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Article

Comparison and Selection of Wet Waste Disposal Modes for Villages in Agriculture-Related Towns Taking Shanghai, China, as an Example

by
Yiyao Ni
and
Zhen Zhang
*
Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(12), 4936; https://doi.org/10.3390/su16124936
Submission received: 30 March 2024 / Revised: 20 May 2024 / Accepted: 4 June 2024 / Published: 8 June 2024
(This article belongs to the Special Issue Critical Issue on Waste Management for Environmental Sustainability)
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Abstract

:
Under the increasingly stringent regulatory constraints, wet waste disposal in agriculture-related towns has become a significant issue. Based on fieldwork in three agriculture-related towns and nine villages located in Shanghai, this paper analyzes the economics and greenhouse gas emissions of three wet waste disposal methods, including centralized disposal, on-site disposal by biogas digesters, and on-site disposal by small-scale biochemical processors, and compares these alternatives by combining various factors, aiming to help decision makers in selecting the optimal alternative. The net present value approach was employed to evaluate the economic aspects of the three disposal methods. The greenhouse gas emissions were calculated using life cycle assessment, life cycle inventory, and the IPCC guidelines. The fuzzy analytic hierarchy process method was employed for a multi-criteria decision-making analysis based on five factors, including economics, greenhouse gas emission control, stability, compliance with environmental regulations, and location feasibility. The results revealed that although centralized disposal is not as good as on-site disposal by biogas digesters in terms of economics and greenhouse gas emissions, centralized disposal is still the optimal alternative according to the results of multi-criteria decision making. The tightening of regulatory constraints will internalize the negative externalities of on-site wet waste disposal. Thus, centralized disposal is the method of wet waste disposal that is most consistent with the regulatory constraints and most sustainable. It is worth emphasizing that policymakers should fully assess the impact of tightening regulations on the selection of wet waste disposal methods, establish a homogenous waste disposal system covering both urban and rural areas, and enhance the management of existing on-site disposal facilities.

