Open AccessFeature PaperArticle

Comparative Life Cycle Assessment of Landfilling with Sustainable Waste Management Methods for Municipal Solid Wastes

Biosystems and Agricultural Engineering, Oklahoma State University, Stillwater, OK 74078, USA

Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK 74078, USA

Author to whom correspondence should be addressed.

Environments 2024, 11(11), 248; https://doi.org/10.3390/environments11110248

Submission received: 11 September 2024 / Revised: 25 October 2024 / Accepted: 28 October 2024 / Published: 11 November 2024

(This article belongs to the Special Issue Waste Management and Life Cycle Assessment)

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Abstract

Municipal solid waste (MSW) generation continues to increase exponentially, leading to the need for better disposal methods. Approximately 50% of the MSW is landfilled in the United States (US). Landfilling is known for its negative effects on the environment and human health. The objective of this study was to conduct a life cycle assessment (LCA) of some of the most common waste treatment methods and propose an alternative and environmentally friendly integrated waste management method (IWM). The LCA was conducted using OpenLCA. Replacing landfilling, incineration, and composting with recycling, gasification, and anaerobic digestion (IWM) reduced the global warming potential from 899 kg CO₂ eq to −14.6 kg CO₂ eq. The same trend was observed for acidification (from 0.21 kg SO₂ eq to −1.1 kg SO₂ eq), ecotoxicity (from 2363.8 CTUe to 1.22 CTUe), eutrophication (from 0.5 kg N eq to 0.3 kg N eq), smog formation (from 4.4 kg O₃ eq to 1.85 kg O₃ eq), ozone depletion (from 2.1 × 10⁻⁵ kg CFC-11 eq to 0 kg CFC-11 eq), respiratory effects (from 2.8 × 10⁻³ kg PM2.5 eq to −7.25 × 10⁻³ kg PM2.5 eq), cancer (from 2 × 10⁻⁵ CTUh to 1.2 × 10⁻⁷ CTUh), and non-cancer effects (from 6 × 10⁻⁵ to 1.4 × 10⁻⁵ CTUh). The results show that an integrated waste management approach with recycling, gasification, and anaerobic digestion can dramatically reduce the environmental and health impacts of municipal solid waste disposal. Policy reforms, technical innovation, economic investment, and social engagement are needed to change waste management paradigm.

Keywords:

MSW management; landfilling; recycling; composting; incineration; gasification

1. Introduction

In 2021, the emissions related to the total municipal solid waste (MSW) landfilling in the United States (US) were equivalent to the emissions from driving 23.1 million gasoline-powered cars for a year or emissions from the energy used in 11.9 million homes for a year [1]. The rapid growth of the population, economic activities, and consumption (such as food waste, packaged goods, disposable goods electronics) have raised concerns about waste management methods and their effects on health, safety, and the environment. These concerns highlight the need for sustainable waste management techniques. The reported effects of waste management methods, including landfilling, composting, anaerobic digestion, recycling, pyrolysis, gasification, incineration, and the combination of the processes, on the environment were studied by others [2,3,4,5,6,7,8,9,10,11,12].

Karaiskakis et al. (2024) highlighted the importance of incorporating region-specific conditions when implementing anaerobic digestion (AD) facilities for food waste treatment [13]. Among the aforementioned methods conducted around the world, landfilling and incineration without energy recovery have shown the highest emission levels [3,4,6,9,11]. Waste management with more than one treatment method combined has also been proposed by several studies. The most environmentally friendly combined treatments were: (1) gasification + plasma conversion + landfilling; (2) landfilling + composting, recycling + composting + landfilling; and (3) recycling + anaerobic digestion + landfilling [2,7,11,12]. These studies indicate that emissions related to waste management and disposal can be reduced using a combination of unconventional processes such as recycling, biological conversion (composting and anaerobic digestion), and thermochemical conversion (pyrolysis and gasification).

LCA is an analysis tool used to evaluate the effect of a process, a product, or a service on the environment and human health. In addition, the technique can be used to optimize a process in terms of energy, material, and water usage and emissions control [8]. For LCA, a system boundary is set to identify which stages of the product’s life cycle are studied. The Cradle-to-Gate boundary is the product manufacturing stage with steps such as raw material extraction, and material and product manufacture [14,15]. The Gate-to-Grave boundary involves product usage (transportation, distribution, and consumption or usage) and end of life (disposal). Therefore, the Cradle-to-Grave boundary includes all product life cycle stages [16].

LCA is a useful tool for waste emissions evaluation. However, the study depends on several factors, such as the geographical location of the study, database and waste type, assumptions, system output, and functional unit; hence, reported results vary. Waste composition changes per location and their study have been conducted in different continents: America, Europe, and Asia [2,3,4,5,6,7,8,9,10,11,12]. Jayawickrama et al. (2024) underscored the importance of methodological consistency and the inclusion of all sustainability dimensions in LCAs of anerobic digestion systems [17]. Nubi et al. (2024) reviewed the use of Life Cycle Sustainability Assessment as a comprehensive tool for evaluating waste-to-energy generation technologies, which has potential to reduce dependence on fossil fuels and minimize landfill usage [18]. Another key variable in LCA is the database and impact assessment used. Most of the studies used SimaPro (SimaPro, 2023) and GaBi (Sphera Solutions, 2023) for the life cycle inventory and CML for the impact assessment [19,20]. The most important variables are the assumptions and system boundaries used. Many studies did not include emissions related to waste transportation, while others included those emissions. Based on these variables and assumptions, comparing the LCA results of different research studies is challenging.

