Carbon emission accounting method based on urban sewage treatment plant

Abstract

The invention discloses a carbon emission accounting method based on an urban sewage treatment plant, and relates to the technical field of sewage treatment carbon emission, the carbon emission accounting method based on the urban sewage treatment plant comprises the following steps: determining an accounting boundary, clarifying a carbon emission activity greenhouse gas generation principle and a carbon emission source, and formulating a greenhouse gas emission list; carbon accounting activities are analyzed, and corresponding accounting methods and accounting principles are confirmed according to different types of carbon emissions; according to an accounting method and principle, an accounting model is built, an accounting system is established, emission factors corresponding to all carbon emission activities are selected, a data monitoring plan is made according to the accounting model / system, carbon emission accounting of all levels is carried out, and the total carbon emission amount is calculated. According to the method, an analysis framework is provided, four carbon reduction types of carbon reduction, carbon substitution, carbon sequestration and endogenous negative carbon are clarified, and a subsequent path scheme is convenient to propose.

Description

Technical Field

[0001] The present invention relates to the technical field of carbon emissions in sewage treatment, and particularly to a carbon emission accounting method based on urban sewage treatment plants. Background Art

[0002] With the advancement of urbanization, sewage treatment plants play an increasingly important role in urban environmental governance. However, a large amount of carbon emissions are generated during the sewage treatment process in sewage treatment plants, which has a non-negligible impact on greenhouse gas emissions and global climate change. To achieve the goal, urban sewage treatment plants need to accurately account for their carbon emissions and take effective emission reduction measures.

[0003] The sewage treatment industry is a key area to achieve the goal. As the core unit of the industry, the research on carbon emission accounting and emission reduction paths of sewage treatment plants is of great significance for promoting the low-carbon transformation of the industry. Carbon accounting is the basis for achieving carbon emission reduction, but there are problems of non-uniformity in the accounting boundary, accounting scope, and emission factors in the existing accounting methods, resulting in limitations in the accuracy and applicability of the accounting results.

[0004] To deal with different sewage treatment accounting situations, the commonly used carbon emission accounting methods in sewage treatment plants mainly include the emission factor method, mass balance method, measurement method, model method, life cycle method, etc. In the existing technology, some domestic and foreign standards and methods have tried to account for the carbon emissions of sewage treatment plants based on the emission factor method and mass balance method. For example, the ISO14064 standard "Greenhouse Gas Accounting and Reporting" provides a general carbon emission accounting framework for enterprises and organizations, but it does not provide a detailed description of the specific processes of sewage treatment plants (ISO 14064-1:2006). In addition, relevant research also shows that carbon emissions during the sewage treatment process mainly come from the following aspects: (1) direct greenhouse gas emissions attributed to or controlled by the activities of the accounting entity itself; (2) indirect greenhouse gas emissions caused by the accounting entity's purchase of electricity, steam, heat / cold sources; (3) other indirect greenhouse gas emissions outside the accounting boundary but caused by the activities of the accounting entity in the value chain (not included in Scope 2), such as purchased consumables and waste disposal. However, there are some neglected carbon emission reductions in the actual carbon emission accounting process: for example, some parts such as energy recovery are often not fully considered, resulting in a higher accounting result. The accounting accuracy is insufficient: the existing emission factors are relatively simple and cannot be accurately adjusted according to specific processes and operating conditions, resulting in a large error in the accounting result.

[0005] Therefore, the technical personnel in this field are committed to developing a perfect carbon emission accounting method for urban sewage treatment plants. Summary of the Invention

[0006] In view of the above-mentioned deficiencies of the prior art, the technical problem to be solved by the present invention is how to conduct high-precision carbon emission accounting for the sewage treatment process.

[0007] To achieve the above object, the present invention provides a carbon emission accounting method based on urban sewage treatment plants, and the method includes the following steps:

[0008] Step 1. Determine the accounting boundary, clarify the greenhouse gas generation principle and carbon emission sources of carbon emission activities, and formulate a greenhouse gas emission inventory;

[0009] Step 2. Analyze the carbon accounting activities, and confirm the corresponding accounting methods and principles according to different types of carbon emissions;

[0010] Step 3. According to the accounting methods and principles, build an accounting model, establish an accounting system, select the corresponding emission factors for each carbon emission activity, and formulate a data monitoring plan according to the accounting model / system to confirm the specific data to be collected or acquired;

[0011] Step 4. Summarize and organize, conduct carbon emission accounting at each level, and calculate the total carbon emissions; the accounting at each level includes calculating the carbon emissions of each emission unit, the carbon emissions from energy consumption of each emission activity, the carbon emissions from material consumption of each emission activity, and the carbon compensation emission reduction amount, and finally summarize to obtain the total carbon emissions, with the unit of carbon dioxide equivalent, that is, CO2-eq.

[0012] In a preferred embodiment of the present invention, the confirmation of the accounting boundary in Step 1 includes:

[0013] 1) Boundary division in the time dimension: In the time dimension, it includes the full life cycle dimension during the operation and maintenance stage, focusing on accounting for the carbon emissions during the operation and maintenance stage of the sewage treatment plant, and at the same time considering the full life cycle carbon emissions of some processes during the operation stage;

[0014] 2) Boundary division in the space dimension: In the space dimension, the accounting boundary is defined as: the entire process of sewage treatment and sludge treatment and disposal.

[0015] In another preferred embodiment of the present invention, the carbon emission sources include direct emissions, indirect emissions, chemical agent consumption, sludge transportation, and carbon emission reduction:

[0016] In another preferred embodiment of the present invention, the direct emissions are non-biogenic CO2, CH4, and N2O gases directly generated during the biochemical reaction during sewage treatment, greenhouse gas emissions generated during the sludge disposal process; fossil energy consumed during the production process.

[0017] In another preferred embodiment of the present invention, the indirect emissions are specifically the carbon emissions indirectly generated due to the consumption of electricity and heat energy during the sewage and sludge treatment process;

[0018] In another preferred embodiment of the present invention, the reagent consumption includes externally added carbon sources, phosphorus removal reagents, and sludge dewatering reagents; among which the sludge transportation includes fuel combustion during the transportation from the sewage treatment plant to the sludge treatment plant.

[0019] In another preferred embodiment of the present invention, the accounting of the carbon emission reduction part includes four categories: "carbon reduction, carbon sequestration, carbon substitution, and endogenous negative carbon".

[0020] The carbon reduction refers to reducing carbon emissions through energy conservation and consumption reduction and process optimization;

[0021] The carbon sequestration refers to the carbon sink effect of the ecosystem;

[0022] The carbon substitution focuses on the use of clean energy to replace traditional energy; the endogenous negative carbon refers to the recycling of internal resources in the sewage treatment plant;

[0023] The recycling specifically includes the utilization of biogas from sludge anaerobic digestion, carbon emission reduction in land use when sludge is used as soil fertilizer, and nutrient recovery.

[0024] The carbon emission accounting method based on urban sewage treatment plants proposed by the present invention demonstrates significant practicality in terms of technical advantages, performance indicators, and production implementation, and has broad industrial application prospects, which are specifically reflected in the following aspects:

[0025] 1. Technical advantages

[0026] (1) Significantly improved accounting accuracy: By refining the selection of emission factors, combining specific processes, and adopting a modular accounting framework, the error problems caused by the simplification of emission factors and the fuzzy boundaries in existing methods are effectively solved.

[0027] (2) Comprehensive coverage of carbon emission reduction paths: Innovatively classifying carbon emission reduction into four categories: "carbon reduction, carbon substitution, carbon sequestration, and endogenous negative carbon", systematically incorporating emission reduction measures such as energy recovery (such as biogas utilization), clean energy substitution (photovoltaic / wind power), and resource recycling (nutrient recovery), ensuring that the accounting results are closer to the actual emission reduction potential. Taking a certain pilot sewage treatment plant as an example, this method increases the accounting coverage rate of carbon emission reduction from the original 60% to 95%.

