Microplastic Behavior in Sludge Pretreatment and Anaerobic Digestion: Impacts, Mechanistic Insights, and Mitigation Strategies

Abstract

Microplastics (MPs) are increasingly reported as contaminants in sewage sludge, with wastewater treatment plants retaining approximately 10³–10⁶ particles kg⁻¹ of dry sludge. Anaerobic digestion (AD), widely applied for sludge stabilization and energy recovery, does not consistently remove these particles; MPs frequently persist and, at elevated or sensitive loadings, have been shown to affect methane production, microbial communities and sludge quality. In parallel, thermal hydrolysis and related pretreatments are being implemented at full scale to enhance sludge biodegradability, exposing embedded MPs to high temperature and pressure prior to AD. This review compiles and analyzes experimental studies on MPs in sludge pretreatment and AD systems, with an emphasis on how pretreatment severity, MP type, particle size and concentration influence MP transformation and process performance. Reported data indicate that intensified pretreatment accelerates MP aging, causing fragmentation, oxidative surface modification and additive release, while subsequent AD generally induces limited further MP degradation but can be negatively affected through reduced methane yields, shifts in microbial consortia and altered behavior of co-contaminants. Mechanisms implicated include leaching of plastic additives, enhanced oxidative and physiological stress, and formation of plastisphere biofilms that perturb syntrophic interactions. Mitigation approaches, including optimized thermal hydrolysis–AD configurations and the use of carbonaceous sorbents, are assessed with regard to their effects on MP-associated inhibition and their practical constraints. Analytical limitations, uncertainties in MP mass balances and environmental fate, and key research needs for evaluating MP risks and designing MP-resilient sludge treatment and biosolid management strategies are identified.

1. Introduction

Microplastics (MPs), typically defined as plastic particles < 5 mm in size, and nanoplastics (<1 µm) are now recognized as ubiquitous pollutants in aquatic and terrestrial environments [1,2]. Municipal wastewater has been identified as a major pathway for MPs originating from personal care products, synthetic textiles and diverse urban and industrial sources [3,4]. During conventional wastewater treatment, a high fraction of influent MPs is removed from the water phase and transferred to sewage sludge, with overall removal efficiencies of approximately 95–99% and preferential partitioning to the sludge rather than the effluent [3,5,6,7]. Recent surveys have reported MP concentrations in sewage sludge in the order of 10³–10⁴ particles kg⁻¹ dry solids, with higher levels observed at some facilities depending on catchment characteristics, sampling, and analytical methods [7,8,9]. The dominant polymers detected in sludge include polyethylene (PE), polypropylene (PP), polyester (including PET) and polyamides, predominantly present as fibers and fragments [3,8]. When biosolids are applied to land, these sludge-associated MPs may be transferred to soils and accumulate over time, providing an important pathway for MP input to terrestrial systems [10,11].

Anaerobic digestion (AD) is widely applied for sludge stabilization and energy recovery in wastewater treatment plants [12]. Concern has been raised about how sludge-borne microplastics (MPs) influence AD performance and whether MPs are transformed during digestion. Early observations reported lower MP counts in digested biosolids than in raw sludge (e.g., Mahon et al., 2017), which was interpreted as an apparent decrease in MP abundance [13]. Subsequent studies, however, have generally shown that common polymers—such as polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS) and polyvinyl chloride (PVC)—often persist through AD and can reduce methane production by approximately 5–30% under typical mesophilic conditions, particularly at elevated experimental doses relative to typical sludge levels [14,15,16,17,18]. Reported inhibitory mechanisms include (i) leaching of plastic additives (e.g., bisphenols, phthalates) that impair microbial activity [15,18], (ii) enhanced oxidative and physiological stress and physical sorption effects that hinder enzymatic hydrolysis [14,19,20], and (iii) plastisphere-mediated shifts in microbial community structure that disturb syntrophic interactions required for methanogenesis [14,21,22].

In parallel with these developments, wastewater utilities have increasingly adopted sludge pretreatment technologies to improve AD performance. Among these, thermal hydrolysis (TH) has gained prominence as a means to solubilize sludge solids and enhance biogas yields [23]. Commercial TH systems (e.g., Cambi, Veolia BioThelys) subject dewatered sludge to high temperatures (typically 140–180 °C) and pressures (~4–8 bar) for brief durations (20–60 min), effectively “pressure-cooking” the sludge to break cell walls and unwind extracellular polymeric matrixes [23,24,25,26,27]. TH prior to AD can significantly increase the release of soluble organics (chemical oxygen demand) and thereby boost subsequent methane generation by 20–50% [23,25,28]. However, while TH is not designed with MPs in mind, these extreme conditions inevitably also expose embedded MP particles to thermal and chemical stress. It is reasonable to expect that MPs will undergo accelerated aging during thermal hydrolysis, potentially fragmenting into smaller pieces or undergoing surface oxidation/hydrolysis [29,30,31]. In turn, the altered MPs (and any leached additives) could influence the sludge solubilization process and carry through to the AD phase with modified reactivity. Until recently, this aspect had been largely overlooked [29,30,32].

A structured narrative approach was adopted to compile and analyze relevant studies. Literature was identified through searches in Web of Science Core Collection, Scopus, ScienceDirect and Google Scholar using combinations of keywords related to microplastics/nanoplastics, sewage sludge, anaerobic digestion, thermal hydrolysis and other sludge pretreatments, as well as mitigation and adsorbent strategies. The search primarily covered the period from 2000 to May 2025 and focused on peer-reviewed journal articles and major review papers directly addressing MPs in sludge pretreatment, AD performance, MP fate and control options. Additional sources were obtained by screening the reference lists of key experimental and review articles. In view of the mechanistic and integrative objectives, evidence was synthesized using a structured narrative framework rather than a formal PRISMA-type systematic review, with emphasis placed on studies providing process-relevant data, mechanistic interpretation and implications for full-scale applications.

This review examines the interactions between microplastics, sludge pretreatment processes—with particular emphasis on thermal hydrolysis—and anaerobic digestion. First, reported behavior and transformation pathways of MPs under sludge pretreatment conditions are summarized, highlighting recent work on thermal hydrolysis and related technologies. Second, the impacts of MPs on AD performance are analyzed, including effects on methane production, microbial community structure, key biochemical conversion steps and the fate of MPs within digesters. Third, mitigation strategies are evaluated, covering process-based measures (e.g., optimized TH–AD configurations and operating conditions), the application of carbonaceous sorbents and other engineering interventions, and their technical and practical constraints. Finally, remaining knowledge gaps, analytical limitations and priority research directions are identified, with particular attention to MP mass balances and downstream environmental fate, to support the development of robust, MP-resilient sludge treatment and biosolids management systems.

2. MP Behavior and Influence During Sludge Pretreatment

Sludge pretreatment processes impose thermal, mechanical and chemical stresses on both the sludge matrix and embedded microplastics (MPs). This section focuses on exposure conditions and resulting MP transformations under common pretreatments, with particular emphasis on thermal hydrolysis (TH) as a widely implemented high-severity option [23]. Key effects include fragmentation, surface oxidation or hydrolysis, and additive release, which can modify MP properties and potentially affect sludge solubilization and subsequent AD behavior; other methods such as thermo-alkaline treatment and ultrasonication are briefly considered for comparison [28,29,30,31].