This study focused on comparing US-based conventional waste management methods with advanced combined waste treatment methods. The objectives of this study were to: (1) evaluate the overall environment and health impact of conventional combined waste management, food waste composting and anaerobic digestion, and mixed waste (wood, yard trimming, textiles, rubber, and leather) gasification and pyrolysis on the environment; (2) propose an alternative and sustainable approach for the MSW treatment based on the findings in objective 1; and (3) compare the impacts of the proposed approach with those of the conventional waste management practices. The findings of this study can help city authorities, planners, and environmentalists to choose management practices for organic and non-organic wastes.

2. Computational Methods

The LCA was developed following the ISO 14040/44 standard, which requires a goal and scope with a functional unit and boundaries, life cycle inventory (LCI), impact assessment, and results’ interpretation, as shown below [21].

2.1. Goal and Scope

The goal of this study was to evaluate and compare the environmental and human health impact of waste management practices in the US to an optimized integrated system (proposed system). The current MSW management system primarily consists of landfilling, recycling, incineration, and composting [22]. Other methods, such as anaerobic digestion, and gasification, are investigated and implemented in an integrated waste treatment system for further analysis. The functional unit of this study was 1 ton of MSW. The LCA format was completed following the ISO 14040 guidelines. The LCI and impact assessment were completed using OpenLCA and TRACI. The missing data were collected from GREET, LandGEM, HELP, AD-Screening tool, the United States Department of Agriculture (USDA) database, the literature, and government reports and imported to OpenLCA. The impact assessment in TRACI was grouped into two categories: environmental and human health impact. The environmental impact included categories such as global warming (kg CO₂ eq), acidification (kg SO₂ eq), ecotoxicity (CTUe), eutrophication (kg N eq), smog (kg O₃ eq), and ozone depletion (kg PM2.5 eq). The human health impact was composed of categories such as respiratory (kg PM2.5 eq), carcinogenic (CTUh), and non-carcinogenic (CTUh) effects.

2.2. Treatments and Scenarios

2.2.1. Treatment 1: Landfilling

Landfilling consists of burying MSW to allow the anaerobic digestion of the waste, leading to the production of substances such as landfill gas (LFG) and leachate. LFG is mainly composed of methane and CO₂ and contains traces of non-methane organic compounds (NMOCs) [23]. The impact of LFG on global warming is a major concern because of its methane content, and therefore, landfill sites opt to flare or convert the LFG into electricity to curb their environmental impact. However, 46.7% of landfill sites in the US do not have an LFG collection or conversion to energy system [24]. Based on these findings, this study conducted LCA of (1) landfilling with no LFG collection, (2) landfilling with flared LFG, and (3) landfilling with energy recovery. The volume of LFG produced and the composition of NMOCs per ton of MSW landfilled were evaluated using the landfill gas emission model (LandGEM) [25]. The leachate yield was estimated using the Hydrologic Evaluation of Landfill Performance (HELP) [26]. According to Title 40 of the Code of Federal Regulations (CFR) part 258, landfills must install liner systems to prevent leachate from leaking into the groundwater [27]. However, liner systems have been proven to not be completely impermeable. Therefore, the study assumed that 5% of the total leachate entered into the groundwater [28]. The leachate composition published reports on leachate for MSW characterization [29]. All landfilling scenarios and boundaries are shown in Figure 1.

Landfilling with no LFG collection assumed that the total LFG (186.6 m³/ton of waste) was released into the atmosphere. In addition, 5% of leachate leaked into the groundwater (same for all landfilling scenarios). Prior to decomposition, MSW was layered in landfill cells using compactors. This study evaluated the diesel consumption based on the compactor model D6T from the manufacturer Caterpillar. The D6T was able to work a maximum of 50 tons per day with a diesel consumption varying from 13.3 L/h to 24.7 L/h [30]. The diesel consumption was considered the same for all landfilling scenarios.

