Enhancing Biogas Production Amidst Microplastic Contamination in Wastewater Treatment Systems: A Strategic Review

: This review highlights the signi ﬁ cant interaction between microplastic (MP) pollution and its impact on wastewater treatment systems, focusing on optimizing biogas production. We explore various sources of MPs, including tire-derived MPs, and their introduction into wastewater environments. This review delves into the mechanical and physicochemical challenges MPs pose in treatment processes, emphasizing the need for comprehensive mitigation strategies. The biological e ﬀ ects of MPs on microbial consortia essential for biogas production are analyzed, particularly how these pollutants interfere with each stage of anaerobic digestion—hydrolysis, acidogenesis, acetogenesis, and methanogenesis—and, consequently, biogas generation. We examine MPs’ quantitative and qualitative impacts on biogas output and production rates, uncovering how MPs disrupt microbial activity in these stages. This review also discusses novel mitigation strategies combining di ﬀ erent sludge pretreatment methods with MPs. Our goal is to enhance the sustainability of wastewater management by promoting e ﬃ cient biogas production and environmental protection in the presence of persistent MP contamination.


Introduction
In the current era characterized by an urgent demand for sustainable energy solutions and the protection of the environment, the production of biogas by anaerobic digestion (AD) stands out as a promising renewable energy source.The European parliament directive on energy neutrality for urban wastewater treatment plants (WWTPs) set an ambitious target, mandating that by 2040, urban wastewater treatment facilities must produce renewable energy equivalent to 100% of their annual energy usage [1].This directive highlights the significance of maximizing biogas production from wastewater as a crucial strategy to meet this goal, underscoring its importance in current research and technological advancements in sustainable energy practices.Microbial consortia are utilized in wastewater treatment systems to efficiently convert organic waste into biogas, primarily methane and carbon dioxide (CO2) [2,3].Utilizing wastewater tackles the pressing problem of handling excess sewage sludge and contributes substantially to mitigating greenhouse gas emissions, representing progress toward attaining worldwide sustainability and climate objectives.Nevertheless, the journey towards maximizing biogas generation is filled with new and pressing environmental obstacles, with microplastic (MP) pollution being particularly prominent and problematic to overcome [4].
MPs, which are plastic particles with a size smaller than 5 mm, have surreptitiously penetrated our ecosystems.The causes of contamination are varied and include industrial discharge, waste from consumer products, and the breaking down of more considerable plastic debris due to environmental pressures.The intrusion of MPs into wastewater treatment systems is particularly problematic [5].MPs pose a significant risk to these systems' functionality, specifically intended to handle and repurpose waste sewage sludge while generating biogas [6,7].The problem is related to the durability and stability of these particles in terms of their physical and chemical properties and their intricate interaction with the biological processes involved in AD.
The release of toxic chemicals or additives from MPs can negatively impact microbial activity in digesters by directly harming microbial cells and inhibiting essential enzymes involved in biochemical processes and oxidative stress defense, thereby disrupting the delicate balance of microbial ecosystems in WWTPs [8].These artificial particles interact with microbial cells and extracellular polymeric substances in various ways, potentially altering the overall structure of microbial communities and impeding critical enzymatic processes required to decompose organic material [9].MPs have diverse physicochemical properties that affect their movement, bioavailability, toxicity, and interactions with other pollutants [10].Moreover, the physicochemical characteristics of MPs, such as their surface charge and hydrophobicity, can lead to the absorption of harmful substances, further exacerbating their ability to hinder methanogenesis.These disruptions have diverse repercussions, including decreases in the production and quality of biogas and presenting significant operational challenges to WWTPs.
Despite the growing number of studies highlighting the harmful impacts of MPs on aquatic ecosystems and human health, more research is needed to understand their exact effects on AD processes.This growing interest is mirrored in the scholarly literature, as evidenced by the annual progression of research publications from 2014 to 2023 focusing on "microplastics" and their intersection with "anaerobic digestion".Figure 1 shows the proportion of studies on MPs that also cover AD, underscoring an increasing trend that reflects the rising awareness of MPs' impacts on AD systems.The data for this analysis were sourced from Google Scholar through annual searches for 'microplastics' in general, as well as 'microplastics' and 'anaerobic digestion' to highlight differences.Additionally, percentages were calculated to elucidate trends.Uncertainties still need to be made about how MPs exert their influence, the potential long-term consequences of their presence in wastewater treatment systems, and the success of current strategies to reduce their impact.While some reviews have addressed the general effects of MPs on AD and biogas production [11][12][13], this review provides a more detailed analysis by examining each stage of AD separately.It consolidates current knowledge on the origins and pathways of MPs in wastewater treatment systems, clarifying their impact on microbial communities and biogas production.Additionally, it explores novel mitigation strategies by investigating the combined effects of different pretreatment methods and MPs on biogas production.

