Evaluating the Environmental and Economic Performance of Municipal Solid Waste Disposal by All-Component Resource Recovery

Abstract

Disposal of municipal solid waste (MSW) has become increasingly challenging. In this study, we used life cycle assessment (LCA) to evaluate environmental impacts and financial performance of a new approach for MSW disposals, namely All-components Resource Recovery (AcRR), which is based on automatic sorting. We compared AcRR with the standardized Waste-to-Energy incineration (WtE) to provide decision-making support for MSW management. The results show that WtE and AcRR are both good MSW resource treatment methods. Through MSW disposal, WtE generates electricity, while AcRR generates secondary resources such as metals, plastics, pulp and organic fertilizers. WtE releases trace amounts of HCl, PM₁₀, heavy metals, dioxins and dust, while AcRR does not produce such pollutants; AcRR produces more odor gases such as SO₂ and H₂S. AcRR produces four environmental issues, i.e., Global Warming, Acidification, Photochemical Ozone Synthesis, and Eutrophication, each of which has a smaller impact than WtE; WtE has two more impacts than AcRR: Human Toxicity and Soot and Ashes. The total environmental impact potential of WtE is 3.38 times that of AcRR, and the greenhouse gas emission equivalent is 6.82 times that of AcRR. The cost of construction and operation of AcRR is lower than that of WtE, while the net profit of AcRR is much higher. In conclusion, AcRR is able to screen the mixed MSW into various secondary resources with less environmental emissions and environmental impacts and better financial performance; it may be a promising MSW disposal approach, especially for small cities, but a corresponding supporting industrial system is needed.

1. Introduction

With the rapid development of the economy and population growth, waste disposal has become a hot issue of global concern. Studies have shown that the annual production of municipal solid waste (MSW) in the world is about 1.3 to 2 billion tonnes, and if all countries continue to generate waste with the current rate of waste production equivalent to that of the high-income countries, the total amount of wastes may reach 5.9 billion tonnes per year [1,2,3,4]. According to the publicly available data, the total amount of MSW generated in China was 248.69 million tonnes in 2021, up 5.6 percent from 2020.

Within recent decades, waste management has changed from being a sector primarily focusing on treatment and final disposal of residual streams from society to now being a sector that contributes significantly to energy provision and secondary resource recovery [5]. Because of the high daily processing capacity, a remarkable reduction in volume and the ability to produce electricity, Waste-to-Energy (WtE) through incineration at dedicated plants has gradually become a waste disposal method that is highly valued [6,7], especially in China. For example, the MSW generation of Guangzhou, one of the most important cities in China, was 22,713 tonnes per day in 2020, of which 5000 tonnes were incinerated for WtE, meaning the amount of MSW by incineration was 2 times higher than in 2017. In addition, three other waste incineration plants have been built and will soon be operational, with a daily capacity of 10,000 tonnes. However, the construction and operating costs of a waste incineration power plant is relatively huge [8,9]. Generally, it needs more than 80,000 tonnes of garbage a year to achieve a balance of revenue and expenditure in China. Due to economic limitations, it is difficult for small cities to choose WtE as a long-term method of household waste treatment. On the other hand, the environmental effects of waste incineration often become a public concern issue. Some studies reported that a lot of vegetables produced near certain incinerators had relatively high levels of dioxins, and the furan equivalent concentration of breast milk in local residents was significantly higher than those exposed to low-pollution concentrations [10,11,12,13,14]. Javier García-Pérez et al., 2013 believed that incineration pollution was associated with increased risk of abortion in surrounding populations [15]. Therefore, it is still very difficult for WtE to control pollution effectively. Choosing a site for a MSW incineration power plant is often incredibly hard because of NIMBY conflict (not in my back yard).