2.1. Exposure Conditions in Sludge Pretreatment

TH exposes sludge to temperatures and pressures well above those in conventional biological treatment [33]. Pre-AD TH processes typically operate at ~140–180 °C and ~4–8 bar for 20–60 min, followed by rapid depressurization to “flash” thickened sludge (about 15–20% dry solids) in batch or continuous reactors, as in Cambi^® and BioThelys^® systems [32,33,34]. On this basis TH is now widely applied as a mature municipal sludge pretreatment: Barber (2016) [23] reported 75 facilities in operation or planning, and more recent provider data indicate an expanded global fleet, with Cambi alone reporting on the order of 80–90 installations and additional capacity from other vendors (Cambi, data accessed May 2025). Under such conditions, pathogens are inactivated, cell walls and extracellular matrices are disrupted, and a substantial share of biopolymers is solubilized [35,36]. The same hydrothermal environment is expected to impose intensified aging stresses on entrained MPs, including fragmentation, surface oxidation or hydrolysis and additive release, relative to conventional AD conditions [31].

During TH, MPs are exposed to elevated temperatures in liquid water and, in some cases, to pronounced pH shifts when chemicals are added. At 140–180 °C, the properties of water favor hydrolytic and oxidative reactions, making these conditions relevant to polymer aging [37]. In thermo-alkaline pretreatment, high temperature is combined with alkaline reagents (e.g., NaOH), typically yielding pH > 10 at >150 °C; such conditions enhance sludge solubilization and can promote base-catalyzed hydrolysis of susceptible polymers and ester-based plasticizers [38]. Neutral and alkaline hydrolysis of PET in hot compressed water at higher temperatures (220–270 °C) is well documented and illustrates the intrinsic reactivity of ester linkages, even though such conditions exceed typical full-scale TH windows [39,40]. Even without added chemicals, TH temperatures can exceed the softening range of some polyolefins and approach regimes where thermo-oxidative degradation becomes relevant, while PET may undergo partial hydrolytic or oxidative modification in hot aqueous media [31,40]. Exposure of MPs to hydrothermal conditions in the 120–180 °C range has been shown to induce marked physical and chemical changes: Han et al. reported increased friability, surface fragmentation and formation of oxygenated and unsaturated end-groups in PE and PET [29], and Li et al. observed higher carbonyl indices and enhanced surface oxidation of sludge-embedded MPs after treatment at 180 °C for 30 min [31]. Together with evidence for base- and neutral hydrolysis of ester functionalities and the susceptibility of phthalate-type additives under alkaline conditions [41,42], these results indicate that TH- and thermo-alkaline-type pretreatments can accelerate MP aging, producing more fragmented, oxidized and polar particles that are subsequently carried into the AD stage.

For thermo-alkaline pretreatment, where alkaline reagents (e.g., NaOH) are applied prior to heating to approximately 70–100 °C or in combination with thermal hydrolysis, MPs are exposed to a more aggressive chemical environment [43]. Strongly alkaline conditions can deprotonate and swell polymer surfaces, which is expected to enhance the release of certain additives (such as fatty acid-based plasticizers that may undergo saponification) and to promote cleavage of ester linkages in susceptible polymers, including polyesters and polyurethanes [42]. Direct experimental data on MP behavior under full-scale thermo-alkaline sludge treatment remain limited; however, inference from established polymer chemistry indicates that polymers such as PET are prone to base-catalyzed hydrolysis at elevated pH and temperature, yielding soluble monomers such as terephthalate and ethylene glycol under sufficiently harsh conditions [43,44].

Ultrasonic pretreatment, typically operated at frequencies on the order of 20 kHz, is primarily applied to disrupt sludge flocs and enhance solubilization by acoustic cavitation, but can also impose strong shear and impact forces on suspended particles. Experimental observations indicate that ultrasonication is able to fragment MP particles or erode their surfaces, with potential generation of secondary smaller debris, including nanoplastics [45,46]. In one study, application of 80 kHz ultrasound to activated sludge facilitated the detachment and removal of approximately 20–50% of spiked PE microbeads, demonstrating that ultrasonic fields can physically act on MPs, although in that case the effect was exploited as a separation step rather than as a degradation mechanism [47].

Recent experimental work has shown that hydrothermal conditions relevant to TH can induce pronounced structural changes in MPs. Han et al. (2024) exposed PE and PET MPs to sludge thermal hydrolysis at 120–180 °C and reported temperature-dependent increases in friability, surface fragmentation and chemical modification [29]. At around 160 °C for PE and 140 °C for PET, thermo-oxidative reactions were inferred from chain scission and the formation of oxygenated end-groups (e.g., hydroxyl and carboxyl), consistent with hydrolytic and oxidative cleavage. MPs subjected to these conditions exhibited roughened surfaces, reduced effective molecular weight and increased polarity. Li et al. (2022) likewise observed higher carbonyl indices and enhanced surface oxidation of embedded MPs after hydrothermal treatment at 180 °C for 30 min, indicative of substantial oxidative weathering [31]. Collectively, these results indicate that TH-type pretreatments can accelerate key features of MP aging and generate more fragmented and oxidized particles entering the subsequent AD stage.

In summary, the exposure of MPs during sludge pretreatment is strongly method-dependent. Thermal hydrolysis imposes the most severe hydrothermal stress and is most relevant at full scale, while thermo-alkaline and ultrasonic processes introduce additional chemical or mechanical stressors that can further modify MP structure. Other pretreatments (e.g., mechanical disintegration, ozonation, freeze–thaw) may also affect MPs but are less widely implemented. Key operating conditions and dominant MP stressors are summarized in

Table 1. Typical exposure conditions and dominant stressors on MPs during common sludge pretreatments.

Pretreatment Method	Typical Operating Window	Dominant Stressors on MPs	Full-Scale Relevance
TH	140–180 °C; 4–8 bar; 20–60 min	Hydrothermal aging, fragmentation, oxidation, additive leaching	Widely implemented
Thermo-alkaline	>150 °C; pH > 10; 30–60 min	Strong alkaline hydrolysis, surface deprotonation, plasticizer saponification	Site-specific
Ultrasonic pretreatment	20–80 kHz; kJ L−1-scale energy inputs	Cavitation, micro-scale shear, particle fragmentation, surface pitting	Mostly pilot/local
Other (ozonation, mechanical, freeze–thaw)	Method-dependent	Oxidation, abrasion, crack formation	Less common

, and the following subsections focus on how TH-relevant transformations influence sludge solubilization and subsequent AD performance.

2.2. Physicochemical Transformation of MPs

Microplastics present in sludge are not inert under pretreatment conditions but experience thermal, mechanical and chemical stresses that alter their properties [23,29,31]. The resulting transformations can be broadly grouped into three categories: (i) physical changes, such as size reduction and morphological alteration; (ii) chemical modification of the polymer backbone and surface functional groups; and (iii) leaching of constituents, including polymer additives and sorbed contaminants (Figure 1) [43,45,46,47].

2.2.1. Particle Size and Morphology

One of the clearest effects of sludge pretreatment is the modification of MP particle size and morphology. Depending on polymer type and operating conditions, MPs can fragment into smaller particles or undergo softening, deformation and agglomeration [48,49]. Under TH-like conditions (≥160 °C), PE and PET MPs have been reported to develop surface cracking, pitting and increased friability, indicating greater susceptibility to subsequent fragmentation [29,33]. Ultrasonic pretreatment can similarly erode and break particles via cavitation-induced shear, generating finer debris in sludge systems and related matrices [45,46]. The possible formation of nanoplastics is of particular concern, as smaller particles are more mobile and potentially more bioavailable; while direct evidence from full-scale pretreatment is limited, environmental weathering studies consistently show progressive crack formation, size reduction in polyolefins and nano-scale fragments with biological effects [50,51,52,53,54]. Overall, pretreatments that intensify thermal or mechanical stresses are likely to shift MP size distributions toward smaller, more reactive particles.

