula (7).

[0071] For non-aerated units, the greenhouse gas emission flux at each point is calculated using formula (1); for aerated units, the greenhouse gas emission flux at each point is calculated using formula (2):

[0072]

[0073] Flux: greenhouse gas diffusion flux at each point, mg·(m 2 d) -1 ;

[0074] Slop: greenhouse gas release rate, ppm·S -1 ;

[0075] F 1 :Standard temperature and pressure of gases in air from ppm to mg·m -3 Conversion factor of

[0076] α: pressure correction coefficient;

[0077] β: temperature correction coefficient;

[0078] C: greenhouse gas concentration, Vol%;

[0079] ρ: density of greenhouse gas at the monitoring temperature, kg / m 3 ;

[0080] Q: Greenhouse gas flow; m 3 / d;

[0081] F 2 : Time conversion coefficient, here it is 86400;

[0082] V: air volume in the static box, m 3 ;

[0083] S: water surface area covered by the box, m 2 ;

[0084] T: atmospheric temperature during on-site monitoring, K;

[0085] P: atmospheric pressure when the gas sample was collected, kPa;

[0086] T 0 : air temperature under standard conditions, 273K;

[0087] P 0 : Air pressure under standard conditions, 101.325 kPa;

[0088] M: gas molar mass, g·mole -1 ;

[0089] R: gas constant, usually 8.3144, J / (K·mol);

[0090] The greenhouse gas emissions of each process unit are calculated as follows:

[0091] E u =Flux×S u (6)

[0092] E u : Greenhouse gas emissions of each process unit, mg / d;

[0093] S u : Total surface area of ​​each process unit, m2 ;

[0094] The direct greenhouse gas emission flux of the sewage treatment plant is calculated as follows:

[0095]

[0096] E: direct greenhouse gas emission flux from sewage treatment plants, kg / d;

[0097] n: the number of process units monitored in the sewage treatment plant, units;

[0098] E uk : Greenhouse gas emissions of the kth process unit, mg / d.

[0099] Example: Actual measurement of total greenhouse gas emissions from sewage treatment plants

[0100] A sewage treatment plant, the current total construction scale is 110,000 m 3 / d, using improved AAO biological treatment process for phosphorus removal and nitrogen removal, the aeration grit chamber of the sewage treatment plant is a closed unit, and the other units are open process units. According to the point distribution principle of the present invention, the process flow of the sewage treatment plant is distributed, and monitoring points are arranged at the water inlet, aeration grit chamber, front end of anoxic tank, middle end of anoxic tank, end of anoxic tank, front end of anaerobic tank, middle end of anaerobic tank, end of anaerobic tank, front end of aerobic tank, middle end of aerobic tank, end of aerobic tank, secondary sedimentation tank and water outlet.

[0101] For closed process units such as aerated grit chambers, a floating gas collection box is placed at the vent; for other open process units, a floating gas collection box is placed above the tank body to monitor the greenhouse gas emissions of the biological pool. After the floating gas collection box is stabilized for 1 minute, the floating gas collection box is connected to the portable greenhouse gas concentration analyzer and the gas sampler with a gas pipe to realize the circulation of the gas in the floating gas collection box between the floating gas collection box and the portable greenhouse gas concentration analyzer and the gas sampler, and the greenhouse gas concentration at this point is monitored. A drying tube filled with anhydrous calcium sulfate is installed in front of the portable greenhouse gas concentration analyzer. The greenhouse gas concentration can be obtained after the greenhouse gas in the floating gas collection box passes through the portable greenhouse gas concentration analyzer, and the rate of change of gas concentration is calculated by the relationship between concentration and time. After the online monitoring mode is determined, it is switched to the offline monitoring mode, and the gas sampler and the gas bag are used to take samples at intervals of 10 minutes, and then the greenhouse gas concentration in the gas bag sample is determined by gas chromatography.

[0102] The gas release rate is calculated through the relationship between the sampling time at a certain point and the greenhouse gas concentration, and the greenhouse gas release flux at a certain point is calculated. The total daily greenhouse gas emissions of the sewage treatment plant during the sewage treatment process can be obtained through the direct greenhouse gas emission flux at each point and the corresponding grid area at each point (such as Figure 4 ). According to calculations, the annual greenhouse gas emissions of the sewage treatment plant are 11463.45 kgCO 2 / d.

