An assessment of the occurrence and removal options of microplastics in wastewater treatment processes at the City Ekurhuleni and Midvaal in South Africa

Abstract

The main aim of the study was to investigate the occurrence and removal of microplastics in wastewater treatment processes in Ekurhuleni and Midvaal in South Africa. The amount of microplastics and chemical composition discharged into the river pose a threat to aquatic systems and human health. The wastewater samples were collected using 24 h autosamplers between 2021 December to 2022 September. The in-situ measurements for pH and temperature were conducted using an advanced digital HACH HQ40d multi-meter. The analysis for chemical oxygen demand (COD) and suspended solids (SS) was done through colorimetric and gravimetric methods. Analysis for heavy metals was done using inductively coupled plasma mass spectrometry (ICP-MS). The polarized light microscopy (POM) was used to generate high-resolution images and Fourier Transform Infrared Spectroscopy (FTIR) identified polymers and functional groups. The quantification of microplastics was done through image J 1.53 K to determine the microplastics particles (MPs) counts and removal efficiencies. The mass of microplastics was obtained and the loading rate was estimated based on the flow rate. The Pearson correlation coefficient was used to establish the relationships between physicochemical water quality, metals and MPs concentrations and loading. The chemical oxygen demand (COD) and SS showed a small positive correlation with r-value ranging between 0.1-0.3. Temperature and pH showed a negative correlation with MPs concentration. Toxic metals and non-metallic parameters monitored in the influent and effluent of WWTWs also did not correlate significantly, only chloride, Fe, S and SO42− showed a smaller correlation at r=0.1-0.3. The MPs vary between 0.01-0.02 mm in the influent and 0.01-0.05 mm in the effluent. The microplastics identified across four seasons were dominated by angular fibres (white), fragments (blue, black and red films (translucent) with concentrations ranging between 145-180 MPs/L in summer, 399-401 MP/L in autumn, 119-798 MPs/L in winter and 152-402 MPs/L in spring. The highest concentrations and loading were observed at WWTW B. The FTIR analysis showed presence of polyethylene (PE), polyethylene terephthalate (PET), polyacrylamide (PAM), polymethyl methacrylate (PPM), polymethyl methacrylate (ABS), polystyrene (PS), polypropylene carbonate (PPC) and thermoplastic vulcanizates (TPV) polymers while the spectra represent functional groups such as saturated aliphatic, alcohol and hydroxyl compounds, carbonyl and alkenes. The scanning electron microscopy/energy dispersive x-ray (SEM/EDX) analysis resulted in carbon and oxygen ranging between 52-73.46% and 15.9-26% with the composition of MPs additives such as Cr, Ca, Fe, Al, Na, Mg, Zn, Cl, P, S and silicone. Continuous monitoring and regulations of MPs is required to minimize microplastics pollution and adverse impact in the environment.