1. Introduction

Shanghai was one of the first cities in China to launch a trial project for the separate collection of municipal solid waste, and it also pioneered the disposal of wet waste in rural areas. The term “wet waste” is also known as organic waste and easily perishable biomass household waste, including food, leftovers, expired food, fruit and vegetable waste, flowers, and plants [1]. Early policies encouraged on-site resource utilization of wet waste in rural areas, while, in recent years, policies have begun to require an increase in the proportion of centralized disposal. As early as around 2011, some townships in Shanghai started to pilot the waste management model involving the natural fermentation of wet waste and the return of organic fertilizer to the fields. The implementation of consecutive policies has improved the functionality and standardization of on-site wet waste disposal while also offering specific financial benefits. The policy was transformed in 2021 when the “Task List of the Shanghai Rural Habitat Environment Optimization Project” put forth the proposal that “the wet waste disposal in over 45% of the administrative villages should be centralized”. Thereafter, the percentage rose to 50% and 55% in 2022 and 2023, respectively. From a policy perspective, the disposal of wet waste in Shanghai is demonstrating a trend toward urban–rural integration. It is crucial for the government and society to carefully reassess and deliberate on the optimal method of rural wet waste disposal in light of a variety of factors.
Rural wet waste can be disposed of through on-site disposal, centralized disposal, and co-disposal. Most of the studies supporting on-site disposal of rural wet waste are motivated by considerations of reducing transportation costs and the treatment costs of landfills and incineration plants [2,3,4,5,6,7]. A variety of on-site wet waste disposal methods are being piloted and applied in rural areas in China [8,9,10]. Aerobic composting and anaerobic digestion are two common technologies employed for on-site disposal. Aerobic composting can be performed naturally in the open air or in biochemical processors. Anaerobic digestion is usually carried out in biogas digesters. There are fewer studies in the literature on the centralized disposal of rural wet waste. These studies suggest that centralized disposal has a scaling effect and can reduce unit disposal costs [11,12]. The literature on the co-disposal of household wet waste and agricultural organic waste is currently focused on case studies [13,14,15], and there is no large-scale application of mature technologies for the time being. In practical applications, fertilizer made from waste is utilized on cropland [16]. However, the production of organic fertilizer from wet waste lacks applicable safety standards and regulations at the national level. According to Organic Fertilizer (NY/T525-2021), wet waste belongs to the assessment class of raw materials, which must pass a safety evaluation before they can be used [17]. The literature advocating the co-disposal of wet waste with agricultural organic waste ignores the safety of wet waste as a raw material for organic fertilizers.
Each treatment method has its unique benefits and drawbacks, and the government is now conducting experiments without reaching a definitive conclusion on the optimal choice among these alternatives. Therefore, it is necessary to study the main indicators that influence decision makers’ choices of wet waste disposal methods and to make a full comparison of the alternatives from a variety of perspectives.
The economic factor is the basic consideration for decision makers when choosing solid waste disposal methods. Domestic studies have calculated and analyzed the cost of using biochemical processors to treat wet waste [18,19,20]. Due to the different research objects, the cost varies between 215 and 2462 CNY/ton. Some studies carried out cost–benefit analysis and financial life cycle analysis on a variety of solid waste disposal methods. Some scholars [21,22,23,24] established economic models using the cost–benefit method to calculate costs under different scenarios in order to obtain the most sustainable method with the highest social net present value and the lowest economic cost. On the basis of LCC, some scholars [25,26] extended the conventional LCC or financial LCC concept, according to which only the internal cost of research objects is calculated, excluding the external environmental cost. Carlsson calculated the present value of the cost of different solid waste disposal methods with the financial LCC method and believed that the value of fertilizer made from wet waste was difficult to determine unless it was marketized enough [25]. In these cost analyses, little consideration has been given to the internalization and visualization of external costs due to the tightening of environmental regulatory constraints. In addition, few studies have compared the cost difference between centralized disposal and on-site disposal for rural areas.
Aside from the economy, we must also take into account other environmental considerations. Currently, there is a global concern about greenhouse gas (GHG) emissions. As a negative externality, GHG emissions from wet waste disposal have environmental impacts. “China’s Policies and Actions to Address Climate Change (2018)” suggests regulating GHG emissions in waste disposal activities as a means to reduce atmospheric pollution. A great deal of research has been carried out on GHG emissions from wet waste disposal, and life cycle assessment (LCA) and life cycle inventory (LCI) are the most commonly used methods [27,28,29,30,31,32,33]. Given the tightening regulatory constraints and the global emphasis on GHG emissions, it is reasonable to assume that GHG emissions could potentially be an influencing factor in the control of solid waste disposal in the future. Therefore, this paper attempts to use GHG emissions as an evaluation criterion of wet waste disposal methods to reflect the social costs that have been made internal by regulatory constraints.
Based on fieldwork, other factors, such as stability, compliance with environmental regulations, and location feasibility, are considered by local government when selecting a wet waste disposal method. Therefore, it is necessary to use multi-criteria decision-making (MCDM) analysis to choose the optimal alternative through a comprehensive evaluation [34]. In the literature, MCDM is widely used to compare different solid waste treatment methods [35,36,37,38,39].
The majority of current research focuses on studying solid waste disposal at the city-wide level, whereas just a few studies examine rural areas. Limited research has been devoted to a comprehensive comparison of centralized and on-site wet waste disposal in the same rural region. Due to limited resources and information, rural areas tend to have low risk tolerance in terms of capital investment and other factors. Consequently, rural areas face challenges in selecting an optimal method for wet waste disposal that is economically viable, environmentally sustainable, technically feasible, and compliant with regulatory constraints.
The rural wet waste management problems faced by villages in agriculture-related towns reflect the complexity and rising costs of rural wet waste management, and the impact of regulatory constraints on disposal methods. In this research, the term “agriculture-related towns” refers to towns that have pure agricultural areas inside their administrative areas. According to the Shanghai Municipal Government, pure agricultural areas are defined as locations where the percentage of permanent basic cropland in the administrative area of a township or village exceeds 20% or 25%, respectively [40]. In pure agricultural areas, agriculture is the main industry, and they are responsible for producing over 80% of Shanghai’s agricultural products. Pure agricultural townships have an average total financial income that is only 43% of the city’s average.
Compared to other villages, villages in agriculture-related towns have a higher proportion of permanent basic cropland, which limits their economic development due to land use control. As a result, their economic level is not as high as that of other villages. This puts pressure on their financial expenditure for waste management. Additionally, these villages also serve as the city’s “rice bag and vegetable basket”, necessitating the protection of cropland, soil quality, and agricultural product quality. In the past, agriculture-related towns had the advantage of being able to utilize composted products in their fields, hence reducing the cost of centralized wet waste disposal. At that time, compared with other towns, it was easier to deal with the wet waste disposal problem in agriculture-related towns. However, with the increasing standards regarding the quality of cropland and fertilizers for agricultural use, it is non-compliant with regulations to use fertilizer without standardized treatment and quality certification as, without these checks, it may cause heavy metal pollution and soil salinization [41,42] and affect the quality of cropland and agricultural products. Therefore, under the control of regulatory constraints, the on-site disposal of wet waste in villages of agriculture-related towns faces challenges, such as difficulties in location selection issues, strict sewage disposal regulations, and restrictions on the utilization of fertilizer made from wet waste.
In conclusion, in the context of the new era of urban–rural integration and rural revitalization, with increasingly stringent regulatory constraints, there is a need for a more in-depth study of the optimal method for rural wet waste disposal.
The aim of this paper was to establish a scientific foundation for further policymaking related to the wet waste disposal systems in villages of agriculture-related towns. Based on fieldwork, this paper focused on nine villages in three agriculture-related towns in Shanghai. This study compared three methods implemented in these villages, including centralized disposal, on-site disposal by biogas digesters, and on-site disposal by small-scale biochemical processors. An analysis of multi-criteria decision making was implemented based on five criteria, including economics, GHG emission control, stability, compliance with environmental regulations, and location feasibility. The alternatives were compared to find the most sustainable one for villages in agriculture-related towns.

