1 Introduction

Plastics have become indispensable across industries due to their versatility, durability, and cost-effectiveness [1,2,3]. However, the exponential increase in plastic production and inadequate waste management have led to pervasive environmental pollution caused by plastic waste [4]. Global plastic production surged to 367 million tons in 2020, with projections indicating a tripling by 2060 [1]. Despite the potential for recycling and reclamation, less than 5% of plastics are currently reclaimed, exacerbating the accumulation of plastic waste and posing a severe threat to the global ecosystem [4,5,6]. Plastic waste undergoes degradation through microbial activity and weathering, resulting in the formation of different fragments, such as macroplastics (> 2 cm), mesoplastics (2 cm–5 mm), microplastics (MPs) (< 5 mm), and nanoplastics (NPs) (< 100 nm) [7]. These plastic particles have been detected in various environmental compartments, such as surface and deep waters, beaches, sediments, and terrestrial ecosystems [8,9,10,11]. A significant portion of globally generated plastic waste, approximately 79%, has been reported to end up in landfills, making soil a major reservoir for MPs [12]. High concentrations of MPs have also been documented in soils from other areas, including industrial zones, farmlands, home gardens, and non-urban natural reserves [13, 14]. Plastic fragments enter the environment through diverse sources, such as discarded containers, detergents, greenhouse covers, and packaging materials [15, 16]. Environmental factors like wind patterns, water currents, and population densities significantly influence the distribution of plastic in the environment [7, 17].

Agricultural practices represent a significant pathway for MPs to enter the soil, alongside other routes such as atmospheric deposition, urban runoff, and flooding [18, 19]. Mulching and greenhouse farming, in particular, are major sources of macro and MP films [20]. Additionally, organic composts contribute significantly to soil contamination with MPs [21]. Manure compost, derived from domestic animals such as sheep, poultry, and pigs, further exacerbates this contamination, depositing hundreds of tons of MPs into agricultural and horticultural soils annually. These MPs ultimately find their way into animal faeces through direct ingestion from the environment and feed. The application of treated wastewater (TWW) and sewage sludge (SS) originating from wastewater treatment plants (WWTPs) in agriculture is another significant yet often overlooked source of MP contamination in soil. WWTPs are not specifically designed to treat MPs. Still, studies have indicated a reduction in MP concentrations between the influent and effluent streams, ranging from 58.8 to 99.9%, depending on the treatment unit [22]. On the other hand, research by Ziajahromi et al. [23] has shown that approximately 79% of the MPs entering WWTPs end up in the sludge phase, which is generally disposed of in landfills or reused in agriculture.

In regions suffering from severe water shortages, such as the Middle East, North Africa, and parts of Asia, reusing TWW provides an alternative water source for agricultural irrigation. This helps alleviate the pressure on freshwater resources and supports agricultural sustainability [24, 25]. Additionally, SS is rich in organic matter and essential nutrients like nitrogen, phosphorus, and potassium, making it an excellent soil conditioner140. Applying SS to agricultural fields can enhance soil fertility, improve soil structure, and increase crop yields, thereby supporting food security in poor soil quality [26, 27]. Reusing TWW and SS also helps manage and reduce urban waste, promoting a circular waste treatment and resource recovery approach. This practice reduces the environmental footprint of waste disposal and supports sustainable urban development [28]. Despite these benefits, MPs in TWW and SS pose a significant and often overlooked environmental risk. When TWW and SS are reused in agriculture, MPs can be introduced into the soil, potentially contaminating terrestrial and aquatic ecosystems. The presence of MPs in agricultural soils raises concerns about their potential uptake by crops, which could compromise food safety and human health [29, 30]. This issue is particularly critical given the lack of stringent regulations and monitoring protocols. An extensive literature search was conducted to ensure a comprehensive and up-to-date review of the topic. Multiple databases were accessed, including Web of Science, Scopus, PubMed, Mendeley, ResearchGate, and Google Scholar. These databases were selected for their broad coverage of scientific publications across various disciplines. Relevant publications were systematically searched using specific keywords. The keywords used included “microplastics” (MPs), “MPs in wastewater,” “MPs in treated wastewater,” “MPs in soil,” “MPs in sewage sludge,” “innovative technologies for MPs removal,” “MPs biodegradation,” “MPs phytoremediation,” “MPs in edible plants,” and “MPs in the human body.” The search yielded a significant number of publications, with 16,083 publications mentioning “microplastics” identified, with 4056 of these published in 2023 alone. While numerous reviews have addressed the presence and impact of MPs in wastewater and sewage sludge on aquatic environments and terrestrial systems, a critical gap remains in the literature concerning the implications of WWTPs’ by-products in soil contamination with MPs, as well as remediation techniques once the soil is contaminated. This review aims to bridge this gap by comprehensively analyzing the hidden risks associated with reusing treated wastewater and sewage sludge in agriculture, specifically focusing on soil contamination with MPs. Additionally, it will explore effective remediation strategies to mitigate soil contamination, filling a significant void in the current body of literature and contributing novel insights to the field.

2 Sources and pathways of microplastics in wastewater treatment plants

WWTPs can serve as receptors for MPs originating from various sources, including stormwater, domestic wastewater, and industrial processes [31]. Understanding these pathways is crucial for comprehensively addressing MP pollution. MPs entering WWTPs can originate from cosmetic products, fibers released from washing machines, tire debris, fragmented roads and urban runoff plastics, and polymeric flocculants used in WWTPs. Domestic laundry facilities, particularly washing machines, significantly contribute to the release of MPs. The most commonly detected polymers in WWTPs include polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS), polyamide (PA), polyester (PES) polyvinylchloride (PVC), polypropylene (PP) and polyurethane (PU) [31]. For instance, PES-cotton blends, PES, and acrylic materials have been shown to release 138,000, 496,000, and 728,000 microfibers, respectively [32]. The analysis of MPs in WWTPs has indicated that white and transparent plastic particles were the predominant ones, comprising 50 to 60% of the total, followed by black (22%), blue/green (13%), and red particles (5%) [33]. To mitigate the issue of microfiber release, manufacturers can integrate high-efficiency filters into washing machines to capture microfibers before they enter the wastewater stream. These filters should be fine enough to trap particles in the size range of MPs, typically less than 5 mm. Regular maintenance and replacement of these filters will ensure their effectiveness. External filter attachments can be installed for existing washing machines. These devices can be added to the washing machine’s drain hose and are designed to capture microfibers as water exits the machine. Environmental organizations can launch informative programs to educate the public about the benefits of using cold water, gentle cycles, and the environmental impact of consumer products. These programs can provide information on the sources of microplastics and harmful chemicals, their effects on ecosystems and health, and ways to reduce their release.

