Waste-to-Energy Technologies in Saudi Arabia: A Case Study and Review of Waste Conversion and Energy Recovery

The annual Hajj pilgrimage draws approximately 3 million Muslims to Makkah, resulting in substantial quantities of MSW generated without sufficient separation or management [27]. Shahzad et al. [37] estimate that the daily waste output during Hajj ranges from 3100 to 4600 tons. A significant portion of this waste is produced in Mina, where pilgrims typically spend four to five days, leading to an estimated 17,000 tons of MSW being deposited in landfills without resource recovery [15]. At the Grand Mosque, daily waste generation exceeds 200 tons [15].

The ritual animal sacrifices associated with Hajj pose considerable challenges for waste management [38]. During Umrah, particularly in Ramadan, waste generation can peak at approximately 190 tons, primarily from food waste [16]. The cumulative waste from Umrah visitors and local residents may total around 5000 tons. On average, Makkah generates roughly 2400 tons of MSW daily, with spikes exceeding 3000 tons during Ramadan and reaching 4500 tons during Hajj [38]. Peak waste generation typically occurs during the last ten days of Ramadan and from the 8th to the 13th of Dhu al-Hijjah, aligning with the Hajj period [39]. In 2014, total MSW generated was approximately 950,000 tons, consisting of about 900,000 tons from residents and 90,000 tons from pilgrims [34,37]. The waste produced during Hajj is varied, comprising household and biological waste [40]. The waste composition during Umrah resembles that of the local population, influenced by visitor interactions, with notable contributions from food disposal and single-use utensils [38]. The extreme heat during Hajj and Umrah exacerbates waste management challenges, particularly due to the significant volume of disposable plastic items, including Zamzam water cups [27]. Additionally, the environmental impact of disposing of animal by-products from ritual sacrifices is considerable [39].

The waste management system in Makkah mirrors those in other Saudi cities, encompassing waste collection, transfer, and landfill disposal [38]. Collection efforts are labor-intensive and costly, especially during Hajj and peak Umrah periods. Waste is collected from street containers three times a day in central areas and less frequently in other locations [27]. Following collection, waste is transported to transfer stations or directly to landfills using compactor trucks [27]. Makkah operates six transfer stations, each with a capacity of 140 m³, designed to consolidate waste for disposal in larger landfill containers [38]. However, similar to many landfills in Saudi Arabia, Makkah’s landfill is deficient in leachate and gas collection systems and protective liners, raising significant concerns about environmental contamination [39]. The landfill is organized into cells that can fill within a week, necessitating rapid landfill expansion and posing regulatory challenges [27]. Effective landfill management is crucial, especially given the absence of alternative waste treatment options. Recycling efforts predominantly rely on the informal sector, which recycles materials such as metals, cardboard, and plastics, achieving only about 10% of total waste recycling [38]. Overall, the waste generated during Hajj and Umrah underscores the urgent need for enhanced waste processing strategies in Makkah to mitigate environmental impacts while accommodating the demands of religious tourism.

Incineration is a widely adopted waste management technology that involves the complete combustion of organic materials in MSW [2]. This method is effective for recovering energy from waste that cannot be recycled, thereby reducing landfill dependency [19]. This method involves the high-temperature oxidation of organic materials, resulting not only in a significant reduction in waste volume but also in the generation of thermal energy, which can be converted into electricity [19]. The relevance of incineration has become increasingly evident, especially in regions experiencing rising waste generation and limited landfill capacity [5]. For instance, in Saudi Arabia, the growing urban population and concomitant waste production have heightened the necessity for effective waste management solutions [5]. The incineration process primarily consists of several stages, including waste feed systems, combustion chambers, and exhaust gas handling [43,62,63].

Generally, incinerators operate at temperatures around 850 °C to ensure complete combustion, resulting in CO₂ and water vapor as primary emissions [63,64]. While this technology effectively reduces waste volume and mitigates the infectious properties of medical and hazardous waste, it also necessitates careful management of non-combustible materials (such as metal, glass, etc.), which can form solid residues known as incinerator bottom ash [19]. Two main methods of incineration are commonly applied: mass burn incineration, which processes unsorted waste, and a pre-sorting method that removes recyclable materials before combustion [5,29]. This approach aims to enhance operational efficiency while limiting negative environmental impacts [65]. Table 3 presents a detailed overview of the advantages and disadvantages of incineration technology.

The thermal energy generated during incineration can be converted into steam, powering turbines for electricity generation [55]. Generally, the energy recovery efficiency of incineration systems ranges from 20% to 30%, contingent upon various operational factors [5,19]. Advanced processing techniques, such as pre-treatment of solid waste, can improve energy efficiency by optimizing the calorific value of the waste materials [65,75]. Despite the benefits of waste volume reduction and the diminishing environmental hazards associated with landfills, incineration raises concerns about pollutant emissions, necessitating effective management strategies to control the release of harmful substances and comply with environmental regulations [19]. As highlighted in Saudi Arabia’s Vision 2030, the transition to incineration is viewed as a strategic approach to curbing GHG emissions and enhancing waste management practices [77]. Investment in incineration technology offers promising opportunities from both economic and environmental standpoints.

The Saudi government, acknowledging the pressing need for efficient waste management, allocated SAR 54 billion for municipal services in 2017, reinforcing its commitment to sustainable practices [31]. Additionally, the implementation of incineration aligns with the objectives of Vision 2030, which aims to reduce GHG emissions and promote cleaner energy production. Ongoing initiatives focus on refining incineration practices and exploring carbon capture technologies to address environmental concerns. Table 4 provides the studies that have evaluated the incineration process within Saudi Arabia.

