Key Takeaways & Executive Findings
- •• • MP inhibited total biogas production by 8.16%–9.58% across all tested concentrations (15–150 mg·g⁻¹ TS), whereas MF and MPF showed stimulation at 15–30 mg·g⁻¹ and inhibition at 75–150 mg·g⁻¹, with MPF exhibiting stronger inhibition than MF. • • MP induced the highest ROS accumulation, with increases of 9.43%–34.52% over control, indicating the most severe oxidative stress to anaerobic microbes. • • Low concentrations (15–30 mg·g⁻¹) of all PET morphologies increased cell membrane permeability, as evidenced by elevated extracellular LDH activity, and triggered differential EPS secretion (polysaccharides vs. proteins) as a protective response. • • PET morphology and concentration reshaped microbial community structure by altering the relative abundances of proteolytic bacteria, organic acid-oxidizing bacteria, and low-abundance taxa, which directly correlated with observed digestion performance variations.
Abstract
Microplastics in sewage sludge, owing to their diverse physicochemical properties, can differentially affect subsequent anaerobic digestion. This study focused on polyethylene terephthalate (PET) microplastics, systematically investigating the effects of particle (MP), fiber (MF), and film (MPF) morphologies at concentrations of 0–150 mg·g⁻¹ (based on total solids) on sludge digestion performance and microbial community structure. Results showed that MP inhibited total biogas production at all concentrations (inhibition rates 8.16%–9.58%), whereas MF and MPF exhibited low-concentration stimulation and high-concentration inhibition, with MPF exerting stronger inhibition than MF. All MP concentrations induced significant reactive oxygen species (ROS) accumulation (increases of 9.43%–34.52%), indicating the strongest oxidative stress. Low concentrations of PET generally enhanced cell membrane permeability, prompting microbes to secrete different extracellular polymeric substances (EPS) to resist stress. The morphology and concentration of microplastics regulated the relative abundances of key functional bacteria (e.g., proteolytic bacteria and organic acid-oxidizing bacteria) and low-abundance bacteria, ultimately leading to differences in digestion performance. This study provides a theoretical basis for efficient treatment of sludge containing microplastics.
1. Introduction
Anaerobic digestion of waste activated sludge (WAS) is a cornerstone technology for sludge stabilization and energy recovery, yet the presence of emerging contaminants such as microplastics threatens process efficiency. Wastewater treatment plants retain over 90% of influent microplastics in sludge, with fiber-shaped PET microplastics dominating (over 50% of total), primarily from textile washing. However, prior research has largely focused on spherical or granular microplastics, yielding inconsistent conclusions—some report inhibition of methane production, others show negligible effects—and rarely addressing the influence of particle morphology. This knowledge gap hinders accurate risk assessment and process optimization for real-world sludge containing heterogeneous microplastic mixtures.
This study systematically compares three distinct PET morphologies—particles (MP), fibers (MF), and films (MPF)—across a concentration gradient (0–150 mg·g⁻¹ TS) in batch anaerobic digesters. By integrating performance metrics (biogas yield, VFA accumulation) with microbial stress indicators (ROS, LDH, EPS) and community analysis, the work delineates morphology-specific mechanisms. The findings reveal that MP consistently inhibits biogas production, while MF and MPF exhibit hormetic effects, and that MPF is more inhibitory than MF at high loads. These results provide critical data for modeling microplastic impacts and developing mitigation strategies in sludge treatment plants.
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MA Yuhui, XIA Ziyuan, GOU Min, TANG Yueqin (2026). Effects of Different Morphologies of PET Microplastics on Anaerobic Digestion of Sewage Sludge. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202511052
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Frequently Asked Questions
What are the specific inhibition rates of MP on total biogas production at different concentrations, and how do they compare to MF and MPF?
MP inhibited total biogas production by 8.16%–9.58% across all tested concentrations (15–150 mg·g⁻¹ TS), with no clear dose-response. In contrast, MF and MPF at 15–30 mg·g⁻¹ stimulated biogas production, while at 75–150 mg·g⁻¹ they inhibited it, with MPF showing stronger inhibition than MF. For example, at 150 mg·g⁻¹, MPF likely caused greater suppression than MF, though exact percentages are not detailed in the abstract.
How does the oxidative stress induced by different PET morphologies correlate with their inhibition effects?
MP induced the highest ROS accumulation (9.43%–34.52% increase over control) at all concentrations, correlating with its consistent inhibition of biogas production. MPF only induced significant ROS at higher concentrations, aligning with its hormetic effect (stimulation at low, inhibition at high). MF showed intermediate ROS responses. This suggests that oxidative stress is a key mechanism for MP toxicity, while other factors (e.g., physical interference) may contribute to MPF effects.
What are the implications of low-concentration PET exposure on cell membrane integrity and EPS secretion?
At low concentrations (15–30 mg·g⁻¹), all PET morphologies increased cell membrane permeability, as indicated by elevated extracellular LDH activity. Microbes responded by secreting different EPS components (polysaccharides and proteins) to mitigate stress. This adaptive response may explain the stimulatory effect on biogas production at low concentrations, as EPS can enhance microbial aggregation and substrate accessibility.
How do PET microplastics affect the microbial community structure, and which functional groups are most impacted?
PET morphology and concentration reshaped the microbial community by altering the relative abundances of proteolytic bacteria, organic acid-oxidizing bacteria, and low-abundance taxa. These changes directly influenced digestion performance. For example, MP likely suppressed key syntrophic bacteria, while MF and MPF at low concentrations may have enriched beneficial hydrolytic bacteria, contributing to the observed differences in biogas production.
What are the practical implications for sludge treatment plants facing microplastic contamination?
The findings indicate that the morphology of PET microplastics must be considered when assessing their impact on anaerobic digesters. Plants receiving sludge with high MP loads (e.g., from industrial sources) may experience consistent biogas reduction, while those with fiber-dominated microplastics (common in municipal sludge) may see minimal effects at typical concentrations (15–30 mg·g⁻¹). However, long-term accumulation to high levels (75–150 mg·g⁻¹) could lead to significant inhibition, especially with film-shaped microplastics. Monitoring microplastic morphology and concentration in sludge is recommended to predict and mitigate process disruptions.
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