Life Cycle Carbon Emission Analysis of a Sludge Dewatering Facility: A South-to-North Water Diversion Case Study

Abstract

To mitigate the impact of global climate change, countries worldwide must take necessary measures to address this environmental issue. China’s goals of carbon peaking and carbon neutrality and the “1 + N” policy framework have provided guidance for various industries and enterprises in advancing carbon accounting, carbon reduction, and green transformation. This study focuses on the sludge dewatering facility of a typical water treatment plant, which uses water from the South-to-North Water Diversion Project as its water source. Based on its construction and operational data, the carbon emissions at different phases were calculated with the emission factor method in the study, and the composition of these emissions was analyzed. The results show that during the three-year construction period, the sludge dewatering facility emitted a total of 1385.57 tons of CO₂-eq, with materials being the main source of carbon emissions. While in the one-year operation period, the facility generated 19.38 tons of CO₂-eq only, with electricity consumption being the primary contributor, followed by chemicals consumption. In conclusion, both the short-term intensive emissions during the construction phase and the long-term cumulative emissions during the operation phase should be considered, so that an integrated mitigation effect can be achieved across the construction and operation stages. This highlights the necessity of adopting a life-cycle perspective in carbon reduction strategies within the defined system boundary, while also supporting the sustainable planning and management of water treatment infrastructure.

1. Introduction

Global warming has become a major environmental challenge threatening sustainable development through sea-level rise, extreme weather events, and ecosystem imbalance [1]. Greenhouse gas emissions, especially carbon dioxide from human activities such as industrial production, energy consumption, and transportation, are the main drivers of this problem. According to the International Energy Agency, global carbon emissions reached approximately 37.4 billion tons of CO₂-eq in 2023, representing an increase of 1.1% compared with 2022 [2]. As the largest developing country and one of the world’s major carbon emitters, China is under increasing pressure to reduce emissions while promoting green and low-carbon development. Against this background, the Paris Agreement and China’s carbon peaking and carbon neutrality goals have provided an important policy basis for carbon accounting and emission reduction research in infrastructure-related sectors [3,4]. In northern China, large-scale inter-basin water diversion projects, such as the South-to-North Water Diversion Project, have effectively alleviated regional water scarcity and improved water supply security [5]. However, while water diversion helps relieve water scarcity, it also introduces additional carbon burdens through energy-intensive transfer processes and sludge generation during subsequent water treatment [6].

As the largest developing country and the largest emitter of carbon in the world, China’s carbon emissions in 2023 were approximately 12.6 billion tons of CO₂-eq, accounting for 34% of the global total [2]. In the context of increasingly urgent global climate change governance, China not only faces immense pressure to reduce emissions but is also actively promoting the transformation to green and low-carbon development. In 2015, the Paris Agreement was officially adopted, with 178 parties worldwide committing to limit the global average temperature increase to below 2 °C and striving to limit it to 1.5 °C within this century [3]. In September 2020, China solemnly announced its carbon peaking and carbon neutrality strategy goals at the 75th United Nations General Assembly, aiming to reach peak carbon emissions by 2030 and achieve carbon neutrality by 2060, marking the country’s entry into the era of climate economy and ushering in a green transformation revolution filled with both opportunities and challenges [4].

Water supply and wastewater treatment plants play an indispensable role as critical urban infrastructure, whose facilities are significant sources of greenhouse gas emissions, and increasing attention has been paid to their environmental impacts during the construction and operation periods. In addition to ensuring basic water security, improving the environmental performance of such infrastructure is also essential for promoting the sustainable development of urban water systems. Life cycle assessment (LCA) has been widely recognized as a standardized methodology for quantifying environmental impacts across the life cycle, making it possible to identify not only direct operational emissions but also the embodied emissions associated with material production, equipment manufacturing, transportation, and construction activities. Existing LCA studies in the water sector have mainly focused on urban water systems, wastewater treatment plants, drinking water treatment facilities, and sludge treatment or disposal pathways. These studies have provided valuable insights into energy consumption, chemical use, and overall environmental burdens, and some have expanded the system boundary to include construction activities [7,8,9]. However, most previous studies have been conducted at the plant scale or process-route scale, whereas relatively limited attention has been paid to specific functional units within treatment plants [7]. As a key functional unit, the sludge dewatering facility plays an important role in sludge reduction, stabilization, harmless treatment, and resource utilization. It is also a critical stage in sludge management associated with electricity consumption, chemical dosing, equipment operation, and maintenance activities, making it an important point for carbon emission control. Unlike conventional buildings, it integrates civil structures, specialized equipment, electricity use, chemical consumption, and maintenance activities, resulting in carbon emission characteristics that cannot be fully represented by general building-level accounting results. At present, studies focusing on sludge dewatering facilities as independent accounting objects remain limited, and a source-oriented carbon accounting framework covering both construction and operation phases is still lacking [10,11]. Compared with existing LCA studies on wastewater infrastructure, the novelty of this study lies in treating the sludge dewatering facility as an independent accounting object and establishing a source-oriented carbon accounting framework that covers both the construction and operation phases, including building materials, specialized equipment, construction machinery, electricity consumption, chemicals, and maintenance consumables.

Therefore, the sludge dewatering facility of a typical northern water treatment plant supplied by the South-to-North Water Diversion Project was selected as the research object. As a representative case of large-scale water infrastructure under inter-basin water transfer conditions, it reflects the combined characteristics of diversion-based water supply, treatment-associated sludge generation, and facility-level carbon emissions. The focus is on carbon emissions generated during the construction phase, including the production and transportation of building materials, equipment procurement, and on-site construction, as well as carbon emissions resulting from energy and material consumption during the operation phase. Through a comparative analysis of carbon emission characteristics across different phases, a scientific basis was provided for the low-carbon design and optimized operation of sludge dewatering facilities, and further support was offered for the sustainable management of sludge treatment facilities and urban water infrastructure.

