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Microfibres versus fragments: differential impacts of polyethylene terephthalate (PET) and polyamide (PA6) microplastics on anaerobic digestion efficiency and microbial ecology
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Scientists tested how tiny plastic particles affect the breakdown of waste in treatment plants that clean our water. They found that fiber-shaped microplastics (like from synthetic clothing) reduced the system's ability to process waste by up to 17%, while plastic fragments actually helped it work better. This matters because if our wastewater treatment plants become less efficient due to microplastic pollution, it could affect water quality and the environment around us.
Rising microplastic (MP) pollution can significantly affect engineered treatment systems such as anaerobic digestion (AD). While prior studies have investigated the influence of individual polymers, varying concentrations and sizes on AD, the role of MP morphology and polymer interactions remains underexplored. This study investigated these factors using polyethylene terephthalate (PET) and polyamide 6 (PA6) MPs, both in isolation and in combination (1:1 ratio), introduced as microfibres (MFs) and fragments at three concentrations, 1, 5, and 15 mg/gTS. Results revealed morphology-dependent effects on methane production. MF exposure inhibited methane yield by 10-17% (p < 0.01), with PET and mixed polymers exhibiting a correlation to MP concentration. In contrast, fragments enhanced methane yield, particularly PA6 and mixed (PET and PA6) polymers increased methane output by 9 and 17% at the highest dose, respectively. Kinetic modelling further revealed that MFs consistently reduced methane production potential, apparent degradation and hydrolysis rate, whereas fragment trends were polymer-driven. Scanning electron microscopy (SEM) micrographs showed greater surface roughness in PA6, which enhanced microbial colonization compared to PET. Elevated reactive oxygen species (ROS) levels with MF addition, especially at the highest concentration, suggested higher oxidative stress and microbial inhibition. Microbial community analysis showed that exposure to MP fragments resulted in similar bacterial shifts across different polymer types, compared to the more diverse effects observed with MFs. Archaeal diversity was more affected by particle shape than polymer composition. All MP treatments favoured a shift toward hydrogenotrophic over aceticlastic methanogenesis. PET and mixed MF addition resulted in a substantial decline in the relative abundance of Actinobacteria (18-20%) from 42% in the control and other methanogenic taxa compared to their fragment counterparts. MF addition disrupted community structure, suppressed additive-degrading taxa, and increased acetogenic groups such as Synergistetes. Overall, the findings suggest that a comprehensive understanding of all influencing factors, including MP morphology, polymer type and concentrations, is important for effective AD system management.
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