Study on Carbon Emissions from an Urban Water System Based on a Life Cycle Assessment: A Case Study of a Typical Multi-Water County in China’s River Network Plain

By inergency On Feb 21, 2024

4.1. Carbon Emission Analysis of Urban Water Systems

The annual average carbon emissions from the urban water system in the study area for its entire lifecycle is 20.48 × 10⁴ tons. The carbon emissions during the operational phase amount to 17.53 × 10⁴ tons, accounting for 86% of the total carbon emissions (as shown in Figure 4). The carbon emissions during the operational phase are significantly higher than those during the construction and dismantling phases. This indicates that although water projects involve a large initial investment of materials and energy during the construction phase, it is the ongoing operation and maintenance of these projects that contribute significantly to the carbon emissions in urban water systems on an annual scale. The carbon emissions during the construction phase are 1.92 × 10⁴ tons in total, with the largest amount coming from the water intake stage. This is because the construction of reservoirs and water diversion projects requires many high-emission materials such as steel bars and cement, resulting in significant carbon emissions. The substantial carbon emissions resulting from the large-scale construction of water projects are not the primary focus of carbon reduction. Increasing the service life of water projects can indirectly achieve the goal of carbon reduction. In the operational phase, the carbon emissions from the water supply stage account for the largest portion, at 43% of the total operational emissions. This is due to the significant water supply volume in the study area and the relatively high energy intensity associated with this stage, resulting in substantial carbon emissions. The carbon emissions in the recycling stage are the lowest, as the amount of recycled water is relatively small, accounting for only 0.01‰ of the water supply volume. The carbon emissions from this stage constitute only 2% of the total carbon emissions during the operational phase. This also indicates that although the energy intensity in the recycling stage is much higher than in the water supply stage, the disparity in water consumption leads to such differences. In the future, an increase in the utilization rate of recycled water will reduce the water intake and supply volume, consequently reducing carbon emissions.

4.2. Analysis of Carbon Footprint Intensity in Multi-Source Water Supply

According to the generalized results of the urban water system in Figure 2, the carbon emission intensities of different subsystems in Yiwu City were calculated. Taking the status of the water supply target as an example, the carbon emission intensity results are shown in Figure 5. The overall carbon footprint intensity of the urban high-quality water subsystem is 0.90 kgCO_2eq/m³, which means that supplying 1 m³ of high-quality water to the distribution network users (residential or industrial users) will result in the emission of 0.90 kg of CO₂. The carbon emission proportions of the water intake, water supply, and wastewater discharge stages in the urban high-quality water subsystem are approximately 2:5:3. During the operational phase, the relatively high energy consumption in the water treatment process leads to a significant contribution of carbon emission intensity in the water supply stage. The overall carbon footprint intensity of the urban general water subsystem in the study area is 0.78 kgCO_2eq/m³, indicating that supplying 1 m³ of general water to the distribution network users (general industrial users) will result in the emission of 0.78 kg of CO₂. The operational phase accounts for the highest carbon emission intensity, constituting 86% of the overall carbon emission intensity. The carbon emission intensity in the water supply stage of the general water subsystem is reduced by 0.08 kgCO_2eq/m³ compared with that of the high-quality water subsystem. This is because industrial water plants have lower water quality requirements for their supply, and the treatment processes are relatively simpler compared with those in conventional water plants. As a result, the energy consumption during the water treatment process is lower, leading to a smaller carbon emission intensity. The carbon emission intensity of the reclaimed water cycle for users such as industrial pipeline networks is 1.38 kgCO_2eq/m³, meaning that supplying 1 m³ of reclaimed water to the distribution network users (general industrial users) will result in the emission of 1.38 kg of CO₂. The operational phase accounts for the highest carbon emission intensity, constituting 95% of the overall carbon emission intensity. Although the carbon emission intensity of the water intake stage in the low-quality water subsystem is reduced compared with that of the high-quality water subsystem, due to the construction and operation of water intake facilities, the energy consumption during reclaimed water treatment in the water supply stage is much higher than that during conventional water treatment. This leads to a higher carbon emission intensity for the reclaimed water subsystem compared with the high-quality water subsystem. Although the carbon emission intensity in the construction and demolition stages for the reclaimed water plant is lower than that for the industrial water plant in Yiwu City, the difference in energy consumption during the water treatment process during the operational phase results in a higher carbon emission intensity for the reclaimed water subsystem.

Due to the different urban water circulation pathways, the carbon emission intensity varies for different water sources supplied to different users (domestic, industrial, agricultural, ecological, etc.). For domestic users with high water quality requirements, the carbon emission intensities for high-quality water, general water, and reclaimed water are 0.90 kgCO_2eq/m³, 1.50 kg CO_2eq/m³, and 1.38 CO_2eq/m³, respectively. Due to the poor quality of the Yiwu River water and reclaimed water, complex treatment processes are required to meet water quality standards, resulting in higher carbon emission intensity. Additionally, since the reclaimed water subsystem lacks a water intake stage, it has a lower carbon emission intensity. For industrial users, the carbon emission intensities for high-quality water, general water, and reclaimed water are 0.90 kgCO_2eq/m³, 0.78 kg CO_2eq/m³, and 1.38 kgCO_2eq/m³, respectively. In Yiwu City, there is an industrial water plant catering to industrial needs, with lower water quality requirements compared with domestic water. Therefore, the carbon emission intensity of the general water subsystem is lower than that for domestic users. For agricultural and ecological users, according to the system generalization results, the urban water circulation pathways only involve the water intake stage. Thus, the carbon emission intensities for high-quality water, general water, and reclaimed water are 0.15 kgCO_2eq/m³, 0.12 kgCO_2eq/m³, and 0.12 kgCO_2eq/m³, respectively.