Landfilling with flared LFG consists of collecting and burning the LFG to reduce its impact on global warming due to the elimination of the methane. LFG collection efficiency varies from 60 to 90% [31]. The LFG collection efficiency was assumed to be 75%. Therefore, 25% of the LFG was released into the atmosphere, and remaining 75% of the LFG was flared to convert methane into CO₂. The CO₂ emitted was calculated based on methane stoichiometric combustion:

{C H}_{4} + 2 (O_{2} + {3.76 N}_{2}) \to {C O}_{2} + {2 H}_{2} O + {7.52 N}_{2}

(1)

Landfilling with energy recovery consists of producing electricity through the combustion of methane. Assuming that 75% of the LFG was collected and converted into electricity and the remaining 25% released into the atmosphere, the methane content in the LFG collected was 70 m³, which is equivalent to a combustion energy of 710.6 kWh. The conversion efficiency of the gas turbine was assumed to be 28%, which led to the generation of 199 kWh of electricity per ton of MSW [6]. Prior to LFG conversion into electricity, the gas impurities were assumed to be removed to avoid the deterioration of the turbine by corrosive compounds, such as hydrogen sulfide (H₂S). The electricity required to upgrade the LFG was estimated based on the data available in Cherubini et al. (2009) [6]. The impacts of electricity consumption and credit due to electricity production were evaluated based on the distributed US Central and Southern Plains Mix in GREET.

2.2.2. Treatment 2: Recycling

According to the EPA, 23% of the US MSW is recycled [22]. There are several types of recycling processes, such as single-stream, mixed-waste, dual-stream, and pre-sorted recycling. This study focused on single-stream recycling as it is the most common method used in the US. In 2010, over 64% of the recycling community used single-stream recycling [32,33]. The single-stream recycling separates waste into old corrugated cardboard (OCC), non-OCC fibers (newsprint and mixed papers), plastic (PET, HDPE, film, and 2–7 class plastic containers), aluminum, and ferrous materials. According to Cascadia Consulting Group (2015), 85% of the materials are recycled and the 15% remaining are contaminants sent to landfills [34]. The single-stream recycling diagram (Figure 2) was designed based on the system presented in the tour given by GreenWaste (2023) and Cascadia Consulting Group (2015) [34,35]. The specifications of recycling depend on waste parameters such as object size, density, shape, optical, and magnetic properties. The MSW is assumed to be collected, weighed, and then discharged on a tipping floor, where the waste is inspected to remove large and/or dangerous objects. The waste is then moved in a metering bin using a front loader that consumes 6.7 L of diesel per ton of waste. The waste is distributed on a conveyor where manual pre-sorting is conducted to remove unopened bags, plastic films, and other contaminants. The unopened bags are sent to a bag breaker to tear the bags and then fed back to the metering bin. After manual sorting, the mixed material proceeds to the OCC screen, where the OCC materials are removed and then manually sorted (quality control) to remove plastic films and other contaminants. The remaining material passes through a debris screen, where glass debris is removed. The rest of the material is processed on three polishing screens. Two polishing screens remove newsprint, and the third screen removes mixed papers. The newsprint and mixed papers streams pass through quality control stations, where manual sorters look for plastic films and contaminants. The stream passes through a vacuum to remove any plastic film contaminant. The bottom stream (metal and plastic containers) is fed to the electromagnetic separator, where ferrous materials are removed and manually sorted to remove contaminants. The rest of the stream is separated using optical sensors (PET, HDPE, and 2–7 class plastic container sensors). Once the plastic containers are removed, the rest of the stream is fed to an eddy current separator to remove aluminum materials. The rest of the stream is classified as contaminants. The contaminants are assembled and sent to a landfill site. OCC, newsprints, mixed papers, PET, HDPE, films, other plastic containers, ferrous, and aluminum are baled separately using a 1-way and 2-way bailer and sent to material processing facilities (mills or plastic processors) or sold on the market. The recycled material composition was obtained from the EPA’s report on assessing trends in materials generation and management [36]. The electricity consumption for each material was obtained based on the data reported [32]. The single stream processes steps and boundaries are defined in Figure 2, and the LCI is shown in Table 1.

2.2.3. Treatment 3: Incineration

Incineration is a common method used to reduce waste volume. This process reduces methane generation and land usage by avoiding landfilling. MSW incineration consists of combusting waste and using the heat to transform water into steam and the steam into electricity using a steam turbine and generator. MSW incineration produces about 550 kWh/ton of MSW [37]. Additionally, the process yields 15–25% of ash by weight of MSW processed. Ash is composed of 10–20% fly ash and 80–90% bottom ash. After incineration, the ash is landfilled, and therefore, leachate is produced in the process. The study assumed that 5% of the leachate contaminates groundwater due to the liner systems not being totally impermeable. Fly and bottom ash compositions were found in Huang and Chuieh (2015) and Birgisdottir et al. (2007), respectively [38,39]. The process inputs are MSW (1 ton) and electricity (the incinerator requires 70 kWh/ton), while the outputs are emissions (to air and water) and electricity (550 kWh/ton).

The emission to water was caused by the leachate from ash landfilling, while the emission to air was due to utility-produced electricity consumed and MSW combustion. This study assumed that materials such as paper and cardboard, food, wood, and yard trimmings (62%) were not petroleum-based materials, and therefore, their CO₂ emissions were considered biogenic [40]. The emission related to MSW combustion was evaluated using data provided in a study by Johnke (2000) [41]. The process flow diagram is shown in Figure 3. The waste is moved into the combustion chamber and converted into heat. The heat is recovered and turned into electricity using a Rankine cycle. The bottom ash is collected at the bottom of the reactor, while the gaseous emissions pass through an air pollution control system to remove fly ash and other impurities [40].