2. Materials and Methods

2.1. Research Subject

This paper focuses on three agriculture-related towns and nine administrative villages located in District S of Shanghai. Shanghai is a megacity with nine agriculture-related districts, one of which is District S. District S is located in the southeastern region of Shanghai. The locations of the studied area are shown in Figure 1. These towns and villages were selected because they face challenges in wet waste disposal and have unique disposal methods. The typicality of the study object is reflected in the following aspects:
Firstly, the agriculture-related towns in District S have a lower level of economic development in contrast to the ones in urban regions, and they are facing more pressure to invest resources in rural domestic waste disposal.
Secondly, the agriculture-related towns in District S are located on the fringe of the administrative area. These towns have inadequate infrastructures and are far away from the centralized wet waste disposal facility in District S. As a result, they have to pay higher transportation costs for centralized disposal. Thus, it is essential to analyze if on-site wet waste disposal offers a distinct benefit over centralized disposal for these villages and towns. The information about the villages can be found in Appendix A (Table A1).
Thirdly, the rural areas in District S have two typical rural production and living styles. On the one hand, there are relatively remote villages where the villagers’ economic activities are based on cultivation, with small populations and dispersed settlements. On the other hand, there are also villages close to industrial zones, farmers’ markets, and market towns, where the settlements are relatively dense and the migrant population far exceeds the household registered population. The migrant workers rent farmers’ houses, resulting in higher non-agricultural economic opportunities and income levels for the villagers. The rural economy of District S is in a stage of rapid development. The population growth and economic development have resulted in the issue of domestic waste management in the rural communities of District S. Therefore, the rural wet waste issue in District S is a combination of rural and urban characteristics, which to a certain extent represents the problem of urban–rural integration and the development of urbanization.
Finally, over the years, the government of District S has been committed to exploring a more suitable mode of wet waste disposal in rural areas. A variety of methods have been tried in District S, both centralized and on-site disposal methods, including open brick composting ponds, closed glass fiber-reinforced plastic composting ponds, biochemical processors, biogas digesters, etc. The effectiveness of various methods is continuously evaluated and adjusted to align with policy changes and keep them optimized. Therefore, taking District S as the research object can reflect the advantages and shortcomings of various wet waste disposal methods in actual operation, which can help this paper summarize the decisive criteria for choosing wet waste disposal modes and provide a reference for other rural areas of the same type.
Comparative analysis has been conducted on three alternatives chosen from the wet waste disposal methods adopted in District S: centralized disposal, on-site disposal by biogas digesters, and on-site disposal by small-scale biochemical processors. After a long period of piloting various disposal methods, these three alternatives have emerged as the most effective and are currently being actively promoted and utilized. The centralized disposal plant utilizes the technological process of pre-treatment, wet anaerobic digestion, and biogas power generation. On-site disposal by biogas digesters employs anaerobic digestion technology to generate biogas, biogas slurry, and biogas residue. The biogas is ignited by an automated torch. On-site disposal by small-scale biochemical processors uses aerobic composting and high-temperature drying technology, resulting in no effluent generation.

2.2. Data Sources

Field surveys and in-depth interviews were conducted to gather data and information on equipment parameters, service conditions, power consumption, disposal processes, costs, and user attitudes and preferences. The Landscaping and City Appearance Administrative Bureau of District S and township and village authorities in District S provided support for the fieldwork. The industry data were derived from authoritative reports and standards. Data that were practically unattainable were acquired from peer-reviewed scientific studies and the default values set by the IPCC. Appendix A contains the comprehensive data and information utilized in the calculations and analyses (Table A2 and Table A3).

2.3. Research Methods

2.3.1. Economic Analysis

Wet waste disposal is a project that requires long-term investment. In order to reflect the time value of capital, the net present value (NPV) method is a more suitable method for this study. Calculating NPV is one of the common methods used in the economic analysis of solid waste disposal [21,25]. The larger the NPV, the better the investment program. For large-scale centralized solid waste disposal facilities, some studies will consider the discount rate, while the literature about on-site wet waste disposal rarely considers the discount rate [21,25,43].
The NPV analysis in this study is based on the following assumptions.
  • Firstly, in order to calculate true costs, this study assumes that there is no subsidy from the superior government.
  • Secondly, for simulation purposes, wet waste generation of 1 Mg·day−1 is used as the unit of calculation. In the fieldwork, the population of each village ranges from 700 to 9000. Wet waste generation in each village ranges from 0.45 to 3.6 Mg·day−1. Therefore, the assumption is reasonable.
  • Thirdly, indirect benefits from the fertilizers made from wet waste are not calculated. According to the fieldwork, the fertilizers from a small biochemical processor are directly taken by the equipment company regularly, and these fertilizers are not used in the village. Additionally, the biogas slurry from a biogas digester is not used in cropland because it is hard to remove salts and heavy metals without specific treatment. Meanwhile, fertilizers made from wet waste should be treated according to the relevant regulations and meet the standards before they can be sold as organic fertilizer products [17]. For the above reasons, this study disregards the indirect benefit of the fertilizers.
  • Finally, three scenarios are designed. To minimize the discrepancy between the analyzed results and the real-world conditions and to account for variations in equipment purchase prices, labor costs, and other factors, this study establishes three scenarios for each cost item. These scenarios represent the best, worst, and medium cost inputs, respectively.

2.3.2. Accounting of GHG Emissions

This paper analyzes and measures the GHG emissions from wet waste disposal by combining the LCA, LCI, and IPCC guidelines, which have been widely used and are highly applicable to this study. The determination of the system boundary of LCA in this study is based on fieldwork and a literature review. The system boundary includes transportation, wet waste treatment, utilization of by-products, and treatment of pollutants.
GHG emissions are categorized into direct emissions, indirect emissions, and carbon offsets. The types of direct emissions include leakage of CH4 and N2O, which are then transformed into CO2 equivalents in this study using the global warming potential over 100 years (GWP-100). Indirect emissions include GHG emissions from electricity consumption of the equipment used; fuel consumption; and chemical dosing, production, and transportation. Carbon offsets include the utilization of biodiesel and the electricity generated from biogas. Direct and indirect emissions are positive, and carbon offset is negative.