2. Methodology

2.1. Identify Sources of Carbon Emissions

The sludge dewatering facility, as municipal infrastructure, can be divided into several life cycle phases, similar to that of public buildings, including raw material acquisition, production and processing, transportation, construction, operation, demolition, and recycling [12]. In this study, the raw material acquisition, production and processing, transportation, and construction phases are collectively referred to as the construction phase. The construction and operation phases of the sludge dewatering facility are the primary sources of its carbon emissions. The research framework is shown in Figure 1.

As shown in Figure 1, the carbon emission sources of the sludge dewatering facility were divided into the construction phase and operation phase according to the defined system boundary. During the construction phase, the main carbon emission sources include materials, equipment, and machinery. Material-related emissions mainly arise from the production and transportation of building materials, including high-energy-consuming materials such as cement, steel, glass, and aluminum profiles. Equipment-related emissions are associated with the production, transportation, and installation of specialized equipment used in the dewatering facility. Machinery-related emissions are mainly generated by the consumption of diesel or electricity during on-site construction activities involving excavators, cranes, concrete mixers, and other construction machinery.

During the operation phase of the sludge dewatering facility, the main source of carbon emissions is electricity consumption, used to drive equipment such as the dewatering machine and auxiliary systems. The production of electricity typically relies on fossil fuels, such as coal or natural gas, which indirectly leads to significant carbon dioxide emissions. Chemicals are also required during the dewatering process to improve sludge dewatering performance. The production and transportation of these chemicals are energy-intensive processes, thus contributing to carbon emissions. Additionally, consumables used for daily equipment maintenance, such as lubricants and coolants, also generate carbon emissions. It is important to note that the treatment and storage of sludge within the dewatering facility may lead to the leakage of greenhouse gases such as methane, particularly when the sludge is not fully stabilized. However, due to the extensive decomposition of gas-producing organic matter in the anaerobic digestion tanks of wastewater treatment plants and the low organic matter content in drinking water plant sludge, the sludge entering the dewatering facility typically generates minimal gas. Moreover, as the greenhouse gas emissions during this process are difficult to detect, this study does not consider the potential leakage of greenhouse gases or other sources in sludge treatment [13].

2.2. Define the Scope of Accounting

To ensure the accuracy and representativeness of carbon emission accounting results, and to avoid double-counting or omissions, it is necessary to reasonably define the scope of the system for accounting. This scope is typically divided into two parts: spatial scope and temporal scope [13].

In terms of spatial scope, the system boundary covers the major activity locations associated with the construction phase of the sludge dewatering facility, including the construction site, material production sites, equipment manufacturing sites, and transportation routes, as well as the relevant operational areas during the use phase of the facility [14,15]. However, emissions associated with employee commuting and minor construction waste management were not included due to limited data availability and their expected relatively small contribution [16].

In terms of temporal scope, carbon emissions may theoretically occur throughout the whole life cycle of the dewatering facility, including the design, construction, operation, demolition, and recycling stages. However, the design and planning stage mainly involves activities such as scheme design, blueprint drafting, and engineering consulting, whose associated carbon emissions are generally small and difficult to quantify accurately. In addition, although the demolition and recycling phase may also generate carbon emissions, previous studies have shown that their contribution generally accounts for a relatively small proportion of building life-cycle carbon emissions [17,18]. Therefore, the quantitative carbon accounting in this study was confined to the construction and operation stages of the sludge dewatering facility.

2.3. Carbon Emission Accounting Methods

The carbon emission accounting method is a core tool for quantifying greenhouse gas emissions, and the emission factor method is widely adopted due to its simplicity and broad applicability [19]. This method calculates the amount of carbon emissions of specific activities or processes by combining activity data (such as energy consumption or production volume) with specific emission factors. The emission factor method is applicable to various scenarios, including industrial production, transportation, and energy consumption, offering strong operability and data availability. Its core formula is shown in Equation (1) [20]:

E = AD × EF

(1)

where E is carbon emissions, usually expressed in units of mass (kgCO₂-eq); AD is activity data that indicates the intensity or amount of activity (e.g., fuel consumption, production output, in units such as kilograms, kilowatt-hours, etc.); EF is the emission factor, which represents the emissions corresponding to a unit of activity data (e.g., kgCO₂-eq/kg of fuel, or kgCO₂-eq/kW·h).

When conducting carbon emission accounting, activity data can be obtained directly from enterprise energy bills, fuel purchase records, production logs, or metering devices (such as electricity meters and flow meters). A critical aspect of the accounting process is the selection of emission factors, as their values directly affect the accuracy and reliability of the results. When using the emission factor method, it is necessary to comprehensively consider activity types, regional characteristics, technological levels, and data availability [21]. Emission factors are preferably obtained from authoritative databases; for example, the IPCC’s Guidelines for National Greenhouse Gas Inventories provide default emission factors covering various fuels and activities at a global average level, which is suitable when local data are lacking [21]. For study objects with regional particularities, national or local emission factors should be referenced, such as China’s Provincial Greenhouse Gas Inventory Guidelines or the grid emission factors published by the National Development and Reform Commission, as these data better reflect local fuel characteristics, energy structure, and technological conditions. The temporal validity of emission factors is important to ensure consistency with the time frame of the activity data, avoiding deviations caused by technological progress or policy changes [22]. Moreover, the units of the selected emission factors and activity data should be consistent; otherwise, unit mismatches can lead to numerical errors, affecting the accuracy of the accounting and potentially overestimating or underestimating carbon emissions, which could mislead the direction of policy or research. In this study, the system boundaries of the adopted emission factors were matched with the corresponding accounting items according to their original sources. For building materials, the emission factors generally cover the processes from raw material extraction and transportation to the production of finished products. For electricity, the emission factor represents the emissions associated with electricity generation. For construction machinery, the emission factors refer to the direct emissions from fuel or electricity use during machinery operation. For chemicals and other consumables, the emission factors generally cover the processes from raw material extraction and transportation to the production of finished products. These boundary definitions were used to ensure consistency between the selected emission factors and the accounting boundary of this study, while avoiding double-counting. The main carbon emission factors are listed in

