Sustainable Sludge Management in China: Quantifying GHG Emissions and Exploring Its Reduction Strategies

1. Introduction

Sludge management has become a critical issue in China due to rapid urbanization and population growth. As a byproduct of wastewater treatment, sludge contains a variety of pollutants, such as heavy metals, pathogens, and organic contaminants, which pose significant risks to human health and the environment if not properly treated [1]. In 2020, China generated approximately 55 million tonnes of wet sludge, and this figure is expected to reach 60 million tonnes by 2025 [2]. The treatment and disposal of such a large volume of sludge not only require substantial energy and financial resources but also contribute significantly to greenhouse gas (GHG) emissions. It is estimated that sludge treatment accounts for 40% of the total GHG emissions from the wastewater sector in China [3]. Therefore, sustainable sludge management strategies that minimize GHG emissions while ensuring safe and efficient treatment are urgently needed.

Recent studies have attempted to quantify GHG emissions from sludge treatment in China. For example, Guo et al. [4] estimated that the total GHG emissions from sludge treatment in China reached 18.6 million tonnes of CO₂ equivalent (CO₂-eq) in 2018, with anaerobic digestion and landfilling being the primary contributors. Similarly, Liu et al. [1] reported that the GHG emissions from sludge incineration in China amounted to 5.2 million tonnes of CO₂-eq in 2017. However, these estimations often have limitations due to unclear system boundaries and methodological inconsistencies. Many studies only consider direct emissions from sludge treatment processes while neglecting indirect emissions from energy consumption, transportation, and disposal of treatment residues [5]. Moreover, the emission factors used in these calculations are often based on default values or limited local data, which may not reflect the actual emission levels under different treatment conditions. Therefore, a comprehensive and systematic re-evaluation of GHG emissions from sludge treatment in China is necessary to provide a more accurate and reliable basis for decision-making.

Traditionally, landfilling and composting have been the most common methods for sludge disposal in China due to their simplicity and low cost. However, these methods have significant drawbacks in terms of environmental sustainability and GHG emissions. Landfilling of sludge can lead to the release of methane (CH₄), a potent GHG with a global warming potential ₂8 times higher than CO₂ [6]. In addition, landfilling requires large areas of land and may cause groundwater contamination if not properly managed [7]. Composting, on the other hand, can reduce the volume of sludge and produce valuable fertilizer, but it also emits GHGs such as CH₄ and nitrous oxide (N₂O) during the decomposition process [1]. To overcome these limitations, various novel sludge treatment technologies have been developed and implemented in recent years. For instance, thermal treatment methods such as incineration, pyrolysis, and gasification can effectively reduce sludge volume and destroy pathogens while recovering energy [4]. Advanced anaerobic digestion techniques, such as temperature-phased anaerobic digestion (TPAD) and anaerobic membrane bioreactors (AnMBR), can enhance biogas production and nutrient recovery from sludge [8]. However, the GHG emission reduction potential of these emerging technologies compared to traditional methods remains largely unknown, hindering their widespread adoption.

This study aims to provide a comprehensive assessment of GHG emissions from the sludge treatment industry in China and explore the potential of novel technologies to reduce these emissions. Firstly, we will estimate the total GHG emissions from sludge treatment in China using a life cycle assessment (LCA) approach, considering both direct and indirect emissions from various treatment processes. The spatial distribution of these emissions across different provinces will be analysed to identify regional hotspots and priorities for emission reduction. Secondly, we will investigate the influence of socio-economic factors, such as GDP, population density, and urbanization rate, on the regional variation of sludge treatment emissions. This analysis will provide insights into the drivers of sludge generation and treatment practices in different parts of China. Finally, we will evaluate the GHG emission reduction potential of selected novel sludge treatment technologies, including thermal treatment, advanced anaerobic digestion, and wet oxidation. The environmental and economic performance of these technologies will be compared with traditional methods to assess their feasibility and sustainability. By quantifying the GHG emissions from sludge treatment and exploring innovative mitigation strategies, this study aims to contribute to the development of a low-carbon and sustainable sludge management system in China.

