Effects of ZnO nanoparticles on flocculation and sedimentation of activated sludge in wastewater treatment process

• Published in: Environmental research 2021-01, Vol.192, p.110256-110256, Article 110256
• Main Authors: Ye, Jinyu, Gao, Huan, Wu, Junkang, Yu, Ran
• Format: Article
• Language: English
• Subjects: Activated sludge; Extracellular polymeric substance; Flocculation; Sedimentation; Sewage; Spectroscopy, Fourier Transform Infrared; Waste Disposal, Fluid; Water Purification; Zinc Oxide; ZnO nanoparticle
• ISSN: 0013-9351; 1096-0953
• DOI: 10.1016/j.envres.2020.110256

Despite the behaviors of ZnO nanoparticles (ZnO NPs) in wastewater treatment processes have been widely explored, the impacts of ZnO NPs on the activated sludge's flocculation and sedimentation performances for solid-liquid separation have rarely been involved yet. In this study, ZnO NPs were observed to exert a dose-dependent negative effect on the sludge's flocculation performance but did not significantly impact the sludge’ sedimentation behaviors. Furthermore, it was NPs themselves rather than the dissolved Zn2+ who impaired on the sludge flocculation performance because the Zn2+ alone would not compromise the sludge's flocculation efficiency. In addition, the sludge flocculation performance was revealed to be inversely related to the extracellular polymeric substances (EPS) content in the sludge and the direct contacts between ZnO NPs and the cells in the sludge should be the prerequisite to stimulate the secretion of the sludge EPS. The poor sludge flocculation performance could also be caused by the reduced protein/polysaccharide (PN/PS) ratio and the zeta (ζ) potential in the loosely bound (LB-EPS) after the sludge exposure to ZnO NPs. Fourier transform-infrared spectra (FT-IR) and three dimensional - excitation emission fluorescence spectra (3D-EEM) analysis further revealed that the decrease of the tyrosine PN-like substance level in the LB-EPS was probably the key reason for the decreased PN/PS ratio and ζ potential in the LB-EPS, which eventually induced the decline of the sludge flocculation performance under the ZnO NP stress. These results could potentially expand the knowledge on sludge flocculation and sedimentation in the presence of ZnO NPs. •ZnO NPs exposure impaired the sludge flocculation rather than its sedimentation performance.•Sludge flocculation performance inhibition could be attributed to the NPs themselves.•EPS content increased by direct contacts between the ZnO NPs and the sludge cells.•Sludge flocculation performance was positively correlated with the PN/PS ratio and the ζ potential in the LB-EPS.•ZnO NPs induced the decrease of the tyrosine PN-like substance content in the LB-EPS.