Membrane technology as a strategy for microplastics removal from landfill leachate: a review

The generation of municipal solid waste has been steadily increasing due to population growth and rapid urbanization. According to the World Bank (2022), approximately 2.01 billion tons of waste are generated annually worldwide. High-income countries account for about 51% of this total waste, with middle- and low-income countries predominantly generating food and green waste (Singh et al. 2023). Among the various types of waste, plastics have emerged as a major environmental concern due to their high volume and slow degradation rate. The production of plastics has risen from 2 million tons in 1950 to 380 million tons in 2015 (Goli & Singh 2022). Yuan et al. (2021) note that this high waste generation, particularly of plastics, is exacerbated by inadequate systematic classification and low recycling rates, a problem that is even more pronounced in developing countries.

The COVID-19 pandemic has further aggravated this situation by significantly increasing the use of plastic-based personal protective equipment, such as face masks and gloves, which has notably impacted the volume and composition of municipal solid waste (Zand & Heir 2020; Albuquerque et al. 2021). It is estimated that during the pandemic, 129 billion disposable face masks made from plastic microfibers and 65 billion gloves were consumed globally each month (Prata et al. 2020). Additionally, a study revealed that over 200 megatons of municipal plastic waste were generated globally in 2020, with 43% of it being managed improperly (Witzig et al. 2020). The inappropriate disposal of this waste poses a significant environmental threat that urgently needs to be addressed.

While several countries have banned landfilling and promoted plastic recycling, landfilling continues to be a common practice in regions that have not yet adopted these initiatives, largely due to its economic feasibility and operational simplicity (Napper et al. 2020). Projections suggest that by 2050, landfills will contain approximately 12 billion metric tons of plastic waste (Sekar & Sundaram 2023), contributing to the formation of microplastics (MPs) – particles smaller than 5 mm.

Shen et al. (2022) analyzed 11 LLs in Finland, Iceland, and Norway and concluded that LLs are significant sources of MPs, with estimated annual emissions ranging from 15 g to 25 kg. Furthermore, the presence of MPs in LL increases the risk of environmental contamination, as these minute particles or fibers can be transported from landfills to surrounding areas (Afrin et al. 2020). MPs can also end up in oceans, rivers, and groundwater, causing pollution and impacting thousands of aquatic lives and microorganisms, representing serious risks to these ecosystems (Parvin & Tareq 2021).

An aggravating factor in MP contamination is their lengthy degradation cycle, allowing them to easily enter natural ecosystems through the food chain via bioaccumulation, which significantly contributes to soil, water, and air pollution (Lin et al. 2022). Collard et al. (2019) conducted a literature review on plastic particle ingestion by freshwater fish, detecting MP accumulation in over 200 species. Excessive MP accumulation can adversely affect fish digestion, reproduction, and development (Windsor et al. 2019; Nanninga et al. 2020). MPs also carry and transport harmful chemicals such as persistent organic pollutants and heavy metals, which can accumulate in human tissues, leading to chronic diseases such as cancer, neurological disorders, and hormonal imbalances (Singh et al. 2023). Furthermore, MPs in leachate can transmit antibiotic-resistance genes, posing significant risks to human and environmental health if not systematically removed (GodvinSharmila et al. 2023).

Despite the known contribution of LLs to MP pollution, the occurrence, fate, health impacts, and removal of MPs from LL are still under-researched (Singh et al. 2023). Without effective management strategies, MPs may escape from landfills and enter the natural environment. Thus, studies are needed to understand the behavior of these pollutants in relation to LL and to develop effective treatment methods. This study aims to comprehensively assess global MP contamination in LL and explore remediation strategies using membrane technology, such as ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), and membrane bioreactors (MBRs). These methods offer promising solutions for mitigating LL toxicity and minimizing associated environmental and human health risks. Research and full-scale applications of membrane technologies for treating LL highlight their potential as solutions for on-site leachate treatment and the production of high-quality effluent (Coppini et al. 2018; Zhang et al. 2020; Lebron et al. 2021; Folino et al. 2024).