MPs in Wastewater Sludge: Sources, Pathways, and Characteristics
MPs vary in form-including fibers, pieces, pellets or beads, and foils-depending on their origin and source, with MP fibers being one of the most prevalent types in water [14].Domestic sewage is crucial in transporting MPs from domestic items and synthetic fabrics into municipal sewage systems [15].The entry of MPs into wastewater treatment systems is a multifaceted environmental problem that involves the interconnection of consumer behavior, industrial practices, and urban infrastructure.MPs originate from various sources, including the breakdown of more extensive plastic materials and intentional discharge into the environment.Studies have shown varying concentrations, colors, and shapes of MPs in sludge [16][17][18][19].Most MPs discovered in wastewater consist of fibers, pieces, and granules, indicating their varied sources.Polyethylene (PE), polypropylene (PP), polyester (PS), and polyvinyl chloride (PVC) are often recognized polymers that have varying effects on the amount of MPs due to their diverse uses and long-lasting nature in the environment.Figure 2 shows the different amounts and types of MPs found in wastewater around the world.In our literature review, it was shown that there were significant variations in the concentration of MP, ranging from 1.21 to 2962 MP/L in the analyzed WWTP.The most commonly identified MPs entering wastewater treatment plants were PET (32 ± 29%), PP (22 ± 15%), PE (16 ± 13%), PS (15 ± 17%), PVC (5 ± 4%), and PA (12 ± 17%).Their concentrations varied significantly, which could be attributed to a range of factors.These factors include the differences in local waste management practices, the efficiency of pre-treatment processes, seasonal variations, and differences in the sources of MPs pollution [20][21][22].Additionally, the physical and chemical properties of the MPs themselves, such as density and degradation rate, may influence their prevalence and concentration in wastewater streams [23].Consumer items, such as toothpaste, face cleansers, and cosmetics, significantly contribute to microbeads in wastewater [42,43].Similarly, synthetic textiles release microfibers when washed, which is a noteworthy contribution considering the worldwide dependence on synthetic fabrics such as polyester and nylon [44].Because of their widespread presence in everyday life, these commonly disregarded fibers constitute a significant proportion of MPs that enter wastewater systems.Figure 3 shows the various sources of microplastic contamination in wastewater systems, highlighting contributions from urban runoff, domestic wastewater, industrial discharge, agricultural runoff, and atmospheric deposition.
Industrial operations contribute to environmental pollution by directly releasing plastic pellets used as raw materials in manufacturing and powders and granules generated by abrasion and cutting activities [45].The escalating issue of plastic pollution worldwide is significantly linked to nurdles or plastic resin pellets, underscoring the importance of understanding their weathering processes, environmental harm, and associated contaminants [46].In many cases, despite having their own wastewater treatment facilities, industrial operations still struggle to manage and eliminate MPs from their discharge effectively.The complexities arise from the diverse array of plastic materials utilized in industrial processes, which can vary widely in composition, size, and dispersal mechanisms.The prevalence of these particles near industrial effluents accentuates the pressing need for enhanced containment and treatment techniques within the industrial sector.Urban runoff is an essential mechanism for MPs to infiltrate wastewater systems.Stormwater transports shattered fragments of more considerable plastic trash, synthetic rubber particles derived from tires, and microfibers from urban dust into sewage systems.Tire-derived MPs deserve particular study because of their widespread presence and the unique difficulties they present.While cars are in motion, the tires gradually deteriorate, releasing minuscule particles covered with an intricate combination of substances, such as heavy metals and organic pollutants.Tire-derived particles, renowned for their durability and intricate chemical composition, introduce additional complexity to wastewater treatment [47].As tires degrade over time, they liberate minuscule particles comprising intricate combinations of polymers and chemical additives [48].These particles, often overlooked in conventional treatment processes, pose a significant challenge to wastewater treatment facilities due to their persistence and potential to exacerbate environmental contamination because they can absorb and carry a mixture of contaminants.Tire-derived MPs are noteworthy due to their intricate composition, which includes synthetic and natural rubber, along with other additives intended to improve tire performance.These particles make up a substantial portion of MPs found in urban wastewater and create distinct difficulties since they can release harmful compounds and interact with microbial populations in wastewater treatment systems.
Chemical pollutants related to MPs are categorized into additives used to improve plastic properties and those absorbed from the external environment [49].On a chemical level, MPs introduce a suite of substances, including polymer-specific additives and absorbed environmental pollutants, into the wastewater matrix.In addition to interfering with microbial cells directly, MPs introduce a wide range of physicochemical disturbances into wastewater treatment processes.Because of these man-made particles, the sludge's rheology, settleability, and dewaterability may change.Wastewater treatment facilities may become less efficient as a result of these alterations, which in turn might raise their operating expenses and energy usage.
A study reported that the effluent from a reclaimed WWTP contained an average of 0.75 ± 0.26 MP/L, with about 98% of these MPs being absorbed and precipitated into the sludge [50].This sludge, rich in MPs, is not only a by-product but also a potential resource for other applications such as biogas production, where its organic content can be utilized to generate energy, although the presence of MPs may affect the efficiency and safety of such processes.Knowledge about the origins, routes, and properties of MPs in wastewater is essential for devising efficient measures to reduce their adverse effects on wastewater treatment and biogas generation.The wide range of MP varieties and their prevalence in numerous environmental sources highlight the intricate nature of tackling MP contamination in wastewater treatment procedures, requiring focused investigation and comprehensive management strategies.

