Identification and quantification of microplastics in wastewater treatment plants by spectroscopic and microscopic techniques in Johannesburg East, South Africa

Across the world, it is estimated that a total of 381 million tons of plastics is produced annually with an increase of 4% (Nhon et al. 2022). Due to limited recycling, plastics end up in the environment. The larger plastics (primary plastics >5 mm) undergo physical processes such as weathering, disintegration by waves in wastewater conveyance systems, and microbial processes to form smaller particles referred to as microplastics (particles <5 mm) (Li et al. 2022). Microplastics that end up in the wastewater treatment works (WWTWs) are not completely removed and, therefore, regarded as the pathway to enter surface water sources (Habib et al. 2020). Microplastics have been identified in the final effluent of WWTWs in Europe, Asia, Africa, and the United States of America (USA) that regulates microplastics in drinking water (Wickham et al. 2019). On average, it is estimated that 4% of global microplastics reach surface water (Rubio-Armendariz et al. 2022).

Although microplastics are known to originate from weathering of larger particles, it is well known that the production of pharmaceutical and personal care products involves the addition of microplastics, e.g. scrubbers, soaps, toothpaste, and cosmetics products (Bashir et al. 2021). During the production processes of primary and secondary microplastics, additives such as plasticisers, flame retardants, antioxidants, acid scavengers, light and heat stabilisers, lubricants, pigments, antistatic agents, slip compounds, and thermal stabilisers are added (Hahladakis et al. 2018). The most common chemical additives of microplastics include bisphenol A (BPA), phthalates, and polybrominated diphenyl ethers (Campanale et al. 2020). The microplastics detected in the final effluent end up in the rivers and, therefore, may act as pathways of toxic additives (Rubio-Armendariz et al. 2022). The additives released from microplastics alter water quality, cause toxicity to aquatic species and may cause endocrine disruption and cancer, and accumulate in organs such as the liver and heart resulting in their failures (Campanale et al. 2020). Traces of microplastics have been found in the digestive tracts of fish and sea birds and that adversely affects their health and increases mortality (Katyal et al. 2020).

The influent of WWTWs originates from diverse sources such as industries, domestic sewage, and storm water (Reddy & Nair 2022). The different sources of wastewater result in the occurrence of a wide array of substances in the wastewater ranging from metallic species including toxic metals to persistent organic compounds (Hejabi et al. 2021). Microplastics' chemical composition is associated with the release of contaminants into the environment, and the hydrophobic characteristics of microplastics play an important role in contaminate distribution (Joo et al. 2021). Seidensticker et al. (2018) confirmed the ability of microplastic polymers such as polyethylene (PE) and polystyrene (PS) to sorption of different contaminants exposed to different pH levels. The results from Seidensticker et al. (2018) showed that PE and PS highly adsorb chemical compounds that do not attract water molecules such as alkanes, hydroxyl compounds and hydrocarbons at neutral pH. Similarly, Wang et al. (2022) studied the ability of PE in the sorption of different pharmaceutical compounds. The study by Wang et al. (2022) was comparable to the study by Seidensticker et al. (2018), where the PE showed high adsorption capacity on hydrophobic pharmaceutical compounds such as sulfamethoxazole, propranolol and sertraline at pH value of 7. This study seeks to identify and quantify microplastics in the influent and final effluent of WWTWs A and B in the Johannesburg East in South Africa.

The WWTW A was designed to treat 63 ML/d and in the year 2021/2022, the work operated at 100% of its capacity (WWTW A Water Quality Report 2021). Although the work operated at its design capacity, it is at a risk of exceeding its design capacity. The treatment works receive its influent from 51 industries scattered around Kempton Park and 53,777 households with a population of 171 575 in Kempton Park. The influent is received in a proportion of 60% domestic and 40% industrial wastewater (WWTW A Manual 2015). There is a potential source of microplastics from industrial and domestic influent which is received in the WWTW.

