Socio-Economic and Environmental Effects of Difference Scale in Sewage Sludge Recycling System Considering Ecological Value in China

Abstract

With China’s ongoing economic development and increasing emphasis on environmental protection, the number of sewage plants is increasing annually and sludge production is increasing. This study analyzed the scale effect of sludge treatment and recycling systems (STRS) with different technologies (i.e., incineration, aerobic composting, used in material, anaerobic digestion) on the performance of environment an socio-economy by life cycle assessment (LCA) and life cycle cost (LCC). The LCA results showed that aerobic composting had a better impact on climate change (−1.53E−03 kg CO₂ eq/p/yr) than other methods, while the whole life cost (WLC) results showed the scenario of using in material had the less cost in four scenarios. Moreover, the socio-economic impact of introducing a carbon ecological compensation mechanism into the STRS to achieve carbon neutrality was analyzed taking Tianjin as an example. In the future, it is recommended to consider and incorporate the environmentally friendly impacts of STRS with various scales into the carbon ecological compensation mechanism.

1. Introduction

The acceleration of economic development and urbanization in China had resulted in a significant increase in the production of dry sludge, reaching 13.69 million tons by 2022, as illustrated in Figure 1. Over the past five years, the disposal ratio had been above 95% on average [1]. The issue of sludge disposal had emerged as a significant challenge in the context of wastewater treatment plants and municipal waste management. In contrast, the philosophy of the “circular economy” viewed waste-activated sludge (WAS) as a renewable resource with high levels of organic matter and nutrients [2]. The STRS system could generate a variety of energy forms and products, which had the potential to avoid greenhouse gas (GHG) emissions [3, 4]. Examples of these included electricity, biogas [5, 6], fertilizer [7, 8], and building materials [9, 10]. It was evident that the current treatment and disposal management of waste activated sludge (WAS) in China lacked a comprehensive and integrated plan that simultaneously considers environmental protection, economic viability, and social acceptance. Consequently, the establishment of a reliable and holistic assessment methodology that encompasses the overall performance of the STRS system was of paramount importance. This would ensure the optimal utilization of WAS, thus reducing its environmental impact, while also ensuring the financial viability of the system and the social acceptability of its operations.

Figure 1.

The situation of sewage sludge produced and treated proportion in China.

In September 2020, China announced at the United Nations General Assembly that it would strive to achieve “carbon peak” by 2030 and “carbon neutrality” by 2060. It was widely acknowledged that the increase in GHG emissions resulting from human activities was the primary driver of global warming. The sector of various waste treatment, such as household waste and wastewater, was contributed 1.7% to total GHG emission of China in 2020, achieved 209.5 billion tones included wastewater treatment. The wastewater treatment had been identified as a significant source of anthropogenic GHGs in the urban.

Wastewater treatment plants (WTTPs) have the capacity to generate carbon dioxide (CO₂), methane (CH₄), and and nitrous oxide (N₂O) through a variety of chemical and biological processes, as well as energy production and combustion. The GHG emission boundaries of wastewater treatment systems had not been clearly defined. Previous research had overlooked the GHG emissions associated with sludge transportation and treatment, which can contribute up to 40% of the total emissions from WTTPs. Due to the vast size of China and the significant disparities in economic development, technological advancement, and industrial structure among its provinces, there were notable variations in sludge disposal strategies, as illustrated in Figures 2 and 3. Consequently, the challenge of reducing carbon emissions in sewage treatment systems varied considerably. It was imperative to develop tailored sludge disposal plans specific to each region.

Figure 2.

The dry sludge produced of each province in China.

Figure 3.

The gross regional product of each province in China.

In the preceding studies, LCA and LCC had been employed extensively in order to evaluate the environmental and economic impacts of various sludge management schemes [11, 12, 13, 14, 15]. However, the combined environmental and economic impacts of the WASR system when considering alternative production scenarios had yet to be fully elucidated in the existing literature. To address these limitations and meet the practical needs of the industry, we employed whole life costing (WLC) in accordance with the guidelines set forth in BSI ISO 15686-5 (2008). This approach integrated the two aspects by including the external cost, which represented the monetized value of environmental pollutants.

Accordingly, the objective of this study was to integrate the environmental and economic evaluation models of four WASR systems in China via LCA and LCC methods and to propose future optimization scenarios. The study also determined the main contribution of pollutants to external costs, which marked the potential social damages caused by pollutant discharge for STRS management. In order to achieve the carbon peak target as soon as possible, and in consideration of the variations in economic development among regions in China, this study calculated the cost of the sludge management scheme to address regional disparities through the carbon ecological compensation mechanism.

