Sustainable Integrated Approach to Waste Treatment in Automotive Industry: Solidification/Stabilization, Valorization, and Techno-Economic Assessment

Abstract

An integrated approach to waste management is based on efficient and safe methods for waste prevention, recycling, and safe waste treatment. In accordance with these principles, in this study, non-hazardous aluminosilicate waste (dust and sand) was used in the solidification/stabilization (S/S) treatment of hazardous waste (coating, emulsion, and sludge) from the automotive industry. Also, the oily component of the waste was valorized and investigated for energy recovery through co-incineration. The two S/S processes were proposed and their sustainability was assessed by utilizing all types of waste generated in the same plant, obtaining stabilized material suitable for safe disposal and oil phases for further valorization, and by techno-economic analysis. The efficiency of the S/S processes was evaluated by measuring unconfined compressive strength, hydraulic conductivity, density, and the EN 12457-4 standard leaching test of S/S products, along with XRD, SEM-EDS, and TG-DTG analyses. The possibility of using the oil phase was assessed based on its calorific value. The techno-economic assessment compared the investments, operating costs, and potential savings of both treatment scenarios. The results show that an integrated approach enables safe waste immobilization and resource recovery, contributing to environmental protection and economic benefits.

1. Introduction

The automotive industry is recognized as the most resource-intensive of all process industries. The fact that automobiles are essential in modern daily lives has led the automotive industry to become a vast and influential sector, introducing significant environmental challenges. In addition to the original equipment manufacturers, the automotive industry includes a great variety of suppliers that support this industry [1]. Automotive parts manufacturing processes involve substantial consumption of raw materials and energy, but also generate various types of industrial waste, especially in casting and mechanical processing operations. These waste types include lubricants, coolants, and cleaning solvents in the form of waste emulsions, as well as paint, scrap metal, plastics, casting sand, and refractory materials. The quantities, concentrations, size distributions, and chemical compositions of these waste types vary depending on the specific manufacturing process [2]. This waste diversity requires the application of different treatments in waste management based on waste type (organic/inorganic), characterization (hazardous/non-hazardous), and state (liquid/solid).

The solidification/stabilization (S/S) process is considered the best available and cost-effective technology for the treatment of large quantities of industrial waste. It is a chemical treatment process that aims to either bind or complex the components of the hazardous waste stream into a stable, insoluble form (stabilization) or to capture the waste in a solid cement matrix (solidification) [3]. The most commonly used binders are cement-based materials. However, to improve the economic and environmental effects of the S/S process, nowadays cement is increasingly being replaced by pozzolanic waste materials that could serve as calcium-alumino-silicate sources, such as fly ash (FA) [4]. FA increases the pH region for immobilization and improves the mechanical properties of the treated waste [5,6]. The choice of additives and binders has a significant impact on the efficiency of the S/S process [7,8]. The binder component (CaO, SiO₂, and Al₂O₃) reacts with water during the pozzolanic reaction to form calcium silicate hydrates (C-S-H) and calcium aluminate hydrates (C-A-H) as the main constituents of the hardened matrix [9]. Both the physical and chemical properties of the hardened matrix depend on the development of this hydrated structure [10,11]. The pozzolanic reaction is influenced by the nature and relative amount of treated waste [12]. The S/S process is very effectively used in the treatment of industrial inorganic waste containing heavy metals [13,14,15].

However, S/S is considered less compatible with hazardous industrial organic waste, such as waste emulsions from casting and mechanical processing operations in the automotive industry. These waste emulsions are mineral oil-based emulsions, commonly used as cutting fluid, cutting oil, coolant, lubricant, or cleaning solvents. After they have lost their efficiency, the waste emulsions are discharged or sent for treatment [16,17]. As hazardous cleaning chemicals, these waste emulsions require special waste management treatments. In the S/S treatment, organic compounds from the waste can inhibit binder hydration [18] and are generally not chemically bound to binder hydration products, so their stabilization is largely dependent on their physical encapsulation or adsorption.

On the other hand, chemical treatment is one of the most useful and utilized techniques in treating waste emulsions [19,20,21]. Chemical treatment usually involves destabilization, coagulation/flocculation process, and separation as a primary treatment, followed by a secondary stage treatment. The most common chemicals used in the treatment of waste emulsions are sulfuric acid, as a demulsifier, and aluminum(III) sulfate, as a coagulant [20,21]. The demulsifiers make the emulsions unstable and in the presence of a coagulant, they create a forced separation of the water and oil phases. Kim et al. [22] proposed a pretreatment methodology for oil waste management in accordance with the waste hierarchy and end-of-waste criteria with the concept of circular economy. The main goal was to valorize the energy content of oil waste using pretreatment methods. Waste-to-energy (WTE) technology would be the best alternative to access alternative fuels [23]. These technologies can generate significant amounts of heat and energy from waste, thereby reducing many of the critical environmental issues associated with industrial waste.