Research on incineration-based WtE systems in Saudi Arabia demonstrates a significant methodological, even though the studies differ in their geographic focus, waste characteristics, and analytical scope. Collectively, these investigations confirm that incineration is a technically viable and environmentally advantageous approach for managing MSW, especially when compared to traditional landfilling practices.

At the local and regional levels, studies conducted in Dammam [50] and the Eastern Province [43,62] underscore the effectiveness of incineration under various operational frameworks. Scenario-based analyses have compared material recovery facilities (MRF), landfilling, and incineration, consistently revealing that incineration provides superior energy recovery and GHG mitigation [50]. For instance, in Dammam, incineration is estimated to generate approximately 1.91 × 10⁹ kWh annually, which corresponds to a reduction in fossil fuel consumption by 1.12 × 10⁶ barrels per year, resulting in a net negative carbon footprint [50]. Complementing these findings, regional studies in the Eastern Province indicate that the complete incineration of mixed MSW could yield up to 254 MW of electricity, sufficient to supply nearly 26,000 households [62], while significantly diminishing CH₄ emissions compared to landfilling [62].

Further supporting the robustness of incineration as a WtE solution are studies from the Western Province, particularly in cities like Jeddah, Makkah, and Madinah [65]. Research by Ouda et al. [65] has shown that incineration can achieve the highest electricity outputs, ranging from 61.3 MW in Madinah to 180 MW in Jeddah. When recycling is incorporated prior to incineration, although there is a notable decrease in energy output, this is offset by enhanced resource recovery and lowered environmental impacts [65]. The objectives of maximizing energy recovery and environmental performance are not always aligned but can be effectively balanced through integrated system designs.

On a national scale, several studies extend these regional insights by connecting trends in MSW generation to future energy demands [5]. While this contribution represents a modest fraction of total national electricity demand, it plays a strategically crucial role in reducing landfill reliance, conserving land area, and decreasing fossil fuel consumption. Notably, scenarios that include recycling yield lower power outputs but provide significantly greater environmental benefits, including crude oil savings of up to 55.6 million barrels annually and GHG emission reductions exceeding 15.2 million metric tons of carbon equivalent each year [78].

Life cycle assessment (LCA) studies conducted in Riyadh further substantiate these conclusions by evaluating the broader environmental impacts of incineration [31]. These studies demonstrate that the combination of incineration and recycling can reduce GHG emissions by 55% to 58% compared to landfill-dominated systems, while also contributing up to 3.17% of household electricity demand [31]. Such findings reinforce the importance of evaluating WtE technologies not only based on their energy yield but also through comprehensive environmental performance metrics.

Overall, regional analyses establish the technical feasibility of incineration, national-scale projections highlight its strategic significance, and LCA studies confirm its environmental sustainability. Together, these works support the assertion that incineration, especially when integrated with recycling and other waste management practices, represents a cornerstone technology for sustainable MSW management and resource recovery in Saudi Arabia.

Plasma arc gasification represents a cutting-edge thermochemical technology aimed at converting carbonaceous waste into valuable energy products [79]. This process operates at extremely high temperatures generated by an electrically powered plasma torch, contrasting with traditional gasification methods [29,79]. Unlike these conventional approaches, plasma arc gasification functions under oxygen-deficient or partially oxidizing conditions [80,81]. This environment facilitates complete molecular dissociation of organic materials rather than merely promoting partial combustion. Consequently, MSW and intricate waste streams are efficiently transformed into synthesis gas (syngas) and an inert vitrified slag [80,81].

Plasma is often referred to as the fourth state of matter and consists of highly ionized gases resulting from electrical discharge at temperatures typically between 2000 and 5000 °C [29,48,79]. Under these harsh thermal conditions, the chemical bonds of organic materials are fully broken down, enabling the conversion of complex hydrocarbons into simpler gaseous molecules, primarily H₂ and CO [82]. Simultaneously, the inorganic portion of the waste melts and solidifies into a glass-like, non-leachable slag upon cooling, which is chemically stable and environmentally friendly [82,83].

The plasma arc gasification system is fundamentally centered around the plasma torch, which facilitates the conversion of electrical energy into high-intensity thermal energy [56]. Plasma torches can operate in either transferred or non-transferred arc configurations [56]. In transferred arc systems, the arc forms directly between the torch electrode and the waste material, achieving high energy transfer efficiency [48,56,79]. In contrast, non-transferred arc systems confine the arc within the torch body, relying on the generated plasma jet to transfer heat indirectly to the waste. Both configurations ensure rapid heat transfer, elevated reaction rates, and nearly complete waste destruction [56].

Plasma arc gasification encompasses several sequential stages: waste pre-treatment (including shredding or size reduction), feeding into the plasma reactor, high-temperature thermal decomposition and gasification, syngas cooling and purification, followed by final energy recovery [46,79]. The performance of the process is significantly influenced by various operational parameters, including reactor temperature, residence time, pressure, and external electrical energy input [82,84]. By meticulously managing these parameters, the composition and heating value of syngas, especially the hydrogen-to-carbon monoxide (H₂/CO) ratio, can be optimized for diverse downstream applications such as electricity generation, heat recovery, H₂ production, or synthetic fuel synthesis [82,84]. Table 5 presents a detailed overview of the advantages and disadvantages of plasma arc gasification technology.