2.3.3. Multi-Criteria Decision Making

The process of MCDM for solid waste disposal typically involves three main steps: criteria selection, weight determination, and alternative ranking [35,36,37,38,39,44].
In this study, economics, GHG emission control, stability, compliance with environmental regulations, and location feasibility are the five main factors identified in the fieldwork as influencing the evaluation of wet waste disposal modes. The interpretations of stability, compliance with environmental regulations, and location feasibility are as follows.
Stability refers to whether wet waste disposal can be operated stably for a long period and effectively address unforeseen circumstances. For example, the equipment is not easily damaged and can withstand the increased volume of wet waste during the summer and holidays. The literature and field research indicate that small biochemical processors have a high failure rate. In such cases, the sanitation department will be required to transport the wet waste to the centralized disposal center. At present, the failure rate of biogas digesters is relatively low. However, District S, being the first district in Shanghai to test a 1 Mg·day−1 scale biogas digester, is less than five years old and thus unable to provide assurances regarding the long-term safety of biogas storage and automated ignition. The centralized disposal facility in District S is presently undergoing a phase II expansion. This expansion enables the facility to cope with challenges such as a significant rise in the volume of wet waste. Furthermore, its operations are overseen by a listed company, which ensures professionalism, stability, and safety.
Compliance with environmental regulations refers to whether wet waste disposal methods meet the requirements of the current environmental policy as well as future environmental policies that may raise the standards and regulations. Outdated on-site disposal methods, which have a significant negative impact on the environment, may be gradually eliminated due to stringent constraints. Hence, when choosing the appropriate method of wet waste disposal in rural areas, decision makers must evaluate the regulatory constraints and accurately forecast potential shifts in policy. For example, small-scale biochemical processors for on-site disposal should meet the standards for sewage discharge and odor emissions, but existing on-site biochemical processors have problems such as excessive sewage discharge due to technical and quality problems. In addition, it is not yet clear whether the compost products of biochemical processors can be used as greening fertilizer [45]. If future environmental protection policies become more stringent, on-site biochemical processors may be phased out and the cost of disposal of compost products will increase. At the same time, for special areas, such as drinking water source protection zones and Yangtze River protection zones, it is not allowed to set up on-site waste disposal facilities according to the Regulations of Shanghai Municipality on Drinking Water Source Protection [46]. On the contrary, centralized disposal plants are strategically located in areas permitted by the municipal government to minimize any disputes in future planning and have a pollution control system to ensure that pollutant emissions meet the standards.
Location feasibility refers to whether a project faces difficulties in selecting a site. A biogas digester with a disposal capacity of 1 Mg·day−1 requires about 130 square meters of land. When choosing a site for such a digester in rural areas near large cities, the main challenges to address are the suitability of the land, the impact of the NIMBY effect, and compliance with regulations regarding the red line of permanent basic cropland. On-site biochemical processors are facing similar difficulties in the selection of locations. For large-scale centralized disposal plants, they are typically designed and organized by the municipal government. Therefore, for agriculture-related towns and villages, the centralized wet waste disposal plant is already constructed and does not need to be considered by the village for site selection.
To determine the weight of the five criteria and rank the three alternatives, fuzzy analytic hierarchy process (FAHP) analysis was used in this study. AHP, TOPSIS, PROMETHEE, VIKOR, and other approaches are widely used in MCDM analysis [35,36,37,38,39,44]. AHP or a combination of AHP and another method is the most commonly employed MCDM method [34,44]. Among these methods, the FAHP method, which combines the fuzzy logic theory with AHP, optimizes the deficiencies of the traditional AHP method.
FAHP can better reflect the importance of multiple influencing factors to the decision maker, solve fuzzy and difficult-to-quantify non-deterministic problems in reality, and turn the qualitative evaluation of things into quantitative evaluation. FAHP is a well-established method, the details of which can be found in the literature [47,48,49,50] and are briefly illustrated in Figure 2.

3. Results

3.1. Economic Analysis

Figure 3 illustrates the cumulative NPVs of the three wet waste disposal methods. Due to the ten-year service life of the on-site disposal equipment, only the initial ten years of data are presented.
In the short term, the cumulative NPVs of centralized disposal and on-site disposal by small-scale biochemical processors (on-site BP) in the first two years are higher than those of on-site disposal by biogas digesters (on-site BD). This is due to the fact that the construction of a biogas digester requires the village to bear the construction costs by itself. However, in the long run, the annual operating cost of on-site BD is significantly lower than those of the other two disposal modes, with cumulative NPVs of CNY −570.67, −841.80, and −1231.55 thousand in the 10th year under the best, medium, and worst scenarios, respectively. These values for the three scenarios are 0.26, 0.38, and 0.56 times that of the centralized disposal mode, respectively, which is CNY −2217.59 thousand. Even in the worst-case scenario, the cumulative NPV of on-site BD is greater than that of on-site BP in the best-case scenario. For the best, medium, and worst scenarios, the cumulative NPV of the on-site BP mode in the tenth year is CNY −1598.92, −2358.69, and −3626.03 thousand, respectively, with only the best scenario outperforming centralized disposal.
Although the cumulative NPV of on-site BD is superior, it requires a higher initial fixed investment compared to the other two methods. Currently, on-site BD does not receive financial subsidies from the government of District S. Thus, assuming that all other requirements are fulfilled (e.g., being able to resolve the land use control and not violating regulations regarding the protection of drinking water sources), if villages can afford the initial investment of the biogas digester, the overall cost will be lower in the long term. However, based on the previous analysis, it is clear that land use control and environmental regulations are difficult issues to resolve. If the operation of the existing biogas digesters is prohibited in the future due to stricter regulatory constraints, this will result in a large loss of fixed assets for the village.

