A global review of factors influencing decision-making on landfill mining and composition of mined residues

Motivations for LFM

Although the cardinal motivation for LFM is to remediate LeLas, several other motivations can play a role with regard to decision-making. In addition, these motivations change with the region, socio-economic fabric, past waste management practices, and time of actual mining/research. For instance, in land-scarce countries like Singapore, the dominant motivations include extending landfill lifespan through the creation of airspace (CA) and recovering resources such as incineration bottom ash for land reclamation, thereby reducing reliance on imported aggregates (Hu et al., 2025; Zhang et al., 2024). Similarly, in Europe, before the existence of the EU landfill directive, 23% of the studies did not mention any specific motivation, whereas another 30% of LFM studies were motivated by CA for operational benefits and landfill rehabilitation (Ortner et al., 2014). This also coincides with the primary motivations, such as CA and landfill rehabilitation of German-specific LFM projects carried out in the 1980s and 1990s (Kruger et al., 2016). However, post-1999, due to the imposition of the EU landfill directive, the focus has shifted towards addressing the concerns about environmental pollution as a result of LeLas (Ortner et al., 2014).

The above discussion emphasises that ‘time of research and/or time of policy’ is a key factor that decides the motivation for research and decision-making on LFM. As many research and field-scale studies are carried out after previous reviews (Calderón Márquez et al., 2019; Krook et al., 2012; Ortner et al., 2014), the key motivations could have differed from the past ones. In addition, we conducted a review of studies that reported motivations only concerning decision-making. For this purpose, we ignored studies that did not report any motivation or reported motivations (such as conducting a field-scale study, motivations for conducting narrow studies, such as the characterisation of LMRs) that do not impact decision-making. The remaining studies reported diverse motivations that have similar effects on the decision-making process and are grouped into a few categories (see Supplemental Table S4): environmental remediation (ER), resource recovery (RR), CA, recovery of land (RL), relocation of waste (RW), to carry out standard landfill operations (SLOs), and miscellaneous (Rest). Motivations of 102 distinct LFM projects/studies (see Supplemental Table S5) are collected using the screening procedure depicted in Supplemental Figure S1.

LFM is often an endeavour with multiple motivations. To reflect this in terms of the relevance of each motivation in decision-making, the relative percentage for occurrence of each motivation is calculated in view of total number of motivations. Accordingly, the relative occurrence of each motivation is determined using motivation-level counts rather than project-level counts. To identify the key motivations, the percentage occurrence of each motivation was first calculated and then ranked from highest to lowest. Motivations were cumulatively added until they account for at least 80% of all reported motivations. The minimum individual contribution required for a motivation to appear within this cumulative 80% was then taken as the threshold for defining a ‘key motivation’. Using this approach, the least percentage contribution needed for a motivation to be included within the 80% cumulative share was found to be 10%. This value emerged consistently across all three scenarios (i.e. worldwide, developed countries, developing countries), indicating that 10% is the data-driven cut-off. This is the case for ER, RR, CA, and RL in the present analysis, when worldwide LFM studies are considered (see Figure 3). Together, these four motivations accounted for 83% of total motivations, with ER and RR contributing to ≈60% of total occurrences. To understand the nature (i.e. specific/general) of these motivations, ER and RR are sub-classified into general environmental remediation (ER-General), protection of water bodies (ER-W), and reduction in gaseous emissions (ER-G), and general resource recovery (RR-General), utilisation of landfill-mined-soil-like-fractions (RR-L), and utilisation of combustibles (RR-C), respectively. The categories ER-General and RR-General may encompass ER-W and ER-G, and RR-L and RR-C, respectively, along with other motivations. Within ER and RR, nearly 60% and 45%, respectively, of occurrences are related to general aspects (see Figure 3(d) and 3(e)). The occurrence of specific motivations is dominated by ER-W (35%) and RR-C (32%) within ER and RR, respectively. The occurrence of ER-W is widely popular with LFM projects in Europe, as the EU landfill directive (1999/31/EC) requires protecting ground and surface water bodies from contamination by LeLas. As it is practically impossible to completely avoid LeLas leachate from entering the subsurface and water bodies, such cases are motivated to go for LFM. The RR-C acted as motivation for LMRs, such as plastics, textiles, wood, and rubber, which contain high calorific values and are capable of replacing conventional fuels, such as coal, in cement kilns and feedstock in WtE plants. In addition, utilisation of combustibles, due to their low density, provides an opportunity to create a huge airspace to extend the landfill operating life span.