2.2.4. Treatment 4: Composting

Composting is the biological degradation of food waste by microorganisms in the aerobic environment [42]. The process yields a useful material (compost) used as fertilizers and volatiles, such as water, CO₂ (biogenic), nitrous oxides, ammonia, and methane. The efficiency of composting depends on the operating conditions, such as moisture, temperature, and oxygen supply. These parameters are controlled by reducing the material size and mechanically turning the waste pile for aeration and uniform temperature distribution. According to the requirements, the study focused on windrow composting, which consists of steps such as waste reception, composting (shredding, windrow formation, and screening), and dispatch. The process flow diagram and boundaries are shown in Figure 4. The receival consists of weighing and inspecting the material for contaminants such as plastic, glass, and metal. The material is moved using front loaders that consume diesel. Prior to the decomposition step (composting), the material is shredded and mixed with water to add moisture. After the windrow formation, the material is screened to remove large particles and contaminants. The contaminants are collected and sent to landfills. The landfilling activity impact assessment was not covered in this study. The main inputs of this overall process are food waste, diesel, electricity, and water, while the outputs are compost and emissions (to air and water). The compost is used as fertilizer. The study included the emission of water caused by the compost. The leachate composition is provided in (Dimitris, 2000) [43]. The pollutants released into the air were evaluated using the data provided in Riitta et al. (2006) [44]. The electricity and diesel inputs were found in Recycled Organic Units and The University of New South Wales (2006) [42].

2.2.5. Treatment 5: Anaerobic Digestion

Anaerobic digestion is the biological decomposition of food waste into biogas (CH₄, CO_2, and traces of other compounds) and digestate (wet solid) in an oxygen-free environment. During anaerobic digestion, the long chains of organic compounds are broken down by hydrolysis, acidogenic, acetogenic, and methanogenic microorganisms [45]. The microorganisms are involved in reactions such as hydrolysis (carbohydrates, proteins, and fats are broken down into smaller chains), acidogenesis (the small chains of organics are fermented into fatty acids, CO_2, and H₂), acetogenesis (fatty acids are converted into acetic acid), and methanogenesis (methanogens convert acetic acid and H₂ into CH₄, CO_2, and traces of H₂S and other elements) [45].

This study focused on a wet system with a mesophilic reactor (digester). A mesophilic reactor is a digester that operates at a medium temperature (35–39 °C) to produce biogas and biofertilizers [46]. As shown in Figure 5, food waste is collected and stored in a tank and transferred to the digester using a pump. The digester is maintained at 30–40 °C, and the biogas collection efficiency was assumed to be 95%. Biogas is collected, treated, and converted into electricity via internal combustion engines/turbines. The biogas generation and composition and digestate production were evaluated using the AD-Screening Tool provided by the US EPA to support their Global Methane Initiative [47]. The biogas composition (59% CH₄, 39% CO₂, 1% N_2, and 1% trace elements) was based on the decomposition of food waste (mostly fats, proteins, sugars, starches, and oils) [47]. The CO₂ emissions were not included in the impact assessment as they are considered biogenic. The AD-Screening Tool biogas estimations were: 139.46 m³/ton (low estimated value), 254.13 m³/ton (high estimated value), and 347.93 m³/ton (theoretical maximum potential). A flaring system is used only when the system is under maintenance. However, the study assumed that 100% of the biogas collected is converted to electricity with a conversion efficiency of 28%. The digestates are collected and used as soil fertilizer. The study assumed the electricity used by the system is approximately 11% of the biogas energy content [48].

2.2.6. Treatment 6: Gasification

Gasification is the partial combustion of MSW or biomass into products such as syngas, ash, and slag. The gasification modeling in this study was based on a scaled-up model of existing downdraft gasifiers at Oklahoma State University [23]. The process comprises (1) MSW pretreatment where the waste undergoes sorting (glass and metal removal) and size reduction, (2) MSW decomposition in the gasifier where waste is moved to the gasifier by a belt conveyor and decomposed into syngas and solid residues, (3) syngas cleaning where impurities are removed from syngas using acetone and water, and (4) power generation, where the syngas is converted into electricity using an internal combustion engine. The gasification process and life cycle inventory details are found in Ouedraogo et al. (2021) [23]. Figure 6 shows the gasification flow diagram and system boundary.

2.2.7. Scenarios 1: MSW Conventional Treatment

The waste management practices selected are landfilling (50%), recycling (23%), composting (15%), and incineration (12%), as shown in Figure 7. The LCA of the MSW conventional treatment consisted of combining the four treatments using the percent contributions mentioned above.

2.2.8. Scenarios 2: Sustainable and Integrated Waste Management System

This study proposes an integrated waste management system (IWM), where waste is treated using a combination of recycling (48.2% of MSW), anaerobic digestion (23% of MSW), and gasification (28.8% of MSW). Figure 7 shows the process flow diagram.