Recently, a back-end sorting method, named All-component Resource Recovery (AcRR) has received a lot of attention. Through the process of electromagnetic separation, eddy current separation, plastic flotation, and step-by-step screening, AcRR automatically classifies the mixed MSW into secondary metals, fibers, plastics, organic matters and inorganic substances. For example, a county-level MSW disposal plant by ACRR, located in central China, has a designed processing capacity of 200 tonnes daily, while its actual processing capacity is 60 tonnes/day. Through the disposal of MSW, the plant can get 0.21 tonnes of metal, 9.48 tonnes of pulp, 6.72 tonnes of plastic, 13.13 tonnes of organic fertilizer, and 3.93 tonnes of inorganic substance a day. AcRR contains both dry and wet selection handlers and can be viewed as an improvement over mechanical biological treatment (MBT). It is a promising MSW disposal method, but there are still some disadvantages and controversies, such as odor and biodegradation efficiency, which need to be improved. Due to the lack of systematic environmental and economic assessments, AcRR is currently difficult for municipalities to accept as an MSW disposal option.

As highlighted by Assamoi and Lawryshyn (2012), a good integrated solid waste management is that, rather than accepting a simple hierarchy, alternatives should be examined systematically so that waste is managed in a resourceful, environmentally friendly and economically viable manner [16]. In recent years, life cycle assessment (LCA) has been widely used to assess the environmental benefits and drawbacks of waste management. LCA is a mature assessment method, and there have been overall assessment guidelines outlining the main assessment principles [5]. Although LCA is a useful tool to evaluate the performance of MSW management systems [5,16], as Laurent et al., 2014 and Astrup et al., 2014 pointed out, there was a lack of consistency between individual LCA studies in the literature [3,5]. Modeling principles, technology parameters, emission levels, choices of impact assessment methodologies and environmental values often resulted in different results between LCA studies for the same scenarios [5], which easily leads to the subjectivity of the decision. China is now vigorously developing MSW incineration facilities; other MSW recycling options are rarely considered by local municipalities [17], but such waste management decisions were often not supported by systematic assessment of the environmental friendliness of the alternatives. Under the increasing pressure of carbon emission reduction, scientific and systematic environmentally and economically friendly analysis of the various MSW disposal approaches needs to be carried out urgently. In this paper, we used LCA to investigate the input, output, and environmental emissions and impacts of AcRR, compared with the current mainstream resource recovery method, WtE. On the basis of the above assessment, the financial performance of the two MSW treatment schemes was evaluated. Our objectives are (1) to assess the environmental and economic practicality of AcRR to provide decision-making support for MSW management; (2) to provide suggestions to enhance the AcRR technology based on LCA results; and (3) to explore the important technical parameters and boundaries of LCA in evaluating AcRR and WtE.

2. Materials and Methods

2.1. Methodology

The LCA method was used to refine the analysis of the treatment process of AcRR and WtE for MSW. The structure of a LCA generally consists of four distinct phases [18]: (1) Goal and scope definition, which defines the studied systems and the options that will be compared; (2) Life cycle inventory (LCI), which focuses on the analysis of mass quantification and energy fluxes; (3) Impact assessment or LCIA, which evaluates the significance of potential environmental impacts; and (4) Interpretation, which analyses the results to reach conclusions and recommendations.

LCA simulations were carried out by using e-Balance software. CML-IA model, a database containing the characterization factors required for life cycle impact assessment, was used for the environmental impact assessment. By calculating pollutants with the same environmental impact effect using equivalent factors (characterization), we integrated them into characteristic pollutants (normalization) and analyzed the environmental impact by using standardised and weighted factors (standardization and weighted assessment), then we were able to visually compare the environmental impact levels of the two domestic waste resource treatment technologies.

2.2. Goal and Scope Definition

2.2.1. Selected Study Site and Function Unit

In this paper, Guangzhou in South China was taken as the selected study site due to its large volume of MSW and rapid growth rate, increasing amount of incineration and increasing number of MSW treatment options, as well as accessible details of its MSW diversion initiatives. The disposal process of 1 tonne of MSW was defined as the functional unit, and its composition is shown in Figure 1.

Guangzhou is located in the south-central part of Guangdong Province, China. Being on the northern edge of the Pearl River Delta and the verge of the South China Sea, Guangzhou is a key city in the Guangdong, Hong Kong, and Macao Bay Area and the Pan-Pearl River Delta Economic Zone. Guangzhou has a total area of 7434 square kilometers, with a permanent population of 18,810,600. The household waste generation of Guangzhou was 20,048 tonnes per day in 2017 and 22,713 tonnes per day in 2020. In 2020, its MSW incineration amount was 5000 tonnes, double that of 2017. In addition, several other processing facilities have been built and will soon be operational, with a daily processing capacity of 10,000 tonnes.