One of the primary advantages of plasma arc gasification is its remarkable feedstock flexibility. The technology is capable of processing various waste materials, including MSW, plastics [46,48], hazardous and medical wastes [79,85], RDF [49], biomass [56], tires [82], and automotive shredder residue (ASR) [47], with minimal pre-treatment required [79]. Unlike traditional thermal processes, high moisture content does not significantly impede system performance due to the extreme operating temperatures, making plasma gasification especially adept at handling heterogeneous and contaminated waste streams [19,80].

The primary product of plasma arc gasification is syngas, which retains a substantial portion of the original chemical energy present in the waste [46,48]. This syngas can be utilized directly in gas engines or turbines for electricity and heat generation or further refined into liquid fuels and chemical products such as methanol and H₂ [46,48]. Additionally, the inorganic fraction is converted into vitrified slag, which is chemically inert, non-hazardous, and suitable for reuse as construction aggregate, enabling a reduction in landfill waste volume by up to 99% [80,81]. From an environmental perspective, plasma arc gasification offers considerable advantages over landfill disposal and conventional incineration. The process results in minimal CH₄ emissions, significantly reduces GHG emissions, and effectively destroys toxic organic compounds at the atomic level [82]. The high-temperature plasma conditions also promote the reduction of hazardous pollutants such as mercury, particularly in plastic-rich waste streams, thereby contributing to the production of cleaner alternative fuels [87].

Nevertheless, plasma arc gasification is not without limitations, primarily concerning economic and operational factors. The technology demands substantial capital investment and operational costs due to the significant electrical power requirements of plasma torches, sophisticated control systems, and the need for specialized labor [84]. Consequently, economic feasibility is highly sensitive to electricity pricing, plant scale, and energy recovery efficiency. As such, this technology has predominantly been deployed in areas with limited landfill capacity and stringent environmental regulations, such as Japan, while remaining largely at the pilot or demonstration stage in other regions [86,87].

Plasma arc gasification has been the focus of three significant studies specifically conducted in Makkah, Saudi Arabia, offering an in-depth examination of the technology within the context of urban waste management and the unique challenges posed by pilgrimage-related waste [48]. These investigations collectively explored the technical feasibility, environmental impact, and economic viability of the technology, especially in light of the rapidly escalating waste generation in the region [48]. Galaly et al. [48] highlighted the growing challenges of waste disposal in Makkah, noting an increase in landfill waste from 168,000 Mg in 1994 to 297,000 Mg by 2022, alongside rising costs associated with environmental pollution. Their research indicated that using air plasma torches for plasma arc gasification of plastic waste could yield approximately 317,000 tons of pyrolysis oil, translating to an energy output of 12.55 × 10⁹ MJ and achieving an efficiency rate of 81%. Economically, this system demonstrated a return on investment of 80%, a payback period of 1.2 years, and a gross profit margin of 129%, all while generating near-zero harmful emissions [48]. Building on these findings, Galaly et al. [46] investigated a larger-scale plasma gasification facility designed to process 200 tons/day of increasing plastic waste in Makkah, which expanded from 224,000 tons in 1994 to 400,000 tons in 2022. This study outlined the plasma treatment methodology, which included pre-treatment stages, high-temperature conversion into syngas, pyrolysis oil, and vitrified slag within a temperature range of 1500–5000 °C, followed by purification and energy recovery processes. The anticipated outputs comprised 317,000 tons of pyrolysis oil, 270,000 tons of diesel-equivalent fuel, and approximately 2.96 million MWh of electricity, all underscoring a commitment to zero emissions and alignment with sustainable development objectives [46].

Moreover, Galaly and Oost [82] explored the Plasma Treatment Project (PTP) in Makkah during peak pilgrimage seasons, when annual waste generation can reach approximately 750,000 tons. Their analysis revealed that a 100 tons per day plasma system could produce up to 23.8 MW of electricity, generating annual profits of about 3.27 million EUR. The study affirmed the capability of plasma arc gasification to process a diverse range of waste streams, while ensuring environmental safety and operational dependability [82]. Table 6 provides a summary of the key characteristics and findings from the plasma arc gasification studies conducted in Makkah, Saudi Arabia.

Gasification is an established thermochemical conversion technology applied in WtE systems. It transforms carbon-rich feedstocks, such as MSW, RDF, biomass, coal, and industrial residues, into a combustible gas mixture known as syngas under conditions of limited oxygen availability [68,90,91]. Unlike direct combustion or incineration, which involves complete oxidation, gasification uses a controlled amount of an oxidizing agent (air, oxygen, or steam) to partially oxidize the feedstock [92]. This process breaks down complex organic molecules into simpler gases, mainly H₂, CO, along with minor amounts of CH₄, CO₂, higher hydrocarbons (tar), and char [92,93,94].

The syngas produced is a versatile energy carrier. It can be directly combusted for electricity or heat generation, utilized in combined-cycle power plants, reciprocating engines, or fuel cells, or serve as a feedstock for chemical production, including methanol, ammonia, urea, or ethanol [91,95]. Gasification is considered an environmentally friendly and efficient alternative to fossil fuel combustion, as it facilitates energy recovery from low-value residues while minimizing harmful emissions [96,97].

The gasification process involves several integrated stages. Pre-treatment ensures stable and efficient operation by sorting, shredding, and drying the feedstock to maintain a uniform particle size and moisture content below 15% [91,97]. The pre-treated feedstock is then fed into a gasifier reactor, where high temperatures, typically around 700–850 °C for biomass and 800 °C for conventional gasifiers, facilitate pyrolysis, char gasification, and reforming reactions to generate syngas [44,90]. Key parameters such as gasification temperature, steam-to-biomass ratio, char split ratio, particle size of the feedstock, and type of gasifying agent greatly influence the composition, yield, and heating value of the syngas [93].