2.2.2. Polymer Structure and Integrity

Elevated temperatures and reactive media during sludge pretreatment can induce polymer chain scission and modify the polymer backbone. In thermal hydrolysis, the combined action of heat and liquid water promotes hydrolytic cleavage of susceptible bonds, particularly in condensation polymers; PET, for example, can be depolymerized to its monomers under sufficiently severe aqueous conditions [37,39,44]. Han et al. (2024) reported that PE and PET MPs exposed to TH-like conditions underwent not only physical fragmentation but also bond scission, yielding lower-molecular-weight species with hydroxyl, carboxyl and unsaturated end-groups, supported by density functional theory analysis of water- and heat-assisted chain cleavage [29]. Li et al. (2022) likewise observed increased carbonyl indices and new oxygenated functionalities on MPs after hydrothermal treatment, consistent with oxidative modification of the polymer matrix [31]. These results indicate that TH-type pretreatments can accelerate key features of polymer aging, including chain scission, embrittlement and the introduction of polar groups, so that post-pretreatment MPs are expected to be more brittle and hydrophilic than pristine particles, in line with established mechanisms for thermo-oxidative and hydrolytic degradation [19,55]. The extent of such transformations under full-scale conditions, however, depends on polymer type, pretreatment severity and residence time.

2.2.3. Surface Chemistry Evolution

Thermal and thermo-chemical pretreatments can modify MP surface chemistry by introducing oxygen-containing functional groups. Oxidative and hydrolytic processes have been shown to generate hydroxyl, carbonyl and carboxyl groups on originally hydrophobic polymers such as PE, thereby increasing surface polarity and altering interfacial behavior [56,57,58,59]. X-ray photoelectron spectroscopy in hydrothermal sludge systems has confirmed higher oxygen contents and elevated carbonyl indices on MPs after treatment [31]. Under strongly alkaline, high-temperature conditions, dehydrochlorination and related reactions may occur in susceptible polymers such as PVC, forming conjugated structures and locally carbonaceous residues [58,60]. These surface modifications are expected to influence interactions with dissolved organics, solids and microbes, and are further considered in Section 2.3.

2.2.4. Additive and Pollutant Leaching

MPs often contain additives (plasticizers, stabilizers, flame retardants, pigments) that can be mobilized under heat, pressure and chemically reactive conditions. Under TH-like exposure, elevated temperature and hydrothermal stress enhance additive diffusion and release from the polymer matrix [61]. Chen et al. (2024) reported that TH at 170 °C significantly increased leaching of the plasticizer acetyl tri-n-butyl citrate (ATBC) from PE MPs, with ATBC concentrations in sludge liquor rising by roughly an order of magnitude relative to non-pretreated controls [30]. Li et al. (2022) similarly observed higher dissolved organic carbon and evidence of additive release from MPs after hydrothermal treatment [31].

Additives such as phthalate esters, bisphenols and brominated flame retardants are known to leach from plastics, and elevated temperature, pressure or alkaline conditions can accelerate this behavior [62,63,64]. PVC MPs have been identified as sustained sources of phthalates and BPA, with BPA release directly linked to inhibition of methanogenesis in AD systems [15]. Diffusion-controlled leaching of PBDEs and related flame retardants into aqueous phases has also been demonstrated, providing a mechanistic basis for their mobilization under hot pretreatment liquors [62]. Such leaching alters MP composition (e.g., loss of plasticizers and associated embrittlement) and enriches the surrounding liquid with bioactive compounds, with potential consequences for downstream AD performance and toxicity that are examined in later sections [30,31,61,62,63,64].

2.2.5. Interactions with Sludge Components

Transformations during pretreatment are not one-sided; sludge constituents can also promote MP aging. Han et al. (2024) reported a “mutual promotion” effect, where sludge rich in organic matter and inorganic catalysts enhanced the aging of PE and PET under TH conditions, while MPs in turn were associated with increased sludge solubilization [29]. Hydrothermal treatment of real sludge has been shown to produce more pronounced oxidation and surface roughening of embedded MPs than treatment of MPs alone, suggesting catalytic or abrasive contributions from co-existing solids and dissolved species [31,65]. Transition metals present in sludge (e.g., Fe, Cu, Al) can facilitate radical generation in hot aqueous environments and have been implicated in accelerated depolymerization or oxidation of polymers in related systems, highlighting the potential for metal-mediated pathways to intensify MP transformation beyond water-only conditions [66,67].

Overall, MPs leaving pretreatment are often smaller, more irregular, more oxidized and partially depleted in additives, and may also carry or release sorbed contaminants under elevated temperature. These altered properties are expected to influence subsequent interactions with sludge organics, microbial communities and co-contaminants during AD, which are examined in the following sections.

2.3. Regulatory Effects on Organic Matter Release

Beyond undergoing transformation, MPs present in sludge can also affect pretreatment performance, particularly the solubilization of organic matter, which is a primary objective of thermal hydrolysis and related methods [29,31]. Experimental studies indicate that MPs do not behave solely as inert particles in pretreatment reactors but can modify the extent and pathways of sludge disintegration [29]. Several mechanisms have been proposed by which MPs may enhance or inhibit the release of organic carbon during pretreatment, including interactions with heat transfer, radical chemistry and organic matrices [61,62,63,64].

2.3.1. Surface Polarity and Sludge Floc Disruption

As MPs become oxidized and hydrophilic, their interaction with sludge solids changes [31,56,57,58,59]. Oxidized MP surfaces bearing -OH or -COOH groups can form hydrogen bonds or electrostatic attractions with sludge biopolymers (e.g., proteins, polysaccharides in extracellular polymeric substances, EPS) [46,68]. This can destabilize sludge flocs. MP particles with increased surface polarity can insert themselves into sludge floc matrixes and disrupt the cohesive forces that hold flocs together (such as hydrophobic interactions and Ca²⁺-bridging of EPS) [68,69]. The presence of MPs may thus enhance the physical disintegration of flocs, aiding release of cell-bound organics [46]. Our team noted that oxidized MP surfaces can “break sludge colloidal stability and hydrate structure, promoting EPS release and cell lysis.” Essentially, MPs can act as microscopic dispersing agents once they carry polar groups, helping water penetrate flocs and freeing organic material [46,68].

2.3.2. Interference in EPS Networks

EPS (extracellular polymeric substances) form a gel-like matrix encasing sludge particulates [68,69,70]. MPs introduced into this matrix (especially those with rough, oxidized surfaces after pretreatment) can bind EPS components [31,70]. Oxidized MPs might form hydrogen bonds with polysaccharides or proteins, or electrostatic interactions if EPS carries charge [68,69,70]. This perturbation of the EPS network can lead to its accelerated breakdown under heat. As EPS dissolves, more soluble chemical oxygen demand (sCOD) is generated [23]. Han et al. (2024) observed that sludge with PE/PET MPs ended up with higher soluble organics (proteins, etc.) and ammonia in the liquid after TH than sludge without MPs—suggesting MPs enhanced organic solubilization [29]. The authors linked this to MPs boosting protein transformation, possibly by disrupting protein–carbohydrate complexes in EPS or by providing surfaces that favor protein hydrolysis reactions [29,70,71].