Impact of MPs on Wastewater Treatment
When MPs enter ecosystems that treat wastewater, they set in motion a chain reaction of events that compromise the biological stability and operational efficiency of these systems.Identifying and categorizing MPs is critical for understanding their impact and devising appropriate mitigation strategies.Currently, there are no standardized methods for separating and characterizing MPs in various streams of WWTPs [51].The common techniques for identifying and categorizing MPs include visual analysis, dynamic light scattering, laser diffraction particle size analysis, scanning electron microscopy-energy dispersive X-ray (SEM-EDX), Fourier transform infrared spectroscopy, Raman spectroscopy, thermal analysis, and mass spectrometry, while quantitative measurements are conducted using methods like flow cytometry, spectroscopy, and the index method [52,53].MPs can also be categorized based on their polymer type, shape (e.g., fibers, fragments, beads), and color, helping to track their sources and behavior in wastewater treatment processes [54].
In a study conducted in China, 18 types of MPs in 10 different colors were identified, with PET-MPs, PS-MPs, and PP-MPs making up over 70% of the detected MPs, and the treatment facility achieved an overall MPs removal efficiency of 95.2%, with removal rates of 58.8% in the aerated grit chamber, 54.5% in A2/O-secondary sedimentation, and 71.7% in advanced treatment processes [55].Due to their small size, some MPs evade capture by screening mechanisms during preliminary wastewater treatment.Unlike heavier solids, they fail to settle in grit chambers, posing challenges for their removal in the initial stages of treatment [56].In the mechanical phase of wastewater treatment, the application of coagulants and flocculants effectively transfers up to 97.3% of MPs to sludge, with the highest removal efficiency of 90% achieved using 2.5 mL/L of PAX, facilitating safe sludge management and reducing the burden on biological treatment stages [57].
In primary sedimentation tanks, the effectiveness of sedimentation diminishes for smaller, less dense MP particles that take longer to settle [58].Due to their buoyancy and size variations, a significant portion remains in the water column, leading to incomplete removal of MPs.The study at the Nash WWTP in South Wales, UK, found that MPs ranging from 1000 to 5000 µm were almost entirely removed from incoming sewage in the primary settler tank, with the majority (83% by mass and 93% by abundance) settling in sludge and a smaller fraction in surface scum, cumulatively accounting for about 1% of the sewage sludge's dry weight [59].
In secondary treatment, microbes adhere to MP surfaces, secrete extracellular polymeric substances (EPSs), colonize the surface, and replicate to create biofilms [60].This process disrupts microbial communities and interferes with biological treatment processes.Additionally, the aeration process may reintroduce previously settled MPs back into the water column, further complicating their removal and affecting the efficacy of the treatment.Due to their durability and resistance to degradation, MPs and plastics, which lack functional groups and hydrolyzable bonds, are unsuitable for microbial attachment and enzymatic reactions [61].Other studies argue that MPs have little to no effect on the functions of important microbes involved in wastewater treatment, including denitrifiers, microorganisms that accumulate polyphosphates, ammonium-oxidizing bacteria (AOB), and nitrite-oxidizing bacteria (NOB), suggesting that their effect on wastewater treatment performance might be overstated [62].
In tertiary treatment, advanced filtration methods such as membrane filtration and UV irradiation effectively remove a high percentage of MPs; however, these techniques are only sometimes implemented due to their high operational costs and maintenance demands.Moreover, MPs can shield pathogenic microorganisms from UV light and disinfectants, diminishing the efficacy of these disinfection processes.A study demonstrated that in urban wastewater treatment, PE-MPs, PS-MPs, and PVC-MPs reduced the effectiveness of sunlight/H2O2 disinfection yet enhanced the efficacy of the solar photo-Fenton (SPF) process, although the SPF improvement did not extend to controlling bacterial regrowth post-treatment [63].