A systematic literature search was conducted using databases including Web of Science, Google Scholar, ScienceDirect, and Scopus with the following keywords: MPs, occurrence, leachate, landfill, treatment, remediation technologies, membrane processes, UF, RO, NF, and MBR.

In the unrefined search of articles, four clusters were identified (Supplementary Figure S1). These clusters were formed based on the affinity and connection of keywords within each theme. The most correlated keywords within these clusters were MPs (12), LL (8), pollution (7), leachate (6), degradation (6), marine environment (6), nanoplastic (5), identification (5), and biodegradation (4). All other keywords had occurrences of three or fewer. In complement, there has been a notable increase in publications concerning the occurrence of MPs in LL, reflecting growing academic interest in understanding their effects on human health and environmental matrices. However, there remains a scarcity of studies focused on the treatment of these emerging contaminants, indicating that research is predominantly centered on understanding the concentration, size, and types of MPs present in LL.

This wide variation in concentration can be attributed to factors such as waste type and volume deposited in landfills, the age and condition of the landfill (whether it is an open dumpsite or controlled landfill), and the different analytical protocols and pre-treatment methods before quantification. Various methods are under investigation for the identification and quantification of MPs, including visual analysis, laser diffraction particle, dynamic light scattering, scanning electron microscopy, Fourier-transform infrared spectroscopy, Raman spectroscopy, thermal analysis, mass spectrometry, and others. A recent document ISO 24187 from 2023 represents a contemporary initiative aimed at standardizing the identification and analysis of MPs in environmental samples. The document provides a framework for conducting MP analyses, offering guidelines on aspects like particle size classification, sampling methodologies, sample preparation techniques, and ensuring representative sample quantities. By establishing minimum requirements, the standard seeks to promote uniformity in MP research methodologies until specific standards tailored to different environmental contexts are developed.

In Figure 2, the inclusion of data from countries in the Americas and Africa would be a valuable addition to understanding the global scope of the phenomenon studied. However, the lack of available data in the literature highlights a significant gap in research in these regions. This scenario underscores the need for future investigations that explore these geographical contexts, aiming for a more comprehensive and inclusive understanding of global patterns.

The data reveal Indonesia, Serbia, Lithuania, and China as the countries with the highest concentration of MPs in LL. According to Banu et al. (2020), China is the largest producer of plastics globally, generating over 60 million tons annually. Landfill from India also shows high concentrations (3–100.7 MP/L). As reported by the Central Pollution Control Board (CPCB 2019), the country produces more than 25,000 tons of plastic waste per day. This substantial generation directly impacts the quality of LL, as depicted in Figure 2. Diverse policies regarding plastic recycling across various countries result in varying proportions of plastics in landfills, consequently influencing the prevalence of MPs in LL. Without proper recycling, reuse, and separation processes at the point of waste generation, plastics degrade in landfills, giving rise to MPs.

The high concentration of MPs in LL is exacerbated by their interaction with other contaminants. Organic, inorganic, and trace metal pollutants present in LL can adhere to the surface of MPs due to their hydrophobic character, imparting greater stability and reduced natural degradation to these compounds. Additionally, MPs can act as vectors for emerging contaminants in the aquatic environment.

When the size of MPs is compared in the different matrices, the domestic sewage exhibited a lower average size of 580.38 ± 1,343.20 μm, with a median of 125 μm. Another important consideration is that sludge generated in wastewater treatment plants (WWTPs) is typically directed to landfills. Studies indicate that between 70 and 98% of MPs are concentrated in WWTP sludge. Consequently, buried sludge can significantly contribute to the presence of MPs in landfill as well (Kong et al. 2023).

Despite the widespread occurrence of MPs in LL, no explicit legislation limits their concentration in LL or industrial wastewater. Most advancements in managing the risks and impacts of macroplastics and larger debris focus on reducing their use and consumption. Examples summarized by Lam et al. (2018) include reducing the use of low-density polyethylene bags, imposing fines, and in more restrictive countries like Kenya, implementing prison sentences for their use, manufacture, and import. More recently, the ban on microbeads in personal care and hygiene products has been another initiative employed by different countries (Santos et al. 2023). For instance, in Australia in 2016, microbeads were found in over 100 personal care products on the market. In response to environmental impacts, the US Federal Government introduced the Microbead-Free Water Act in 2015 to prohibit the manufacture and sale of microbeads in cosmetic products (Lam et al. 2018).