2. Materials and Methods

2.1. System Boundary and Data Sources

A life-cycle sludge treatment industry begins with sludge collection from WWTPs, followed by treatment and disposal, as depicted in Figure 1. Traditional sludge treatment and disposal methods include landfill, incineration, building materials application, and land application. In China, due to limited data, the primary methods are considered landfill, incineration, and land application, referred to as solid waste treatment [9]. CH₄ emissions from sludge AD are not calculated in this study due to their minimal application (nearly 3%) at WWTPs in China [10]. The geographical scope covers the 31 provincial administrative districts in mainland China. Based on regional economic zones, China is divided into six regions in this study:

Northern region: Beijing, Tianjin, Hebei, Shanxi, Inner Mongolia, and Shandong.

Northeastern region: Liaoning, Jilin, and Heilongjiang.

Eastern region: Shanghai, Jiangsu, Zhejiang, Anhui, and Fujian.

Central region: Jiangxi, Henan, Hubei, Hunan, Chongqing, and Sichuan.

Southern region: Guangdong, Guangxi, Hainan, Guizhou, and Yunnan.

Northwestern region: Tibet, Shaanxi, Gansu, Qinghai, Ningxia, and Xinjiang.

The base year of 2017 was used for GHG emission quantification from the sludge treatment industry in this study. Data on urban population, population density, GDP, daily water consumption, fixed investment, sewer density, sludge production, and sludge disposal methods for 660 Chinese cities were obtained from the Statistical Yearbook of Urban Construction [9]. These data were used to calculate GHG emissions from wastewater collection and sludge disposal. Additionally, the regional power grid baseline emission factor for six regions in 31 provinces was collected by the Ministry of Ecological Environment [11].

2.2. GHG Emission Calculation

The total GHG emissions from the sludge treatment industry are quantified using a LCA method, as described by Equation (1):

$$E=AD\times EF$$

(1)

where E is the total GHG emissions; AD represents the activity amount (e.g., volume of treated wastewater); EF is the emission factor of different processes.

On-site GHG emissions originate from direct CO₂, N₂O and CH₄ emissions during landfill, incineration, and composting. Off-site emissions are due to electricity, chemicals, and fuel consumption during wastewater treatment, sludge transportation, and disposal [12]. GHG emissions for N₂O and CH₄ are expressed in CO₂ equivalent (CO₂-eq), with global warming potentials (GWP) of 1, 25, and 298 for CO₂, N₂O and CH₄ respectively [6].

Total GHG emissions from different sludge disposal methods (R1: landfill; R2: incineration, and R3: composting + land application) in various cities are calculated using:

$${GHG}_t={\sum }_i^{660}GHGR1\times K_{1,i}+GHGR2\times K_{2,i}+GHGR3\times K_{3,i}$$

(2)

where, K₁, K₂, and K₃ are disposal method proportions.

Specific emissions for each method are calculated as:

$$\mathrm{R}1: {GHG}_{R1}={GHG}_t +{CO}_{2-electricity}+{GHG}_{diesel}+{GHG}_{CH4-1}+{CO}_{2-bio}$$

(3)

$$\mathrm{R}2: {GHG}_{R2}={GHG}_t +{CO}_{2-electricity}+{GHG}_{coal}+{GHG}_{CH4-2}+{GHG}_{N2O}$$

(4)

$$\mathrm{R}3: {GHG}_{R3}={GHG}_t +{CO}_{2-eq1}+{CO}_{2-electricity}+{GHG}_{coal}+{CO}_{2-avoid}$$

(5)

where, GHG_CH4-1 is the CO₂-eq produced during anaerobic digestion during landfill, 60.6 kg/t dry sludge (DS) [13]. CO_2-bio is the biogenic CO₂ emitted from landfills, 17.2 kg/t DS [14]. GHG_CH4-2 and GHG_N2O are the equivalent CO₂ emissions of CH₄ and N₂O emissions from sludge incineration, 0.049 and 0.99 kg CO₂-eq/t DS (IPCC, 2007). CO_2-eq1 is the equivalent CO₂ emitted during the sludge aerobic compost, 210 kg CO₂-eq/t DS [16]. CO_2-avoided is the CO₂-eq from the subsequent sludge land utilization in the form of replacement chemical fertilizer, 66.8 kg CO₂-eq/t DS [17]. GHG_t is the CO₂-eq from sludge transportation, which is presented as followed:

$$GHGt=(f_1+f_2\times 25+f_3\times 298)\times D_t$$

(6)

where D_t is the transportation distance from WWTPs to each site of three methods, which is assumed as 30 km, 70 km, and 30 km for landfill, incineration, and land application (Liu et al., 2013). The distance of incineration is longer than that of sludge landfill because incineration ash will be transported to landfill. f₁, f₂, and f₃ are the effective EFs of average GHGs for a heavy-duty diesel truck with a 20-ton load under Chinese emission standard IV, which was 2.6 kg CO₂, 1.4×10^-4 kg CH₄, and 1.4×10^-4 kg N₂O for each liter of diesel consumption, 0.2 L/km [18].

GHG_diesel is the CO₂-eq of diesel use from aerobic compost and incineration, as shown in followed:

$${GHG}_{diesel}=T_1\times (f_4+f_5\times 25+f_6\times 298)$$

(7)

where T₁ is the diesel consumption from the sludge landfill, 3.1 L diesel/ton (Chen and Kuo et al., 2016). f₄, f₅, and f₆ are the effective EF of GHGs from diesel consumption, which was 7.4×10^-2 kg CO₂, 2.5×10^-5 kg CH₄, and 4.5×10^-4 kg N₂O for each liter of diesel consumption [19].

CO_{2-electricity} is the CO₂-eq of electricity consumption from sludge landfill, incineration, and aerobic compost, which is calculated in Equation (8):

$${CO}_{2-electricity}=f_7\times T_2$$

(8)

Where f₇ is the EF of electricity, which was 0.785 kg CO₂/kWh [16]. T₂ is the electricity consumption from sludge landfill, incineration, and aerobic compost, which was 219 kWh/t DS, 70 kWh/t DS, and 730 kWh/t DS, respectively [16].

GHG_coal is the CO₂-eq of coal used from aerobic compost and incineration, as shown in followed:

$${GHG}_{coal}=T_3\times (f_8+f_9\times 25+f_{10}\times 298)$$

(9)

where T₃ is the amount of coal used from aerobic compost (65 MJ/t DS) (Murray et al., 2008) and incineration (8,049 MJ/t DM) [18]. f₈, f₉, and f₁₀ is the effective emission factor of CO₂, CH_4, and N₂O of coal, which are assumed to be 0.098 kg/MJ, 0.25×10^-4 kg/MJ, and 0.45×10^-3 kg/MJ, respectively [16].

2.3. Principal Component Analysis

Because the varying measurement scales of evaluation indicators affect total GHG emissions from the sludge treatment industry, data standardization is essential. Principal Component Analysis (PCA) effectively reduces dimensionality while retaining significant information by transforming data into a new subspace with high variance [20]. The new coordinate axes are oriented based on the maximum variance from the original data. Five socio-economic indicators (population density, construction land, fixed investment, GDP, and daily water consumption) and sludge treatment characteristics were selected as input parameters. Additionally, infrastructure and operational parameters, including sewer pipeline density, treated wastewater volume, and treated sludge volume, were included.

3. Results and Discussion

3.1. Statement of Sludge Treatment Industry in China

Figure 2 illustrates the treated sludge volumes and disposal methods across 31 provinces in China [9]. The provinces of Guangdong, Beijing, and Zhejiang treated the most sludge, accounting for about 40% of the national total. Landfilling was the most common disposal method, used for 56.5% of the sludge, consistent with Zhang et al.'s (2016) finding of 50%. Nearly ₄0% of sludge was incinerated, while only 3.5% was composted. Cities with high GDP and large sludge production volumes tend to prefer incineration, as it effectively reduces volume despite higher costs for fuel and equipment. Notably, Jiangyin, Wuxi, Shihezi, and Dali achieved a 100% incineration rate. In East China, incineration rates were particularly high, with Jiangsu at 74.2%, Fujian at 62.3%, Zhejiang at 56.7%, Anhui at 54.7%, and Shanghai at 48.6%. Additionally, Hainan and Yunnan exhibited significant incineration rates of 68.9% and 55.2%, respectively.