[0028] (3) Adaptability to process diversity: By establishing a differentiated emission factor library for different treatment processes (AAO, SBR, oxidation ditch, etc.), supporting personalized accounting schemes, meeting the needs of various sewage treatment scenarios, and solving the limitations of the existing standard of "one-size-fits-all".

[0029] 2. Performance indicators

[0030] (1) High efficiency and accuracy: Using a standardized data collection template and accounting model to reduce the accounting cost.

[0031] (2) Dynamic monitoring ability: By formulating a real-time data monitoring plan (such as daily water intake, energy consumption, chemical dosage), it supports the generation of carbon emission reports on a monthly or quarterly basis, providing data support for dynamically optimizing process parameters.

[0032] 3. Feasibility of production implementation

[0033] (1) Low implementation cost: No additional hardware investment is required. Only need to embed an accounting module in the existing management system, or realize data entry and calculation through a standardized Excel template.

[0034] (2) Ease of operation: Provide clear accounting process guidelines and parameter databases to lower the technical threshold. Description of the drawings

[0035] Figure 1 is the complete flowchart of the implementation of the present invention;

[0036] Figure 2 is a schematic diagram of the carbon emission accounting boundary of the urban sewage treatment plant in Example 1;

[0037] Figure 3 is a line chart of the monthly carbon emissions of sewage treatment plant A from 2021 to 2023 in Example 1;

[0038] Figure 4 is a bar chart of the monthly carbon emissions of sewage treatment plant A from 2021 to 2023 in Example 1;

[0039] Figure 5 is a distribution chart of carbon emissions from different emission sources of sewage treatment plant A from 2021 to 2023 in the first group of Example 1;

[0040] Figure 6 is a percentage contribution chart of carbon emissions from different emission sources of sewage treatment plant A from 2021 to 2023 in Example 1;

[0041] Figure 7 is a Sankey diagram of carbon emissions of sewage treatment plant A in 2023 in Example 1;

[0042] Figure 8 is an analysis of the carbon emission intensity and carbon emission influencing factors at each emission site of sewage treatment plant A in 2023 in Example 1. Detailed implementation manners

[0043] The following introduces multiple preferred embodiments of the present invention with reference to the accompanying drawings of the specification, making its technical content clearer and easier to understand. The present invention can be embodied in many different forms of embodiments, and the protection scope of the present invention is not limited to the embodiments mentioned in the text.

[0044] In the accompanying drawings, components with the same structure are denoted by the same numerical labels, and components with similar structures or functions everywhere are denoted by similar numerical labels. The dimensions and thicknesses of each component shown in the drawings are arbitrarily shown, and the present invention does not limit the dimensions and thicknesses of each component. To make the illustration clearer, the thicknesses of some components are appropriately exaggerated in the drawings.

[0045] Provided is a carbon emission accounting method based on an urban sewage treatment plant. The method is specifically as follows:

[0046] 1. Determine the accounting boundary, clarify the greenhouse gas generation principle and carbon emission sources of carbon emission activities, and formulate a greenhouse gas emission inventory.

[0047] (1) Confirm the accounting boundary (time and physical boundary):

[0048] 1) Boundary division in the time dimension: In the time dimension, the entire life cycle dimension within the operation and maintenance stage is selected, that is, the carbon emissions during the operation and maintenance stage of the sewage treatment plant are mainly accounted for. At the same time, for some processes during the operation stage (such as chemical consumption), the carbon emissions throughout their life cycles are considered. The specific basis is as follows: First, the carbon emissions during the construction and demolition stages account for a small proportion. Research shows that the carbon emissions during the construction and demolition stages account for about 5% of the total life cycle carbon emissions of the sewage treatment plant, and the carbon emission intensities per unit treatment volume of sewage treatment plants of different construction types do not differ significantly. Second, the pertinence of the research goal. This article focuses on the carbon emission reduction assessment of existing sewage treatment plants that have been built and put into operation. Considering the carbon emissions during the construction and demolition stages has limited practical significance for the implementation of carbon emission reduction work.

[0049] Although the carbon emissions during the construction and demolition stages are not considered, when calculating the carbon emissions of chemicals during the sewage treatment process, the carbon emissions throughout the life cycle of chemical production and manufacturing (including processes such as fuel transportation) are still included to ensure the comprehensiveness of the accounting. And this process is also included within the operation and maintenance stage of the sewage treatment plant.

[0050] 2) Boundary division in the space dimension: In the space dimension, the accounting boundary is defined as: the entire process of sewage treatment and sludge treatment and disposal. First, the sewage treatment process within the sewage treatment plant, including greenhouse gas emissions generated during the sewage treatment process, indirect emissions such as energy consumption, and indirect emissions of energy and raw material consumption during the ventilation and deodorization process. Second, the sludge treatment and disposal process outside the plant boundary, including carbon emissions from sludge transportation, composting, incineration and other disposal methods. At the same time, the carbon emissions of the receiving water body are not considered. The emission accounting of the receiving water body is relatively complex, and its direct correlation with the operation of the sewage treatment plant is relatively weak, so it is not included in the accounting scope.

[0051] (2) In this accounting method, the basis for the division range of carbon emission sources and the specific division content are as follows:

[0052] 1) Scope 1: Direct emissions, including non-biological CO2, CH4, and N2O gases directly generated during the biochemical reaction in sewage treatment, greenhouse gas emissions during sludge disposal, and fossil energy (such as oil, coal, gas, etc.) consumed during the production process.

[0053] 2) Scope 2: Indirect emissions refer to the carbon emissions indirectly generated due to the consumption of electricity and thermal energy during the sewage and sludge treatment process. (Equivalent carbon emissions generated from purchased electricity and steam).

[0054] 3) Scope 3: Chemical agent consumption (chemical agents include externally added carbon sources, phosphorus removal agents, and sludge dewatering agents); sludge transportation (fuel combustion during the transportation process from the sewage treatment plant to the sludge treatment plant).

[0055] Table 1

[0056]

[0057] 4) Carbon emission reduction: The accounting of carbon emission reduction mainly includes four categories: "carbon reduction, carbon sequestration, carbon substitution, and endogenous negative carbon". In this classification, "carbon reduction" refers to reducing carbon emissions through energy conservation and consumption reduction and process optimization; "carbon sequestration" refers to the carbon sink function of the ecosystem; "carbon substitution" focuses more on the utilization of clean energy (such as photovoltaic power generation, wind power generation, etc.) to replace traditional energy; "endogenous negative carbon" specifically refers to the recycling of internal resources in the sewage treatment plant (such as the utilization of biogas from sludge anaerobic digestion, carbon emission reduction in land use when sludge is used as soil fertilizer, and nutrient salt recovery).

[0058] (3) The greenhouse gas emission inventory is as follows:

[0059] 1a Fossil-source CO2 generated by biochemical reactions during sewage biological treatment

[0060] 1b CH4 generated by biochemical reactions during sewage biological treatment

[0061] 1c N2O generated by biochemical reactions during sewage biological treatment

[0062] 1d CH4 escaping from the lift pump and grille chamber

[0063] 1e Fossil-source CO2 generated by the mineralization of externally added carbon sources during sewage biological treatment

[0064] 2a Emissions generated during the anaerobic digestion of sludge treatment

[0065] 2b Emissions generated during the aerobic composting of sludge treatment

[0066] 2c Emissions generated during the incineration of sludge treatment

[0067] 2d Emissions generated by pyrolysis carbonization or gasification of sludge treatment

[0068] 2e. Emissions generated during the sanitary landfill of sludge disposal

[0069] 2f. Emissions generated during the land use of sludge disposal

[0070] 3a CO2 generated from fossil fuels such as diesel, natural gas, and coal used in the sludge process

[0071] 4a Indirect carbon emissions caused by energy consumption (purchased electricity and heat)

[0072] 5a Indirect carbon emissions caused by chemical agent input

[0073] 2. Analyze carbon accounting activities, and confirm the corresponding accounting methods and principles according to different types of carbon emissions (for example, using a modular approach to facilitate the formulation of subsequent emission reduction plans)

[0074] 3. According to the accounting methods and principles, build an accounting model / establish an accounting system, select the corresponding emission factors for each carbon emission activity, and formulate a data monitoring plan according to the accounting model / system to confirm the specific data to be collected / collected.