The WWTW A utilises a conventional treatment technology. It comprises inlet works equipped with three mechanical fine screens and degritting chambers for the screen, sand and grit removals as a preliminary treatment process (WWTW A Manual. 2015). Preliminary treatment is followed by four primary settling tanks (PST) aimed at solid and COD reduction to reduce the loading into the activated sludge processes (Archer 2018). The supernatant from PST overflows into activated sludge processes which are configured as University of Cape Town (UCT configuration) in treatment sections 1–3 and Modified Ludzac Etinger processes in treatment section 4 (WWTW A Manual 2015). Although treatment configurations are different in the sections, the secondary treatment of the BNR process remains the same across the treatment units. In the secondary treatment, microorganisms are used to oxidise nutrients and organic matter. Overflow from the BNR is subjected to tertiary treatment using final settling tanks (FSTs). This is the final polishing step for the removal of SS before the effluent is disinfected using chlorine gas (WWTW A Water Quality Report 2021). The wastewater residue such as waste-activated sludge (WAS) and primary sludge from PSTs is treated in cold open digesters equipped with rotor mixers. However, the anaerobic digestion process is not heated (Eustina et al. 2018).

The final effluent and sludge treated at WWTW A are expected to comply with the Water Use License (WUL) granted in terms of the National Water Act (Act 36 of 1998) under Section 21. Although there is WUL in place, microplastics are not regulated.

WWTW B is designed to treat a maximum of 155 ML/d (6458,33 m³/h) from primarily domestic households with an average population of 111,612 and 38,046 households (WWTW B Water Quality Report 2021). Only 5.41 ML/d (4%) of wastewater is contributed by industries. The wastewater treated is from domestic households in Everton and surrounding communities. The plant is comprised of four treatment modules with three modules treating 35 ML/d and one treating 50 ML/d. The conventional treatment process employed in the plant with primary, secondary, and tertiary treatment mainly as the final stage (WWTW B Water Quality Report 2021). Primary treatment consists of six mechanical screens and six pre-screen grits for screen and debris removal. It is also equipped with six grit chambers for sand and grit removal (WWTW B Process Audit 2019). Four modules comprised of four primary settling tanks for further removal of SS and sand which escaped from grit chambers. Solids settle as raw sludge and are further treated in an anaerobic hydrolysis process, filter belt pressers, and paddies. The activated sludge process is used for secondary treatment for oxidation of nutrients in the presence of microorganisms (Eustina et al. 2018). It also entails secondary clarification for further removal of SS and polishing of final effluent. The last treatment process is disinfection for the removal of microorganisms using chlorine gas.

The WWTW B is also regulated by the WUL. The final effluent discharged into Klip River must comply with the conditions of WUL (WWTW B WUL audit report 2021).

Samples were collected from December 2021 to September 2022 to give representative samples over four seasons. A total of 16 samples were collected from the wastewater treatment plant; from the influent before the inlet works fine screens and final effluent sampling points after disinfection. Microplastic sampling was done using the MS 10 composite autosampler, which is equipped with a 3-L sampling bottle. The MS 10 composite autosamplers equipped with a vacuum pump, which allowed the suction of water at 10–20 min intervals were placed in the inlet and effluent of the WWTWs. The sampler timer was set to collect an equivalent of 2.5 L of the representative sample over 24 h. A sample volume equivalent to 1 L of composite sample was collected by opening the autosampler dispenser pipe to fill up the glass bottle to its maximum capacity. The samples were preserved in a cooler box and transported to the Council for Scientific and Industrial Research (CSIR) laboratories for further analysis.

The microplastics were pre-treated prior to analysis by filtering using a Millipore xx2004708 stainless steel sieve with 5-mm pores purchased at Merck group. Materials retained from a 5-mm stainless steel sieve were discarded. The filtering of samples was done to remove particles greater than 5 mm which were not considered microplastics.