Protection of humans and the environment from the negative impacts of waste is one of the high-priority requirements today [24]. Industrial waste streams have traditionally been managed separately, with wastewater, solid residues, and gaseous emissions treated in isolation, often leading to inefficiencies and limited opportunities for resource recovery [25]. Integrated waste management in process industries is a system aimed at reducing, collecting, recycling, and disposing of waste products [26]. Integral waste treatment comprises three core principles: multi-process coordination, resource recovery optimization, and circular economy implementation, all working together to minimize waste and recover resources. This involves the use of sequential or parallel physical, chemical, biological, and thermal treatments that are specifically tailored to different types of waste, including recovery of energy, nutrients, and materials, to enhance the economic viability of waste treatment processes [27]. Recent studies demonstrate the potential of integrated approaches, such as eco-industrial parks [28] and multi-stream valorization technologies [29], to enhance efficiency and circularity. Energetic valorization of municipal solid waste, evaluated via techno-economic and environmental criteria, represents a key model for integrated waste treatment [30]. Similar frameworks have guided the assessment of sewage sludge management technologies [31] and thermal treatment of plastics through pyrolysis and gasification, highlighting both technical efficiency and small-scale viability [32,33]. Integrated systems, such as mechanical-biological treatment combined with hydrogen production [34], and optimized wastewater treatment in eco-industrial parks [35], further demonstrate the benefits of combining processes, while life cycle assessments support sustainable biomass recovery [36]. Parallel research addresses oily waste streams, particularly emulsions. Demulsification methods using ultrasound, electrostatic, and microwave-assisted processes have been evaluated through techno-economic studies [37]. Nanocomposite fibrous mats, including magnetic and electrospun materials, show high oil-removal efficiency and scalability [38,39]. Techno-economic and life cycle analyses have also been applied to renewable diesel production from waste feedstocks [40] and lubricating oil recycling [41], highlighting both environmental and economic viability. The automotive industry context underscores the importance of such strategies for circular economy implementation [42]. Collectively, these studies indicate that future waste treatment requires integrated approaches combining advanced materials, innovative separation methods, and sustainability assessments to ensure both technical effectiveness and long-term viability. Sustainable waste management increasingly relies on integrated approaches combining technological, economic, and environmental considerations. Within this framework, solidification/stabilization (S/S) processes, waste valorization, and sustainability assessments supported by techno-economic analysis are particularly relevant, enabling environmental risk reduction and resource recovery.

This study explores an integrated approach that combines solidification/stabilization (S/S), chemical demulsification, and valorization of oil phases, supported by a techno-economic assessment to sustainably manage waste generated from casting and mechanical processing operations in the automotive industry. Namely, research objectives were consistently targeted (a) safe disposal of hazardous waste, (b) valorization of useful by-products, and (c) integration of circular economy and techno-economic viability into waste management systems.

2. Materials and Methods

2.1. Waste Samples

The specific types of hazardous and non-hazardous industrial wastes generated in an automotive industry plant in Serbia were selected and analyzed for further treatment. Hazardous waste streams were: oil emulsion, sludge, and Zr-based coating, while non-hazardous streams were dust, sand, and refractory material.

These types of waste are generated in the casting and machining processes of parts in automotive production: the waste oil emulsion was used as a coolant in machining processes; the waste sludge was created in the treatment process of liquid organic waste by vacuum distillation, Zr-based waste coating was used in the quality control process (automotive parts); waste dust and sand were created in the process of casting steel parts (in the technological process of mold preparation), as well as refractory material from a furnace.

The characterization of the hazardous waste oil emulsion and sludge, presented in

Table 1. Waste characterization.

	Hazardous Waste			Non-Hazardous Waste
Parameter	Sludge	Emulsion	Zr-Coatings	Dust	Sand	Refractory Material
Moisture content [%]	73	32	53	0.5	0.14	4.3
LOI (550 °C) [%]	90	99	1.9	15	0.5	1.0
Total PAH [mg/kg]	<6.0	<6.0	<6.0	<1.0	<1.0	<6.0
Total BTEX [mg/kg]	<1.0	<1.0	<1.0	<1.0	<1.0	<1.0
Total hydrocarbons [%]	2.9	12.0	<0.05	<0.05	<0.05	<0.05
Metals content [mg/kg]
Pb	<0.2	3.1	2.0	49	1.1	2.9
Cd	<0.09	<0.1	<0.1	<0.1	<0.1	<0.1
Zn	34.0	96	9.6	90	3.1	2.8
Cu	9.0	13	7.4	33	2.7	4.2
Cr	<0.1	20	7.1	119	24	6.1
Mn	0.75	15	1.2	97	135	7.2
Ni	0.94	13	2.9	89	139	24
As	<0.9	<0.9	<0.9	2.1	<0.9	<0.9
Hg	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05
Phase compositions
ZrSiO2 [%]	-	-	75–95	-	-	-
SiO2 [%]	-	-	-	60–90	70–95	-
Al2O3 [%]	-	-	-	1–5	-	70–90

, shows a high moisture content (73% and 33%) and expectedly high organic matter content (indicated by LOI of 90% and 99%), respectively. The content of total hydrocarbon (2.9 and 12%), higher than the reference value (2%) [43,44], indicates that the sludge and emulsion pose a characteristic that merits regulation as hazardous waste. The concentrations of metals were far lower than the reference values for hazardous waste. Also, the concentrations of total PAH (<6 mg/kg) and BTEX (<1 mg/kg) were well below the reference values (100 and 500 mg/kg, respectively) [43]. Waste Zr-coating was also characterized as hazardous waste due to its generic form and origin. Its analysis, presented in Table 1, shows a 33% moisture content and low organic matter content (indicated by LOI of 5.2%). The concentrations of selected metals were far lower than the reference values for hazardous waste. The concentrations of total PAH (<6 mg/kg) and BTEX (<1 mg/kg) were well below the reference values (100 and 500 mg/kg, respectively) [43]. The content of total hydrocarbons was significantly lower than the reference value [43]. The XRD analysis shown in Figure 1 confirms that the waste coating is based on zirconium silicate ZrSiO₄ (55–75%).