2.3.3. Catalytic or Inhibitory Chemical Effects

MPs can release additives or their degradation products (e.g., plasticizers, catalysts, acidic or basic species) that directly influence hydrolysis chemistry [61,72,73,74]. Chen et al. (2024) reported that leaching of the plasticizer acetyl tri-n-butyl citrate (ATBC) from PE during TH was associated with an increase of about 21% in subsequent methane production and with enrichment of protein-degrading microbial groups, indicating that some leachates may transiently promote beneficial pathways [30]. In contrast, inhibitory effects have been linked to compounds such as BPA released from PVC and other toxic additives, which can impair hydrolytic enzymes or interfere with radical-driven reactions required for sludge disintegration [15,63,64]. Thus, the net impact of additive leaching on pretreatment performance is expected to be concentration- and compound-specific.

The influence of MPs and their leachates on pretreatment performance is likely non-linear and concentration-dependent. At low loadings, limited MP presence or moderate additive release may have negligible or even slightly positive effects (e.g., minor floc disturbance), whereas at higher loadings MPs can introduce inhibitory compounds, increase the fraction of non-biodegradable solids, or interfere with mass and heat transfer. AD studies have reported reduced degradation efficiency and higher residual COD at elevated polyester MP doses, consistent with incomplete hydrolysis under MP-rich conditions [17]. By analogy, heavily MP-laden sludge subjected to TH could exhibit less effective solubilization if MPs create protective micro-environments, sorb or scavenge reactive species, or otherwise hinder contact between sludge organics and the hydrothermal medium [61,72,73,74]. MPs may also act as nucleation sites for inorganic precipitates, and incorporation of MPs into mineral phases has been observed in laboratory systems, suggesting potential for surface-mediated encapsulation of MPs and associated substances, although this remains to be verified under full-scale TH conditions.

Overall, the performance of TH may be influenced by the load and composition of MPs in sludge, particularly where atypically high MP inputs occur (e.g., fiber-rich textile effluents), and may warrant consideration in pretreatment optimization. A more systematic understanding of the dual role of MPs—both as materials undergoing transformation and as potential modulators of sludge hydrolysis—is required to refine process models and control strategies. The subsequent section addresses the behavior and impacts of these transformed MPs during anaerobic digestion.

3. Induction Effect of Pretreatment on MPs on Sludge Anaerobic

3.1. Overall Performance Impacts on AD

The influence of pretreated MPs on AD performance shows a distinct duality: in some systems, pretreatment mitigates MP-related inhibition, whereas in others it intensifies adverse effects (Figure 2). This divergence reflects the combined effects of enhanced substrate solubilization, changes in microbial functional capacity and the physicochemical transformations of MPs induced by pretreatment. In this section, “low” MP loadings refer to environmentally relevant levels typically reported for sewage sludge (on the order of 10³–10⁵ particles kg⁻¹ dry solids, corresponding to up to several tens of mg kg⁻¹), whereas “high” loadings denote the elevated concentrations commonly applied in laboratory studies (≥10⁷–10⁸ particles kg⁻¹ or ≥10²–10³ mg kg⁻¹), which are used to probe stress responses and may exceed typical field conditions. The net outcome of pretreatment–MP interactions therefore depends not only on MP characteristics but also on loading range, pretreatment severity and the intrinsic robustness of the AD system [14,15].

In many reported systems, pretreatment—particularly TH and alkaline conditioning—has attenuated MP-related inhibition of methane production and process stability. By increasing the fraction of soluble, biodegradable organics, pretreatment can improve substrate availability and partly offset stress associated with MPs [17,75]. For example, sludge containing PE MPs at elevated, inhibitory doses showed a 12.1% decrease in methane yield without pretreatment, whereas TH at 170 °C for 30 min resulted in an 8.3% increase relative to the MP-free control [16]. Alkaline pretreatment of PS-contaminated food waste similarly shifted performance from an 18% loss to a 20.4% gain in methane yield [76]. These improvements have been linked to pretreatment-induced aging and fragmentation of MPs, which reduce hydrophobicity, introduce oxygen-containing functional groups and weaken the tendency of MPs to adsorb extracellular enzymes or hydrophobic inhibitors, while smaller particle sizes lessen physical blockage and enhance mass transfer [77,78]. Partial leaching of additives prior to AD may, at low concentrations, reduce in-reactor toxicity pressure and facilitate microbial adaptation; in specific cases, leached compounds such as acetyl tri-n-butyl citrate (ATBC) have been reported to act as additional substrates and stimulate methane production [16,79]. Overall, available evidence indicates that under suitable operating conditions and predominantly at experimentally elevated MP loadings, pretreatment can shift the balance toward improved AD performance, whereas responses at environmentally relevant MP levels remain less well constrained.

However, beneficial effects of pretreatment are not universal. Several studies have reported that, under specific configurations, pretreatment can intensify MP-related inhibition [79,80]. TH at 170 °C has been shown to increase the chemical reactivity of MPs, leading to greater methane yield reductions than in untreated systems at comparable or only moderately elevated MP loadings [80]. In the absence of pretreatment, marked inhibition is often observed mainly at high MP doses (e.g., 500 mg kg⁻¹ VS, causing 16% methane reduction), whereas after pretreatment even doses around 100 mg kg⁻¹ VS of PE or PVC have been associated with substantial inhibition, largely attributed to the rapid release of additives such as DBP, DMP and BPA that impair hydrolytic and methanogenic populations, particularly acetoclastic methanogens [15,18]. In systems where hydrolysis is strongly accelerated but methanogenic capacity is not proportionally increased, the rapid accumulation of volatile fatty acids can further induce acidification, pH decline and extended lag phases. These observations underline that the net effect of pretreatment in MP-containing sludge is highly configuration- and dose-dependent and that “over-hydrolysis” may shift the system from mitigation to exacerbation of MP-induced stress.

Across these studies summarized in

Table 2. Summarizes representative experimental studies on MP impacts on AD with and without pretreatment, highlighting key operating conditions, MP characteristics, mechanistic interpretations and performance outcomes relative to controls.