2.3. Life Cycle Inventories (LCIs)

The LCIs of the studied processes are shown in Table 1, Table 2, Table 3 and Table 4. Each table presents the inputs and outputs of the treatment methods. The LCI was completed using primary and secondary data sources. More than 80% of the data were from US-based sources. As LCA studies are highly dependent on their LCI, the accuracy of the results presented in this study is directly bound to the data collected from the previously mentioned sources.

Table 1. Recycling and thermal conversions LCI for 1 ton of MSW.

Input	Recycling	Source
Electricity (kWh)	4.9	[32]
Diesel (L)	6.71	[30,49]
Output
Recyclables (ton)	0.85	[34]
Contaminants (ton)	0.15	[34]
Inputs	Incineration
Electricity (kWh)	70	[10]
Outputs
Electricity generated (kWh)	550.0	[37]
Total ash (kg)	200
Bottom ash (kg)	170.0
Fly ash (kg)	30
Inputs	Gasification
MSW (ton)	1
Electricity (kWh)	117.139	[23]
Propane (L)	0.05
Wood charcoal (kg)	1.36
Acetone (L)	3.22
Outputs
Electricity generated (kWh)	584.3	[23]
Solid residues (kg)	102.4	[23]

Table 2. Landfilling treatments LCI for 1 ton of MSW.

	Landfilling	Landfilling and Flaring	Landfilling and ER	Source
Inputs
Diesel (L)	6.71	6.71	6.71	[30,49]
Electricity (kWh)	-	-	78.2	[6]
Outputs
Total LFG (m³)	186.5	186.5	186.5	LandGEM
Collected LFG (m³)	-	140	140
Uncollected LFG (m³)	186.5	46.5	46.5
Collected CH₄ (kg)	-	46.0	46.0
Collected CO₂ (kg)	-	130.8	130.8
Uncollected CH₄ (kg)	61.29	15.3	15.3
Uncollected CO₂ (kg)	184.71	43.6	43.6
Electricity generated (kWh)	-	-	199	Calculated
Leachate produced (m³)	0.166	0.166	0.166	HELP
Leaked leachate (m³)	0.83	0.83	0.83	Calculated

Table 3. Composting LCI for 1 ton of MSW.

Inputs	Composting	Source
Electricity (kWh)	0.1	[42]
Diesel (L)	5.5	[42]
Output
Volatiles (kg)		[44,50]
Compost leachate composition (kg)		[43]

Table 4. Anaerobic digestion LCI for 1 ton of MSW.

Inputs	Anaerobic Digestion	Source
Electricity consumption (kWh)	123	[48]
Outputs
Total biogas	197	*
Collected biogas (m³)	187	*
Non-collected biogas (m³)	10	*
Biogenic CO₂ (m³)	276	calculated
Electricity produced (kWh)	314	calculated
Digestates (ton)	0.9	*
Dry sludge	0.7	*
Liquid effluent	0.2	*

* (Global Methane Initiative, 2022) [47].

2.4. Assumptions and Limitations

The following are the assumptions made and limitations of this study:

All wastes were collected the same way and transported to equal distances for all treatments. Hence, for comparison purposes, emissions related to transportation would be the same for all treatments, and therefore, were not included. Emissions related to infrastructures (building) and equipment manufacture were not included in the LCA as the goal of the study was to compare the effects of the actual waste treatment processes. During LFG flaring, 100% of the methane was converted into CO₂ and water.

Due to the lack of sufficient data on energy consumption for tipping floor and office areas (light, office equipment, AC, and heating), all plants were assumed to have the same electricity consumption, and therefore, for comparison purposes, the emissions related to the resource consumption were not included. Processes with energy recovery were assumed to produce electricity and were given credit for avoiding emissions related to producing US mixed utility electricity.

After recycling, metal, paper, cardboard, glass, and plastic are sorted, processed, and purified prior to being sold on the market. Due to the lack of data on the aforementioned processes and the study’s boundaries, credit for recycling was not added to the study at this time. Processes such as recycling, incineration, composting, anaerobic digestion, and gasification should be given credit for avoiding landfilling. However, the study compares the processes to landfilling, and therefore, credits related to avoiding landfilling were considered.

Paper and cardboard, food, wood, and yards trimmings are considered non-fossil-based materials; therefore, CO₂ emissions related to the materials were assumed to be biogenic and were not included in the LCI of incineration, gasification, composting, and anaerobic digestion. Additionally, the resources (energy and raw materials) used to fabricate the material (paper and cardboard, food, wood, and yard trimmings) were not included.

The contaminants removed from recycling and composting were landfilled; however, these landfilling emissions were not included. The anaerobic leachate composition was assumed to be similar to leachate produced in landfills as both reactions are operated in oxygen-free environments.

