Flow cytometry as a tool for the rapid enumeration of 1-μm microplastics spiked in wastewater and activated sludge after coagulation-flocculation-sedimentation

Considering the limited literature and the difficulty of quantifying 1-μm micro-nanoplastics (1-μm MNP) in complex aqueous matrices such as wastewater and sludge, the removal rate of these very small particles in wastewater treatment plants (WWTP) represents a major challenge. In this study, coagulation-flocculation-sedimentation (CFS) with aluminum salts was investigated to evaluate the removal of 1-μm MNPs spiked in tap water, raw wastewater, pre-settled wastewater, and activated sludge. Quantification of 1-μm MNP was performed using the high-throughput flow cytometry (FCM) analysis which takes only a few minutes and produces results with high accuracy and reproducibly. The results indicated that the 1-μm MNPs were highly stable in pure water and unable to settle rapidly. In raw wastewater, sedimentation without coagulants removed less than 4% of 1-μm MNP. Conversely, CFS treatment showed a significant improvement in the removal of 1-μm MNP from wastewater. At dosages of 0.3–3 mg Al3+/L, the removal of MNPs in wastewater reached 30% and no flocs were observed, while floc formation was visible with increased dosages of 3–12 mg Al3+/L, obtaining MNP removal greater than 90%. CFS in activated sludge with a solids content of 5800 mg MLSS/L registered the highest removal efficiency (95–99%) even for dosages of 0.3–60 mg Al3+/L and pH dropping to 5. However, activated sludge showed extremely high removal efficiency of MNPs (97.3 ± 0.9%) even without coagulants. The large, dense flocs that constitute activated sludge appear particularly efficient in capturing 1-μm MNPs during the sedimentation process even in the absence of coagulants. [Display omitted] •High-throughput flow cytometry to enumerate 1-μm microplastics in few minutes.•Coagulation-flocculation to investigate microplastics removal from wastewater and sludge.•1-μm microplastics in wastewater are removed by 30% with 0.3–3 mgAl3+/L.•1-μm microplastics in activated sludge are removed by 99% with 0.3–3 mgAl3+/L.•Activated sludge showed removal of 1-μm plastics of 97% even without coagulants.