Waste dust, sand, and refractory material from a furnace were characterized as non-hazardous wastes with moisture content, LOI, metals content, PAH, BTEX, and total hydrocarbon, far lower than the reference values for hazardous waste [43]. XRD analysis of these materials showed that the dust and sand contained quartz (SiO₂), while the refractory material consisted of alumina (Al₂O₃) (Figure 1 and Table 1). They were used in the S/S treatment in addition to FA and bentonite as pozzolan material and silica and alumina sources.

2.2. Selected Additives

The fly ash from thermal power plants in Serbia and commercial-grade activated Ca-bentonite were used as selected additives in the treatment. Prior to experiments, the required amount of FA was dried at 105 °C for 24 h and sieved (−100 µm fraction). According to ASTM C618 standard [45], the FA used in this research belongs to Class F FA (SiO₂ + Al₂O₃ + Fe₂O₃ > 70%). Their chemical compositions are presented in

Table 2. Chemical composition of agents.

Component	CaO [%]	SiO2 [%]	Al2O3 [%]	Fe2O3 [%]	MgO [%]
FA	7.52	55.15	13.45	4.33	4.06
B	3.41	52.23	24.33	5.21	1.28

.

2.3. Treatment Process

2.3.1. One-Stage Treatment S/S Process

The one-stage treatment was an S/S process that involved direct mixing of the waste streams with a stabilization agent. Waste emulsion and sludge were mixed with bentonite for better adsorption of organic components and further stabilization by physical encapsulation in a hardened matrix. After that, the other waste components were added and mixed with FA and water to obtain a homogeneous paste. Fresh S/S mixtures were poured into plastic molds (5 cm × 5 cm × 5 cm) and vibrated to remove excess water and entrapped air. S/S product specimens were cured for 28 days according to ASTM C109 standard [46]: 24 h under wet towels to meet the requirement of 99% moisture, and after 3 days, they were removed from the molds and left to cure in the air before further testing.

2.3.2. Two-Stage Treatment Process

The two-stage treatment process consisted of chemical demulsification (CD) and centrifugation of the waste sludge in the first stage. The second stage involved the solidification/stabilization (S/S) process of the CD sludge obtained from the demulsification process with other types of waste.

Chemical demulsification of waste sludge was conducted with the addition of sulfuric acid (p.a. H₂SO₄) as demulsifier and aluminum(III) sulfate (p.a. Al₂(SO₄)₃) as coagulant in an amount corresponding to 2% of the mixture. The mixture was homogenized by mixing at a speed of 700 rpm for 5 min and left to stand for 5 min between the addition of acid and flocculant. To achieve an effective phase separation process, the mixture was centrifuged for 5 min at 5000 rpm. This process resulted in an efficient separation of the oil and CD sludge phases. This part of the research aimed to utilize the organic component of the waste as an alternative fuel, reduce the content of oil and water in treated waste, and increase the efficiency of the S/S process. The oil phase was characterized for valorization, and the CD sludge phase from the demulsification process was used in the S/S process, as the second stage in the two-stage treatment of waste.

All fresh S/S products were obtained by mixing the waste samples with B and FA (

Table 3. Compositions of S/S mixture.

Type of Waste	One-Stage Treatment	Two-Stage Treatment
	Demulsification
CD sludge * [%]	/	1
	S/S process
Emulsion [%]	6	5
Waste sludge ** [%]	5	/
Coatings [%]	6	3.6
Silicate dust [%]	28	25
Sand [%]	8	7.5
Refractory material [%]	-	12.3
FA [%]	30	27.4
B [%]	17	18.2

* CD sludge—sludge generated in chemichal deemulsification process; ** Waste sludge—sludge created in the treatment process of liquid organic waste by vacuum distillation.

) following water addition and stirring vigorously until homogeneity was achieved. The S/S products were poured and cured according to the already described procedure of ASTM C109 standard (2.4.1.) [46]. A total of two different S/S mixtures with different waste content were tested to assess S/S treatment efficiency.

The process flow of the treatment process of selected types of waste from the automotive industry is shown in Figure 2.

2.4. Characterization of Treatment Products

All products of one-stage (solidified material) and two-stage processes (oil, CD sludge phase, and solidified material) were sent for waste characterization. The characterization included determining the pH value, moisture content, loss on ignition (LOI) at 550 °C, content of metals, PAH, BTEX, halogen elements, sulfur, and total hydrocarbons (C10–C40), as well as the EN 12457-4 standard leaching test [43,47]. The analysis of the oil phase involved measuring the ignition point, calorific value, and viscosity to assess its potential as an alternative fuel [43].

2.5. Analytical Methods

Mineralogical analyses by X-ray powder diffractometry (XRD) were applied to identify and monitor the main components in a solidified specimen using powder diffractometer “PHILIPS”, model PW-1710, with the characteristic CuKα radiation (λ = 1.54178 Å) and a scintillation detector, at 40 kV and 30 mA, over the 2Θ range 10–60°, in steps of 0.02° and within the time of 1 s.