The wet peroxide oxidation procedure for removal of organic contaminants to isolate microplastics in wastewater samples was followed to reduce interferences that might cause errors during microplastic identification. 100 mL of wastewater sample was taken from the filtered sample and catalysed by adding 20 mL of iron sulphate at (0.05 M of Fe (II)), which was allowed to stand for 5 min at room temperature prior to heating at 75 °C on a hotplate and later allowed to stand for 30 min at 23 °C. The organic compounds were degraded and only microplastics remained due to their resistance to peroxide oxidation. The solution of wet H₂O₂ was then transferred into a density separator and allowed to settle over 24 h at room temperature of 23 °C. The settled organics were discarded, and the clear sample was transferred into the 250-mL glass beaker for drying that was placed in the Memmert vacuum oven-PM400 at 90 °C for 72 h to ensure proper drying. Microplastics remained on the surface of the glass beaker. In order to avoid contamination from ambient air and cross-contamination, the glass beaker was covered with an aluminium foil.

The identification of microplastics included the physical and chemical properties of microplastics as well as their quantities in wastewater samples. Physical identification was done using scanning electron microscopy (SEM) and the polarised optical microscope (POM). To do an elemental analysis of microplastics, Fourier transform infrared (FTIR) spectroscopy was used to identify polymers and the SEM coupled with energy-dispersive X-ray (EDX) was used to perform an elemental analysis of microplastics. The identification and classification of microplastics was carried out using a POM and SEM.

For the physical identification of the microplastics, a POM was used. The sample preparation was done by submerging the microplastic particles in the refractive index oil, this was done to allow clear visibility of the sample. The magnification of the POM was adjusted to 40× using the top light. Microplastics were then placed on the POM stage micrometre with a rectangular size of 25 mm 75 mm. The dried microplastics on the micrometre were observed and images were taken to categorise particles based on the colour and shapes of fragments, fibre, film, micro-beads, and foam.

For the physical identification of shapes and sizes of microplastics, a SEM was used. The first procedure was to open the specimen chamber on the SEM to allow the specimen stage to be visible. Microplastic particles were transferred from the petridish into the specimen stage of the SEM coated with carbon at 50 nm to avoid conflicting with peaks of other compounds. The chamber on the SEM containing the specimen was closed and the pressure equalisation was maintained by switching on the vacuum pumps for 5–10 s at 10⁻³ Torr. The SEM coupled with EDX was operated at 15 kV voltage. To produce primary electron, the magnification was set between 400 and 6,400× and the sample was scanned with a beam at high energy with a map resolution of 512 × 384. The SEM coupled with energy-dispersive X-ray was operated at backscattered imaging mode where the primary electron produced was allowed to interact with the samples to produce secondary electrons, backscattered electrons, and X-rays. SEM and X-ray detectors were allowed to collect signals from electrons and X-rays to create images at a pixel size of 0.06 μm and a maximum magnification of 5,200 which were displayed on the computer monitor.

To identify the spectra of samples, a Perkin Elmer Spectrum 100 spectrometer was used. To achieve the required signal-to-noise ratio, FTIR spectroscopy spectra of all microplastic samples were measured in the range of 550–4,000 cm⁻¹ and 32 scans were made per spectra, which were co-added and averaged. In all the samples, the spectra resolution was maintained at 16 cm⁻¹. A systematic procedure was followed in the sample preparation and observation to minimise contamination and optimise the identification of the polymer. First, the detection unit was cleaned with alcohol and a clean cloth free from fibre to avoid contamination. The microplastics were placed on a rectangular glass micrometre with dimensions 25 mm75 mm. The control joystick was used to position the specimen and microplastics were scanned on the sample table. An optical photo of the specimen was taken and the location where the sample was characterised was marked. The spectrum of the measured particle was matched to the database of spectra in the library to identify the closest match and, therefore, the polymer types on the microplastics. FTIR spectroscopy was composed of various reference spectra of polymer which included PE, polyethylene terephthalate (PET), PS, poly(methyl methacrylate) (PMM), cellulose, polypropylene (PP), polyvinylchloride (PVC), polycarbonate carboxylic acid amine salt film and other spectra not identified on the samples.