[0075] Scope 1 carbon accounting in the sewage treatment process

[0076] (1) CH4 carbon emissions

[0077] The CH4 generated from sewage treatment mainly comes from the anaerobic decomposition of organic matter. The global warming potential (GWP) of methane is 28 times that of carbon dioxide. Therefore, its emissions have a significant impact on climate change. The methane gas generated during the sewage treatment process mainly includes the methane generated by biochemical reactions and the methane escaping from the sewage transportation and pretreatment links (such as lift pumps and grille rooms), while deducting the methane amount recovered or removed by treatment. That is, the CH4 emission composition = the amount of CH4 gas generated by biochemical reactions - the amount of CH4 gas recovered or removed by treatment.

[0078] It should be noted that the carbon dioxide equivalent of methane emissions in the sewage treatment section is calculated according to formula (1):

[0079]

[0080] In the formula:

[0081] The carbon dioxide equivalent of CH4 emissions on the mth day in the sewage treatment section of the sewage treatment plant, kg CO2-eq;

[0082] Q in,m : The influent volume of the sewage treatment plant on the mth day, m 3 ;

[0083] COD in,m: Average influent COD concentration of the sewage treatment plant on the m-th day, mg / L;

[0084] COD out,m : Average effluent COD concentration of the sewage treatment plant on the m-th day, mg / L;

[0085] Emission factor of CH4 in sewage treatment, kg CH4 / kg COD, conventional recommended value

[0086] 0.0040 - 0.0075, take the larger value when there is sludge deposition in the structure;

[0087] CH4 emission amount dissipated from the lift pump and grille chamber on the m-th day, kg; it is recommended to measure actually, if there is no

[0088] actual measurement condition, calculate according to 20% of the emission amount during the treatment process;

[0089] Global warming potential of CH4.

[0090] (2) N2O carbon emissions

[0091] During the sewage treatment process, especially in the biological nitrogen removal stage, the emission of N2O as a greenhouse gas cannot be ignored. The generation of nitrous oxide mainly comes from the incomplete reactions in nitrification and denitrification. During the nitrification process, some intermediate products such as nitrite (NO2 - ) and hydroxylamine (NH2OH) will be converted into nitrous oxide, while in the denitrification process, due to insufficient carbon source supply or incomplete reaction, NO3- cannot be completely converted into nitrogen gas (N2), resulting in some reactions staying at the N2O stage. Therefore, accurately accounting for the emission amount of nitrous oxide during the sewage treatment process is crucial for greenhouse gas emission assessment and environmental impact analysis.

[0092] The carbon dioxide equivalent of the nitrous oxide emitted during the biological nitrogen removal process of sewage treatment is calculated according to the formula in the "Assessment Standard", but when the sewage treatment plant removes N2O gas through recovery or treatment, it should be deducted from the result:

[0093]

[0094] In the formula:

[0095] Carbon dioxide equivalent of N2O emission from the sewage treatment section of the sewage treatment plant on the m-th day, kg

[0096] CO2-eq;

[0097] Q in,m : Water inflow of the sewage treatment plant on the m-th day, m3 ;

[0098] TN in,m : The average influent TN concentration of the sewage treatment plant on the m-th day, mg / L;

[0099] TN out,m : The average effluent TN concentration of the sewage treatment plant on the m-th day, mg / L;

[0100] Emission factor of N2O in sewage treatment, kg N2O-N / kg N; recommended value 0.016

[0101] kg N2O-N / kg N, either calculated by mathematical model method or obtained by field measurement method;

[0102] 44 / 28: Conversion factor, kg N2O-N / kg N2O;

[0103] Recover or treat to remove N2O gas;

[0104] Global warming potential of N2O, 256.

[0105] (3) Fossil source CO2 emissions

[0106] In the secondary treatment stage of the sewage treatment plant, organic pollutants in the sewage are decomposed by microbial metabolism, which is the core link for pollutant removal. In this process, organic pollutants (such as biochemical oxygen demand BOD5 and chemical oxygen demand COD) are taken up and utilized by microorganisms. Specifically, part of the organic matter is oxidized to CO2 to provide energy for microbial metabolism; the other part is assimilated into the cell substances of microorganisms. In addition, microorganisms decompose the organic matter stored in their bodies through endogenous respiration and also release CO2. However, in the process of oxidizing ammonia nitrogen (NH4+) to nitrate (NO3-), autotrophic nitrifying bacteria use CO2 as an inorganic carbon source. This process effectively reduces the net CO2 emissions in the system and is therefore called the "carbon sink effect". Therefore, the CO2 absorbed by nitrifying bacteria has been deducted from the total CO2 emissions of the sewage treatment plant.

[0107] The CO2 emitted during sewage treatment can be further divided into biogenic CO2 and fossil source CO2. Biogenic CO2 mainly comes from natural organic matter, such as food residues, plant and animal residues, and human excreta. These natural organic matters are part of the natural carbon cycle, and the CO2 released by microbial metabolism of them is usually considered to have no additional contribution to carbon emissions in the atmosphere. Therefore, the IPCC guidelines state that biogenic CO2 is not included in the greenhouse gas emission accounting.

[0108] The emissions of fossil - sourced CO2 mainly come from two parts. One is the synthetic organic compounds contained in the influent, and the other is the carbon sources externally added during the sewage treatment process. Synthetic organic compounds may be contained in domestic sewage, such as detergents, cleaners, cosmetics, and industrial chemicals discharged from households. These compounds are usually extracted or synthesized from fossil fuels, and the CO2 released during their decomposition is classified as fossil - sourced CO2. In addition, during the sewage treatment process, to optimize biological denitrification reactions (such as reducing nitrates to nitrogen gas), external carbon sources (such as methanol or acetic acid) are often added. These carbon sources also mainly come from fossil fuels, and the CO2 released during their degradation will be directly included in the accounting scope of fossil - sourced emissions. To accurately account for the emissions of fossil - sourced CO2, it is necessary to calculate by combining the proportion of fossil - sourced organic compounds and the total CO2 emissions. The general method is to first determine the content of organic compounds in the sewage and the proportion of fossil - sourced organic compounds, then calculate the total CO2 emissions during the treatment process, and multiply the two to obtain the emissions of fossil - sourced CO2.

[0109] In summary, the emissions of fossil - sourced CO2 in sewage treatment plants mainly include three parts: CO2 generated by the decomposition of fossil - sourced organic compounds under aerobic conditions, CO2 generated by the endogenous respiration of microorganisms metabolizing fossil - sourced carbon, and CO2 directly released by the decomposition of externally added carbon sources. The classification and accurate accounting of these emissions are of great significance for the formulation of carbon emission reduction strategies and process optimization in sewage treatment plants.

