Assessing Greenhouse Gas Emission Factors in Wastewater Treatment

Abstract

In the context of combating climate change, accurately evaluating the environmental impact of wastewater treatment is of great significance for sustainable development. This study centers on two methods for determining greenhouse gas emission factors in wastewater treatment. One approach calculates per-unit-volume emission factors by utilizing measured greenhouse gas data and the volume of treated water. When measured data are unavailable, an alternative method is adopted to obtain empirical values. Wastewater treatment plant A, with its relatively large scale and certain monitoring capabilities, can acquire partially measured data on greenhouse gas emissions from its treatment units. Thus, both the emission factor measurement method and the empirical value calculation method were utilized to analyze the greenhouse gas emission characteristics and compare the differences in accounting results. For this plant, the average measured values of CH₄ and N₂O emissions were 0.0304 kg CO₂-eq/m³ and 0.0343 kg CO₂-eq/m³, respectively. In contrast, the empirical values were 0.0505 kg CO₂-eq/m³ for CH₄ and 0.0711 kg CO₂-eq/m³ for N₂O. Wastewater treatment plant B, due to its smaller scale, currently lacks the conditions for on-site greenhouse gas measurement. Consequently, only the empirical value calculation method could be used to analyze its greenhouse gas emission characteristics. Its empirical CH₄ and N₂O values were 0.0645 kg CO₂-eq/m³ and 0.1135 kg CO₂-eq/m³, respectively.

1. Introduction

In recent years, human activities have led to a rise in the levels of greenhouse gases like CO₂, CH₄, and N₂O in the atmosphere. This is a major cause of global warming [1]. In response to this challenge, researchers worldwide are actively engaged in studies aimed at reducing greenhouse gas emissions during production processes across various industries [2,3]. As a responsible country, China has announced its aim to peak carbon dioxide emissions by 2030 and achieve carbon neutrality by 2060, while actively establishing and improving a greenhouse gas emission accounting system across various sectors.

Wastewater treatment plants are significant contributors to anthropogenic greenhouse gas emissions [4,5,6,7,8]. Research shows that these facilities are responsible for approximately 1–2% of global greenhouse gas emissions [9], with this percentage increasing annually [10,11,12]. According to the related literature [13], direct emissions amounted to 15.221 million t CO₂-eq, with CH₄ emissions accounting for 6.653 million t CO₂-eq and N₂O emissions for 8.568 million t CO₂-eq—collectively representing 45% of total emissions. In contrast, indirect emissions were recorded at 18.939 million t, accounting for the remaining 55%. Overall emissions from wastewater treatment reach 24.624 million t CO₂-eq, while emissions from sludge treatment and disposal are 9.536 million t CO₂-eq, resulting in a ratio of approximately 7:3. This article focuses on quantifying greenhouse gases emitted directly during the wastewater treatment process. Non-carbon dioxide greenhouse gases, particularly CH₄ and N₂O released by urban WWTPs, are significant contributors [14,15]. Due to the increasingly stringent wastewater discharge standards, the emissions of CH₄ and N₂O are gradually increasing [16,17]. According to the IPCC 2014 report, CH₄ has a Global Warming Potential (GWP) 28 times greater than that of CO₂ over a century. CH₄ is generated during the anaerobic digestion phase of wastewater treatment [18,19,20,21,22]. N₂O also possesses a GWP significantly higher—265 times that of CO₂ over a century—and is released during biological treatment through nitrification and denitrification processes [18,23,24,25,26].

Furthermore, according to IPCC guidelines of 2006, CO₂ produced from biochemical oxygen demand (BOD) during wastewater or sludge treatment is considered part of the natural carbon cycle and should be excluded from greenhouse gas emissions calculations. Most assessments of greenhouse gas emissions from WWTPs have not included emissions from fossil-sourced CO₂ [27,28,29]. Consequently, this study excludes fossil-sourced CO₂ emissions from its calculations and indicates the need for further research on local wastewater discharges.

