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Article

Energy Utilization Assessment of Municipal Sewage Sludge Based on SWOT-FAHP Analysis

by
Lu Xiang
,
He Li
*,
Yizhuo Wang
,
Linyan Qu
and
Dandan Xiao
School of Civil Engineering, Southeast University, Nanjing 211189, China
*
Author to whom correspondence should be addressed.
Water 2023, 15(2), 260; https://doi.org/10.3390/w15020260
Submission received: 6 December 2022 / Revised: 3 January 2023 / Accepted: 5 January 2023 / Published: 8 January 2023
(This article belongs to the Section Water Resources Management, Policy and Governance)
Browse Figures
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Abstract

:
Recently, due to the abundance of carbon, nitrogen and phosphorus in municipal sewage sludge (MSS), the energy potential of MSS has become increasingly prominent. Economically developed regions possess more financial and policy support advantages for the development of MSS energy recovery technology; hence, the selection of the appropriate sludge treatment and disposal technologies to maximize the energy potential of MSS is of great importance. In this study, the energy recovery potential of MSS was evaluated on the basis of regional economies, sludge analysis, a main sludge energy reuse technology review and legislative profile analysis. As the most commonly adopted technology, incineration had a lower energy potential, which may be ascribed to the high moisture content, high disposal costs and difficulties in in situ energy recovery. In contrast, the energy potential for anaerobic digestion, pyrolysis and gasification was relatively high. By conducting a SWOT-FAHP analysis, management evaluation of these four technologies was carried out from the following four perspectives: problem solving, technology development, ecological environment and laws and regulations. Pyrolysis was evaluated to be the most suitable technology from the technical and environmental perspectives because its products had high energy potential and avoided heavy metal problems. The obstacles and problems that pyrolysis technology might encounter in commercial applications in the future are discussed. With this energy-generating, low energy consumption, low-pollution sludge energy reuse technology, the potential of sludge pyrolysis would be high in the long run. These results revealed the factors affecting the energy recovery potential of sludge, and comprehensively evaluated the technologies from the aspects of problem solving, environmental impact, technology development and law, the optimal solution obtained could provide reference on the management decision of sludge disposal technology for economically developed areas in the future.

1. Introduction

Municipal sewage sludge (MSS) is the by-product derived by treating wastewater from various sources such as household sewage, industrial wastewater, medical facilities sewage and street runoff, etc., in wastewater treatment plants (WWTP). With the continuous advancement of industrialization and urbanization, the production of MSS in WWTP presented an explosive growth curve in China. In 2016, the annual generation rate of MSS in China reached about 30 million tons of wet sludge (moisture content of 80%) [1]. The large amounts of MSS increase the need for the safe and appropriate disposal of sludge, due to the fact that MSS contains pathogens, parasites and chemical pollutants, which could threaten the health of the public and pollute soil, water, etc. [2]. Nevertheless, due to the complexity of sewage sources and the unevenness of composition [3], coupled with sludge disposal costs [4], the disposal of MSS is a complex and expensive process, meaning that the rational disposal of sludge presents a serious challenge.
Sludge could be reused as fertilizer in landfill and agriculture, due to the abundant useful elements, such as carbon, nitrogen and phosphorus [5]. Meanwhile, sludge could also be recovered as an energy source, such as sludge–bituminous briquettes and sludge-generated biogas [6,7,8]. Given the moisture content and contaminant concentration in MSS, land application, utilization in building materials and incineration were recognized as potential routes for the disposal of sewage sludge in the United States, Japan and European Union [9]. The agricultural utilization rate of dry sludge after anaerobic digestion in the United States reached about 55% [10]. Correspondingly, this rate reached 75% in Australia [11]. In 2018, agriculture application and incineration became the two dominant routes of sludge disposal in the European Union (EU), accounting for 37% and 35%, respectively [3,12]. Due to the limitation of land resources, more than half of the accumulated sludge was reused to produce building materials after incineration in Japan [13,14].
The content of heavy metals and organic pollutants in sludge would cause secondary soil pollution and could even be harmful to the ecosystem, which indicates that the reuse of sludge in agriculture may be restricted by legislation [15]. Thermochemical technologies such as gasification and pyrolysis were considered as more latent substitutes to sludge disposal for their outstanding energy recovery capacity compared to the traditional MSS disposal routes [16]. Nevertheless, factors such as the feedstock, operating factors and reactors that were decisive for sludge pyrolysis products [17], coupled with the tar-induced pipeline blockage during the gasification [18] would limit the viable development of sludge thermochemical technologies.
SWOT analysis is a proven tool for evaluating processes and strategies that helps decision makers to assess the viability of strategies by understanding the internal strengths, weaknesses and external factors such as opportunities and threats when adopting a strategy [19,20]. Currently, the SWOT analysis method has been applied in sludge management in several regions. Srivastava et al. reviewed municipal solid waste management based on SWOT analysis, which was conducted from a stakeholder perspective and did not take environmental considerations into account [21]. Samolada and Zabaniotou made a comparative evaluation of the thermal treatment methods of sludge, but external opportunities and barriers were not included in the SWOT analysis, which are discussed separately following the result of the SWOT analysis [20]. Chen et al. conducted a SWOT analysis on sludge treatment in Tibet, focusing on the influence of sludge organic matter and heavy metal content on the selection of treatment methods, and concluded that anaerobic digestion combined with pyrolysis was the most suitable route for sludge treatment in Tibet [22].
Since the qualitative analysis of SWOT relies on the subjective judgment of decision makers and does not consider the priority of various factors on decision making, it was necessary to introduce quantitative methods. An Analytic Hierarchy Process (AHP) was widely employed to determine the weighting of factors in SWOT analyses such as energy planning and tourism management strategies due to its simplicity and transparency [23]. Fuzzy AHP (FAHP) combines fuzzy logic and AHP in order to solve the fuzziness and uncertainty in the processing problem, and is second only to AHP in terms of versatility [24]. Adar et al. evaluated a variety of sludge treatment methods and summarized that supercritical water gasification had the highest application value by developing the SWOT-FAHP analysis, and concluded that the availability of technology in sludge management was not as important as the criteria for solving the problem [25].
There are several studies on SWOT and SWOT-FAHP analyses of sludge management, but the research area in these studies is relatively specific, e.g., Tibet’s fragile ecological environment and low oxygen content, where the applicability was found to be lacking for the economically developed city; therefore, it was necessary to establish a specific analysis method for sludge management in these regions. An integrated methodology that included SWOT analysis and a Fuzzy Analysis Hierarchy Process (FAHP) was employed in this study, and anaerobic digestion, incineration, gasification and pyrolysis technologies were evaluated in terms of problem solving, environmental impact, technological development and legal regulatory aspects, to determine the feasibility of the energy utilization of the technology, with the aim of providing a reference for managers to make appropriate decisions according to actual needs. In general, the purpose of this study was to evaluate the energy recovery potential of sludge disposal technologies based on regional economic, sludge analysis, technical analysis and legislative analysis. The SWOT-FAHP analysis was also employed to provide further data support for sludge management decisions in economically developed areas.