Impact of MPs on Biogas Production
Diverse microbial communities are essential to the efficiency of wastewater treatment methods, particularly those that generate biogas by AD [64].In a precisely coordinated biochemical process, these microbes degrade organic materials into biogas, a sustainable energy source.However, the ubiquitous presence of MPs disrupts this delicate microbial equilibrium, leading to physicochemical disturbances that may diminish both the production and quality of biogas.Biogas production progresses through four distinct stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis [65].MPs can uniquely affect each stage, highlighting the importance of examining their specific impacts on the overall biogas generation process (Figure 4).To understand these effects, existing studies were analyzed to determine the impact of MPs on critical indicators such as methane production potential, hydrolysis coefficients, volatile fatty acid (VFA) production, hydrogen and acetate levels, and microbial activity under varying MP concentrations and types.

Effect on Hydrolysis
The primary organisms involved in this fermentation stage are mostly facultative and anaerobic bacteria.Because it consists of the transformation of insoluble organic compounds (carbohydrates, lipids, and proteins) into soluble organic compounds (sugars, long-chain fatty acids, and amino acids), hydrolysis is typically considered a rate-limiting step in biogas production [66].Varying doses of PE-MPs (from 0 to 200,000 particles/kg of activated sludge) were analyzed for their impact on the hydrolysis phase of AD, demonstrating a decrease in both methane production potential and hydrolysis coefficient across nearly all conditions, causing a reduction in methane production ranging from 88% to 95% compared to the control after 59-day digestion [7].The 50 µm PE-MPs did not significantly alter biogas production across a range of concentrations (1 g/L to 4 g/L), but they decreased the hydrolysis coefficient compared to the control, suggesting an inhibitory effect of smaller MPs on the hydrolysis stage of AD [67].This effect was likely due to their larger surface area, which increased the adsorption of inhibitory substances and limited anaerobic microorganism activity.At concentrations over 20 particles/g total solids (TS), PVC-MPs reduced the hydrolysis coefficient and methane potential of wasteactivated sludge (WAS) [6].These findings highlight the inhibitory effects of MPs on the hydrolysis stage of AD, impeding the essential breakdown of complex organics needed for effective biogas production.