Based on the described background and requirements, this study focuses on two wastewater treatment plants of varying scales, both adopting the A²/O process for nitrogen and phosphorus removal. Wastewater treatment plant A is relatively large-scale, and its managers have certain monitoring capabilities, enabling them to obtain some measured data on greenhouse gas emissions from the treatment units. By using both the measured and empirical methods, they can comprehensively analyze the plant’s greenhouse gas emission characteristics and compare the accounting results. From 2019 to 2021, they measured the amounts of COD and TN removed, the volume of treated water, and the greenhouse gas emissions [30], obtaining some measured data. In contrast, wastewater treatment plant B is smaller in scale. Due to the current technological and resource limitations, its managers do not yet have the means to measure greenhouse gas emissions on-site. However, they have collected monthly data from 2018 to 2021 on the amounts of removed COD and TN, as well as the volume of treated water. Since a direct measurement of emissions is not possible at present, based on these existing data, an empirical method was employed to estimate the greenhouse gas emissions. The managers of both plants are striving for low-carbon operations, in accordance with the national goals of “carbon peaking and carbon neutrality”, contributing to environmental protection and the sustainable development of the wastewater treatment industry.

To account the greenhouse gas emissions at the national level, the emission factor method provided by the Intergovernmental Panel on Climate Change (IPCC) is widely used in wastewater treatment studies [31,32,33,34]. This paper establishes a provincial emission factor method based on recommended CH₄ emission factors by Cai Bofeng et al. [35] to calculate CH₄ emissions resulting from COD removal. In this paper, the recommended values of N₂O emission factors for wastewater treatment plants, which were obtained by Aliya Ablimit et al. [36] through literature-based statistics, are utilized to calculate the N₂O emissions resulting from TN removal.

2. Data and Methods

2.1. Wastewater Treatment Process Overview of Wastewater Ttreatment Plants A and B

2.1.1. Wastewater Treatment Plant A (600,000 m³/d) Process Overview

Wastewater treatment plant A has a designed daily treatment capacity of 600,000 m³/d. It is located in north China. The relevant data are sourced from on-the-spot research. Due to confidentiality principles, the plant’s name and its location cannot be disclosed. The wastewater treatment primarily employs the A²/O process for nitrogen and phosphorus removal. The treated wastewater can be utilized for applications such as agricultural irrigation, urban river and lake landscaping, and industrial water supply. Figure 1 illustrates the process flow diagram of the wastewater treatment plant with a capacity of 600,000 m³/d (A).

2.1.2. Wastewater Treatment Plant B (40,000 m³/d) Process Overview

Wastewater treatment plant B officially commenced operations in August 2005, with a designed treatment capacity of 40,000 m³/d. The relevant data from this wastewater treatment plant are obtained from the official website of a sewage treatment plant in north-west China. In July 2016, it underwent an upgrade to enhance its capabilities, also employing the A²/O process for nitrogen and phosphorus removal and achieving an effluent quality that meets the Class A standard of the “Discharge Standard for Pollutants from Urban Wastewater Treatment Plants” [37]. This plant is primarily responsible for collecting and treating municipal wastewater and industrial effluent, as well as regenerating urban wastewater. Figure 2 illustrates the process flow diagram of the wastewater treatment plant with a capacity of 40,000 m³/d (B).

2.2. Greenhouse Gas Emissions Produced During the Wastewater Treatment Process

The accounting of greenhouse gas emissions generated during the wastewater treatment process includes emissions of CH₄ and N₂O produced during the anaerobic treatment operations. For wastewater treatment plants using the A²/O process and capable of direct greenhouse-gas measurement, this study adopts a Gas Chromatograph-MS for measurement. Stainless-steel sampling probes are used to collect gas samples from key treatment areas, such as anaerobic, anoxic and aerobic tanks, as well as primary and secondary sedimentation tanks. These samples are then conveyed via PTFE tubes to the Gas Chromatograph-MS for analysis. Regular calibration with standard gas mixtures ensures measurement accuracy. However, if measured data are unavailable, emissions can be estimated using the recommended emission factors based on the amount of pollutants removed, calculated using Formula (1):

$$E_i=A{D}_i\times E{F}_i\times {GWP}_i$$

(1)

where $E_i$ is the emission amount of CH₄ or N₂O generated during the wastewater treatment process (expressed in a carbon dioxide equivalent) (kg CO₂-eq); $A{D}_i$ is the amount of pollutants (COD and TN) removed during the wastewater treatment process (kg); $E{F}_i$ is the emission factor for CH₄ or N₂O during the wastewater treatment process (kgCH₄/kg COD or kg N₂O/kg TN), using recommended emission factor data from the relevant literature or guidelines; ${GWP}_i$ is the Global Warming Potential values for CH₄ and N₂O, which are 28 for CH₄ and 265 for N₂O; i indicates CH₄ or N₂O.