Scanning electron microscopy (SEM) was conducted to analyze the development of hydrated structures in the obtained solidified products. Before the analysis, the samples were coated with gold using a sputter coater (Polaron SC502, Polaron/Quorum Technologies Ltd., Laughton, UK). Microstructures were analyzed on a JEOL JSM-5800 scanning electron microscope at 20 kV. Energy Dispersive Spectrometer (EDS) Isis 3.2, with a SiLi X-ray detector (Oxford Instruments, Abingdon, UK) connected to SEM, and a computer multi-channel analyzer, was used to qualitatively and semi-quantitatively determine the elemental composition of the samples.

Thermogravimetric analysis was performed using an SDT Q-600 instrument (TA Instruments, New Castle, DE, USA) for simultaneous TG/DTG analysis. The samples were heated in alumina (Al₂O₃) vessels and in an air stream at a flow rate of 100 cm³/min, at a heating rate of 10 °C min⁻¹, in the temperature range from 25 to 800 °C.

2.6. Effectiveness of the S/S Process

The effectiveness of the S/S process is usually determined by measuring the hydraulic conductivity of materials (ISO 17313:2004) [48], UCS (EPA/530-SW-86-016) [49], and the leaching resistance (standard leaching test EN 12457-4) [47] of the gained solidified product. ISO 17313:2004 applies to compacted specimens that have a hydraulic conductivity between 1 × 10⁻⁵ m/s (1 × 10⁻³ cm/s) and 1 × 10⁻¹¹ m/s (1 × 10⁻⁹ cm/s). UCS provides basic information on pollutant stabilization within a solidified matrix. The UCS of the solidified products was measured after 28 days of curing, using a servo-hydraulic testing machine type INSTRON 1332-retrofitted Fast track 8800 by gradual loading to the breaking point of the specimens. There are numerous recommendations for the minimum required UCS for the safe disposal of stabilized waste. The U.S. Environmental Protection Agency (USEPA) requires a UCS of 0.35 MPa for S/S treated waste [49]. However, the national regulations of the Republic of Serbia do not define this parameter for waste disposal. The leaching resistance is assessed by conducting standard leaching tests on treated waste and measuring the concentration of contaminants in the solution after the tests, which should be below defined values. Standard leaching test EN 12457-4 [47] is a stage batch test at L/S ratio of 10 L/kg for materials with particle size below 10 mm, and is a part of the standard waste characterization procedure. It is performed to evaluate the suitability of treated waste to be disposed of as hazardous, non-hazardous, or inert waste.

2.7. Techno-Economic Analysis (TEA)

For each S/S process, TEA was performed to evaluate financial performance from a cost-minimization and cost-benefit angle. Key financial indicators such as net present value (NPV) and cost-benefit ratio were assessed. Economic valuation considers the time value of money and metrics such as the payback period and the return on investment are calculated.

3. Results and Discussion

3.1. Characterization of Solidified Products

3.1.1. XRD Analysis

XRD analysis was used to identify crystalline phases in the solidified products of the one- and two-stage processes aged for 28 days (Figure 3). The major phases in the products were quartz (SiO₂) at 21° and 26° (2θ) from silica sources (dust and sand). Quartz acts as an inert filler that contributes positively to the mechanical properties of the binder matrix. Quartz’s crystalline nature provides structural reinforcement to the material by enhancing packing density and contributing to resistance against compressive forces, effectively improving the strength of the overall composite. However, quartz particles are chemically inert under both hydration and alkali activation, meaning they do not participate in the chemical reactions responsible for forming binding phases.

XRD analysis of both solidified products showed the absence of the amorphous phase, whose development was expected during the S/S process. This result is attributed to the dominance of crystalline phases (e.g., quartz, ZrSiO₄), which mask the weak signals of poorly ordered hydration products, and to the limited amorphization under the applied S/S conditions. For the product from the one-stage process, the peaks had lower intensity, which could indicate better reactivity of quartz in the high alkalinity system.

3.1.2. SEM-EDS Analysis

The solidified products obtained by S/S processes were crushed and dried for 24 h prior to SEM-EDS analysis. A micrograph of the solidified material obtained by one-stage S/S processes is presented in Figure 4a, with EDS spectra of selected points shown in Figure 4b–e. The micrograph shows grains of different sizes where the development of a hydrated C-S-H structure, specific to the S/S treatment, can be observed. This is confirmed by the EDS spectrum in Figure 4e. The micrograph also shows the structures specific to the starting waste materials: iron particles remaining from the waste emulsion (EDS shown in Figure 4b), zirconium silicate from waste coating (EDS in Figure 4c), and Al₂O₃ as a residue of refractory material from the furnace (EDS in Figure 4d). This indicates a poorly developed, hydrated structure of the solidified product, which is probably inhibited by the presence of organic matter in the waste. The components of the initial waste are stabilized by physical encapsulation within this solidified structure.

A micrograph of the solidified product obtained by two-stage S/S processes is presented in Figure 5a. Figure 5b–e shows EDS spectra of selected points of the solidified material. The micrograph shows small grains of hydrated structure, confirmed by the EDS spectrum in Figure 5e. Particles of the initial waste material can also be observed in the micrograph: zirconium silicate from the waste coating (Figure 5b), silicate sand (Figure EDS in Figure 5c), and Al₂O₃ from the refractory material (Figure 5d). The grains of the hydrated structure are smaller than the residual waste particles, which indicates an insufficient degree of hydration and development of the C-S-H structure in this solidified sample.

