Stage-wise analysis of microplastic removal in an industrial wastewater treatment plant: A case study from Colombo District, Sri Lanka

Abstract

Microplastics, defined as plastic debris smaller than 5 mm in size, have become a global concern due to their widespread distribution in various environmental compartments and their potential toxicity. Wastewater Treatment Plants (WWTPs), although designed for treating wastewater through various physical, chemical, and biological techniques, are considered point sources of aquatic microplastic pollution. However, in the Sri Lankan context, there is limited understanding regarding the role of WWTPs in removing microplastics from industrial wastewater. To address this knowledge gap, this study presents the stage-wise microplastic removal efficiency at different treatment stages of an industrial WWTP in the Colombo District, Sri Lanka, across six sampling months. Additionally, temporal variations in microplastic abundance and removal efficiencies within the six sampling months are also highlighted. Wastewater samples were collected from the inlet, the equalisation tank, the oxidation ditch, the settlement tank, and the final discharge point from January to June 2025. Microplastics were extracted through sieving, digestion, and density separation. Extracted particles were visually quantified using a stereomicroscope, followed by the calculation of stage-wise microplastic removal efficiency. Fourier- Transform Infrared spectroscopy (FTIR) was used for polymer identification. Microplastics were found in all the treatment stages of the plant. The findings revealed a notable microplastic abundance in the raw influent (63.77±20.9 particles/L) and a gradual decline across subsequent treatment stages, reaching a low microplastic abundance at the final discharge point (2.14±1.57 particles/L). This shows a progressive reduction of microplastics across the treatment stages of the WWTP. The lowest microplastic removal efficiencies were noted from the inlet to the equalization tank, especially in March (1.49%). The highest microplastic removal efficiency was associated with the settlement tank (mean>76%). FTIR revealed the presence of polyethylene, nylon, and polyethylene terephthalate. Temporal variations demonstrated a clear microplastic reduction across the treatment stages over the six sampling months. Temporal inconsistency in influent microplastic load was further confirmed by the high standard deviations at early treatment stages. The presence of microplastics in the final effluent shows the inefficiency of existing wastewater treatment technologies, necessitating advanced microplastic-focused treatment techniques and wastewater policy frameworks.