2. Materials and Methods

2.1. Treatment and Disposal of MSS in Jiangsu, China

The focal researched area was Jiangsu, which is located on the eastern coast of China, between 30°45’~35°08’N and 116°21’~121°56’E, with a total area of 107,200 square kilometers. There were 215 sewage treatment plants in Jiangsu Province, with a daily sewage treatment capacity of 15.96 million tons and a sludge output of about 16,000 tons/day (moisture content of 80%) according to the 2021 China Urban and Rural Statistical Yearbook. Based on the “2020 Jiangsu Urban Drainage and Sewage Treatment Development Report”, the proportion of sludge treatment and disposal methods in urban sewage treatment plants in Jiangsu (in terms of sludge treatment volume) was as follows: incineration accounted for 59.8%, 15.1% of MSS was used for building materials utilization, composting for 13.5%, sanitary landfill for 1.5%, and others accounted for 10.1%.
The choice of sludge disposal technology was affected by economic factors such as regional GDP and industrial structure. In 2021, the annual GDP of Jiangsu reached RMB 11636.42 billion [26], accounting for 10.2% of China’s GDP [27]. In addition to the strong economic foundation, Jiangsu Province was very concerned about the treatment and disposal of sludge, which was proposed to accelerate the capacity building of sludge treatment and improve the technical specifications and standards for sludge treatment, disposal and resource utilization. Economic and support policies mean greater freedom in the selection of sludge treatment and disposal technologies, which provides a certain reference for other cities that would not be as affected by economic factors and policy in the selection of sludge treatment and disposal technology.

2.2. Composition and Characteristics of MSS

As the solid or semi-solid waste generated by the sewage treatment process, sludge mainly includes microorganisms, incompletely digested organic matter, inorganic matter, heavy metals and pathogenic bacteria; the main components are presented in Figure 1.
The original pollution load of MSS and the sewage treatment process affect the yield and properties of sludge [28]. In previous studies, the heating value of sludge in different regions of Jiangsu Province was found to vary greatly. Considering the site of the sewage treatment plant, the type of influent and the treatment process of the sewage treatment plant, a total of 20 sewage treatment plants in Jiangsu Province were selected as the source of MSS. The distribution of these 20 sewage plants is shown in Figure 2, and the measurement of elemental analysis, industrial analysis, calorific value and moisture content of these 20 sewage plants are listed in Table S1.
The elemental analysis of sludge from 20 typical sewage treatment plants in Jiangsu is shown in Figure 3, where the average C content reached 42.51% with H content reached 9.71% , which was higher than that in other parts of the country (Shenzhen, Tianjin, Xi’an), indicating that the sludge might have a relatively high organic matter content [22]. However, the N content was relatively high, reaching 9.76%, which might result in the nitrogen oxide emission problem. The industrial analysis of the sludge indicated that the volatile solids content of the sludge in Jiangsu could reach 47.60% with an ash content of 44.32%, which was not conducive to the operation and maintenance of incinerators, pyrolysis furnaces and other equipment.