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Figure 3.

Percentage share of occurrence –for motivations reported by LFM project/studies conducted (a) worldwide, (b) in developed countries, (c) in developing countries, –for sub-classification motivations related to (d) environmental remediation, a€(e) resource recovery.

The underlying motivations differ based on the socio-economic conditions. Per capita income, GDP growth, and population growth are a reflection of socio-economic conditions, as evident from previous studies on MSW generation and management (Nguyen et al., 2020). Hence, an effort has been made to differentiate the key motivations for LFM in developed and developing countries. The key motivations for LFM in developed countries are ER, RR, CA, RL, and RW (see Figure 3(b)). In developing countries, the key motivations are identified as ER, RR, and RL (see Figure 3(c)). Overall, the environmental protection aspects (i.e. ER) are driving LFM in developed and developing countries, with a relative occurrence of 32% and 34%, respectively. Within ER, ER-General occurrence as motivation for LFM in developed countries (≈47%) is much lower compared to developing countries (i.e. 100%; see Figure 3(d)). Higher occurrence of ER-General in developing countries is a result of adverse impacts from existing dumpsites on all abiotic systems (i.e. soil, water, and air), which justifies not having any specific system as a target for ER. In other words, the overarching motivation is the remediation of all of the environmental media – soil, water, and air. RR-related motivations have occurred 24% and 31%, respectively, in developed and developing countries. Despite having more adverse landfilling conditions, decision-making on LFM in developing countries is relatively highly motivated by RR aspects and might be due to the high demand for alternative secondary resources in energy and infrastructure development sectors. For instance, in a country like India, using combustible waste as refuse-derived fuel (RDF) and converting landfill-mined-soil-like-fractions (LFMSFs) into compost, provided they comply with regulatory standards, can significantly reduce reliance on imported fossil fuels and chemical fertilisers (TET, 2024; USEIA, 2014). A relatively lower occurrence of RR for developed countries is due to less or negative environmental benefits from the substitution of combustibles as a result of less fossil-dependent background heat and electricity systems (Laner et al., 2016). Within RR, the relative occurrence of RR-L in developed countries is high due to the case-specific nature of those studies (see Figure 3(e)). For instance, LFM at Shawano County and Clovis LeLas in the United States was motivated by RW and RR-L, wherein the recovered LFMSF has been used in berm and road construction, and daily and intermediate cover construction in lined cells (Dhar, 2015). Similarly, LeLas in Hauge of New York State (the United States) and Tel-Aviv (Israel) were mined for RR-L as compost in 1994 and 1953, respectively (McCaffrey, 2010; Ortner et al., 2014). Later studies in developed countries did not report the occurrence of RR-L for compost as motivation, due to the stringent compost quality standards for microplastics and persistent organic pollutants, wherein most LFMSF samples might not qualify. When RR is not possible due to the inferior quality of recovered materials, even if LFM is compelled by policy (such as shifting of waste from unlined to lined cells), then RW becomes a motivation, as is the case for developed countries (see Figure 3(b)). The occurrence of RW and SLO as motivations is limited to developed countries due to the strict policy implementation and well-developed waste management and landfill monitoring systems (RAWFILL, 2021). LFM studies in developing countries are highly motivated (≈29%) by RL compared with developed countries (≈12%) as most LeLas in developing countries (i) occupy limited land available with urban local bodies for waste processing, treatment, and disposal (Pushkara, 2021), and (ii) are near densely populated areas wherein infrastructure development is key for accommodating more citizens (Pinto, 2025). Based on the relative occurrence of motivations, the priority-wise key motivations, including sub-classes, for LFM in developed and developing countries are proposed (see Supplemental Table S6). This differentiation underscores the distinct motivations guiding LFM practices in developed versus developing countries.