Emissions from landfills occur over an extended period (approximately 100 years) and at different rates, which makes it difficult to compare with instantaneous processes such as incineration, gasification, recycling, composting, and anaerobic digestion (Laner, 2009) [51]. To address this challenge, the temporal space difference among the processes was not considered. The LCI of landfilling was completed over time. For example, the total LFG generated was calculated over 140 years. However, since the functional unit of the study is 1 ton of MSW, an average LFG generated per ton of MSW was used even though its release occurred over 140 years. This is because the estimated lifespan of CO₂ in the atmosphere is 300–1000 years per NASA’s Jet Propulsion Laboratory.

Additional limitations of this study include the analysis of only the selected waste treatment methods within the US because of the data used for this study.

3. Results and Discussion

3.1. Comparative LCA of Waste Treatment Methods in the US

3.1.1. Global Warming Potential

Landfilling scenarios had the highest impact on global warming compared to other waste treatment methods (composting, anaerobic digestion, recycling, incineration, and gasification). The LCA of three different landfilling methods showed that the fate of LFG highly affects the overall landfill global warming potential. Landfilling without LFG collection global warming was 1749 kg CO₂ eq compared to 719 and 628 kg CO₂ eq per ton of waste for landfilling with flared LFG and landfilling with energy recovery (ER), respectively. Flaring the LFG without energy reduced the impact of global warming by 59%, while converting the LFG to electricity reduced the impact of global warming by 64%. The global warming potential decreased when the LFG was flared or converted into electricity because the CH₄ in the LFG was converted into CO₂ in both cases. The impact of methane on global warming was 25 times higher than CO₂, thus the impact reduction [52]. Composting and recycling global warming impacts were ranked behind landfilling with 115 and 34 kg CO₂ eq per ton of waste, respectively. Anaerobic digestion, incineration and gasification global warming potentials were −56 kg CO₂ eq, −5 kg CO₂ eq, and −63 kg CO₂ eq per ton of waste. The negative values meant that the processes had no impact on global warming and avoided the emission of 56 kg CO₂ eq, 5 kg CO₂ eq, and 63 kg CO₂ eq per ton of waste for anaerobic digestion incineration and gasification. These processes avoided global warming because of credits from avoided electricity, soy, and corn production. Credits are emissions avoided by producing products that are usually made using petroleum resources. For example, credits applied to anaerobic digestion and gasification were due to the electricity not being produced; thus, the credit was for the emissions avoided by not producing the conventional US electricity mix. Table 5 shows the global warming potential of all processes. Gasification was the most environmentally friendly process in terms of global warming.

3.1.2. Acidification Potentials

Incineration and composting had the highest acidification potentials (Table 5). Incineration and composting acidification potentials were followed by landfilling without LFG collection and with flared LFG, recycling, anaerobic digestion, landfilling with energy recovery, and gasification. Acidification is caused by the emission and release of compounds such as ammonia, nitrogen oxides (NOx), nitric acid, and sulfuric acid in the environment, leading to acid precipitation (rain and snow) [52]. The high acidification potential of incineration was due to the release of NOx during the combustion of the MSW. The acidification potential of composting, on the other hand, was mainly due to the release of ammonia during waste decomposition. The impacts of NOx and ammonia were 0.7 SO₂ kg eq/kg of NOx and 1.88 kg SO₂ eq/kg of ammonia, respectively.

3.1.3. Ecotoxicity Potential

The impact on ecotoxicity was the highest in landfilling, followed by composting, incineration, and gasification (Table 5). Anaerobic digestion and recycling had no impact on ecotoxicity. Ecotoxicity measures the toxicity level of compounds on freshwater and land. Ecotoxicity is mainly caused by metals such as zinc, copper, arsenic, lead, barium, and nickel [52]. The landfilling ecotoxicity impact was due to the metal content of the leachate. The ecotoxicity potentials of composting, incineration, and gasification were caused by the leachate from compost, ash landfilling, and gasification residue landfilling, respectively. Preventing the leachate from leaking into the groundwater can significantly reduce the ecotoxicity potential.

3.1.4. Eutrophication Potential

Anaerobic digestion and landfilling were among the processes with the highest eutrophication potential followed by composting, incineration, and gasification (Table 5). Recycling has shown no impact on eutrophication. Eutrophication is caused by an excessive release of nutrients (phosphorus, ammonia, nitrate) in the aquatic environment leading to an overgrowth of aquatic plants, such as algae [52]. The eutrophication potential of landfilling is explained by the presence of ammonia and phosphorus in the leachate. On the other hand, nitrate and ammonia from the anaerobic digestate caused the eutrophication potential of the anaerobic digestion.

3.1.5. Smog Formation

Smog formation was the highest for the thermal conversion of MSW, such as incineration and gasification (Table 5). Processes such as landfilling, recycling, composting, and anaerobic digestion had significantly lower impacts on smog formation (Table 5). Landfilling with energy recovery and anaerobic digestion (−1.56 kg O₃ eq and −2.52 kg O₃/ton of waste, respectively) avoided smog formation because the collected gases were cleaned and converted into electricity. Smog formation is mainly caused by NOx and VOCs. The impact category is known for causing respiratory issues [52]. The smog formation impacts of incineration and gasification were due to the emission of NOx and VOCs during the MSW total and partial combustion, respectively.