2.3. Treatment and Disposal of MSS

2.3.1. Anaerobic Digestion

Anaerobic digestion (AD) is a widely adopted biological technology for stabilizing MSS by microorganisms, which involves a series of biochemical stages such as hydrolysis, acid production (fermentation), acetic acid formation and methane production [29]. The biogas generated by anaerobic digestion is composed of methane CH4 (60~70%), carbon dioxide CO2 (30~40%), H2S and other gas products; the gas heating value could be as high as 28.03~38.92 MJ/m3 [30,31]. The biogas produced by anaerobic digestion could be used as the energy source for powering gas engines and for the operation of wastewater treatment plants [32]. However, AD had several limitations in actual operation, including the 10~30 days required to convert 30~60% of organic matter, the energy conversion rate of anaerobic digestion being low and the digestive sludge being not only difficult to biodegrade but also containing a large amount of unconverted energy [4,33]. In addition, relying solely on anaerobic digestion could not completely eliminate toxic and harmful substances such as pathogens and heavy metals in sludge, which also hindered the further development of anaerobic digestion.

2.3.2. Incineration

Incineration is a thermochemical treatment method for MSS, usually with temperatures of up to 800~1000 °C, which could help to achieve stabilization and the reduction in MSS, eliminate pathogenic microorganisms and convert toxic organic compounds into carbon dioxide, water and heat energy [34]. The heat energy generated by incineration could be recovered and recycled for power generation and heating, and the ash residue generated by incineration could also be used in building materials [25].
However, municipal sludge has a high moisture content and low heating value, so it is difficult to meet the heating value requirements by incineration separately [35]. In order to improve the combustion characteristics of MSS, municipal sludge could be mixed with coal, agricultural waste such as rice husk straw, livestock and poultry breeding manure and municipal solid waste such as domestic waste, cement raw materials, etc., to reduce the energy loss in the combustion process [36]. Another problem of sludge incineration is the production of dangerous compounds such as dioxins and furans, which pollute the environment, and cause a decrease in air quality and concern among the public; nevertheless, this issue could be solved by increasing the temperature to 850 °C [37]. In addition, the emission of NOx could be solved by phased air combustion and low-oxygen dilution combustion. The addition of chlorine-containing compounds could reduce the volatilization of heavy metals and polychlorinated biphenyls [38].
At present, 64% of China’s sludge marketization projects employ incineration as their sludge disposal technology, and 70% of the wastewater treatment plants in this study adopted incineration for the disposal of MSS, where the strategy was mainly co-combustion with coal. According to the manager, the treatment cost of sludge incineration was relatively high, at an average RMB 200~400/ton, meaning the costs override the economic benefit. Furthermore, the amount of MSS in sewage treatment was so great that the thermal power incineration plant could not treat all the MSS received in winter, which resulted in sludge accumulation problems.

2.3.3. Pyrolysis

Pyrolysis technology is considered to be a zero-waste technology with great potential to replace the sustainable energy required in all social, economic and environmental problems, and has gradually become a research hotspot for the development of sludge treatment worldwide [39,40]. Pyrolysis involves the thermal decomposition or degradation of fuel without adding any oxidant in an inert environment, which is also academically referred to as sludge carbonization technology [41].
Compared with incineration and other technologies, sludge pyrolysis showed certain advantages in effective energy utilization and greenhouse gas emission reduction. In addition, pyrolysis technology could be applied to the recycling of sludge, micro-algae, lignocellulose and organic waste; the breadth of raw materials made pyrolysis technology more advantageous in practical application [42,43]. Pyrolysis products include pyrolysis gas (mainly organic matter with a small molecular weight and water vapor), bio-oil, and solid carbide (mainly fixed carbon and inorganic matter) that organic matter is able to decompose during the pyrolysis process [44]. Among them, the heating value of bio-oil was generally more than 30 MJ/kg, meaning it could be used as fuel for heating or power generation or as liquid fuel after purification. In addition, pyrolysis gas could not only be burned as fuel, but also upgraded to syngas, which could be processed into liquid fuel or chemical products [16]. Moreover, the biochar resulting from the MSS pyrolysis could also be used for the adsorption of CO2 and other pollutants in sewage, which was very attractive for sewage treatment plants [45,46]. Although the further usage of pyrolysis is promising, its feasibility depends on the sludge’s energy potential, heavy metal content and nutrient content [40].

2.3.4. Gasification

Gasification is carried out at high temperatures (>700 °C) and at an atmospheric pressure to convert biomass into combustible gas [47]. Gasification produces non-condensing gas such as H2, CO, CO2, and light hydrocarbons such as CH4, C2H2, C2H4, C2H6, C3H8 and C3H6 [48]. The heating value of the gas produced by gasification was about 4 MJ/m3, but it required further treatment to be used for heating and power generation, or to upgrade syngas for liquid fuels and chemical synthesis [49]. The ash obtained in the process could be disposed in landfills or used for agriculture and building materials. Gasification avoided the environmental problems caused by incineration, did not require the addition of additional fuel under stable conditions and had high energy conversion efficiency. However, gasification had high requirements for the pretreatment of sludge that the gasification works best when the dry solid content of sludge reached 90% [50].