[0110] The carbon emissions generated by fossil - sourced carbon mineralization are calculated according to the formula in the "Accounting Guidelines". In the subsequent calculation process, since the proportion of fossil - sourced organic compounds in the average influent of sewage treatment plants in Shanghai is relatively low, the FCF parameter is corrected to 5% here:

[0111]

[0112] In the formula:

[0113] The emission intensity of fossil - sourced CO2 for sewage treatment on the m - th day, kg CO2 - eq / m 3 ;

[0114] MFCF: The proportion of fossil - sourced CO2 emissions, %, which can be calculated by formula (2 - 4);

[0115] 1.1: The CO2 production from the mineralization of BOD5;

[0116] B in，m The average influent BOD5 concentration of the sewage treatment plant on the m - th day, mg BOD5 / L;

[0117] B ex，m : The additional carbon source artificially added during the operation, mg BOD5 / L;

[0118] Beff，m Average effluent BOD5 concentration of the sewage treatment plant on the mth day, mg BOD5 / L;

[0119] 1.47: Ratio of actual BOD concentration to measured BOD5 concentration;

[0120] 1.42: BOD5 equivalent of microbial cells, kg BOD5 / kg MLVSS;

[0121] 0.67: Absolute yield coefficient, kg MLVSS / kg BOD5;

[0122] K d : Decay coefficient, d -1 , which can be calculated by formula (2-5);

[0123] SRT: Mean cell residence time, d;

[0124] 1.947: Endogenous CO2 respiration yield of sludge, kg CO2 / kg MLVSS;

[0125] HRT: Hydraulic retention time of the biological reaction tank, d;

[0126] MLVSS: Average concentration of mixed liquor volatile suspended solids in the biological tank, mg MLVSS / L;

[0127] 4.49: Mass of CO2 fixed per unit mass of ammonia nitrogen nitrification, kg CO2 / kgNH4 + -N;

[0128] TKN in : Average influent total Kjeldahl nitrogen concentration of the sewage treatment plant, mgN / L;

[0129] TKN eff : Average effluent total Kjeldahl nitrogen concentration of the sewage treatment plant, mg N / L;

[0130] 0.124: Mass ratio of N in microorganisms;

[0131] FCF: Proportion of fossil-source organic matter in the average influent of the sewage treatment plant, generally taken as 10%, which can be

[0132] corrected according to the region and changed between 0 and 10%;

[0133] T b : Water temperature, °C.

[0134] (4) Direct carbon emissions from fuel combustion

[0135] During the sewage treatment process, to meet the requirements of boiler operation, hot steam production, equipment operation, etc., it is usually necessary to consume fixed energy fuels. The combustion of these fuels will directly release carbon dioxide, which is one of the important sources of direct carbon emissions in sewage treatment plants. The carbon emissions generated by energy fuel consumption are calculated according to formula (6):

[0136]

[0137] In the formula:

[0138] CE w-fc，m : Carbon emissions generated by the combustion of fixed fuels in the sewage treatment plant on the mth day, kg CO2-eq;

[0139] Mf c，j，m : Combustion amount of the jth type of fuel on the mth day, TJ;

[0140] EF fc，j : CO2 equivalent emission factor of the jth type of fuel, kg CO2-eq / TJ.

[0141] Carbon emission accounting for Scope 1 in the sludge treatment process

[0142] (1) Carbon emissions from sludge anaerobic digestion

[0143] When the sewage treatment plant uses anaerobic digestion to treat the excess sludge, the organic matter in the sludge is degraded under the action of microorganisms and finally converted into CO2 and CH4 gases. Since it is usually difficult to avoid the leakage of some CH4 gases during the biogas collection process, there may also be carbon emissions caused by incomplete combustion of the biogas torch. The leaked methane should be accounted for and converted into carbon dioxide equivalent according to the methane emissions for statistics.

[0144] Therefore, this study provides two accounting methods: (1) Theoretical calculation method: Use the methane potential of the excess sludge for theoretical calculation, and estimate the theoretical methane production based on the degradation amount of VSS (volatile suspended solids) in the sludge and the volume ratio of methane in the biogas. See Method 1 for details. (2) Actual measurement method: When the actual methane production can be obtained, the collected biogas data can be directly used, combined with the biogas leakage ratio, to account for the actual emissions to ensure better compliance with the on-site operation conditions. See Method 2 for details.

[0145] Method 1: Can directly use the methane production potential of the excess sludge for theoretical calculation

[0146] The formula is:

[0147]

[0148] In the formula:

[0149] Anaerobic digestion CH4 emission intensity, kg CO2-eq / m 3 ;

[0150] F: Biogas leakage ratio, %, the recommended value by IPCC can be adopted;

[0151] Q ss,m : Total amount of excess sludge produced by the sewage treatment plant on the mth day, m 3 ;

[0152] VSS in,ad,m : VSS of the influent sludge in the anaerobic digester, kg / m 3 ;

[0153] VSS out,ad,m : VSS of the effluent sludge in the anaerobic digester, kg / m 3 ;

[0154] 44: Molar mass of CO2, constant, g / mol;

[0155] 28: Global warming potential of CH4, kg CO2-eq / kg CH4;

[0156] 16: Molar mass of CH4, constant, g / mol;

[0157] Volume ratio of CH4 in biogas, %;

[0158] Q in,m : Total amount of domestic sewage treated by the sewage treatment plant on the mth day, m 3 .

[0159] Method 2: When the methane production collected in actual production activities can be directly obtained:

[0160]

[0161] In the formula:

[0162] Carbon dioxide equivalent of CH4 emissions during the anaerobic digestion of sludge on the mth day, kg

[0163] CO2-eq;

[0164] VSS biogas,m : Biogas production on the mth day, m 3 ;

[0165] Proportion of CH4 in biogas, %;

[0166] F: Biogas leakage ratio, ranging from 0 to 10%, with a default value of 5%. If the technical standards of the biogas plant can ensure that all accidentally leaked CH4 is burned by the flare, then the CH4 emissions can be approximated to 0;

[0167] 16 / 22.4: Conversion coefficient of the molar mass and molar volume of CH4, kg / m 3 ;

[0168] Global warming potential of CH4.

[0169] The corrected leakage ratio is F / (1 - F). When considering the leakage ratio, directly using the leakage ratio to calculate the methane content will overestimate the methane production. Because the leaked biogas part is not collected, the actual amount of methane produced will be larger than that reflected by the methane ratio in the collected biogas.

[0170] (2) Carbon emissions from aerobic sludge fermentation

[0171] During the aerobic sludge fermentation process, while microorganisms convert organic matter into stable products, the greenhouse gases produced mainly include CO2, N2O, and CH4. The greenhouse gas emissions during the aerobic sludge fermentation process include carbon dioxide, nitrous oxide, and methane. Among them, the emission of carbon dioxide mainly comes from the complete oxidation of organic matter, while nitrous oxide and methane are respectively related to the nitrification / denitrification process and the microbial metabolism in the local anaerobic environment. Although aerobic fermentation aims to avoid the formation of anaerobic conditions through sufficient oxygen supply, local anaerobic environments may still lead to the generation of methane. The generation of methane usually occurs at the bottom of the fermentation pile or in cases of poor aeration. Under these conditions, the anaerobic decomposition of microorganisms will convert organic matter into methane. The schematic of this reaction process is: C6H 12 O6 → 3CO2 + 3CH4. Although the generation of methane under aerobic conditions is relatively small, its generation depends on specific operating conditions and pile structure. It should be noted that the carbon dioxide included in the calculation here mainly refers to the fossil source carbon part.

[0172] The "Assessment Criteria" and the "Accounting Guidelines" respectively omit the calculation of carbon emissions of two gases, fossil source carbon and methane. The greenhouse gases generated by the biochemical reactions during the aerobic sludge fermentation process referred to in this article include CO2, N2O, and CH4, and their carbon dioxide emission equivalents are calculated according to the following formula:

[0173] 1) Emission of CH4:

[0174]

[0175] In the formula:

[0176] Carbon dioxide equivalent of CH4 emissions during aerobic sludge fermentation on the mth day, kg

[0177] CO2-eq;

[0178] M ss,m : Amount of sludge treated by aerobic fermentation on the mth day, kg of sludge;

[0179] Emission factor of CH4 during aerobic sludge fermentation, g CH4 / kg of sludge;

[0180] Global warming potential of CH4, 28.

[0181] 2) N2O emissions:

[0182]

[0183] Where:

[0184] Carbon dioxide equivalent of N2O emissions during aerobic sludge fermentation on the mth day, kg

[0185] CO2-eq;

[0186] M ss，m : Amount of sludge treated by aerobic fermentation on the mth day, kg of sludge;

[0187] Emission factor of N2O during aerobic sludge fermentation, kg N2O / kg of sludge;

[0188] Global warming potential of CH4, 28;

[0189] Global warming potential of N2O, 256.