Gasification offers several advantages over traditional WtE methods. Operating under low-oxygen conditions reduces the production of hazardous by-products, such as dioxins and furans, which are often generated in incineration processes [63]. This process is also energy-efficient, consumes less oxygen, generates smaller volumes of gas, and simplifies pollutant capture due to higher contaminant partial pressures in the gas phase [63]. Additionally, gasification significantly decreases waste volume, typically reducing it by around 90% and leaving only 8–12% residual ash, which is less than that from conventional incineration [91,98]. The technology’s adaptability allows it to process heterogeneous waste streams, including MSW, RDF, biomass, coal, sludge, black liquor, PVC, tires, and ASR, provided that pre-processing removes inorganic contaminants [91,97,99,100]. This versatility enhances its appeal for urban WtE applications and supports circular economy strategies by recovering energy from otherwise wasted materials [91,99].

Nevertheless, gasification also presents challenges. The syngas produced may contain tars, alkaline compounds, halogens, and heavy metals, which can impact operational stability and pose environmental risks if not adequately treated [98,101]. For instance, alkalis in fluidized-bed gasifiers can lead to agglomeration, tars can harm filtration and catalytic systems, halogens may cause corrosion, and uncontrolled heavy metals can accumulate [95]. Additionally, setting up gasification plants typically demands a significant capital investment, along with operational complexity and a skilled workforce [14,18,95,96]. Despite these challenges, advancements in dual fluidized-bed (DFB) gasifiers, gas cleaning technologies, and process integration are enhancing operational efficiency, syngas quality, and environmental performance [19,102,103]. Table 7 summarizes the primary benefits and drawbacks of the gasification process.

In Saudi Arabia, gasification has emerged as a versatile WtE technology well-suited to addressing the diverse challenges of waste management, from urban MSW to agricultural residues and microalgae. The research conducted by Rehan et al. [25], Ali et al. [57], Findley et al. [105], and Al-Rabiah et al. [106] collectively illustrate the practical applications of this thermochemical conversion technology across various contexts within the country. These studies highlight gasification’s adaptability, environmental benefits, and potential for energy recovery. The type of waste processed directly influences the by-products generated. As shown in Table 8, which provides an overview of gasification for MSW in Saudi Arabia, MSW and date palm fronds (DPF) primarily yield combustible syngas with minimal residual ash, while camp waste generates biochar that is beneficial for soil enhancement. In contrast, microalgae conversion produces both syngas and methanol, alongside manageable residuals. These variations underline how the characteristics of different feedstocks dictate the overall outputs from the gasification process.

Rehan et al. [25] focus their investigation on a simulated 50 t/day gasification plant targeting urban MSW in Jeddah. This study underscores the environmental advantages and economic feasibility of gasification at the city level, revealing a significant reduction of 100 kg of CO₂-eq emissions for every ton of waste processed. This work emphasizes traditional WtE applications, particularly regarding the production of syngas for electricity generation, which aligns with broader efforts to address climate change and enhance energy security in urban settings. In parallel, Ali et al. [57] explore the potential of agricultural residues, specifically DPF, in a downdraft gasifier. Although this study operates on a smaller scale, it provides critical insights into the optimization of gasification parameters, revealing that temperature plays a role in determining syngas composition. By maximizing H₂ and CO output, this study highlights how the choice of feedstock influences not only process design but also syngas quality for chemical applications. The findings from Ali et al. complement those of Rehan et al. by demonstrating that different feedstock types can be effectively utilized to produce high-value energy products, reinforcing the adaptability of gasification technology across diverse contexts.

The flexibility of gasification is further exemplified in the work of Findley et al. [105], who introduce a modular, transportable gasifier designed for use in remote locations. This study diverges from the urban and agricultural focuses of Rehan et al. [25] and Ali et al. [57] by emphasizing operational flexibility and waste volume reduction. By processing organic camp waste and kitchen scraps in small batches, this system achieves over 95% waste volume reduction, recovering energy through internal syngas combustion, and managing by-products safely through the production of non-toxic biochar suitable for soil enhancement, thereby contributing to effective waste management in remote settings. This adaptability shows that gasification can effectively operate in decentralized or temporary settings, highlighting its practicality across various Saudi contexts. In a more industrial context, Al-Rabiah et al. investigate the use of microalgae biomass in a DFB gasifier for the concurrent production of methanol and electricity [106]. They utilize a DFB gasifier to process microalgae from the Red Sea, achieving a biomass conversion efficiency of 45.7% to produce methanol while generating 38 MW of electricity. This study illustrates how gasification can optimize value extraction from specialized feedstocks. This research extends the scope of previous studies by integrating chemical production with energy generation, illustrating the multifaceted benefits of gasification in an industrial setting.

Together, these studies emphasize gasification’s strengths in Saudi Arabia, including its adaptability to various feedstocks, such as MSW, DPF, microalgae, and camp waste, significant energy recovery potential, and substantial waste volume reduction. These attributes align closely with environmental and circular economy objectives. Critical operational parameters, such as gasifier temperature, feedstock preparation, and syngas management, consistently emerge as essential for maximizing process efficiency and product quality, regardless of whether the goal is electricity generation, H₂ production, or methanol synthesis. Gasification in Saudi Arabia represents a flexible, scalable, and environmentally beneficial WtE technology capable of transforming diverse organic materials into valuable energy and chemical products. This approach contributes to reducing landfill dependency and aligns with the renewable energy goals outlined in Vision 2030.