3. Legislation of MSS Management in China and Jiangsu, China

MSS is categorized as non-hazardous solid waste by China [51]. The treatment and disposal of MSS has attracted great attention from the government in China; the Action Plan for the Prevention and Control of Water Pollution (2015) [52], the Implementation Plan for the Shortcomings of Urban Domestic Sewage Treatment Facilities (2020) [53] and the “14th Five-Year Plan for the Development of Urban Sewage Treatment and Energy Utilization” (2021) [54] have continuously expanded on the requirements for MSS treatment and disposal, and clearly put forward the need to “solve the difficulties of sludge disposal, realize harmlessness and recycling treatment of MSS”. In addition, cities in the eastern region are required to accelerate the reduction in the scale of MSS landfills and promote the energy utilization of MSS. In order to strengthen the disposal and management of MSS, the Ministry of Housing and Construction and the Ministry of Ecology and Environment formulated a number of national industry standards on sludge land use, incineration, building materials utilization, etc.
Incineration is at a disadvantage in terms of legislative priorities, even though it has had a long period of development as a sludge thermal treatment process. The “Jiangsu Province Environmental Infrastructure Three-Year Construction Plan (2018–2020)” [55] limited sludge incineration to the periodic treatment and disposal of MSS when there were industrial kilns available for use alone due to the high toxic and harmful substances content in MSS. In addition, “Disposal of sludge from municipal wastewater treatment plant—Quality of sludge used in mono-incineration” (GB/T 24602 2009) clearly stipulates the heavy metal content and emission flue gas limit. Moreover, incineration fly ash was listed as hazardous waste for incineration disposal residue in the National Hazardous Waste Catalogue [56]. Overall, the legislation on sludge incineration is strict.
As a sustainable and energy-efficient sludge treatment method, anaerobic digestion (AD) has received regulatory support due to the generation of biogas. In the “14th Five-Year Plan for the Development of Urban Sewage Treatment and Energy Utilization” [54], the use of anaerobic digestion for harmless MSS treatment was encouraged. Jiangsu Province also proposed policy subsidies for products of AD, such as biogas [55]. However, due to the risk of the incomplete elimination of pathogenic bacteria and heavy metal contamination [57], AD is not fully supported by regulations; the agricultural use of digested sludge is limited due to limits on heavy metals and pathogenic bacteria content.
Energy utilization had been raised to an unprecedented height in sludge treatment and disposal. Pyrolysis, which can convert MSS into energy products such as bio-char [20], is strongly supported by legislation; therefore, use of pyrolysis technology for sludge disposal is clearly encouraged. Furthermore, another emerging thermal treatment technology, gasification, could also help to produce energy through bio-oil and bio-gas. In addition, pathogenic bacteria could be completely sterilized during pyrolysis and the gasification process, thereby eliminating the risk of disease transmission and ensuring the treated sludge meets sludge disposal standards [40]. Critically, pyrolysis could stabilize heavy metals such as arsenic, cadmium and cobalt to infuse them in pyrolytic carbon with an inactive state, thereby reducing the toxicity of heavy metals [58].

4. The Energy Recovery Potential Assessment

In order to reuse energy in MSS, it is necessary to make a specific assessment of the energy recovery potential of the sludge after treatment. For the evaluation of the energy recovery potential of sludge, there are models used to evaluate anaerobic digestion and the incineration of sludge [59]; however, methods to assess the energy recovery potential of pyrolysis and gasification still need to be studied to understand the complex reactions and product diversification.

4.1. Energy Recovery Potential Assessment of Anaerobic Digestion

According to the results of the sludge elemental analysis shown in Table S1, the average value of the elemental analysis results of 20 sewage treatment plants was taken as the representative value, and the chemical composition was C213H576O131N41S1. Buswell’s calculation method [60] was employed to calculate the molar fraction of biogas produced by anaerobic digestion (CH4, CO2, NH3, H2S), and the chemical composition of the sludge; per unit mole of sludge could theoretically lead to biogas containing 66 mol CO2, 147 mol CH4, 41 mol NH3 and 1 mol H2S by anaerobic digestion, suggesting the sludge without pretreatment could produce biogas that contains 22.88% CO2 and 57.65% CH4 through anaerobic digestion. The heating value of the mixed gas was calculated as 17.78 MJ/kg. Combined with the industrial analysis of the sludge samples, the volatile content V was estimated, assuming that the content of degradable organic carbon was 77% [61], digestion efficiency was 60%, and the efficiency of the gas engine was 30% [62]. Hence, the average energy recovery potential of sludge anaerobic digestion was calculated to be 1482 kWh/t dry sludge. Karagiannidis explored the potential of sludge energy recovery in Greece and showed that anaerobic digestion produced about 1400 kWh/t dry sludge [63]. The energy recovery potential of sludge anaerobic digestion was basically the same. Nevertheless, studies have shown that when the treatment capacity of the sewage plant is less than 18,900 m³/d, the amount of biogas generated cannot meet the electricity demand [64]. When the organic matter content in the sludge is less than 50%, single-digestion energy recovery might no longer be feasible [65]. For the 20 wastewater treatment plants in Jiangsu included in this survey, only about 35% could meet the scale requirements to achieve the electricity consumption needs through anaerobic digestion.