3.2. Accounting of GHG Emissions

The LCA system boundaries and LCIs of the three wet waste disposal modes are depicted in Figure 4. The material flow of centralized disposal begins with the transportation of wet waste and then goes through each functional unit, including pre-treatment, anaerobic digestion, biogas power generation, biogas slurry and residue treatment, and dry sludge treatment. In this paper, GHG emissions are measured in CO2 equivalents.
Table 1 displays the summarized inventories of GHG emissions resulting from the three disposal methods.
Figure 5 shows the net GHG emissions. The net GHG emissions for the three methods, namely, centralized disposal, on-site BD, and on-site BP, are 193.67, 64.71, and 263.37 kg CO2eq/d, respectively. While centralized disposal can reduce GHG emissions by generating biogas (−93.28 kg CO2eq/d) and reusing biodiesel (−40.09 kg CO2eq/d), the overall GHG emissions are not significantly reduced due to the high electricity consumption of the equipment and the direct emissions of CH4 and N2O. In terms of electricity consumption, the incineration of biogas residue and dry sludge has the highest GHG emissions from electricity consumption (59.05 kg CO2eq/d). In terms of direct emissions, the incineration segment also has the highest carbon equivalent from N2O leakage (69.41 kg CO2eq/d). GHG emissions from on-site BP are dominated by electricity consumption, mainly due to the fact that the technology of this equipment is high-temperature drying and completely non-sewage generating. The main reason for the use of non-sewage generating technology is to comply with high environmental standards. The utilization of this technology leads to high electricity costs and high GHG emissions, reflecting the tightening of environmental control policies that make social costs internal.
The GHG emissions of the two on-site disposal methods do not consider the utilization or disposal of the fertilizer made from wet waste. If the fertilizer is utilized, it can only be used for gardening or greening, but it lacks compliance and has the risk of polluting the land and salinizing the soil. If the fertilizer is mainly disposed of, for instance, by the incineration of biogas residue, its net GHG emissions will continue to increase.

3.3. Multi-Criteria Decision Making

Table 2 presents a pairwise comparison matrix of the five primary factors, namely, economics, GHG emission control (GHGEC), stability, compliance with environmental regulations (CER), and location feasibility (LF), as determined through fieldwork and subjective judgment.
Upon verifying the consistency of the pairwise comparison matrix (CR = 0.0978 < 0.1), it can be concluded that the factor importance results are reliable [51].
In Table 3, the weight of each primary factor is detailed. The respective weights of the economic, GHGEC, stability, CER, and LF factors are 0.20, 0.13, 0.23, 0.18, and 0.26. The weights suggest that, in terms of importance, the five primary factors are ranked as follows: LF > stability > economics > CER > GHGEC.
Table 4 displays the FAHP analysis of the three disposal modes. The evaluation results for disposal modes are categorized as either “good” or “bad”. The membership values range from 0 to 1, and the total sum of all memberships for each factor is equal to 1. If a disposal mode exhibits a notable advantage for a specific factor, then the evaluation result for the “goodness” of this disposal mode is 1. If there is no disposal method that obviously performs better than others in relation to a given factor, the score will be decided based on the current circumstances [52]. The membership for each factor in FAHP is determined by the following criteria:
  • Economics. According to the previous cumulative NPV results, with a ten-year service life, the score is determined by the area between the X-axis and the cumulative NPV curve (medium scenario). The smallest area is scored as 1; the largest area is scored as 0. For the second-largest area, the score is determined by the ratio of the difference between it and the minimum value to the difference between the maximum value and the minimum value.
  • GHG emission control. Based on the results of the previous analysis, the net GHG emissions of the three disposal modes were used to determine the scores. The lowest net GHG emission is scored as 1; the highest is scored as 0. For the second-largest net GHG emission, the score is determined by the ratio of the difference between it and the maximum value to the difference between the minimum value and the maximum value.
  • Stability. According to the literature and fieldwork [20,53,54], small-scale biochemical processors have a high failure rate and exhibit poor operational stability. The long-term stability of biogas digesters remains unproven. The centralized wet waste disposal plant is professionally managed and demonstrates good stability.
  • Compliance with environmental regulations. According to the literature and fieldwork [20,53,54], it is challenging to effectively regulate the sewage and odor emissions from small-scale biochemical processors. At the same time, it is difficult to ensure the quality of the compost products from on-site wet waste disposal. These compost products cannot be utilized as agricultural fertilizer, and it is difficult to meet the policy standards. In the future, with the strict enforcement of the regulations on drinking water source protection zones and other rural environmental protection policies, on-site disposal facilities may be at risk of being demolished [46]. In contrast, the centralized disposal plant is selected by the municipal government, which is unlikely to happen in the case of planning contradictions. The centralized plant is equipped with pollution control equipment, follows standardized management practices, and meets environmental standards.
  • Location feasibility. The location feasibility for a biogas digester is challenging due to the huge space it occupies and the necessity of ensuring that it is located far away from underground rivers. Additionally, the location is further constrained by the red line of basic cropland and land use control. A small-scale biochemical processor covers a small area, and the site is usually near the refuse room. However, it is still subject to land use control, such as drinking water source protection zones, and the NIMBY effect needs to be considered. If villages choose centralized wet waste disposal, there is no need to consider location feasibility, as the wet waste disposal plant has already been planned, sited, and constructed by the municipal government.
According to the maximum membership principle, the final evaluation results (final memberships) are shown in Figure 6. Centralized disposal is the best among the three modes. The final membership of the centralized disposal mode in the “good” category is 0.75, which means it is moderately good. The main shortcomings of the centralized disposal mode for agriculture-related towns and villages are the high transportation and disposal costs. The main shortcomings of on-site BD (0.57) are the challenges associated with selecting a suitable location and the lack of demonstrated long-term operational stability. On-site BP (0.13) lacks advantages in five key areas: it has the lowest cumulative NPV, the highest net GHG emissions, poor stability, a risk of non-compliance with environmental regulations, and difficulties in location selection.