The elemental analysis was done using the SEM equipped with energy-dispersive X-ray. This was done to identify elements such as heavy metals that are linked to the presence of microplastics in water. The presence of elements such as chloride, aluminium, manganese, zinc, iron, silicon and heavy metals in microplastics present the potential of microplastics to distribute contaminants. The step-by-step systematic procedure for SEM and EDX has been explained above on morphological characterisation. The identification of elements was based on the reaction of primary electrons that interacted with samples to create secondary electrons, scattered electrons and X-rays. The X-ray detector was used to detect elements.

The ImageJ 1.53 K java-based application by the National Institute of Health (NIH) of the USA was used to perform particle counts to determine the microplastic concentration in the influent and final effluent of WWTW. First, the sequence images were imported and uploaded to ImageJ 1.53 K and then processed to accentuate the image edges for ease of automated particle count and analysis. The image was edited by setting the threshold colour to white and saturation ranges of 120–255. The image size was interpolated as bilinear at a pixel width of 481 and height of 332 with a constrained aspect ratio and average when downsizing. The image contrast and colour balance were adjusted to 50%. The particles were processed by setting the image to smooth and sharpened mode. The edges on the image particles were determined automatically on the ImageJ application. The particles were analysed by setting the pixel from 0 to infinity and circularity ranging between 0.001 and 1. The application analysed the particles automatically and the results summary table was displayed with particle counts of microplastics in MPs/L. The edited image was compared with the original image from the POM and SEM.

The concentrations of microplastics vary seasonally across the sampling points. According to Figure 2 and Table 1, during the summer season, the influent concentrations at WWTW A contained 145 MPs/L fibres and 148 MPs/L fragments while the effluent contained 54 MPs/L fibres and 138 MPs/L fragments. The composition of WWTW B did not vary in the influent during summer; the influent concentrations ranged between 79 and 180 MPs/L and a total of 180 MPs/L were fibres (see Figure 2 and Table 2). The effluent at WWTW B during summer contained 78 MPs/L fragments and 21 MPs/L fibres. During summer both WWTWs influent and effluent were composed of fragments and fibres. The concentrations of microplastics are generally higher in the influent and lower in the effluent across all sampling points with WWTW A receiving higher concentrations of microplastics during the summer season.

The concentrations of microplastics during autumn slightly increased in the influent and effluent of both WWTW A and B. With reference to Figure 2 and Tables 1 and 2, the influent concentrations of microplastics at WWTW A during the autumn season ranged between 219 and 180 MPs/L, in which a total of 399 MPs/l was composed of fragments. The effluent concentration ranged between 129 and 138 MPs/L and a total of 298 MPs/L were observed as fragments.

The WWTW B influent microplastics concentrations ranged between 170 and 231 MPs/L, with a total of 401 MPs/L composed of fragments. The effluent concentration also consisted of 210 MPs/L predominantly fragments. During autumn, the concentrations of microplastics were only composed of fragments in both WWTWs. The influent microplastic concentrations for WWTW B were higher than WWTW A.

During winter, the concentrations of microplastics continued to increase on the influent and effluent of WWTW A and B, with the highest peaks being 250 MPs/L and 798 MPs/L in the influent and 189 MPs/L and 316 MPs/L in the effluent. A morphology analysis showed that the influent at WWTW A contained 250 MPs/L angular and 203 MPs/L fibres-shaped particles. Although the effluent concentrations dropped, the angular and fibre-shaped particles were identified with concentrations of 134 MPs/L and 189 MPs/L respectively. The WWTW B influent microplastics concentrations were 354 MPs/L angular and 798 MPs/L fibrous particles. Of these concentrations, the effluent microplastics were also composed of angular and fibre-shaped particles with concentrations of 119 MPs/L and 316 MPs/L respectively. During the winter season, WWTW B received the highest influent concentrations of microplastics resulting in high effluent concentrations. This is likely due to activated sludge processes being unable to handle higher microplastic loading.

