Comparative Environmental Evaluation of Sewage Sludge Treatment and Aggregate Production Process by Life Cycle Assessment

Abstract

This study evaluated the environmental impact of landfill, incineration, and lightweight aggregate production for sewage sludge management techniques and compared the utilization of sewage-produced lightweight aggregates with natural aggregates in terms of building material production. Two scenarios were established for the life cycle assessment (LCA) of the sludge and associated product that was carried out after sludge generation. Sludge incineration and landfill deposition includes emissions from the drying, transportation, incineration of sludge, and landfill, and the production of lightweight aggregates and natural aggregates includes transportation to manufacturing facilities, the manufacturing processes themselves, and transportation of the produced aggregates to construction sites. We calculated the amount of pollutant emissions for each process in each scenario and analyzed the environmental impact index considering the environmental impact of each pollutant using the Open LCA program. The parameters used for the environmental impact index analysis for sludge management are potential acidification, climate change, eutrophication potential, human toxicity, photochemical oxidation, and stratospheric ozone depletion. The environmental impact values of lightweight aggregates (LWA) are GWP 100 441 kg CO₂_Eq, AP 2.73 × 10⁻² kg SO_{2_}Eq, EP 4.46 × 10⁻³ kg PO₄⁻_Eq, HTP 4.15 × 10⁻² kg, 1,4-DCB_Eq, POCP 1.64 × 10⁻³ kg CH₄_Eq, and ODP 3.41 × 10⁻⁷ kg CFC-11_Eq. We found that these values were low compared to landfill and incineration as a sewage sludge disposal method and compared to natural aggregate as a construction material production method. The environmental impact index analysis during LCA of lightweight aggregates produced from the sludge verified its positive environmental impact compared to the other potential methods of sludge management.

1. Introduction

The amount of sludge production from domestic wastewater treatment facilities is increasing every year due to the rapid increase in population and the need to improve water quality. This not only increases the operational cost of treatment systems but is also responsible for the leaching of toxic waste and sludge into the environment [1]. A survey conducted by the Korean Ministry of Environment in 2021 revealed showed that compared to 2018 the generation of sewage sludge increased by 10.6% and was the largest volume of byproduct from sewage treatment (Appendix A). Moreover, the sewage sludge contained 0.25~12% of solids, untreated pathogenic, toxic substances, and chemical contaminants. These chemicals/contaminants would have the potential to cause health hazards if not handled correctly. Thus, it is necessary to have significant sludge treatment before final discharge into the environment. The necessary sludge treatment methodology needed to reduce the amount of organic solids and the moisture content sufficiently to reduce the environmental burden [2].

Methods used for treating sewage sludge include landfill, incineration, marine dumping, and recycling [3]. However, the “Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter: London Convention” held in 2011 stated that marine dumping is prohibited due to its significant impact on the marine ecosystem, and that incineration, landfill disposal, and recycling of waste must be considered suitable options for sludge management. The techniques included in recycling sewage sludge are composting, fueling, and using cement raw materials. In addition to the suitability of recycling, we considered landfill disposal and incineration of the sludge to be a better option for large-scale applicability due to ease of operation. However, significant care is required while considering these methods. Improper disposal of sludge in landfills generates into the environment undesirable emissions of gases and leachate of heavy metals, which have a potential impact on polluting air, agricultural soil, and groundwater [4,5,6]. Incineration has the advantage of minimizing the mass and volume of sludge and obtaining energy from waste gas, but it can potentially cause health risks that can be harmful to humans [7]. The potential polluting gases produced from the incineration of sludge include SO₂, NO_x, and dioxin, which are toxic and injurious to health [8]. Moreover, during incineration, organic pollutants can be thermochemically converted, but heavy metals are not thermochemically destroyed, thus making it difficult to recycle and dispose of them [9]. Therefore, recycling and reusing sludge is considered to be the most suitable and convenient option for sludge management, which includes organic recycling, the production of lightweight aggregates (LWA), and energy-generation from the sludge (biogas) [10]. Choosing the right method for sludge disposal is important for meeting the critical environmental needs of current and future generations at the same time, without constraining the complex economic aspects and the feasibility of future generations [11]. The reuse of sewage sludge is economical and environmentally sustainable compared to waste disposal and landfills [10]. Out of this option, LWA is considered the better option for sludge management, as it not only reduces the environmental burden of the sludge while reducing the amount of landfill disposal but also reduces the toxicity. Moreover, LWA has a low self-weight, adequate strength, and excellent durability, which allows it to be used for purposes such as the construction of high-rise buildings and offshore platforms [12], and is utilized for other environmental applications.

Globally, consumption of aggregates as a building material reached 55 billion tons per year in 2020, and if this trend is sustained, it will double in the next 10 years, leading to a shortage of natural aggregates (NA) [13]. As one of the solutions to the aggregate shortage, LWA has a variety of uses [14,15,16]. Natural raw materials such as slate and waste clay such as sewage sludge, washed aggregate sludge, waste glass, fly ash, etc., can be used as LWA [17]. The use of LWA produced from waste offers environmental benefits due to its recycling of waste materials [18]. Additionally, the construction market of Korea uses 200 million tons of aggregate annually, so recycling waste and using artificial lightweight aggregates can reduce the environmental load.