The United States, the European Union, and China have started implementing legislation primarily focused on rinse-off cosmetics and personal care products (Halfar et al. 2021). For example, California, New York, and Illinois have enacted measures to prevent primary sources of MPs. The H.R.1321 – Microbead-Free Waters Act of 2015 banned MPs in rinse-off cosmetics. However, this law has been criticized for its limited scope and lack of support for biodegradable alternatives. The European Chemicals Agency (ECHA) proposes significant reductions in MP use, with the potential for an 85% reduction in emissions by 2042.

The data summarized by Lam et al. (2018) regarding legislation on MPs have been updated by Conti et al. (2021), Halfar et al. (2021), and Santos et al. (2023), reaffirming that most initiatives aim to limit plastic materials as a preventive measure against MP formation. Prevention is often more environmentally friendly than remediation due to the long-term negative impacts of MPs. Prevention also contributes to resource conservation and is often more cost-effective than remediating effects. However, prevention is a long-term solution to a problem that currently affects society. Thus, this review highlights the occurrence of MPs in LL and emphasizes the need for their regulation in treated leachate to prevent environmental contamination.

Lam et al. (2018) proposed a collaborative approach involving all levels of society, from governments and plastic manufacturers to consumers, waste management organizations, and researchers, focusing on changes in manufacturing, consumption behaviors, and waste management strategies (emphasizing recycling and energy recovery practices). Since there is no explicit regulation, educational programs and voluntary campaigns toward plastic litter control are alternatives to prevent MP formation and occurrence in LL.

In Portugal, the introduction of fees for plastic bag use resulted in 52% of supermarket customers changing their behavior, leading to a 64% decrease in bag consumption (Perestrelo & Spínola 2010). The study showed that charging even a symbolic price (€0.02 per bag) led to a 37% reuse rate compared with no reuse in supermarkets without the fee. This voluntary example can be complemented by other initiatives promoted by local and international associations or institutes. Educational campaigns for young people, such as the ‘Junker APP’ and the ‘Tide Turners Plastic Challenge Badge’, supported by the UK Department of Environment, Food, and Agriculture (Singla et al. 2021), are effective. Movies such as ‘Plastic Ocean’, one of the most awarded environmental documentaries of 2017, also aim to raise public awareness and improve recycling systems.

The literature suggests that including MPs as a parameter for monitoring in leachate or industrial wastewater is complex and lengthy. The issues related to MPs in LL and their environmental contribution have been recognized, indicating the necessity for regulation. The next steps involve specialists and stakeholders proposing drafts and understanding the potential impacts. Stakeholders should be able to monitor MP concentrations, representing a challenge due to the lack of standard protocols for identification and quantification. The high costs could impede widespread monitoring, especially for facilities with limited financial capacity. Encouraging new monitoring parameters would be impractical if landfill facilities lack the means to implement and sustain them.

A comprehensive review of existing literature shows a lack of legislation providing guidance for treating LL to ensure compliance with safe limits for MPs before disposal. Although such parameters are not universally prescribed, landfills must acknowledge and address their contribution to MP contamination in the environment. Given the well-documented adverse impacts associated with the presence of MPs in the environment, it is prudent to explore technologies capable of effectively removing MPs from LL, as advocated by the scientific community.

A web search of Science and Scopus was performed to quantify the number of publications on membrane technologies for the treatment of LL using the keywords LL, membrane, and treatment. The search results revealed a total of 199,843 articles that mentioned the use of membranes in the title, keywords, and abstract of the manuscripts (Supplementary Figure S2). This indicates that the use of membrane technologies for LL treatment has been growing considerably in recent years. However, it is noteworthy that most of these studies are still at a laboratory scale.