Table 1. Main sources of carbon emission factors.

Type	Emission Factor (Unit)	Data Source
Electricity	0.7252 kgCO2-eq/kW·h	Ministry of Ecology and Environment of the People’s Republic of China [23]
C10 Concrete	107 kgCO2-eq/m3	Standard for Building Carbon Emission Calculation (GB/T 51366-2019) [24]
C30 Concrete	295 kgCO2-eq/m3	Standard for Building Carbon Emission Calculation (GB/T 51366-2019) [24]
C50 Concrete	385 kgCO2-eq/m3	Standard for Building Carbon Emission Calculation (GB/T 51366-2019) [24]
Medium Sand	2.51 kgCO2-eq/t	Standard for Building Carbon Emission Calculation (GB/T 51366-2019) [24]
Crushed Stone	2.18 kgCO2-eq/t	Standard for Building Carbon Emission Calculation (GB/T 51366-2019) [24]
Cement	735 kgCO2-eq/t	Standard for Building Carbon Emission Calculation (GB/T 51366-2019) [24]
Polyaluminium Chloride (PAC)	0.53 kgCO2-eq/kg	Guidelines for Carbon Accounting and Emission Reduction in the Urban Water Sector [25]
NaClO	0.99 kgCO2-eq/kg	Guidelines for Carbon Accounting and Emission Reduction in the Urban Water Sector [25]
FeCl3	0.26 kgCO2-eq/kg	Guidelines for Carbon Accounting and Emission Reduction in the Urban Water Sector [25]

Notes: C10, C30, and C50 denote strength grades of ready-mixed concrete, where the number refers to the compressive strength class. PAC = polyaluminium chloride; NaClO = sodium hypochlorite; FeCl₃ = ferric chloride.

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2.4. Carbon Emission Accounting Model for Sludge Dewatering Facility

2.4.1. Construction Phase

Carbon emissions during the construction phase can be divided into three components: materials, equipment, and machinery. Building materials are one of the primary sources of carbon emissions in the construction phase, including basic materials such as cement, steel, sand and gravel. The carbon emissions from equipment are mainly associated with production, transportation, and other related processes of the dewatering facility equipment. Emissions from construction machinery originate from the use of excavators, trucks, bulldozers, and other equipment during on-site construction.

(1)

The construction of the sludge dewatering facility involves a wide variety of materials, and calculating the emission factor for each individual material from first principles would be highly time-consuming and impractical. Therefore, in this study, the required carbon emission factors were primarily obtained from existing databases with relatively high applicability to the Chinese context, including the China Products Carbon Footprint Factors Database (CPCD, https://lca.cityghg.com (accessed on 22 April 2026)), the Donghe Building Carbon Emission Calculation and Analysis Software V3.0 (https://carbon.seuicc.com (accessed on 22 April 2026)), and Qingyue Data (https://data.epmap.org (accessed on 22 April 2026)). When corresponding factors were not directly available from these databases, relevant industry standards, technical guidelines, and published literature were used as supplementary sources. The material inventory data were derived from project quantity documents and engineering records of the sludge dewatering facility. Based on the selected emission factors and material quantities, the carbon emissions from material consumption during the construction phase were calculated using the emission factor method, as shown in Equation (2) [20].

$$E_{Mat}=\sum _{i=1}^n{AD}_{Mat,i}\times {EF}_{Mat,i}$$

(2)

Here, E_Mat is the carbon emissions resulting from material consumption during the construction phase (kgCO₂-eq); AD_Mat,i is the consumption of the i-th type of material during the construction phase $\left(\mathrm{k}\mathrm{g}\right)$; EF_Mat,i is the carbon emission factor of the i-th material (kgCO₂-eq/kg); i is the material type index; and n is the total number of material types involved in the construction phase.

(2)

The carbon emission factors for specialized equipment are largely absent from existing databases and have been studied little in the literature, so specialized equipment is analyzed separately. In this study, the sludge dewatering machine is used as an example to examine its “cradle-to-gate” carbon emissions. The process involves defining the system boundary and determining the life cycle scope of the machine, collecting data such as the bill of materials required for its production and related energy consumption (including electricity and fuels), determining emission factors based on authoritative sources such as the IPCC for relevant materials and energy during the manufacturing process (including carbon dioxide, methane, and nitrous oxide), and calculating carbon emissions for each activity and energy consumption using Equation (3) [20]:

$$E_{Equ}=\sum _{j=1}^m\left(E_{P,j}+E_{T,j}+E_{O,j}\right)$$

(3)

where E_Equ is the total carbon emissions generated by the equipment during the “cradle-to-gate” process in the construction phase, in kgCO₂-eq; E_P,j is the net carbon emissions generated during the production phase of the j-th type equipment; E_T,j is the carbon emissions generated during the transportation phase of the j-th equipment; E_O,j is the carbon emissions generated during the processes such as installation and commissioning of the j-th equipment; j is the equipment type index, ranging from 1 to m; and m is the total number of equipment types.