Environmental impacts

LFM should focus on assessing its environmental impacts. Several indicators are used to assess the environmental impacts as part of life cycle assessment (LCA) studies. Among these, the global warming potential (GWP) is a key consideration. During LFM, activities such as levelling, excavation, transportation, drying, sorting, composting, recycling, and incineration will have positive CO₂ emissions (Frändegård et al., 2013). The key benefits are from avoided emissions due to (i) replacement of fossil fuels, raw materials, and metals, which are otherwise mined and processed, (ii) recovered heat and electricity, and (iii) future emissions, which are associated with LeLa in case it is not mined. The complete valorisation of each tonne of LMRs through composting, WtE, and metal and glass recycling would provide ≈50 kg CO₂e of benefits to GWP as compared to composting and sanitary landfilling of LFMSF and remaining fractions, respectively (Cheela et al., 2023). In addition, up to 40% of future GHG emissions can be mitigated by WtE of combustibles, with a higher fraction of textile and paper, due to their ability to produce methane in the long term when placed in landfills (Liu et al., 2024). The savings from fossil fuel replacements and avoided emissions could potentially turn the entire LFM activity into a net saving of CO₂e compared to not mining the landfill (Cheela et al., 2023; Laner et al., 2016).

Trends associated with other impact categories of LCA can widely vary and are often guided by scope, background, and foreground processes considered during the study. For instance, the impacts associated with photochemical oxidation are higher for LFM scenarios compared to the reference scenario (Cheela et al., 2023). The post-mining sorting operations, such as profiling, windrow pile formation, and excavation, release non-methane volatile organic compounds, carbon monoxide, and nitrogen oxides (Balaban et al., 2023; Nordahl et al., 2023), which are ozone precursors and negatively impact (i.e. positive emissions) photochemical oxidation. The emissions from these operations were typically not taken into consideration by LCA-LFM studies in the past.

The contribution of each operation or process to different impact categories of LCA will depend on its range of parameters. The transportation distance of waste from (i) the LFM site to the sorting facility, and (ii) sorting facility to respective LFMSF and RDF valorisation facilities, or metal and glass recycling facilities, is one such parameter. In literature, these distances are considered between 1 and 500 km based on case-specific information and the distance anticipated between different operating and final utilisation facilities (Cappucci et al., 2020; Frändegård et al., 2013; Jain et al., 2014). An LFM study conducted in Sri Lanka showed that transportation processes (with an average distance of 150 km) could relatively contribute to >50% of total impacts towards ozone depletion, human toxicity, terrestrial ecotoxicity, and marine ecotoxicity (Maheshi et al., 2015). Given the importance of reducing environmental impacts, it would be better to develop a relationship between transportation distance and different impact categories to establish environmentally favourable distances. Other parameters that would impact the environmental benefits of LFM are the heating value of RDF and the composition of LMRs, especially recyclables such as metals and glass (Laner et al., 2016). The higher the heating value of RDF and the content of recyclables, the greater the replacement of conventional fuels and virgin raw materials, respectively, which otherwise need to be acquired through mining and processing. Mining and processing of natural resources are highly energy-intensive and have a high impact on acidification, GWP, and ecotoxicity (Jain et al., 2014). Hence, the effective valorisation of LMRs will enhance environmental performance and increase the tendency for LFM.

The management strategy of LFMSF, often the largest fraction of LMRs by mass, is key to the environmental impacts of LFM, which is guided by regional waste management policies. For instance, an LCA-LFM study of India considered the utilisation of LFMSF as compost (Cheela et al., 2023), unlike the European case study, which considered their re-landfilling with the recovery of heat and electricity from LFG (Laner et al., 2016) in accordance with respective policy frameworks. However, the impacts associated with these two management pathways are guided by different characteristics of LFMSF. For instance, benefits from LFMSF use as compost can be enhanced when macronutrients (Nitrogen, Phosphorous, Potassium) are available in substantial quantities. Similarly, re-landfilling with heat and electricity substitution from LFG yields higher benefits for LFMSF with a considerable amount of biodegradable organic carbon. Moreover, with recent policy changes in Germany regarding restrictions on landfilling of untreated waste, LFMSF management would become case-specific. When the methane generation potential or calorific value of LFMSF is above 20 L/kg in 21 days or 6000 kJ/kg, respectively, an additional stage of biological or thermal treatment is needed before landfilling (DepV, 2009). Therefore, the impact of LFMSF characteristics should be considered for optimising environmental benefits to make informed decisions on LFM and LFMSF utilisation strategy.