Effect on Acidogenesis
In the acidogenesis stage of biogas generation, bacteria transform soluble products from hydrolysis into short-chain organic acids (such as acetic, propionic, and butyric acids), alcohols, and gases (including CO2 and hydrogen), which serve as essential substrates and energy sources for methanogens [68].The following steps in the biogas production process, especially acetogenesis and methanogenesis, rely on VFAs as an essential intermediary.Existing data on the impact of MP on VFA composition are often contradictory due to varying conditions in each study; thus, it is essential to develop a standardized protocol for assessing VFA production [69].By standardizing the process, we can better understand the effects of different pretreatment techniques and foreign particles, such as MPs, on VFA yields and composition.Previous studies have shown that low concentrations of polystyrene (30 particles/g TS) significantly increased the production of VFAs by 12.8% due to enhanced solubilization and enzymatic activity, whereas high concentrations (90 particles per gram) reduced VFA production by about 17%, despite initially boosting organic matter release, due to inhibited microbial activities [70].At high concentrations, the harmful effects of reactive oxygen species (ROS) and excess leached sodium dodecyl sulfate (SDS) increased the toxicity, inhibiting microorganism activities and VFA production.PS-MPs at 0.25 g/L reduced glucose breakdown and considerably impeded acidification, resulting in a 5.5% drop in efficiency relative to the control [71].Compared to lower doses (0.05, 0.1, and 0.2 g/L), which did not significantly differ from the control group, it was suggested that this effect could have been due to the MPs releasing more leachates that were toxic to microorganisms.

Effect on Acetogenesis
Hydrogen and acetate are produced during the acetogenic phase by breaking down longer fatty acids.This process needs low levels of hydrogen and acetate to keep going and also involves homoacetogenesis, where hydrogen is used to turn CO2 into more acetic acid [72].In addition to acidogenesis, acetogenesis can create hydrogen by converting butyric acid to acetic acid [73].PET-MPs added during WAS fermentation strongly impeded key activities, including hydrolysis, acidogenesis, and acetogenesis, which are vital for hydrogen; the breakdown of butyrate dropped from 20% to 17% when the amount of PET-MPs increased from 0 to 20 particles/g TS.By increasing the dosage to 60 particles/g TS, the degradation rate decreased dramatically to 16%, indicating a dosedependent inhibition of activities associated with hydrogen formation [73].The disruptive influence of PET-MPs on these processes could be attributed to the MPs acting as a physical barrier, adsorbing to the microbial cells or enzymes, thereby stymying their metabolic activity or potentially releasing toxic compounds that inhibit microbial function and enzyme activity.