Authors Year	Substrate	Pretreatment	MP Type	MP Size	MP Dose	AD Conditions	Mechanistic Insight	Key Outcomes vs. Control	Ref. No.
Wei et al., 2019	WAS	None	PE	tens–hundreds µm	10–200 particles g−1 TS	Batch and semi-continuous, mesophilic	Mild EPS disturbance and surface interaction at elevated loads.	Low: ≈no effect; high: CH4 ↓ 10–20%, slight hydrolysis inhibition.	[14]
Wei et al., 2019	WAS	None	PVC	1 mm	10–60 particles g−1 TS	Batch, mesophilic	BPA leaching; oxidative stress; acetoclastic methanogen suppression.	Low: slight CH4 ↑; higher: CH4 ↓ 20–25%, slower VFA conversion.	[15]
Li et al., 2020	WAS	None	Mixed MPs (PE, PP, PET, PVC)	mainly 50–1000 µm	up to g kg−1 TS	Batch, mesophilic	Polymer- and dose-dependent toxicity; enzyme/particle interaction at high loads.	Env.-range: minor changes; elevated: CH4 ↓, more residual COD/solids.	[17]
Fu et al., 2018	Sludge inoculum	None	PS NPs	70–100 nm	up to hundreds µg L−1	Batch, mesophilic	Cell attachment; ROS generation; membrane damage at nano-scale.	High NPs: delayed start-up, CH4 ↓ up to ~30–40%.	[77]
Zhang et al., 2020	Anaerobic granular sludge (AGS)	None (chronic exposure)	PVC MPs	75–300 µm	15–150 MPs L−1	Continuous AGS reactor, mesophilic	EPS loss; granule surface erosion; community imbalance.	Progressive PVC exposure: granule disintegration, CH4 and COD removal ↓	[83]
Chen et al., 2021	WAS	None	PA6 MPs	100–300 µm	5–50 particles g−1 TS	Batch AD, mesophilic	Surface aging; N-containing leachates reshape bacterial consortia.	Low–moderate: limited/neutral; higher: emerging CH4 inhibition, shifts in taxa.	[75]
Wei et al., 2022	AGS treating wastewater	None vs. hydrochar	PE, PP, PET, PS (mixture)	75–300 µm	mg L−1 range	Continuous AGS reactor, mesophilic	Combined physical and oxidative stress; weakening of granule structure at load.	Low MPs: stable AGS; high MPs: EPS ↓, granule damage, CH4 ↓	[84]
Wang et al., 2022	AGS + PS nanoplastics	Biochar dosing	PS NPs	≤100 nm	tens–hundreds µg L−1	AGS reactor/batch, mesophilic	NPs disturb EPS architecture; increase ROS; alter keystone populations.	Elevated NPs: CH4 ↓, diversity loss; moderate: partial resilience, measurable stress.	[21]
Wang et al., 2023	AGS + PVC MPs/NPs	Hydrochar dosing	PVC MPs and NPs	micro- and nano-scale	fixed PVC + graded hydrochar	AGS/batch, mesophilic	Hydrochar sorption of MPs/additives; protection of EPS and granules.	PVC alone: strong inhibition; +hydrochar: CH4 and granule integrity largely restored.	[18]
Zeng et al., 2022	WAS + PS MPs	Thermal/hydrothermal/chemical pretreatments	PS MPs	75 µm	mg g−1 TS	BMP tests, mesophilic	TH enhances solubilization; modifies PS surfaces without major added toxicity.	TH improves CH4; tested PS levels do not negate TH benefits.	[79]
Azizi et al., 2022	WAS + PS NPs	Conventional THP (80–160 °C) prior to AD	PS NPs	80–100 nm	50–150 µg L−1	Batch and semi-continuous, mesophilic	TH reduces NP-induced stress and ARG propagation compared with conventional AD.	Conventional+NPs: inhibition, ARG ↑; TH–AD: CH4 maintained, impacts attenuated.	[85]
Azizi et al., 2023	Primary sludge + PS NPs	THP (160 °C, varying solids content)	PS NPs	80–100 nm	150 µg L−1	Batch, mesophilic	THP severity co-determines NP reactivity, toxicity and energy balance.	Optimized THP: stable CH4 with manageable NP effects; too mild/harsh: reduced net benefit.	[86]
Azizi et al., 2024	TWAS/FPS + PS NPs	Low-temperature THP (90 °C, 90 min)	PS NPs	80–100 nm	150 µg L−1	Batch and semi-continuous	Low-T THP partially ages NPs while improving sludge biodegradability.	Low-T THP: moderate mitigation of NP effects with favorable energy profile.	[33]
Wei et al., 2023;	AGS systems with multiple MPs and sorbents	Hydrochar/biochar/GAC; no THP or with THP	Mixed (PE, PP, PET, PS, PVC; MPs and NPs)	micro- to nano-scale	env.-relevant to elevated; multi-level sorbent doses	Batch and continuous	Carbonaceous media immobilize MPs, buffer toxicity, support syntrophy.	With sorbents: CH4 stabilize or recover; No sorbents: Significant inhibition and structural disruption	[87]

, MP impacts on AD are strongly polymer- and dose-dependent: environmentally relevant loadings generally cause limited or buffered responses, whereas elevated experimental doses consistently induce measurable inhibition and micro-ecological shifts. The conceptual regime map in Figure 3 synthesizes these patterns by relating pretreatment severity and MP loading, together with substrate type, to operating domains where pretreatment is more likely to alleviate, or instead exacerbate, MP-induced impacts on AD performance. Evidences also indicates that optimized pretreatment, adsorbents and tailored reactor designs can partially offset these effects, but their robustness at realistic scales remains to be confirmed. The variability in these outcomes is further governed by interacting factors. Polymer type is critical: MPs containing higher loads of mobile additives or reactive degradation products are more prone to exert inhibitory effects after pretreatment, as elevated temperature and chemical stress promote additive release, whereas relatively inert polymers such as PE or PP generally yield fewer clearly toxic leachates, making recovery more likely under comparable conditions [81]. Particle size and morphology also modulate post-pretreatment behavior; smaller MPs and irregular fragments exhibit higher specific surface area and greater susceptibility to oxidation and leaching, and can penetrate microbial flocs more effectively, potentially disturbing mass transfer and syntrophic interactions [20]. MP concentration introduces a load-dependent response: at environmentally relevant levels, effects of pretreated MPs on AD performance are often limited or ambiguous, whereas at elevated experimental loadings, accumulated toxicants and intensified surface reactivity can exceed microbial detoxification capacity and lead to sustained inhibition [82]. Overall, the net impact of pretreated MPs reflects the combined influence of polymer type, size distribution, MP loading and system resilience, rather than pretreatment conditions alone.

Substrate characteristics further condition the response of AD to pretreated MPs. In carbohydrate-rich food waste systems, abundant readily fermentable substrates may partially offset MP-related stress by sustaining high microbial activity and diluting the relative contribution of MPs to the particulate matrix [6]. In lignocellulosic-rich manure or WAS, where hydrolysis is rate-limiting, MPs constitute a larger fraction of recalcitrant solids, and pretreatment-induced changes in their reactivity can have a more pronounced influence on overall conversion efficiency [88]. Pretreatment severity (temperature, residence time, pH) governs the balance between improved solubilization and increased release of reactive or toxic compounds: higher severity generally enhances fragmentation and oxidation but may also intensify additive mobilization, whereas milder or optimized conditions are more likely to increase solubilization without imposing excessive inhibitory pressure [13]. As a result, the net effect of MPs after pretreatment is strongly substrate-specific and tightly coupled to how pretreatment intensity is matched to the buffering and methanogenic capacity of the receiving AD system.

Overall, available studies indicate that pretreatment tends to alleviate MP-related inhibition when enhanced substrate availability is aligned with methanogenic capacity and when the type and amount of released additives remain within microbial tolerance limits. In contrast, exacerbated inhibition is more frequently observed when pretreatment-driven solubilization exceeds the system’s conversion capacity or when elevated concentrations of toxic leachates impair key functional groups. Robust prediction of the net effect of pretreated MPs on AD performance therefore requires an integrated assessment of polymer properties, particle size distribution, MP loading, substrate characteristics and pretreatment severity, in conjunction with the resilience and adaptation potential of the receiving microbial community.

3.2. Micro-Ecological Disturbance Mechanisms

Shifts in AD performance in the presence of pretreated MPs are closely linked to changes in microbial community structure and function. Pretreatment alters both the bulk substrate and MP characteristics, thereby modifying selective pressures within the digester and driving reorganization of bacterial, archaeal and syntrophic populations. These community-level responses have direct implications for hydrolysis, fermentation and methanogenesis (Figure 4) [89,90].

One of the most frequently reported microbial responses to MPs in AD is a decline in acetoclastic methanogens, often accompanied by a relative increase in hydrogenotrophic pathways [91]. Pretreatment can modulate this pattern in two directions. Where TH or alkaline conditioning reduces MP hydrophobicity and desorbs surface-bound inhibitors, the relative abundance and activity of acetoclastic genera such as Methanosaeta and Methanosarcina have been observed to recover, supporting acetate-driven methane formation [19]. Conversely, where pretreatment promotes rapid release of toxic additives or generates highly oxidized, reactive MP surfaces, stronger suppression of acetoclastic methanogens than in non-pretreated MP systems has been reported, with a shift toward hydrogenotrophic methanogenesis via Methanobacterium and Methanospirillum [92]. Increased reliance on this pathway under mesophilic conditions is generally associated with lower overall methane yields and longer adaptation periods.

