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Configuration-driven microplastic fate in full-scale sewage sludge treatment and opportunities for system-level mitigation
Summary
Researchers examined how different treatment configurations in wastewater plants shape the types and concentrations of microplastics that end up in sewage sludge — the solid byproduct that is frequently spread on agricultural land as fertilizer. Microplastic concentrations in final sludge ranged from 617 to 936 particles per gram of dry solids, with fine fragments under 100 micrometers dominating, and the specific sequence of thickening, digestion, and dewatering steps significantly influenced which polymer types were retained. Since sludge application is a major route for microplastics to enter farmland, optimizing treatment processes could meaningfully reduce environmental contamination.
The accumulation of microplastics (MPs) in sewage sludge has major implications for land application and biosolid sustainability. We assessed microplastics across three wastewater treatment plants with different sludge line treatment configurations to determine how thickening, digestion, and dewatering shape the particle abundance, size, and polymer profiles. The study observed that the treatment process configuration strongly influenced the fate of microplastics. The Microplastic concentrations ranged from 617 to 936 MP/g of dry solids. Most of the particles were predominantly fibrous and mainly within the >10-100 μm range, however, in dewatered sludge, the proportion of fragments became dominant. Polymer-specific analysis showed that configurations in which WAS entered the treatment train at later stages promoted the enrichment of low-density polymers, such as polyethylene (PE) and polypropylene (PP), particularly in the fine-particle fraction. Although PE dominate across all stages, polyvinyl chloride (PVC), polyurethane (PU), and polyethylene terephthalate (PET) exhibited preferential retention during digestion and dewatering. Moreover, despite operational differences, the final dewatered sludge converged at similar concentrations, dominated by fine MPs (<100 μm), with total loads of 1.36-3.59 × 10 MP/d. While mechanical dewatering can modify the microplastic profiles in the final treated sludge, these characteristics are also strongly influenced by the type of feed entering the dewatering process and the point at which it enters the solids line. Accordingly, the centrate generated during dewatering remains an important intervention point for limiting terrestrial transfer.
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