Effect on Methanogenesis
MPs influence the microbial consortia at multiple stages of AD, particularly during methanogenesis [74].At this critical stage, when methanogenic archaea convert CO2, acetate, and hydrogen into methane, it is particularly susceptible to disruption by MPs.It was observed that the impact of PS-MPs on methane production varies with their concentration.Lower concentrations (20-40 particles/g TS) increased methane yield by 3.38-8.22%,while higher concentrations (80-160 particles/g TS) reduced methane yield by 4.78-11.04%[75].The study also indicated that PS-MPs can enhance sludge processing at lower levels but inhibit key processes and shift methanogenic pathways at higher concentrations due to oxidative stress.PS-MPs in AD systems reduce acetoclastic methanogenesis, which hinders the reaction that converts acetate into methane, reducing methane production [76].MPs presence in reactor digestate was also found to retain more organic matter and nutrients, slightly hinder dewaterability, and inhibit methane production due to incomplete digestion [7].
The presence of PE-MPs of different sizes at a dosage of 100 mg/g TS significantly impacted the stages of biogas production, leading to reductions in methane production by 6.1% and 13.8% in the presence of PE-MPs-180 µm and PE-MPs-1 mm, respectively [48].In an AD of WAS, the addition of PET-MPs at doses ranging from 1 to 6 mg/g TS revealed that a medium concentration of 3 mg/g TS was most effective, enhancing methane production by 15% due to improved sludge solubilization and microbial biofilm formation, while the higher dose showed no significant changes, potentially due to the leaching of harmful additives from the plastics [77].
A study investigating the effects of different aged MPs and their leachates on anaerobic methanogenesis of sludge suggested that the inhibition of methanogenesis by MPs is primarily due to the leachates, which induce oxidative stress, damage microbial cells, and reduce microbial activity [78].High concentrations (0.5 g/kg VS) of PVC-MPs, especially when combined with hydrothermal pretreatment, reduced methane production by 16.2% due to accelerated release of toxic substances like dibutyl phthalate, dimethyl phthalate, and Bisphenol A, impacting microbial activity [79].It was observed that, unlike typical MPs, which inhibit AD, adding 10 particles of Polyamide 6 MPs/g TS enhanced methane production by 39.5% due to the effect of leached caprolactam from Polymide 6 MPs [80].The leaching of caprolactam improved acidification and methanogenesis by promoting key enzyme activities, thereby increasing methane production.PVC-MPs at concentrations of 1 and 10 particles/g TS did not affect methane production but reduced cadmium toxicity [81].
Low concentrations of polyester fibers (10 mg/L) did not affect methane generation, but it rose by 7% and 17%, respectively, at 100 mg/L and 1000 mg/L compared to the control.Additionally, compared to the control, methane generation was 20% to 27% higher in the presence of polyamide fibers at concentrations of 10, 100, and 1000 mg/L [82].
Recent findings indicate that 150 µm PE-MPs at a concentration of 1 g/L increased biogas production by 12%, likely serving as an additional carbon source and enhancing AD.However, at higher concentrations of 4 g/L, these MPs reduced biogas production by approximately 7%, likely because organic matter adsorbing onto the MP surfaces made them less bioavailable, thus decreasing biogas generation efficiency [67].The presence of PE-MPs of different sizes at a dosage of 100 mg/g TS significantly impacted the stages of biogas production, leading to reductions in methane production by 6.1% and 13.8% in the presence of PE-MPs-180 µm and PE-MPs-1 mm, respectively, primarily by inducing changes in the microbial population [83].

Effect on Digester Microbiome
Diverse microbial communities are essential to the efficiency of wastewater treatment methods, particularly those that generate biogas by AD [84].MPs may become less bioavailable as organic matter adsorbs onto their surfaces, thus decreasing biogas generation efficiency.MPs can alter the composition of microbial communities, which could benefit microbes that are either less efficient at producing biogas or more resilient to the stresses caused by plastic.The presence of PVC-MPs alongside cadmium (Cd) was found to mitigate Cd's bioavailability and its inhibitory effects on anaerobes, evidenced by enhanced carbon flux towards biomethane and recovery of biogas production from 58.8% to 89.7%, likely due to PVC-MPs' higher adsorption capacity for Cd compared to sludge substrates [81].The protective role of PVC-MPs suggests that they may shield anaerobic microbes from Cd toxicity by adsorbing the metal, thus preserving the microbial activity essential for biogas production.After incubation, SEM revealed morphological alterations in PE-MPs, particularly in 150 µm MPs, which may impact microbial interaction and degradation efficiency due to the increased surface erosions and grooves [67].The retention of PVC-MPs in sewage sludge affected AD by leaching Bisphenol A, which altered microbial metabolism and shifted the community away from hydrolysis-acidification and methanation, reducing methane production rates by 5.9% to 24.2% at dosages from 10 to 60 particles/g TS [6].Elevated levels of polycarbonate (PC)-MPs (200 particles/g TS) resulted in heightened generation of intracellular ROS, diminished enzyme activity and cell viability, and hindered hydrolysis and acidification processes [85].PC-MPs leached 1.26 mg/L of Bisphenol A at 30 particles/g TS, reducing intracellular ROS production, enhancing enzyme activity and biomass viability, and increasing the abundance of Methanobacterium sp. and Methanosarcina sp., ultimately boosting methane production [85].Polyamide 6 MPs enhanced vital enzyme activities in AD, particularly impacting Coenzyme F420.The activity of Coenzyme F420 peaked at 200% of control levels at a concentration of 10 particles/g TS and then declined to 120% at a concentration of 50 particles/g TS [80].During the AD of sludge, the co-presence of polyamide and ofloxacin MPs significantly increased the relative abundance of antibiotic resistance genes such as tetA, sul1, and intI1, with increases ranging from 12.3% to 79.2%, indicating enhanced antibiotic resistance genes dissemination linked to augmented microbial interactions and gene transfer mechanisms facilitated by the co-exposure [86].