Overall, LFM will have more environmental benefits when applied to dumpsites, as fugitive emissions, such as methane, that are directly emitted to the atmosphere can be avoided. The LFM of dumpsites and subsequent utilisation of combustibles in WtE with heat and electricity substitution or in cement kilns as a substitute for coal are beneficial to GWP compared to the reference scenario. Fugitive emissions from a dumpsite account for 500–1600 kg CO₂e per tonne of LMRs, whereas the utilisation of combustibles alone could result in net negative GWP, despite releasing considerable fossil CO₂ during incineration in cement kilns and WtE plants (Maheshi et al., 2015). During composting and subsequent land application of LFMSF, the GWP of fugitive emissions is negligible compared to emissions from the dumpsite (Cheela et al., 2023) as a result of aerobic decomposition. Nevertheless, the impact of organic matter content in LFMSF on fugitive emissions during its utilisation in compost and land applications must be established separately through detailed investigations. The environmental benefits from land recovery and its utilisation for future waste processing activities, such as material recovery and landfilling, are yet to be understood. Particularly, in large cities where it is hard, if not impossible, to acquire land for new waste processing and disposal facilities at environmentally and economically feasible distances, LFM would be beneficial. Adding avoided emissions from future transportation to the environmental benefits of recovered land would make decision-making on LFM more rewarding. However, a thorough understanding of future utilisation pathways for recovered land is required to propose a reliable emission savings estimate.

Most LFM studies conducted in developed countries, particularly in Europe, have not demonstrated clear environmental benefits largely due to relatively lower gains from (i) avoided future emissions and (ii) substitution of heat, energy, and raw materials. In other words, the presence of scientific landfills (i.e. gas and leachate management systems), along with a less fossil-intensive heat and electricity mixes and material production technologies, reduces the potential environmental advantages of LFM in these regions. On the contrary, developing countries often have inadequately managed landfill sites and rely more heavily on fossil-based heat, electricity, and material production systems. These conditions increase the potential environmental benefits of LFM. Importantly, developing countries can learn from the European experience, as LFM tends to be more beneficial when it is adopted early, before the transition to scientifically engineered landfills and cleaner material production and renewable energy systems. This insight is crucial for shaping a strategic LFM policy framework for developing countries, ensuring that the process remains environmentally sustainable. Moreover, the experience gained from European studies can also support other developing countries in Asia, such as Singapore, in planning their LFM strategies more effectively.

Economic impacts

Economic impact studies involve the estimation of monetary benefits and burdens from LFM by considering the different processes involved in it. Similar to LCA, the scope and boundary conditions of the study and processes considered would influence outcomes to a great extent. Most studies (Frändegård et al., 2013; Kieckhäfer et al., 2017; Zhou et al., 2015a) on the economic performance of LFM considered a holistic view, which includes both excavation, sorting and utilisation aspects under different scenarios to compare the net present value. One of the general conclusions is that the integration of LMRs valorisation with excavation and sorting will improve the economic performance of LFM. A comprehensive investigation carried out by Laner et al. (2019) on the influence of 12 different factors belonging to site-, project-, and system- levels revealed that only ≈19% (99,821 out of 531,441) scenarios are profitable in the European context. These findings are also supported by conclusions from most case studies (Danthurebandara et al., 2015; Kieckhäfer et al., 2017; Wolfsberger et al., 2016) carried out in Europe, which revealed that LFM is hardly profitable.

Contrary to this, a case study on LFM of 0.5 million metric tonnes of waste present in Yingchun landfill, China, revealed that the entire project costs (6.36 million USD) were ≈45% (of 13.99 million USD) of monetary benefits from valorisation of LMRs (Zhou et al., 2015a). Higher benefits are due to the (i) lower operational costs for incineration (24.2 USD/tonne) compared to studies (Danthurebandara et al., 2015; Laner et al., 2019) conducted in Europe, which considered incineration gate fees or investment and operation costs between 70 and 140 USD/tonne, and (ii) ≈10 fold higher revenues from recovered electricity (0.54 USD/kWh) compared to studies (Danthurebandara et al., 2015; Laner et al., 2019) conducted in Europe, wherein the average benefit from recovered electricity was between 0.05 and 0.08 USD/kWh. Apart from these benefits, the avoided landfill management (aftercare) costs and revenues from recovered land utilisation will make LFM economically attractive to the operators and help in making decisions in favour of LFM. Another study conducted in Sri Lanka suggested that LFM is not economically beneficial (Maheshi et al., 2015), which is due to consideration of high investment and operation costs (together 117 USD/tonne) for RDF incineration and low net electricity efficiency (22%). In this study, investment and operation costs are bound to be overestimated, as the data is taken from Ducharme (2010), which was carried out for the United States. Hence, relevant data considerations are necessary to understand the economic impacts of LFM to help with decision-making.