In sludge management studies, the literature is focused on evaluating the performance of management methods, including landfill disposal, incineration, and recycling [19,20,21]. However, no studies have been conducted to compare the environmental impact of sludge management techniques including the comparison of the impact of landfill disposal, incineration, and LWA production. Moreover, it is also necessary to compare the environmental impact of LWA and NA through LCA to provide the guidelines for efficient utilization for construction purposes and reduction of environmental burden.

Thus, this study quantifies and compares the environmental impact of sewage sludge treatment (landfill disposal, incineration, and LWA), and the production of LWA using recycled sewage sludge with NA for construction purposes. We conducted an LCA of sludge treatment methods using existing literature and market data. For the unit process (landfill disposal and incineration) for sludge management included in the scenario, we used literature data. However, for the construction material production process using sewage sludge (LWA), we set the process used in an actual site as a functional unit. This study provides information on the environmental impacts of the management of sewage sludge and the production of building materials. Moreover, we expand the boundaries of LCA studies by comparing sewage sludge treatment and building materials production.

2. Materials and Methods

2.1. Manufacturing Lightweight Aggregate Using Sewage Sludge

Lightweight aggregate using sludge is produced by mixing and molding sludge and clay, drying the molded aggregate for more than 24 h in an environment of 105 °C, and then sintering them in a rotary kiln at a high temperature of 1100–1200 °C. An important point in the production of lightweight aggregate is the black core formation process to increase the volume of the aggregate. For the formation of a black core, an oxidation–reduction reaction within the aggregate and an adhesion phenomenon due to the viscous behavior of the molded body are necessary, which occur during the sintering process. For this process, Fe₂O₃ and carbon are used, and Park et al. [22] explained that the formation of the Black Core is achieved by a reduction reaction of Fe₂O₃ due to carbon. Fe₂O₃ is an important element related to the expansion and adhesion of aggregates during the sintering process and is one of the components of sludge aggregation. The adhesion phenomenon caused by Fe₂O₃ causes the blackening of the unburned carbon core inside the lightweight aggregate and causes the clay molded body to swell, expanding the molded body and reducing its specific gravity. Additionally, a large amount of organic matter contained in sludge may form a fine pore structure due to oxidation, thereby reducing the specific gravity of the aggregate. In other words, the formation of black core is a key part of the manufacturing of lightweight aggregate and has a significant impact on the properties of the lightweight aggregate.

There are several influencing factors for the production of lightweight aggregate. For example, CaO, MgO, FeO, K₂O, Na₂O, Al₂O₃, etc., substances in flux, play a role in lowering the melting point so that the viscous behavior of the clay molded body can be smoothly achieved during the sintering process [23]. The mixing ratio of clay and flux is an important part of determining the properties of lightweight aggregate, and the optimal conditions for the mixing ratio of sludge and clay and the sintering temperature were established through previous research [24,25].

2.2. Life Cycle Assessment Method

Life cycle assessment (LCA) is a technique for assessing the environmental aspects and potential impacts associated with a product (in this study, the product is sewage treatment sludge). The LCA of produced sludge covers the qualitative and quantitative assessment methods for assessing the environmental impacts and alternative utilization of the products produced from the sludge and processes that would be used for the production of these products [26]. The LCA of the sludge or any other products consists of four main phases:

• Define the purpose and scope: In this, the goal and boundaries of the work must be defined. In this study, a comparison of the environmental impact and aspects of the various methods of sludge treatment (landfill, incineration, and aggregate production) were analyzed.
• Analyze the lifecycle inventory (LCI): This includes emission data for each unit process related to the inputs and outputs of energy and mass flow of the sludge treatment process. This step involves calculating both material and energy inputs and outputs to create a numerical representation.
• Life cycle impact assessment (LCIA): this step assesses potential environmental impacts and estimates resources, which includes the LCI results and manufacturing/process characterization of sludge treatment methods.
• Interpret the entire lifecycle: This step provides the overall conclusion through the interpretation of important environmental issues arising during the LCA of the sludge management techniques. In this study, the total environmental impact of the treatment of 1 ton of anaerobic digestion sludge was quantified without estimating the entire life cycle effect within the time span.

2.3. Goal and Scope Definition

Our study had two objectives: (a) to evaluate and compare the environmental impact of three sludge treatment methods: landfill, incineration, and lightweight aggregate (LWA) manufacturing; (b) to compare the environmental impact of LWA made of waste sewage sludge and natural aggregates (NA).

2.4. Description of System Boundaries

System boundaries of the targeted goals were established by assessing the environmental impact of the process used for sludge treatment with the omission of the treatment process that produced the sludge.