(3)

When accounting for carbon emissions from construction machinery, the first step is to determine the activity data (AD), which is usually the fuel consumption or operating conditions of the machinery. Fuel consumption can be obtained from construction logs, equipment operation records, and machinery fuel efficiency [26]. Appropriate emission factors (EF) should then be selected; for diesel-powered machinery, the diesel emission factors provided by the IPCC can be used, while for electrically powered machinery, local grid emission factors should be adopted. So the total carbon emissions generated by construction machinery during the construction phase can be calculated using Equation (4) [20].

$$E_{Mach}=\sum _{k=1}^p{AD}_{Mach,k}\times {EF}_{Mach,k}$$

(4)

Heere E_Mach is the carbon emissions from construction machinery during the construction phase, in kgCO₂-eq; AD_Mach,k is the energy consumption of the k-th type of construction machinery. For fuel-driven machinery, the unit is L, and for electrically driven machinery, the unit is kW·h. EF_Mach,k is the corresponding carbon emission factor. For fuel-driven machinery, the unit is kgCO₂-eq/L, and for electrically driven machinery, the unit is kgCO₂-eq/kW·h, as shown in Appendix

Table A1. Emission Factors for Energy Use in Construction Machinery.

Energy Type	Emission Factor	Unit	Source *
Diesel	3.096	kgCO2-eq/kg	IPCC (2006) [3]
Gasoline	2.92	kgCO2-eq/kg	IPCC (2006) [3]
Natural Gas	2.75	kgCO2-eq/Nm3	IPCC (2006) [3]

* Notes: Carbon emission factors for fossil fuels are derived from the IPCC 2006 Guidelines for National Greenhouse Gas Inventories.

. k is the construction machinery type index, ranging from 1 to p, and p is the total number of construction machinery types used in the construction phase.

2.4.2. Operational Phase

During the operation phase of the sludge dewatering facility, the carbon accounting mainly focuses on two major parts: electricity consumption and consumable consumption.

During the operation phase, a large amount of electricity is consumed, and the corresponding carbon emissions can be calculated using the relevant emission factors. The calculation formula is shown in Equation (5) [20].

$$E_{Elec}={AD}_{Elec}\times {EF}_{Elec}$$

(5)

Here, E_Elec denotes the carbon emissions from electricity consumption during the operation phase (kgCO₂-eq); AD_Elec denotes the electricity consumption of the sludge dewatering facility during the operation phase (kW·h); and EF_Elec denotes the electricity emission factor of the local power grid (kgCO₂-eq/kW·h), as shown in Appendix

Table A2. Average CO₂ Emission Factors of Provincial Electricity in 2022.

Province	Emission Factor (kgCO2-eq/kW·h)
Beijing	0.558
Tianjin	0.7041
Hebei	0.7252
Shanxi	0.7096
Inner Mongolia	0.6849
Liaoning	0.5626
Jilin	0.4932
Heilongjiang	0.5368
Shanghai	0.5849
Jiangsu	0.5978
Zhejiang	0.5153
Anhui	0.6782
Fujian	0.4092
Jiangxi	0.5752
Shandong	0.641
Henan	0.6058
Hubei	0.4364
Hunan	0.49
Guangdong	0.4403
Guangxi	0.4044
Hainan	0.4184
Chongqing	0.5227
Sichuan	0.1404
Guizhou	0.4989
Yunnan	0.1073
Shaanxi	0.6558
Gansu	0.4772
Qinghai	0.1567
Ningxia	0.6423
Xinjiang	0.6231

Notes: The data are derived from the Ministry of Ecology and Environment of the People’s Republic of China.

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During the operation phase of the sludge dewatering facility, flocculants are commonly added to improve the solid–liquid separation of the sludge and enhance dewatering efficiency. In addition, routine maintenance of the dewatering facility requires consumables such as lubricants, cleaning agents, and replacement parts. By accounting for all types and quantities of consumables and obtaining the corresponding carbon emission factors, the carbon emissions can be calculated, as shown in Equation (6) [20]:

$$E_{Cons}=\sum _{t=1}^y{AD}_{Cons,t}\times {EF}_{Cons,t}$$

(6)

where E_Cons is the carbon emissions during the operation phase resulting from chemicals and maintenance consumables, in kgCO₂-eq; AD_Cons,t is the consumption of the t-th type chemical or consumable, in kg or L; EF_Cons,t is the carbon emission factor of the t-th chemical or consumable, in kgCO₂-eq/kg or kgCO₂-eq/L; t is the consumable type index, ranging from 1 to y; and y is the total number of consumable types involved in the operation phase.

3. Results and Discussion

3.1. Carbon Emission Accounting in Construction Phase

The case water treatment plant is located in Hebei Province, China, and uses water from the South-to-North Water Diversion Project as its source. The plant covers a total area of 7.8 hectares and has a design capacity of 150,000 m³/d. Based on the collected data and using the carbon emission calculation model presented in Section 2.4.1, the carbon emissions of the sludge dewatering facility during the construction phase were calculated separately for equipment, materials, and construction machinery. The quantities of construction materials were derived from available engineering documents, including the bill of quantities, construction budget documents, and completion records. The results are shown in

Table 2. Carbon Emissions of the Sludge Dewatering Facility during the Construction Phase.