4.2. Energy Recovery Potential Assessment of Incineration

According to the heating value analysis results shown in Table S1, the arithmetic average value of the heating value of the 20 sewage treatment plants was taken as the representative value, and the total energy was calculated using Tchobanoglous’s method [66]. The average energy recovery potential of MSS incineration was 549 kWh/t dry sludge, which is less than the value found in Karagiannidis’s assessment, where sludge incineration led to about 1400–1700 kWh/t dry sludge [63], and was also far less than that of anaerobic digestion. The reason might be that the moisture content of sludge in this study was relatively high, and the organic matter content and heating value was low; at the same time, mono-incineration would cause energy losses. Additionally, sludge disposal in Jiangsu is mostly conducted through external incineration. The average cost of sludge external incineration was RMB 270/ton (including sludge disposal fees and transportation costs), because the distance between the thermal electric incineration plant and the sewage treatment plant was generally about 20 km, as it was difficult to reuse the heat energy generated for the sewage treatment plant. Therefore, for sewage treatment plants, it is almost impossible to obtain direct economic benefits unless thermoelectric incineration is realized in the sewage plant.

4.3. Energy Recovery Potential Assessment of Pyrolysis

Pyrolysis has not yet been applied on a large scale; most of the research is still in the laboratory stage. Studies have shown that pyrolysis cannot directly produce electricity, and that this method can lead to sulfur dioxide and carbon dioxide emissions; furthermore, when the sludge organic matter content was 40%, the power generated by the combustion of pygas and bio-oil could not meet the heat input of the whole process, and the environmental burden was 5.4 times that of anaerobic digestion. Although the self-sustaining system could combust when the sludge organic matter content increased to 60%, it still needed external power support, and the environmental burden also increased along with an increase in sludge organic matter, due to the increase in NOx and SO2 emissions. This increased the demand for gas treatment [67]. Zhang et al. simulated the application of sludge pyrolysis in bench experiments, and found that after optimizing the pyrolysis operation process, 50% dry reducing sludge and 30~40% liquid could be obtained, the calorific value of the products was 26.0~33.0 MJ/kg, and the pyrolysis gas and bio-oil combustion could meet the energy requirements of the whole system, where the energy required for drying was not included [68]. The results obtained varied greatly due to the different pyrolysis product yields and the different contents contained in the energy balance analysis of each system. In the research of Zhang et al., the self-supply of energy of the pyrolysis system was mainly realized with the help of system optimization, which provided strong support for the practical application of pyrolysis in the future.

4.4. Energy Recovery Potential Assessment of Gasification

Gasification has not yet been applied on a pilot-scale, whereas laboratory-phase studies have shown that syngas could be further processed for chemical or liquid fuel synthesis in addition to power generation, and the generated ash could be reused in agriculture or construction. Azize et al. obtained about 1 kWh of electricity by gasifying 1.2 kg of municipal sludge. Considering Turkey’s renewable energy legislation, the electricity revenue was estimated to be 133 USD/MWh and the biochar sales revenue was evaluated to be 0.05 USD/kg; hence, the return on investment of the gasification equipment was estimated to be 3.3 years [69]. However, when the ash content in the sludge is too high, it could adversely affect the operation of the gasifier, such as during sintering, agglomeration and clinker formation, which would lead to the frequent shutdown of the reactor for maintenance [70]. In addition, the main challenges of sludge gasification include the high inorganic content, tar minimization and ash-related problems caused by the sludge’s composition (moisture, heavy metals, nitrogen and sulfur), which would limit further applications of sludge gasification [34]. It will only be possible for gasification to be applied in the field once these problems are solved.

5. Comparative Evaluation by SWOT-FAHP Analysis

5.1. SWOT Analysis

The successful application of technology for sludge treatment depends on a series of factors such as legislation, ecological impact and economic development. A SWOT analysis was employed to evaluate sludge treatment and disposal technologies to select the most suitable technology. To evaluate the feasibility of anaerobic digestion, incineration, gasification and pyrolysis technologies for the energy utilization of sewage sludge, four issues were raised, which are problem solving, ecological environment, technological development and laws and regulations.
(1)
Problem solving: Was the relevant energy utilization method sufficient to treat sewage sludge or is additional treatment required?
(2)
Ecological environment: Were greenhouse gas emissions produced under the background of carbon neutrality, and could a reduction in greenhouse gas emissions be achieved?
(3)
Technological development: Has the method been applied in Jiangsu Province, and at what stage is it currently in (site scale, pilot scale or laboratory scale)?
(4)
Laws and regulations: Were there any corresponding laws and regulations related to this method? Was it a national standard or an industry standard?
Table 1 presents the SWOT analysis of internal advantages (S), disadvantages (W), external opportunities (O) and threats (T) of each scheme under the above four issues.

5.2. Fuzzy Hierarchy Model and the Introduction of the Trapezoidal Fuzzy Function Method

The decision hierarchy was built by using weight values obtained as results of mutual evaluation criteria and scenarios, which are shown in Figure 4. The binary comparison process was based on the opinions of 20 experts in sewage sludge treatment, who were responsible for the management of the 20 sewage treatment plants at the time and had a practical understanding of the treatment and disposal technology of MSS, to determine the relative importance of the elements at each level. The main goals and criteria that affect lower-level decisions in the hierarchy are placed at the top of the hierarchy, while the decision alternatives can be seen at the bottom.
The importance of different indicators was measured according to the importance scale proposed by Walczuk [80], and different scales are used to determine the distribution of importance, as shown in Table 2. The evaluation form presented in Table S1 in this study was completed by these 20 experts for SWOT analysis. These experts evaluated the (a), (b), (c) and (d) criteria and (1), (2), (3) and (4) methods, respectively, to construct the fuzzy assessment matrix.
The fuzzy set theory was proposed by Zadeh [81] to develop simplified models and analyze complex real-world systems. Fuzzy logic can be applied when there is uncertainty or when an optimal decision needs to be made in the case of incomplete information. Fuzzy logic is a multivariate theory that uses averages such as “medium”, “high”, “low” instead of classical variables such as “yes” “no” and “right” “wrong”. To this end, by introducing trapezoidal fuzzy numbers into the analytic hierarchy method, the weights and ranking problems of energy utilization decisions were calculated and studied to ensure the accuracy of the ranking. Therefore, this study referred to the calculation method mentioned by Solangi et al. [19], and employed MATLAB software to carry out the SWOT-FAHP analysis based on the scores of the 20 sewage treatment plant management experts.

