Acute Exposure to Polystyrene Nanoplastics Inhibits the Flocculation of Activated Sludge

The flocculation performance of activated sludge (AS) determines the efficiency of solid-liquid separation and effluent quality in wastewater treatment processes. However, the effect of nanoplastics (NPs) on flocculation performance is still unclear. This study investigated the flocculation of AS under the acute pressure (10 h) of polystyrene nanoplastics (PS-NPs). The results indicated that the flocculation of AS was reduced by 1.0%, 8.9%, 30.5%, and 69.3% after 0.1, 0.5, 1, and 5 mg/mL PS-NP exposure compared with the control, respectively. The adsorption of a large number of plastic particles on the surface of AS was observed, which changed the surface characteristics and increased the electronegativity of sludge. A significant decrease in zeta potential of AS was found after exposure to 1 and 5 mg/mL PS-NP, which was − 9.9 and − 16.0 mV, respectively, compared with the control (−7.3 mV). The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory proved that the increase of repulsive potential barrier dominated by electronegativity was the main factor hindering flocculation when the concentration of PS-NPs was higher than 1 mg/mL. Furthermore, PS-NP exposure decreased the contents of proteins and polysaccharides in extracellular polymeric substance (EPS). PS-NPs interacted with the functional groups of EPS including hydrogen bonds, C-H, and CO, which led to the reduction of hydrophobicity and loose structure of EPS. These changes also became the potential factors to inhibit flocculation. In summary, the results reveal the effects of PS-NPs on the flocculation performance of AS and provide new insights for controlling the potential risk of NPs to sewage treatment plants.