Conversely, provinces with over 50% landfill rates, particularly in Northwest China, had rates exceeding 60%, with Shaanxi and Qinghai reaching 100% and 98.1%, respectively. This trend is attributed to the availability of extensive land resources and the relatively low costs associated with landfilling. Sludge composting was the least prevalent disposal method, with rates ranging from 0 to 17.3%, the highest being in Beijing (17.3%), Heilongjiang (12.6%), and Liaoning (8.2%). These trends highlight regional disparities in sludge treatment methods and volumes, reflecting the economic, geographical, and infrastructural differences across China's provinces, and emphasizing the need for tailored sludge management strategies to address regional challenges effectively [16].

3.2. Quantification of GHG Emission

In 2017, total GHG emissions from sludge treatment industry was 18.54 Mt. Various countries reported that GHG emission from sludge treatment industry ranged from 0.15 to 1.2 t CO₂-eq per functional unit (e.g., per ton of dry matter or per m³ of treated wastewater) [12,14,22,23,24,25]. The main contributors to these emissions include electricity consumption, direct emissions from treatment processes, and specific treatment methods such as incineration, landfilling, and anaerobic digestion. The variations in reported emissions can be attributed to differences in treatment methods, energy sources, and country-specific factors. These findings underscore the need for considering local conditions and treatment practices when assessing the carbon footprint of sludge management and developing mitigation strategies.

Figure 3 reveals that incineration contributes the highest total GHG emissions among the three sludge treatment methods, with a total of 1,011.53 kg CO₂-eq per ton of dry sludge. This is primarily due to the substantial CO₂ emissions from incineration (897.56 kg CO₂-eq), which are approximately 38% higher than the CO₂ emissions from landfill (647.25 kg CO₂-eq) and more than three times the CO₂ emissions from land application (276.17 kg CO₂-eq). Landfill has the second-highest total GHG emissions (717.51 kg CO₂-eq), followed by land application (276.41 kg CO₂-eq). The composition of GHG emissions and the contribution of different emission sources vary among the three methods, highlighting the importance of considering these factors when assessing the environmental impact of sludge treatment. The reliance on coal in incineration, the high CH₄ emissions from landfills, and the potential for emission offsets in land application are notable differences that should be considered when developing sustainable sludge management strategies.

The composition of GHG emissions varies among the three methods. While CO₂ is the primary GHG emitted by all three methods, landfills also have significant CH₄ emissions (617.65 kg CO₂-eq), which account for 86.1% of their total emissions. In contrast, CH₄ emissions from incineration (0.₄7 kg CO₂-eq) and land application (0.24 kg CO₂-eq) are minimal, representing only 0.05% and 0.09% of their total emissions, respectively. N₂O emissions are relatively low for all three methods, with incineration having the highest N₂O emissions at 113.10 kg CO₂-eq (11.2% of total emissions), followed by landfill at 7.01 kg CO₂-eq (0.98% of total emissions) and land application at 0.0002% of total emissions. The contribution of different emission sources varies among the treatment methods. Incineration heavily relies on coal, which contributes 7.44 kg CO₂-eq to its total emissions, whereas coal is not applicable for landfill and land application. Electricity consumption is highest for landfills (0.29 kg CO₂-eq), followed by incineration (0.09 kg CO₂-eq) and land application (0.01 kg CO₂-eq). Transportation emissions are highest for incineration (0.15 kg CO₂-eq), followed by landfill (0.08 kg CO₂-eq).

The total GHG emissions from sludge treatment vary significantly among Chinese provinces, as shown in Figure 4. Guangdong has the highest total emissions at 2,741.94 t CO₂-eq, followed by Zhejiang (1,371.97 t CO₂-eq) and Shandong (1,367.49 t CO₂-eq). In contrast, Tibet has the lowest total emissions at 22.87 t CO₂-eq, followed by Qinghai (93.23 t CO₂-eq) and Ningxia (145.73 t CO₂-eq). The composition of GHG emissions also differs among provinces. CO₂ is the primary contributor across all provinces, ranging from 56.7% to 91.6% of total emissions. Yunnan has the highest share of CO₂ emissions at 91.6% (786.68 t CO₂-eq), while Ningxia has the lowest at 56.7% (82.62 t CO₂-eq). CH₄ emissions vary more significantly among provinces, with Ningxia having the highest share at 36.75% (53.54 t CO₂-eq) and Hainan the lowest at 10.₂5% (24.49 t CO₂-eq). N₂O emissions generally account for a smaller portion of total emissions, ranging from 0.03% in Tibet (0.01 t CO₂-eq) to 6.64% in Shaanxi (18.80 t CO₂-eq).