UF operates using hydraulic pressures ranging from 0.1 to 5 bar and features pore sizes between 2 and 10 nm (Karimi et al. 2019). UF membranes are typically employed for retaining bacteria, viruses, and high molecular weight organic compounds (Jacquet et al. 2021). In leachate treatment, UF is used to retain suspended solids, which can be present in concentrations ranging from 3,015 to 17,850 mg/L (Lindamulla et al. 2022).

The characteristics of UF membranes, including surface charge, pore size, manufacturing material, and operational conditions, are crucial for understanding their capacity for MP removal (Pizzichetti et al. 2021). Few studies have investigated the removal of MPs from leachate using UF. Kara et al. (2022) examined their occurrence and removal from LL in Istanbul, Turkey, using UF integrated with a biological tank in an MBR. The study reported a 95.6% removal efficiency of MPs by the UF membrane, which was made of polyvinylidene difluoride (PVDF) with a pore size of 0.03 μm, resulting in low MP concentrations in the permeate. The biological treatment alone removed 62.3% of MPs, leaving 147 MP/L, whereas the UF treatment further reduced this to 6.5 MP/L in the permeate. Additionally, a similar system incorporating NF after UF achieved a comparable removal rate of 96.3%.

Zhang et al. (2021) evaluated MP removal from leachate using a treatment system composed of an equalization tank, an MBR stage (including two-stage anoxic/oxic (AO) processes), UF, NF, and RO. The UF membrane effectively retained 75% of particles with diameters less than 1 mm, resulting in a UF permeate with approximately 0.1 MP/L.

Another example of the use of UF membranes was presented by Nayahi et al. (2022). The authors studied MP removal from leachate using a sequence of aeration lagoon, equalization tank, sedimentation, UF, and RO. Contrary to expectations, they observed an increase in MP concentration after UF. The effluent from the sedimentation process contained 3.9 MP/L, which increased to 19.7 MP/L in the UF permeate. The concentration of polyacrylonitrile (PAN), a polymer used in some UF membranes, increased from 2.0 to 15.0 MP/L, suggesting that membrane abrasion and wear might contribute to the elevated MP levels. Overall, UF membrane-based treatments have demonstrated satisfactory results in MP removal. However, potential MP release from the membranes themselves, as noted by Nayahi et al. (2022), requires further experimental investigation.

MBRs combine membranes such as microfiltration or UF with biological treatment processes. These reactors can be configured in aerobic, anaerobic, anoxic, or hybrid configurations (Lindamulla et al. 2022). Aerobic MBRs are the most common due to their well-established efficiency in removing contaminants such as organic matter and ammonia (N-NH₃) from LL, regardless of leachate composition (Abuabdou et al. 2020; Brasil et al. 2021).

Sun et al. (2021) investigated MP removal using an aerobic MBR using a submerged UF membrane made of polyvinyl chloride (PVC) with a pore size of 0.02 μm. The biological system significantly reduced the MP concentration from 235.4 MP/L in raw leachate to 3.8 MP/L in the MBR effluent, demonstrating a clear advantage over conventional biological treatments, which achieved a maximum removal efficiency of 62.3% (Figure 4).

Zhang et al. (2021) also highlighted the potential of MBRs in removing MPs from LL, reporting a 50% reduction in MP concentration and achieving 0.5 MP/L in its effluent. The appearance of polystyrene in the MBR effluent, not seen in earlier treatment stages, suggests possible MP release from the membranes. However, the authors did not provide details on membrane characteristics, and the presence of polystyrene could also originate from other contamination sources within the treatment plant. Sun et al. (2021) also observed the presence of two polymers (polyethylene-co-acrylic acid – EEA and polyacrylamide – PAM) after MBR treatment, but they were not detected in the raw leachate. According to Zhang et al. (2021), this release may be associated with the material of synthetic membranes, since they can be made of PES, PVC, PP, and PVDF, and the possible release of MPs by membranes cannot be ruled out. Santos et al. (2023) suggest that, in addition to the release of low concentrations of MPs by membranes, their presence in the permeate may be associated with fouling, due to the different sizes and shapes of MP that favor the passage to the permeate.

