Nanoplastic Removal From Secondary Wastewater Effluent By Granular Filtration

Lauryn Guerrissi

doi:10.7273/000005241

The use of plastic materials has proliferated globally due to their characteristic versatility and durability. However, their widespread use has resulted in the excessive generation of plastic wastes which often end up in the environment. Of growing concern are micro and nanoplastic (MP and NP) wastes that are generated from the breakdown of larger plastic debris or are purposefully produced for use in personal care products. Chronic exposure to MPs and NPs may have deleterious health effects for environmental biota and humans. Recently, wastewater treatment plants have been identified as a significant source of MPs and NPs to the environment. Conventional wastewater treatment processes do not remove all MPs/NPs from influent streams; thus, advanced (or tertiary) treatment solutions are necessary. One simple and economical method which has shown promise for MP/NP removal in full-scale wastewater treatment plants (WWTPs) is tertiary granular filtration. Yet, tertiary granular filtration of MPs/NPs remains largely unexplored at the laboratory scale. Therefore, the objectives of this research were to investigate the removal efficiency of model NPs (885nm) from secondary wastewater effluent by bench-scale granular media filters, determine whether coagulation pre-treatment or surface-modified filtration media could improve NP removal in granular media filters, and understand how wastewater chemistry affects the behavior and fate of NPs in a filtration system. Combined jar tests and column filtration studies were carried out for aluminum chlorohydrate (ACH) coagulant doses from 0-12mg/L for three different filtration media: quartz sand, aluminum oxide coated sand (AOCS), and granular activated carbon (GAC). A subset of experiments was conducted in ultra-pure water to compare against experiments conducted in wastewater. Results of this study showed that tertiary filtration may only be effective for NP removal when a coagulant aid is used, with maximum achieved removal efficiencies of 85.6%, 78.5%, and 86.7% at a dose of 12 mg/L ACH for sand, AOCS, and GAC, respectively. With no coagulant aid, AOCS showed enhanced removal (27.1%) compared to sand (12.7%) and GAC (9.3%) indicating that surface modification of granular media has the potential to improve NP removal performance if modification can be optimized. Overall performances of sand and GAC were similar, suggesting no advantage in choosing one media over the other for targeted NP removal. Experiments conducted in ultra-pure water demonstrated that behavior of NPs differs markedly in the presence of wastewater chemical constituents due to steric and bridging interactions with organic matter, and electrostatic interactions with salts. Finally, by examining experimental results in the context of fundamental colloid filtration and Derjaguin–Landau–Verwey–Overbeek (DLVO) theories, this study determined that the dominant mechanisms of NP removal for the coagulation-filtration system included particle destabilization and floc precipitation by ACH, followed by physical straining and screening of particles in filtration media. The results of this work provide new knowledge that can be utilized to understand fate of NPs in tertiary filtration systems, and to improve upon current filter design for enhanced NP removal.

Nanoplastic Removal From Secondary Wastewater Effluent By Granular Filtration

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