Type	Unit	Equipment	Materials	Machinery	Total
Carbon Emissions	kgCO2-eq	76,174.12	1,278,966.41	30,429.79	1,385,570.32
	tCO2-eq	76.17	1278.97	30.43	1385.57
Proportion	%	5.50%	92.30%	2.20%	100.00%

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From the data in the table, the total carbon emissions of the sludge dewatering facility at the case water treatment plant amount to 1,385,570.32 kgCO₂-eq. Among them, the carbon emissions from materials are the highest, reaching 1,278,966.41 kgCO₂-eq, accounting for 92.30%, indicating that material production and transportation are the major contributing factors. This is likely related to the extensive use of high-energy, high-carbon materials such as cement and steel [27]. In comparison, the specialized equipment required generates 76,174.12 kgCO₂-eq, accounting for 5.50%, reflecting a relatively lower carbon footprint from equipment manufacturing and transportation, yet still having a notable impact. The carbon emissions from construction machinery used during the construction process are the lowest, at only 30,429.79 kgCO₂-eq, representing 2.20% of the total.

To improve comparability with other studies, the construction-phase carbon emissions were further normalized by the total gross floor area of the sludge dewatering facility. According to the construction drawings, the sludge dewatering facility has a total gross floor area of 1509 m². Based on the total construction-phase carbon emissions of 1,385,570.32 kgCO₂-eq, the carbon emission intensity during construction was calculated as 918.87 kgCO₂-eq/m². This value is higher than the 508.57 kgCO₂-eq/m² reported for an industrial building by Rodrigues et al. [28], but falls within the range of 757–1022 kgCO₂-eq/m² reported for construction-stage building carbon emissions in China, summarized by Sandanayake et al. [29]. Such differences should be interpreted with caution, because carbon emission intensity per unit floor area is strongly affected by building function, system boundary, material composition, and whether specialized process equipment is included. As the sludge dewatering facility is an integrated functional unit combining civil structures with dedicated treatment equipment, its construction-stage carbon intensity may differ from that of conventional buildings.

During the entire construction phase of the sludge dewatering facility, a total of 103 types of materials were used, which can be categorized into five groups: building materials, steel and metal products, pipes and fittings, electrical and cable materials, and other materials. Among them, building materials mainly include concrete, bricks, mortar, and foam boards, which are essential for constructing the facility, totaling 18 items. Steel and metal products include rebar, steel plates, metal bolts, etc., totaling 8 items. Pipes and fittings mainly consist of drainage pipes, joints, elbows, and similar components, totaling 60 items. Electrical and cable materials mainly consist of cables and wires, totaling 8 items. Other materials include water, gasoline, diesel, fire extinguishers, and similar items, totaling 9 items. The proportion of carbon emissions for each material category is shown in Figure 2.

Cement, concrete, and other building materials generate the highest carbon emissions, totaling 716,164.23 kgCO₂-eq, accounting for 56% of all material-related emissions. Among them, the major contributors include ready-mixed concrete with different strength grades (such as C15, C30, and C40), lightweight aggregate concrete of grade LC5.0, and aerated concrete blocks. The next highest contributor is steel and metal products, producing 528,191.18 kgCO₂-eq, or 41.3%. The production processes of building materials and steel/metal products are energy-intensive and rely heavily on fossil fuel consumption, resulting in a relatively large carbon footprint for these two categories [30]. Pipes and fittings, electrical and cable materials, and other types of materials contribute 14,056.40 kgCO₂-eq, 651.13 kgCO₂-eq, and 19,903.47 kgCO₂-eq, respectively, accounting for 1.1%, 0.04%, and 1.56% of the total material-related carbon emissions.

For the equipment category, a total of 48 major pieces of equipment were accounted for, generating 76,174.12 kgCO₂-eq, which represents 5.50% of the total carbon emissions during the construction phase. Among them, the centrifugal dewatering machine contributes the most to carbon emissions, with 16,774.86 kgCO₂-eq. This is because its manufacturing process requires a large amount of high-energy industrial raw materials, such as stainless steel and carbon steel, and involves a complex production process including casting, machining, welding, and surface treatment, all of which are energy-intensive.

Compared with equipment and materials, the carbon emissions generated by construction machinery during the construction of the sludge dewatering facility account for only 2.20% of the total carbon emissions in the construction phase. The main source of these emissions is the energy consumed by construction machinery, such as electricity, diesel, and gasoline.

Table 3. Investment Costs during the Construction Phase of the Sludge Dewatering Facility.

Type	Equipment	Materials	Machinery	Total
Cost (CNY)	5,143,441.7	3,535,472.5	192,655.95	8,871,570.1
Proportion	57.98%	39.85%	2.17%	100.00%

presents the investment costs during the construction phase. When combined with the carbon emissions shown in Table 1, a significant imbalance between economic input and carbon output is evident. The total construction cost amounts to 8.8716 million CNY, of which equipment costs reach 5.1434 million CNY, accounting for 57.98% of the total investment. In contrast, the corresponding carbon emissions from equipment account for only 5.50% of total construction-stage emissions. This contrast suggests that equipment costs and carbon emissions are driven by different factors: equipment investment is largely influenced by technical complexity, functional requirements, and procurement price, whereas carbon emissions are more closely related to material composition, manufacturing processes, and the applied emission factors. Therefore, a high cost share does not necessarily correspond to a high carbon emission share. Materials contribute 92.30% of the carbon emissions, making them the primary source of emissions; however, their economic input is 3.5355 million CNY, or 39.85% of the total cost. In addition, construction machinery costs 192,655 CNY, representing 2.17% of the total cost, with carbon emissions accounting for 2.20%, indicating a relatively minor impact on both overall emissions and economic expenditure. Using the total construction-stage investment of 8.8716 million CNY as a unified benchmark, the carbon emissions normalized by total investment are 3.618 tCO₂-eq/10,000 CNY for materials, which is much higher than those for equipment (0.148 tCO₂-eq/10,000 CNY) and machinery (1.579 tCO₂-eq/10,000 CNY). This indicator is used to compare the relative carbon contribution of different categories under the same total investment benchmark during the construction stage, and it does not account for differences in service life between equipment and building materials. The carbon emissions, costs, and normalized carbon emissions of these three categories are illustrated in Figure 3.

