The overlooked pathway: A systematic review on sewage sludge treatment as a critical secondary source of terrestrial micro(nano)plastics

Sewage sludge has long been believed as an essential sink for microplastics (MPs), with concentrations up to 1380 particles/kg dry weight. Considering that MP residues in present sewage sludge are predominantly aged, their surface characteristics have been modified, with specific surface area ranging from 1 m²/g to 5 m²/g and a carbonyl index increasing from 0.2 to 1.8, which corresponds to greater toxic effects. Nevertheless, the formation mechanisms and potential ecological risks of those aged MPs to sludge ecology have yet to be unraveled. This paper provides an overview of the effect of sludge treatment processes (e.g., mechanical pressing, sludge dry/wet heat oxidation, anaerobic digestion, composting) on the plastic fragmentation and MPs formation based on the existing techniques, including scanning electron microscopy, Fourier-transform infrared spectroscopy. Temperature-catalyzed heat oxidation in physically, microbial-dominated biodegradation, and reactive oxygen species (hydroxyl radicals and superoxide anions)-mediated chemical oxidation are preliminary contributors to MP weathering, and able to induce up to 76 % reduction in plastic number and 97 % reduction in plastic molecular mass. Moreover, adverse environmental threats of aged MPs to the ecology of sewage treatment plants via the release of toxic oligomers to surrounding sludge; shifting the sludge microbial community structure; and altering the sludge function were also summarized herein. Overall, the obtained results elucidate formation pathways and environmental fate of sludge-based MP (pristine/aged variants), establishing a scientific basis for ecological risk assessment and predicting their cross-media migration.