2.2.1. Activity Data

Activity data refer to the characterization values that represent the quantity of activities leading to greenhouse gas emissions. The activity data obtained in this article consists of raw data, including the volume of water treated by the wastewater treatment plant, the concentrations of pollutants in influent and effluent, and the greenhouse gas emissions from each treatment unit.

2.2.2. Emission Factor

Emission factors refer to coefficients that characterize the amount of greenhouse gas emissions per unit of treatment or consumption activity. Currently, the wastewater treatment industry lacks corresponding standard specifications for emission factors, and most wastewater treatment plants do not have emission factor data that reflect their own greenhouse gas emissions, resulting in the absence of a systematic dataset of emission factors. Therefore, Formula (1) utilizes the recommended emission factor data from the other relevant literature or guidelines, based on the amount of pollutants removed, as the basis for calculating greenhouse gas emissions. Such emission factor data can be divided into domestic recommended values and foreign recommended values, with domestic recommended values being more closely aligned with the actual operating conditions of wastewater treatment plants in China, thereby taking precedence over foreign recommended values.

2.3. Accounting Methods for Emission Factors Based on Measured and Empirical Values per Unit of Treated Water Volume

2.3.1. Accounting Methods for Measured Emission Factors in the Wastewater Treatment Process per Unit of Treated Water Volume

When the wastewater treatment plant has actual measured data $E_t$ on greenhouse gas emissions from its treatment units, the following method is provided for calculating the average value of the measured greenhouse gas emission factor ${EF}_a$ per unit of treated water, as shown in Formulas (2) and (3).

$${EF}_a={\displaystyle \frac{\sum _1^m{EF}_j}{m}}$$

(2)

$${EF}_j={\displaystyle \frac{\sum _1^n{E}_t}{Q_j}}$$

(3)

where ${EF}_a$ is the average value of the measured greenhouse gas emission factor for the wastewater treatment plant (kg CO₂-eq/m³); ${EF}_j$ is the greenhouse gas emission factor for the wastewater treatment plant calculated in the j-th measurement (kg CO₂-eq/m³); m is the total number of measurements conducted; j is the j-th measurement (1 ≤ j ≤ m); $E_t$ is the greenhouse gas emissions from the treatment unit t (kgCO_2-eq); $Q_j$ is the volume of water treated by the wastewater treatment plant during the j-th measurement (m³); t is the t-th treatment unit (1 ≤ t ≤ n); n is the total number of treatment units.

2.3.2. Accounting Methods for Empirical Emission Factors in the Wastewater Treatment Process per Unit of Treated Water Volume

When the wastewater treatment plant lacks measured data $E_t$ on greenhouse gas emissions from its treatment units, the greenhouse gas emissions can first be calculated using Formula (1). Subsequently, the method for calculating the average value of the empirical greenhouse gas emission factor ${EF}_b$ per unit of treated water is provided, as shown in Formulas (4) and (5).

$${EF}_b={\displaystyle \frac{\sum _1^m{EF}_j}{m}}$$

(4)

$${EF}_j={\displaystyle \frac{E_j}{Q_j}}$$

(5)

where ${EF}_b$ is the average value of the empirical greenhouse gas emission factor for the wastewater treatment plant (kg CO₂-eq/m³); ${EF}_j$ is the greenhouse gas emission factor for the wastewater treatment plant obtained from the j-th calculation (kg CO₂-eq/m³); m is the total number of calculations conducted; j is the j-th measurement (1 ≤ j ≤ m); $E_j$ is the greenhouse gas emissions from the wastewater treatment plant calculated in the j-th assessment, which can be obtained using emission factor data recommended by other literature or guidelines (kg CO₂-eq); $Q_j$ is the volume of water treated by the wastewater treatment plant during the j-th calculation (m³).