[0190] (3) Carbon emissions from sludge incineration

[0191] Gases such as fossil-source CO2, CH4, and N2O are generated during sludge incineration.

[0192] 1) Carbon emissions from fossil-source CO2:

[0193] Carbon emissions of fossil-source CO2 from sludge incineration are calculated according to formula (11):

[0194]

[0195] Where:

[0196] Carbon emissions of fossil-source CO2 during sludge incineration on the mth day, kg CO2-eq;

[0197] M ss,m : Dry basis weight of sludge incineration on the mth day, kg of dry sludge;

[0198] CF: Carbon content ratio in dry matter, %; Calculated as 40% - 50% of wet sludge;

[0199] FCF: Fossil carbon ratio in sludge, 5% - 20%, taking a higher value when there is petrochemical wastewater in the influent of the sewage treatment plant or

[0200] when a large amount of external carbon source for denitrification is added;

[0201] OF: Oxidation factor. Assuming complete reaction during sludge incineration, the oxidation factor is taken as 100%;

[0202] 44 / 12: Molecular mass ratio of CO2 to C.

[0203] 2) Carbon emission of CH4:

[0204] The carbon dioxide emission equivalent of CH4 during sludge incineration is calculated according to formula (12):

[0205]

[0206] Where:

[0207] The carbon dioxide emission equivalent of CH4 emission during sludge incineration on the mth day, kg CO2-eq;

[0208] M ss,m : Dry basis weight of sludge incineration on the mth day, kg of dry sludge;

[0209] Emission factor of CH4 during sludge incineration, g N2O / kg sludge;

[0210] Global warming potential of N2O, 256.

[0211] 3) Carbon emission of N2O:

[0212] The carbon dioxide emission equivalent of N2O during sludge incineration is calculated according to formula (13):

[0213]

[0214] Where:

[0215] The carbon dioxide emission equivalent of N2O emission during sludge incineration on the mth day, kg CO2-eq;

[0216] M ss,m: Dry weight of sludge incinerated on the mth day (calculated as SS), kg;

[0217] Emission factor of N2O during sludge incineration, kg N2O / t dry sludge (calculated as SS), the recommended value by IPCC (0.99 kg N2O / t dry sludge) can be adopted;

[0218] Global warming potential of N2O, 256.

[0219] (4) Carbon emissions from sanitary landfill

[0220] When dewatered residual sludge is disposed of by sanitary landfill, due to the lack of oxygen in the landfill environment, the organic matter in the sludge will degrade under anaerobic conditions, generating CO2 and CH4. The gas emissions from this process can be estimated and calculated by the mass balance method. It should be noted that part of the source of organic matter can be traced back to fossil-source pollutants in the sewage. At the same time, considering that the gas emissions after sanitary landfill are a long-term process, reasonable estimation can be carried out according to the law of gas emissions. For example, the content of degradable organic carbon in the sludge decreases with the time after landfill, but how to measure the gas volume generated by the landfill sludge every day specifically remains to be considered.

[0221] 1) Carbon emissions of fossil-source CO2:

[0222] For the CO2 generated by the degradation of these fossil-source organic matters, it should be included in the carbon emission accounting, and the calculation formula (14) is as follows:

[0223]

[0224] In the formula:

[0225] Fossil-source CO2 emission intensity on the mth day of sanitary landfill, kgCO2-eq / m 3 ;

[0226] MFCF: Proportion of fossil-source CO2 emissions, %, which can be calculated by formula (2-4);

[0227] M ss,m : Dry weight of sludge treated on the mth day (calculated as SS), kg;

[0228] DOC: Content of degradable organic carbon in the sludge, kg C / kg dry sludge;

[0229] DOC f : Proportion of decomposable DOC, %, which can be taken as 50% in sanitary landfill;

[0230] MCF: CH4 correction factor, which can be taken as 1 in anaerobic landfill;

[0231] F: The proportion of CH4 in landfill gas. The recommended value of the IPCC can be adopted, 50%.

[0232] 44 / 12: The molecular weight ratio of CO2 to C

[0233] Q m,m : The total amount of sewage treatment in the sewage treatment plant on the mth day, m 3 .

[0234] 2) Carbon emissions from CH4:

[0235] When using the sanitary landfill method to treat the excess sludge of the sewage treatment plant, due to the lack of oxygen, the organic matter in the sludge will generate methane CH4 and CO2 through anaerobic reactions, and finally produce CO2 and CH4 gases. If the landfill is properly managed and the top of the landfill sludge is covered with a breathable material (such as soil, etc.), part of the CH4 will be oxidized by the microorganisms in the cover layer before entering the atmosphere. Therefore, the calculation of this part of CH4 should be excluded. At this time, the mass balance method can be used for related calculations. The calculation formula (15) is as follows:

[0236]

[0237] In the formula:

[0238] The emission intensity of fossil source CH4 from sanitary landfill on the mth day, kgCO2-eq / m 3 ;

[0239] M ss,m : The dry weight of the sludge treated on the mth day (calculated as SS), kg;

[0240] DOC: The content of biodegradable organic carbon in the sludge, kg C / kg dry sludge;

[0241] DOC f : The proportion of decomposable DOC, %, which can be taken as 50% in sanitary landfill;

[0242] MCF: CH4 correction factor, which can be taken as 1 in anaerobic landfill;

[0243] F: The proportion of CH4 in landfill gas. The recommended value of the IPCC can be adopted, 50%;

[0244] OX: The proportion of CH4 oxidized before release, which can be taken as the recommended value of the IPCC, 0.1 (when properly treated) or 0

[0245] (when not properly treated);

[0246] 16 / 12: The molecular mass ratio of CH4 to C

[0247] 1 / 28: Global warming potential of CH4, constant, kg CO2-eq / kg CH4;

[0248] Q m,m : Total amount of sewage treatment in the sewage treatment plant on the mth day, m 3 。

[0249] (5) Sludge pyrolysis carbonization or gasification carbonization

[0250] The direct carbon emissions during the sludge pyrolysis carbonization or gasification process are similar to those of incineration, mainly including fossil-source CO2 emissions, CH4 and N2O emissions. Currently, the pyrolysis carbonization technology is in its infancy in China. According to the IPCC 2019 revised edition, for the pyrolysis system using a rotary kiln, the CH4 and N2O emission factors (calculated based on wet sludge) are 5.4 g / t wet sludge and 8.38 g / t wet sludge respectively. However, with the development of technology and the increase in domestic sludge pyrolysis treatment engineering cases, the emission factors need to be further optimized.

[0251] 1) Emission of fossil-source CO2:

[0252] The fossil-source CO2 emissions from sludge incineration are calculated according to formula (16):

[0253]

[0254] In the formula:

[0255] Fossil-source CO2 carbon emissions during the sludge pyrolysis carbonization or gasification carbonization process on the mth day, kg CO2-eq / m 3 ;

[0256] M ss,m : Dry basis weight of sludge pyrolysis carbonization or gasification carbonization treatment on the mth day, kg dry sludge;

[0257] CF: Carbon content ratio in dry matter, %, calculated as 40% - 50% of wet sludge;

[0258] FCF: Fossil carbon ratio in sludge, 5% - 20%, taking a higher value when there is petrochemical wastewater in the influent of the sewage treatment plant or

[0259] the amount of externally added denitrification carbon source is large;

[0260] OF: Oxidation factor. Assuming complete reaction during sludge pyrolysis carbonization or gasification carbonization, the oxidation factor

[0261] is taken as 100%;

[0262] 44 / 12: Molecular mass ratio of CO2 to C.

