Microplastic removal, identification and characterization in Chennai sewage treatment plants

Sewage treatment plants (STPs) act as either sinks or sources of microplastic (MP) contamination in the environment. This study examined and assessed the occurrence, removal efficiencies, abundance and characteristics of MPs in two STPs in Chennai, India. Large volumes of influent and effluent water were collected and filtered on site via a filter in a series system. The samples were later treated in the laboratory to isolate the MPs from other organic and inorganic particles. The MPs were analysed via Fourier Transform Infra-Red (FTIR) spectroscopy and Raman spectroscopy to analyse the chemical composition of the isolated microplastics. Pollution load index (PLI) and EU classification, labelling and packaging (CLP) standard was incorporated to assess the pollution risk of MPs in STP. According to the results obtained from this research work, the MP concentrations in the influent waters were high for both STPs (5443 MPs/L and 4800 MPs/L). Although the MP removal efficiency of the STPs were quite high (~96 % and ~93 %), the pollution load indices at Kodungaiyur and Koyambedu STPs were observed to be 0.272 and 0.208 respectively, which were moderately contaminated. PORI scores revealed that Kodungaiyur Plant is in danger level I with the hazard score of 9.25 and Koyambedu plant is in danger level II with the hazard score of 12.78. The estimated quantity of the MPs discharged from the monitored STPs was approximately 28.4 & 28.2 billion MPs/day.