2.2.2. System Boundary

In this study, two MSW management scenarios were investigated. MSW is transported directly from the transfer station to the disposal plant for subsequent processing by the pick-up truck without pre-classification.

AcRR realizes the automatic separation of all components of the mixed MSW through electromagnetic sorting, eddy current sorting, foam flotation, and screening of the mixed MSW, then metal, paper, plastic, wood and textiles, gravel, and other easily degradable organics wastes like kitchen waste will be further processed to obtain six major categories of secondary resources including metal, pulp, plastics, organic fertilizers, residual derived fuel (RDF) and inorganics.

In the process of AcRR, MSW is transported to the automatic sorting workshop for secondary resources including metal, pulp, plastics, organic fertilizers, RDF and gravel separation. The mixed MSW first enters the tearing machine to break the bags, then to electromagnetic sorting system, where the big metal pieces are sorted out and compacted after washing. The remaining waste is discharged into the storage pit for biodegradation. Then, a dewatering equipment is used to separate metal, residue and organic liquid concentrate. Subsequently, specific gravity is used to separate sand and other inorganic materials, pulp, plastics, wood and textile stalks and other combustible materials from the residue, meanwhile, the organic concentrated liquid is pumped into a biogas biological treatment tank and fermented. The biogas slurry and residues are concentrated and turn into organic fertilizer, and the biogas enters the combustion chamber to dry the products.

For WtE incineration, MSW is discharged into the storage pit for fermentation and dehydration; leachate generated in this process will be disposed of by the treatment system and tested to meet the standards for water recycling or harmless discharge. MSW after dehydration is sent to the combustion chamber by grab bucket. Combustion gas like SO₂, CO₂, H₂O, flue gas, and noncombustible residues are generated during incineration. In order to mitigate effects originating from the incineration of MSW, the combustion temperature is controlled at more than 850 ℃ and the flue gas will be kept in the incinerator for at least 2 h. Flue gas with high temperature is then sent to the heat exchanger by heating up water for generating electricity via the Rankine cycle, at the same time, flue gas and noncombustible residues are treated to scrub away pollution by gas cleaning processes. Bottom ash and fly ash generated during the incineration process are treated separately accordingly without reuse in this study. The energy generated can only be recovered as electricity, of which 20% will be used for internal purposes and the rest is sold to the grid. Auxiliary fuels required to run the facility are not included in this study.

System boundaries of the two MSW management approaches are shown in Figure 2.

2.3. Environmental Assessment with LCA

2.3.1. Key Assumptions

In the LCA process, only fuel consumption for transportation and energy consumption for equipment operation are taken into account, as well as polluting gases generated by the anaerobic fermentation of biogas and by combustion. Except for the above, any consumption and emissions are not considered.

The condensed water produced by the concentration of organic fertilizers enters the circulating water system after being processed, not discharged.

The power consumption is provided by coal-fired power, and the emission factor is selected from the data of the China Southern Power Grid, which means every time 1 kW·h electricity is generated, 0.72 kg CO₂, 0.005 kg NO_x, and 0.008 kg SO₂ are emitted to the atmosphere [19].

The fuel consumed for transportation is provided by diesel. The average fuel consumption of the domestic waste transfer vehicle is 15.00 L of diesel fuel per 100 km for a load of 8 tonnes, and the average transfer distance is the same for the two resource treatment technologies (25.00 km).

The MSW disposal system is airtight, and the gases produced by anaerobic fermentation all enter the biogas combustion chamber, in which CH₄ and CO are converted into CO₂ after combustion.

2.3.2. Data Issues

In the process of AcRR, there are only three parts that have pollution emissions, namely the collection and transportation of MSW, the operation of processing, and the anaerobic fermentation gas combustion and emission process. The pollutants in these three processes are derived from diesel combustion, thermal power generation, and anaerobic fermentation of organic matter and combustion of combustion gases. The relevant data sources and the emission factor of pollutants are shown in

Table 1. Data source.