Comparing the different greenhouse gases, CO₂ emissions dominate across all provinces, with a total of 20,796.94 t CO₂-eq. The top three provinces for CO₂ emissions are Guangdong (2,562.98 t CO₂-eq), Zhejiang (1,265.26 t CO₂-eq), and Jiangsu (1,179.90 t CO₂-eq). CH₄ emissions amount to 3,502.17 t CO₂-eq nationwide, with Guangdong (172.81 t CO₂-eq), Shandong (135.95 t CO₂-eq), and Jiangsu (117.80 t CO₂-eq) being the top contributors. N₂O emissions are comparatively lower, totalling 371.84 t CO₂-eq. The provinces with the highest N₂O emissions are Guangdong (52.23 t CO₂-eq), Hubei (22.18 t CO₂-eq), and Jiangsu (21.07 t CO₂-eq).

At the regional level, the Northern region has the highest total GHG emissions at 6,176.14 t CO₂-eq, followed by the Eastern region (5,836.77 t CO₂-eq) and the Central region (5,209.68 t CO₂-eq). The Northeastern and Northwestern regions have significantly lower emissions at 1,5₂0.5₂ t CO₂-eq and 1,122.65 t CO₂-eq, respectively. CO₂ emissions are the main contributor across all regions, with the Northern region emitting 5,148.80 t CO₂-eq, followed by the Eastern region (4,969.63 t CO₂-eq) and the Central region (4,418.78 t CO₂-eq). The Northern region also leads in CH₄ emissions (902.83 t CO₂-eq), followed by the Eastern region (774.10 t CO₂-eq) and the Central region (726.02 t CO₂-eq). N₂O emissions are highest in the Northern region (124.51 t CO₂-eq), followed by the Eastern region (93.04 t CO₂-eq) and the Central region (64.88 t CO₂-eq). The Southern region ranks fourth in total emissions (4,805.19 t CO₂-eq), with 4,195.72 t CO₂-eq from CO₂, 568.49 t CO₂-eq from CH₄, and 40.98 t CO₂-eq from N₂O.

Figure 5 presented is a Lorenz curve showing the cumulative proportion of total GHG emissions from China's sludge treatment industry across 660 cities. The curve reveals a highly uneven distribution of emissions among cities, with a small number of cities contributing to a significant portion of the total emissions. Interestingly, 7 cities (approximately 1% of the total) account for a staggering 34.2% of the total GHG emissions. These cities are Beijing, Shenzhen, Shanghai, Changsha, Hangzhou, Shaoxing, and Inner Mongolia. Expanding the scope, the top 188 cities (28.5% of the total) are responsible for 90% of the total emissions. Furthermore, 50% of the emissions come from just 18 cities, which include the aforementioned 7 cities along with Suzhou, Guangzhou, Chengdu, Chongqing, Qingdao, Nanjing, Xiangyang, Wuhan, Tianjin, Zhengzhou, and Shijiazhuang.

The frequency results indicate that the average total GHG emission of the sludge treatment industry per city is 0.11 Mt CO₂-eq, with 77.3% of the cities emitting between 0.0001 and 0.10 Mt CO₂-eq. The spatial distribution shows that the top 1% of cities, which are mainly cosmopolises and metropolises, contribute to more than 20% of the total emissions. Beijing, as the biggest emitter, accounts for 5.8% of the total GHG emissions with 4.26 Mt CO₂-eq. These high-emitting cities are either municipalities directly under the Central Government or economically developed coastal cities, characterized by large populations, higher GDP, and developed industries, which result in higher wastewater production and, consequently, higher GHG emissions. The next 167 cities (top 1.1% to 26.3%) generate 43.83 Mt CO₂-eq, contributing to 60% of the total emissions. Most of these cities are located in Eastern China, with a few in Western China, such as Ataylor, Urumqi, Kunming, Honghe, Yinchuan, Lanzhou, Xining, Deyang, Mianyang, Nanchong, and Leshan. In contrast, the remaining 486 cities, which are mainly underdeveloped or have relatively low urbanization levels and are in the Northwestern and Southeastern regions, only emit 20% of the total GHG emissions.