4.3. Analysis of Carbon Emission Reduction Potential

4.3.1. Scenario Setting

Scenario exploration is crucial for analyzing carbon emission reductions in urban water systems under different operational modes. In this study, three different water system operation scenarios were primarily set as follows:

(1): Baseline scenario. The baseline scenario mainly predicts the carbon emissions from urban water systems in 2030 based on the development trends of water resource utilization intensity in water intake, water supply, and wastewater treatment stages under the natural operation state of the study area’s urban water system over the years. Based on the development indicators of the population, industry, agriculture, and other sectors at different levels in the study area, as well as water consumption quotas, and using statistical data on total water consumption, water supply, and water usage over the years, the water demand for each sector in 2030 is predicted (as shown in Table 3). The total urban water demand can reach 3.92 × 10⁸ m³. If the water supply pattern in 2030 remains basically the same as the current situation. Among them, all of the domestic water supply will be provided by high-quality water sources (reservoir water and water from outside the area), 17% of the industrial water supply will come from river water, agricultural water supply will be provided by low-grade water, and the ecological water supply will be provided by reclaimed water. The expansion of urban scale and population growth in the future determines the increase in water consumption and wastewater discharge for production and daily life. Therefore, based on the proportional relationship between water supply and wastewater treatment volume in 2021, the projected wastewater treatment volume for 2030 is estimated to be 2.66 × 10⁸ m³. The energy intensity of wastewater treatment (0.32 kWh/m³) and reclaimed water treatment (1.14 kWh/m³) is assumed to remain at the 2021 level.

(2): Moderate low-carbon scenario. Based on the Yiwu City water resources planning document, the substitution rate of low-quality water is expected to increase for various sectors, and the utilization rate of unconventional water resources in the urban area is expected to reach 30% by 2030. Assuming that the water supply volume in Yiwu City remains unchanged according to the baseline scenario in 2030, with the upgrading of future raw water, wastewater, and reclaimed water treatment equipment and technology, the energy intensity of the water production process is expected to decrease to 0.29 kWh/m3, the energy intensity of wastewater treatment can reach the average level of cities in East China (0.22 kWh/m³) [28], and the energy intensity of reclaimed water reuse can reach the typical level of municipal water treatment in the United States (0.43 kWh/m³) [29].
(3): Highly low-carbon scenario. This scenario is mainly set based on the moderate low-carbon scenario. It assumes that the actual water consumption in the study area in 2030 exceeds the annual total control target by 19%, and the water supply network leakage rate can reach the domestic average level (7.6%). For the wastewater treatment process, further optimization of the wastewater treatment equipment is assumed, leading to a decrease in the energy intensity of wastewater treatment to the average level of cities in South China (0.194 kWh/m³) [28].

4.3.2. Comprehensive Evaluation

According to the baseline scenario, the predicted carbon emissions of the water system in the study area in 2030 are 29.65 × 10⁴ t, which is an increase of 9.93 × 10⁴ t compared with 2021 (Figure 6). The main reason is the continuous expansion of urban-scale and socioeconomic development in the study area in recent years, which has led to a year-by-year increase in urban water demand. In addition, in 2023, Yiwu City was selected as one of the first batch of national typical areas for pilot projects in reclaimed water utilization, and the amount of reclaimed water reuse has been increasing year by year. The utilization of unconventional water has risen from 1.91 million cubic meters in the base year to 43.49 million cubic meters. At the same time, the energy intensity of the water supply process and reclaimed water utilization process is relatively high, leading to a year-on-year increase in carbon emissions from the urban water system. In terms of carbon emission growth rate, apart from the reclaimed water utilization process, both the water supply and wastewater sectors in Yiwu City are experiencing rapid growth. In the baseline scenario, the carbon emissions from these two systems in 2030 are approximately 1.5 times and 1.4 times higher than those in 2021, respectively. The significant increase in water supply volume and reclaimed water reuse leads to the growth in carbon emissions in these two sectors. As for the rapid growth in carbon emissions in the wastewater sector, it is mainly due to the fast annual increase in urban sewage discharge volume in Yiwu City, coupled with a continuous rise in the urban sewage treatment rate.