[0103] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for greenhouse gas monitoring and direct emission flux accounting in a sewage treatment plant, characterized by: The steps include: Step 1: Before monitoring greenhouse gases in sewage treatment plants, it is necessary to investigate the background information of the sewage treatment plants, and fully understand the process flow, sewage type, and sewage pipe network of the sewage treatment plants. On this basis, monitoring points are arranged along the sewage treatment from water inlet to water outlet according to the actual situation of the sewage treatment plants; Step 2: Use the thermometer configured on the floating gas collection box device to read the internal temperature of the collection box in real time; use the anemometer and barometer to measure the on-site wind speed and air pressure in real time; use the flow meter and pressure gauge configured on the air pipe to monitor the gas flow and pipeline pressure in real time; use a pressure-free air bag to obtain the surface gas flow of the aerated grit chamber, aerobic tank and other units containing aeration facilities; Step 3, connecting the floating gas collection box with a portable greenhouse gas concentration analyzer, obtaining concentration changes of the greenhouse gas in the floating gas collection box after passing through the portable greenhouse gas concentration analyzer, calculating the gas concentration release rate according to the change of greenhouse gas concentration over time, and realizing online monitoring of greenhouse gases in the sewage treatment plant; in addition, connecting the air sampler to the floating gas collection box to collect the gas on the tank body, and realizing offline monitoring of greenhouse gases in the sewage treatment plant by gas chromatography, after the floating gas collection box is stabilized for 1 minute, sampling is performed by using a gas sampler and an air bag every 10 to 20 minutes, and then the greenhouse gas concentration in the air bag sample is determined by gas chromatography, and the greenhouse gas release rate in the process unit is calculated according to the relationship between the sampling time and the change of greenhouse gas concentration; Step 4: Based on the changes in greenhouse gas concentrations, weather conditions, and temperature at different monitoring points, the following greenhouse gas direct emission flux calculation formula based on the monitoring system is established: For non-aerated units, the greenhouse gas emission flux at each point is calculated using formula (1); for aerated units, the greenhouse gas emission flux at each point is calculated using formula (2): Flux: greenhouse gas diffusion flux at each point, mg·(m 2 d) -1 ; Slop: greenhouse gas release rate, ppm·S -1 ; F1: Standard temperature and pressure of gases in air from ppm to mg·m -3 Conversion factor of α: pressure correction coefficient; β: temperature correction coefficient; C: greenhouse gas concentration, Vol%; ρ: density of greenhouse gas at the monitoring temperature, kg / m 3 ; Q: Greenhouse gas flow; m 3 / d; F2: time conversion coefficient, take 86400; V: air volume in the static box, m 3 ; S: water surface area covered by the box, m 2 ; T: atmospheric temperature during on-site monitoring, K; P: atmospheric pressure when the gas sample was collected, kPa; T0: air temperature under standard conditions, 273K; P0: air pressure under standard conditions, 101.325 kPa; M: gas molar mass, g·mole -1 ; R: gas constant, taken as 8.3144, J / (K·mol); The greenhouse gas emissions of each process unit are calculated as follows: AND u =Flux×S u (6) E u : Greenhouse gas emissions of each process unit, mg / d; S u : Total surface area of ​​each process unit, m 2 ; The direct greenhouse gas emission flux of the sewage treatment plant is calculated as follows: E: direct greenhouse gas emission flux from sewage treatment plants, kg / d; n: the number of process units monitored in the sewage treatment plant, units; E uk : Greenhouse gas emissions of the kth process unit, mg / d.

2. The method for greenhouse gas monitoring and direct emission flux calculation of a sewage treatment plant according to claim 1 is characterized by: The principles of monitoring point layout in step 1 include: ① overall monitoring of greenhouse gas emissions during sewage treatment, covering all process units through which sewage flows; ② water quality monitoring of the most important nodes in each process unit, including the front, middle and back ends of the unit, to examine the generation and transformation of greenhouse gases when sewage flows through each unit, and to grasp the regional greenhouse gas emission rules; ③ under the dual constraints of considering convenience and meeting needs, the point should be convenient for the installation and layout of floating gas collection boxes; ④ for process units with aeration and anaerobic digestion processes, the number of monitoring points in this part should be increased; ⑤ under the setting of multiple number of points, limited monitoring equipment is arranged on important monitoring elements to obtain key information and achieve the best monitoring efficiency; ⑥ under the constraint of economic cost investment, the number of monitoring points arranged as much as possible can determine the optimal monitoring effect that can be achieved under this condition; ⑦ according to the preliminary implementation and experiment of the monitoring plan, the monitoring plan also needs to be optimized and adjusted, and the required remaining monitoring point monitoring data is continuously supplemented according to the existing monitoring data.

3. The method for greenhouse gas monitoring and direct emission flux calculation of a sewage treatment plant according to claim 2 is characterized by: In step 2, a fan is installed above the floating gas collection box.

4. The method for greenhouse gas monitoring and direct emission flux calculation of a sewage treatment plant according to claim 3 is characterized by: In step 2, the floating gas collection box and the pressure-free air bag should be completely submerged in the water.

5. A method for greenhouse gas monitoring and direct emission flux accounting of a sewage treatment plant according to claim 4, characterized in that: In step 3, a drying tube is installed in front of the portable greenhouse gas concentration analyzer.

6. A method for greenhouse gas monitoring and direct emission flux accounting of a sewage treatment plant according to claim 5, characterized in that: In step 3, the online measurement and offline measurement modes are coupled to switch between different monitoring modes. The greenhouse gas concentration error in the two modes should be less than 10%.

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