Major processes contributing to the costs are waste treatment, re-landfilling, excavation, and sorting. Activities such as avoiding landfill aftercare costs, material sales, and airspace recovery are major drivers of revenue generation (Winterstetter et al., 2015). Considering multiple revenue pathways beyond traditional sources, such as material sales and land recovery, is key to turning LFM economically profitable. Extending existing sources of revenues to avoided costs from (i) damage to the climate and soil from LeLas, (ii) transportation of future fresh waste to another landfill site, and (iii) avoided soil and groundwater remediation costs due to LFM is worth considering. It would be practically relevant to compute the monetary values of avoided costs from damages to climate change using per unit cost of carbon credits. Most economic performance studies are limited to the EU, which represents the context of developed countries. Though a few studies have been carried out in the context of developing countries (Maheshi et al., 2015; Sabour et al., 2020; Zhou et al., 2015a), the factors considered were limited due to the case-specific nature of these studies, unlike EU cases, which are more generic and considered a broader range of parameters (Esguerra et al., 2021; Laner et al., 2019). Extending these frameworks to developing countries is necessary to establish a comprehensive understanding of the factors guiding the economy of LFM to help with decision-making.

Usability of LMRs

The usability of LMRs is crucial for environmental (see ‘Environmental impacts’ section) and economic sustainability (see ‘Economic impacts’ section), for decision-making on LFM. The schemes of LMRs utilisation are guided by existing legislation and guidelines. In the EU, the valorisation of LFMSF as construction aggregates would be possible only when they meet the criteria laid out by local governments. For instance, in Portugal, waste compliant with NP EN 13242:2002+A1:2010 can be used as aggregates for civil engineering work and road construction (Blasenbauer et al., 2020). Though this would protect the environment, the operators will be compelled to make favourable decisions to LFM on LeLas with LFMSF that pass regulatory norms for heavy metals, salts, and organics. In addition, practitioners may select the locations where the local governments impose less stringent selection and testing criteria (Esguerra et al., 2021). The Central Pollution Control Board (CPCB) of India suggests cleaning of lightweight fractions before handing over to incineration units or cement kilns for valorisation as RDF (CPCB, 2019). This would incur additional capital and maintenance costs, which are not lucrative for the operators. Thus, interest in marketing RDF can recede quickly.

The valorisation of LMRs without adhering to environmental norms can have short-term economic benefits but create long-term burdens if LMRs become a secondary source of contamination. Particularly, LFMSF, due to its inherent high sorption and cation exchange capacity (Goli et al., 2022a), has become a sink for pollutants such as heavy metals, polychlorinated biphenyls, benzocynides, per- and polyfluoroalkyl substances, polybrominated diphenyl ethers, and microplastics (Goli and Singh, 2023a, 2023b; Goli et al., 2024; Hölzle, 2019; O’Kelly et al., 2021, 2024; Somani et al., 2020; Tolaymat et al., 2023; Zhang et al., 2021). Hence, its ex-situ utilisation in compost and geotechnical fill applications, unless proven safe against long-term leaching and against causing adverse impacts by microplastics and persistent pollutants, is not recommended. Such investigations should be carried out even on stabilised LFMSF matrices, which are established as safe against the leaching of contaminants in the short-term, as per existing guidelines (Qin et al., 2023). The leaching of toxic additives and the release of microplastics is a proven threat to the aquatic environment and soil biota (Ding et al., 2022; Yu et al., 2024). Prima facie, LFMSF should not be an exception to having such harmful impacts on the ecosystems.

Additionally, it is essential to emphasise the importance of studying the engineering properties of LMRs, as such research directly impacts their potential reuse applications and economic value. In particular, long-term studies on organic-rich LMRs such as LFMSF are essential, given their susceptibility to further biodegradation under varying environmental conditions. Extending the reuse scenarios of LMRs through engineering evaluation not only enhances their economic viability but also reduces the need for natural resources, promoting sustainable waste management practices. Such studies should be conducted systematically to ensure the safe and effective reuse of LMRs in diverse geotechnical and engineering applications. For instance, testing properties like the California Bearing Ratio and unconfined compressive strength can evaluate LMRs’ suitability for road construction, whereas simple shear tests, triaxial tests, and permeability measurements can validate their performance and value for land reclamation or embankment construction (Zhang et al., 2023). However, most LFMSF samples are rich in organic matter (Bokade and Singh, 2025), which undergoes significant compression and degradation during testing, making it necessary to modify conventional testing tools before their geotechnical properties can be reliably evaluated.