Pyrolysis is an advanced WtE thermochemical conversion method that thermally decomposes carbonaceous materials in an oxygen-free or limited oxygen environment. This process typically occurs at temperatures between 300 and 1000 °C, depending on the specific operational setup and target product distribution [45,107,108,109]. Through this thermal treatment, MSW and other waste types are transformed into three primary products: solid char, liquid pyrolysis oil (bio-oil), and gaseous products known as syngas [45,110,111,112,113].

The yield and composition of pyrolysis products are significantly affected by various operational factors, including feedstock type and composition, particle size, heating rate, reaction temperature, vapor residence time, and pressure conditions [114,115]. Lower temperatures tend to favor liquid product formation, while higher temperatures promote reactions such as volatile cracking and decarboxylation, leading to increased syngas output with reduced yields of bio-oil and char [115,116,117]. Typically, syngas from pyrolysis contains CO, H₂, CO₂, and CH₄, while the liquid product is composed of complex oxygenated hydrocarbons, and the solid product is mainly carbon-rich char [45,111,112].

Pyrolysis offers considerable operational flexibility, allowing adjustments to process parameters to optimize yields based on energy or material recovery goals [110]. The pyrolytic liquid can be used directly in boilers, furnaces, and engines with minimal upgrading, or can be refined into transportation fuels and chemical feedstocks [118,119]. The char generated is valuable due to its high heating value and can be utilized for power generation, soil enhancement, carbon sequestration, or as a precursor for materials like activated carbon and biochar [120,121]. The production of char from pyrolysis has a long history in the creation of coke and charcoal from biomass and coal, indicating its technological maturity [121].

Pyrolysis can be classified into three main types based on heating rate, temperature, and residence time: slow, fast, and flash pyrolysis. Slow pyrolysis operates at heating rates of 0.1–2 °C/s and around 550 °C, which encourages char and tar production. Fast pyrolysis uses moderate temperatures (586–977 °C) and short residence times (0.5–10 s) to yield high amounts of bio-oil. Flash pyrolysis, which occurs at very high temperatures (777–1027 °C) with rapid heating rates, primarily produces gas-rich products suitable for alcohol and gasoline production [19]. Fast pyrolysis is the most commonly utilized for WtE applications due to the fuel-like characteristics of its products and their compatibility with existing energy infrastructures [19].

However, applying pyrolysis to MSW involves significant challenges because of the heterogeneous nature of municipal waste [122]. The yield and quality of products from MSW pyrolysis can vary greatly depending on feedstock composition, with gas yields often falling below 1 Nm³/kg MSW, equating to a calorific value of about 15 MJ/Nm³ [123]. Additionally, the liquids obtained from mixed MSW typically contain high water content and complex chemical compounds, requiring advanced treatment prior to disposal or reuse, which compromises overall energy and material recovery efficiency [123]. Although MSW-derived char holds substantial energy potential, contamination from heavy metals and toxic organic pollutants necessitates careful handling and treatment before use [120]. Table 9 summarizes the primary benefits and drawbacks of the pyrolysis process.

To improve process efficiency and power generation capacity, pyrolysis systems necessitate consistent and well-prepared feedstock. Typical feedstocks include MSW, RDF, plastics, biomass, sewage sludge, tires, coal, ASR, and polymer-rich wastes [45,115]. Preprocessing tasks such as sorting, shredding, and drying are vital for removing inorganic materials, reducing particle size below 50 mm, and lowering moisture content to approximately 20%, thus ensuring stable reactor operation and enhanced energy recovery [19]. Sorted plastic waste, in particular, is better suited for oil production than heterogeneous MSW [123,138,139]. The overall efficiency of pyrolysis is comparable to gasification, with both technologies able to treat a wide range of municipal wastes. However, pyrolysis is less technologically mature than traditional incineration and AD methods [140]. Nevertheless, the syngas and pyrolysis oil produced can be combusted to generate thermal energy, steam, and electricity, making pyrolysis a promising option for integrated energy recovery and resource valorization in sustainable waste management systems [19,115,138,139].

Pyrolysis has increasingly been recognized as a versatile WtE technology in Saudi Arabia, capable of converting various waste streams into valuable fuels and materials, aligning with national objectives aimed at energy diversification and environmental protection. At the national level, Nizami et al. [33] conducted an evaluation of different WtE technologies and identified pyrolysis as especially suitable for plastic waste, estimating a theoretical electricity generation potential of approximately 1.03 TWh per year. Table 10 presents a summary of several studies on the pyrolysis of MSW in Saudi Arabia. While these national estimates are promising, they remain largely conceptual, underscoring the need for localized, site-specific investigations.

In urban settings, where waste composition changes seasonally, Rehan et al. [16] investigated the pyrolysis of mixed plastic waste in Makkah. Operating temperatures between 300 °C and 500 °C allowed the process to yield up to 80% liquid fuel with high energy content, indicating significant potential for electricity generation. This study went beyond national estimates by incorporating projections for future waste generation. However, it made assumptions about consistent feedstock quality and uninterrupted plant operation—conditions that may not accurately represent the variability present in urban MSW.

