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Microplastics in Wastewater Treatment Plant Sludges: Detection Methods, Seasonal Variation, and Effects on Anaerobic Digestion
Summary
Scientists found that microplastic pollution in wastewater treatment plants spikes in spring, likely from snowmelt and runoff, and these plastics often carry other contaminants along with them. The study also found that tiny plastic fibers can disrupt the treatment process itself, while plastic fragments actually boosted it, showing that shape and type matter for how we manage this pollution before it reaches our environment.
Wastewater treatment plants (WWTPs) are an important pathway for microplastics (MPs), whose distribution is influenced by numerous factors impacting different treatment processes. Addressing MP pollution requires reliable and standardized detection protocols, which are currently lacking. This thesis addresses these challenges by investigating MP analysis methods, exploring the understudied impact of seasonal variations on MP characteristics in sludges, and evaluating their effects on downstream treatment, anaerobic digestion (AD). Two micro-flow imaging (MFI) based particle counters were evaluated for rapid, in-situ MP identification and enumeration. MFI could automate the measurement of particle features (size, shape descriptors, colour, intensity), offering improved reliability over manual techniques. Transparent MPs, often missed by microscopy, were effectively detected. Despite limitations with high concentrations and reflective particles, MFI offers promise for real-time MP monitoring. To classify MPs based on their polymer nature, hyperspectral imaging (HSI) with perClass Mira software was studied under environmentally relevant conditions. MPs of varying sizes, colours, and polymer types were subjected to physical and chemical treatments. Results indicated polymer and size-dependent spectral changes, with smaller MPs more prone to degradation. This study establishes a foundation for rapid, efficient MP classification with minimal sample preparation using HSI. Seasonal variations of MPs across primary, secondary, and digested sludge streams at a municipal WWTP in Ontario were studied. Spring samples showed the highest MP concentrations (up to 2.5×), including tire and road wear particles, likely due to snowmelt and stormwater runoff. While polymer composition remained stable, FTIR analysis revealed the presence of contaminants on MPs, underscoring their role as pollutant vectors and the need for seasonal monitoring. Polyethylene terephthalate (PET) and polyamide 6 (PA6) microfibres (MFs) and fragments at varying concentrations were used to assess the impact of MPs on AD. MFs inhibited methane production by up to 17%, increased reactive oxygen species (ROS), and disrupted microbial communities. Fragments of the same polymers enhanced methane output, indicating morphology-dependent effects. Microbial shifts and ROS levels were influenced by MP polymer and shape. This research advances the understanding in MP detection, reveals seasonal impacts in sludges and underscores the importance of polymer and shape-dependent MP mitigation strategies in AD systems.