MSW Disposal		Resource Consumption	Major Pollutants	Data Sources
S1 a	MSW transportation	Diesel	CO2, NOx, SO2, CO, VOC	Zhou et al., 2012 [20]
	Resource separating	Electricity	CO2, NOx, SO2	2016 China Regional Power Grid Carbon dioxide baseline emission factor BM [21]
				Dong et al., 2007 [22]
	The combustion of biogas		CO2, NOx, SO2, N2O, NH3	Yang et al., 2014 [23]
S2 b		Diesel and Electricity	CO2, NOx, SO2, CO, VOC	GB 18485-2014 Pollution Control Standard for Incineration of Municipal Solid Waste [24]
				GB 2414554-93 for Emission Standard for Odor Pollutants [25]
				Zhao et al., 2016 [26]

^a S1 stands for all-component resource recycling (AcRR). ^b S2 stands for incineration (WtE).

. Considering the fact that the emission of flue gas generated by incineration during the power generation is required to meet the standards, the relevant pollutant emission data is set to 50% of the 24 h average limit value in the “GB 18485-2014 Pollution Control Standard for Incineration of Municipal Solid Waste” and “GB 2414554-93 for Emission Standard for Odor Pollutants”. Other data are sources from the relevant literature listed in Table 1.

Life cycle inventory (LCI) refers to the quantification and rationalization of resource and energy consumption and environmental emissions for all processes within the system boundaries identified in the previous phase (e.g., extraction of raw materials, processing, product manufacturing, packaging, transportation, consumption, recycling and disposal, etc.) and the development of an inventory table, i.e., input and output tables. Based on the above data and the data we observed, the LCI list of the two methods can be obtained. For each scenario, a detailed LCI has been used to determine the environmental emissions. The emissions produced from the construction of facilities are not included in this study.

In addition to the gaseous pollutants other than NH₃, the equivalent factors were used to convert the pollutants with the same environmental impact effect into standard reference materials, then the normalization coefficients and the relevant weighting coefficients of the reference emissions of World 2000 in CML-IA, were used for environmental impact analysis. Based on the pollutant discharges from the two disposal methods, we selected six types of environmental impacts including Global Warming, Acidification, Photochemical Ozone Synthesis, Eutrophication, Human Toxicity, and Soot and Ashes, and the values of each parameter and the environmental impacts were analyzed. The equivalent factor, normalization coefficient, and weighting coefficients are shown in

Table 2. Environmental impact factors.

Environmental Impact	Pollutants	Equivalent Factor	Normalization Coefficient	Weight Coefficient
Global warming	CO2	1	8700 kg CO2 eq/(person·a)	0.83
	N2O	296
	CO	2
Acidification	SO2	1	36 kg SO2 eq/(person·a)	0.73
	NOx	0.7
Photochemical ozone synthesis	VOC	0.6	0.65 kg C2H4 eq/(person·a)	0.53
	CO	0.03
Eutrophication	NOx	1.35	62 kg NO3 eq/(person·a)	0.73
Human toxicity	2,3,7,8-TCDD	1,933,982,792	8760 kg 1,4-DCB eq/(person·a)	1.99
	Cd2	289
	Hg3	6008
	Pb2	467
Soot and Ashes	PM10	1	1.8 kg eq/(person·a)	0.61

Data are derived from the relevant literature [10,23,26,27,28,29,30].

.

2.4. Financial Assessment

In order to carry out a more complete comparison of the two technologies, we performed a financial analysis with generic discounted cost, in addition to environmental LCA analysis. The costs include operational and maintenance costs and inputs, the revenues are comprised of electricity sales and secondary resources sales. For comparison purposes, we set the daily processing capacity of both AcRR and WtE to be 100 tonnes.

The operating costs and benefits of AcRR are calculated based on the market prices of various secondary resources in Guangzhou: plastic particles 2000 yuan/ton, pulp 1200 yuan/ton, organic fertilizer 700 yuan/ton, scrap metal 500 yuan/ton, RDF and sandstone prices were negligible. According to the industrial electricity price benchmark for Guangzhou, the electricity price was chosen as 0.6642 yuan/kW·h [31], diesel as 6.4 yuan/liter, and water as 4.86 yuan/m³. According to the “Notice of the National Development and Reform Commission on Perfecting the Policy of Garbage Incineration Price”, the online price of waste incineration benchmark is chosen as 0.65 yuan/kW·h within 280 kW·h, and 0.4505 yuan for excess [32]. The subsidy fee of 120 yuan formulated by the City Administration Committee was not considered.