3.1.3. Thermogravimetric Analysis

The results of the thermogravimetric analysis (Figure 6 and Figure 7) showed that the thermal properties of the solidified product depend on its initial composition. As expected, the total mass loss during heating up to 800 °C is higher in the sample from the one-stage process (7.6%) compared to the solidified sample from the two-stage process (4.5%). Both samples lost mass during the s four steps. Only in the first step, which involves the dehydration of surface-bound water, is the mass loss similar in both samples and amounts to about 1%. The sharper DTG maxima and the greater mass loss observed in the sample from the one-stage process, within the temperature range of 100 to 500 °C, are attributed to a higher moisture content. Additionally, these results indicate that the decomposition of organic components present in the waste sludge from the VD process occurs at these temperatures.

The smaller peaks on the DTG curve of the two-stage S/S process are a consequence of the lower content of organic components in the two-stage sample, which results in a reduced decomposition rate per unit time. It is evident that, in the first step of the two-stage process, during demulsification and centrifugation, the organic components decomposing at around 180 °C and 400 °C were removed. Consequently, the peaks are significantly smaller, confirming the efficiency of the first step in the treatment process. The DTG curve further indicates that the components decomposing at approximately 600 °C cannot be removed by demulsification and subsequent centrifugation, which highlights the importance of solidification as the second phase of the two-stage S/S procedure.

3.1.4. The Mechanical Properties of the Solidified Products

The mechanical properties of the solidified products, including UCS, hydraulic conductivity, and density, were the primary criteria in evaluating the efficiency of the S/S processes. The values were measured for hardened solidified products aged for 28 days (

Table 4. The values of unconfined compressive strength, hydraulic conductivity, and density of solidified products.

Product	UCS [MPa]	Hydraulic Conductivity [m/s]	Density (g/cm3)
One-stage S/S process	0.79	2.58 × 10−7	1.6
Two-stage S/S process	0.53	9.42 × 10−9	1.4

).

The results shown in Table 4 indicate the influence of the organic matter content on the mechanical properties of S/S products. The higher value of UCS (0.79 MPa) and hydraulic conductivity (2.58 × 10⁻⁷ m/s) were measured for solidified products with higher organic phase content (one-stage process). This result is in correspondence with the indicated hydrated structure of the solidified products and physical encapsulation of the initial waste within this solidified structure (Figure 4). The solidified products with a lower content of organic phase (two-stage process) had the lower values of UCS (0.50–0.53 MPa) and hydraulic conductivity (9.42 × 10⁻⁹ m/s). This was confirmed by SEM analysis and an insufficient degree of hydration and development of the C-S-H structure in this solidified sample (Figure 5).

The relatively low compressive strengths obtained for both one-stage (0.79 MPa) and two-stage (0.53 MPa) S/S products can be explained by their lower density and higher porosity. The density is considerably lower, approximately 1.4–1.6 g/cm³, which is in accordance with the high porosity observed in SEM images and the interference of the oily fractions. For comparison, ordinary cement pastes and mortars typically reach dry densities of 2.0–2.2 g/cm³, with porosities closer to 20–25% [50]. This reduction in density reflects the incomplete development of hydration products, as confirmed by SEM–EDS analysis, where residual waste particles and poorly connected hydrated structures were observed. The incorporation of oil emulsions and sludge further contributed to increased porosity and weakened the matrix, since these components neither react with the binder nor contribute to load transfer, but instead form inclusions that hinder hydration. The result is a discontinuous and less compact structure, leading to substantially lower strength compared with conventional binders. The link between density and mechanical performance indicates that the main limitation of the studied S/S products lies in the insufficient densification of the matrix. Strategies to address this may include controlling the oil fraction in waste mixtures (the issue of the studied S/S products lies in the insufficient densification of the matrix). Strategies to address this may include controlling the oil fraction in waste mix improvements, which could increase the packing density, reduce porosity, and thus improve the mechanical stability of the S/S products while maintaining their environmental safety.

The addition of the refractory material from a furnace as a source of alumina (2.5 and 12.3% in compositions of raw S/S mixtures) did not have a good effect on the USC of the material. Touite et al. [51] investigated the influence of alumina powder (Al₂O₃) on the absorption of oil emulsion with solid material. Their research aimed to examine and improve the mechanical strength of the final products to understand the immobilization mechanism and the performance of the cementation process based on the S/S protocol for the immobilization of oil waste. Significant differences in the USC of that matrix compared to the solidified products analyzed in this study are in a very complex system of a real waste mixture whose treatment is examined in this paper. However, all analyzed solidified products met the basic criteria of the S/S process efficiency. The values of measured USC were higher than 0.35 MPa [49] and hydraulic conductivity was lower than ∼10⁻⁹ m/s [48].

3.1.5. Physico-Chemical Characterization of Solidified Products

The results of solidified products characterization are shown in

Table 5. Waste characterization of S/S products.