Expanding on this, Nizami [34] framed pyrolysis within the concept of a waste-based biorefinery, assessing aspects beyond energy recovery, including fuel quality, GHG reduction, and the potential for fossil fuel substitution. The resulting pyrolysis oil, characterized by low sulfur content and a high cetane number, supports applications in both power generation and transportation. Despite these advantages, the analysis, like many others, relied significantly on theoretical modeling instead of operational data. Rehan et al. [141] introduced a more integrated approach in Madinah, combining AD for organic waste with pyrolysis for plastics. This highlighted pyrolysis as a targeted solution for high-carbon, non-biodegradable fractions, while also addressing the economic and operational challenges often overlooked in energy-centric studies.

Experimental research has further validated the feasibility of pyrolysis for specific waste types. Dawoud [142] demonstrated the effectiveness of batch pyrolysis for scrap tires, successfully producing fuel oil and carbon black with measurable yields and acceptable quality. However, the constraints of batch-scale operations limit the direct application of these findings to industrial-scale scenarios. Yahya et al. [143] tackled these limitations by examining vacuum pyrolysis of scrap tires within a techno-economic and environmental framework, revealing that reactor design and process optimization significantly enhance fuel yield, economic performance, and payback periods. The adaptability of pyrolysis is also showcased in the processing of biomass residues. Yahya [15] investigated fast pyrolysis of date palm waste, an abundant agricultural by-product, confirming its capability to produce bio-oil, biochar, and syngas with favorable energy and economic outcomes.

Collectively, these studies illustrate a strong consensus on the technical potential of pyrolysis as a flexible WtE technology capable of processing diverse waste streams in Saudi Arabia, consistently delivering energy and economic benefits. However, most research relies on modeled scenarios or small-scale experiments, revealing a critical gap in large-scale, long-term operational data under the heterogeneous and seasonally variable conditions typical of urban areas in the country. Addressing this gap is essential for transitioning pyrolysis from a promising theoretical solution into a reliable and sustainable component of municipal waste management strategies aligned with Saudi Arabia’s Vision 2030.

Pre-treatment is a crucial phase in thermochemical WtE conversion technologies, significantly impacting feedstock quality, energy efficiency, and the stable operation of processes like combustion and pyrolysis [33]. MSW designated for thermochemical conversion typically undergoes several preparatory processes, including mechanical size reduction, material separation, crushing, and drying, prior to energy recovery [29,33]. These processes are vital for heterogeneous waste streams that include plastics, paper, and other combustible materials, as they enhance feedstock uniformity and mitigate operational variability [29]. Pre-treatment effectively lowers moisture content, eliminates non-combustible and inert fractions, and boosts the calorific value of the waste, thereby improving conversion efficiency and minimizing technical challenges during thermal processing. In systems utilizing RDF, pre-treatment is essential for transforming mixed MSW into a standardized, high-calorific fuel that is appropriate for use in power generation facilities and co-combustion applications [29,33].

AD is a well-established biochemical WtE conversion technology widely applied to treat biodegradable waste streams such as the organic fraction of municipal solid waste (OFMSW), food waste, agricultural residues, animal manure, and slaughterhouse waste [19,63,149]. This process involves the biological and chemical degradation of organic waste into biogas and digestate [150,151]. Biogas typically contains 60–80% CH₄ and can be used for electricity generation, heating, or upgraded to biomethane [136,139,152,153]. Biogas, mainly composed of CH₄ and CO₂, can be directly used for heat and electricity generation or upgraded to biomethane for injection into gas grids or transport applications [19,136,150]. The digestate, enriched with carbon and nitrogen compounds, can be reused as an organic fertilizer, thereby closing nutrient loops and supporting circular economy principles [4,55]. The conversion process occurs through a sequence of interdependent biochemical stages, includes hydrolysis, acidogenesis, acetogenesis, methanogenesis, and sulfur reduction, each mediated by specific microbial communities, as illustrated in Figure 5 [19,136,150]. The strong interdependence between these stages explains the sensitivity of AD to operational disturbances and feedstock variability [150,151]. Stable reactor performance therefore requires strict control of operating parameters, including temperature (typically 35–70 °C), pH, moisture content, and hydraulic retention time [19,63].

AD offers several advantages that justify its widespread consideration for organic waste management. As summarized in Table 13, AD achieves substantial landfill diversion while simultaneously recovering renewable energy, with reported biogas methane concentrations ranging between 60% and 80% [136,152,153]. Compared to thermochemical technologies, AD generally results in lower air emissions, reduced odor generation, and lower overall GHG impacts [136,154,155]. The environmental benefits extend beyond energy recovery, as diverting organic waste from landfills significantly reduces leachate formation and uncontrolled methane emissions, which are major contributors to soil, groundwater contamination, and climate change [19,154].

Various types of anaerobic digesters exist, determined by factors such as feedstock properties (wet or dry waste), feedstock loading systems (continuous or batch processes), operating temperature conditions (thermophilic or mesophilic), selected technology (single-stage, two-stage/phase, or multistage processes), and feedstock retention time [4]. The strengths and limitations of AD, including its waste treatment capacity, energy recovery potential, environmental performance, and operational challenges, are encapsulated in Table 13.

However, despite these advantages, AD faces several limitations. The process is sensitive to feedstock variability, operational complexity, and requires skilled labor and efficient emission control systems [63]. It is also inherently slow and affected by variations in feedstock composition and contamination levels, which can impact process stability and CH₄ yield [63,136]. Additionally, AD does not fully degrade all waste components, potentially leading to residual waste streams or digestate containing contaminants that limit market value [136]. The implementation of AD encounters additional constraints related to space availability, feedstock logistics, and odor management. These factors partially explain its lower adoption rate compared to incineration-based WtE systems in densely populated areas [158]. Furthermore, while AD generally incurs lower capital and operational costs than thermal technologies, the initial investment for reactors, preprocessing units, and gas handling systems remains substantial, potentially hindering large-scale deployment [87,136].