4. Discussion

Based on the study of three agriculture-related towns and nine administrative villages in District S, this paper examines the cumulative NPV and GHG emissions of three different wet waste disposal methods and uses FAHP to construct an MCDM analysis and comprehensively compares the wet waste disposal methods to identify the optimal alternative.
From an economic perspective, on-site BD has the highest cumulative NPV over a 10-year operating period, while the cumulative NPV of on-site BP (the medium scenario) is lower than that of centralized disposal. This contradicts the common belief that on-site disposal is a low-cost option. It is important to note that on-site BD requires a significant initial investment from local government. Therefore, careful budget planning is essential for village authorities. Furthermore, as the utilization of fertilizer made from wet waste for cropland or greening without quality testing is not in accordance with regulations, such as Organic Fertilizer [17], the authors assert that not only should the revenue from the sale of fertilizer not be considered as income, but also the cost of disposing of solid residue should be factored in. In the near future, as environmental regulations become stricter, the cost of disposing of solid residue will steadily climb, and the expense of certifying the quality of fertilizer will also rise.
In terms of GHG emissions, on-site BD offers a significant advantage. On-site BP exhibits the highest levels of GHG emissions, reflecting the impact of environmental regulatory policies on GHG emissions as a manifestation of the internalization of social costs. The net GHG emissions of the on-site disposal methods are likely to increase further as environmental regulatory policies become more stringent. As for centralized disposal, the estimated level of GHG emissions falls within the moderate range. However, Shanghai has conducted research and demonstrated technologies for the low-carbon utilization of wet waste. The decarbonization of centralized disposal is an important task for the municipal government. This suggests that GHG emissions from centralized disposal will be further reduced in the future.
According to the MCDM analysis, centralized disposal is the most favorable alternative. The tightening of regulatory constraints can internalize the negative externalities of on-site wet waste disposal, which will lead to a significant increase in the cost of the on-site disposal method [55,56,57]. Thus, considering the various factors that influence selection, it can be concluded that centralized disposal is the method that is most consistent with the regulatory constraints and the most sustainable way of disposing of wet waste for villages in agriculture-related towns. Centralized disposal represents the cost of wet waste disposal more realistically and internalizes externalities more comprehensively. It also reminds us that there is a justification for higher disposal costs as a result of higher regulatory standards; in other words, it is incorrect to dismiss a particular disposal method simply because of its high cost.

5. Conclusions

The implementation of increasingly stringent environmental regulations has resulted in the internalization of the external social costs associated with disposing of wet waste in rural areas. This has led to a standardization of wet waste disposal practices and an increase in disposal costs. Consequently, the perception that wet waste should be disposed of on site is being challenged. Centralized disposal accurately captures the true cost of wet waste disposal and fully incorporates external costs. Therefore, it is essential for management to accurately assess disposal expenses and set aside funds.
This study examines wet waste disposal methods in agriculture-related towns and villages to offer suggestions for other rural areas and challenge the stereotype that wet waste should be disposed of on site. This study emphasizes the influence of regulatory policies on decisions about wet waste disposal methods, which has been ignored by much of the previous literature.
Based on the results of this paper, the following suggestions may be helpful to the government.
Firstly, decision makers should give an adequate estimation of the tightening of regulatory constraints, which have a great impact on wet waste disposal. The regulatory constraints include standardized land use control, environmental protection regulations, and quality standards for fertilizers. The basic trend in the development of the relevant regulations is to become more robust and stricter. Rural domestic waste disposal methods should be in line with the regulations and have a certain degree of foresight. In terms of comparing the centralized and on-site disposal methods, not only the transportation costs should be considered, but also the differences in environmental restoration expenditures of waste storage sites and disposal sites, as well as the compliant use or disposal cost of fertilizer, should be taken into account. Typically, pollution control equipment and centralized disposal plants are scale-effective. Therefore, centralized disposal may be more cost-efficient for the whole process.
Aside from the effects of stricter enforcement of current regulations, it is crucial to consider the potential consequences of regulatory developments in the future. For instance, under the “carbon peaking and carbon neutrality” target, if carbon emission allowances are imposed on waste disposal facilities, this could render a previously favored disposal method completely impractical. Another instance is the increased criteria for pollutant emissions in areas of water source protection zones. In this regard, it is important to consider the extra expenses associated with retiring facilities and equipment, particularly to prevent premature retirement of recently acquired facilities and equipment before they have fully served their purposes.
Secondly, a homogenized waste collection and disposal system including both urban and rural communities should be established. Homogenization refers to the same quality of waste collection and disposal services. Currently, the urban–rural gap should be narrowed by improving the service quality of rural waste collection and disposal systems. For cities that have implemented waste classification, the waste transportation and centralized disposal system should cover all rural communities. Rural areas have absorbed industries and inhabitants that have been displaced or moved from metropolitan areas due to the process of urbanization. However, the development of infrastructural facilities for waste collection and disposal has been progressing slowly. Given the low population density in rural areas, it is inevitable that waste collection and transportation in rural areas will require higher investment, and it is impractical to expect that urban and rural waste management systems could have the same operational efficiency. Furthermore, centralized rural communities should align their domestic waste management with that of urban communities. This mainly refers to rural areas where there is a concentration of residential zones following a village merger or where it is common to rent rooms to migrant workers. Given the similarities in living styles and population densities, it is advisable to design waste management systems for rural communities based on those used in urban communities. Additionally, homogenization helps to compare disposal costs more accurately, enabling a more rigorous evaluation of disposal methods and preventing the inclusion of unrealistically low disposal costs resulting from lesser standards, which may mislead selection among disposal alternatives.
It should be emphasized that the homogenized disposal system should be managed by the municipal government. The government should be unconstrained by the administrative boundaries between districts and follow the proximity principle to allow inter-district disposal of wet waste. Geographically, the centralized disposal facility in District M is closer to the southern towns of District S. Therefore, disposal by the centralized disposal plant in District M would be more cost-effective and reduce GHG emissions.
Thirdly, the management level of the existing on-site disposal facilities should be strengthened. The existing on-site disposal facilities should be regarded as temporary solutions to meet the demand for wet waste disposal. An on-site disposal facility usually has a service life of approximately 10 years. During this period, Shanghai must progressively enhance its centralized disposal capacity. In addition to making the best use of on-site disposal facilities, the Shanghai municipal authorities and equipment suppliers should take action. On the one hand, efforts should be made to reduce the GHG emissions and other secondary environmental impacts of existing on-site disposal facilities. On the other hand, the authorities should encourage users of on-site disposal facilities to improve the management of facility operations. This can be achieved by increasing users’ understanding of their environmental obligations and providing training for facility operators.