Given this uneven distribution of emissions, GHG emission mitigation strategies should prioritize developed cities with higher total emissions. Upgrading WWTPs with energy recovery technologies can help reduce off-site GHG emissions and potentially offset the energy consumption demand for their operation. For larger-scale cities with substantial sludge production, sludge thermal conversion technologies can be employed to reduce both sludge volume and GHG emissions. For example, a carbonization co-digestion sludge system could achieve net 219 GJ energy and reduce GHG emissions by 25%, with zero N₂O emissions by utilizing biochar biochar [26]. Integrated pyrolysis systems can also achieve net energy output when the volatile solids in total solids (VS/TS) ratio is greater than 63%. However, these technologies are currently studied at a lab scale and require significant investment in equipment and operational costs, making them more suitable for cities with higher GDP and fixed investment.

3.3. Impacts of GHG Emissions

The PCA loading plot illustrates the influences of various factors on GHG emissions from the sludge treatment industry. The first two principal components (PC1 and PC2) explain 60.8% of the variation in the input data, indicating their significance in understanding GHG emissions across different cities. Indicators such as GDP, daily water consumption, and construction land, located in the first quadrant, positively affect total GHG emissions, suggesting that higher values of these indicators correlate with increased emissions. Specifically, GDP and treated sludge show the highest correlations with GHG emissions, reflecting the impact of economic and treatment scale factors. Sewer density and population density primarily influence PC2 but in opposite directions, with higher sewer density linked to increased emissions and higher population density showing a mitigating effect. First-tier cities like Beijing, Shanghai, and Shenzhen are significantly influenced by GDP and treated volumes of wastewater and sludge due to their extensive treatment infrastructure and economic activities. Tianshui and Sanya are notably affected by sewer and population densities, respectively, due to their unique geographic and demographic characteristics. To mitigate GHG emissions, particularly in developed cities, adopting novel technologies like sludge pyrolysis and wet oxidation can effectively reduce sludge volume and emissions.

Figure 6. Principal component analysis.

Figure 6. Principal component analysis.

4. Potential of GHG Emission Mitigation

Sludge treatment is a significant contributor to greenhouse gas (GHG) emissions in the sludge treatment industry. To achieve China's goal of carbon neutrality by 2060, it is crucial to evaluate and optimize sludge treatment technologies to minimize their carbon footprint. This analysis focuses on the GHG emission mitigation potential and cost analysis of various sludge treatment methods, including thermal conversion technologies (pyrolysis, gasification, and incineration), AD, hydrothermal processes (HTP), and wet oxidation (WO). Table 1 shows the comparison of reduction potential and cost analysis of these sludge treatment technologies.

Incineration is a widely used method for sludge treatment, but it emits the largest GHG emissions among different disposal methods due to the demand for coal during the process and the electricity consumption for sludge drying [32]. Other thermal technologies, such as carbonization, gasification, and pyrolysis, can also be used for sludge volume reduction with net GHG emission [8,26]. However, most of these technologies are applied at a lab-scale without large-scale applications and require sludge drying to almost 30% water content, which necessitates higher energy and chemical consumption. A 15% reduction in energy and material recovery in pyrolysis would result in an additional 195.96 kg CO₂-eq of carbon emissions, emphasizing the need to focus on stabilizing the yield of high calorific value products [31]. Gasification of sewage sludge can reduce GHG emissions by 0.33-0.46 t CO₂-eq/t dry solids compared to incineration [22].