The first process of the sludge treatment is the landfill treatment, with the process flowing from the drying of the sludge, to transportation to the landfill site, to depositing to the landfill. Similar to the landfill process, the incineration of the sludge includes the following process: drying of the sludge, transportation to the incineration plant, and incineration. However, LWA production from the system includes different steps from the landfill treatment and incineration of the sludge. The process flow of the LWA production from the sludge treatment includes sludge drying, transportation to the LWA manufacturing facility, the manufacturing process (molding, drying, and rotary kiln), and transportation of the LWA to the construction site. The overall system boundaries for the treatment option of sludge are illustrated in Figure 1. The 1 ton of sewage sludge generated in the sewage treatment plant was set as a functional unit and its environmental impact was evaluated for landfill, incineration, and production of LWA. Moreover, the environmental impact on aggregate production was evaluated by considering the same amount of NA as LWA generated from 1 ton of sludge as a functional unit. The process outlook for the comparative LCA analysis of the LWA with NA is shown in Figure 2.

2.5. Life Cycle Inventory (LCI) Analysis

The LCI includes sludge collection and emission data for each unit process for the inputs and outputs of the system covered in this study and should include calculating both the inputs and outputs of materials and energy [27]. In this study, three product systems of sludge treatment methods and a comparative analysis of two product systems of building material generation were analyzed and quantified according to the amount of discharged substances. The data of each process were calculated and quantified in the following way, and the values are shown in

Table 1. Calculation summary related to CO₂ emission coefficients and energy consumption estimation during different scenarios/processes of transportation/LWA production by sludge.

	Name	Fuel Type	CO2 Emission Coefficient	Energy Consumption	Reference
Transport	Heavy-duty truck (Loaded)	Diesel	0.1052 kg CO2/t·km	-	[28]
	Heavy-duty truck (Empty)	Diesel	0.0488 kg CO2/t·km	-	[28]
	Ship	Crude oil	1.2682 kg CO2/t·km	-	-
LWA	Rotary kiln	LNG	2.17 kg CO2/m3	350 m3/h	Environmental Labeling
	Dehydrator	Electricity	4.95 × 10−1 kg CO2/kWh	3.1 kW	Environmental Labeling
	Molding machine	Electricity	4.95 × 10−1 kg CO2/kWh	14.92 kW	Environmental Labeling

.

2.5.1. Drying Sludge

In the case of the sludge-drying process, which is common to the sludge treatment method, CO₂ is calculated as in Equation (1) [29].

$$\begin{gathered} {CO}_2 emissions\left(kg {\mathrm{C}\mathrm{O}}_2\right)=Amount of oil\times energy content of oil\times \\ {\mathrm{C}\mathrm{O}}_2 emission coefficient \end{gathered}$$

(1)

The amount of oil is the oil required to dry 1 ton of sludge (kg_oil/t_sludge) and energy content shows the energy potential of the oil used for the sludge drying (GJ/t). The CO₂ emission coefficient represents the equivalent amount of the CO₂ produced during energy consumption (g CO₂ eq/MJ). According to Piippo et al. [29], the amount of oil required to dry 1 ton of sludge was 51.5 kg_oil/t_sludge, energy content was 42.5 GJ/t_oil, and the carbon emission factor of combustion due to oil was 74.3 g CO₂ eq/MJ.

2.5.2. Transportation

The carbon emissions generated during transportation required for each process were calculated [28] by considering the carbon emission factors of trucks loaded with sludge after drying and emptying the trucks [28]. Equation (2) calculates the CO₂ emissions generated during transportation, where $i$ represents the means of transportation, $f_{i_l}$ is the CO₂ emission factor of the loaded vehicle, $f_{i_e}$ is the CO₂ emission factor of the empty vehicle, $k$ is the type of material, $m_k$ is the weight of the material, and $l_k^i$ is the transportation distance [28].

$$e_{t_l}={\sum }_i\left(\right(f_{i_l}+f_{i_e})\times {\sum }_{k=1}^k{m}_k\times l_k^i)$$

(2)

For the transportation of sludge by other means, particularly by ship, emissions were calculated using Equation (3) [30], where P_transientME is the main engine power, t is the time, and SFOC represents specific fuel oil consumption.

$$\begin{gathered} Fuel consumption\left(g\right)=P_{transientME}\times t\times SFOC \\ Emissions\left(g per pollutant\right)=Fuel consumption\times emission factor \end{gathered}$$

(3)

In addition, based on CO₂ emissions, the proportion value was derived from the amount of sulfur dioxide (SO₂), oxides of nitrogen (NO_x), carbon monoxide (CO), particulate matter (PM), non-methane volatile organic compounds (NMVOC), and CH₄ generated during transportation with reference to Chua et al. [31]. The amount of CFC-11 was calculated based on Hu et al. [32]. Furthermore, Table 1 provides the estimated CO₂ emission coefficient from the transportation by various means used for the transportation of the goods using diesel and crude as the fuel source for trucks (landfill, incineration, and LWA) and ships for NA.