5.3. Results of SWOT-FAHP Analysis

Four standards were used for the SWOT-FAHP analysis to determine the energy utilization of sewage sludge, and the weight values obtained by the binary comparison of each standard and the energy utilization mode are shown in Table 3.
According to the weight value, in terms of standards, the hierarchy is as follows: problem solving > ecological environment > technology development > laws and regulations; in terms of energy utilization methods, the hierarchy is as follows: pyrolysis > anaerobic digestion > gasification > incineration, which is basically consistent with the conclusions of Adar [82]. The difference is the weight of the ecological environment in the calculation results, which was higher than that of technological development. This might be attributed to the advancement of a series of ecological policies such as carbon neutrality and carbon peaking.
In addition, the weight values of the sludge energy utilization method on the four standards in the decision hierarchy are shown in Table 4. When the weights of individual standards were determined by fuzzy hierarchy (FAHP), some standards were zero weight so the weight calculation was performed by using the traditional AHP method. Therefore, the incineration method weight values under the ecological and environmental criteria were calculated by the conventional AHP method.
A major difference is shown for the weight value. Pyrolysis had the highest weight value in problem solving and ecological environment standards, the energy utilization potential of carbide and pyrolysis gas were high and the secondary pollution of heavy metal was minimal; however, it is still in the development stage, so the weight value in the technical development standard is significantly lower than that of anaerobic digestion and incineration technologies. Anaerobic digestion had the highest weight value in the technical development standard, as it could stabilize perishable organic matter, kill the pathogenic bacteria, reduce the sludge volume and recover energy; furthermore, it had implemented field-scale application in sewage treatment plants. Although incineration could completely eliminate pathogens and odor, the NOx, N2O, SOX, dioxin and furan generated by incineration would lead to a number of pollution problems, so its weight in ecological and environmental standards was 0.0000 [82]. Gasification had the lowest weights because of its strict pretreatment requirements, operational conditions and the development of technology, and there are fewer field-site scale applications in sewage treatment plants. Since incineration is still the main sludge treatment and disposal method in Jiangsu Province, the weight of incineration in this study was higher. In addition, for the above four sludge energy utilization methods, there was no obvious difference in the weight value of laws and regulations, and the weight of incineration was slightly higher than that of other methods due to the strict requirements for physical and chemical indicators such as the heating value of incineration.
Overall, the weight values showed the following trend: pyrolysis > anaerobic digestion > incineration > gasification. Samolada M.C. et al. demonstrated that pyrolysis was the optimal technique compared to incineration and gasification in Greece [20]. The combination of anaerobic digestion and pyrolysis is the better decision out of the five methods of anaerobic digestion, incineration, gasification, pyrolysis, anaerobic digestion and pyrolysis in Tibet [22]. Elanur Adar et al. concluded that the sludge energy utilization method is suitable for Turkey, and weighted the methods as follows: supercritical water gasification > anaerobic digestion > gasification > pyrolysis > incineration [82]. Considering the four sludge energy utilization technologies, pyrolysis and anaerobic digestion were the better energy utilization methods, which is consistent with the above study.

6. Potential Deployment Barriers to Sludge Pyrolysis

From the perspective of problem solving and ecological environment standards, pyrolysis has great advantages related to its zero-waste production, lower gas-waste emissions and energy and fuel generation. Furthermore, the pyrolysis carbon produced could be reused in sewage treatment plants for pollutant adsorption, which demonstrates its advantages as a carbon-negative technology. With the gradual rise of pyrolysis technology, the WuJiang sewage treatment plants adopted low-temperature pyrolysis equipment, which could lead to a sludge treatment capacity of 30 m³/d, with a reduction of 90% of MSS. Additionally, energy consumption could be reduced by about 40% compared with the original anaerobic digestion process. The operation of pyrolysis technology in technology and commerce could demonstrate its effectiveness. However, the further promotion of pyrolysis technology still faces the following obstacles:
(1)
Sales of pyrolysis products: The market for pyrolysis products needs to be further expanded to effectively utilize pyrolysis products.
(2)
Technical risk issues: If the technology is not applied on a large scale before implementation, the relevant parties would be generally unwilling to bear any unknown risks.
(3)
Financial support issues: It was difficult to obtain project funding due to the lack of operational experience.
Although these obstacles might be difficult to overcome in the short term, for economically developed and policy-supported regions, the motivation to promote such energy-producing, low pollution, lower energy consumption technology would undoubtedly be high in the long run. To this end, this study makes the following recommendations:
(1)
Countermeasures to manage the market problems of pyrolysis products: pygas and bio-oil could be applied for boiler power generation, fuel production, chemical raw materials, etc. Biochar could be employed to produce activated carbon, for soil remediation, in situ adsorption of pollutants in sewage plants, etc.
(2)
Countermeasures to manage large-scale application problems: the government could be the initiator and invest a certain amount of research funds and establish sludge pyrolysis pilots.