2.3.3. Limitations and Application Discussions of the Measured and Empirical Methods

When determining greenhouse gas emission factors in wastewater treatment, the measured and empirical methods have distinct features. Knowing their limitations and applicable scenarios is crucial for accurate emission assessment.

The measured emission factor in our study comes directly from the experimental measurement of greenhouse gas emissions at wastewater treatment plants. We use professional equipment to measure emissions from each treatment unit and calculate the factor with the treated water volume. It can accurately reflect the actual emissions.

The empirical emission factor in our study is calculated based on the recommended literature data, usually related to pollutant removal. First, we calculate greenhouse gas emissions and then divide by the treated water volume to get the factor for the plant. This method is used when on-site measurement data are lacking.

The measurement method is of great importance as it can directly obtain the actual emission data of wastewater treatment plants. These data accurately reflect real-world emissions under specific conditions, supporting precise assessment and targeted process optimization. However, this method has several limitations. It highly depends on the professional equipment and technicians, resulting in high costs. For example, a high-precision CH₄ and N₂O gas monitor is quite expensive, and the expenses for calibration and maintenance are also considerable. Moreover, to obtain reliable data, long-term and high-frequency monitoring is essential. This not only consumes a significant amount of time but also restricts the immediate analysis and application of data, making it difficult to promptly support treatment processes that require rapid response and adjustment.

In contrast, the empirical method is a practical approach when data collection is challenging. It utilizes the emission factor data recommended in the literature, which is related to the amount of pollutant removal, to provide a feasible way for calculating greenhouse gas emissions. This enables a rapid estimation of emissions, especially for wastewater treatment plants lacking on-site measurement conditions. However, it should be noted that due to the differences in wastewater quality, treatment processes, operation and management levels, and environmental conditions among different wastewater treatment plants, the greenhouse gas emission factors recommended in the literature may not be fully applicable to each plant. For example, in some regions, the wastewater may contain special substances that affect the generation of CH₄ and N₂O. If the emission factors recommended in the literature are directly used for calculation, the results may deviate to some extent, which is currently unavoidable.

In practice, choose the method according to the wastewater treatment plant’s situation. Large-scale plants with strong capabilities and complete monitoring systems should use the measured method for process optimization and emission reduction. Small-scale plants or those in the initial research stage can use the empirical method to quickly estimate emissions.

2.4. Priority in Selecting Greenhouse Gas Emission Factors in the Wastewater Treatment Process

By comparing the recommended emission factors used in Formula (1) with the measured and empirical values obtained from Formulas (2)–(5), the priority for selecting emission factors for the wastewater treatment process, as shown in

Table 1. Priority for selecting greenhouse gas emission factors.

Emission Factors Type	Description	Priority
Measured emission factors	Measured data of the wastewater treatment plant’s emission factors per unit of treated water	High
Empirical emission factors	Empirical data of the wastewater treatment plant’s emission factors per unit of treated water	Medium
Recommended emission factors	Emission factors data recommended by the relevant literature or guidelines	Low

, can be determined.

3. Results and Discussion

Wastewater treatment plant A has obtained partial measured data on greenhouse gas emissions from its treatment units. Therefore, both the emission factor measurement method and the empirical value calculation method are used to analyze its greenhouse gas emission characteristics and compare the differences in the accounting results. Due to the smaller scale of wastewater treatment plant B, it currently lacks the conditions for on-site measurement of greenhouse gases, which means that no measured data on greenhouse gas emissions are available. Therefore, the empirical value calculation method is employed to analyze its greenhouse gas emission characteristics.

The steps for calculating greenhouse gas emission factors are shown in Figure 3.

3.1. Greenhouse Gas Emission Accounting in Wastewater Treatment Plant A Using Measured and Empirical Value Methods for Emission Factors

3.1.1. Accounting for Measured Greenhouse Gas Emissions and Greenhouse Gas Emissions Caused by the Removal of Pollutants (COD, TN)

For wastewater treatment plant A, statistical analysis was conducted on the CH₄ emissions from the primary sedimentation tank, biological tank, and secondary sedimentation tank between 2019 and 2021. Additionally, the N₂O emissions from the primary sedimentation tank and biological tank between 2019 and 2021 were also analyzed. The results showed that the measured greenhouse gas emissions were 11 days for CH₄ and 13 days for N₂O.