3. Results and Discussion

3.1. Resource Input and Output

After evaluation, a LCI list of the two methods can be obtained (

Table 3. Life cycle inventory of the two disposal methods for MSW.

One Ton of MSW Disposal		S1		S2
Energy/material input	Diesel/L	0.94		0.94
	Electricity/kW·h	50		61.6
	Water/m3	0.6		2.3
Energy/material recovery		Metal/kg	3.5	Electricity/kW·h	342
		Pulp/kg	158
		Plastics/kg	122
		Organic fertilizers /kg	168
		RDF/kg	160
		Inorganics/kg	65.4
Greenhouse gases emission/kg	CO2	37.57		256.21
	N2O	0.0005		-
Other gas pollutant emissions/kg	CO	0.006		0.345
	SO2	0.47		0.285
	NOx	0.35		0.75
	NH3	0.01		0.012 (Ⅲ Standard)
	VOC	0.0009		0.18 × 10−4
	H2S	0.15		1.08 × 10−4
	HCl	-		7.36 × 10−4
	Hg	-		1.50 × 10−4
	Cd+Tl	-		3.00 × 10−4
	Sb + As + Pb + Cr +	-		3.00 × 10−3
	Co + Cu + Mn + Ni
	PCDD/DFs (kg TEQ)	-		3.00 × 10−10
Solid pollutant emissions/kg	Fly ash	-		37.76
	Bottom ash	-		134.25

).

As shown in Table 3, to process one ton of MSW, WtE consumes 61.6 kW·h of electricity and 2 tonnes of fresh water, and produces 342 kW·h of electricity; AcRR consumes 50 kW·h of electricity and 0.6 tonnes of fresh water, and produces 3.5 kg of metal, 122 kg of plastics, 158 kg of pulp, 168 kg of organic fertilizers, etc.

Guangzhou currently incinerates about 5000 tonnes of waste a day. Based on the above net generating capacity per ton of waste, WtE could generate about 500 million kW·h of electricity a year. Guangzhou currently consumes about 100 billion kW·h of electricity a year, then the electricity generated from WtE could meet 0.51 percent of Guangzhou’s electricity needs. If all MSW is treated by WtE, the electricity generated could meet 2.3 percent of Guangzhou’s needs. Therefore, WtE is a good way of MSW energy recovery [6,8].

AcRR does not generate electricity, but produces secondary resources such as metals, plastics, pulp and organic fertilizers (Table 3). With the development of economy in the new century, people have to face more and more serious problems of resource shortage and environmental pollution. The utilization of increasing MSW for resource purposes can be helpful for us to solve the problems of energy shortage and environmental protection simultaneously to a certain extent [33]. If all MSW in Guangzhou treated with AcRR, a large number of secondary resources such as metal, plastics, pulp and organic fertilizer can be generated annually, which are about 28,000 tonnes, 1,000,000 tonnes, 1,250,000 tonnes and 1,350,000 tonnes, respectively. On the one hand, it can be seen that AcRR is a good resource and an environmentally friendly way to dispose of MSW. On the other hand, the production of one ton of metal, plastics, pulp and organic fertilizer, respectively requires about 29.88, 250, 593.71 and 155 kW·h of electric energy, and 403.7, 1, 18.25, 20 tonnes of water [34,35,36,37]. If all household waste in Guangzhou is disposed by AcRR, it is equivalent to an annual saving of 1.21 billion kW·h of electric energy (i.e., 1.21 percent of Guangzhou’s annual electricity consumption and 52 percent of the electricity recovered by WtE) and 51.64 million tonnes of water.