2. Methodology

2.1 Life cycle assessment and life cycle cost

2.1.1 Goal and scope definition

The objective of this research was to evaluate the environmental and economic efficiency of four prevalent technologies for treating and recycling WAS in China and to analyze them under various policy scenarios. The study focused on processing 1 ton of WAS with 80% moisture content as the functional unit. The system encompassed transportation, drying, pretreatment, production, and air pollution control measures. The outputs from the STRS, such as electricity, fertilizer, bricks, and biogas, were considered as substitutes for conventional products in the market. System expansion was implemented to prevent the allocation of any by-products during the process (Figure 4).

Figure 4.

System boundaries of four scenarios of WAS recycling system.

2.1.2 Data source and life cycle inventory

The data inventory for various processes such as incineration, aerobic composting, brick production, and anaerobic digestion were gathered from enterprises via environmental impact assessment (EIA) reports. The life cycle inventories of the original main products were derived from the Chinese Life Cycle Database by software Ebalance and previous literature [16, 17, 18, 19]. Factors such as socioeconomic conditions, technology efficiency, and the quality of WAS could significantly impact both environmental and economic performance. Therefore, a case-by-case evaluation of each WAS from wastewater treatment plants was necessary [14]. To facilitate a comparative analysis, it was assumed that the quality of WAS adheres to the standard GB 24188-2009, and regional differences did not substantially influence the evaluation outcomes in this study.

Economic data regarding budget costs were sourced from market research and EIA reports of individual enterprises. Operational costs considered in this study encompassed expenses related to transportation, raw materials, energy consumption, and labor. Sales incomes were inclusive of all revenues generated from the sale of final products. Prices of final products and raw materials were obtained from publicly available sources, reflecting their market rates in 2019. The lack of a standardized WAS disposal subsidy across Chinese cities led this study to adopt the subsidy amount in Chongqing (205 CNY/t). The assumed distance for transportation between each technology was set at 100 km.

The application of the external cost valuation method from Japan to China was deemed unfeasible due to differing economic development stages and environmental perspectives [20]. Nonetheless, the external costs were determined based on the Environmental Protection Law in China and CREO 2017. In cases where data on external costs were unavailable, the monetization value of per unit total emissions was referenced from prior studies [16].

2.1.3 Environmental and economic evaluation

This research assessed and measured the environmental performance of WASR through a Life Cycle Assessment (LCA) using the ReCipe 2008 [14, 21]. LCA was chosen as the preferred approach due to its comprehensive coverage of potential environmental impacts [22]. The study focused on quantifying the total environmental impact of climate change (EPtotal) resulting from both direct and indirect greenhouse gas emissions during the operation of the WASR system. Additionally, the avoided environmental impact of climate change (EPavoid) was determined, which represented the environmental impact of products replacing an equivalent number of traditional products. The net environmental impact of climate change (EPnet) was calculated to provide a more accurate assessment of the true environmental impact of each scenario.

$${\mathrm{EP}}_{\mathrm{net}}={\mathrm{EP}}_{\text{total}}-{\mathrm{EP}}_{\text{avoid}}$$E1

The calculation of the life cycle cost (LCC) was conducted and evaluated based on the net present value, as Eq. (2). The cash inflow (C_I) represented the total amount of monetary inflows in the initial year, including the WAS subsidy, revenue from final product sales, and income from carbon compensation in each scenario. Conversely, the cash outflow (C_O) encompassed the total amount of monetary outflows in the initial year, comprising capital expenditures, operational expenses, and tax payments [15].

$$\mathrm{LCC}={\mathrm{C}}_{\mathrm{I}}-{\mathrm{C}}_{\mathrm{O}}$$E2

The WLC analyzed LCC and external cost considerations to evaluate the comparative performance of different scenarios. The study applied the concept of “true cost” as outlined in the BSI ISO 15686-5 standard. The external costs for each scenario were calculated using the Ecotax 2002 method in Sweden, as described in reference [23, 24]. This involved multiplying the quantity of emissions k (${\mathrm{e}}_{\mathrm{j},\mathrm{k}}$) by the unit price of emissions k (${\mathrm{P}}_{\mathrm{k}}$) for each scenario. The currency exchange rate utilized in the research was 1 USD = 6.804 RMB.

$${\text{Externality cost}}_{\mathrm{j}}={\sum }_{\mathrm{j}=0}^4{\mathrm{t}}_{\mathrm{j}}·\sum _{\mathrm{j},\mathrm{k}}{\mathrm{e}}_{\mathrm{j},\mathrm{k}}·{\mathrm{P}}_{\mathrm{k}}$$E3

The external costs in each scenario primarily were considered the financial damages caused by water quality, human health, climate change, and indeterminate issues [20, 23, 24]. The external costs related to climate change involve the quantified worth of CO₂ and CH₄.