The following table presents a comprehensive overview of the occurrence and concentrations of MPs in wastewater, sewage sludge, and soil across various countries. It highlights the different types of MPs detected, the geographic locations of the studies, and the concentration levels observed (Table 1). This information underscores the global scale of MP pollution in wastewater and the variation in contamination levels across different regions. The varying levels of MP pollution across different regions worldwide highlight this environmental issue’s complex and multifaceted nature. Each region presents unique challenges and contributions to the global MP burden, from bustling urban centres to pristine coastlines. Europe, known for its stringent environmental regulations and advanced waste management infrastructure, exhibits varying levels of MP pollution across its countries. In Sweden (Lysekil), MP concentrations have been measured at 15,100 particles per cubic meter, suggesting comparatively lower pollution levels, possibly due to stringent environmental regulations and effective wastewater treatment practices [34]. Germany has reported 9050 particles per cubic meter, reflecting effective industrial and municipal waste management practices that help mitigate MP pollution [35]. Finland (Viikinmäki) and Denmark have shown moderate levels, with 6900 particles per cubic meter and 7.22 million particles per day, respectively, indicating varying degrees of MP contamination influenced by regional industrial activities and urbanization [36, 37]. France has reported between 260,000 and 320,000 particles per cubic meter, indicating considerable MP contamination likely influenced by both urban centers and industrial activities [38]. With 467,160 particles per cubic meter, Russia has shown a high MP load, possibly influenced by industrial discharge and extensive urbanization [39]. The Netherlands and Poland have exhibited varying levels, from 68,000 to 910,000 and 19,000 to 552,000 particles per cubic meter, reflecting localized factors such as urbanization and industrial practices [40, 41]. In Spain (Madrid), the daily release of 300 million particles has underscored significant MP pollution. This high figure can be attributed to large urban populations and extensive industrial activities, which contribute to elevated MP concentrations in wastewater discharges [42]. Similarly, Northern Italy has reported a substantial daily release of 160 million particles, indicative of densely populated urban areas and intensive industrial sectors that discharge MPs into water bodies [43]. The United Kingdom (Glasgow) has also stood out with a daily release of 65 million particles, reflecting industrial activities and densely populated urban centers contributing to high MP loads [44]. China has demonstrated a wide range of MP concentrations in Asia, from 6600 particles per cubic meter [45] to a staggering 168 trillion particles per year [46]. The high levels reflect extensive industrialization, rapid urbanization, and substantial plastic usage in various sectors. South Korea has recorded 30,000 particles per cubic meter [47], indicating moderate MP contamination. Factors contributing to these levels include industrial activities, urbanization, and coastal development. Compared to Europe and China, regions across America, Africa, and the Middle East have exhibited diverse levels of MP pollution shaped by urbanization, industrial outputs, and environmental policies. Studies have revealed significant daily releases in the United States, with Southern California and Detroit reporting 1.1 million and 10 million particles, respectively [48, 49]. Conversely, New York has reported lower daily counts ranging from 11,500 to 25,000 particles, underscoring effective waste management practices and stringent regulatory measures in dense urban settings [50]. Similarly, Sydney, Australia, has recorded 2,200 particles per day, showcasing the effectiveness of rigorous environmental policies and public awareness campaigns in mitigating MP pollution [23]. Mauritius has reported 276,300 particles per day in Africa, reflecting moderate pollution levels influenced by tourism, local industries, and coastal activities [51]. Meanwhile, Turkey has recorded between 2 and 4.8 million particles daily in the Middle East, driven by industrial sectors and urban expansion [52]. Canada, specifically Vancouver, has been facing substantial challenges, estimating 1.76 trillion particles annually, translating from approximately 4.82 billion to 10.96 billion per day [53]. This figure highlights the significant impact of industrial activities and coastal dynamics on MP pollution, emphasizing the critical need for enhanced waste management strategies in coastal regions worldwide.

Table 1 Types and concentrations of microplastic particles in wastewater, sewage sludge, and soil matrix across different countries
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These regional variations in MP contamination reflect differences in local industrial activities, waste management practices, and environmental regulations. Urban and industrial areas are hotspots for MP pollution, indicating higher anthropogenic activities and insufficient waste management infrastructure. Regions with robust waste treatment infrastructure exhibit lower concentrations of MPs, as seen in some European countries like the Netherlands, Germany, and Denmark. In contrast, regions with inadequate waste management systems experience higher pollution levels, as observed in some areas of Turkey and Poland. The significant daily emissions in certain regions raise concerns about long-term ecological impacts and the potential entry of MPs into the human food chain, necessitating urgent mitigation measures tailored to each regional context. This data underscores the critical need for comprehensive strategies to manage and mitigate MP pollution, emphasizing the importance of targeted interventions to address the unique challenges faced by each region.

Nonetheless, the lack of data on the occurrence of MPs in WWTPs in the Middle East, Africa, and India raises significant concerns regarding the understanding and management of plastic pollution in these regions. Despite being among the top contributors to mismanaged plastic waste globally [54], limited research exists on the presence and concentration of MPs in WWTPs within these regions. This knowledge gap impedes the development of effective mitigation strategies tailored to local contexts, exacerbating environmental and health risks associated with plastic pollution.

The widespread distribution of MPs in wastewater presents significant environmental challenges, as these particles can persist in the environment for extended periods, accumulate in aquatic ecosystems, and potentially harm aquatic organisms. For instance, Magni et al. [43] have conducted a study on one of the largest WWTPs in Northern Italy, estimating that 160 million MP particles are released daily into the aquatic system. Similarly, Edo et al. [42] have observed that WWTPs in Spain discharge around 300 million MP particles into the Henares River daily, corresponding to a load of 350 particles per cubic meter of water. In the United Kingdom, Murphy et al. [44] have reported that a WWTP serving a population of approximately 650,000 releases 65 million plastic particles into receiving waters daily. Similarly, in the USA, Carr et al. [48] have estimated that approximately 900,000 plastic particles are discharged daily into receiving waters from a WWTP that processes approximately 1.06 million cubic meters of wastewater daily. Moreover, Cheung and Fok [46] have estimated that over 210 trillion microbeads are discharged annually into the aquatic environment in mainland China, with more than 80% of these particles attributed to wastewater effluents. However, it is crucial to recognize that seasonal factors, such as increased rainfall or snowmelt during winter months, can influence the release of MPs. When WWTPs cannot treat all incoming wastewater due to capacity limitations or other operational constraints, the untreated influent, which may contain high concentrations of MPs, is directly discharged into nearby rivers, lakes, or coastal waters. This can lead to the introduction of MPs into aquatic ecosystems, potentially impacting the environment and wildlife [44, 46]. Therefore, researchers and policymakers need to consider these seasonal variations in WWTP performance when evaluating the overall effectiveness of wastewater treatment in reducing MP pollution. To ensure consistent and effective MP removal, strategies to improve WWTP capacity and resilience during high flow or extreme weather events may be necessary.

3 Removal efficiency of microplastics from wastewater

As an essential component of safeguarding environmental and human health, WWTPs employ multiple treatment stages to reduce the presence of contaminants, including MPs, in effluent wastewater. Figure 1 provides valuable insights into the effectiveness of different treatment processes, shedding light on the challenges and opportunities in mitigating MP pollution in wastewater.