2.5.3. Landfill

For the landfill process, the anaerobic digestion of sludge was the main contributor to the environmental impact as it produces CH₄ instead of CO₂ because landfilled sludge discharged through the digestion process produces 60.6 kg CH₄/t sludge, which has a global warming potential of 21 times greater than CO₂ [33]. Thus, the overall CO₂ generated by anaerobic digestion is considered to have no environmental impact, and the N₂O emissions from landfills are considered negligible [19].

In this study, for the analysis, the landfill included the transportation distance from Bucheon to Gimpo, South Korea, which is the location of one of the largest landfill sites in Korea for sludge deposition. Moreover, the emission factors were calculated by a method provided by the IPCC. When 1 ton of sludge was completely digested, the total CH₄ emissions were calculated using Equation (4) [33].

In Equation (4), x in CH₄ Emission_x,T represents the type of sludge and T is the time the sludge is generated. The oxidation rate was mainly calculated as 0.1 according to the information provided by the IPCC [33].

$${CH}_4 Emission={\sum }_x[\mathrm{C}{\mathrm{H}}_4{ Emission}_{x,T}-Recovered {CH}_4]\times (1-OxidationRate)$$

(4)

2.5.4. Incineration

$$\begin{array}{cc}{CO}_2 Emission coefficient& \\ & =Drying substance content\times Carbon content\\ & \times Fossil carbon content\times Oxidation coefficient\times \left(\frac{44}{12}\right)\end{array}$$

(5)

For incineration, CH₄ emissions were very low, whereas CO₂ emissions were the highest, followed by N₂O. Therefore, the emission factors of CO₂ and CO₂ emissions generated in the incineration process were calculated through Equations (5) and (6) [33], and CH₄ and N₂O other than CO₂ generated in the incineration process were also calculated through the emission factors provided by the IPCC.

$$\begin{gathered} {\mathrm{C}\mathrm{O}}_2 Emission=Incineration treatment amount\times \\ {\mathrm{C}\mathrm{O}}_2 Emissioncoefficient \end{gathered}$$

(6)

The CO₂ emission coefficient represents the equivalent amount of CO₂ produced during energy consumption (g CO₂ eq/MJ). The carbon content represents the carbon content of the material (kg_C/t_waste), and in this study, the carbon content of the sludge. Fossil carbon content represents the amount of carbon a fuel contains.

In the case of incineration, only exhaust gases using theoretical formulas were not included in the data. In order to look at other environmental impacts, we added on-site data from sewage treatment facilities.

2.5.5. Lightweight Aggregate (LWA)

In the case of the LWA manufacturing process from sludge treatment as mentioned in Lee et al. [24], CO₂ emissions were calculated by Equation (7) [28]. In consideration of the energy consumption and working time of the equipment, $i$ is the type of machine, $f_i$ is the CO₂ emission factor of the energy used in the equipment, $v_i$ is the hourly energy consumption of the equipment, and $t_i$ is the total working time of the equipment. In molding machines, dryers, and rotary kilns, electricity and LNG are used, so the carbon emission factors of each type of energy were calculated [28].

$$e_{c_l}={\sum }_i{f}_i\times v_i\times t_i$$

(7)

As the firing process is carried out at a high temperature of 1200 °C, which is higher than the heating temperature of 850 °C used in incineration, most of the N₂O discharged from the incineration is decomposed into N₂ and NO [34]. Thus, the emissions of N₂O were considered for the LWA. Moreover, CH₄ is also pyrolyzed into C and H₂ at a high temperature of 1200 °C if produced during the LWA production process, so CH₄ emission was also not considered for the impact assessment [35]. Table 1 presents the CO₂ emission coefficients and energy consumption by different machines or processes used for the LWA depending on the fuel type.

2.5.6. Natural Aggregate (NA)

The source of the NA for construction purposes is sea sand, which has been covering the shortfall in the supply of aggregate in Korea. It is produced by extraction and then moved to land using ships. Moreover, the mined sea sand is produced in various sizes according to market demand through a crushing and sorting process [36]. The exhaust gas generated in this process was calculated using the same formula as mentioned in Section 2.5.5. LWA.

2.6. Life Cycle Impact Assessment

In this study, LCA from various sludge treatment methods was analyzed by considering the environmental impact of the acidification index (AP), global warming potential (GWP), eutrophication index (EP), human toxicity (HTP), photochemical oxidation (POCP), and ozone depletion index (ODP) using the CML impact assessment method of open LCA 1.11.0. The environmental impact index was selected because substances emitted from the process act as major influencing factors. The CML 2001 (suspended) impact assessment method is widely used for the LCA due to its ease of utilization and provides comparisons for most categories that would be necessary for the analysis [37].

3. Result and Discussion

3.1. Assessment of Input and Output Parameters Used for Different Sludge Treatment Methods

Table 2. Emission summary of various gases during different sludge treatment methods along with their impact potential (additionally emission during NA process is also summarized).