In 2021, the current supply of high-quality water in the study area was 2.79 × 10⁸ m³, and according to the predicted trend in each industry sector (baseline scenario), the total urban supply is projected to reach 3.92 × 10⁸ m³ by 2030. Furthermore, in 2023, as one of the first selected national pilot cities for reclaimed water allocation in China, the study area witnessed a gradual increase in the volume of reclaimed water reuse. In 2021, the volume of reclaimed water treatment and reuse was relatively low, reaching only 1.91 million m³. With the expansion of reclaimed water reuse initiatives, by 2030 (baseline scenario), the urban unconventional water utilization is projected to reach 43.49 million m³. At the same time, the high unit energy intensity factors in these two links together will lead to a year-by-year increase in carbon emissions from urban water systems. Furthermore, in terms of the carbon emission growth rate, the supply, drainage, and reclaimed water utilization sectors in the study area have shown a faster growth rate. The carbon emissions from these two systems in 2030 (baseline scenario) are projected to be approximately 1.5 times and 1.4 times the levels of 2021, respectively. Among them, the reclaimed water utilization sector has the fastest growth rate of carbon emissions, followed by the water supply and drainage sectors. In addition to the significant increase in water supply and reclaimed water reuse, the rapid growth of carbon emissions in the drainage sector is due to the annual increase in urban wastewater discharge and the continuous improvement of urban wastewater treatment rates.

In the future operation of urban water systems, a series of water-saving and energy-saving measures can be implemented to achieve carbon emission reduction scenarios for water systems. In 2030, the carbon emissions from urban water systems under the moderate low-carbon scenario and the highly low-carbon scenario are projected to be 23.62 × 10⁴ tons and 18.31 × 10⁴ tons, respectively. Moreover, the carbon emission reductions compared with the baseline scenario are 6.03 × 10⁴ tons and 11.34 × 10⁴ tons, respectively. The highly low-carbon scenario has greater potential for carbon emission reduction in the operation of urban water systems, with carbon emissions even lower than the total carbon emissions generated by urban water systems in 2021 (20.48 × 10⁴ tons). By further strengthening planning and policy constraints on the development and utilization of urban water and energy resources, focusing on improving the efficiency and technological level of water and energy utilization in various stages of urban water system operation, particularly emphasizing reclaimed water treatment technology and efficiency enhancement, the carbon emission reduction effect of the water system becomes more pronounced.

Table 4 presents the carbon emission reduction and contribution in different sectors under different scenarios. From the table, it can be observed that the water supply sector has the greatest carbon emission reduction potential in Yiwu City’s water system. Compared with the baseline scenario, the moderate low-carbon scenario and highly low-carbon scenario contribute to carbon emission reductions of 41.5% and 42.0%, respectively. In the water supply link, the moderate low-carbon scenario reduces energy intensity, while the highly low-carbon scenario decreases the total water supply volume. The carbon emission reduction potential in the water withdrawal phase is the smallest, which aligns with the previous analysis results, indicating that large-scale water source projects do not significantly contribute to carbon emissions. Due to the significantly higher energy intensity in the reclaimed water treatment process compared with other phases, the carbon emissions increase as the rate of reclaimed water utilization rises. According to the table, the main reason for the significant carbon emission reduction potential of the urban water system operation mode in the highly low-carbon scenario is, on the one hand, due to the water quantity factor. Due to the implementation of water-saving measures and the promotion of reclaimed water reuse, the urban water system has significantly reduced its intake of fresh water, resulting in a decrease in water consumption in subsequent stages such as water supply from water plants and wastewater discharge from treatment plants. On the other hand, it is due to the energy consumption factor. With significant improvements in water treatment processes for water plants, wastewater treatment processes, and reclaimed water treatment processes, the energy intensity of these previously high-energy-consuming stages has decreased, resulting in a reduction in carbon emissions.

4.4. Rationality Analysis

During the operational phase of the urban water system in the study area, there are significant differences in carbon emissions intensity among different links. The ranking of carbon emission intensity from high to low is as follows: water reuse > drainage > raw water treatment > tap water distribution > water diversion > water storage > river water lifting. Among them, the energy consumption of the water reuse process is the highest, at 0.90 kgCO_2eq/t, which is much higher than that of other processes (Figure 7).

Currently, there is relatively limited direct research on the “water–carbon” nexus, with more research results in China focusing on the “water–energy” relationship. Energy intensity is identified as a crucial factor influencing carbon emissions across different sections of urban water systems. In comparison with existing studies, the energy intensity values determined in this study fall within the range of values reported in previous research [30]. In a study by Yu et al. [26] that analyzed the water system of Zhengzhou City, China, in terms of “water–energy–carbon”, significant differences were noted in the water withdrawal process (attributed to most water sources in Zhengzhou City being groundwater, leading to higher energy consumption during groundwater extraction), while the energy intensity in other sections was similar to that of this study. Zhu et al. [5] investigated the overall energy consumption in the social water cycle in the Beijing–Tianjin–Hebei region, showing similarities in the energy consumption ratio between the water withdrawal and supply phases compared to this study. Xiang [23] conducted a study on the “water–energy” relationship in the existing social water cycle in China, with energy intensity values for each phase closely aligning with those in this study. He [31] researched energy consumption in the social water cycle process in Beijing, with energy intensity values in most phases similar to those in this study, except for slightly lower energy consumption in reclaimed water treatment due to advanced processes. Therefore, the energy intensity values determined in this study for the operational phases fall within a reasonable range.