3.2. Carbon Emission Accounting in Operation Phase

3.2.1. Electricity Consumption

The operational data of the sludge dewatering facility at the case water treatment plant are based on the actual records of electricity consumption, types of chemicals used, and their corresponding dosages.

Based on the 2022 operational data of the sludge dewatering facility at the case water treatment plant, the monthly treated water volume (m³), electricity consumption (kW·h), and carbon emissions (kgCO₂-eq) were analyzed to reveal seasonal variations in operational efficiency. According to the plant’s operational records, the total water output in 2022 reached 6,197,036 m³, with a total electricity consumption of 2,886,142 kW·h. The sludge dewatering facility consumed 13,988.61 kW·h over the year, accounting for 0.48% of the plant’s total electricity usage, as shown in Figure 4.

Specifically, the treated water volume peaked in September at 689,058 m³ and reached its lowest in February at 205,987 m³, exhibiting a trend of higher volumes in summer (June–September) and lower volumes in winter (November–December). However, the monthly electricity consumption did not follow the same pattern. The highest consumption occurred in November at 2093.1 kW·h, accounting for 17.9% of the annual total, while the lowest was in February at 626.2 kW·h, or 5.4% of the total, showing relatively stable consumption from January to October (average around 801 kW·h) and a significant increase in November–December. Based on the local grid’s electricity carbon emission factor of 0.7252 kgCO₂-eq/kW·h, the carbon emissions from electricity consumption in the sludge dewatering facility in 2022 were calculated to be 8462.94 kgCO₂-eq. Since the electricity emission factor remained constant in the calculation, the monthly variation in electricity-related carbon emissions was consistent with the pattern of electricity consumption, with the highest value occurring in November and the lowest in February.

The monthly electricity-related carbon emissions showed the same trend as electricity consumption, with the highest value occurring in November at approximately 1517.92 kgCO₂-eq, followed by December at around 915.27 kgCO₂-eq. This indicates that fluctuations in electricity consumption had a direct influence on the operational carbon footprint of the sludge dewatering facility. It should be noted that the monthly electricity consumption of the sludge dewatering facility was derived from actual operational statistics and does not necessarily vary in direct proportion to plant water output. In addition to water production, electricity use may also be affected by sludge generation characteristics, equipment operation scheduling, and intermittent or concentrated dewatering activities during specific periods.

Since similarly high values were not observed in January or February, the November peak may not be attributed solely to winter conditions, but may also be related to specific operational arrangements or equipment loading characteristics during that month. Therefore, the operational management of the sludge dewatering facility should pay close attention to periods of elevated electricity consumption and optimize equipment scheduling or process parameters to improve energy efficiency, thereby balancing treatment capacity and environmental impact.

3.2.2. Consumable Consumption

During operation, the sludge dewatering facility requires the use of PAC to improve sludge settling and enhance dewatering performance; FeCl₃ serves as an auxiliary chemical for the pretreatment of hard-to-dewater sludge; lubricant is used for mechanical components of centrifuges or filter presses to ensure long-term equipment operation; coolant absorbs and dissipates heat generated by motors or bearings during operation, preventing overheating and maintaining equipment stability; and tap water is used for equipment maintenance. Based on the plant’s operational energy monitoring records, the carbon emissions associated with these chemicals and consumables are presented in

Table 4. Consumables and carbon emissions of the sludge dewatering facility in 2022.

Consumable	Quantity	Unit	Carbon Emission Factor	Carbon Emission (kgCO2-eq)
Polyaluminium Chloride	13,951.17	kg	0.53 kgCO2-eq/kg [25]	7394.12
FeCl3	4748.33	kg	0.26 kgCO2-eq/kg [25]	1234.57
Lubricant	250	L	4.05 kgCO2-eq/L [25]	1012.5
Coolant	75	L	1.65 kgCO2-eq/L [25]	123.75
Tap Water	6850	m3	0.168 kgCO2-eq/m3 [31]	1150.8
Total	-	-	-	10,915.74

.

In 2022, the total carbon emissions from consumables in the sludge dewatering facility amounted to 10,915.74 kgCO₂-eq. The use of chemicals contributed the most, accounting for 79.05% of the total emissions. Specifically, PAC, as the primary coagulant, had a consumption of 13,951.17 kg and contributed 7394.12 kgCO₂-eq, representing 67.74% of the total, making it the largest source of carbon footprint from consumables during the dewatering process, as shown in Figure 5. FeCl₃ had a consumption of 4748.33 kg, resulting in 1234.57 kgCO₂-eq, or 11.31% of the total. In contrast, lubricant, coolant, and tap water contributed 1012.5 kgCO₂-eq (9.28%), 123.75 kgCO₂-eq (1.13%), and 1150.8 kgCO₂-eq (10.54%), respectively.