Process	Input	Quantity	Unit	Output	Quantity	Unit	Impact Potential
Landfill process
Drying	Sludge	1.00	Ton
	Diesel	51.5	Kg	CO2	162	Kg	GWP
Transportation	Diesel	54.4	Kg	CO2	16.6	Kg	GWP
				SO2	5.61 × 10−3	Kg	AP, HTP
				NOx	5.84 × 10−2	Kg	AP, EP, HTP
				CO	8.82 × 10−2	Kg	GWP, POCP
				PM	6.02 × 10−3	Kg	-
				NMVOC	4.72 × 10−2	Kg	-
				CH4	2.33 × 10−2	Kg	GWP, POCP
				CFC-11	1.17 × 10−6	Kg	GWP, ODP
Landfill	D. Sludge	430	Kg		26.1	Kg	GWP, POCP
Incineration process
Drying	Sludge	1.00	Ton
	Diesel	51.5	Kg	CO2	162	Kg	GWP
Transportation	Diesel	91.4	Kg	CO2	27.8	Kg	GWP
				SO2	9.42 × 10−3	Kg	AP, HTP
				NOx	9.82 × 10−2	Kg	AP, EP, HTP
				CO	1.48 × 10−1	Kg	GWP, POCP
				PM	1.01 × 10−2	Kg	-
				NMVOC	7.93 × 10−2	Kg	-
				CH4	3.92 × 10−2	Kg	GWP, POCP
				CFC-11	1.97 × 10−6	Kg	GWP, ODP
Incineration	D. Sludge	430	Kg	CO2	188	Kg	GWP
				CH4	4.17 × 10−3	Kg	GWP, POCP
				N2O	3.87 × 10−1	Kg	GWP, EP
				SOx	1.00 × 10−2	Kg	-
				PM	2.00 × 10−4	Kg	-
				NOx	1.00 × 10−2	Kg	EP, HTP
				CO	4.00 × 10−3	Kg	POCP
				HCl	1.90 × 10−3	Kg	HTP
	Coal	1.40 × 103	MJ	CO2	138	Kg	GWP
LWA
Drying	Sludge	1.00	Ton	D. Sludge	430	Kg	-
	Diesel	51.5	Kg	CO2	162	Kg	GWP
Transportation	Diesel	12.0	Kg	CO2	3.64	Kg	GWP
				SO2	1.23 × 10−3	Kg	AP, HTP
				NOx	1.29 × 10−2	Kg	AP, EP, HTP
				CO	1.94 × 10−2	Kg	GWP, POCP
				PM	1.32 × 10−3	Kg	-
				NMVOC	1.04 × 10−2	Kg	-
				CH4	5.13 × 10−3	Kg	GWP, POCP
				CFC-11	2.57 × 10−7	Kg	GWP, ODP
Molding	Electricity	14.9	kWh	CO2	7.39	Kg	GWP
	D. Sludge	430	Kg	M. Sludge	7.16 × 102	Kg	-
	Clay	286	Kg				-
Drying	Electricity	74.4	kWh	CO2	36.9	Kg	GWP
	M. Sludge	716	Kg	M. Sludge (Dried)	716	Kg	-
Calcination	LNG	4.64 × 103	MJ	CO2	228	Kg	GWP
	M. Sludge (Dried)	716	kg	LWA	716	kg	-
Natural aggregates (NA)
Transportation (Ship)	Crude Oil	104	Kg	CO2	334	Kg	GWP
				CO	2.89 × 10−1	Kg	GWP, POCP
				N2O	1.57 × 10−2	Kg	GWP, EP
				CH4	6.27 × 10−3	Kg	GWP, POCP
				NOx	9.11	Kg	AP, EP, HTP
				NMVOC	3.22 × 10−1	Kg	-
				SOx	2.76	Kg
Drilling	Electricity	75.6	kWh	CO2	37.4	Kg	GWP
Excavating	Electricity	75.6	kWh	CO2	37.4	Kg	GWP
				NA	716	Kg	-
Crushing	Electricity	75.0	kWh	CO2	37.1	Kg	GWP
	NA	716	Kg	NA	716	Kg	-
Screening	Electricity	22.0	kWh	CO2	10.9	Kg	GWP
	NA	716	Kg	NA	716	Kg	-
Transportation (Truck)	Diesel	2.76 × 103	MJ	CO2	36.9	Kg	GWP
	NA	716	Kg	CO	1.97 × 10−1	Kg	GWP, POCP
				CH4	5.20 × 10−2	Kg	GWP, POCP
				CFC-11	1.57 × 10−6	Kg	GWP, ODP
				NOx	1.3 × 10−1	Kg	AP, EP, HTP
				NMVOC	1.05 × 10−1	Kg	-
				SOx	1.23 × 10−2	Kg	-
				NA	716	Kg	-

shows the comparative results for the input/output parameters of the unit processes used for the sludge treatment using the environmental impact index result values in open LCA 1.11.0. The results shown in Table 2 show that SO₂, NO_x, and HCl are the main influencing factors for AP, and CO₂, N₂O, CH₄, and CO are the main influencing factors for GWP [38]. Similarly, for EP, N₂O and NO_x act as the main influencing factors, and for HTP, NO_x, SO₂, and HCl act as the main influencing factors. Moreover, the POCP is characterized by SO₂, CO, and CH₄ as the main influencing factors and ODP is a major influencing factor of CFC-11.