Fig. 1
figure 1

Microplastic removal efficiency across wastewater primary, secondary and tertiary treatment stages

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In the initial stage of wastewater treatment, known as primary treatment, several key components are employed to remove large solid materials and sediments [79]. Screening mechanisms are primarily used to filter out larger debris such as plastics, paper, and other solid objects, preventing them from progressing into treatment. Following screening, the wastewater undergoes sedimentation, where gravitational settling separates suspended solids from the water. During this phase, heavier particles settle to the bottom of settling tanks as sludge, while lighter materials like grease and oil rise to the surface and are skimmed off. Primary treatment processes have demonstrated efficacy in removing a considerable portion of plastic fragments and fibrous residues, reducing the presence of plastics in the pre-treated effluents [44, 48, 61, 79]. Fiber removal rates can reach up to 93% [53], and studies have shown that between 70 and 90% of plastics sized 300 µm or greater are effectively eliminated [36]. However, it’s important to note that this process cannot effectively remove smaller-sized and low-density MPs, as reported by Azizi et al. [80].

In the secondary treatment process of WWTPs, several key steps are typically involved to purify the water further after primary treatment. Aeration is a standard method used in secondary treatment, where oxygen is introduced into the wastewater to promote the growth of aerobic bacteria. These bacteria consume organic matter and other pollutants in the water, helping break them down into simpler compounds. Another common secondary treatment method is the activated sludge process. In this process, a portion of the settled sludge from the primary treatment stage, known as activated sludge, is mixed with incoming wastewater in aeration tanks. Here, the aerobic bacteria present in the activated sludge continue to break down organic matter and pollutants in the water through biological oxidation. Notably, the activated sludge process has shown an average removal rate of 16% for MPs [36]. According to Sadia et al. [31], MPs in settling tank sludges generally have a high density and fiber and fragments form less than 1 mm in size. In some WWTPs, coagulation and flocculation are incorporated into the secondary treatment process to enhance the removal of suspended solids and colloidal particles. Coagulation involves adding chemicals, such as aluminum or iron salts, to the wastewater to destabilize and aggregate fine particles into larger floc structures. Flocculation then promotes the collision and agglomeration of these floc particles, forming larger, settleable masses that can be more easily separated from the water [81]. Coagulation and flocculation can be effective methods for removing MPs, but their efficiency varies depending on their size and density. Alum coagulant, in particular, is efficient for smaller and higher-density MPs, with a removal rate of 70.7%, as reported by Shahi et al. [82]. Additionally, flocculants like ferric sulfate have effectively eliminated small-sized MPs (106 to 300 μm) [47]. However, the elimination remains incomplete, with millions of MPs being released into the environment even after treatment. It is important to note that the efficiency of coagulation and flocculation for MP removal is influenced by factors such as coagulant type, dosage, and the characteristics of the treated MPs. Optimizing these parameters and combining coagulation/flocculation with other treatment steps is necessary for more effective MP removal in WWTPs.

Tertiary treatment in WWTPs involves advanced processes to improve treated water quality further before it is discharged into the environment or reused for different purposes. Chlorination, filtration, and advanced oxidation processes (AOPs) can be employed in tertiary treatment to remove residual contaminants, improve water quality, and meet stringent regulatory standards for wastewater discharge. Chlorination is a low-cost disinfection method commonly used for pathogen removal in water and wastewater treatment. It involves the addition of chlorine or chlorine-based compounds, such as sodium hypochlorite or chlorine gas, to the water or wastewater. Although its effectiveness in eliminating MPs has not been investigated, this process leads to the formation of harmful chlorinated by-products [83]. To address this issue, AOPs, such as UV treatment, ozonation, photocatalysis, and adsorption techniques, enhance the quality of TWW before reuse [83]. AOPs have effectively removed MPs from wastewater, with efficiencies ranging from 60% to over 90% depending on the specific AOP technique and treatment conditions [19]. Hou et al. [84] have recently showcased the effectiveness of various advanced oxidation processes, including H2O2/UV, H2O2/ultrasound (US), O3/UV, photo-Fenton, and UV/US, in removing MPs from wastewater. Photo-Fenton process has also shown superior removal of PVC and PP, particularly at higher temperatures (above 100 °C), with reaction times of 5 h or more, and at pH of 3 or lower, achieving up to 96% removal [85]. The same authors have demonstrated that ozonation increases the fragmentation of MPs, resulting in the formation of new and smaller MPs. Further studies should focus on the toxicity of these newly formed MPs after oxidative treatments. Despite AOPs being a promising alternative for the degradation of MPs, there is still limited research on this subject. Studies on AOPs addressing the degradation of MPs with realistic and detailed operational parameters are needed. Filtration techniques are the most efficient tertiary treatment for removing organic and inorganic contaminants from wastewater [19]. Commonly used filtration techniques in wastewater treatment include sand, activated carbon (biochar), and membrane filtration. Siipola et al. [86] have noted that biochars emerge as efficient filter materials for MPs owing to their exceptional adsorption capacities. The porosity, surface charge, and other inherent characteristics of biochar play significant roles in the retention of colloidal MPs. Membrane filtration encompasses a variety of techniques, including membrane bioreactors (MBR), dissolved air flotation (DAF), disc filters (DF), rapid sand filters (RSF), and dynamic membrane (DM) systems [23, 87]. Among these, MBRs emerge as highly effective methods, achieving up to 99.9% removal, followed by DAF with a removal efficiency of 95%. DF has exhibited removal rates ranging from 40 to 98.5%, while RSF has achieved a removal efficiency of 97% [88]. Despite these advancements, effluents from reverse osmosis (RO) or microfiltration (MF) treatment still contain significant quantities of plastic particles. For instance, Michielssen et al. [49] have found that RO-treated effluents release approximately 10,000,000 particles per day, equivalent to 0.21 plastic particles per liter. In comparison, MF-treated effluents contained around 0.25 plastic particles per liter. To address this issue, researchers have proposed combining RO membranes with ultrafiltration (UF) membranes, a hybrid approach that has shown promise in almost completely removing MPs and NPs from wastewater [23]. Additionally, dynamic membrane (DM) technology has emerged as an alternative to RO for removing MPs due to its highly permeable membrane with large pores [60]. The formation of a cake layer with large suspended solids on the DM acts as a selective barrier, effectively trapping MPs and preventing their passage through the membrane. Nanofiltration (NF) membranes present a compelling alternative with properties lying between UF and RO membranes [23]. They feature selective porous regions and denser sections that facilitate the effective rejection of MPs from water without significant membrane fouling. However, the efficacy of MP removal by NF membranes hinges on pore size. According to Xu et al. [89], the removal of MPs depends on the pore size of the NF membrane. Smaller pore sizes (0.128 nm) effectively reject MPs more than larger ones (1.28 nm). Numerous researchers across the globe are currently developing new membranes with exceptional permeability. These membranes have effectively eliminated light organic molecules and salts smaller than most MPs and NPs [89]. Their exceptional properties make these membranes highly promising for addressing micro and nano-plastic pollution. Additionally, they boast high water production rates and exhibit low costs associated with producing clean water. However, it is important to note that many remain in the laboratory-scale phase and are not ready for industrial production. As a result, they cannot be immediately implemented as a solution to the MP pollution issue. Recently, natural-based wastewater treatment systems, particularly constructed wetlands (CW), have gained more attention than conventional methods due to their energy efficiency, ecological restoration capabilities, and minimal secondary pollutant generation [90, 91]. CW demonstrate optimal removal efficiency for various pollutants, including MPs [90]. Moreover, the interplay among wetland vegetation, substrate materials, soil, and their associated microbial communities has significantly contributed to the removal of MPs [92]. However, there is a lack of studies elucidating the mechanisms underlying MP removal by CW. Further research should delve into the migration and fate of MPs within CW to unravel the intricate response mechanisms involved.