Saudi Arabian case studies presented in Table 14 further confirm the technical feasibility and energy recovery potential of AD under local conditions. National-scale assessments indicate that optimized AD of food waste could generate up to 2.99 TWh/year of electricity [33], while city-scale applications in Makkah and Madinah demonstrate electricity outputs of 164.5 GWh/year and 15.64 MW of continuous power, respectively [34,141]. These results are strongly linked to the high and relatively consistent organic content of Saudi MSW, where food waste represents more than 50% of total waste in major cities [33,34] and around 40% in Madinah [141]. Such feedstock consistency provides a structural advantage for AD by enabling predictable biogas yields and stable reactor operation. Energy recovery through AD is consistently reported as reliable under conventional mesophilic conditions, with electricity conversion efficiencies around 35% [34,141]. The effectiveness of AD is tied to the quantity and quality of segregated organic waste, making large-scale adoption sensitive to social and operational variables such as household participation, collection systems, and seasonal fluctuations (e.g., Hajj and Ramadan peaks).

Economic analyses consistently portray AD as competitive relative to other WtE technologies. Nizami et al. [34] estimated low capital costs ($0.1–0.14 per ton) with minimal labor requirements, and Rehan et al. [141] and Abdel Daiem and Said, [10] affirm that net economic benefits can be realized via electricity generation, landfill diversion, and carbon credits. However, this economic optimism must be tempered, the feasibility of AD in practice is contingent on effective pre-treatment, reliable feedstock supply, and supportive policies. Without systemic coordination, cost savings may be overestimated, particularly in urban centers where space constraints and high organic contamination rates may compromise process stability. Life cycle and environmental assessments also consistently show that AD outperforms landfilling and some thermochemical options [34,148,160]. Almansour and Akrami, [148] shows substantial reductions in global warming potential, acidification, eutrophication, and ozone depletion for Riyadh scenarios involving AD, while Nizami et al. [34] confirms maximal reductions in CH₄ emissions and GWP in Makkah compared to pyrolysis and RDF. Lakhouit [160] further quantifies the long-term environmental burden of landfilling in Tabuk, highlighting the mitigation potential achieved through AD-mediated biogas recovery.

Despite these advantages, the AD faces significant technical, spatial, and operational challenges that limit its scalability, particularly in dense urban environments. One of the primary constraints is the strong dependence of AD performance on feedstock quality. Variations in organic content, contamination with heavy metals or plastics, and seasonal fluctuations can disrupt microbial activity, reduce methane yields, and compromise digestate quality [63,136]. These issues are particularly relevant in urban settings where effective source separation is difficult to enforce.

The multi-stage biochemical nature of AD requires continuous monitoring and precise control of temperature, pH, moisture, and retention time [19,63,159]. This complexity translates into a need for skilled labor, advanced monitoring systems, and robust process control, which can increase operational costs and reduce reliability when such expertise is limited [158,159]. Furthermore, AD is inherently slower than thermochemical technologies, making it less responsive during peak waste generation periods such as Hajj and Ramadan, when waste quantities increase sharply [34,141].

Spatial constraints also play a critical role in limiting urban deployment. AD facilities require substantial land for reactors, preprocessing units, digestate storage, and odor control systems [158]. These spatial and logistical requirements partially explain the lower adoption of AD compared to incineration-based WtE systems in densely populated cities [158]. Odor management and community acceptance further complicate siting decisions, especially in residential or mixed-use urban areas [158]. While AD is often described as a low-cost technology, Table 13 and the Saudi case studies in Table 14 indicate that initial capital investment remains significant. Costs associated with reactor construction, feedstock preprocessing, gas handling, and emission control infrastructure can hinder large-scale deployment if not supported by coordinated waste collection systems and enabling policies [87,136]. As a result, the economic feasibility of AD may be overestimated when logistical and spatial constraints are not fully accounted for, particularly in metropolitan contexts.

Recent advances in process optimization provide promising pathways to address some of these limitations. For example, the addition of locally available natural zeolites, such as mordenite, has been shown to enhance CH₄ production by adsorbing inhibitory ammonium ions and stabilizing microbial activity, thereby improving reactor resilience under variable operating conditions [126]. Such locally tailored optimization strategies highlight the importance of adapting AD systems to regional waste characteristics rather than relying on generic reactor designs.

Overall, the Saudi case studies establish AD as a technically feasible, economically attractive, and environmentally beneficial approach to managing organic MSW. However, the success of AD hinges not solely on the reactor technology but on integrated waste management systems, effective source separation, continuous monitoring, and site-specific optimization.

Transesterification is a chemical conversion process that transforms waste oils and fats into biodiesel and glycerol through reactions with alcohols in the presence of catalysts [33,161,162]. During this conversion, organic waste is transformed chemically into liquid biofuels, which can be used for energy generation either in their pure form or blended with conventional diesel [161,162]. Biodiesel produced via this route is sulfur-free, biodegradable, and can be used directly or blended with conventional diesel, reducing dependence on fossil fuels and lowering GHG emissions [163,164], as shown in Figure 6.

The benefits and challenges associated with transesterification, such as energy recovery, waste reduction, environmental advantages, and operational limitations, are discussed in detail in Table 15.