To further explore the monthly variation in coagulant consumption, the monthly usage of PAC and FeCl₃ in the sludge dewatering facility was analyzed based on the plant’s 2022 operational data, as shown in Figure 6. The results indicate that both PAC and FeCl₃ exhibit noticeable fluctuations over the year, with relatively higher consumption in several summer months compared to autumn and winter. Overall, PAC consumption shows considerable variation throughout the year, particularly rising from May to August. FeCl₃ exhibits a similar trend, gradually increasing with a peak in September, followed by a slight decrease. The high consumption of PAC, combined with its relatively high carbon emission factor, makes it a key contributor to the carbon footprint of the sludge dewatering facility. However, because monthly sludge treatment volume data were not available in this study, it was not possible to determine whether these fluctuations were due to changes in sludge treatment volume or variations in dosage intensity per unit treatment. Therefore, the underlying causes of the observed monthly variation require further investigation based on more detailed operational data. To reduce the overall carbon footprint of sludge treatment, PAC dosing may be optimized through precise application or dynamic adjustment under improved data support. Additionally, exploring low-carbon alternative coagulants is a potential strategy to further lower carbon emissions in the sludge treatment process.

3.2.3. Analysis of Total Carbon Emissions in the Sludge Dewatering Facility

According to the accounting results, the total annual carbon emissions of the sludge dewatering facility were 19,378.68 kgCO₂-eq. Carbon emissions from electricity consumption amounted to 8462.94 kgCO₂-eq, accounting for 43.67% of the total operational emissions, as shown in Figure 7. While emissions from consumables were 10,915.74 kgCO₂-eq, representing 56.33%. Overall, electricity consumption and consumables made similarly important contributions to operational carbon emissions. Although consumables were slightly higher than electricity in the present accounting results, the difference between the two was small and may be influenced by uncertainties in activity data and emission factors. Among consumables, chemicals (PAC and FeCl₃) were the largest contributors, producing 8628.69 kgCO₂-eq, or 44.53% of the total.

On a monthly basis, carbon emissions in January and February were dominated by electricity, accounting for about 70% (Figure 8). From March to May, material-related emissions gradually increased, with April showing nearly equal contributions from both sources. Between June and August, material emissions rose significantly, reaching 63–67% of total emissions, with June peaking at 1483.89 kgCO₂-eq due to surges in PAC and FeCl₃ usage. In September and October, total emissions decreased slightly, but material emissions remained higher than electricity emissions. November saw the annual peak in total emissions (2699.20 kgCO₂-eq), primarily caused by a sharp rise in electricity emissions to 1517.92 kgCO₂-eq, while December experienced a notable drop. Overall, seasonal variations in raw water quality likely drove higher chemical dosing in summer, increasing material emissions, whereas the November electricity peak may relate to higher equipment loads under low temperatures.

For carbon reduction in the sludge dewatering facility during operation, several strategies can be implemented. First, regarding electricity consumption, high-energy-consuming equipment can be replaced with energy-efficient dewatering machines, and operational scheduling can be optimized. More importantly, integrating renewable energy sources such as solar and wind power to partially supply electricity can significantly reduce the carbon footprint. Second, concerning the consumption of chemicals and maintenance consumables, reduction efforts should focus on optimizing chemical usage and selecting low-carbon alternatives. Precise dosing and process optimization can minimize excessive chemical use, while prioritizing the use of environmentally friendly, low-carbon chemicals can replace traditional high-carbon-footprint products.

3.3. Comparison of Carbon Emissions in Different Phases and Carbon Reduction Strategies

To comprehensively understand the carbon footprint of the sludge dewatering facility across its entire lifecycle, a systematic comparison and synthesis of the construction and operation phase emissions is necessary. By analyzing the total emissions, structural differences, and mitigation potential across phases, the key contributors to carbon emissions can be identified, providing a basis for subsequent low-carbon optimization.

Table 5. Analysis of carbon emissions from the construction and operation phases of the sludge dewatering facility.

Phase	Emission Source	Carbon Emission (Unit: kgCO2-eq)	Contribution to Total Emissions
Construction Phase	Equipment	76,174.12	5.42%
	Material	1,278,966.41	91.03%
	Machinery	30,429.79	2.17%
	Total	1,385,570.32	98.62%
Operation Phase	Electricity	8462.94	0.60%
	Consumable	10,915.74	0.78%
	Total	19,378.68	1.38%
Total	-	1,404,949	100%

Notes: The percentages in Table 5 are based on the three-year cumulative construction emissions and the one-year operational emissions in 2022, rather than the 50-year design service life estimate discussed later in the text.

presents a clear summary of carbon emissions in both the construction and operation phases, including contributions from various emission sources.

As shown in Table 5, the carbon emission characteristics of the sludge dewatering facility in the water plant differ significantly between the construction and operation phases. During the three-year construction period, the sludge dewatering facility generated a total of 1,385,570.32 kgCO₂-eq, whereas the annual carbon emissions during operation were 19,378.68 kgCO₂-eq. Based on the annual operational carbon emissions under the 2022 operating conditions, a simplified static estimation was conducted for comparative purposes. Under the assumption that the 2022 operating conditions remain unchanged, the cumulative carbon emissions of the operation phase over a 50-year design service life are estimated at approximately 968,934.00 kgCO₂-eq, accounting for about 41.15% of the combined carbon emissions from the construction and operation phases (2,354,504.32 kgCO₂-eq), while the construction phase accounts for 58.85%. These results indicate that the two phases exhibit distinct temporal characteristics: carbon emissions from the construction phase are concentrated within a short period, whereas those from the operation phase are continuous and cumulative. Therefore, although the construction phase occupies the shortest period in the overall life cycle, its carbon emissions are substantial and should not be overlooked.