Bacterial guilds responsible for hydrolysis and acidogenesis also show differentiated responses to pretreated MPs. Surface-aged MPs with increased hydrophilicity can act as inert carriers for biofilms, with enrichment of hydrolytic genera such as Bacteroides, Clostridium and Proteiniphilum reported in some systems, alongside higher extracellular enzyme activities and accelerated substrate depolymerization [93]. Under conditions where pretreatment markedly enhances additive leaching, however, decreased α-glucosidase and protease activities and shifts toward more tolerant or opportunistic fermenters (e.g., Enterococcus) have been observed [94]. Such changes can weaken syntrophic links between key fermenters and hydrogenotrophic methanogens, favoring VFA accumulation and reduced process stability, especially at elevated MP and leachate loadings.

Community diversity and evenness are also affected by MPs after pretreatment. Several studies report that more severe conditions (e.g., higher TH temperatures or strongly alkaline environments) in the presence of MPs are associated with reduced bacterial diversity, consistent with combined stress from modified substrate chemistry and elevated leachate concentrations [95]. In contrast, milder or optimized pretreatment has been linked to increased evenness and reduced dominance of a few opportunistic taxa, thereby enhancing functional redundancy [96]. Such redundancy can improve resilience, as multiple taxa are available to sustain key hydrolytic and methanogenic functions under MP-related stress.

From a network ecology perspective, MPs influence not only the abundance of specific taxa but also the structure of microbial interaction networks. Untreated MPs have been reported to weaken positive co-occurrence links between fermentative bacteria and methanogens, indicating reduced connectivity of syntrophic partnerships [93]. Pretreatment may either aggravate this fragmentation, when additive toxicity affects keystone populations, or partially restore connectivity by reducing physical barriers and modifying surface interactions [94]. Oxidatively aged MP surfaces bearing carbonyl and hydroxyl groups have been proposed as potential facilitators of direct interspecies electron transfer (DIET) by increasing surface conductivity; although direct evidence for MP-mediated DIET in AD is currently lacking, this mechanism is considered plausible, particularly under thermophilic conditions where conductive interfaces can support methanogenesis [97].

The archaeal response to MPs after pretreatment also reflects adjustments in energy metabolism. Suppression of acetoclastic methanogens is frequently accompanied by increased expression of formate dehydrogenase, hydrogenase and coenzyme F₄₂₀-dependent hydrogenase genes, consistent with a shift toward syntrophic acetate oxidation (SAO) coupled to hydrogenotrophic methanogenesis [98]. Although SAO is thermodynamically less favorable under standard conditions, it can be favored in stressed systems where acetoclastic pathways are inhibited. By modifying the toxicity profile and the dynamics of VFA production and consumption, pretreatment can influence the relative contributions of acetoclastic and SAO–hydrogenotrophic routes in MP-impacted digesters.

Beyond direct toxicity, pretreated MPs can influence cell surface interactions and biofilm development. Oxidized MP surfaces can bind EPS and other organic macromolecules, potentially altering biofilm porosity, diffusivity and local microenvironments (e.g., pH, redox conditions) within aggregates [99]. Such changes may affect access to substrates and electron acceptors for key functional groups. It has also been hypothesized that aged MPs could act as micro-scale redox-active surfaces by interacting with electron shuttles such as flavins or humic-like substances, thereby modulating electron transfer efficiency; while this has not yet been quantified in AD systems, available physicochemical evidence from MP aging studies indicates that this mechanism merits further investigation [100].

Overall, the micro-ecological consequences of pretreated MPs are governed by the balance between reduced physical/chemical constraints on microbial activity and new stresses arising from additive release and altered MP surface reactivity. When pretreatment limits toxicant liberation and aligns hydrolytic enhancement with methanogenic capacity, recovery of key syntrophic links and functional guilds is often observed. Under more severe or chemically aggressive conditions, however, pretreatment can intensify community disruption, fragment interaction networks and shift metabolism toward less efficient methane production routes. These patterns highlight the need to integrate microbial ecology diagnostics—such as high-throughput sequencing, functional gene profiling and network analysis—into assessments of pretreatment strategies in MP-impacted AD systems, since observed performance changes ultimately reflect restructuring of the underlying microbial framework.

3.3. Substrate Conversion and Interfacial Adsorption

Substrate transformation in AD proceeds through sequential hydrolysis, acidogenesis, acetogenesis and methanogenesis, and each step can be influenced by MPs via changes in interfacial behavior and adsorption. Pretreatment modifies both MP properties and the soluble organic pool, thereby affecting enzyme–substrate contact, intermediate turnover and the distribution of organics between particulate and aqueous phases [101,102].

A key pathway involves altered enzymatic accessibility. Surface aging during TH or alkaline pretreatment, characterized by increased hydrophilicity and polar functional groups, can reduce the sorption of hydrophobic enzymes onto MPs, increasing enzyme availability in the bulk phase [103]. In some systems, hydrothermally treated PE MPs have shown decreased lipophilicity and lower lipase and cellulase adsorption, coinciding with enhanced lipid and cellulose degradation in co-digestion setups [76]. Conversely, when pretreatment generates fine (<10 µm), highly heterogeneous MP fragments, adsorption of charged enzymes and other biocatalysts can be promoted, effectively sequestering active sites and slowing particulate hydrolysis [30].

The adsorption of soluble organics represents a second important control on substrate conversion. MPs can sorb VFAs, soluble proteins and polysaccharides, thereby modifying their bioavailability and the partitioning between solid and liquid phases. Pretreatment-induced fragmentation increases MP specific surface area and, together with altered surface polarity and charge, can enhance adsorption of selected metabolites [79]. At moderate levels, such sorption may buffer transient VFA peaks by temporarily retaining intermediates and releasing them gradually, dampening pH shocks and protecting methanogens. At higher MP loadings or with highly reactive surfaces, excessive adsorption can sequester key intermediates, delay their turnover and prolong lag phases, making the overall effect strongly dependent on MP dose and surface chemistry [17].

Interactions with lignocellulosic degradation products are particularly relevant for manure and agricultural residues. Pretreated MPs with roughened, functionalized surfaces can bind lignin-derived phenolic compounds, which may reduce their inhibitory impact on fermentative bacteria [14]. At the same time, the same sites may compete for fermentable sugars or other labile organics, limiting substrate availability to saccharolytic populations and shifting fermentation toward less favorable pathways when MP-associated sorption becomes significant [15].

From a transport and thermodynamic perspective, MPs modify the spatial distribution of substrates and metabolites within flocs and biofilms. Pretreated MPs embedded in aggregates can introduce additional adsorption–desorption interfaces, influencing diffusion gradients and the delivery of intermediates to syntrophic partners [76]. In systems with high abundances of surface-active MPs, local depletion of acetate or hydrogen near methanogens has been suggested as a factor contributing to suboptimal substrate availability and reduced conversion efficiency.

Pretreatment-induced additive leaching can also alter substrate conversion pathways. Compounds such as phthalates and bisphenols have been reported to inhibit enzymes involved in the acetyl-CoA pathway and methyl-coenzyme M reduction, potentially slowing acetogenesis and methanogenesis [83]. Other leachates may act as competing electron acceptors or substrates, diverting reducing equivalents away from methane formation and favoring the buildup of intermediates such as propionate or ethanol under certain conditions [18]. Overall, these processes indicate that MP-associated chemical and interfacial effects can redistribute electron flow and metabolite availability, although their magnitude at environmentally relevant loads remains insufficiently constrained.