2.2 The effect of implement scale of sewage sludge recycling system

Previous studies had found that the correlation between the implementation scope, ecological emissions, and financial expenditures of STRS were consistent with the least squares method. By conducting a cost-benefit analysis that took into account the impact of the implementation scale, the most effective system and cost-balanced scale could be determined. Following an in-depth examination, it was found that the correlation between environmental emissions and economic expenditures was closely related to the implementation scope. This relationship could be measured by a power function, as shown in Table 1 [25].

Scenario	The implement scale of baseline	Unit cost	Unit GHG emission
Incineration	200 t/d	0.438	0.476
Aerobic composting	400 t/d	0.515	0.350
Used in brick	15,000 t/a; (60% water content)	0.606	0.595
Anaerobic digestion	50 t/d	0.576	0.479

Table 1.

The implement scale of STRS in economic and GHG emission sector.

The environmental performance by climate change of each province was calculated by Eq. (4).

$$\mathrm{EP}=\sum \frac{{\mathit{EP}}_{i,\mathit{\text{base}}}}{{\left(\frac{S_{\mathit{\text{base}}}}{S}\right)}^{x_{\mathit{GHG}}}}$$E4

In this context, $\mathrm{EP}$ referred to the environmental performance of each province in climate change. ${\mathrm{EP}}_{\mathrm{i},\text{base}}$ referred to the environmental performance of the baseline for scenario i in climate change. $S_{\mathit{\text{base}}}$ was the implementation scale of the baseline. $S$ was the implementation scale of the target scenario. $x_{\mathit{GHG}}$ was the scaling exponent of environmental performance for scenario i.

2.3 Carbon ecological compensation mechanism

Carbon ecological compensation, a new area of ecological compensation research, aims to adjust the interest dynamics between ecological protection areas and beneficiary regions by facilitating the flow of funds between them. This approach helps narrow the development gap between regions, alleviate financial pressures on ecological protection areas to some extent, and ultimately achieve ecological justice. It has emerged as a crucial strategy for numerous countries worldwide to address climate change and promote coordinated regional development. In carbon ecological compensation, the central government is responsible for establishing a horizontal carbon ecological compensation mechanism. The primary objective is to promote the coordinated development of the regional economy and ecological environment, as well as to incentivize and guide provinces in utilizing sludge resources. The provincial governments aim to optimize the environmental and economic benefits of the sludge treatment system within the region by leveraging existing resources under the horizontal carbon ecological compensation mechanism. Assuming that the forest carbon sink in the region, shown in Table 2, serves as the baseline emission, if the carbon emissions from the sludge treatment system exceed the baseline emission, carbon credits must be purchased. Conversely, if the carbon emissions are lower than the baseline, carbon credits will be earned.

Province	Forest carbon sequestration	Province	Forest carbon sequestration
Beijing	217.74	Henan	853.55
Tianjin	38.60	Hubei	1912.75
Hebei	657.60	Hunan	2045.85
Shanxi	866.41	Guangdong	2847.40
Inner Mongolia	4137.13	Guangxi	4313.57
Liaoning	1139.17	Hainan	1522.92
Jilin	2046.77	Chongqing	1616.11
Heilongjiang	5162.03	Sichuan	4547.38
Shanghai	65.82	Guizhou	2335.63
Jiangsu	266.04	Yunnan	5961.94
Zhejiang	1658.99	Shaanxi	1994.37
Anhui	1027.61	Gansu	1003.75
Fujian	3020.66	Qinghai	155.81
Jiangxi	2440.29	Ningxia	55.28
Shandong	157.50	Xinjiang	1809.94

Table 2.

Forest carbon sequestration by province in China in 2017 (unit: 10⁴ tone).

3. Result and discussion

In terms of climate change for scenario 1, CO₂ contributed impact of climate change by about 97.51%. The main source is energy consumption, of which low calorific value and high water content are the main influences. The finding of this analysis is in alignment with those of Guo et al. [26]. Compared with aerobic composting, the method of incineration consumed more energy to reduce the water content. Energy consumtion was the main contributor of climate change and operation cost. According to the results of the climate change in Table 3, scenario 2 has a favorable effect on the environment. The reduction in N₂O is the primary factor contributing to the favorable environmental impact of scenario 2 in climate change. The process of making mineral fertilizers releases N₂O, which has a significant impact on climate change. Compared to the manufacture of mineral fertilizer, scenario 2 does not result in the generation of N₂O. Analysis of the components of climate change indicates that CO₂ is the primary pollutant responsible for climate change. The primary sources of CO₂ are energy use and the amount of organic matter present. On the one hand, the sludge’s organic matter level needs to be decreased. Sludge treatment and recycling, on the other hand, ought to prioritize energy conservation. The main influencing constituents of climate change in scenario 4 are CO₂ (94.27%) as same as in scenario 1.

Category	Unit	S1	S2	S3	S4
Climate Change	kg CO2 eq/p/yr	4.94E−03	−1.53E−03	6.96E−04	7.41E−05

Table 3.