Of course, the secondary resources obtained from AcRR are generally of poor quality. For example, pulp has short fiber and health concerns. However, with the rapid development of e-commerce and express delivery industry today [38], if combined with rural straw treatment, its products can be good packing and filling materials, with good market prospects and economic value. Another example is organic fertilizer. Due to the monsoon climate, soil in South China is generally short of organic matters, and there is a great demand for organic fertilizers [39]. However, at present, the output from mixed domestic wastes composting is often regarded as nothing more than a “technical compost,” which cannot meet the demand of fertilizer quality due to contamination, and it cannot be accepted in agriculture [40]. As a matter of fact, organic fertilizer obtained from AcRR is high-quality organic fertilizer in line with relevant standards because various inorganic and organic pollutants have been separated in the treatment process.

3.2. Environmental Emissions and Environmental Impacts

For each ton of MSW processing, WtE emits 7.36 × 10⁻⁴ kg of HCl, 0.019 kg of PM10, 3.0 × 10⁻⁴ kg of Cd + Tl, 1.5 × 10⁻⁴ kg of Hg, 3 × 10⁻³ kg of Pb and other heavy metals, 3.0 × 10⁻¹⁰ kg of TEQ dioxins PCDD/DFs, 37.76 kg of fly ash and 134.25 kg of bottom ash, while AcRR does not produce the above pollutants; CO₂ and CO emissions of WtE are 256.21 kg and 0.345 kg, respectively, which are 6.82 and 57.5 times that of AcRR; the NO_x emission of WtE is 0.75 kg, which is twice that of AcRR (Table 3).

Among all monitored pollutant emission indexes, only SO₂ and H₂S are higher in AcRR than WtE. Sulfur dioxide, hydrogen sulfide and ammonia are the main sources of odor in the process of domestic waste disposal [41]. Nowadays, the waste incineration power plants in Guangzhou are neat and beautiful without any odor, and there is no uncomfortable perception when walking in or around the factories. Such a fact can be reflected in the odor gas emissions in this study, and the same situation is also reflected in many cases of waste incineration power plants around the world [16].

According to the actual emissions, six indicators including global warming, acidification, photochemical ozone synthesis, eutrophication, human toxicity and soot and ashes were selected to analyze the environmental impacts of the two MSW disposal methods based on the CML-IA database. The results are shown in

Table 4. Environmental impact emission equivalent of the two methods.

Environmental Impact	Pollutants	Equivalent Factor	Normalization Coefficient	Weight Coefficient	Total Emission/kg		Emission Equivalent/kg
					S1	S2	S1	S2
Global warming	CO2	1	8700 kg CO2 eq/(person·a)	0.83	37.57	256.21	37.734	256.9
	N2O	296			0.0005	_
	CO	2			0.006	0.345
Acidification	SO2	1	36 kg SO2 eq/(person·a)	0.73	0.47	0.285	0.715	0.81
	NOx	0.7			0.35	0.75
Photochemical ozone synthesis	VOC	0.6	0.65 kg C2H4 eq/(person·a)	0.53	0.0009	0.001	0.00072	0.01095
	CO	0.03			0.006	0.345
Eutrophication	NOx	1.35	62 kg NO3 eq/(person·a)	0.73	0.35	0.75	0.473	1.0125
Human toxicity	2,3,7,8-TCDD	1933982792	8760 kg 1,4-DCB eq/(person·a)	1.99	-	3E-10	-	2.968
	Cd2	289			-	0.0003
	Hg3	6008			-	0.00015
	Pb2	467			-	0.003
Soot and Ashes	PM10	1	1.8 kg eq/(person·a)	0.61	-	0.06	-	0.06

, Figure 3 and Figure 4.

As can be seen from Table 4, WtE produces all six types of environmental impacts, while AcRR only has four types of environmental impacts including global warming, acidification, photochemical ozone synthesis, and eutrophication. After converting greenhouse gases CO and N₂O into CO₂ emission equivalents according to CO₂ emission equivalent factors, the normalized CO₂ emission equivalent of AcRR is 37.734 kg/t, which is 14.69% of WtE. The difference between the two methods for acidification is not significant; AcRR is slightly lower than WtE. Photochemical synthesis and eutrophication responses of AcRR are 6.58% and 46.72% of WtE, respectively.

According to Table 4 and the equivalent value in Figure 3, the emission equivalent of global warming and pollutants in AcRR is lower than WtE, and the environmental impact of AcRR is much lower than that of WtE.