Recent Studies have highlighted the tangible potential of transesterification as a chemical WtE technology in Saudi Arabia. Nizami et al. [33] observed that MSW in the Kingdom contains a substantial proportion of fats and oils, notably from food and restaurant waste, which can be effectively valorized through transesterification. This process not only recovers energy from the lipid waste fraction but also aids in reducing CH₄ emissions, diverting waste from landfills, and reinforcing circular economy initiatives [33]. In Makkah, transesterification converted approximately 64,000 t/year of lipid-rich food and slaughter waste into 62.5 kt/year of biodiesel and 6.38 kt/year of glycerol, achieving an impressive 98% conversion efficiency and generating 244.2 GWh of electricity, leading to a net economic gain of 76.5 million SAR per year [34].

Further research conducted in Jeddah has underscored the effectiveness of natural zeolites as catalysts in enhancing transesterification performance, demonstrating that their thermal stability, porosity, and surface area significantly boost biodiesel production from used oils and animal fats [126]. Additionally, Yaqoob and Ali [163] illustrated the application of biodiesel sourced from waste chicken fat in high-pressure diesel engines. Their findings indicated that a 10% biodiesel blend (DB10) increased torque and brake power while simultaneously reducing emissions of CO, CO₂, and hydrocarbons (HC), thereby affirming the practicality of transesterification as a viable Chemical WtE strategy that yields renewable fuel, supports circular economy practices, and does not necessitate modifications to existing engines. An overview of the implementation of transesterification for MSW management in Saudi Arabia is presented in Table 16.

Transesterification contributes to landfill diversion of lipid-rich waste, reduces methane emissions, and supports circular economy initiatives by converting waste into valuable fuels and by-products [34,166]. Experimental engine studies further confirm the practical usability of biodiesel blends without requiring engine modification, while achieving reductions in CO, CO₂, and HC emissions [163].

Despite its advantages, transesterification faces distinct logistical and operational limitations that restrict its broader application. The technology is highly dependent on the availability, purity, and consistent collection of lipid-rich waste streams [33,34,165]. Unlike AD, which can process mixed organic waste, transesterification is limited to fats and oils, making feedstock collection and segregation a critical necessary. Urban implementation requires well-established collection networks for used cooking oils from households, restaurants, and food processing facilities [165]. In the absence of organized collection systems, feedstock losses and contamination can significantly reduce conversion efficiency and economic viability. Additionally, the process is sensitive to water content and impurities, which can impair reaction efficiency and increase downstream treatment requirements [33,34,165].

Transesterification requires precise control of reaction parameters, including temperature, alcohol-to-oil ratio, and catalyst dosage, increasing system complexity and skilled labor demand [33,165]. Capital costs associated with reactors and downstream purification units remain relatively high, particularly when compared to biological treatment options [33,165]. Collectively, these studies demonstrate that transesterification serves as a crucial chemical WtE pathway for enhancing the lipid fraction of MSW in urban Saudi Arabia. It complements other WtE technologies conversion for recyclables, thereby facilitating a comprehensive waste-based biorefinery approach. This synergy reveals the dual benefits of transesterification.

The development of WtE facilities in Saudi Arabia primarily emphasizes the conversion of waste into usable fuel rather than electricity generation. Notably, the Saudi Investment Recycling Company (SIRC) operates facilities that process approximately 3 million tons of MSW annually, focusing on high-energy waste streams, particularly plastics, which constitute about 23% of the total MSW. SIRC successfully converts 35% of this waste into RDF while recycling additional fractions [170,171]. This operation alleviates landfill pressure, substitutes conventional fuels, and aligns with the circular economy goals outlined in Vision 2030 [172]. While electricity generation capabilities are currently limited, several thermal WtE plants are in planning stages to directly generate electricity from MSW. For instance, the proposed Jeddah WtE Plant aims to process approximately 3500 tons of waste daily, with an expected output of around 100 MW of electricity [173]. This initiative is designed to minimize landfill usage and reduce associated GHG emissions while enhancing the city’s electricity supply [173].

Similarly, the Riyadh WtE Plant is anticipated to manage about 1.3 million tons of MSW each year, generating roughly 200 MW of electricity for the capital area. Both projects will be implemented under public–private partnerships (PPP) and will utilize advanced thermal WtE technologies to optimize energy recovery, aligning with the Kingdom’s goals for energy diversification and sustainability [173]. In line with these efforts, the Saudi government has initiated the King Abdullah City of Atomic and Renewable Energy (KACARE) program, aimed at establishing renewable energy sources through science, research, and industry [33]. The plans include generating 54 GW of energy from various renewable sources, including nuclear, wind, solar, and WtE facilities [33]. Table 18 provides an overview of the major operational and planned WtE projects in the Kingdom.

WtE projects in Saudi Arabia offer numerous advantages. Environmentally, they help reduce reliance on landfills and lower CH₄ emissions, thereby enhancing air quality and promoting better public health outcomes [170,172]. In terms of energy, thermal WtE plants and the production of RDF contribute additional sources of electricity and fuel, aiding in the diversification of the Kingdom’s energy mix [171]. Economically, these projects are expected to draw both domestic and international investments, while also creating job opportunities in waste collection, plant operation, and maintenance sectors [170,171].

Despite their benefits, WtE projects face several challenges in Saudi Arabia. The diverse composition of MSW, characterized by high organic content and fluctuating calorific values, can hinder energy recovery efficiency [170,171]. Additionally, the high capital and operational costs present financial hurdles, necessitating the adaptation of advanced WtE technologies to local waste characteristics and climate conditions for optimal effectiveness [171]. Furthermore, a robust regulatory framework, including feed-in tariffs and gate fees, is essential to ensure the economic feasibility of these projects.