Seasonal variation and removal efficiency of microplastics in wastewater treatment: a year-long study across three municipal water reclamation plants

Abstract

Municipal wastewater treatment plants (WWTPs) serve as major recipients of microplastics (MPs) before these contaminants are discharged into local waterbodies. However, detailed investigations into MP pollution from wastewater in Australia remain limited. Therefore, understanding the occurrence and fate of MPs in WWTPs is crucial for developing effective control strategies. To address this gap, a year-long study was conducted to investigate the occurrence of MPs, seasonal trends, their removal efficiency, and the potential ecological risks posed by MPs from three water reclamation plants (WRPs) in Victoria, Australia. Nile red staining, Fourier transform infrared spectroscopy and scanning electron microscopy were used to detect and validate MPs in all wastewater samples. Results revealed that synthetic fibers were the dominant shape, accounting for 52–57% of all MPs. The influent predominantly contained MPs >500 μm, while effluent MPs were mostly sized between 300–100 μm. Five major polymers were identified: polyester, polyethylene terephthalate, polypropylene, polyethylene, and polystyrene. The MPs load in wastewater ranged from 25–37 MPs per L in the influent to 0–2 MPs per L in the effluent, demonstrating removal efficiencies ranging from 92% to 100% across the primary, secondary, and tertiary treatment processes. Notably, over 60% of MPs were removed during the primary treatment phase alone. Despite this high removal efficiency, between 3.63 × 10⁶ and 1.7 × 10⁸ MPs per day were released through WRP effluent, posing a low to medium ecological risk to the surrounding environment. Overall, this study underscores the effectiveness of current wastewater treatment processes in removing MPs and highlights the importance of long-term monitoring and sampling for managing MP pollution.