Mitigation Strategies: Enhancing Biogas Production Amid MP Contamination
The pervasive issue of MP contamination in wastewater treatment systems necessitates developing and implementing effective mitigation strategies.These strategies aim to reduce the adverse impacts of MPs on the microbial consortia responsible for biogas production and to improve the overall efficiency of wastewater treatment processes.Current approaches range from pretreatment techniques designed to remove MPs from wastewater streams before they reach the biological treatment stages to innovative bioreactor modifications and advancements in microbial engineering that enhance the resilience of microbial consortia to MP-associated stressors.

Pretreatment
Prior research has consistently investigated the impacts of MPs through direct introduction into AD systems, neglecting the customary pretreatment procedures that sludge typically undergoes before its introduction into AD systems [87].Pretreatment methods, encompassing ultrasonication, high temperature, acid, thermo-chemical, low temperature, alkaline, ozonation, and thermo-mechanical techniques, are crucial in optimizing biogas production efficiency [88].Table 1 presents a synthesis of studies incorporating various sludge pretreatment methods to assess their combined effect with MPs' presence on the efficiency of AD.PET-MPs, subjected to 0.5 M alkali pretreatment for two days followed by thermal hydrolysis at 127 °C for 120 min and added in sizes of 250-500 µm at doses of 0, 1, 3, 6 mg/g TS, produced 22% more methane than those without pretreatment, across all doses [77].A study showed that thermal hydrolysis (TH) pretreatment at 170 °C for 30 min reduced the inhibition of methane production by PE-MPs in AD of WAS, lowering the inhibitory effect from 12.1% to 8.3%, a 31.4% reduction [89].The same study also found that the TH pretreatment increased the leaching of additives, such as acetyl tri-n-butyl citrate, from PE-MPs, with concentrations reaching 124.0 µg/L and resulting in a 21.4% boost in methane production, indicating that TH pretreatment can effectively mitigate the impact of MPs on biogas production.In a reclaimed WWTP, effluent MP concentrations were reduced by 41% through combined treatments, including thermal hydrolysis, AD, and plate and frame dewatering, with thermal hydrolysis being the most effective [50].In another study, various pretreatment methods on WAS containing 0.2 g/L PS-MPs significantly influenced methane production, solubilization, and microbial dynamics.Alkaline pretreatment most effectively increased solubilization and VFA production, notably enhancing methane output by 20.4%, and showed the highest microbial activity favoring methanogenesis [90].The alkaline, Fenton, and thermal pretreatment processes favored the dominance of the Firmicutes phylum in the sludge microbial community.In contrast, the control and ultrasonically pretreated sludge were predominantly composed of Chloroflexi and Proteobacteria, indicating a shift from Gram-positive to Gram-negative bacteria due to the varying resistance of their cell walls to the pretreatment methods.Conversely, Fenton and thermal treatments also improved VFA production, but to a lesser extent, with respective methane increases of 18.7% and 17.5%.Ultrasonic treatment, however, had minimal impact [90].Ozone pretreatment enhanced gas production by 12% at a lower concentration (20 mg/L) of microfibers by increasing their surface area for adsorption of soluble substances, which microorganisms could utilize.However, this effect was not observed at higher concentrations (100 mg/L-1000 mg/L), likely due to the inhibitory effects of excess microfibers on microbial activity [82].An investigation on enhancing biomethane production through anaerobic co-digestion of synthetic primary sludge with biodegradable bioplastics at municipal water resource recovery facilities was performed, examining the impact of thermal and chemical pretreatments on polyhydroxy butyrate and polylactic acid bioplastics, resulting in significant improvements in biochemical methane potential, with increases exceeding 100% [91].