3.2. Assessment of Environmental Impact Parameters Used for LCA of Sludge Treatment Methods

Landfill and incineration of sludge showed a higher impact in GWP with maximum impact on climate change with more CO₂ production, as elucidated in

Table 3. Summary of various LCA parameters in different sludge-management techniques.

LWA vs. Landfill vs. Incineration
Indicator	LWA	Landfill	Incineration	Unit
acidification potential (AP)	2.73 × 10−2	4.65 × 10−2	9.68 × 10−2	kg SO2_Eq
climate change (GWP 100)	441	831	445	kg CO2_Eq
eutrophication potential (EP)	4.46 × 10−3	7.59 × 10−3	1.19 × 10−1	kg PO4−_Eq
human toxicity (HTP)	4.15 × 10−2	7.06 × 10−2	1.33 × 10−1	kg 1,4-DCB_Eq
photochemical oxidation (summer smog) (POCP)	1.64 × 10−3	1.59 × 10−1	5.30 × 10−3	kg CH4_Eq
stratospheric ozone depletion (ODP)	3.41 × 10−7	1.17 × 10−6	1.97 × 10−6	kg CFC-11_Eq

. However, LWA production from sludge has a minimum impact on the GWP potential in terms of CO₂ production. Chen et al. [19] evaluated GWP through greenhouse gas emissions from sewage sludge management and found that incineration had a higher impact among landfill and incineration. In contrast, compared to all processes used for sludge management in this study, landfill has a higher GWP potential followed by incineration and LWA production. Similar results were observed for the POCP production for landfill, incineration, and LWA production from wastewater treatment sludge. As for landfills, GWP and POCP showed the highest values of 831 kg CO₂_Eq and 1.59 × 10⁻¹ kg ethylene, respectively, which appears to be the dominant factor in CH₄ emissions from the landfill process among the processes. In the case of POCP, the effect of CO among the substances generated in this study was 2.7 × 10⁻² times. It was due to the high presence of CH₄ from landfill sites as 1 kg of dried sludge produced 3.636 × 10⁻⁴ kg of CH₄ which had a significant impact on the production of CO as more than 95% (98.2468) of the CH₄ gas generated in landfills contributed to production of CO. Moreover, it can be seen from Table 3 that GWP and POCP affect the environment due to the generation of high amounts of CO₂ and CH₄ in the landfill process rather than transportation, as confirmed in Table 2.

In the case of incineration, AP, EP, HTP, and ODP showed the highest values of 9.68 × 10⁻² kg SO₂_Eq, 1.19 × 10⁻¹ kg PO₄⁻_Eq, 1.33 × 10⁻¹ kg 1,4-DCB_Eq, and 1.97 × 10⁻⁶ kg, respectively. Moreover, for the ODP index, considering the effect of transportation, it was found that CFC-11 generated by diesel used in transportation directly affects ODP. However, this observation was missing in the previous work of Deviatkin et al. [39], who did not consider the effect of CFC-11 generated in the transportation process on his study related to evaluating the impact of the ODP index. CFC-11 and N₂O have a greater global warming potential than CH₄ [33], but their emissions are very low, so they have a minimal impact on GWP. Moreover, CO₂ had the most impact at 73.80% of all the substances generated in the processes of incineration. In the case of AP, NOx had the largest impact of 7 × 10⁻¹ with 78.21% due to the burning of fuel during incineration. Moreover, incineration was the most unsuitable process for sludge management as it contributes more to damaging the environmental factors due to the higher generation of NOx, which was almost equal to 88.31% of the total NOx generation during the studied process. In the case of HTP, during incineration, NOx was the largest contributor with more than 95% contribution as compared to the other process options.

However, different impacts were observed for the LWA production from the wastewater treatment sludge as the amount of CO₂ generated through LNG utilization in the rotary kiln during the LWA manufacturing process contributed 51.69% GWP, which is the highest of the LWA manufacturing process. However, LWA contributed 25.38% of the total GWP in the overall process comparison with landfill and incineration. In addition to that, production of CH₄ in terms of POCP was minimal in LWA due to pyrolysis at a high temperature of 1200 °C. Moreover, AP and EP, which were high in the incineration process, were also lower in the LWA production system, as illustrated in Figure 3. These data show that landfills and incineration emit more emissions than the LWA process for sludge management, indicating a lower environmental impact despite the fact that LWA production required more manufacturing steps.

Table 4. Summary of various LCA parameters of different aggregate-producing techniques.