Using the weight coefficients to further analyze the respective environmental impact contributions of these two resource treatment methods (Figure 4), it can be seen that the total environmental impact potentials of AcRR and WtE are 0.024 and 0.081, respectively; WtE is 3.38 times of AcRR.

The degree of the four kinds of environmental impact caused by AcRR from high to low is acidification, eutrophication, global warming, photochemical ozone synthesis. The degree of the six environmental impacts of WtE from high to low is global warming, soot and ashes, acidification, eutrophication, photochemical ozone synthesis and human toxicity.

Nowadays, people are more and more concerned about the discharge of pollutants. Our results show that AcRR has a much smaller environmental impact than WtE.

WtE generates energy while releasing a large amount of greenhouse gases, and its greenhouse gas emission equivalent is 6.82 times that of AcRR, which is undoubtedly a more environmentally friendly MSW treatment technology in today’s world of great pressure for carbon reduction [42]. Taking into account the carbon emission reduction targets set by China under the framework of the Paris Agreement, AcRR is undoubtedly a suitable approach to MSW disposal.

For the general public, the main concern is human toxicity and dust. WtE releases trace amounts of environmental pollutants such as PCDD/DFs and heavy metals during the treatment process (Table 3), which affects the surrounding environment mainly as atmospheric deposition. A study from Zhejiang, China, showed that soil PCDD/DFs levels increased yearly after the operation of a medical waste incineration plant, and the average soil dioxin level around the incineration plant reached 7.05 ng/kg I-TEQ in 2014, which was 6.47 times higher than that in 2007 (the year the study started) [43]. Li et al. (2018) evaluated the carcinogenic risk values of dioxins in the atmosphere and soil environment around a WtE plant, they found that the risk values for children, adolescents and adults were 1.24 × 10⁻⁶, 9.06 × 10⁻⁷ and 4.41 × 10⁻⁶, respectively. Though the values were within the acceptable range, the main source of pollution in the surrounding atmospheric environment was the incineration plant, and the pollution risk is still of concern [27]. It was reported that some agricultural products from around some WtE plant sites showed significantly higher PCDD/DFs equivalent concentrations, and local residents’ breast milk also had significantly higher furan equivalent concentrations than the population exposed to low pollution concentrations [44]. A survey from the Pearl River Delta region of China concluded that, 21% of the atmospheric mercury in the region come from waste incineration [45]. WtE can reduce a large amount of MSW while generating electricity, which is a good way to treat MSW. However, at the same time, we note that WtE still generates more than 15% of fly ash and bottom ash, which are hazardous wastes and require additional special treatment [46]. The characteristic pollutants of fly ash are PM10 and PM2.5. During the environmental impact analysis in this study, the human toxicity of PM2.5 was not included in the environmental impact calculation results because of the lack of characterized, normalized and standardized data on human toxicity in the CML-IA database. However, we can never ignore this effect. Li et al., 2016 reported that PM2.5 contributed significantly to stroke, ischemic heart disease, chronic obstructive pulmonary disease, and lung cancer, contributing up to 15.5% of total deaths [47].

3.3. Financial Performance

AcRR is able to produce 3.5 kg of metal, 158 kg of pulp, 168 kg of organic fertilizers and 65.4 kg of inorganics (

Table 5. Financial assessment of the two MSW disposal methods.

Project	S1				S2
			Consumption/ Output	Unit Price/ ¥			Consumption/Output	Unit Price/ ¥
Expenditure	Input	Diesel/L	0.216	6.4	Input	Diesel/L	0.276	6.4
		Electricity/kW·h	50	0.6642		Electricity/kW·h	61.6	0.6642
		Water/m3	0.6	4.86		Water/m3	2.3	4.86
		Management costs		220		Management costs		220
		Depreciation		8.22		Depreciation		12.33
Operating costs	265.73				286.29
Income	Output/kg	Plastics	122	2	Output/kW·h	Electricity	280	0.65
		Pulp	158	1.2			62	0.4505
		Organic fertilizers	168	0.7
		Metal	3.5	0.5
		RDF	160	0
		Inorganics	65.4	0
Product revenue	552.95				209.931
Net income per tonne/¥	287.22				−76.359
Annual total MSW/ton	36,500
Annual net income/¥	104,835,000,000				−278,710,000,000
Total investment/¥	30,000,000				45,000,000