Furthermore, despite the high efficacy of MBR systems in removing MPs, the significant retention of MPs by the membrane leads to their accumulation in biological sludge, impacting biological performance. Sun et al. (2021) observed a 150-fold increase in MPs within the biological treatment tank compared with raw leachate, attributing this to MP accumulation in sludge due to membrane retention. Maliwan et al. (2021) found that MP reduces sludge floc size, floc hydrophobicity, and external polymeric substance molecular size while increasing their concentration. Liu et al. (2023a, 2023b) showed that MP accumulation in biological sludge can reduce the removal of organic matter and ammonia and impair methane production in anaerobic systems. Additionally, their presence in biological sludge can increase membrane fouling, necessitating more frequent backwashing or chemical cleaning, potentially releasing MPs and affecting the system's performance (Kabir et al. 2023). Therefore, studies need to assess MP accumulation in MBRs, its impact on microbial activity and membrane fouling and exploring strategies to mitigate this accumulation.

NF typically operates at pressures ranging from 3 to 20 bar and features an average pore size of 0.5–2 nm (Acarer 2023). These membranes, often composed of polymeric films, are utilized in LL treatment to retain sulfate and other multivalent ions, meeting high-quality water standards (Lebron et al. 2021). NF membranes also contribute to MP removal. Furthermore, the combination of NF membranes with other processes such as MBRs can achieve high removal rates, greater than 72% (Singh et al. 2023). The majority of retained MPs were larger than 100 μm and fibrous in form. Due to the efficiency of NF, the MP concentration in the final effluent was reduced to 2 MP/L. Shen et al. (2022) reported that most MPs in LL are larger than the pore size of NF membranes, suggesting a high removal rate for this technology.

However, this is not always observed. Previous sections indicated that UF membranes sometimes achieved greater MP removal than NF membranes. Zhang et al. (2021) explained these unexpected results by investigating MP removal across various membrane technologies. They found that NF not only failed to remove MPs effectively but also increased their concentrations in the permeate. This was attributed to the release of MPs from the NF membrane itself, as cellulose nitrate (CN), a common membrane material, was detected in the NF permeate but not in the raw LL. Despite the potential release of MPs, the concentration in the NF effluent remained low (0.3 MP/L), which is 75% less than in the raw LL. While the authors did not detail the cleaning procedures used for the membranes, it is important to note that intensive or frequent chemical cleaning can enhance MP release due to membrane degradation.

Gan et al. (2021) demonstrated that cleaning agents like sodium hydroxide, sodium hypochlorite, and citric acid can cause MP release after prolonged membrane exposure. Therefore, cleaning protocols must balance flux recovery with membrane integrity. Further research is needed to develop cleaning procedures that minimize MP release. Bodzek & Pohl (2023) also highlight the possibility of using NF as a tertiary treatment, after treatments such as sedimentation, coagulation, and aerobic or anaerobic processes. In this way, it would be possible to increase the efficiency of the process.

Some critical factors, such as membrane characteristics, operating conditions, and cleaning procedures, were not fully explored in the reviewed studies. The effectiveness of membrane-based MP removal relies heavily on optimal operating and maintenance conditions. Therefore, additional studies are necessary to identify the most effective conditions for enhancing the performance of these treatment systems in removing MPs.

RO is a dense membrane technology with pore sizes <0.5 nm, typically operated at high pressures exceeding 5 bar. Some researchers also describe them as dense and non-porous, highlighting that among all membrane processes, RO exhibits the greatest rejection efficiency. This high rejection efficiency is expected due to the membrane's dense structure and small pore size, which effectively excludes a wide range of contaminants, including MPs. RO is also recognized as a robust and highly efficient technology for treating LL due to its resilience against variations in its composition (Lebron et al. 2021). The literature demonstrates a high compound retention rate for these membranes, exceeding 99.6% for chemical oxygen demand, total organic carbon, color, ammonia, and iron (Tałałaj et al. 2021).