[0263] 2) CH4 carbon emissions:

[0264] The carbon dioxide emission equivalent of CH4 during the sludge incineration process is calculated according to formula (17):

[0265]

[0266] In the formula:

[0267] The carbon dioxide equivalent emissions of CH4 during the pyrolysis carbonization or gasification carbonization process of sludge on the mth day, kgCO2-eq / m 3 ;

[0268] M ss,m : The dry weight of the sludge treated by pyrolysis carbonization or gasification carbonization on the mth day (calculated as SS), kg;

[0269] The emission factor of CH4 during the pyrolysis carbonization or gasification carbonization process of sludge, g CH4 / kg sludge, can adopt the recommended value of IPCC 2019, that is, 5.4 g / t sludge (calculated as wet basis);

[0270] The global warming potential of CH4, 28.

[0271] 3) Carbon emissions of N2O:

[0272] The carbon dioxide emission equivalent of N2O during the sludge incineration process is calculated according to formula (18):

[0273]

[0274] In the formula:

[0275] The carbon dioxide equivalent emissions of N2O during the pyrolysis carbonization or gasification carbonization process of sludge on the mth day, kgCO2-eq / m 3 ;

[0276] M ss,m : The dry weight of the sludge treated by pyrolysis carbonization or gasification carbonization on the mth day (calculated as SS), kg;

[0277] The emission factor of N2O during the pyrolysis carbonization or gasification carbonization process of sludge, kg N2O / t dry sludge

[0278] can adopt the recommended value of IPCC 2019, that is, 8.38 g / t sludge (calculated as wet basis);

[0279] The global warming potential of N2O, 256.

[0280] Carbon emission accounting for Scope 2 in the sewage sludge treatment process

[0281] The carbon emission accounting for Scope 2 in the sewage sludge treatment process mainly refers to the indirect carbon emissions caused by electricity consumption and heat consumption during the treatment process. The electricity-consuming links in the sewage treatment process mainly include lift pumps, aeration equipment, etc. In the sludge treatment link, the electricity consumption of all links within the treatment plant area needs to be considered, such as sludge thickening, dewatering, drying, incineration, flue gas treatment, etc.

[0282] 1) Indirect carbon emissions caused by electricity consumption:

[0283] The carbon emissions from the electricity consumption of the equipment operating in the sewage and sludge treatment sections are calculated according to formula (19):

[0284] CE w-ec,m =E ec,m ×EF ec,p (19)

[0285] In the formula:

[0286] CE w-ec，m : The carbon emissions from the electricity consumption on the mth day in the sewage treatment section of the sewage treatment plant, kg CO2-eq;

[0287] E ec，m : The electricity consumption on the mth day in the sewage treatment section of the sewage treatment plant, kWh;

[0288] EF ec，p : The average CO2 equivalent emission factor of the power generation industry in region p where the sewage treatment plant is located, kg

[0289] CO2-eq / kWh.

[0290] Heat consumption mainly refers to the heat such as purchased steam in the sewage and sludge treatment. At present, there are differences in the attribution of heat consumption in many accounting methods. It should be noted that if this heat is directly purchased from the outside, it belongs to Scope 2; but if natural gas or coal is directly burned in the boiler room in the plant to generate heat, this emission activity directly emits greenhouse gases into the atmosphere and belongs to the direct emissions of Scope 1.

[0291] 2) Indirect carbon emissions caused by heat consumption:

[0292] The carbon emissions from the heat consumption of the equipment operating in the sewage and sludge treatment sections are calculated according to formula (20):

[0293] CE w-ec,m =E he,m ×EF he,p (20)

[0294] In the formula:

[0295] CE w-ec，m : The carbon emissions from the heat consumption on the mth day in the sewage treatment section of the sewage treatment plant, kg CO2-eq;

[0296] E he，m : Heat consumption on the m-th day in the sewage treatment section of the sewage treatment plant, kWh;

[0297] EF he，p : Average CO2 equivalent emission factor of the heat production method in area p where the sewage treatment plant is located, kg

[0298] CO2-eq / kWh.

[0299] Carbon emission accounting for Scope 3 in the sewage sludge treatment process

[0300] Indirect carbon emissions brought about by the production process of chemical and biological agents added in the sewage and sludge treatment section, including carbon sources, phosphorus removers, sludge thickening and dewatering agents, disinfectants, etc. Their carbon emissions are calculated according to formula (21):

[0301]

[0302] In the formula:

[0303] CE w-cc，m : Carbon emissions consumed by chemical agent addition on the m-th day in the sewage and sludge treatment section of the sewage treatment plant,

[0304] kg CO2-eq;

[0305] Mc c，j，m : Consumption of the j-th chemical agent added on the m-th day in the sewage and sludge treatment section of the sewage treatment plant, kWh;

[0306] EF cc，j : CO2 equivalent emission factor of the j-th chemical agent on the m-th day in the sewage and sludge treatment section of the sewage treatment plant, kgCO2-eq / kWh.

[0307] Accounting for the carbon emission reduction part

[0308] The carbon emission reduction amount in the sewage sludge treatment section is calculated according to formula (22):

[0309]

[0310] In the formula:

[0311] CA: Carbon emissions of all emission reduction parts on the m-th day of sewage sludge treatment in the sewage treatment plant, kg CO2-eq;

[0312] CA reduction，m : Carbon emissions of the carbon reduction measures on the m-th day of sewage sludge treatment in the sewage treatment plant, kg CO2-eq;

[0313] CA replace,m : Carbon emissions of the carbon substitution measures on the m-th day of sewage sludge treatment in the sewage treatment plant, kg CO2-eq;

[0314] CA sequestration,m : Carbon emission of carbon sequestration measures on the m-th day of sewage sludge treatment in sewage treatment plant, kg CO2-eq;

[0315] CA Negative，m : Carbon emission of endogenous negative carbon measures on the m-th day of sewage sludge treatment in sewage treatment plant, kg CO2-eq.

[0316] Emission factor selection:

[0317] (1) N2O emission factors in different sewage treatment processes

[0318] The N2O emission factors in different sewage treatment processes are shown in Table 2.

[0319] Table 2 N2O emission factors in different sewage treatment processes

[0320]

[0321] Data source: "Technical Guidelines for Carbon Accounting and Emission Reduction in Urban Water Service Systems"

[0322] (2) CH4 emission factors in different sewage treatment processes

[0323] The CH4 emission factors in different sewage treatment processes are shown in Table 3.

[0324] Table 3 CH4 emission factors in different sewage treatment processes

[0325]

[0326]

[0327] Data source: "Technical Guidelines for Carbon Accounting and Emission Reduction in Urban Water Service Systems"

[0328] (3) CH4 emission factor of municipal solid waste incineration

[0329] The CH4 emission factor of municipal solid waste incineration is shown in Table 4.

[0330] Table 4 CH4 emission factor of municipal solid waste incineration

[0331]

[0332] Data source: 2006 IPCC Guidelines

[0333] (4) Biogas leakage ratio of anaerobic digester

[0334] The biogas leakage ratio of anaerobic digester is shown in Table 5.

[0335] Table 5 Methane Leakage Ratio of Anaerobic Digestion Ponds

[0336]

[0337] Data Source: 2006 IPCC Guidelines

[0338] (5) Emission Factors of Common Chemical Agents

[0339] A large amount of chemical agents are used in the process of sewage sludge treatment, and the main categories include carbon sources, phosphorus removers, neutralizers, disinfectants, coagulants, etc. The emission factors of common chemical agents are shown in Table 6.

[0340] Table 6 Emission Factors of Common Chemical Agents

[0341]

[0342]

[0343] Continued Table 6

[0344]

[0345]

[0346] Data Source: 1 Carbon Emission Reduction Assessment Standard for Urban Sewage Treatment Plants

[0347] 2 Technical Guide for Carbon Accounting and Emission Reduction Paths in Urban Water Systems

[0348] (6) Emission Factors of Fossil Fuels

[0349] The emission factors of fossil fuels are shown in Table 7.

[0350] Table 7 Emission Factors of Fossil Fuels

[0351]

[0352]

[0353] Data Source: Technical Guide for Carbon Accounting and Emission Reduction in Urban Water Systems

[0354] (7) Carbon Emission Factors of Regional Electric Energy in China

[0355] The carbon emission factors of regional electric energy in China are shown in Table 8.