Upgrading WWTPs is critical for improving the removal efficiency of MPs and mitigating their release into the environment. Traditional WWTPs, which typically include primary, secondary, and tertiary treatment stages, are not fully equipped to handle MPs’ minute size and diverse nature. Therefore, adopting advanced technologies and optimizing existing processes are essential to enhance MP removal. Upgrading WWTPs also involves the implementation of robust monitoring systems to assess the performance of MP removal technologies continuously. Regular maintenance of treatment units and timely replacement of membranes are crucial to maintain high removal efficiencies and prevent operational issues.

4 Microplastic in sewage sludge: occurrence and elimination processes

Although WWTPs effectively remove a substantial portion of MPs from the water phase, small and heavy-density MPs are not eliminated [93]. Instead, these particles are transferred and accumulate in the solid phase, particularly within SS [28, 74]. This section explores the occurrence of MPs in SS and the processes involved in their transfer from the water phase to the solid phase during wastewater treatment. Table 1 summarizes various studies that have quantified the presence of MPs in sewage sludge from different regions. The data provided in Table 1 indicates a wide range of SS MP concentrations, underscoring this issue’s global scale and variability. In North America, there is a notable range of MP contamination. New York has recorded 1000 particles per kilogram [55], while Los Angeles has recorded a significantly higher count at 5000 particles per kilogram [48]. Canada has also shown variation, with 4400 to 14,900 particles per kilogram [53]. This range may be attributed to different urban densities, waste management practices, and industrial activities across these regions. Chile (Mellipilla) has presented some of the highest recorded MP contamination levels, ranging from 18,000 to 41,000 particles per kilogram [59]. This high range in South America could be influenced by extensive industrial activities and less stringent waste management regulations. Europe exhibits a broad spectrum of MP contamination levels in soil, with notable high and low spots reflecting the continent’s diverse environmental policies, industrial activities, and waste management practices. High contamination levels have been observed in regions like Poland, ranging from 6700 to 62,600 MP particles per kilogram [41]. Similarly, Finland has shown significant variation, with contamination levels ranging from 23,000 to 170,900 particles per kilogram [57], reflecting its industrial output and the challenges in managing MP pollution despite stringent environmental regulations. Moderate contamination levels have been observed in Ireland, Sweden, and Germany, reaching 15,400, 16,700, and 25,000 particles per kilogram, respectively [34, 35, 56]. Spain presents a stark contrast within its regions. Madrid exhibits highly high contamination levels of 165,000 to 185,000 particles per kilogram, indicative of intensive urban activities and possibly less effective waste management practices [42]. On the other hand, eastern Spain reports much lower contamination levels, ranging from 18 to 32 particles per kilogram, suggesting better waste management and fewer industrial pollutants [63]. In contrast, some European regions demonstrate lower levels of MP contamination. The Netherlands, for instance, has one of the lowest recorded contamination levels at 510 particles per kilogram, indicating highly effective waste management and environmental policies [40]. Similarly, the United Kingdom’s Glasgow shows a lower contamination level of 1000 particles per kilogram [44], which suggests effective urban waste management practices. In China, general assessments have recorded concentrations ranging from 1565 particles per kilogram [58] to a broader range of 1600 to 56,400 particles per kilogram [60, 65]. Notably, one study reported an exceptionally high concentration of 240,300 particles per kilogram [61]. This wide range suggests significant disparities in the sources of pollution, the efficiency of WWTPs, and perhaps differences in the methodologies used for MP detection and quantification. In Beijing specifically, the concentration of MPs in SS is notably lower, with a reported value of 95.2 particles per kilogram [62]. This marked difference from the national averages could indicate more stringent local regulations, more effective treatment processes, or different sources of wastewater input. The concentration of MPs in South Korea has also shown variation, but within a narrower range compared to China. Reported values have ranged from 1620 to 13,275 particles per kilogram [47]. This range indicates a relatively lower upper limit compared to the extremes observed in China, which might reflect differences in industrial activities, waste management practices, and the effectiveness of WWTPs in removing MPs. In Taiwan, the concentration of MPs has been reported to range from 1000 to 7000 particles per kilogram [64]. This range is comparable to the lower to mid-range concentrations observed in South Korea and the lower end of the spectrum reported in China.

Plastic particles in sewage sludge samples exhibit various shapes, including fibers, films, fragments, lines, glitters, flakes, spheres, shafts, and beads [56]. Among these, fibers and fragments are the predominant plastic shapes detected in sewage sludge, accounting for an average percentage range of 60% to 85%. According to Magni et al. [43], the chemical composition of the plastics most frequently detected in SS has revealed the presence of acrylic, acrylonitrile-butadiene, ethyl acrylate, polycarbonate, PA, PS, PES, PE, PP, and PET. The variety of MP types and their polymer compositions found in SS suggest that comprehensive approaches are needed to address the full spectrum of MP pollution. Furthermore, the findings underscore the necessity for developing and implementing advanced wastewater treatment technologies that can effectively remove MPs from both the water and solid phases.