Given the typical size of MPs found in LL, the necessity of employing RO for MP removal might be questioned. However, RO is frequently used in LL treatment, and consequently, indirectly remove MPs as well. Sun et al. (2021) analyzed an integrated system of MBR, followed by NF and RO and found that the membrane processes removed approximately 89.4% of MPs. The residual concentration of MPs (0.4 MP/L) found in the final effluent was attributed to the release from the membrane. It consisted of poly(ether-ether-ketone) and polyethylene terephthalate, initially absent in the raw LL.

Zhang et al. (2021) also observed potential MP release from RO membranes in a study involving an integrated treatment train (equalization tank, MBR, biological treatment, UF, NF, and RO) with RO being a polishing step for NF permeate. They noted an increase in MP concentration from 0.3 MP/L in the NF permeate to 0.5 MP/L after RO treatment. The polymers identified in the RO permeate were not detected in the raw LL neither.

Nayahi et al. (2022) assessed MP removal from LL collected from controlled landfills and open dumps using UF and RO, following pre-treatment with an aerated facultative pond and sedimentation. The membrane-based system achieved a 37.1% MP removal, resulting in a final effluent concentration of 2.47 MP/L. The lower removal efficiencies were attributed to MP release from the UF membrane material, though details about the membrane material were not provided, limiting comprehensive evaluation and discussion.

The overall outcomes suggest that a proper system operation is crucial to prolong RO membrane lifespan and maintain MP removal efficiencies, without additional release to the treated effluent. The literature suggests that operating RO systems under elevated pressures may exacerbate membrane abrasion, favoring MP release. These conditions were investigated by Qi et al. (2016), which also assessed the chemical and thermal stability of RO membranes. The authors concluded that the membrane was chemically stable for cleaning agents such as EDTA, sodium hydroxide (NaOH), and citric acid but sensitive to pressure changes above 30 bar and feed solution temperatures above 65 °C, which could enlarge membrane pores.

Even if MPs are released by the RO membrane, their concentration in the effluent is likely to be significantly lower than that in raw LL. Employing UF membranes as a pre-treatment is a strategic approach, as they can efficiently remove MPs, thereby reducing fouling of the RO membrane. This reduction in fouling decreases the need for frequent cleaning and high operating pressures, ultimately improving the overall performance and efficiency of the treatment system.

There are few studies that have investigated the occurrence, concentration, particle size, and removal of MPs in concentrates of membrane systems treating LL. Kara et al. (2022) identified particles ranging from 100 to 2,000 μm in NF concentrate, including fibers of various colors such as transparent, black, blue, pink, red, and yellow. Investigating MPs in LL concentrates is essential, as pollutants can increase by 25–50% more than the initial contaminants in the feed (Kallel et al. 2017; İskurt et al. 2022). This is evident in the study by Sun et al. (2021), where the concentration of MPs in raw LL was 235.4 MP/L, but it reached 3.51 × 10⁴ MP/L in the biological tank of the MBR. This concentrated stream results from the high rejection rates of MPs by membranes (Almeida et al. 2022).

Studies indicate that due to their hydrophobic nature, MPs can adsorb and serve as carriers for other pollutants (Li et al. 2018; Fan et al. 2022). Santos et al. (2023) demonstrated that the interaction of MPs with metals and pharmaceuticals can intensify the toxicity of these compounds. Micropollutants commonly found in LL, such as pesticides, pharmaceuticals, and polycyclic aromatic hydrocarbons (Shanmuganathan et al. 2017; Kumar et al. 2023), have the potential to accumulate in membrane concentrates. The accumulation of toxic compounds and MPs should be further investigated since the interaction between these pollutants can increase their environmental risks.

Proper management of membrane concentrates is challenging due to their complex quality. For raw LL, which typically presents lower concentrations of contaminants, Singh et al. (2023) highlighted that the complex quality of LL poses challenges to MP removal due to elevated levels of organic matter, ammonia, and heavy metals. Zhou et al. (2022) reinforced these findings, demonstrating that MPs can negatively influence the removal of other pollutants. Specifically, the authors found that polystyrene particles (100 nm) in concentrations above 1 mg/L reduced the biological removal of ammonia by 24.5% due to the acute toxicity they cause to microorganisms.

