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Assessment of an air-assisted wet separation process for microplastic reduction in sewage sludge
Summary
Sewage sludge (the leftover solid waste from treating wastewater) is often used as fertilizer on farms, but it's frequently contaminated with tiny plastic bits called microplastics that can end up in soil, crops, and eventually our food. Researchers found a simple, chemical-free method using air bubbles to pull about 80% of these microplastics out of dried, shredded sludge, working best on larger plastic fragments. This kind of low-cost, no-chemicals-needed process could help make sludge safer to reuse as fertilizer, potentially reducing how much microplastic pollution makes its way into farmland and,
Microplastics (MPs) in sewage sludge limit safe sludge handling, agricultural reuse, and circular valorization. This study develops and evaluates an air-assisted wet separation (wet aeration/flotation) step that promotes selective bubble–particle attachment to transfer hydrophobic MPs to an overflow stream without chemical additives while limiting sludge entrainment. Bench-scale tests with municipal sewage sludge spiked with representative secondary MPs (PET, PP, and HDPE fragments, and PET fibers) were used to identify required sludge preconditioning, to quantify operating windows, and to assess the effects of MP properties. Drying and size reduction of sewage sludge were essential to enable biphasic behavior and selective flotation. Untreated wet sludge formed homogeneous suspensions that prevented separation. Under optimized conditions (60 min, W/S = 40, air flow rate = 0.5 L/min), approximately 80% MP separation was achieved with sludge losses <2.5%. Separation increased with particle size (>90% for MPs >3 mm), while polymer type and morphology had a limited influence relative to particle size. Validation with sludges from different wastewater treatment plants showed consistent performance across the tested matrices, although thermally dried pelletized sludge reduced separation efficiency from 86.82% to 72.65%. The process provides a reagent-free, design-relevant separation–loss trade-off and supports integrated pollution control via internal process-water reuse and a small MP-enriched solid stream for downstream handling.