Additionally, using Formula (1) as a reference, greenhouse gas emissions arising from the removal of COD and TN will be calculated and compared with the measured values. The recommended CH₄ emission factor is based on values from Cai Bofeng et al. [35], which is 0.0055 kg CH₄/kg COD. The recommended N₂O emission factor is based on values from Aliya Ablimit et al. [36], which is 0.00852 kg N₂O/kg TN.

The variations in greenhouse gas emissions obtained from field measurements and those calculated using the recommended emission factors, along with the changes in pollutant removal amounts, are illustrated in Figure 4 for different calculation days.

Figure 4 shows that the calculated values for CH₄ and N₂O emissions using the emission factor method are greater than the field-measured values. The average accounting value of CH₄ emissions is 23,330 kg, which is 1.65 times the average measured value of 14,170 kg. Additionally, the average accounting value of N₂O emissions is 33,568 kg, 2.09 times the average measured value of 16,085 kg. The measured CH₄ emissions in wastewater treatment plant A generally increase with an increase in COD removal and decrease with a reduction in COD removal. Similarly, the measured N₂O emissions tend to rise with an increase in TN removal and decline with a decrease in TN removal.

A correlation analysis was conducted between the measured values of COD removal and CH₄ emissions, as well as between TN removal and N₂O emissions in wastewater treatment plant A. The results of this analysis are presented in

Table 2. Correlation analysis between measured greenhouse gas emissions and pollutant removal amounts for wastewater treatment plant A.

		COD removal amount
CH₄ emission measured value	Pearson correlation	0.621 ①
	Significance (two-tailed)	0.041377
		TN removal amount
N₂O emission measured value	Pearson correlation	0.871 ②
	Significance (two-tailed)	0.000104

Note: ① Correlation is significant at the 0.05 level (two-tailed). ② Correlation is very significant at the 0.01 level (two-tailed).

.

The correlation analysis shows that in wastewater treatment plant A, CH₄ emissions and COD removal have a significance level < 0.05 and a positive Pearson correlation coefficient (r = 0.621), indicating a strong positive correlation. For N₂O emissions and TN removal, the significance level is <0.01 and the Pearson correlation coefficient (r = 0.871) is also positive, showing a strong positive correlation. Thus, controlling biodegradable organic matter in wastewater can reduce greenhouse gas emissions [6].

As shown in Figure 4, the average measured value of CH₄ emissions is 14,170 kg CO₂-eq/d, while that of N₂O emissions is 16,090 kg CO₂-eq/d. This indicates that the N₂O emissions are approximately 1.14 times that of CH₄ emissions, suggesting that the contribution of N₂O to greenhouse gas emissions from this plant is greater than that of CH₄. Since the GWP of N₂O is 265—far exceeding CH₄’s GWP of 28—even if N₂O emissions are lower than those of CH₄, its overall impact on the greenhouse effect could be more significant. Therefore, while paying attention to CH₄ emissions, we should also focus on the environmental impact of N₂O emissions.

To reduce CH₄ emissions, optimizing the anaerobic digestion process can be effective. By precisely controlling parameters such as temperature, pH, and the ratio of organic matter in the anaerobic digestion stage, the production of CH₄ can be minimized. Regarding N₂O emissions, adjusting the aeration strategy in the biological treatment process can be beneficial. Controlling the dissolved oxygen concentration in the nitrification and denitrification tanks can regulate the metabolic activities of microorganisms.

It should be noted, that although this study has discovered significant positive correlations between CH₄ emissions and COD removal, as well as between N₂O emissions and TN removal through correlation analysis, this analytical method has certain limitations. Correlation analysis can only reflect the degree of association between variables, is unable to reveal causal relationships, and has not quantified the relative impacts of variables such as COD and TN removal on CH₄ and N₂O emissions. Future research could consider using methods such as regression analysis or generalized additive models (GAMs) to further explore the relationships among these variables.