Several treatment processes are employed in WWTPs for SS, including stabilization, dewatering, drying, thickening, composting, anaerobic digestion, and thermal treatment. Lime stabilization, a widely used technique, reduces the volume and weight of the sludge while also managing odor and pathogen content. However, conventional sludge treatment methods like dewatering, drying, and thickening have shown limited efficacy in eliminating MPs [94, 95]. In contrast, composting has emerged as a promising approach that may alter the surface structure of MPs, leading to their reduction. Zhang et al. [93] have reported a 50% decrease in MPs from 353.3 ± 97.0 items/kg in raw SS to 245.6 ± 84.1 items/kg in composted SS. During composting, organic matter undergoes natural aerobic biodegradation mediated by microorganisms, converting fresh organic material into stable organic fertilizer [21]. As the composting process progresses, temperatures gradually rise from ambient levels to approximately 45 °C, reaching thermophilic conditions exceeding 70 °C. This temperature elevation has been shown to facilitate the degradation and fragmentation of MPs, contributing to their reduction in the final compost product. Thermal treatment is a common method for SS management, as it can effectively reduce the mass and volume of the sludge while eliminating certain contaminants. However, research has shown that this treatment process can also significantly impact the characteristics of MPs in the sludge. According to Mahon et al. [56], thermal treatment can cause MPs to meld, wrinkle, and fracture. Similarly, Ducoli et al. [96] have found an increase in the abundance of MPs after thermal hydrolysis of the sewage sludge, indicating that the process can lead to the cracking and breaking of MPs into smaller particles. Furthermore, the residual MPs that remain after thermal treatment can still pose a potential threat to the natural environment. Ducoli et al. [96] have reported that these residual MPs in the bottom ash, often reused in agriculture or construction, can further spread environmental MP pollution. Anaerobic digestion emerges as the predominant method for sewage sludge stabilization, extensively adopted across twenty-four European Union (EU) countries, as highlighted by Gianico et al. [97], and in the United Kingdom, as reported by Harley-Nyang et al. [94]. This widespread utilization of anaerobic digestion in SS treatment underscores its efficacy and reliability as a preferred method for managing SS. Research by Mahon et al. [56] and Horton et al. [8] has demonstrated the effectiveness of anaerobic digestion in reducing the presence of MPs in SS. Mahon et al. [56] have found that the presence of MPs in SS samples was lower after anaerobic digestion (3.9 MPs/g) compared to lime stabilization (12 MPs/g). Similarly, Horton et al. [8] have reported lower MP concentrations in anaerobically digested sludge (301 particles/g) compared to lime-stabilized sludge (10,380 particles/g). However, it’s worth noting that the MP concentrations reported in these studies have been higher than those reported across the broader literature. This suggests that while anaerobic digestion is generally effective in reducing MPs in sewage sludge, the extent of MP removal may vary depending on the specific treatment conditions, the characteristics of the sewage sludge, and the types of MPs present.

Recent studies have introduced advanced treatment technologies that have demonstrated effectiveness in eliminating MPs from SS without simply breaking them down into smaller particles. These strategies include pyrolysis, biodegradation, hydrothermal carbonization (HTC), hydrothermal liquefaction (HTL), vermin-wetland, and enzymatic degradation. Pyrolysis involves the decomposition of sewage sludge at elevated temperatures under anaerobic conditions, yielding high-calorific-value liquid and gaseous products alongside biochar, which can be beneficial as a soil fertilizer [98, 99]. Ni et al. [98] have observed a significant reduction in the concentration of MPs like PE, PP, PS, and PA using pyrolysis. The best removal rates have been obtained at 350 °C (99.1–99.4%) and 450 °C (99.8%), with a heating rate of 10 °C/min and a holding time of 30 min. HTC is another promising technology for converting SS into a nutrient-rich carbonaceous hydrochar under moderate temperature (180–260 °C) and autogenous pressure for several hours. Recent studies have shown the effectiveness of HTC in removing MPs from SS. Xu and Bai [100] have reported that HTC treatment at 260 °C reduced the abundance of MPs in the sludge by 79.71%. The study found that HTC could completely remove certain types of MPs, such as PET, PA, PS, and PU, from the sludge. However, the reduction rates have been lower for other MPs like PP (79.34%) and PE (55.93%). HTL is an emerging technology for the treatment of wet SS. The process involves subjecting the sludge to high temperatures (250–400 °C) and pressures (4–22 MPa), which converts the organic matter in the sludge into a valuable bio-crude oil [101, 102]. An important aspect of HTL is its ability to reduce the risk posed by micro-pollutants, such as MPs, present in the SS. According to a study by Chand et al. [102], the HTL process under supercritical conditions (temperature: 400 °C; pressure: 30 MPa) has significantly reduced the mass of MPs by as much as 97%.The high-temperature and high-pressure environment of the HTL process appears to be effective in breaking down and degrading the MPs present in the SS. This is a significant advantage of HTL over other sludge treatment methods, as it converts the organic matter into a valuable bio-crude and effectively mitigates the environmental risks associated with MPs in the treated sludge. The findings from Chand et al. [102] have highlighted the potential of HTL as a promising technology for managing SS, particularly in addressing the growing concern over MP pollution. Further research and development in this area could lead to the widespread adoption of HTL as a sustainable and effective solution for sewage sludge treatment and mitigating MP contamination. Vermi-wetlands are an environmentally friendly and economical method that uses the synergistic action of earthworms, microorganisms, and plants to recycle excess SS. In this process, the plant rhizospheres play a crucial role in intercepting sludge flocs and providing oxygen for the earthworms and microorganisms, thereby enhancing sludge degradation [92]. In a recent study, Ragoobur et al. [103] observed significant reductions in the abundance of PE and PP of different size ranges (146–500 and 1650–2000 μm) after 14 weeks of vermicomposting. FTIR analysis has also revealed a 34% and 11% reduction in the absorbance of alkane groups for PE and PP, respectively. Similarly, Nie et al. [104] have investigated the performance of vermi-wetlands in removing MPs from excess sludge in the laboratory. The authors have reported a 100% reduction in 500 μm MPs, 95.44–99.52% reduction in 100 μm MPs, and 86.62–95.69% reduction in 1 μm MPs, suggesting an effective interception of MPs in the vermi-wetland system. Furthermore, the study has found that all the MPs were detected in the earthworm excrement. In contrast, only 1 μm MPs have been found in the earthworm’s digestive organs, indicating the important role of bioturbation in mobilizing MPs within the vermi-wetland. Another promising treatment technology is bioremediation through bacterial and enzymatic degradation processes. For instance, Chen et al. [105] have investigated the biodegradation of PS using inoculum at 70 °C for 56 days, achieving a degradation rate of 7.3%, significantly higher than conventional thermophilic composting at 40 °C. This enhanced efficiency was attributed to the prevalence of Thermus, Geobacillus, and Bacillus bacteria, which have effectively degraded plastic structures. Vinay et al. [95] reported the enzymatic degradation of HDPE beads using three hydrolytic enzymes: lipase, cellulase, and protease. Protease was found to be the most effective, removing 4% of the initial bead mass at a thermophilic condition (55 °C) with an enzyme concentration of 88 mg per liter in a 3-day batch experiment. Increasing the enzyme concentration and temperature further enhanced the degradation of HDPE beads. These advanced and eco-friendly treatment technologies offer promising solutions for effectively removing MPs from SS without simply breaking them down into smaller particles. Further research and development in this area are crucial for developing sustainable and efficient strategies to mitigate MP pollution from SS.