7. Conclusions

The large amount of sludge and the need to further improve the quality and efficiency of sludge treatment and disposal make it necessary to evaluate sludge treatment and disposal. At the same time, the high content of carbon, nitrogen and phosphorus in MSS indicates the great energy recovery potential of MSS and provides an opportunity to realize the energy utilization of MSS. The representative MSS and the main energy recovery MSS treatment and disposal technologies, along with the legislative management of these four technologies in China and Jiangsu, were analyzed. Although more than 70% of sewage treatment plants in this study applied thermoelectric incineration technology, the energy potential of sludge incineration was not high due to the high water content. The energy potential of anaerobic digestion could achieve 1400 kWh/t dry sludge; when the organic matter content was higher than 50% and the treatment capacity of the sewage plant was larger than 18,900 m³/d sewage, the system could achieve the self-supply of energy. Due to the variability of the operation process and the complexity of the product, pyrolysis and gasification technologies are still in the experimental stage. The SWOT-FAHP analysis was employed to evaluate the four energy recovery technologies for sludge treatment from the following four perspectives: problem solving, ecological environment, laws and regulations and technology development. Pyrolysis was identified to be a promising sludge energy recovery technology as its products had a high energy value and it could avoid heavy metal problems. However, the technical risk, product market and capital problems would impede the practical application of pyrolysis; therefore, these problems must be solved, otherwise pyrolysis technology would not be successful commercially. Regardless, for economically developed and policy-supported regions, the motivation to promote such an energy-producing, low pollution, lower energy consumption technology will be high in the long run. Consequently, pyrolysis is likely to become the first choice for sludge disposal in economically developed areas in the future.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w15020260/s1, Table S1: The situation of sewage treatment plants.

Author Contributions

Conceptualization, L.X. and H.L.; methodology, L.X.; software, L.X.; validation, L.X.; formal analysis, L.X.; investigation, D.X. and Y.W.; resources, Y.W.; data curation, L.X.; writing—original draft preparation, L.Q.; writing—review and editing, H.L.; visualization, L.X.; supervision, H.L.; project administration, H.L.; funding acquisition, H.L. 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