3.1.2. Analysis of Measured and Empirical Greenhouse Gas Emission Factor Results per Unit of Treated Water Volume

Using Formulas (2)–(5), the measured and empirical values of greenhouse gas emission factors for CH₄ and N₂O in wastewater treatment plant A were calculated based on the volume of treated water. The results of these calculations are presented in Figure 5.

The calculation results indicate that the measured CH₄ emission factor in wastewater treatment plant A ranges from 0.0267 kg CO₂-eq/m³ to 0.0336 kg CO₂-eq/m³, with an average value of 0.0304 kg CO₂-eq/m³ and a variance of 4.61 × 10⁻⁶. The empirical CH₄ emission factor ranges from 0.0348 kg CO₂-eq/m³ to 0.0688 kg CO₂-eq/m³, with an average of 0.0505 kg CO₂-eq/m³ and a variance of 8.83 × 10⁻⁵. The measured N₂O emission factor ranges from 0.0153 kg CO₂-eq/m³ to 0.0529 kg CO₂-eq/m³, with an average value of 0.0343 kg CO₂-eq/m³ and a variance of 1.17 × 10⁻⁴. The empirical N₂O emission factor ranges from 0.0356 kg CO₂-eq/m³ to 0.1057 kg CO₂-eq/m³, with an average of 0.0711 kg CO₂-eq/m³ and a variance of 4.97 × 10⁻⁴. Based on the variance comparison presented earlier, it is evident that the measured emission factors for both CH₄ and N₂O are more stable than their empirical counterparts in wastewater treatment plant A. The lower variance of the measured data indicates less fluctuation compared to the empirical data. From the comparison of average values, the measured values are lower. The average empirical CH₄ emission factor is about 1.66 times the average measured CH₄ emission factor, and the average empirical N₂O emission factor is roughly 2.07 times the average measured N₂O emission factor. This suggests that using measured emission factors might lead to a more conservative estimate of greenhouse-gas emissions from the plant. In the future monitoring of greenhouse gas emissions at this plant, obtaining more measured emission factor values can further enrich the dataset for this facility, providing additional data for future calculations.

3.2. Greenhouse Gas Emission Accounting in Wastewater Treatment Plant B Using Empirical Value Methods for Emission Factors

3.2.1. Accounting for Greenhouse Gas Emissions Caused by the Removal of Pollutants (COD, TN)

Due to the lack of measured values for CH₄ and N₂O emissions from wastewater treatment plant B, the provincial emission factor method in Formula (1) can be used to calculate the greenhouse gas emissions caused by the removal of COD and TN in wastewater treatment plant B. The recommended CH₄ emission factor is based on the values from Cai Bofeng et al. [35], which is 0.0087 kg CH₄/kg COD. The recommended N₂O emission factor is based on values from Aliya Ablimit et al. [36], which is 0.00852 kg N₂O/kg TN.

Using Formula (1), the emissions of CH₄ and N₂O caused by the removal of COD and TN in wastewater treatment plant B from 2018 to 2021 are calculated. Figure 6 shows the relationship between the pollutant removal amounts and the associated greenhouse gas emissions in wastewater treatment plant B from 2018 to 2021.

According to the results illustrated in Figure 6, the average monthly emissions of CH₄ from wastewater treatment plant B were 37,916.87 kg CO₂-eq in 2018, and the average monthly emissions of N₂O were 50,431.50 kg CO₂-eq. In 2019, the average monthly emissions of CH₄ increased to 54,433.51 kg CO₂-eq, while the average for N₂O rose to 73,309.83 kg CO₂-eq. In 2020, the average monthly emissions of CH₄ reached 62,870.97 kg CO₂-eq, and the average N₂O emissions were 120,867.29 kg CO₂-eq. By 2021, the average monthly emissions of CH₄ further increased to 75,749.48 kg CO₂-eq, with N₂O emissions averaging 160,949.51 kg CO₂-eq. It is evident that from 2018 to 2021, the emissions of CH₄ and N₂O from wastewater treatment plant B have shown a yearly upward trend. The total greenhouse gas emissions caused by pollutant removal (COD and TN) at wastewater treatment plant B increased from 1,060,180.42 kg CO₂-eq in 2018 to 2,840,387.91 kg CO₂-eq in 2021, representing a growth rate of 167.92%. The situation regarding greenhouse gas emission reduction is quite severe.