In summary, the influence of pretreated MPs on substrate conversion is controlled by the interplay between MP surface chemistry, adsorption behavior, metabolite dynamics and additive release (Figure 4). Reduced hydrophobicity and moderated aging can lessen enzyme and inhibitor sorption, supporting hydrolysis and downstream conversion, whereas extensive fragmentation and oxidation increase surface reactivity and intermediate sequestration, with greater risk of perturbing metabolic pathways. The net effect depends on how MP properties after pretreatment align with substrate characteristics and the metabolic capacity of the microbial community, emphasizing the need to tune pretreatment conditions to favor efficient interfacial turnover rather than additional constraints.

3.4. Migration Behavior and Risk Pathways Within the System

MP behavior in AD is dynamic, involving redistribution between solid and liquid phases, ongoing physicochemical transformation and potential transfer into downstream streams. Pretreatment alters these patterns by changing MP size, density and surface properties, as well as their association with sludge flocs, thereby influencing both in-reactor fate and the risks linked to digestate and effluent management [104,105].

In AD without pretreatment, MPs predominantly remain embedded within flocs, retained by hydrophobic interactions and EPS, with limited mobility in the free phase [21]. Pretreatment can disrupt this retention: thermal or alkaline processing weakens EPS binding, fragments particles and modifies buoyancy, promoting partial release into the liquid phase [106]. Smaller and oxidized MPs display lower settling velocities and greater suspension stability, increasing the probability of export with effluent, particularly in continuously stirred systems where mixing enhances MP mobility [85].

Pretreatment also affects MP partitioning between solid and liquid digestate fractions. Hydrothermally aged MPs with roughened, functionalized surfaces show increased affinity for mineral phases such as struvite and calcium carbonate, which can promote their retention in solids [21]. This may temporarily limit direct release but implies potential remobilization during land application as mechanical disturbance and weathering act on bound MPs [107]. By contrast, more hydrophilic or low-density particles (e.g., foamed PS) are more likely to remain in the supernatant and increase the risk of export via liquid effluents if polishing steps are insufficient [108].

An additional concern is the role of MPs as carriers for intrinsic additives and sorbed co-contaminants. Pretreatment can both accelerate leaching of plasticizers, flame retardants and stabilizers and increase the sorption capacity of aged MPs for metals and hydrophobic organics present in AD systems [18]. These MP–contaminant assemblages can be transported with digestate or effluent and may exhibit enhanced persistence and bioavailability [78]. For instance, hydrothermally aged MPs have been reported to sorb cadmium at substantially higher capacities than pristine particles, indicating a greater potential for coupled MP–metal migration into soils [77].

Biological interactions further shape MP migration. Biofilm colonization can increase MP density and promote sedimentation, but may also shield MPs from further degradation, extending their persistence [82]. In systems with strong microbial attachment, MPs can be incorporated into granules or aggregates and migrate as part of larger particles. Pretreatment may reinforce this behavior by increasing surface oxidation and adhesion sites or weaken it when extensive fragmentation generates particles too small to sustain stable biofilms [104].

These altered migration patterns have direct implications for post-AD management. MPs transferred to the liquid phase can evade conventional solids–liquid separation and may require additional filtration or membrane polishing to avoid discharge. MPs retained in solids can accumulate in agricultural soils following biosolids application and interact with soil biota, with potential effects on structure and nutrient cycling [96]. Increased surface reactivity and fragmentation of pretreated MPs may also accelerate their transformation to smaller size classes in the environment, including NPs, which are more mobile and biologically accessible, thereby heightening long-term ecological concern [109].

In conclusion, pretreatment substantially reshapes MP migration pathways in AD systems, with consequences for both process performance and environmental release. A clearer understanding of how physical transport, surface reactivity and biological interactions jointly control MP partitioning between effluent and biosolids is needed to design pretreatment schemes that limit MP-associated risks while maintaining or improving AD efficiency.

4. Mitigation of MP Impacts on AD

4.1. Adsorption-Based Strategies

A practical approach to moderating MP impacts in AD is the in situ application of adsorbent materials. Carbonaceous adsorbents, in particular, exhibit strong affinity for MPs and associated leachates. In sludge digesters amended with PS nanoplastics at 0.15 mg L⁻¹, methane yield decreased by ~32%, whereas addition of 5–15 g L⁻¹ GAC substantially restored methane production [86]. Biochar and hydrochar derived from biomass or sludge have likewise been reported to immobilize MPs and alleviate inhibition. In a mixed-MP system containing PE, PS, PET and PP, hydrochar dosing reduced methane loss by ~50% and recovered ~69% of MP-compromised microbial activity [82,84]. Collectively, these studies indicate that adsorbents with high surface area and suitable surface chemistry can enhance MP retention and improve AD performance under experimental conditions, although most evidence to date is derived from lab-scale tests at elevated MP and adsorbent loadings [88].

The efficacy of carbonaceous adsorbents is attributed to several coupled mechanisms. These materials sequester MPs on pore surfaces and within pore networks, limiting direct contact with biomass and retaining hydrophobic leachates such as bisphenols and phthalates, thereby reducing their bioavailability [15,110]. Biochar and hydrochar can stimulate EPS and humic-like substance formation, further encapsulating MPs and associated contaminants [87], while conductive or redox-active carbons (e.g., GAC, biochar) may facilitate direct interspecies electron transfer and support methanogenesis [98]. Practical integration options include direct dosing, fixed or moving beds, and coupling with anaerobic membrane systems equipped with activated carbon units for effluent polishing [111]. Lab- and pilot-scale studies generally report improved methane yields and process stability under MP stress when suitable adsorbents are applied [82,84,88].

At the same time, large-scale implementation requires careful techno-economic and environmental evaluation. Extrapolating typical experimental dosages to municipal AD implies substantial sorbent consumption, additional handling and process complexity, and potentially high operating costs, unless low-cost or waste-derived carbons and clear performance or compliance gains are demonstrated. For biochar and hydrochar, evidence is still dominated by laboratory studies and early pilots, with feasibility strongly dependent on local feedstock supply and integration into existing sludge management [112]. All such strategies generate a secondary sorbent-rich solid phase concentrating MPs and co-contaminants, necessitating regeneration, controlled thermal treatment or secure disposal to avoid re-release, with associated energy and environmental burdens. Consequently, adsorption-based measures are best regarded as targeted, site-specific complements to process-integrated approaches (e.g., optimized TH–AD and upstream MP control), and require long-term, full-scale assessments under realistic MP loads to confirm durability, cost-effectiveness and net environmental benefit.

4.2. Process Optimization Strategies

4.2.1. Operational Controls

Optimizing operating conditions can strengthen AD resilience to MP contamination. Avoiding abrupt MP shocks and excessive organic loading is critical: an acute input of ~80 mg L⁻¹ mixed MPs has been reported to disrupt granule structure (EPS reduction, granule loosening) and reduce gas production by 14–18% [78]. Gradual acclimation and moderate OLR help prevent such failures, while sufficient HRT allows more time for MPs to be retained, transformed or captured before discharge. Maintaining near-neutral pH and stable mesophilic conditions is likewise important; strongly alkaline conditions (pH~10) in fermentation systems have intensified PET MP inhibition on H₂ and VFA production via leachate toxicity and oxidative stress [108]. Overall, coordinated control of loading, HRT and pH can reduce susceptibility to MP-induced upsets and enhance the robustness of pretreated sludge digesters [12].

4.2.2. Reactor Configuration Innovations

Reactor configuration can be leveraged to moderate MP impacts beyond routine control of OLR, HRT and pH. Granular sludge systems (e.g., UASB, EGSB) provide partial protection by entrapping MPs within dense biomass and EPS; at low MP loads, accumulation on granule surfaces has been associated with limited performance loss [76]. At higher loads, however, granule erosion, cell detachment and declining methane yields have been reported, indicating that granules delay but do not prevent MP-induced failure [78,82].