3.1.6. Ozone Depletion Potential

Ozone depletion potentials were only found in landfilling scenarios (Table 5). The impact category measures the impact of substances such as chlorofluorocarbons (CFCs) on the destruction of the stratospheric ozone level [52]. Ozone depletion can cause cancer and human cataracts. Ozone depletion in these scenarios was due to the NMOCs present in the LFG.

3.1.7. Carcinogenic Effects

Carcinogenic effects were significant in all landfilling scenarios compared to gasification, composting, and incineration (Table 5). Recycling and anaerobic digestion production have shown no carcinogenic impact. The carcinogenic effects measure the cancer potential of substances [52]. Metals such as chromium, nickel, mercury, cadmium, and lead caused cancer potentials. These metals were mainly found in landfill leachate. NMOCs have shown cancer potential; however, their impact contributed to less than 1% of the cancer potential of landfilling.

3.1.8. Non-Carcinogenic Effects

Non-carcinogenic effects were the highest in landfilling scenarios and gasification. Composting and incineration had lower non-carcinogenic impacts (Table 5). Recycling and anaerobic digestion production have shown no impact in the non-cancerogenic impact category. The non-carcinogenic effects measure the impact of substances on human health [52]. The non-carcinogenic effects of landfilling were due to metals (zinc, mercury, chromium, lead, nickel, cadmium, and copper) in the leachate and organic compounds (hexane, toluene, xylene, and benzene) in the LFG. Gasification also showed a high impact in the category. The mercury emitted during gasification and metals leached during the solid residues landfilling caused the non-carcinogenic effects.

3.1.9. Respiratory Effects

Composting had the highest respiratory effect. followed by recycling, landfilling with LFG flaring, and landfilling without LFG recovery (Table 5). Gasification, incineration, anaerobic digestion, and landfilling with energy recovery avoided most respiratory effects (Table 5). The impact category measures the impact of particulate matter and precursors on human health [52]. Bare et al. (2012) affirmed that NOx and SOx cause respiratory issues. Composting showed a high impact in the category because of substances such as ammonia, PM 2.5, PM10, NOx, and CO [52]. Although landfilling, recycling, incineration, and gasification emit similar compounds, the amount of ammonia released during composting was much higher than the other treatments.

3.2. Comparative LCA of MSW Management Methods and Integrated Waste Management

The IWM system processes were selected because of their low emissions, as discussed in Section 3.1. Figure 8 shows the result of a comparative LCA of the conventional waste treatment and the proposed waste management suggested by the study. The IWM’s impact on the environment and human health was significantly lower than the conventional waste treatment route. For example, the ecotoxicity, eutrophication, smog formation, ozone depletion, and carcinogenic and non-carcinogenic potentials of the IWM were 99.9%, 41.9%, 58.3%, 100%, 99.4%, and 75.8% lower than the conventional MSW treatment route, respectively. On the other hand, the global warming, acidification, and respiratory effects of the IWM method were negative. The negative value indicates that the IWM system has a positive impact on the categories. For example, IWM showed a global warming potential of −14.6 kg CO₂ eq. This result means that the IWM of 1 ton of MSW removes 14.6 Kg CO₂ from the atmosphere. Overall, limiting landfilling activities and promoting targeted treatment of waste classes have shown a significant decrease in the impact of MSW disposal measures on the environment and human health.

4. Conclusions

The present study proposed and examined the LCA of MSW landfilling, composting, anaerobic digestion, recycling, incineration, and gasification. The LCA of the single processes was used to evaluate and compare the impact of the US conventional waste treatment route and the proposed integrated waste management (IWM) method on the environment and human health.

Landfilling had the highest impact on global warming, ecotoxicity, ozone depletion, and carcinogenic and non-carcinogenic effects. Acidification was significant for composting, while anaerobic digestion showed a higher impact on eutrophication. The impact of incineration on smog formation was the highest among all treatments. Overall, recycling, anaerobic digestion, and gasification were found to be the most environmentally friendly processes for recyclable materials, food waste, and biomass. Additionally, the ecotoxicity, eutrophication, smog formation, ozone depletion, and carcinogenic and non-carcinogenic potentials of the integrated waste management were 99.9%, 41.9%, 58.3%, 100%, 99.4%, and 75.8% lower than the conventional MSW treatment route, respectively. The comparative study of the conventional waste treatment and IWM method revealed that the IWM has the potential to reduce global warming, respiratory effects, and acidification potential by removing 14.6 kg CO₂ eq, 7.2 × 10⁻³ kg PM2.5 eq, and 1.2 × 10⁻² kg SO₂ eq per ton of waste treated, respectively. Overall, limiting landfilling activities and promoting targeted treatment of waste classes has shown a significant decrease in the impact of MSW disposal measures on the environment and human health.

This study can be used as a guide for waste management planners, city authorities, and environmentalists for the choice of waste treatment methods considering environmental and health impacts. Moreover, the availability of extensive data regarding the operation, emission, and sustainability impacts of various waste treatment methods would also enable a comprehensive analysis and for the public to make informed decisions regarding waste treatments.

