Does size matter? Quantification of plastics associated with size fractionated biosolids

This study investigated the occurrence and contribution of plastic particles associated with size fractionated biosolids to the total concentration in biosolids (treated sewage sludge) samples collected from 20 wastewater treatment plants (WWTP) across Australia. This was achieved through sequential size fractionation of biosolids samples to quantify the mass concentration of 7 common plastics across a range of biosolids size fractions, including below 25 μm which has not been assessed in many previous studies. Quantitative analysis was performed by pressurized liquid extraction followed by pyrolysis coupled to gas chromatography - mass spectrometry. Of the total quantified plastics (Σ₇plastics), the greatest proportion (27%) of the total mass were identified in the nominal <25 μm sized biosolids fraction. Polyethylene dominated the polymer mass in every size fraction, even though profiles varied between WWTPs. When comparing the sum of all sites for each sized biosolids fraction, the plurality of the polyethylene, polyvinyl-chloride, polystyrene, polypropylene, polycarbonate, and polyethylene-terephthalate concentrations were associated with the smallest size fraction (<25 μm). We confirm for the first time the presence of plastic particles in biosolids below a size fraction that is not captured by many methods. This is important, because of the potential greater significance of plastics in the low sizes to environmental and human health.