LWA vs. NA
Indicator	LWA	NA	Unit
acidification potential (AP)	2.73 × 10−2	8.40	kg SO2_Eq
climate change (GWP 100)	441	604	kg CO2_Eq
eutrophication potential (EP)	4.46 × 10−3	1.56	kg PO4−_Eq
human toxicity (HTP)	4.15 × 10−2	14.4	kg 1,4-DCB_Eq
photochemical oxidation (summer smog) (POCP)	1.64 × 10−3	1.64 × 10−2	kg ethylene_Eq
stratospheric ozone depletion (ODP)	3.41 × 10−7	1.57 × 10−6	kg CFC-11_Eq

reveals that LWA production from wastewater sludge has fewer environmental impacts and is a more eco-friendly option. Moreover, LWA also has additional usage in terms of building materials and construction sites; thus, it is necessary to do a comparative analysis of the environmental impact of the LWA with NA. A comparison of the LCA parameter results shown in Figure 4 illustrated that LWA production still has a more positive environmental impact compared to NA. The results show that the NA environmental impacts of AP, EP, and HTP are 8.40 kg SO₂_Eq, 1.56 kg PO₄⁻_Eq, and 14.4 kg 1,4-DCB_Eq, respectively, with NOx generated during ship transportation contributing more than 95%. This is because a smaller amount of SO_x, another substance, was emitted compared to NO_x during ship operation. However, the LWA environmental impacts of AP, EP, and HTP are 2.73 × 10⁻² kg SO₂_Eq, 4.46 × 10⁻³ kg PO₄⁻_Eq, and 4.15 × 10⁻² kg 1,4-DCB_Eq, respectively. Moreover, similar results were observed for the GWP, POCP, and ODP evaluation which confirmed the lower emission of the pollutants to the environment in LWA production as compared to NA. Furthermore, the higher contribution of the environmental impacts in NA was mainly in transportation where ships played a significant role in the production of CO₂. On the other hand, POCP and ODP were more affected by trucking than by ship, because the emissions of CO, CH₄, SO₂, and CFC-11 generated by trucking are higher, but overall, still lower than those of ship transportation. From these results, we concluded that the use of ships had a great influence on the environmental impact of NA production from sea sand.

3.3. Limitation/Sensitivity Analysis

The limitations of the work during the evaluation of different processes were related to other products generated in the sludge-management process. Incineration of sludge generates heavy metals, including Cu, Zn, As, Pb, Cd, and Ni, which have a significant impact on humans. In addition to the heavy metals, polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) were also generated [40] along with endocrine disruptor compounds, which are harmful to the reproductive system and thyroid hormones [40,41]. In the case of the landfill process, the uncertainty about the mechanisms by which organic pollutants would biodegrade during the process was not discussed [9]. The substances from these incinerations and landfills have a significant effect on HTP levels (e.g., 95.8 times for Zinc and 3.5 × 10⁴ times for Nickel). However, in the case of LWA, 90~99% of heavy metals such as Cd, Cu, Pb, and Zn can be removed by using the temperature of the rotary kiln (1100 ± 100 °C) by mixing chloride and water [42]. There are other techniques that can be used for the reduction of heavy metals generated from landfill deposition and incineration of sewage sludge. One of them is composting [43]. However, LWA is more effective in reducing the discharge of heavy metals in comparison with studied methods having more eco-friendly parameters in terms of producing pollution in the environment.

In addition to that, previous studies showed that uncertainty analysis is not typically performed in LCA [44,45,46]. However, in this study, uncertainty analysis was performed for two scenarios, evaluating the uncertainty due to the difference in transportation distance and the difference in treatment methodology. Transportation distance of raw materials has been shown to be an important factor in the production of LWA [47]. In the case of LWA, uncertainty analysis was performed including the transport amount of sludge, which is a raw material for landfill and incineration, and the transport amount of lightweight aggregate made through a rotary kiln. Additionally, in the case of NA, aggregate collection time and NA transport volume by ships were included. The Monte Carlo Simulation method was selected to perform the uncertainty analysis [48]. The simulation was performed 10,000 times with the Oracle Crystal Ball, and the results on the impact of variation in the environmental aspects through transportation are summarized in

Table 5. Transportation uncertainty results summary in terms of mean and standard deviation of the environmental aspect’s variation.

