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The overlooked pathway: A systematic review on sewage sludge treatment as a critical secondary source of terrestrial micro(nano)plastics
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This systematic review examines sewage sludge as an overlooked pathway for microplastics to contaminate land, with concentrations reaching over 1,300 particles per kilogram. When this sludge is applied to farmland as fertilizer, aged and chemically modified microplastics enter agricultural soil, where they may be more toxic than fresh particles and can potentially be taken up by crops.
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.
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This review summarizes research on microplastics in sewage sludge and soil, noting that when contaminated sludge is used as fertilizer, it turns farmland into a major reservoir for microplastic pollution. The accumulated microplastics can alter soil properties, harm soil organisms, and potentially enter crops and groundwater, creating pathways for human exposure through food and drinking water.
Sewage sludge as a source of microplastics in the environment: A review of occurrence and fate during sludge treatment
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This review assessed how wastewater treatment plants concentrate microplastics from influent into sludge, estimating that sewage sludge applied to agricultural land represents a major secondary pathway for MPs entering terrestrial ecosystems, with estimated releases of millions to billions of particles per hectare.
An Overlooked Entry Pathway of Microplastics into Agricultural Soils from Application of Sludge-Based Fertilizers
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Researchers analyzed sludge-based fertilizers applied to agricultural soils and found high microplastic concentrations (hundreds to thousands per kilogram of dry weight) that were transferred to soils after application, identifying this as an important but overlooked pathway for terrestrial microplastic contamination.
Microplastics in Sewage Sludge: A Known but Underrated Pathway in Wastewater Treatment Plants
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This review finds that wastewater treatment plants effectively transfer microplastics from effluent into sewage sludge, creating a significant but underrated pathway for MP contamination when sludge is applied to agricultural soils.
Variation in microplastic concentration, characteristics and distribution in sewage sludge & biosolids around the world
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Researchers systematically reviewed 65 studies on microplastics in sewage sludge and biosolids from wastewater treatment plants around the world. They found that while treatment processes remove 57% to 99% of microplastics from wastewater, the removed particles concentrate in sludge that is often applied to agricultural land. The review highlights that land application of biosolids may be a significant, underappreciated pathway for microplastic pollution in soils.
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