Parameter	One-Stage Process	Two-Stage Process	Oil Phase
Moisture content [%]	1.66	<1	17
LOI [%]	4.95	2.78	98.5
Ignition point [°C]	-	-	>66.5
Calorific value [MJ/kg]	-	-	37.8
Total hydrocarbons C10-C40 [mg/kg]	16,468.1	3142.0	>100,000
Total PCB [mg/kg]	<0.1	<0.01	<0.1
Total PAH [mg/kg]	<0.1	<0.1	1.67
Total BTX [mg/kg]	<0.01	<0.01	0.049
Sulfur content [%]	-	-	0.037
Total halogens, Cl [%]	-	-	1.62
TOC [%]	2.98	1.06	-
Metals content [mg/kg]
Cu	28.3	18.1	3.11
Hg	<0.2	<0.2	<0.2
Cd	28.3	<0.6	<0.6
Mn	198	201	-
Ni	154	65.3	3.04
Pb	22.8	17.7	<0.6
Cr	41.8	33.6	1.60
Zn	47.2	42.2	22.6
Viscosity [mPa·s]			185.4
pH value	9.90	10.5	-
EN 12457 test (Metals content) [mg/kg]
Cu	0.80	0.42	-
Hg	<0.003	<0.003	-
Cd	<0.01	<0.01	-
Mn	0.25	0.18	-
Ni	0.24	<0.1	-
Pb	<0.07	<0.07	-
Cr	1.32	0.82	-
Zn	<0.4	<0.4	-
TDS 105 °C [mg/kg]	12,600	18,300	-
DOC [mg/kg]	4913	1829	-
Cl− [mg/kg]	460.9	567.2	-

. The measured value of total hydrocarbons was higher for the solidified product from the one-stage process (16,468.1 mg/kg). However, this value is lower than the reference value (20,000 mg/kg) [43]. A decrease in the content of total hydrocarbons in the solidified products from the two-stage process (3142.0 mg/kg) was expected due to a lower content of the organic phase in treated waste mixtures.

The higher content of organic components in the samples was accompanied by a higher concentration of dissolved organic carbon (DOC) (Table 5). The value of DOC above the reference value for disposal on landfill (2400 mg/kg) [52] was measured for the solidified product obtained by one-stage S/S (4913 mg/kg). The DOC value (1829 mg/kg) appropriate for the disposal of solidified at the landfill [43] had an S/S product with sludge from CD with a lower content of organic phase and water.

According to these results, it can be concluded that in the S/S process of this complex mixture of real industry waste streams, the content of waste oil emulsions and sludge must be controlled in the mixture (up to 6%). In a few studies, a low-viscosity and non-reactive mineral oil was successfully incorporated in a proportion of 20 vol.% in alkali-activated pastes, in the water-glass phase, and the geopolymer paste to form stable composite materials (oil immobilization) [53,54].

Physico-chemical characterization confirmed that both processes immobilized heavy metals and hydrocarbons below reference values, underscoring the effectiveness of the integrated treatment for inorganic pollutant control. However, the results also underline the necessity of controlling oil content in the mixtures (≤6%) to ensure environmentally acceptable leachability levels. This finding reinforces the complexity of treating heterogeneous real-industrial waste mixtures, where interactions between organic and inorganic phases strongly influence the properties of solidified matrices.

3.2. Utilization of Organic Component

Demulsification and coagulation are the techniques that are recommended in the treatment of waste emulsions [20,21,55]. This paper presents the two-stage treatment of sludge generated in the VD process. The first phase of the process included chemical demulsification (H₂SO₄), coagulation (Al₂(SO₄)₃), and centrifugation. The purpose of this stage was to destabilize the waste emulsions and to valorize the oil phase.

The obtained results of oil phase characterization (Table 5) showed the following values: the content of water was 17%, the upper caloric value (37.8 MJ/kg) was above the reference value (8 MJ/kg), and the concentration of chlorides and total halogens was below the reference value (20,000 mg/kg). It was concluded that the obtained phase could be used as a fuel substitute [43]. The waste volume for disposal was decreased and a hierarchy in oil waste treatment (recycling, burning, and landfilling) was implemented.

The valorization of the organic phase in the two-stage process represents a critical advantage. The recovered oil fraction displayed a high calorific value (37.8 MJ/kg), acceptable halogen content, and ignition properties consistent with fuel standards. These characteristics suggest strong potential for its utilization as an alternative fuel, in line with circular economy principles. While this reduces the volume of waste requiring disposal and enables partial energy recovery, further research is needed to evaluate the combustion performance and emissions of the recovered oil under industrial conditions. The relatively high moisture content (17%) also indicates that pre-treatment may be necessary before large-scale energy application.

When comparing the two proposed approaches, the one-stage process demonstrated superior mechanical properties due to higher encapsulation of organic matter within the matrix. However, this option resulted in excessive dissolved organic carbon (DOC) levels (4913 mg/kg), surpassing the regulatory limit for landfill disposal (2400 mg/kg), and therefore cannot be considered environmentally safe despite its technical advantages.

In contrast, the two-stage process, although resulting in solidified material with lower mechanical properties (UCS of 0.53 MPa and 9.42 × 10⁻⁹ m/s hydraulic conductivity), ensured full compliance with landfill acceptance criteria (DOC 1829 mg/kg) and offered additional benefits through oil recovery. The recovered oil fraction, with a high calorific value (37.8 MJ/kg), represents a valuable secondary fuel resource, thereby reducing the overall disposal burden and contributing to circular economy principles. The density (1.4–1.6 g/cm³) of the two-stage S/S products indicates higher porosity and lower strength than ordinary binders, but the UCS values remain above the minimum required value (0.35 MPa).