		Landfill	Incineration	LWA	NA
CO2	mean	179.15	516.55	440.34	576.94
	Standard deviation	2.36	3.91	0.59	42.97
SO2	mean	5.60 × 10−3	1.94 × 10−2	3.29 × 10−3	6.47 × 10−3
	Standard deviation	8.00 × 10−4	1.32 × 10−3	4.07 × 10−4	9.16 × 10−4
NOx	mean	5.83 × 10−2	1.08 × 10−1	3.43 × 10−2	11.9
	Standard deviation	8.33 × 10−3	1.38 × 10−2	4.24 × 10−3	1.17
CO	mean	8.81 × 10−2	1.52 × 10−1	5.18 × 10−2	4.78 × 10−1
	Standard deviation	1.26 × 10−2	2.08 × 10−2	6.41 × 10−3	3.98 × 10−2
PM	mean	6.01 × 10−3	1.03 × 10−2	3.53 × 10−3	1.39
	Standard deviation	8.59 × 10−4	1.42 × 10−3	4.37 × 10−4	1.36 × 10−1
NMVOC	mean	4.71 × 10−2	7.93 × 10−2	2.77 × 10−2	4.72 × 10−1
	Standard deviation	6.73 × 10−3	1.12 × 10−2	3.43 × 10−3	4.19 × 10−2
CH4	mean	26.1	4.33 × 10−2	1.37 × 10−2	3.5 × 10−2
	Standard deviation	3.33 × 10−3	5.51 × 10−3	1.69 × 10−3	3.90 × 10−3
CFC-11	mean	1.17 × 10−6	1.96 × 10−6	3.42 × 10−7	8.09 × 10−7
	Standard deviation	1.17 × 10−7	1.96 × 10−7	2.70 × 10−8	8.14 × 10−8

.

The results summarized in Table 5 revealed that CFC-11 had the greatest impact on the transport distance of 90% or more in environmental aspect variation. Moreover, for landfill and incineration, seven types of exhaust gases generated during transportation, except CFC-11, tend to have a small difference in sensitivity between the amount of transportation and the distance transported. In the case of landfills, the contribution of CFC-11 emission was 50.3%, and in the case of incineration, it was 49.3%. However, in the case of LWA and NA in Building Material Generation Scenarios, transportation for aggregate generation and two processes of applying the generated aggregate to the construction site were considered and analyzed. In the results for LWA, the volume of CO₂ during the sludge transportation was 39.6%, the sludge transportation distance was 37.8%, the aggregate transportation distance was 11.7%, and the aggregate transportation volume was 10.9%. Except for CFC-11 and CO₂, the six exhaust gases accounted for 9.3% of the sludge process, 65.6% of the sludge transportation distance, and 25.1% of the aggregate process, and the aggregate transportation distance was not affected. NA production results showed that in the case of CH₄, the emission through transportation was 48.6% and processing contributed to 47.2% of the volume. Moreover, in the case of CO emissions, ship transportation contributed 87.3% and truck transportation contributed 6.4% of the overall emission. Furthermore, truck transportation covered 50.6% of SO₂ emissions in comparison with the processing of sludge with a 49.4% contribution. However, 96% of SO₂ emissions were during transportation mainly through ship transportation.

By comparing the total amount of substances that can be generated during the entire process of LWA, NA, landfill, and incineration depending on the transportation and the amount of sludge, the environmental impact assessment is estimated to increase or decrease by ±10 for CO₂ and CFC-11, and by ±15 for SO₂, NOx, CO, PM, NMVOC, and CH₄. In terms of sludge management, in the case of GWP and POCP, LWA has a three times lower environmental impact compared to the incineration process. Moreover, in the case of EP, LWA and landfill have two times lower impact than incineration, with a higher difference for LWA production. However, we concluded that LWA is much more environmentally friendly than NA in terms of the production of building materials.

4. Conclusions

In this study, we performed LCA for three sludge treatment methods: landfill, incineration, and LWA production. Moreover, we compared the LCA of NA and LWA to determine the environmental impact of building material production as LWA through sludge treatment and NA from sea sand. According to this study, landfills contribute to the highest environmental impact in terms of GWP in comparison with others which have 54% and 53% lower GWP for incineration and LWA production. Furthermore, they have a higher impact in terms of POCP in comparison with the others which are 3% lower for incineration and 1% lower for LWA. Results revealed that GWP and POCP had small differences in incineration and LWA and lower impact on the environment. In the evaluation of AP, EP, HTP, and ODP, incineration was the highest with a difference of 2% for landfill and 6% for LWA in the case of EP, and similar results were observed for the others. From these results, we concluded that LWA has a minimum impact in all environmental categories with LWA having a 20%, 22%, and 42% lower impact in terms of AP, HTP, and ODP than landfill, respectively. Furthermore, the comparison of environmental aspects of LWA with NA for the production of building materials confirmed that LWA has a lower impact with the highest of 27% lower for GWP. Furthermore, uncertainty analysis results confirmed that transportation of the materials has a significantly higher impact on analyzed environmental aspects. Transportation contributes 48.6% and processing contributes 47.2% of CH₄ emissions in the NA analysis, and transportation through ships has a higher impact compared to trucks.

Through these results, our study concluded that LWA is a more eco-friendly option for sludge management than landfills and incineration. Moreover, through numerical evaluation of NA and LWA production for building materials, it was confirmed that LWA production using sludge can contribute to eco-friendly production and circular economy. In the future, it is believed that more precise results will be obtained if environmental impact assessments are conducted by conducting research on exhaust gases by decomposition mechanisms in landfills, incineration, LWA, and NA.