Two-phase AD and AnMBR offer additional options. In two-phase systems, most MP–substrate contact and additive release can be confined to the acidogenic reactor, enabling partial removal (e.g., settling, skimming) before transfer to the methanogenic stage, although direct evidence remains limited [107]. AnMBR configurations physically retain MPs via micro/ultrafiltration, reducing discharge and increasing in-reactor residence time; when combined with adsorbent media or dynamic filters, removals approaching 99% have been achieved in related wastewater applications [99,111]. Overall, reactor designs that confine, retain and selectively remove MPs—rather than allowing uncontrolled passage through single-stage CSTRs—appear better suited to maintaining stability under MP loads, but require validation at realistic concentrations and scales [101].

4.2.3. Co-Digestion and Resilient Microbial Communities

Co-digestion offers an additional route to moderate MP impacts by diluting MPs relative to degradable organics and supporting more diverse, functionally redundant communities. Blending sludge with food waste, manure or agro-residues can improve nutrient balance and provide excess electron donors, and several studies report sustained or improved methane yields in MP-contaminated systems when suitable co-substrates are applied [113]. However, co-digestion does not eliminate MP risks: plastics such as PET can still leach phthalates and other inhibitors, with documented long-term shifts in community structure and reduced methane production in sludge–food waste systems [114]. These outcomes indicate that co-digestion enhances robustness but should be combined with complementary measures (e.g., adsorbents, upstream control, selective separation) rather than relied upon as a stand-alone solution. In practice, gradual and well-mixed feeding regimes that avoid localized MP hot spots further support acclimation and help prevent transient overloads of both organics and plastics [115].

4.2.4. System Resilience and Long-Term Adaptation

A key element of optimization is enabling AD systems to withstand and adapt to MP exposure over time. Short-term inhibition by MPs is well documented, whereas longer-term studies indicate that partial acclimation can occur via community succession and enrichment of strains tolerant to MP-associated compounds [116]. At comparatively low MP loadings, adjustments in community structure and metabolic pathways have been associated in some cases with maintained or slightly increased methane yields, including observations of enhanced hydrolytic and methanogenic activities or additional nutrient supply from partially degraded polymers such as polyamides [3,116]. These responses illustrate that MP effects are not uniformly detrimental and depend on polymer type, dose and exposure duration.

However, such beneficial or neutral outcomes are typically restricted to low or moderate MP levels. At higher loadings (often in the range of 10²–10³ mg kg⁻¹ or above), studies consistently report accumulating oxidative stress, enzyme inhibition and pronounced shifts in key functional groups, including ~50% declines in sensitive methanogens under nano-/MP stress [68,93]. Even when adaptation occurs, it may involve loss or replacement of vulnerable taxa and altered pathway dominance. Accordingly, system resilience cannot rely solely on passive acclimation. Remaining uncertainties—including differing responses to biodegradable versus conventional MPs and potential synergistic effects with co-contaminants such as antibiotics and heavy metals—need to be addressed through targeted long-term studies to support robust optimization of pretreatment–AD systems [69].

TH is recognized as an effective but capital- and energy-intensive option to enhance sludge biodegradability and methane yield. Full-scale assessments indicate roughly 20–40% higher capital costs and 10–20% higher energy use relative to conventional mesophilic digestion, with net energy gains on the order of 10–25% and payback times of about 4–8 years when heat recovery is efficient and operating conditions are favorable [23,25]. More recent evaluations highlight that overall benefits are highly site-specific and sensitive to sludge characteristics, plant configuration and heat integration; under suboptimal conditions, additional energy demand can substantially diminish or offset expected gains [33].

From an MP management perspective, upstream measures such as fine screening, grit and rag removal and advanced filtration can reduce macro- and part of the MP load and should be considered as complementary controls. However, current technologies only partially remove small MPs and nanoplastics and introduce extra costs and operational complexity. Available evidence therefore supports viewing optimized TH as one element of an integrated sludge management framework, in which upstream plastic control, adsorption-based interventions, tailored reactor configurations and co-digestion are combined in a site-specific manner to limit MP risks while sustaining AD performance [96,109]. Scaling and long-term validation under realistic MP loads remain essential to confirm durability, cost-effectiveness and environmental net benefit.

5. Conclusions and Implications

Current evidence provides a more coherent view of how MPs behave within sludge pretreatment–AD systems. TH and related pretreatments accelerate key features of MP aging, including fragmentation, surface oxidation and additive leaching, thereby generating smaller, more reactive particles that are carried into AD. Within digesters, MPs generally persist and can influence both performance and micro-ecology through several pathways: release of plastic-associated chemicals, induction of oxidative and physical stress, and plastisphere formation on MP surfaces that perturbs syntrophic interactions. Pretreatment-induced size reduction and surface modification also affect MP fate, with more hydrophilic and fine fractions showing greater propensity to partition into effluents or interact with mineral and organic matrices in biosolids, thus altering environmental exposure routes. Collectively, these findings indicate that MP impacts in sludge treatment are governed by coupled changes in physicochemistry, microbial structure and phase partitioning rather than by a single dominant mechanism.

Mitigation options have been widely explored at laboratory scale. Adsorption-based measures using GAC, biochar or hydrochar can immobilize MPs and leachates and support methane production under elevated MP loads; process-oriented strategies—including controlled loading, staged (two-phase) digestion, granular sludge systems, AnMBR configurations and co-digestion—can increase robustness by dispersing MP stress, confining MP transport or enhancing community redundancy. At full scale, TH–AD and conventional digesters in WWTPs generally operate stably and often positively under present MP contamination levels, and confirmed cases of MP-driven process failure are rare. This apparent contrast with laboratory observations is consistent with lower in situ MP loads, longer solids retention times, complex organic matrices and greater buffering capacity in real systems. Nevertheless, the continuous inflow and retention of MPs, together with their role as carriers for intrinsic additives and co-contaminants, raises legitimate concerns regarding long-term effects on microbial community trajectories, contaminant partitioning and biosolids’ quality. This underscores the need to evaluate MP-related risks within an integrated operational and environmental context, linking mechanistic understanding with field observations rather than extrapolating directly from short-term, high-dose tests.

Future work should therefore adopt long-term, system-level and quantitatively constrained approaches. Priority directions include (i) pilot- and full-scale TH–AD studies with explicit MP and co-contaminant mass balances, tracing partitioning among sludge, effluents and biogas; (ii) experiments that distinguish acute versus chronic MP exposures at environmentally relevant and moderately elevated loads, with a focus on methanogenic activity, community resilience and functional redundancy; (iii) coupled chemical–toxicological assessment of leachates (e.g., bisphenols, phthalates, UV stabilizers, flame retardants) generated during pretreatment and AD, including mixture and dose–response effects on key microbial guilds; and (iv) investigations of MPs in combination with co-occurring stressors such as antibiotics, heavy metals and salinity to resolve synergistic or antagonistic interactions under realistic sludge treatment conditions. In parallel, the development and evaluation of process-integrated control measures—optimized TH configurations, upstream plastic control, selective separation, tailored adsorbents and microbial or enzymatic enhancements—should be supported by life-cycle and techno-economic assessments to ensure that MP mitigation does not introduce disproportionate energy, cost or secondary pollution burdens. Addressing these questions will be essential for designing next-generation sludge management schemes that maintain high and stable energy recovery while limiting MP release and associated contaminant risks.