The spring season was accompanied by a slight drop in the influent concentrations. However, the effluent concentrations remained high in both WWTWs. The influent microplastics concentrations at WWTW A comprises of 271 MPs/L fragments and 402 MPs/L angular-shaped particles while the final effluent comprises 211 MPs/L films and 296 MPs/L angular particles. The WWTW B influent in spring comprises 287 MPs/L angular and 420 MPs/L fragmented particles. The concentrations dropped in the effluent with 152 MPs/L angular and 225 MPs/L fragmented particles. Both WWTWs concentrations of microplastics consisted of angular and fibres at the influent and effluent. Across the four seasons, films were only noticeable during the spring season at WWTW A. The concentrations of microplastics at WWTW A influent were higher than at WWTW B. However, WWTW A recorded the highest concentrations in the final effluent indicating insufficient treatment of microplastics.

Microplastics concentrations were quantified based on the particle count of different shapes such as angular, fragments, fibres and film. According to the literature, various shapes of microplastic concentrations have been detected in the influent and effluent of WWTWs (Kang et al. 2018; Conley et al. 2019). Although various studies used similar techniques, the concentrations of microplastics observed were different. In a study done by Conley et al. (2019) the microplastic concentrations in the influent had a difference factor of 2.5 count/L while the effluent factor was 4.8 count/L. There were no seasonal variations observed. The findings in the current study show variations in the microplastic influent and effluent concentrations. However, lower concentrations were observed in summer and autumn while high concentrations were observed in winter and spring. Conley et al. (2019) did not quantify microplastics based on their shapes while the current study quantified the number of angular, fragments, fibres and films identified as seasonal as shown in Tables 1 and 2.

Long et al. (2019) reported that microplastic concentrations ranging from 1.57–13.69 items/L and 0.2–1.73 items/L in the influent and final effluent respectively. Long et al. (2019) further quantified the concentrations based on shapes such as pellets, fragments, fibres and granules. The study done by Long et al. (2019) is comparable to the findings of the current study in that the concentration of microplastics was high in the influent and lower in the final effluent with different shapes identified. Although the two studies are comparable, the microplastic samples in the study done by Long et al. (2019) were collected over 2 days in September, therefore, seasonal differences in the concentrations could not be quantified. The reported high concentrations in the influent and low concentrations in the effluent suggest that microplastics are reduced in other treatment units of the activated sludge process. In the current study, the microplastic concentrations varied across the seasons. The variations of microplastics across the seasons are associated with the changes in polymer compositions and changes in human behaviours during specific seasons (Jiang et al. 2022). Microplastics discharged in wastewater end up in the rivers, where seasonal variations may determine the abundance and accumulation of different types of microplastics. According to Xia et al. (2021) the distribution of microplastics during the dry season is higher than in the rainy season, wet weather conditions result in the distribution of smaller microplastics along river banks. Therefore the toxicity of microplastics may differ across seasons based on the polymers present attributed to changes in flow patterns, water quality and changes in human habits that lead to diverse and complex microplastic characteristics across the seasons (Jiang et al. 2022).

The results of the current study were comparable with the findings of Rosal (2021) where dominant microplastics identified were fragments and fibres with various colours ranging from white, blue and black, however, the differences were observed in the sizes of microplastics which ranged between 0.0016 and 5 mm.

Williams et al. (2020) conducted a study on microplastics quantification comparing two WWTWs. The findings of the study revealed that the two studied WWTWs contained polymers such as PP, PE, PET and silicon in the influent and effluent over 10 months of monitoring. The current study showed a complex range of polymers identified in the influent and effluent of WWTW A and B. The influent and effluent of two WWTWs in the current study were comprised of polymers such as PE, acrylonitrile butadiene styrene, PAM, polymethyl methacrylate, PS, cellulose, acrylic, PP carbonate, thermoplastic vulcanisates, poly(methyl methacrylate) and poly(ethyl cyanoacrylate). Contrary to Williams et al. (2020), the current study showed that the microplastic polymer types observed in the influent and effluent tend to vary seasonally. Studies such as Olesen et al. (2017) and Stockin et al. (2021) showed similar findings to the current study.