Author Contributions

Conceptualization, Y.N. and Z.Z.; methodology, Y.N.; software, Y.N.; validation, Y.N. and Z.Z.; formal analysis, Y.N. and Z.Z.; investigation, Y.N. and Z.Z.; resources, Z.Z.; data curation, Y.N.; writing—original draft preparation, Y.N.; writing—review and editing, Y.N. and Z.Z.; visualization, Y.N.; supervision, Z.Z.; project administration, Z.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

Thanks to the Landscaping and City Appearance Administrative Bureau of District S and the investigated township and village authorities in District S for their support.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Table A1. Information on the investigated villages.
Table A1. Information on the investigated villages.
TownVillagePermanent Household PopulationMigrant Worker PopulationVillage Area (km2)Proportion of Migrant Worker PopulationPopulation Density (Person/km2)Wet Waste Disposal Mode
AA1289213005.131.0%822On-site disposal by biogas digester
A210803003.8821.7%356On-site disposal by closed composting ponds
A3300060007.7866.7%1157On-site disposal by closed composting ponds
BB1300328005.648.3%1036Mainly through on-site disposal by small-scale biochemical processors, a small portion through centralized disposal
B2650434.926.2%141
B36453532.235.4%454
CC11556524.2880.8%189Only centralized disposal
C2289213005.131.0%822
C310803003.8821.7%356
Table A2. The parameters and data used in the calculation of NPV.
Table A2. The parameters and data used in the calculation of NPV.
Disposal MethodParameterValueUnitSource
Disposal demand1Mg·day−1Assumption
Centralized disposalTransportation cost180CNY/tFieldwork
Disposal cost573CNY/t[58]
Environmental compensation fee30CNY/t[59]
On-site disposal by biogas digesterConstruction and acquisition of equipmentBest370,000CNY/t design capacityFieldwork and [45]
Medium435,000
Worst500,000
Land priceBest60.75CNY/m2[60]
Medium68.85
Worst86.55
Land area of design disposal capacity per tonBest110m2Fieldwork and estimation
Medium130
Worst150
Additional labor salary of disposal capacity per tonBest10,000CNY/a
Medium12,500
Worst15,000
Strain and maintenance cost of disposal capacity per tonBest10,000CNY/a
Medium15,000
Worst20,000
Electricity cost of disposal capacity per tonBest5000CNY/a
Medium7500
Worst10,000
Actual disposal capacityBest1Mg·day−1
Medium0.85
Worst0.7
Discount rate4.90%People’s Bank of China interest rates for loans over five years
On-site disposal by biochemical processorLand priceBest60.75CNY/m2[60]
Medium68.85
Worst86.55
Land area of design disposal capacity per 0.5 tBest15m2Fieldwork and estimation
Medium20
Worst25
Rental of equipment (including maintenance services) of design capacity per 0.5 tBest110,000 for the first year and 70,000 for each subsequent yearCNY/a
Medium120,000 for the first year and 80,000 for each subsequent year
Worst130,000 for the first year and 90,000 for each subsequent year
Electricity cost of disposal capacity per 0.5 tBest18,000CNY/a
Medium24,000
Worst30,000
Additional labor salary of disposal capacity per 0.5 tBest10,000CNY/a
Medium12,500
Worst15,000
Actual disposal capacityBest0.5Mg·day−1
Medium0.4
Worst0.3
Discount rate4.90%People’s Bank of China interest rates for loans over five years
Table A3. The parameters and data used in the calculation of GHG emissions.
Table A3. The parameters and data used in the calculation of GHG emissions.
Disposal MethodParameterValueUnitSource
Disposal demand1000kg/dAssumption
Centralized disposalFuel consumption of transportation vehicle0.000125L/kg·km[61,62,63]
Average transportation distance31.4kmMeasured by e-map
Density of diesel fuel0.86kg/L[64]
Emission factor of diesel3.209kg CO2/kg diesel[64]
Factor of biogas slurry generation0.000971m3/kg waste treated[65]
COD concentration of biogas slurry8.2kg/m3[66]
Biogas yield0.065m3/kg waste treated[65]
Electricity generated from waste disposal per ton0.12kWh/kg waste treated[65]
Biogas residue yield33% [65]
Dry sludge yield2% [65]
Factor of crude oil extraction1.75% [65]
Natural gas consumption of incineration0.0335Nm3/kg solid residue incinerated[67]
Emission factor of natural gas2.184kg CO2/Nm3[64]
Electricity consumption of pre-treatment0.01366kwh/kg waste[68]
Electricity consumption of anaerobic digestion equipment0.035kwh/kg waste[69]
Electricity consumption of dehydration equipment0.0107kwh/kg biogas residue[70]
Electricity consumption of biogas slurry treatment equipment0.215kWh/m3[71,72]
Electricity consumption of incineration0.3375kWh/kg[73,74,75]
Factor of chemical PAC addition0.05kg/kg dry sludge[61]
Emission factor of chemical PAC22.7kg CO2/kg[72]
Factor of chemical PAM addition0.002kg/kg dry sludge[27]
Emission factor of chemical PAM1.5kg CO2/kg[72]
Factor of chemical lime addition0.2kg/kg dry sludge[61,74]
Emission factor of lime0.683kg CO2/kg[76]
NaOH consumption of dry sludge incineration0.019kg/kg dry sludge[74,75]
Emission factor of NaOH1.145kg CO2/kg[77,78]
Ca(OH)2 consumption of dry sludge incineration0.00941kg/kg dry sludge[74]
Emission factor of Ca(OH)20.975kg CO2/kg[77]
CH4 emission factor of anaerobic digestion0.001kg/kg waste[79]
BOD/COD0.45 Industry experience
CH4 emission factor of biogas slurry treatment0.099kg/kg BOD[76,79]
Total nitrogen concentration of biogas slurry3kg/m3[66]
N2O emission factor of biogas slurry treatment0.02kg/kg TN[27,76]
CH4 emission factor of biogas generation2% Industry experience
CH4 emission factor of incineration0.000002425kg/kg solid residue incinerated[79]
N2O emission factor of incineration0.00072kg/kg solid residue incinerated[79]
GWP CH4-10029.8 [80]
GWP N2O-100273 [80]
Conversion efficiency of crude oil to biodiesel0.85 [81]
Density of biodiesel0.88kg/L[82]
Heat value of biodiesel36,615kJ/L[82,83]
Emission factor of electricity0.7773kg CO2/kWh[84]
Diesel generator efficiency30% Industry experience
On-site disposal by biogas digesterBiogas yield0.054m3/kg waste[66]
Concentration of methane66% [66]
Density of methane0.717kg/m3General standard
Electricity consumption of equipment0.034kWh/kg wasteFieldwork
Emission factor of electricity0.7773kg CO2/kWh[84]
CH4 emission factor of biogas generation5% Industry experience
GWP CH4-10029.8 [80]
On-site disposal by biochemical processorsCH4 emission factor of aerobic composting0.002kg/kg waste[79]
N2O emission factor of aerobic composting0.00012kg/kg waste[79]
Electricity consumption of equipment0.22kWh/kg wasteFieldwork
GWP CH4-10029.8 [80]
GWP N2O-100273 [80]
Emission factor of electricity0.7773kg CO2/kWh[84]