Based on these findings, the two-stage solidification/stabilization process should be considered the most suitable option for managing complex waste streams in the automotive industry. While it is less favorable in terms of mechanical strength, it ensures environmental safety, regulatory compliance, and long-term sustainability by reducing leachability risks and valorizing the organic phase. Future optimization should focus on enhancing matrix densification in the two-stage process, thereby improving mechanical performance without compromising environmental quality.

3.3. Techno-Economic Analysis (TEA)

This paper also attempts to investigate which solution would be the most appropriate for the company from a financial perspective. TEA is aimed at quantifying the economic and technical performance of each of the two scenarios (one- and two-stage processes).

Calculations of the materials, fluids, additives, and energy balance data from the S/S processes for each of two scenarios were used to size, map the equipment, and build the capital and operating cost profiles. Additionally, the capital costs for equipment were collected on the market.

In regular operation, the plant generates 1.872 tons of different categories of industrial waste. Costs of waste disposal and transportation during disposal were collected on the market in accordance with the valid prices of waste operators on the territory of the Republic of Serbia (data in

Table 6. The disposal cost of industrial waste generated by the current operation of the plant.

Type of Waste	t/Year	Price €/t	Total, €
Sludge from the VD process	132	1.455	192,000
Emulsion	168	1.500	252,000
Zr-based coatings	180	500	90,000
Silicate dust	804	100	80,400
Sand	240	100	24,000
Refractory material	348	100	34,800
Total			673,200

and

Table 7. Transportation costs for industrial waste disposal.

Waste Name	Transport Cost, €/km	Number of Batches of 20 t	Average km	Total, €
Industrial waste	1.5	93.6	150	21,060

).

For the mentioned types of industrial waste, the treatment, including the process of solidification and stabilization (S/S process), is provided. The goal of the treatment is to obtain final waste disposal that can be permanently disposed of in landfills under specially defined conditions or can have its use value.

Based on the obtained results of testing the solidified products of the S/S processes at the laboratory level, two variant solutions for the treatment of waste generated by the automotive industry plant are proposed:

• R1—one-stage S/S treatment of waste;
• R2—two-stage S/S treatment of waste with pre-treatment of waste sludge from the VD process.

A more detailed calculation of the techno-economic analysis is given in Supplementary Materials (Table S1. Estimation of equipment costs for options R1, R2. Table S2. Estimation of additional equipment costs necessary for R2 processes. Table S3. Depreciation calculation. Table S4. Staff costs. Table S5. Energy costs. Table S6. The costs of additives).

Financial balances and key performance indicators for one- and two-stage processes are shown in

Table 8. Financial balances for one- and two-stage processes.

Year	2025	2026	2027	2028	2029	2030
Disposal costs without investment (current situation), €	694.260	0	0	0	0	0
R1—one-stage process		0	1	2	3	4
Investment in equipment, €		88,800	0	0	0	0
Equipment maintenance 3%, €		2,664	2.664	2.664	2.664	2.664
Depreciation, €		5.328	5.328	5.328	5.328	5.328
Staff costs, €		57.984	57.984	57.984	57.984	57.984
Energy costs, €		15,296.6	15,296.6	15,296.6	15,296.6	15,296.6
The costs of additives, €		83.160	83.160	83.160	83.160	83.160
Costs of final disposal, €		294.624	294.624	294.624	294.624	294.624
TOTAL, €		459,056.6	459,056.6	459,056.6	459,056.6	459,056.6
Cash flow, €	−694,260	−547,856.6	−459,056.6	−459,056.6	−459,056.6	−459,056.6
Savings, €	0	146,303.4	235,203.4	235,203.4	235,203.4	235,203.4
R2—two-stage process		0	1	2	3	4
Investment in equipment, €		136.100	0	0	0	0
Equipment maintenance 3%, €		4.083	4.083	4.083	4.083	4.083
Depreciation, €		8.166	8.166	8.166	8.166	8.166
Staff costs, €		57.984	57.984	57.984	57.984	57.984
Energy costs, €		18,743.96	18,743.96	18,743.96	18,743.96	18,743.96
The costs of additives, €		59,310.6	59,310.6	59,310.6	59,310.6	59,310.6
Costs of final disposal, €		393.408	393.408	393.408	393.408	393.408
TOTAL, €		541,695.56	541,695.56	541,695.56	541,695.56	541,695.56
Cash flow, €	−694,260	−677,795.56	−541,695.56	−541,695.56	−541,695.56	−541,695.56
Savings, €	0	16,464.44	152,564.44	152,564.44	152,564.44	152,564.44

and

Table 9. Key performance indicators for one- and two-stage processes.

	R1—One-Stage Process	R2—Two-Stage Process
NPV, €	1,699,347.76	1,257,781.81
Discount rate, %	9	9
IRR, %	298	320
Payback, years	0.38 years 4 months 17 days	0.89 years 10 months 20 days
CBR	0.51	0.28

.

When assessing which solution is the best for the company in terms of cost minimization, the focus is on savings (or avoided costs) that a project or option will provide, rather than on revenues. In this techno-economic analysis, the following key performance indicators (KPIs) are utilized [56].