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 15 00260 g001 550
Figure 1. Composition of municipal sewage sludge.
Figure 1. Composition of municipal sewage sludge.
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Figure 2. Distribution of 20 sewage treatment plants in Jiangsu.
Figure 2. Distribution of 20 sewage treatment plants in Jiangsu.
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Figure 3. (a) Elemental analysis of MSS in Jiangsu; (b) industrial analysis of MSS in Jiangsu.
Figure 3. (a) Elemental analysis of MSS in Jiangsu; (b) industrial analysis of MSS in Jiangsu.
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Figure 4. The decision hierarchy model of municipal sewage sludge.
Figure 4. The decision hierarchy model of municipal sewage sludge.
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Table
Table 1. SWOT analysis matrix.
Table 1. SWOT analysis matrix.
SchemeIssueInternal Advantages (S)Internal Disadvantages (W)External Opportunities (O)External Threats (T)
Anaerobic digestionProblem solving(1) No need to dry or dewater.
(2) Produces biogas with high energy potential (natural gas, fuel and fertilizer production) [71].
(3) Low external input energy requirements.
(4) Sludge stabilization and reduction.		(1) Long reaction time: >14 days [4].
(2) Toxicity of exogenous pollutants at high levels [72].
(3) Biogas requires subsequent treatment before it can be utilized [73].
(4) High system operation requirements.		(1) The biogas produced could replace fossil fuels.
(2) The organic fertilizers could replace nitrogen and phosphate fertilizers.		(1) Needs large amount of land.
(2) Competition with other methods.
(3) Affected by the seasons.
Ecological environment(1) Emission control system available.(1) CO2 emissions.--
Technological development(1) Mature technology.
(2) Could be coordinated with biomass anaerobic digestion.		(1) High investment costs.
(2) Low energy conversion efficiency.
(3) Heavy metals, persistent organic pollutants, etc., could not be eliminated and require subsequent treatment. 		(1) Wider applicability than other energy utilization technologies.(1) The economic environment is unstable.
Laws and regulations-(1) There are no complete laws or regulation framework.(1) Environmental awareness is gradually increasing.
(2) The support of policy.		-
IncinerationProblem solving(1) Sludge is completely reduced and harmless.
(2) Heat could be recycled for power generation and heating [74].
(3) Minimizes odor generation.
(4) Ash residue could be used for building materials and the production of phosphoric acid [25].		(1) The sludge needs to be dewatered/dried [75].
(2) Energy efficiency is low, and mono-incineration had corresponding requirements for the heating value of sludge [36].		(1) Government subsidies are available.(1) Competition with other thermal technologies.
(2) Higher investment.
Ecological environment(1) Emission control system is available.(1) Production of chloro-compounds.
(2) Air pollution problems (NOX, N2O, SOX, dioxin and furan emissions) [76].
(3) Increased demand for gas treatment and strict environmental emission control. 		(1) Environmental problems due to high-risk emissions.
Technological development(1) Mature technology.
(2) Coordinated incineration could utilize a variety of solid wastes.		(1) Pretreatment of the sludge required.
(2) Generally suitable for large sewage plants to ensure economic viability.
(3) Low energy conversion efficiency.
(4) The problem of heavy metal content in ash treatment [77].		(1) Technological progress and safety factor improved. (1) The economic environment was unstable.
(2) High cost of blended fuels.
Laws and regulations(1) Laws and regulations exist and have been adopted.(1) Sludge incineration requires strict legal control.(1) Environmental awareness is gradually increasing.(1) Strengthen legal controls.
GasificationProblem solving(1) No additional energy required after stable operation.
(2) Syngas could be used as a feedstock for the production of natural gas, hydrogen and chemical synthesis [78].
(3) Ash residue could be used for building materials.		(1) The moisture content of the sludge should be less than 30%.
(2) The composition and efficiency of the gasification products may vary depending on the operating parameters, and improper handling could produce a large number of by-products such as tar [79]. 		(1) Increased research and investment.(1) Financing difficulties.
(2) Competition with other thermal processing processes.
Ecological environment(1) Avoids NOX, SOX, dioxin and furan emissions.(1) Releases large amounts of organic pollutants. -
Technological development(1) High energy efficiency and carbon balance achieved.
(2) Could be co-treated with biomass.		(1) Heavy metal content in ash treatment [77].
(2) At present, it has not been promoted and applied.
(3) High investment and operating costs.
(4) The synthesized gas contains tar, coke and hydrogen carbon that may contaminate the device.		(1) Renewable technology research and development cooperation.(1) The economic environment was unstable.
(2) Commercial feasibility is unknown.
Laws and regulations-(1) There are no perfect laws or regulations.(1) Environmental awareness is gradually increasing.(1) Lack of environmental standards.
PyrolysisProblem solving(1) Bio-oil, pyrolysis gas and biochar have market potential.
(2) Short processing cycle, large equipment processing capacity and small footprint.
(3) Sludge is completely reduced and harmless [40].		(1) Need to be dewatered or dried.
(2) The composition and efficiency of the pyrolysis product depends on the operating parameters and sludge characteristics [58].		(1) Increased research and investment.-
Ecological environment(1) Reduced greenhouse gas emissions.
(2) Avoids NOX, SOX, dioxin and furan emissions.		(1) CO and CO2 emissions.
Technological development(1) High energy efficiency and carbon balance achieved.(1) It has not been promoted and applied.
(2) Most of the technologies are proprietary abroad.		(1) Renewable technology research and development cooperation.(1) The economic environment is unstable.
(2) Commercial feasibility needs to be further studied.
Laws and regulations (1) There are no perfect laws or regulations.(1) Environmental awareness is gradually increasing.(1) Lack of environmental standards.
Table
Table 2. Importance scale.
Table 2. Importance scale.
Scale ValuesMeaningAnnotation
0.5Equally importanti was equally important to j
0.6Moderately importanti was slightly more important than j
0.7Obviously importanti was obviously more important than j
0.8Strongly importanti was much more important than j
0.9Extremely importanti was extremely important than j
0.1, 0.2, 0.3, 0.4Compare insteadThe two elements i and j are compared in reverse, rij = 1−rji
Table
Table 3. Binary comparison weight values for each standard and energy utilization method.
Table 3. Binary comparison weight values for each standard and energy utilization method.
StandardWeight ValueEnergy Utilization MethodsWeight
Problem solving (a)0.4945AD (1)0.3366
Ecological environment (b)0.2336Incineration (2)0.1443
Technological developments (c)0.1665Gasification (3)0.1756
Laws and regulations (d)0.1054Pyrolysis (4)0.3435
Table
Table 4. Weight values according to the standard energy utilization method.
Table 4. Weight values according to the standard energy utilization method.
Problem Solving (A)Ecological Environment (B)Technological Developments (C)Laws and Regulations (D)Weight Value
AD (1)0.25230.24140.43440.27350.2974
Incineration (2)0.15450.00000.36110.31240.2032
Gasification (3)0.17130.32560.04490.18400.1731
Pyrolysis (4)0.43190.43300.15960.23010.3163
Weight value0.37540.25570.22860.14031.0000
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Xiang, L.; Li, H.; Wang, Y.; Qu, L.; Xiao, D. Energy Utilization Assessment of Municipal Sewage Sludge Based on SWOT-FAHP Analysis. Water 2023, 15, 260. https://doi.org/10.3390/w15020260

AMA Style

Xiang L, Li H, Wang Y, Qu L, Xiao D. Energy Utilization Assessment of Municipal Sewage Sludge Based on SWOT-FAHP Analysis. Water. 2023; 15(2):260. https://doi.org/10.3390/w15020260

Chicago/Turabian Style

Xiang, Lu, He Li, Yizhuo Wang, Linyan Qu, and Dandan Xiao. 2023. "Energy Utilization Assessment of Municipal Sewage Sludge Based on SWOT-FAHP Analysis" Water 15, no. 2: 260. https://doi.org/10.3390/w15020260

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