Global warming impact of each scenario for STRS.

The primary expenses of the system were further examined to offer recommendations for enhancement, as depicted in Figure 5. The findings reveal that the primary cost associated with the system is the construction cost as shown in Figure 6. Within these expenses, transportation costs constitute over 56% of the operational expenses. Therefore, it is advisable for future management of the WAS system to strategically plan the reuse location and optimize transportation routes. In scenarios 2 and 3, in addition to transportation costs, water expenses play a significant role, with the increasing moisture content of WAS upon exiting the sewage treatment plant necessitating attention. It is crucial to strike a balance between transportation and water costs in future management, as the volume is correlated with the moisture content of WAS, and transportation expenses escalate with the volume of WAS. In alternative scenarios, apart from transportation costs, energy expenditures such as electricity, natural gas, and coal also serve as major cost contributors, and their financial performance can be improved by enhancing energy efficiency.

Figure 5.

The WLC of each scenario for STRS.

Figure 6.

The optional cost of each scenario for STRS.

The government promotes the use of sludge as a resource, which can generate job opportunities for citizens and boost public environmental consciousness by involving them in environmental protection. Large-scale recycling systems have the potential to contribute significantly to job creation in the waste management sector. By employing a larger workforce to manage and operate these systems, job opportunities are created, thereby stimulating economic growth. On the other hand, small-scale systems promote local entrepreneurship and community involvement by encouraging individuals or small businesses to engage in waste recycling activities. These systems not only empower local communities but also foster a sense of environmental responsibility. Additionally, medium-scale systems can enhance regional economic development through resource recovery projects that generate revenue and promote sustainable practices.

Taking Tianjin as an example, there were 12 sludge treatment plants in 2021, including facilities for industrial sludge treatment. The dry sewage sludge produced in Tianjin was 170,212 tons. The environment performance of climate change of total sludge treatment plants in Tianjin was −0.021 tCO₂ eq calculated by Eq. (4) as depicted in Figure 7, P1, P4, P10, and P12 exhibited negative environmental impacts. The main reason is that these four treatment plants use aerobic composting technology with a daily processing capacity of more than 300 tons. This technology replaces traditional fertilizers with sludge, significantly reducing greenhouse gas emissions from the entire system.

Figure 7.

The effect of climate change of each sludge treatment plants in Tianjin.

This research utilized the mean transaction price of carbon emission rights in the national carbon trading market as the benchmark for carbon compensation, a common practice in contemporary studies on carbon ecological compensation. In 2022, the average annual transaction price in the national carbon trading market stood at a mere 55.3 yuan/t. For instance, in the case of Tianjin’s sludge resource utilization, 2135.7 yuan in subsidies was obtained through the reduction of carbon emissions over the course of a year, which significantly undervalues the worth of carbon. The persistent setting of ecological compensation standards at a low level could have adverse effects, such as dampening the drive for ecological development in carbon compensation regions and potentially leading carbon-paying regions to view it as a cost-effective production approach. This could diminish incentives for emission reduction and impede the establishment of effective constraints on carbon-paying regions.

In the future, it is recommended to consider and incorporate the environmentally friendly impacts of STRS with various scales into the carbon ecological compensation mechanism. For example, large-scale systems play a crucial role in decreasing greenhouse gas emissions by effectively handling organic waste and harnessing methane for energy generation. This not only helps combat climate change but also aids in the production of renewable energy. Conversely, small-scale systems reduce pollution related to transportation by treating waste on-site, thereby lessening the carbon footprint linked to waste transportation. Additionally, medium-scale systems support biodiversity and soil health by incorporating ecological principles into their recycling methods, such as utilizing compost as a soil enhancer to enhance soil fertility and structure.

4. Conclusion

Considering the ecological value in sewage sludge recycling systems is essential for promoting sustainability and resilience in waste management practices. Implementing circular economy principles, such as recycling and reusing resources, can maximize resource efficiency and minimize waste generation in these systems. Furthermore, incorporating ecological design elements, such as green infrastructure and nature-based solutions, can enhance ecosystem services and resilience in recycling processes. Establishing partnerships between government, industries, and local communities is crucial for promoting sustainable sewage sludge management practices that prioritize ecological value, ensuring a harmonious balance between socio-economic development and environmental conservation. In conclusion, the scale at which sewage sludge recycling systems operate in China has far-reaching socio-economic, environmental, and ecological implications. By understanding and harnessing the socio-economic benefits of different scales, mitigating environmental impacts, and prioritizing ecological value, China can build a more sustainable and resilient sewage sludge management system that promotes long-term prosperity and environmental health. It is imperative for stakeholders to work together to implement innovative solutions that address the complex challenges posed by sewage sludge management, ensuring a greener and more sustainable future for all.

Conflict of interest

The authors declare no conflict of interest.