Advanced AD coupled with thermal hydrolysis is a promising solution to expand methane yield, achieving a net energy production of 372 kWh/t DS [33]. If 60% ~ 70% of organic matter is converted into biogas energy and recycled, it can meet the energy demand of a WWTP [4]. AD can reduce GHG emissions by 0.09-0.3 t CO₂-eq/t dry solids compared to conventional sludge treatment [28]. The cost of AD ranges from 100-200 USD/t dry solids [27]. Hydrothermal processes, such as HTP and ATH, can also effectively treat sludge while recovering valuable resources. However, the high energy consumption in the hydrolysis process leads to substantial carbon emissions. When the energy and resource consumption is reduced by 15%, GWP is reduced by 194.04 kg CO₂-eq and 181.98 kg CO₂-eq, and HTP is reduced by 75.55 kg CO₂-eq and 44.5 kg CO₂-eq for two different routes [31,34]. HTP can reduce GHG emissions by 0.2-0.4 t CO₂-eq/t dry solids compared to incineration [23]. The cost of HTP ranges from 300-500 USD/t dry solids [28]. Wet oxidation of sludge could be considered as a possible solution to reduce GHG emissions from sludge drying, as it can directly oxidize organics in sludge to realize volume reduction under the liquid phase. Moreover, it facilitates product recovery (e.g., acetic acid, P, and N), which could be reused during biological treatment processes at WWTPs. Wet oxidation can reduce GHG emissions by 0.1-0.3 t CO₂-eq/t dry solids compared to incineration [27]. The cost of wet oxidation ranges from 150-300 USD/t dry solids [4].

LCA results indicate that emerging sludge treatment routes can provide favorable environmental benefits through the utilization of sludge resources. However, they still require improvement compared to traditional sludge treatment routes in effectively controlling both environmental pollution and carbon emissions. Optimization strategies, such as the development and utilization of resource-based clean chemicals, the selection of clean heat sources, and the optimization of heat recovery systems, are promising options to demonstrate the carbon emission reduction benefits of these technologies [31,34]. While thermal conversion technologies, AD, hydrothermal processes, and wet oxidation offer potential for GHG emission mitigation in sludge treatment, further research and optimization are necessary to enhance their environmental performance and economic feasibility. The cost analysis reveals that AD is the most cost-effective option, followed by wet oxidation, pyrolysis, gasification, and HTP. However, the actual costs may vary depending on factors such as plant scale, location, local regulations, and market conditions. By implementing these technologies and adopting optimization strategies, the sludge treatment industry can significantly contribute to achieving China's carbon neutrality goal.

5. Conclusions

This study provides a comprehensive assessment of GHG emissions from China's sludge treatment industry and explores the potential of novel technologies to mitigate these emissions, promoting sustainable sludge management practices. The quantification of GHG emissions using a life cycle assessment approach reveals that the total emissions from sludge treatment in 2017 were 18.54 Mt CO₂-eq, with incineration contributing the highest emissions, followed by landfill and land application. The spatial distribution of emissions across cities highlights the need for targeted mitigation strategies that consider local sustainability challenges and regional disparities in economic development and treatment infrastructure. PCA analysis demonstrates that economic factors, such as GDP and treatment scale, significantly influence GHG emissions from sludge treatment. This finding emphasizes the importance of integrating sustainability goals with economic development and adopting technologies that can effectively reduce emissions while promoting resource recovery and circular economy principles. The evaluation of various novel sludge treatment technologies, including thermal conversion, anaerobic digestion, hydrothermal processes, and wet oxidation, shows that these methods offer GHG emission reductions ranging from 0.09 to 0.46 t CO₂-eq/t dry solids compared to conventional methods. However, further optimization is necessary to enhance their environmental and economic performance, ensuring long-term sustainability. To achieve China's carbon neutrality goal and foster sustainable development in the sludge treatment industry, a multi-faceted approach is required. This includes implementing novel technologies, adopting optimization strategies, and promoting policy incentives that encourage the adoption of sustainable sludge management practices. Future research should focus on further refining the life cycle assessment methodology, incorporating more granular data on regional sludge treatment practices, and evaluating the long-term environmental and economic impacts of novel technologies. Moreover, investigating the social acceptance and policy implications of sustainable sludge management strategies can provide valuable insights for decision-makers and facilitate the widespread adoption of sustainable practices.