• Net present value (NPV)—calculates the present value of all future savings or avoided costs minus the initial cost of the investment.
• Payback period—The time it takes to recover the initial investment from the accumulated savings.
• Internal rate of return (IRR), i.e., discount rate at which NPV = 0. This indicator shows the efficiency of the investment in percentages.
• Cost-benefit ratio (CBR)—cost-benefit ratio, useful for comparing alternatives. In the case of this study, the ratio of total benefits (savings) to total costs.

In this study, it was assumed that the lifetime of projects is 15 years and the discount rate is 9% [57].

Option R1—one-stage process is an option that brings a relatively low investment, a high and positive net present value, an extremely high internal rate of return, the fastest payback period compared to the second option, but the lowest cost benefit ratio.

Option R2—two-stage process is the option with a longer payback period and a slightly lower rate of return than the R1 option, but still very high and positive parameters.

Both options discussed in the financial context are highly profitable for companies that generate significant amounts of waste each year. According to current market prices, these companies incur an annual cost of €694,260. Given that both alternative solution for the S/S process and investment in equipment within the complex has a payback period of less than a year, it is necessary to additionally consider the choice of waste treatment process in accordance with the most technologically adequate option.

The techno-economic analysis illustrates the practical implications of the two waste treatment scenarios. Both scenarios demonstrated high profitability compared to current disposal practices, with payback periods of less than one year. The one-stage process offered superior financial metrics, including lower capital costs, faster payback, and higher net present value. However, its environmental shortcomings—particularly the excessive DOC levels—render it less suitable for regulatory compliance. Conversely, the two-stage process, although requiring higher capital investment and exhibiting lower mechanical strength of solidified product, ensured safe disposal and enabled oil valorization, thereby providing additional benefits that were not fully monetized in the present analysis. Incorporating potential revenues from fuel substitution, avoided environmental penalties, or carbon credits could strengthen the economic justification for the two-stage process.

Overall, the findings demonstrate that the integrated approach to waste management in the automotive industry is technically viable and economically attractive. Nevertheless, achieving an optimal balance between UCS development and leachability resistance of the solidified products with cost-effectiveness of the treatment process requires careful adjustment of waste mixture composition and further development and optimization of the two-stage process. Future work should also evaluate the long-term stability of treated materials under variable environmental conditions and conduct life-cycle assessments to quantify the broader environmental benefits of waste valorization.

4. Conclusions

This study has shown that an integrated approach to waste management in the automotive industry, combining hazardous (emulsions, sludges, Zr- based coatings) and non-hazardous (dust, sand, refractory) waste streams through stabilization/solidification (S/S) and chemical demulsification, ensures: (a) safer disposal of hazardous waste, (b) valorization of useful by-products, and (c) integration of the circular economy and techno-economic viability into waste management systems.

The novelty of the research is reflected in:

• The synergistic use of non-hazardous waste from the plant as a source of alumino-silicates for immobilization of hazardous fractions, thereby closing the internal material loops.
• Two-stage S/S waste treatment with oil valorization, where pre-treatment of sludge by chemical demulsification improved the safe disposal of the S/S product (reducing DOC below landfill limits) and generated an oil phase with a high calorific value (37.8 MJ/kg), suitable as an alternative energy source. This represents a practical application of circular economy principles in hazardous waste management.
• Integration of technical and economic assessments, where the study links laboratory-scale characterization of products (mechanical and physico-chemical properties) with techno-economic indicators (NPV, IRR, payback). Both the one-stage and two-stage options proved highly profitable, with payback periods shorter than one year.
• The scientific significance is reflected in demonstrating that:
• Industrial hazardous wastes with high organic content can be safely incorporated into solidified matrices at controlled levels (≤6%), overcoming the typical limitations of S/S processes.
• Recovered oil phases from waste streams can meet regulatory requirements and be reintroduced into energy systems, thereby reducing disposal volumes and creating added value.
• The developed methodology provides a scalable model applicable to other process industries facing complex mixed waste streams.

It was concluded that the two-stage solidification/stabilization process is considered a suitable option for managing complex waste streams in the automotive industry, despite higher capital costs and a slightly longer payback period. It ensures environmental safety, regulatory compliance, and long-term sustainability by reducing the risk of leaching and valorizing the organic phase. Further research will focus on optimizing the S/S process to obtain a higher proportion of hydrated phase in the matrix and increasing the density of the S/S matrix in the two-stage process, thereby improving mechanical performance without compromising environmental quality.

Finally, the proposed integrated approach advances sustainable industrial waste management by combining safe disposal, resource valorization, and economic feasibility. This work bridges the gap between experimental research and industrial practice, showing that the automotive industry can transform hazardous waste challenges into opportunities for resource efficiency and circular economy implementation.

Although the findings of this research provide valuable insights into integrated waste treatment in the automotive industry, some limitations should be noted. The experiments were carried out at laboratory scale, and therefore, the long-term stability of the treated materials under real landfill conditions has not yet been assessed. In addition, the variability of waste composition in industrial practice may affect the reproducibility of the proposed treatment process. Finally, the techno-economic analysis was based on current market prices and laboratory data, and additional sensitivity analyses will be needed to account for potential fluctuations in costs and performance at an industrial scale.