[0356] Table 8 Carbon Emission Factors of Regional Electric Energy in China

[0357]

[0358] Data source: "China Regional Grid Baseline Emission Factors for Emission Reduction Projects in 2019" by the Ministry of Ecology and Environment, updated with official data.

[0359] Note: When calculating for a specific region, local grid emission factors can be preferentially used. For example, the electricity emission factor in Shanghai can be revised to 0.42 kg CO2-eq / kWh according to the "Notice on Adjusting the Numerical Values of Relevant Emission Factors in the Greenhouse Gas Emission Accounting Guidelines of this Municipality" issued by the Shanghai Municipal Ecology and Environment Bureau.

[0360] (8) Carbon emission factors of electricity in different regions of our country

[0361] The global warming potential values of greenhouse gases are shown in Table 2.8.

[0362] Table 9 Global warming potential values of greenhouse gases

[0363]

[0364] 4. Summarize and organize, conduct carbon emission accounting at all levels, and calculate the total carbon emissions. The accounting at all levels refers to calculating the carbon emissions of each emission unit, the carbon emissions from energy consumption of each emission activity, the carbon emissions from material consumption of each emission activity, and emission reduction amounts such as carbon compensation. Finally, the total carbon emissions are summarized, with the unit being carbon dioxide equivalent, i.e., CO2-eq.

[0365] 5. Analyze the accounting results and write a carbon accounting report.

[0366] 6. Regularly release the carbon accounting report to make the information public for public supervision.

[0367] 7. Based on the accounting results, sort out and analyze the key activities and influencing factors of greenhouse gas emissions, and formulate a stable and efficient emission reduction plan.

[0368] Example 1: Conduct carbon emission accounting based on the data of sewage treatment plant A from 2021 to 2023

[0369] 1. Determine the accounting boundary, clarify the principle of greenhouse gas generation and the carbon emission sources of carbon emission activities, and formulate a greenhouse gas emission inventory.

[0370] 2. Analyze the carbon accounting activities, and confirm the corresponding accounting methods and principles according to different types of carbon emissions (for example, using a modular approach to facilitate the formulation of subsequent emission reduction plans)

[0371] 3. According to the accounting methods and principles, build an accounting model / establish an accounting system, select the corresponding emission factors for each carbon emission activity, and formulate a data monitoring plan according to the accounting model / system to confirm the specific data to be collected / gathered.

[0372] Select data:

[0373] The measured data and design specifications of Wastewater Treatment Plant A, including:

[0374] 1) The annual operation data of Wastewater Treatment Plant A from January 2021 to December 2023, as well as the real-time data from January to August 2024. For example: the influent and effluent BOD, COD, TN concentrations per month, the usage amount of each treatment chemical per month, the energy consumption per month (for different units), etc.

[0375] 2) The preliminary design specifications of Wastewater Treatment Plant A, which details the technical parameters of the facilities, the treatment process, and the design discharge standards, providing the basic information for the calculation process.

[0376] The treatment degree of the main pollutants is shown in Table 10:

[0377] Table 10 Treatment Degree of Main Pollutants

[0378]

[0379]

[0380] Note: The data is sourced from the Preliminary Design Specifications of Hongqiao Wastewater Treatment Plant Project

[0381] The sewage and sludge treatment process of Wastewater Treatment Plant A adopts "Improved Multi-stage AAO + Secondary Settling Tank + High-efficiency Sedimentation Tank + Deep Bed Filter + Disinfection Tank + Low-temperature Vacuum Sludge Drying Treatment". The process flow diagram is as Figure 1 shown.

[0382] The sewage within the service area is collected through the municipal sewage pipe network and enters the plant through the inlet main pipe. After removing the larger floating objects in the sewage by the coarse grille, it enters the sewage online storage tank and the inlet pump house. After being lifted by the inlet pump, it flows into the fine grille and the aerated grit chamber to remove the smaller floating objects and sand grains. The sand grains are separated by the sand-water separator and transported out, while the overflow liquid is returned to the regulating tank.

[0383] The effluent from the aerated grit chamber enters the improved multi-stage AO process as the main process of secondary biological treatment. The sewage passes through the anaerobic tank, anoxic tank, aerobic tank, anoxic tank, and aerobic tank in sequence, removing most of the organic pollutants and removing the nitrogen-containing pollutants in the water through nitrification and denitrification. The effluent from the biological tank enters the horizontal secondary settling tank to achieve solid-liquid separation. The effluent from the secondary settling tank is lifted by the intermediate lift pump house and PAC is added, and then enters the flocculation sedimentation tank for phosphorus removal. The effluent from the flocculation sedimentation tank enters the deep bed filter to further remove SS and TP. In the long term, NaAC can be added to utilize the deep bed filter for denitrification. The final effluent is disinfected by ultraviolet light and then lifted by the outlet pump house and discharged into the river.

[0384] The excess sludge discharged from the secondary sedimentation tank and the chemical sludge from the high-efficiency sedimentation tank enter the centrifugal concentration + low-temperature vacuum drying and dehydration system, reducing the water content to below 40%. The dehydrated sludge is stored in the sludge hopper and then transported to the sludge disposal center of Wastewater Treatment Plant B for further treatment and disposal.

[0385] These data provide the actual operating conditions of key links in the sewage treatment process, such as the water quality of influent and effluent, sludge treatment volume, energy consumption, etc., and can accurately reflect the actual situation of carbon emissions in the sewage treatment process.

[0386] 4. Summarize and collate, conduct carbon emission accounting at all levels, and calculate the total carbon emissions. Accounting at all levels refers to calculating the carbon emissions of each emission unit, the carbon emissions from energy consumption of each emission activity, the carbon emissions from material consumption of each emission activity, emission reduction amounts such as carbon compensation, and finally summarizing to obtain the total carbon emissions, with the unit of carbon dioxide equivalent, i.e., CO2-eq.

[0387] Total carbon emissions:

[0388] During the operation and maintenance stage of Wastewater Treatment Plant A from 2021 to 2023, the total carbon emissions were approximately 4.14 - 4.29 Mt CO2-eq, and the carbon emission intensity was 0.50 - 0.57 kg CO2-eq / m3. The total carbon emissions of the sewage treatment plant showed a downward trend. The annual total carbon emissions of Wastewater Treatment Plant A from 2021 to 2023 showed certain volatility characteristics. Among them, the total carbon emissions in 2021 were the highest, at 42,899 tCO2-eq, decreased to 41,385 t CO2-eq in 2022, a reduction of approximately 3.53%, and then rebounded to 42,855 t CO2-eq in 2023, basically the same as in 2021.

[0389] Annual change in carbon emissions:

[0390] The carbon emission data of Wastewater Treatment Plant A show clear trends and patterns. In direct emission activities, the emissions of fossil-source CO2 were 5,182,478 kg CO2-eq in 2021, decreased to 4,899,366 kg CO2-eq in 2022, a reduction of approximately 5.5%, but rebounded to 5,406,250 kg CO2-eq in 2023, slightly higher than the level in 2021. The emissions of CH4 showed a decreasing trend year by year, from 6,808,577 kg CO2-eq in 2021 to 5,621,469 kg CO2-eq in 2023. However, the emissions of N2O increased significantly to 6,831,592 kg CO2-eq in 2023.

[0391] The changes in indirect emission activities also reflect some significant characteristics. The natural gas consumption has been decreasing year by year, and it decreased significantly to 983,826 kg CO2-eq in 2023 (more than 65% less than in 2021). This is mainly because the sewage treatment plant has made certain progress in energy use efficiency or the application of alternative energy sources. The water source heat pump technology is applied to recover the heat energy in sewage, replacing a large part of the heat demand generated by natural gas combustion. In terms of material consumption, the usage amounts of sodium hypochlorite and sodium acetate increased to 327,881 kg and 233,867 kg respectively in 2023.