In a broader context, the impact of complex composition could be exacerbated in membrane concentrates due to higher concentrations of MPs and ammonia. Despite this, more studies are needed to evaluate the removal of MPs and other pollutants in membrane concentrates, given the limited investigations available. Analyzing the occurrence, colors, particle sizes, morphology, and types of polymers in these concentrates is crucial for proposing effective treatment strategies and mitigating environmental contamination risks that could harm public health.

The main limitation of using membrane technology for MP removal is membrane fouling, which can affect membrane performance and lifespan (Pan et al. 2021; Pizzichetti et al. 2023). Fouling by MPs can occur through pore blockage, cake layer formation, and adsorption on the membrane surface, depending on the characteristics of the MPs and membranes, and operational conditions (Wang et al. 2021a, 2021b; Parvin et al. 2022). For example, Zhang et al. (2023) showed that fouling of UF membranes (PVDF; 0.1 μm) in an MBR worsened at higher concentrations of polyethylene terephthalate, resulting in flux declines of 14, 24, and 31% for concentrations of 10, 30, and 50 mg/L, respectively. This is indicative that membranes used for LL treatment may be subject to more severe fouling due to high concentrations of MPs.

Enfrin et al. (2019) emphasized that particles larger than the membrane pore size can lead to pore blockage or cake layer formation, while particles smaller than the pore size can cause internal and irreversible fouling. Additionally, Li et al. (2021) showed that reducing the size of polystyrene particles can aggravate fouling of hollow fiber membranes (PVDF; 0.03 μm).

The shape and polymer type of MPs also influence fouling characteristics. Pizzichetti et al. (2023) evaluated fouling by polyamide and polystyrene MPs on a microfiltration membrane (cellulose acetate; 5 μm). Greater fouling occurred after polyamide filtration, attributed to the polymer's greater hydrophobicity and smaller particle size. The authors highlighted those repulsive electrostatic interactions between polystyrene particles and the negatively charged membrane, combined with the high shape irregularity of polystyrene particles, formed a looser cake layer.

Understanding the characteristics of the membrane and MPs during LL treatment is essential to minimize fouling damage. Efficient cleaning strategies are also crucial for permeate flux recovery. Enfrin et al. (2021a) evaluated the cleaning of commercial UF membranes (PSF, MWCO 30 kDa) fouled by polyethylene terephthalate fibers (200 nm diameter at 10 mg/L), using gas scouring as a physical cleaning procedure. This method reduced fiber adsorption on the membrane by up to 80% with a gas injection rate of 0.3, applied every 15 min. Similarly, Enfrin et al. (2021b) evaluated fouling mitigation of UF membranes with polyethylene fragments averaging 93 ± 1 nm. Among the strategies used – forward flushing and gas scouring – gas scouring was the most efficient, with a gas injection rate of 0.2. Although a rate of 0.3 was marginally better, a rate of 0.2 had less impact on the membrane coating layer.

Chemical cleaning is also important for mitigating fouling, but it must be conducted to remove fouling while maintaining membrane integrity. Prolonged exposure to chemical agents may lead to MP release (Gan et al. 2021). However, few studies evaluate the cleaning of MP-fouled membranes, and no studies specifically address this topic in the context of LL treatment, highlighting the urgent need for experimental investigations.

The final objective of the review paper was to identify studies in a full scale that considered membrane-based systems for LL treatment, in order to indirectly infer their contribution to MP removal. For that, a refined search of the literature was conducted. The search employed keywords such as LL, treatment, full scale, membrane, microfiltration, UF, NF, RO, and MBRs. After excluding duplicate studies, 169 manuscripts were identified.

Membrane technology has been extensively studied and applied to LL treatment on a full scale. Monitoring these studies is crucial not only for evaluating and controlling the release of treated effluents but also for demonstrating the efficiency of these processes to facilitate their broader application.