Author Contributions

Conceptualization, A.S.O., A.K., R.F. and K.A.S.; methodology, A.S.O.; software, A.S.O.; validation, A.K., R.F. and K.A.S.; formal analysis, A.S.O.; investigation, A.S.O.; resources, A.K.; data curation, A.S.O.; writing—original draft preparation, A.S.O.; writing—review and editing, A.K., R.F. and K.A.S.; visualization, A.S.O., A.K., R.F. and K.A.S.; supervision, A.K., R.F. and K.A.S.; project administration, A.K.; funding acquisition, A.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported, in part, by the Hatch Multistate Research Fund, project award no. S1075, from the U.S. Department of Agriculture’s National Institute of Food and Agriculture.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author(s).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Landfilling scenarios’ flow diagram and boundary.

Figure 2. Single-stream recycling flow diagram and boundary.

Figure 3. Incineration flow diagram and boundary.

Figure 4. Windrow composting flow diagram and system boundary.

Figure 5. Anaerobic digestion flow diagram and system boundary.

Figure 6. Gasification flow diagram and system boundary.

Figure 7. (a) Conventional treatment of MSW. (b) Sustainable and integrated waste management system.

Figure 8. Impact assessment of conventional MSW treatment and integrated treatment.

Table 5. Impact assessment of MSW disposal methods in the US.

Impact Category	Units	Landfilling	Landfilling and Flaring	Landfilling and ER	Composting	Anaerobic Digestion	Recycling	Incineration	Gasification
Global warming	Kg CO₂ eq	1.75 × 10⁺³	7.19 × 10²	6.27 × 10²	1.15 × 10²	−5.60 × 10¹	3.42 × 10¹	−5.15	−6.33 × 10¹
Acidification	Kg SO₂ eq	1.76 × 10⁻²	2.20 × 10⁻²	−1.10 × 10⁻¹	7.63 × 10⁻¹	−6.29 × 10⁻²	4.63 × 10⁻³	7.75 × 10⁻¹	1.43 × 10⁻³
Ecotoxicity	CTUe	4.64 × 10³	4.64 × 10³	4.64 × 10³	1.65 × 10²	-	-	1.65 × 10²	4.22
Eutrophication	Kg N eq	9.94 × 10⁻¹	9.94 × 10⁻¹	9.92 × 10⁻¹	1.35 × 10⁻¹	1.31	1.16 × 10⁻⁴	6.63 × 10⁻²	1.52 × 10⁻²
Smog Formation	Kg O₃ eq	2.93 × 10⁻¹	3.67 × 10⁻¹	−1.56	5.95 × 10⁻²	−2.57	6.62 × 10⁻²	3.55 × 10¹	8.36
Ozone depletion	Kg CFC-11 eq	4.29 × 10⁻⁵	5.36 × 10⁻⁵	1.07 × 10⁻⁵	-	-	-	-	-
Carcinogenic	CTUh	4.11 × 10⁻⁵	4.11 × 10⁻⁵	4.10 × 10⁻⁵	2.46 × 10⁻⁷	-	-	2.46 × 10⁻⁷	4.30 × 10⁻⁷
Non-carcinogenic	CTUh	1.18 × 10⁻⁴	1.18 × 10⁻⁴	1.18 × 10⁻⁴	3.68 × 10⁻⁶	-	-	3.68 × 10⁻⁶	5.04 × 10⁻⁵
Respiratory effects	Kg PM2.5 eq	1.06 × 10⁻⁵	1.33 × 10⁻⁵	−6.12 × 10⁻³	2.71 × 10⁻²	−8.20 × 10⁻³	2.49 × 10⁻⁴	−1.04 × 10⁻²	−1.91 × 10⁻²

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Ouedraogo, A.S.; Kumar, A.; Frazier, R.; Sallam, K.A. Comparative Life Cycle Assessment of Landfilling with Sustainable Waste Management Methods for Municipal Solid Wastes. Environments 2024, 11, 248. https://doi.org/10.3390/environments11110248

AMA Style

Ouedraogo AS, Kumar A, Frazier R, Sallam KA. Comparative Life Cycle Assessment of Landfilling with Sustainable Waste Management Methods for Municipal Solid Wastes. Environments. 2024; 11(11):248. https://doi.org/10.3390/environments11110248

Chicago/Turabian Style

Ouedraogo, Angelika Sita, Ajay Kumar, Robert Frazier, and Khaled A. Sallam. 2024. "Comparative Life Cycle Assessment of Landfilling with Sustainable Waste Management Methods for Municipal Solid Wastes" Environments 11, no. 11: 248. https://doi.org/10.3390/environments11110248

APA Style

Ouedraogo, A. S., Kumar, A., Frazier, R., & Sallam, K. A. (2024). Comparative Life Cycle Assessment of Landfilling with Sustainable Waste Management Methods for Municipal Solid Wastes. Environments, 11(11), 248. https://doi.org/10.3390/environments11110248

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