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Figure 1. The locations of the studied villages and the centralized wet waste disposal facility. The letters represent the towns, and the numbers represent the villages attributed to the towns.
Figure 1. The locations of the studied villages and the centralized wet waste disposal facility. The letters represent the towns, and the numbers represent the villages attributed to the towns.
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Figure 2. The calculation process of FAHP.
Figure 2. The calculation process of FAHP.
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Figure 3. The cumulative NPVs of the three wet waste disposal methods.
Figure 3. The cumulative NPVs of the three wet waste disposal methods.
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Figure 4. The LCIs and system boundaries of the LCAs for the three wet waste disposal methods.
Figure 4. The LCIs and system boundaries of the LCAs for the three wet waste disposal methods.
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Figure 5. The net GHG emissions of the three wet waste disposal methods.
Figure 5. The net GHG emissions of the three wet waste disposal methods.
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Figure 6. The final evaluation results (final memberships) of three wet waste disposal modes.
Figure 6. The final evaluation results (final memberships) of three wet waste disposal modes.
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Table 1. The inventories of the GHG emissions for three wet waste disposal modes *.
Table 1. The inventories of the GHG emissions for three wet waste disposal modes *.
Direct EmissionsIndirect EmissionsCarbon Offsets
FC 1EC 2CD 3
Centralized164.4436.6899.8026.11−133.37
On-site BD 438.28/26.43//
On-site BP 592.36/171.01//
1 FC—GHG emissions from fuel consumption; 2 EC—GHG emissions from electricity consumption of the equipment used; 3 CD—GHG emissions from chemical dosing, production, and transportation; 4 On-site BD—on-site disposal by biogas digester; 5 On-site BP—on-site disposal by small-scale biochemical processor. * The quantity of GHG emissions is denoted in kg CO2eq/d.
Table 2. A matrix of pairwise comparisons among the five primary factors.
Table 2. A matrix of pairwise comparisons among the five primary factors.
EconomicsGHGECStabilityCERLF
Economics0.50.70.40.60.3
GHGEC0.30.50.20.30.1
Stability0.60.80.50.70.4
CER0.40.70.30.50.3
LF0.70.90.60.70.5
Table 3. The weights of five primary factors.
Table 3. The weights of five primary factors.
EconomicsGHGECStabilityCERLF
Weight0.200.130.230.180.26
Table 4. An evaluation matrix of three wet waste disposal modes.
Table 4. An evaluation matrix of three wet waste disposal modes.
CentralizedOn-Site BDOn-Site BP
GoodBadGoodBadGoodBad
Economics0.200.801001
GHGEC0.350.651001
Stability100.500.5001
CER100.400.600.300.70
LF100.200.800.300.70
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Ni, Y.; Zhang, Z. Comparison and Selection of Wet Waste Disposal Modes for Villages in Agriculture-Related Towns Taking Shanghai, China, as an Example. Sustainability 2024, 16, 4936. https://doi.org/10.3390/su16124936

AMA Style

Ni Y, Zhang Z. Comparison and Selection of Wet Waste Disposal Modes for Villages in Agriculture-Related Towns Taking Shanghai, China, as an Example. Sustainability. 2024; 16(12):4936. https://doi.org/10.3390/su16124936

Chicago/Turabian Style

Ni, Yiyao, and Zhen Zhang. 2024. "Comparison and Selection of Wet Waste Disposal Modes for Villages in Agriculture-Related Towns Taking Shanghai, China, as an Example" Sustainability 16, no. 12: 4936. https://doi.org/10.3390/su16124936

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