5 Soil contamination by microplastics following the reuse of treated wastewater and sewage sludge

Reusing TWW and SS contaminated with MPs poses significant risks to soil ecosystems. Studies have shown that applying SS containing MPs as a fertilizer in agricultural soils can lead to substantial MP pollution, potentially making these lands one of the largest global reservoirs of MP contamination. Table 1 summarizes the concentrations of various types of MPs found in soil across different global locations following the reuse of TWW and SS. In America, various regions have reported diverse concentrations of MPs in soil. Zubris and Richards [55] have been the first to precisely report the presence of MPs in New York, with 1235 particles per kg. In Canada (Ontario), levels have ranged from 4 to 541 particles per kg [74]. Huerta Lwanga et al. [13] have reported 2770 particles per kg in Mexico, specifically Campeche. In Chile, Corradini et al. [59] have found an even more alarming concentration of up to 3500 MP items per kilogram of agricultural soil in Chile after 10 years of sewage sludge disposal. The study has also observed an increasing trend in MP abundance with higher sludge application rates. In China, studies have revealed varying levels of MP concentrations in soil. In Shanghai, MP levels have ranged from 68 to 2131 particles per kg [61, 75]. A study by Zhang et al. [71] has found an alarming concentration of approximately 18,760 MP items per kilogram of soil in areas with intensive agriculture that utilized sewage sludge (applied at around 23 tons per hectare annually) and wastewater irrigation. Further research by Zhang et al. [93] in Guilin City, China, has revealed that the total abundance of MPs in soils amended with 30 tons per hectare and 15 tons/ha of composted sludge were 545.9 and 87.6 items per kilogram, respectively. These values have been significantly higher than the 5.0 items per kilogram found in soils without sewage sludge application. The issue is not limited to China; European studies have investigated the extent of soil pollution with MPs following the reuse of TWW and SS. Horton et al. [68] have reported 660 particles per kg in the United Kingdom. Switzerland’s soils contained 593 particles per kg [14]. In eastern Spain, MP levels in soil have been reported to vary between 930 and 3060 particles per kg [63]. Another study in Spain (Madrid) has shown the presence of 1013 particles per kg [42]. Recent studies have highlighted the significant impact of municipal sludge on agricultural land, particularly in Germany. Weber et al. [78] have investigated the long-term effects of SS application on the dispersion of MPs in Germany. The study has examined sites that had received sludge application at rates of 120–200 tons per hectare over 30 years (1973–1986, with a 3-year application interval). The results have shown that the average MP concentration at the sludge-treated sites ranged from 9.10 to 31.80 particles per kilogram of soil. Importantly, the MPs have been primarily accumulated in the upper soil layer (0–30 cm), highlighting the limited vertical mobility of these particles within the soil profile. The color distribution of the plastic items has been predominantly white (41.1%), followed by blue (14.0%), transparent (13.5%), black (7.8%), and smaller amounts of green, grey, orange, pink, red, yellow, and multi-colored fragments. Further analysis using micro-Fourier Transform Infrared Spectroscopy (µFTIR) has revealed a dominance of PS, PP, and PE in the soil samples. Other polymers have been identified, including PET, PVC, polysulfone, nylon 6, and various synthetic resins. Tagg et al. [77] have studied the accumulation of MPs in soil samples following the application of SS at 190 tons/ha. The results have shown that the soil sample with the highest MP load has been detected at the surface of the sludge-applied soil, with a total MP load of 14.6 MP/g. Importantly, this sludge-applied field is a source of contamination for the surrounding areas, echoing similar observations made by Weber et al. [78]. These findings underscore the potential for further uncontrolled contamination when municipal sewage sludge is applied to agricultural land. This highlights the need for future research better to understand the extent of these effects on real farms, ultimately informing agrarian policy.

In Africa, research on MPs remains limited compared to other regions, accounting for only 4% of global studies. One notable study by Boughattas et al. [76] has focused on the contamination of Tunisian agricultural soils in North Africa resulting from using TWW for a decade. The concentrations of MPs have ranged from 13.21 to 852.24 items per kilogram, with a prevalence of small-sized particles measuring between 0.22 and 1.22 µm. The dominant types of MPs identified in the soil samples were PE and polybutyrate adipate terephthalate (PBAT). PBAT is a relatively new biodegradable polymer that has gained significant attention recently as a potential alternative to conventional non-biodegradable plastics. However, the presence of PBAT in the environment, particularly in agricultural soils, has not been widely reported in the literature until recently. The findings from this study highlight the importance of further research and monitoring efforts to understand the extent of MP contamination in African soils, particularly in regions where wastewater reuse is common. Addressing the challenges posed by MP pollution in agricultural systems requires a comprehensive approach that integrates sustainable wastewater management practices and effective strategies to mitigate the impacts of MPs on soil ecosystems and human health.

As reported in these studies, the wide variations in MP concentrations across different regions of the same continent likely reflect diverse environmental and anthropogenic factors that influence MP pollution levels. Lower MP concentrations in certain areas might be attributed to effective waste management practices and lower industrial discharge. Conversely, higher concentrations suggest localized hotspots of MP pollution, likely influenced by intensive industrial activities, poor waste disposal practices, or heavy urbanization in those regions. These discrepancies in reported MP levels can also be partially explained by differences in sampling locations, such as rural versus urban areas, agricultural lands versus industrial sites, and varying MP extraction and quantification methodologies across the different studies. In future research, standardized sampling protocols and analytical techniques will be important for accurately comparing MP pollution levels between regions.

6 Potential remediation techniques in soil contaminated with microplastics

The transfer of MPs from soil to plants can have severe implications for human health. Plants can ingest MPs, entering the food chain through contaminated crops. This can lead to the accumulation of MPs in the human body, potentially causing adverse health effects. Understanding soil remediation techniques to mitigate the risks associated with soil contamination by MPs is essential. Soil remediation involves removing or degrading contaminants from soil to restore its natural functioning. The primary goal of soil remediation is to eliminate or reduce the concentration of MPs in the soil to prevent their transfer to plants and humans. Biodegradation and phytoremediation are two techniques that can be used to eliminate MPs from soil. Biodegradation is a natural process in soil, where soil fauna, such as earthworms, snails, and mealworms, break down organic pollutants, including MPs. These soil animals ingest MPs and use specific enzymes with the help of their gut microbiota. These enzymes, which include laccase, esterase, peroxidase, oxidoreductase, and hydrolases, help to break down the larger polymer chains into smaller units. Through this enzymatic action, MPs are ultimately mineralized into carbon dioxide (CO2), water (H2O), and methane (CH4), effectively removing them from the soil environment. Bioreactors can enhance this process, providing a controlled environment for microorganisms to thrive. The table below (Table 2) summarizes selected studies on the degradation of MPs by various microorganisms. Pseudomonas sp. has shown a significant ability to degrade pre-treated PP, with weight loss percentages ranging from 0.6 to 1.5% [106]. Additionally, Pseudomonas sp. has demonstrated a 17.3% decrease in PP over 40 days [107]. Pseudomonas putida has completely degraded PVC in less than 2 days [108]. Rhodococcus ruber has shown a small reduction in PS (0.8% within 8 weeks) and a 7.3% decrease in PP within 40 days [107]. Actinobacteria have also garnered significant attention for their ability to degrade rubber. These microorganisms contain genes or enzymes, specifically rubber oxygenase, known as latex-clearing protein (LCP) [109]. This enzyme cleaves the isoprene double bond in rubber, breaking it down into carbon and energy sources for the bacteria. Rosleateles depolymerans strain TB-87 has shown a slow rate of degradation for polybutylene succinate, polyethersulfone, and polycaprolactone, but not for PLA and polyhydroxybutyrate-co-hydroxyvalerate [110]. Enterobacter asburiae and Exiguobacterium, isolated from the guts of Indian meal moths, have demonstrated efficiency in degrading PE, with an 8% and 7.4% weight loss after 60 days of incubation, respectively [111]. Exiguobacterium sp. YT2, isolated from mealworm guts, was found to degrade PS within 12–24 h [112]. The larvae of Tenebrio molitor Linnaeus (mealworms) have been shown to degrade PS foam within 12–24 h [13] and reduce PE by 40.1% and PS by 12.8% within 32 days [113]. As per Wang et al. [114], larvae of superworms (Zophobas atratus Fab.) and yellow mealworms (Tenebrio molitor Linn.) can thrive on diets consisting solely of plastic.