The ratio of N₂O emissions resulting from TN removal to CH₄ emissions resulting from COD removal during the treatment process of wastewater treatment plant B from 2018 to 2021 was analyzed, and the results are presented in Figure 7.

As shown in Figure 7, the ratio of carbon dioxide equivalent emissions from TN removal to that from COD removal in wastewater treatment plant B shows a consistent increasing trend, rising from 1.33 in 2018 to 2.12 in 2021. This indicates that N₂O emissions due to TN removal dominate the total greenhouse gas emissions resulting from pollutant removal at the wastewater treatment plant. Although N₂O emissions during the wastewater treatment process may be low, their high GWP means that their contribution to the greenhouse effect should not be underestimated [38]. Therefore, it is recommended that wastewater treatment plant B pays attention to N₂O emissions, during the operation of wastewater treatment and when formulating its carbon reduction target scheme.

3.2.2. Analysis of Empirical of Greenhouse Gas Emission Factor Results per Unit of Treated Water Volume

Using Formulas (4) and (5), the empirical values of greenhouse gas emission factors for wastewater treatment plant B were calculated, resulting in a dataset of CH₄ and N₂O emission factors per unit of treated water. Figure 8 illustrates the relationship between the monthly empirical values of greenhouse gas emission factors of wastewater treatment plant B and the corresponding pollutant removal amounts.

According to Figure 8, the empirical values of CH₄ and N₂O emission factors from wastewater treatment plant B generally increased with the rise in pollutant removal amounts. After linear fitting, the Pearson correlation coefficient (Pearson’s r) between the CH₄ emission factor and COD removal amount was found to be 0.8858, while the Pearson correlation coefficient between the N₂O emission factor and TN removal amount was 0.9453. Both correlation coefficients are close to 1, indicating a strong correlation. By analyzing the empirical values of greenhouse gas emission factors from wastewater treatment plant B, a preliminary dataset of CH₄ and N₂O emission factors has been constructed, with average empirical values of 0.0645 kg CO₂-eq/m³ for CH₄ and 0.1135 kg CO₂-eq/m³ for N₂O. The relationship between the empirical values of CH₄ and N₂O emission factors and the corresponding COD and TN removal amounts is shown in

Table 3. Dataset of CH₄ and N₂O empirical values of emission factors from wastewater treatment plant B.

COD Removal (kg)	CH₄ Emission Factors (kg CO2-eq/m3)	TN Removal (kg)	N₂O Emission Factors (kgCO2-eq/m3)
100,000~150,000	0.0266~0.0390	10,000~20,000	0.0350~0.0386
150,000~200,000	0.0343~0.0568	20,000~30,000	0.0415~0.0785
200,000~250,000	0.0560~0.0848	30,000~40,000	0.0718~0.1480
250,000~300,000	0.0611~0.1159	40,000~50,000	0.1068~0.1460
300,000~350,000	0.0744~0.0863	50,000~60,000	0.1321~0.1797
350,000~450,000	0.0954~0.1011	60,000~70,000	0.1536~0.1938
		70,000~80,000	0.1554~0.1782
		80,000~90,000	0.1841~0.2082

.

3.3. Comparison of Data Between Wastewater Treatment Plant A and Wastewater Treatment Plant B

When comparing the data obtained from wastewater treatment plant A and wastewater treatment plant B, distinct differences emerge in terms of greenhouse gas emission factors and emissions trends.

In terms of emission factors, there are clear disparities between the two plants. The average measured CH₄ emission factor in wastewater treatment plant A stands at 0.0304 kg CO₂-eq/m³, whereas wastewater treatment plant B has an average empirical CH₄ emission factor of 0.0645 kg CO₂-eq/m³. For N₂O, wastewater treatment plant A’s average measured emission factor is 0.0343 kg CO₂-eq/m³, while wastewater treatment plant B’s average empirical value is 0.1135 kg CO₂-eq/m³. Evidently, wastewater treatment plant B’s empirical emission factors for both CH₄ and N₂O are significantly higher than wastewater treatment plant A’s measured values.