The above results indicate that carbon emissions of the sludge dewatering facility exhibit distinct phase-specific characteristics: emissions during the construction phase are concentrated and high in intensity but short in duration, whereas those during the operation phase are lower in intensity yet continuous and cumulative. From a lifecycle perspective, the gap in total carbon emissions between the two phases is not absolute and tends to converge over extended service years. Therefore, lifecycle-oriented carbon management strategies should integrate both “early-stage control” and “long-term optimization”.

On the one hand, during the construction phase, besides unavoidable structural emissions, there are non-essential emissions that can be mitigated through early-stage planning and technical optimization. For example, prioritizing low-carbon materials and optimizing construction processes in design and material selection can significantly reduce emissions associated with material production and transportation. The results of this study show that materials dominate the carbon emissions during construction, accounting for 92.3%, consistent with Arab et al. [32], who reported that materials contribute 80–90% of total embodied emissions in buildings. Thus, implementing reduction measures in this segment has both strong justification and high potential. Additionally, during construction, reasonable scheduling of processes and minimizing repeated temporary facilities can effectively reduce energy consumption and construction waste.

On the other hand, during the operation phase, promoting energy-efficient operation modes and smart management systems can precisely match treatment loads and reduce redundant energy use. Simultaneously, exploring low-carbon alternative chemicals, such as bio-based coagulants (e.g., chitosan and plant-based coagulants) or hybrid reduced-dosage conditioning strategies, can lower material-related emissions [33,34]. Considering the persistent nature of operational carbon emissions, integrating renewable energy sources and carbon offset initiatives also presents practical and strategic value.

Although this study is based on a single case, the results still provide useful references for similar sludge treatment units or auxiliary functional units in water treatment plants. For infrastructure units with a relatively limited building volume but intensive use of concrete, steel, and metal components, embodied carbon from material production often remains the dominant source of carbon emissions during the construction phase. During the operation phase, carbon emission sources tend to exhibit a diversified pattern. In addition to electricity consumption, the contribution of chemicals and maintenance consumables may also increase significantly with changes in sludge characteristics, process conditions, and operational modes, reaching a level comparable to or even higher than that of electricity. In view of these characteristics, low-carbon management of such units should be carried out in a coordinated manner across both the construction and operation phases, incorporating early-stage material control, low-carbon procurement of specialized equipment, optimized electricity scheduling, and precise chemical dosing into an integrated management framework. These findings can provide references for the low-carbon design, operation optimization, and subsequent benchmarking of similar facilities.

It should be noted that the carbon accounting in this study is based only on the actual data from 2022. Therefore, the results reflect the specific operating conditions of that year and cannot fully capture possible inter-annual variations in raw water quality, sludge production, chemical demand, or electricity consumption. In addition, the long-term comparison based on a 50-year design service life is a simplified static estimation under current conditions and does not take into account future changes such as grid decarbonization, equipment aging, technological upgrades, or changes in chemical formulations. These factors may affect the long-term carbon emission characteristics of the sludge dewatering facility. Moreover, the present study does not include benchmarking against other sludge dewatering technologies or other sub-processes within water treatment plants, which limits the comparative interpretability of the results. Future studies should further explore these issues through uncertainty analysis, sensitivity analysis, scenario modelling, and comparative studies of different sludge dewatering technologies or functional units.

4. Conclusions

The sludge dewatering facility of a typical water plant, which sources water from the South-to-North Water Diversion Project, is selected as the accounting object. A carbon emission accounting framework was established for both the construction and operation phases of the dewatering facility, with clearly defined system boundaries and emission sources. Using the emission factor method, a quantitative analysis of lifecycle carbon emissions was conducted, and potential reduction pathways were explored. The results indicate that during the three-year construction period, the sludge dewatering facility generated a total of 1,385,570.32 kgCO₂-eq, with materials contributing the largest share at 92.3%, thus dominating the construction phase emissions. During a one-year operational cycle, total carbon emissions amounted to 19,378.68 kgCO₂-eq, primarily from electricity consumption (43.67%), followed by chemicals and maintenance materials. Based on a 50-year design lifespan for the plant, construction-phase emissions account for 58.85% of the total carbon emissions estimated for the construction and operation phases, while operation-phase emissions account for 41.15%.

The results show that the carbon emissions of the sludge dewatering facility exhibit clear phase-specific characteristics. The construction phase is short in duration but high in emission intensity, whereas the operation phase has lower annual emissions but substantial cumulative emissions over a long service period. Therefore, carbon reduction efforts should address both phases simultaneously. In the construction phase, priority should be given to reducing the use of carbon-intensive materials, optimizing material selection, improving material utilization efficiency, and strengthening carbon control in material production, transportation, and on-site construction processes. Since materials account for the vast majority of construction-phase emissions, targeted control of material-related sources should be the primary focus of emission reduction in this phase.

In the operation phase, practical mitigation measures should focus on improving electricity-use efficiency and refining chemical management. Specifically, operational carbon emissions may be reduced by optimizing equipment scheduling, improving the operating efficiency of key devices, and reducing unnecessary electricity consumption during high-load periods. At the same time, since PAC and FeCl₃ are important contributors to operation-phase emissions, more precise dosing management and the exploration of relatively low-carbon alternatives may help reduce the carbon footprint of the sludge dewatering facility within the current research boundary. Overall, the findings of this study provide a useful basis for identifying major emission sources and formulating targeted low-carbon management strategies for sludge dewatering facilities under similar conditions.