Availability of alternative remediation strategies

Apart from LFM, LeLas can be remediated using techniques such as landfill closure and containment, pump-and-treat of contaminated leachate plumes, microbial methane oxidation, among others (Ezyske and Deng, 2012; Somani, 2025). Although each technique has its pros and cons, in practical situations, the decision-making on the adoption of a particular technique will be based on environmental and economic benefits, coupled with policy intervention. Though in an ideal situation, LFM will act as a final remedy and have the most environmental benefits, not all LeLa conditions are suitable for LFM. So, alternatives available for the remediation of a particular LeLa can support or oppose the decision-making on LFM. This section illustrates these aspects with the help of two field case studies conducted in India and Estonia. The selected case studies are based on the availability of data from field-scale implementations and the presence of distinct waste management setups.

Gorai dumpsite is located in the western Suburbs of Mumbai in India, where mixed solid waste was disposed of on marshy land between 1972 and 2008 (Bhardwaj and Inocentes, 2011). The dumpsite was spread over an area of 19.6 ha with an average height of ≈15.5 m (Rodic and Gupta, 2012). The leachate from dumpsite is released into Gorai Creek, which drains into the Arabian Sea. The LFG is emitted into the surrounding atmosphere, contributing to global warming. Although the first research study on LFM in India was conducted back in 2003 (Kurian et al., 2003), the city of Mumbai did not have sufficient infrastructure for waste processing by then, which is essential for LMRs management, and the LFM technology was not sufficiently mature (until the demonstration at the Kumbakonam site in 2016). In addition, promotion of dumpsite closure projects under the Clean Development Mechanism (CDM), a programme that aims to fund emissions reduction projects in developing countries to earn Certified Emission Reduction (CER) credits under the Kyoto Protocol (UNFCCC, 2006), has benefited the decision-making for Gorai dumpsite closure and the capture of LFG for electricity generation. At the time of project planning, it was estimated that between 2009 and 2019, the Gorai dumpsite closure could reduce 1,240,289 tonnes of CO₂e emission (CDM-PDD, 2009). The tradability of CERs to developed countries to meet their emission reduction targets made the closure project economically viable with an internal rate of return of 19.5%, which is higher compared to the benchmark return of 11.5% set by the Reserve Bank of India (CDM-PDD, 2009). As LeLas closure projects were common in Europe and the United States by then, technology readiness was not in question. International project partner (Vander Weil, Strotgas BV, The Netherlands) agreed to share and transfer technology (Rajeev, 2010), which helped in attracting funds from the Asian Development Bank (CDM-PDD, 2009). In this case, apart from the promotion of dumpsite closure projects under CDM, the lack of sufficient infrastructure for LMRs processing acted as a barrier for decision-making in favour of LFM.

Kudjape dumpsite is located on the island of Saaremaa in Estonia, which was dumped with mixed waste between 1970 and 2009 (Bhatnagar et al., 2017). The dumpsite is spread over an area of 4.2 ha with an average height of 12 m. In 2009, as per the National policy to close non-EU-compliant landfills (European Parliament, 2017), the Kudjape dumpsite was proposed as a closure project. The dumpsite closure needs application of a 1.5 m thick final cover composed of clay or any other low-permeable material, which was not available locally (Pehme and Kriipsalu, 2018). As the dumpsite poses low risk, an alternative methane oxidation cover in place of a conventional cover has been proposed. The material required for the methane oxidation cover is extracted from the partially decomposed fine fraction (i.e. LFMSF) obtained through LFM (Pehme et al., 2020). Lack of clay or fine soil required for dumpsite remediation through closure (i.e. alternative strategy) benefited decision-making in favour of LFM at Kudjape dumpsite.