Regarding emissions trends, from 2018–2021, the emissions of CH₄ and N₂O from wastewater treatment plant B showed a continuous upward trend. However, wastewater treatment plant A, with its relatively stable operation and more advanced treatment processes, may have more stable emissions, although this study only presents a limited number of measured data points.

The differences in data between the two plants can be attributed to several factors. Firstly, the scale of operation plays a crucial role. Wastewater treatment plant A has a much larger treatment capacity of 600,000 m³/d, compared to wastewater treatment plant B’s 40,000 m³/d. Larger-scale plants often have more resources to invest in advanced treatment technologies and in equipment that can optimize the treatment process and reduce greenhouse gas production per unit of treated water.

Secondly, the monitoring capabilities of the two plants vary greatly. Wastewater treatment plant A can obtain partial measured data on greenhouse gas emissions, allowing for a more accurate assessment of actual emissions. In contrast, wastewater treatment plant B lacks on-site measurement conditions and has to rely on empirical methods. The empirical values are calculated based on general recommended factors from the literature, which may not precisely reflect the specific conditions of wastewater treatment plant B. Local wastewater quality, treatment process details, and operational management levels can all deviate from the assumptions in the literature, leading to differences in the calculated emission factors.

By understanding these differences, we can better evaluate the greenhouse gas emissions from different scale wastewater treatment plants and develop more targeted strategies for emission reduction.

4. Conclusions

Based on the measured data of greenhouse gas emissions and the emission factor method, we propose a calculation approach for empirical and measured greenhouse gas emission factors per unit treated water volume, specifically for the wastewater treatment process.

The average measured CH₄ and N₂O emission factors in wastewater treatment plant A are 0.0304 kg CO₂-eq/m³ and 0.0343 kg CO₂-eq/m³, respectively. In contrast, the average empirical values are 0.0505 kg CO₂-eq/m³ and 0.0711 kg CO₂-eq/m³, respectively. The average empirical CH₄ emission factor is about 1.66 times that of the measured value, and the average empirical N₂O emission factor is roughly 2.07 times that of the measured value. Clearly, the measured values are consistently lower than the empirical values.

Due to the small scale of wastewater treatment plant B, which lacks the capability to monitor greenhouse gas emissions data, we employed the empirical emission factor calculation method to derive the CH₄ and N₂O emission factors corresponding to various pollutant removal ranges. The average empirical values determined are 0.0645 kg CO₂-eq/m³ for CH₄ and 0.1135 kg CO₂-eq/m³ for N₂O.

Generally speaking, for wastewater treatment plants using the A²/O core process, the larger the treatment capacity, the smaller the emission factors. This is because larger-scale plants often have a more advanced technology and better-designed processes, which can reduce greenhouse gas emissions per unit of treated water. For wastewater treatment plants with a treatment capacity between 40,000 and 600,000 m³/d, adopting the A²/O core process, in the absence of measured greenhouse gas emissions data, this paper provides reference calculation results. The recommended ranges for CH₄ and N₂O emission factors are from 0.0304 to 0.0645 kg CO₂-eq/m³ and from 0.0343 to 0.1135 kg CO₂-eq/m³, respectively. Our study’s emission factor calculation methods and reference ranges provide a scientific basis for the policymakers. They can use this to create more targeted policies, like setting different emission reduction goals for different scale plants. Plant operators can also use this information to optimize their operations and meet the national standards, actively joining China’s carbon neutrality effort. Overall, our research aids the wastewater treatment industry in contributing to China’s carbon neutrality strategy. Based on the research findings of this paper, due to the extremely high GWP value of N₂O, practitioners in the wastewater treatment industry should attach importance to N₂O emissions. This can be achieved through measures such as optimizing treatment processes, enhancing monitoring frequency, and adjusting operational parameters to effectively contribute to greenhouse gas carbon reduction.

In conclusion, the findings of this study contribute to the growing body of knowledge on sustainable wastewater treatment. By providing accurate emission-factor calculation methods and reference ranges, we hope to empower decision-makers, plant operators, and researchers to develop and implement more effective strategies for reducing greenhouse gas emissions from wastewater treatment. This, in turn, will help move the industry closer to achieving its sustainable development goals, aligning with the global imperative to combat climate change.