Table 2 Selected studies on the degradation of different types of microplastics by microorganisms
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Superworms were observed to consume 49.24 mg of PS per larva and 26.23 mg of PU per larva cumulatively, which were 18 and 11 times higher than the consumption levels of yellow mealworms, respectively. Lumbricus terrestris (earthworms) has been reported to facilitate the size reduction of PE within 4 weeks [13]. These findings highlight the varying effectiveness of different microorganisms in degrading various types of plastics. The findings underscore the importance of selecting appropriate microbial strains and treatment conditions for the efficient degradation of different MPs. While some microorganisms can achieve rapid and significant degradation, others may need more optimized conditions to achieve similar efficacy [115]. Notably, Pseudomonas sp. and Tenebrio molitor Linnaeus have demonstrated particularly high efficiency in degrading several MPs. The data suggests that leveraging these microorganisms could be a promising approach to mitigate MP contamination in different environments. A novel approach to mitigating MPs’ risks on soil microbial rzhizosphere involves incorporating biochar into the soil. Wu et al. [116] have found that combining MPs with biochar significantly increased total biomass, indicating that biochar can reduce the inhibitory effects of MP accumulation on sugarcane growth. This finding is consistent with other studies, which have shown that biochar can alleviate the adverse effects of PVC on crop yield, soil enzyme activity, and microorganisms [20, 98, 99]. Furthermore, research has demonstrated that biochar addition can enhance shoot dry matter production in soils contaminated with various concentrations of PVC. However, the effects and mechanisms of biochar application on sugarcane growth, soil nutrients, and microbial communities in soils contaminated with various concentrations ofMPs remain unclear and require further investigation.

Phytoremediation is an additional technique used for soil remediation, specifically in the context of plastic contamination. This method uses vegetation and associated microbiota to eliminate, confine, or render environmental contaminants harmless [117, 118]. Recent studies have demonstrated the potential of plants to extract MPs from contaminated soils [30, 60]. Phytoextraction or phytoaccumulation is a technique that involves the uptake of MPs by plant roots from the soil and their translocation to the above-ground tissues of the plants. Li et al. [60] have demonstrated the entrapment of 0.2 μm PS microbeads in the root cap of an edible lettuce plant, emphasizing the importance of root contact in phytoremediation techniques. Phytostabilization is another phytoremediation technique that aims to immobilize MPs in the soil. Certain plants are used to absorb and accumulate MPs onto their roots or precipitate them in the rhizosphere, reducing their mobility and making them less available or unavailable for leaching into the soil profile and potentially contaminating groundwater [119]. This approach prevents the mobility of MPs and minimizes their transfer through the food chain. While there is currently no information on the immobilization of MPs in terrestrial plants, hydrophytes have been documented to adsorb MPs from water. For instance, Rozman et al. [120] have reported that MPs had no significant effect on Lemna minor (duckweed) growth over 12 weeks. Similarly, Arikan et al. [121] have found no growth restriction in Lemna minor under 100 mg per liter of small-sized PS exposure. Phytofiltration, also known as rhizo-filtration or blasto-filtration, is a technique in which plant roots or seedlings absorb or adsorb MPs, similar to phytoextraction. After absorption or adsorption, the roots are harvested and safely disposed of, removing the MPs from the environment [122]. Rhizoremediation has been identified as a promising approach for managing MPs and NPs. This method involves the action of rhizosphere microorganisms, which play a crucial role in detoxifying plants from MPs by promoting plant growth. Roy et al. [123] have explained that plants resist the MP uptake process by releasing mucilaginous exudates rich in organic acids and amino acids that envelop the MPs and prevent their entry into the plant. Omidoyin and Jho [124] have reported the efficiency of various microorganisms, including Cyanobacteria, Actinobacteria, Bacteroidetes, Gemmatimonadetes, Proteobacteria, Ascomycota, and Basidiomycota, in conjunction with root exudates, in offering promising potential for MP rhizoremediation. These microorganisms can potentially utilize the MPs as a carbon source, enhancing their biodegradation and removal from the environment.

Several environmental factors substantially influence bioremediation, including pH, moisture content, and the molecular weight of plastic particles. pH influences the survival of microorganisms and the hydrolytic cleavage of plastic fragments, with higher moisture content enhancing the hydrolytic activity of microbes [122]. Higher molecular weight corresponds to a lower degradation rate, whereas low molecular weight MPs like PES are more easily biodegraded [125]. The biodegradation process also depends on the mode of action of the microorganisms involved and their optimal growth conditions in soil. The biodegradation of polymers can be influenced by their intrinsic properties, such as flexibility, friability, glass transition temperature, melting temperature, modulus of elasticity, molecular weight, nature of functional groups, attached substituents, and the presence of additives [126].

7 Impact of microplastics on the soil ecosystem and human health

MP particles are pervasive pollutants in soil environments, originating from various sources, including using TWW and SS. These particles interact with soil components, affecting soil health and potentially posing risks to human health through the food chain. Figure 2 illustrates the complexity of MP contamination in soil, depicting their sources, interactions, and ultimate fate. This figure underscores the multifaceted nature of MP pollution and highlights the critical pathways through which these particles can influence the soil ecosystem and human well-being.

