Occurrence and Removal of Microplastics in Activated Sludge Treatment Systems: A Case Study of a Wastewater Treatment Plant in Thailand

Domestic wastewater treatment plants involved as a point source of microplastics (MPs) distribution to the environment. The conventional wastewater treatment systems were not designed for MPs removal. Due to immense wastewater effluent discharge, the rate of MPs discharge is possibly high and thus needs to be evaluated. Therefore, a conventional activated sludge wastewater treatment plant at Mahidol University Salaya Campus was selected to investigate the occurrence of MPs and their removal efficiency. The influent and effluent samples were collected and filtered with 100 µm filter bags by using a pump system. The samples were treated with Fenton’s reagent, and sodium iodide was used to remove interferences in the environmental matrix. The MPs were manually picked and grouped by size and shape using a stereomicroscope followed by determining the chemical composition using Attenuated Total Reflection-Fourier Transform Infrared ATR-FTIR spectroscopy. The overall concentration of MPs was found to be 0.40 MPs/L in the influent and 0.05 MPs/L in the effluent sample. The treatment plant showed 86.5% efficiency in MP removal. The predominantly detected shapes of MPs were fibers with 86 % and fragments with 85 % removal rate. The detected MPs with the size range of 600-1100 µm were in high abundance in the effluent sample. Additionally, the concentration of larger-sized MPs was significantly lower in the effluent with good removal efficiency. The chemical composition of the detected types of MP fragments revealed PP, PMMA, cellophane, and PET fibers in the effluent. Interestingly, high-density polymers such as PVC, blend PC/PE, and PTFE/P microbead were not detected in the effluent. The results indicate that the small MPs are still released with the effluents and a few types of MPs could be retained in the sludge. Therefore, the understanding of how MPs are released along with effluent wastewater and their composition may help in determining the potential sources of MPs in incoming wastewater to treatment plants.