Despite the limited number of studies that specifically investigated MP removal efficiency at full-scale plants, it is anticipated that most membrane-based systems would effectively contribute to MP removal, as observed in bench-scale studies. However, additional studies are required to validate the performance of membrane-based systems in full-scale applications.

Economic aspects should also be considered while transferring research findings to full-scale applications and should evaluate both costs and potential benefits. The technological advancements have led to the development of more cost-effective membranes with improved durability and energy efficiency, potentially lowering these costs. From an efficiency standpoint, the use of membrane technologies can offer benefits such as treated water quality and robustness, preventing the occurrence of MPs and other contaminants in the final treated stream regardless of the leachate composition. Almeida et al. (2020) estimated the costs of LL treatment by using RO reporting operational costs around 8.58–10.09 US$/m³. The contributions were mainly associated with membrane replacement, suggesting that researches should advance in order to develop membranes of greater resistance, or prolong the lifespan of existing membranes. Despite the opportunities for improvements, it was also reported by the authors that these costs were lower than the ones when the RO system was not employed (21 US$/m³), and are comparable with the costs reported by other authors as Calabrò & Grosso (2018), 17–44 US$/m³, and Kurniawan et al. (2006), 2–30 US$/m³. This suggests that the use of membrane systems for LL treatment is already a feasible solution and should prevent the introduction of MPs into the environment by LL disposal.

As the global concern regarding MP pollution intensifies, it becomes imperative to develop innovative strategies to mitigate its environmental impact. This review paper identifies several key areas warranting special attention for future research endeavors aimed at advancing the removal of MPs from LL using membrane technology. Specifically, focus should be placed on investigating the operating conditions that influence both the retention and release of MPs by membranes, exploring destructive technologies for MP degradation from membrane concentrate, and validating these techniques through full-scale applications.

Future investigations must prioritize understanding the influence of operating conditions, such as pressure, temperature, and flow rate, on the efficiency of MPs removal by membranes, as well as the potential for MPs release during membrane processes. Furthermore, there is a critical need to advance our understanding of fouling mechanisms and develop effective anti-fouling strategies to prolong membrane lifespan and minimize the need for frequent cleaning intervals. Additionally, the development of membrane materials with specific affinity for MPs is essential to prevent their deposition and fouling.

In managing membrane concentrate, technologies capable of destroying MPs rather than simply transferring them between different phases should be explored. Examples of such technologies include advanced oxidation processes like ozonation and photocatalysis, as well as the application of ultraviolet (UV) irradiation and sonication to facilitate MP breakdown. Another promising approach involves the integration of biological degradation methods utilizing enzymes or microorganisms to target MPs.

Ultimately, pilot-scale studies or full-scale assessments are necessary to evaluate the scalability and feasibility of membrane-based MP removal technologies. Long-term monitoring of treated effluent will provide valuable insights into the efficacy and sustainability of removal techniques over time. Furthermore, comparative analyses of different membrane configurations and treatment processes in real-world LL treatment plants are essential for identifying optimal approaches for MP remediation.

The presence of MPs in LL raises significant environmental concerns due to the wide variability in concentrations and the diverse range of particle sizes and polymer compositions. Currently, legislation regarding MP concentration limits in LL treatment is lacking, underscoring the necessity for comprehensive guidance to mitigate MP contamination in the environment. Membrane technologies such as UF, NF, RO, and MBRs have demonstrated effectiveness in reducing MP concentrations. However, these technologies also encounter challenges such as membrane fouling and MP release. The accumulation of MPs in the biological sludge of MBRs adversely affects biological performance. Although the size of MPs found in LL may raise questions about the necessity of employing NF and RO membranes for MP removal, these processes are commonly utilized in LL treatment across many landfills, acting as barriers to MP retention. Further research is imperative to address several aspects related to MP removal in LL treatment. This includes understanding membrane–MP interactions, assessing the accumulation of MPs in MBRs and their impact on microbial activity and membrane fouling, exploring cleaning strategies in LL treatment and their effects on MP release from membranes, and studying the integrity of membranes after continuous exposure to LL under varied operating conditions.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.