Fig. 2
figure 2

Various pathways through which microplastics impact soil ecosystems

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MPs in soil can adversely affect various soil properties, including physical characteristics such as structure, aggregation, bulk density, and water-holding capacity [70]. Studies have shown that MPs can interact with soil organic matter and minerals, leading to their persistence in the soil longer than nutrients, potentially impacting soil ecosystems [127]. Furthermore, certain MPs, like PE, have been observed to alter soil pH, consequently increasing the mobility and bioavailability of specific contaminants such as trace metals [119, 128]. Recent research has also highlighted the ability of MPs to attract a wide range of microbial pathogens that could pose risks to human health and potentially exhibit resistance to antimicrobial treatments [129]. In a study by Wagstaff et al. [130], the researchers confirmed MPs’ capacity, particularly PA, to adsorb pharmaceuticals, with a stronger affinity observed for more hydrophobic compounds. Similar observations were made by Zhang et al. [131], showing the adsorption of residual organochlorine pesticides on PE due to plastic’s extensive surface area and hydrophobic characteristics. These findings underscore the ability of MPs to act as vectors for the transport and accumulation of a wide range of toxic contaminants in the environment, potentially impacting both aquatic and terrestrial ecosystems. MPs can be transported from soil to surface and groundwater bodies, which humans often exploit for drinking, irrigation, and fishing. Additionally, MPs can enter plant tissues and accumulate in different plant organs, such as leaves, stems, flowers, and even fruits [60, 116, 123]. Recent research by Yu et al. [30] has shed light on the issue of micro and NP accumulation in crops, which pose serious food safety risks. The study found that  micro and NPs can accumulate in the roots of root crops, such as carrots, radishes, onions, and potatoes. This is particularly alarming, as humans commonly consume these root vegetables and can directly transfer the plastic particles into the food chain. Furthermore, the researchers have also observed these tiny plastic particles accumulating in the aboveground parts of general crops, including wheat, rice, and lettuce. This indicates that the contamination is not limited to root vegetables but can also affect a wide range of agricultural produce we consume regularly. These findings highlight the urgent need for comprehensive strategies to address MP pollution and its impacts on soil health and agricultural sustainability. Despite these concerns, the research results haven’t mentioned any specific regulations or guidelines for the presence of MPs in SS or TWW. While EU countries have restrictions on applying sewage sludge to agricultural land, these are primarily focused on other contaminants like heavy metals and organic pollutants, not MPs [78]. The lack of targeted regulations highlights the need for policymakers to address this emerging environmental issue. The regulation of reclaimed wastewater and sewage sludge reuse is of utmost importance, especially in the absence of established threshold values for soil MPs. Therefore, initiatives aimed at removing MPs from wastewater treatment plants and developing robust legislation and policies are urgently needed to confront this issue and ensure the preservation of soil and water quality. Additionally, comprehensive monitoring programs should be implemented to track the presence of MPs in TWW and SS, enabling informed decision-making regarding their reuse in agricultural and other applications while mitigating the potential risks of leaching and groundwater contamination. Overall, a concerted and multifaceted approach involving scientific research, policy development, and public awareness initiatives is essential to effectively tackle the pervasive issue of MP pollution and safeguard the integrity of our soil and water resources for future generations.

Ingestion of MPs through contaminated water or food can potentially harm human health, although the long-term impacts are still being investigated. A recent study by Leslie et al. [29] has raised significant concerns about MPs in human blood. The researchers have investigated the presence of plastic particles as small as 700 nm in blood samples from 22 healthy volunteers in the Netherlands. The study has found that the mean sum of the quantifiable concentration of plastic particles in blood was 1.6 μg/ml. This is a concerning finding, suggesting that MPs are present in the human body and circulating in the bloodstream. Various types of plastics were identified in the blood samples. Half of the samples contained PET, commonly used in drink bottles. One-third of the samples contained PS, used for packaging food and other products. A quarter of the samples contained PE, from which plastic carrier bags are made. Another study by Zhao et al. [132] reported MPs’ presence in reproductive organs and bodily fluids, specifically in testis and semen samples. The average concentration of MPs has reached 0.23 particles per mL in semen and 11.60 particles per gram in testis. MPs in the testis are primarily composed of PS, accounting for 67.7%. In contrast, PE and PVC were the dominant polymers in semen samples, most falling within the 20–100 μm size range. Research has revealed that MPs have been detected in 15 human body parts, including the lungs, liver, and placenta [133]. According to Lee et al. [134], exposure of human cells to MPs such as PE and PS has been found to cause inflammation, oxidative stress, disturbances in lipid metabolism, gut microbiome imbalance, and even neurotoxicity. As a society, we must take action to mitigate the risks posed by MPs to human health and the environment [135,136,137,138,139]. This includes implementing stricter regulations on plastic production and waste management, investing in research to develop safer alternatives, and educating the public about reducing plastic consumption.

8 Conclusions and recommendations

MPs have become a pervasive environmental pollutant, and their presence in the TWW and SS poses significant risks to ecosystems and human health. This review provides a comprehensive analysis of the sources, occurrence, and fate of MPs in WWTPs, as well as the subsequent contamination of agricultural soils following the reuse of TWW and SS. Studies have reported diverse concentrations of MPs in soil, ranging from 1000 to 18,760 particles per kilogram, with the application of SS being a major contributing factor. Reusing TWW and SS contaminated with MPs can lead to substantial MP pollution in agricultural soils, potentially making these lands one of the largest global reservoirs of MP contamination. The review also highlights the efficiency of various treatment technologies in removing MPs from water and sludge. Membrane filtration, particularly MBRs, can achieve up to 99.9% removal of MPs. AOPs like UV, ozonation, and photocatalysis can remove 60–90% of MPs, while combining RO with UF can almost completely remove MPs and NPs. Soil remediation techniques, such as biodegradation and phytoremediation, offer promising solutions for eliminating MPs from contaminated soils. Microorganisms like Pseudomonas sp. and Tenebrio molitor Linnaeus (mealworms) demonstrate high efficiency in degrading several plastic types. Biochar addition can also alleviate the adverse effects of MP pollution on the soil ecosystem. However, the review also raises concerns about the presence of MPs in human blood, with a mean sum of quantifiable concentrations of 1.6 μg/ml. MPs have been detected in 15 human body parts, including the lungs, liver, and placenta. Exposure to MPs such as PE and PS has been found to cause inflammation, oxidative stress, disturbances in lipid metabolism, gut microbiome imbalance, and even neurotoxicity.

In this regard, legislation and regulations must be strengthened to recognize MPs as contaminants and establish source control, waste management, and clean-up measures. This includes regulating the application of sewage sludge on agricultural fields to minimize microplastic contamination. Upgrading wastewater treatment plants is crucial for efficiently removing MPs; membrane technology shows promise. However, further research is needed, especially in regions with limited data on MP presence. Specific regulations should be developed to characterize MPs in wastewater and sewage sludge, enabling effective monitoring and management of contamination levels in agricultural soils. Prohibiting the addition of MPs to consumer products and encouraging the use of biodegradable alternatives can prevent further contamination. Public awareness campaigns and responsible consumption initiatives are essential for reducing plastic demand consumption and promoting recycling. Government intervention through environmental taxes and awareness campaigns can incentivize responsible plastic use. Additionally, efforts should focus on preventing plastic contamination at both household and industrial levels. By implementing these recommendations, stakeholders can mitigate the risks posed by MP contamination in soil and protect environmental and human health.