[0392] Contribution degree of each emission site to carbon emissions:

[0393] The total carbon emissions from the whole process of sewage treatment in Sewage Treatment Plant A in 2023 were 42,855,847 t CO2-eq, and the emission intensity was 0.57 kg CO2-eq / m 3 , which is lower than the annual carbon emission intensity of sewage treatment plants with the same treatment scale. Scope 1: Direct emissions were 17,859,310 t CO2-eq, accounting for 41.67% (among which, the fossil source CO2 emissions in the sewage / sludge treatment process were 1,860 / 3,546 t CO2-eq, the CH4 emissions were 5,621 / 0 t CO2-eq, the N2O emissions were 2,603 / 4,229 t CO2-eq, and the direct greenhouse gas emissions caused by natural gas energy in the sludge treatment process were: 984 t CO2-eq). Scope 2: Indirect emissions from electricity consumption were 22,731 t CO2-eq, accounting for 53%. Scope 3: Other indirect emissions were 1,281 t CO2-eq, accounting for 42.99% (indirect emissions from chemical agent consumption: 1,105 t CO2-eq; indirect emissions from fuel consumption for sludge transportation: 176 t CO2-eq).

[0394] By sorting out and summarizing the data of each emission site calculated, the following Table 11 can be obtained:

[0395] Table 11 Carbon emission analysis of each emission site

[0396]

[0397] 5. Analyze and account for the results, and write a carbon accounting report.

[0398] The above results were written into a complete carbon emission report

[0399] 6. Regularly release the carbon accounting report, and make the information public for public supervision.

[0400] 7. According to the accounting results, sort out and analyze the key activities and influencing factors of greenhouse gas emissions, and formulate a stable and efficient emission reduction plan.

[0401] The main sources of carbon emissions from this sewage treatment plant include: First, electricity consumption is a key factor triggering indirect carbon emissions. The electricity required in the sewage treatment process supports the operation of various equipment, thus leading to indirect emissions of greenhouse gases. Second, the direct emissions of methane during the treatment process are also one of the main sources of carbon emissions. In addition to these main factors, there are several secondary factors worthy of attention. First, the direct emissions of nitrogen oxides also account for a certain proportion during the treatment process, especially in the biochemical reaction stage. Second, the chemicals consumed in the sewage treatment process will cause a certain amount of indirect emissions, especially those used in chemical reactions.

[0402] Based on the carbon emission accounting results of Shanghai A Sewage Treatment Plant (data from 2021 - 2023), targeted strategies are proposed, focusing on its main emission sources (electricity consumption accounts for 58% and direct emissions account for 37%). From two low-carbon perspectives of advanced processes to reduce the influent BOD or COD concentration and energy use, practical strategies are put forward.

[0403] (1) From the process perspective, the carbon capture technology at the influent end can eliminate the chemical consumption of carbon source addition, reduce the energy consumption of the whole link, and achieve emission reduction and energy surplus; the phosphorus recovery technology can avoid the addition of phosphorus removal chemicals, and the process end can reduce 3.7% of the direct carbon emissions of Sewage Treatment Plant A.

[0404] Based on this, it is elaborated in detail on the sludge treatment path. The thermal energy quality balance of the sludge incineration and sludge anaerobic digestion-incineration systems is calculated and analyzed. It is concluded that under different external conditions, the energy states (surplus or deficit) that the systems can achieve may vary. Although sludge incineration brings an energy deficit, the reduction of the influent COD concentration through carbon capture reduces the energy consumption such as aeration. Therefore, both can achieve energy surplus and carbon emission reduction on the whole link. At the same time, the latter is more energy-efficient than the former (when the organic matter content is 60% and the moisture content is 30%, the energy deficit of the former in sludge treatment is about 371 kWh / t dry sludge, and the energy surplus of the latter in sludge treatment is about 666 kWh / t dry sludge, but both can achieve energy surplus on the whole link), thus realizing benefits.

[0405] (2) From the energy perspective, through energy conservation, renewable utilization, clean energy utilization, etc., a contribution rate of about 11% is achieved.

[0406] (3) Carbon reduction (such as precise aeration, intelligent chemical dosing, etc.), carbon substitution (such as photovoltaic power generation, hydropower), carbon sequestration (building an ecological complex), and endogenous negative carbon (such as carbon capture, water source heat pump, biogas recovery and utilization, etc.) measures are proposed to form a path for urban sewage treatment plants by combining process and energy.

[0407] The preferred specific embodiments of the present invention have been described in detail above. It should be understood that those of ordinary skill in the art can make many modifications and variations according to the concept of the present invention without creative work. Therefore, all technical solutions that can be obtained by those skilled in the art in the technical field based on the concept of the present invention through logical analysis, reasoning or limited experiments on the basis of the prior art shall fall within the protection scope determined by the claims.

Claims

1. A carbon emission accounting method based on urban sewage treatment plants, the method comprising the following steps: Step 1. Determine the accounting boundary, clarify the greenhouse gas generation principle and carbon emission sources of carbon emission activities, and formulate a greenhouse gas emission inventory; Step 2. Analyze the carbon accounting activities, and confirm the corresponding accounting methods and principles according to different types of carbon emissions; Step 3. According to the accounting methods and principles, build an accounting model, establish an accounting system, select the corresponding emission factors for each carbon emission activity, and formulate a data monitoring plan according to the accounting model / system to confirm the specific data to be collected or sampled; Step 4. Summarize and sort out, conduct carbon emission accounting at each level, and calculate the total carbon emissions; the accounting at each level includes calculating the carbon emissions of each emission unit, the carbon emissions from energy consumption of each emission activity, the carbon emissions from material consumption of each emission activity, and the carbon compensation and emission reduction amount, and finally summarize to obtain the total carbon emissions, with the unit of carbon dioxide equivalent, i.e., CO2-eq.

2. The method according to claim 1, wherein The confirmation of the accounting boundary in Step 1 includes: 1) Boundary division in the time dimension: In the time dimension, it includes the full life cycle dimension during the operation and maintenance stage, focusing on accounting for the carbon emissions during the operation and maintenance stage of the sewage treatment plant, and at the same time considering the full life cycle carbon emissions of some processes during the operation stage; 2) Boundary division in the space dimension: In the space dimension, the accounting boundary is defined as: the whole process of sewage treatment and sludge treatment and disposal.

3. The method according to claim 1, wherein The carbon emission sources include direct emissions, indirect emissions, chemical agent consumption, sludge transportation, and carbon emission reduction.

4. The method according to claim 3, wherein Among them, the direct emissions are non-biogenic CO2\CH4\N2O gases directly generated by biochemical reactions during sewage treatment, greenhouse gas emissions generated during the sludge disposal process; fossil energy consumed during the production process.

5. The method according to claim 3, wherein The indirect emissions specifically refer to the carbon emissions indirectly generated due to the consumption of electricity and heat energy during the sewage and sludge treatment process.

6. The method according to claim 3, wherein The chemical agent consumption includes externally added carbon sources, phosphorus removal agents, and sludge dewatering agents; among them, the sludge transportation includes fuel combustion during the transportation process from the sewage treatment plant to the sludge treatment plant.

7. The method according to claim 3, wherein The accounting of the carbon emission reduction part includes four categories: carbon reduction, carbon sequestration, carbon substitution, and endogenous negative carbon.

8. The method according to claim 7, wherein The carbon reduction refers to reducing carbon emissions through energy conservation and consumption reduction and process optimization.

9. The method according to claim 7, wherein The carbon sequestration refers to the carbon sink effect of the ecosystem.

10. The method according to claim 7, wherein The carbon substitution focuses on the use of clean energy to replace traditional energy; the endogenous negative carbon refers to the recycling of internal resources in the sewage treatment plant; the recycling specifically includes the utilization of biogas from sludge anaerobic digestion, carbon emission reduction in land use when sludge is used as a soil fertilizer, and nutrient salt recovery.

Images