Site-specific factors

Age

The age of LMRs, which indicates the time provided for their degradation, collected from LeLas varied between a few months and as long as 40 years. The weight percentage distribution of individual LMR fractions with change in age is depicted in Figure 5. The percentage of LFMSF varied over a wide range (between 9% and 91%) and increased exponentially up to 15 years (see Figure 5(a)). A similar but opposing trend can be observed for weight percentages of plastics, textiles, P&C, glass, and miscellaneous fractions of LMRs (see trendlines of Figure 5(b)–(d) and (i)). No significant difference in weight percentage of metals, wood, and inorganic mineral fractions with age can be noticed (see Figure 5(e)–(h)) as they neither undergo recognisable degradation nor their concentrations in LeLas are high enough to measure accurately during sorting operations. The lack of significant differences in percentage of these fractions is also established through p-values (<0.05 for all fractions) measurement (see Supplemental Table S8).

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Figure 5.

Weight percentage distribution of LMRs with age.

To understand the quantitative variation in the composition of LMRs with age (in years), the LeLas are grouped under four different categories: young (0 < age ⩽ 5), medium (5 < age ⩽ 10), old (10 < age ⩽ 20), and mature (20 < age ⩽ 40; see Figure 6). This classification is based on the general understanding of MSW degradation in landfills (Eleazer et al., 1997). Although the degradation rate depends on the operation type and climatic conditions of LeLas, older LMRs are generally more degraded. In line with this hypothesis, the percentage of LFMSF in LMRs increased from ≈42 in young LeLas to ≈61 in mature LeLas (see Table 1). An increase in the percentage of LFMSF in mature LeLas is due to the (i) downsizing of organic matter during biodegradation (Hossain et al., 2009), and (ii) degradation of medium and hardly biodegradable fractions such as P&C, wood, and textiles (Reddy et al., 2009). Although to a lesser extent, the LFMSF content may also be enhanced by (i) the disintegration of C&DW, stones and glass into smaller particles due to mechanical loads and chemical reactions with leachate (Miller and Maher, 1972), and (ii) the precipitation of calcite from microbial activities (Fleming et al., 1999). Studies on the percentage contribution of each of these factors to LFMSF will provide a better idea of dominant factors and the extent of influence on their physicochemical and engineering characteristics. For instance, calcite originating from C&DW (non-biogenic) and microbial precipitation (biogenic) will have a different microstructure, which can influence their engineering properties (Manning, 2001). So, their relative percentages in LFMSF will, in turn, influence the engineering properties during geotechnical fill application. The contribution of microbial calcite precipitation, which indicates waste degradation and stabilisation, to LFMSF will depend on the age of the LeLa (Stolarski and Mazur, 2005). The reduction in individual percentages of textiles, wood, and P&C, which can degrade up to 24%, 10%, and 60%, respectively (Eleazer et al., 1997; Moazzem et al., 2021; Ximenes et al., 2018), with time (see Figure 5(c), (d), and (h)) is an evidence of their biodegradation in landfills. The rate of increase in LFMSF percentage is higher between young and medium age LeLa groups, which is in line with the peak biogas production (Mebarki et al., 2015) and indicates the dependency of LFMSF’s abundance on waste biodegradation.

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Figure 6.

Average composition of LMRs in (a) young, (b) medium, (c) old, and (d) mature LeLas.

The percentage of plastic waste which is supposed to be non-biodegradable reduced with an increase in age (see Figure 5(b)). This can be attributed to lesser consumption of plastic in past decades and hence, less contribution to MSW (Geyer et al., 2017). Another factor contributing to a lower percentage of plastic waste in older LeLas is their fragmentation under mechanical and hydrolytic stresses to form microplastics, which will either exit with leachate or mix with LFMSF (Fei et al., 2022; Goli and Singh, 2023b; Kabir et al., 2023). In this context, the simulated landfill experiments conducted by Lu et al. (2023) and Wang et al. (2021) revealed that mass loss in low-density polyethylene (LDPE) and expanded polystyrene (EPS) plastics can be up to 45  μ g/g under a load of 500 kPa (stress induced by ≈5 m high landfill) within the first 300 days. Although mass loss values appear to be smaller, these findings suggested that degradation in plastics follows an exponential growth trajectory with time and thus aggravates the mass loss. So, the contribution of plastics’ degradation in LeLas to the observed relationship between plastics percentage and age needs to be investigated. On the contrary, Fei et al. (2023) opined that steady degradation of easily biodegradables leads to an increase in plastic waste content in LMRs with age. This could be attributed to the shorter age of plastic waste considered for grouping the samples (two groups with <1 and >1 year) by Fei et al. (2023), wherein significant biodegradation of food waste and P&C occurs in landfills, whereas the changes in plastics content in disposed waste are negligible. Overall, the data suggest that the combustibles in LMRs decrease with an increase in the age of LeLa.