). On the basis of the reference price, the project income statement of the MSW recycling project (100 tonnes daily processing capacity) in Guangzhou is obtained (Table 5). Based on Table 5, it can be seen that the net profit per unit of MSW resource recycling for AcRR is 363.58 yuan higher than that of WtE. Not considering the subsidy fee provided by government, the incineration plant cannot achieve a surplus. In the case of a daily processing capacity of 100 tonnes, the total revenue of AcRR is 13.27 million yuan higher than that of WtE. Moreover, the investment for the processing system per ton is 150,000 yuan lower than that of the WtE, making AcRR an undoubtedly good choice for small and medium cities with low daily solid waste production.

However, due to the need to sell the different types of secondary resources for sorting and integration to the corresponding secondary resources industry factory, it requires a lot of manpower and material resources. Therefore, a lot of investigations are needed in the site selection process to make a scientific and reasonable plan.

From an economic point of view, the performance of AcRR is superior to WtE. Gao et al. (2018) calculated the total social cost of MSW treatment in another important city in the Pearl River Delta region of China, and concluded that the economic cost of incineration power generation and air pollution were 37.23 yuan and 51.75 yuan/ton, respectively, while the economic cost of terminal classification combined with biological treatment was 86.20 yuan/ton [48]. Song et al. (2017) reported that the total process management cost of MSW incineration treatment in Beijing was 2253 yuan per ton, including pre-collection and transportation and harmless treatment. In addition, due to the increase of dioxin emissions, there will be a huge health loss, that may be 4476 yuan/ton [49]. Therefore, there is a risk that the social costs of waste incineration will spiral out of control.

Waste sorting can reduce the difficulty and cost of resource regeneration to a great extent. However, due to the characteristics of high organic matters content, high humidity and low calorific value of MSW due to diet and lifestyle, as well as the lack of motivation in system, law, education, awareness, etc., it will take a long time for the whole society to build an effective waste sorting system of MSW in China. AcRR is able to play to its strengths in such a situation and reflect great value.

4. Conclusions and Suggestions

This research was conducted based on five strict assumptions and has some limitations. We are able to draw some informative conclusions and we believe that the study can be more widely applied in the current economic development situation and provide some useful reference values for further research in the future.

WtE and AcRR are both good MSW resource treatment methods. Through MSW disposal, WtE generates electricity, while AcRR generates secondary resources such as metals, plastics, pulp and organic fertilizers. WtE releases trace amounts of HCl, PM10, heavy metals, dioxins and other pollutants; AcRR does not produce these pollutants, but releases SO₂, H₂S and other odor gases. WtE produces all six selected environmental effects including global warming, acidification, eutrophication, photochemical ozone synthesis, human toxicity and soot and ashes, while AcRR only produces the first four types of environmental effects. The greenhouse gas emission equivalent of WtE is 6.82 times that of AcRR, and the total environmental impact latent value of WtE is 3.359 times that of ACRR.

Although AcRR has the advantages of low pollution, low emissions, high resource utilization, and low investment cost, there are still certain deficiencies in the actual promotion process, mainly reflected in the lack of policy support, restriction by waste composition and insufficient development of the secondary resources industry. In response to these issues, the following suggestions are made:

(1)

Strengthen policy support. The state should increase assistance to other resource treatment methods while supporting waste incineration, improving the law and issuing relevant documents to promote China’s resource level, and strengthen the development of a recycling economy to further realize China’s ecological and environmental protection.

(2)

Pre-analysis of MSW’s components. AcRR is designed for MSW without waste sorting, so when the composition of the waste is less reproducible, its economic benefits will be affected. Therefore, it is necessary to scientifically analyze the components of local municipal solid waste before selecting this treatment method and formulate a reasonable construction plan based on the analysis results.

(3)

Enrich the industrial system of secondary resources. The various secondary resources obtained after AcRR treatment need to be matched with the corresponding secondary resources industrial system, so that the secondary resources can be further transformed into products and enter the market, adding a powerful driving force for the recycling economy.