Advancements in Microbial Engineering Techniques
Recent advancements in microbial engineering offer exciting possibilities for improving biogas production in the presence of MPs.Genetically engineered microorganisms, designed to express enzymes capable of degrading plastic polymers, represent a cutting-edge approach to tackling MP pollution.In the landmark review [93], the bacterium Ideonella sakaiensis 201-F6 emerges as a pivotal microorganism, owing to its enzymes PETase and MHETase-enhanced by mutations like R61A, L88F, and I179Fwhich exhibit a dramatically improved capacity to degrade the resilient polymer PET, paving the way for innovative biotechnological applications in plastic waste management.Currently, the mechanisms related to polymer degradation are quite comprehended, while those concerning MPs degradation remain less explored, with the corresponding degradation pathways still unclear [94].The development of consortia of engineered microbes that can synergistically degrade different types of plastics while simultaneously producing biogas is a particularly promising area of research.Furthermore, the application of CRISPR-Cas systems for the targeted enhancement of microbial tolerance to MP-associated stressors could lead to more robust microbial consortia in anaerobic digesters [95].Through the application of cutting-edge genetic engineering, particularly CRISPR-Cas genome editing, scientists are optimizing acetogens like Clostridium ljungdahlii for superior biogas production by enhancing their C1 gas conversion capabilities, with systems like AlphaFold2 aiding in the meticulous design of the Wood-Ljungdahl pathway enzymes to transform emissions into value-added biochemicals sustainably [96].

Conclusions
To effectively manage MP contamination in wastewater treatment systems, a comprehensive approach is required.This approach should combine technological advancements with robust regulatory frameworks and public policy initiatives.Most research has focused on the effect of MPs on methanogenesis by reporting their impact on accumulated methane gas, yet fewer studies have detailed the effect of MPs on each specific stage of AD.Analyzing these stages individually can help identify the exact mechanisms of MP interference and develop targeted mitigation strategies.Interestingly, MPs have been shown to improve biogas production at environmentally reasonable concentrations.Additionally, few studies have analyzed the combined effect of different sludge pretreatment methods and MPs.More studies are needed to investigate these combined effects to develop comprehensive strategies for enhancing biogas production in the presence of MPs.This review supports the discussion on sustainable wastewater management and biogas production by promoting integrated management solutions.It emphasizes the importance of collaborative efforts to improve biogas production, making it more efficient and capable of withstanding environmental obstacles such as MP contamination.By doing so, it contributes significantly to the ongoing efforts to achieve worldwide sustainability and climate objectives.

Figure 1 .
Figure 1.Trend analysis of MPs research in AD (data sourced from Google Scholar).

Figure 2 .
Figure 2. The scope of MP contamination; (A) maps the concentration of MPs across various global water bodies, with the intensity of the blue color and the size of the circles representing the amount of contamination; (B) a comparative analysis of the types of polymers found in wastewater; the dots are individual data that were collected for analysis, and × is the average of all collected points [24-41].

Figure 3 .
Figure 3. Sources of MP contamination in wastewater treatment systems.

Figure 4 .
Figure 4. Impact of MPs on AD process.

Table 1 .
Impact of sludge pretreatment on methane yield in AD amid varying MPs loads and types.