Depth

Most LMRs, except for glass, showed no clear trend in their weight percentages with a continuous change in the depth of sampling (see Supplemental Figure S2). The lesser glass consumption in the past resulted in less glass in MSW (Gallucci et al., 2021; Westbroek et al., 2021) at greater depths, as older waste is disposed of first in LeLas or a particular cell (Singh and Chandel, 2020). However, studies conducted in Brazil (Guimaraes et al., 2023), Thailand (Chiemchaisri et al., 2010; Prechthai et al., 2008), India (Kurian et al., 2003), and China (Zhou et al., 2015b) do not follow this trend due to lower glass waste in MSW of these countries compared to their counterparts in Europe and NA (UNEP, 2024). To further understand the quantitative variation in the LMRs, the samples are grouped into five different categories: (i) 0 < depth ⩽ 3 m (surface samples which can be collected by excavators through open pits), (ii) 3 < depth ⩽ 6 m (shallow depth samples which can be collected by long boom excavators through open pits), (iii) 6 < depth ⩽ 10 m (intermittent samples which manual augers can collect through borehole digging), (iv) 10 < depth ⩽ 15 m (deeper samples which can be collected by machine augers through borehole digging), and (v) 15 < depth ⩽ 25 m (terminating depth samples which can be collected by machine augers through borehole digging with protection against borehole collapse). The groups are chosen based on the instrument in use, which is an operational factor that often decides the sampling depth during feasibility studies.

When analysing the average composition of LMRs, the mean depth was used as a proxy for the overall sampling depth range, which is then grouped into depth-based groups mentioned earlier. Studies that reported very broad sampling depth ranges (greater than 5 m) were excluded from the depth-wise composition analysis to avoid introducing excessive variability (see Figure 7). In the depth-wise assessment of LMR composition, several fractions including LFMSF, inorganic mineral fractions, and miscellaneous materials, have shown a clear trend with increasing depth. The average percentage of LFMSF in LMRs increased from ≈43 to ≈72 with a change in sampling depth from the surface to the termination level (see Figure 7). In LeLas, wherein the waste has been placed in horizontal layers, this trend can be primarily attributed to the longer degradation time provided for deeper samples. Apart from longer degradation time, factors such as (i) the formation of fine particles due to fragmentation or disintegration of waste by increased overburden mechanical stresses at greater depths (Zekkos et al., 2006), (ii) mixing of inorganic clay liner and dense soil daily cover minerals with LMRs during excavation (Qin et al., 2023), and (iii) disposal of soil and C&DW in deeper layers of LeLas due to changes in waste management regulations and practices (mixed disposal vs separate compartments) also plays a role. In India, C&DW was disposed of in the same LeLas as MSW (Rodic and Gupta, 2012), which was later changed due to the introduction of C&DW management rules in 2016 (MoEFCC, 2016a). Nevertheless, the latter reasoning is contrary to the observation concerning the concentration of inorganic mineral fractions (including C&DW), which decreased with an increase in the sampling depth (see Supplemental Figure S2(g)). Higher concentration (between 15% and 30%) of inorganic mineral fractions in top layers (between 0 and 2 m) is due to their use as a final closure cover for LeLas, as implemented at Gericino landfill in Brazil (Nunes and Mahler, 2024).

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Figure 7.

Average composition of LMRs collected from (a) surface, (b) shallow, (c) intermittent, (d) deeper, and (e) terminating depths.

With regard to the percentage of combustibles, except for samples obtained from shallow depths, the rest have shown a decreasing trend with an increase in depth. However, the change in combustibles is very negligible in surface samples in comparison with intermittent samples (see Figure 7(a) and (c)). No clear trend can be observed in percentages of other constituents of combustibles and recyclables with respect to sampling depth as established by a significance test (see Supplemental Table S8). The p-values for all combustible and recyclable fractions are >0.05, implying no significant difference in the weight percentage between groups (i.e. change in depth). Such unclear trends reinforce that knowing the history of waste placement and rearrangement, if any, in LeLas is a must for obtaining reliable depth-wise data.

System-specific factors