In the current study, different polymers were identified from the FTIR spectroscopy library while the spectra were used to identify the functional group region. According to Bayo et al. (2021) there are two regions of FTIR spectroscopy spectra that are the fingerprint region (between 600 and 1500 cm⁻¹) and the functional group region. However, the functional group is composed of single bond region (2500–4000 cm⁻¹), triple bonds (2,000–2,500 cm⁻¹), and double bond region (1,500–2,000 cm⁻¹) wavelength. The fingerprint region is complex and is composed of a large number of peaks, therefore, it was not used as opposed to the functional group region (Aurelio Ramírez-Hernández & Universidad 2019). In this study the functional group regions were between 1,500 and 3,400 cm⁻¹ as in the results obtained by FTIR spectroscopy.

The functional groups identified represented various compounds such as ethylene (>CH₂), methyl (−CH₃), methyne (>CH−) carbonyl compounds, olefinic (alkene), alcohol and hydroxy compounds identified between 1,500 and 4,000 cm⁻¹ wavelength. According to Shekoohiyan & Akbarzadeh (2022) microplastic particles can be consumed by fauna and may end up in human bodies through the hierarchy of the food web and this is likely to affect the health of human beings. The identified saturated aliphatic, alcohol and hydroxyl compounds as well as alkene in the current study form part of polymer synthetic and are regarded as microplastic additives substances. The presence of hydroxyl compounds and alkenes indicates the presence of hydrophobic compounds, therefore, the likelihood of adsorption on polymer surfaces is high, exposing the aquatic species to a more toxic environment and likely to change the water quality regime (Seidensticker et al. 2018; Wang et al. 2022).

It is also documented that different polymers have sorption capacity on various compounds, for example, a study conducted by Sheng et al. (2021) investigated the ability of different polymers such as PE, PP and polyvinyl chloride on the ability to adsorb triclosan (TCS) and health impacts on the Zebrafish. The results of the study by Sheng et al. (2021) indicated that all polymers adsorbed TCS, however, PP microplastics highly adsorbed TCS as compared to other polymers increasing the distribution and concentrations of TCS in the tissues and livers of Zebra fish, followed by polyvinyl chloride then PE.

Kang et al. (2018) observed that the occurrence of polymer types such as PS in wastewater may indicate domestic sewage as the main source since it is largely fibres that disintegrate during washing in laundries. When comparing the influents and effluents of both WWTW, we observe similar types of polymers with close % (PS 33% for A and 32.7% for B; PET at 22.4% for A and 21.9% for B) and other different polymers. The main polymers identified in both WWTW influents are PE, PET, and PS while the effluent components are PC and PP.

Figure 9(a) shows the SEM images for influent at WWTW A with particle sizes at 0.005 mm (5 μm). The sample was composed of PE showing fibrous-shaped microplastics. Figure 9(b) shows the SEM image for effluent at WWTW A with particle sizes at 0.05 mm (50 μm). The shapes of particles are long translucent fibres and fragmented materials containing PAM polymer.

Figure 9(c) shows the SEM images for influent at WWTW B with particle sizes averaging 0.025 mm (25 μm). The microplastics appear to be thick fibres of PET. SEM image for effluent at WWTW B confirms the presence of fibrous particles, but the fibres appear to be bio fouled, this could be attributed to microplastics having passed through different treatment stages in the activated sludge processes such as anaerobic zone, anoxic zone, and aeration. The polymer composition of the WWTW B effluent sample was PE (see Figure 9(d)) for reference.

Figure 10(a) presents the elemental analysis of PE particles on the influent of WWTW A. The elemental analysis shows highly intense peaks between 0 and 3 KeV which are indexed to elements such as Carbon (C) and Oxygen (O). Other peaks noticeable such as Calcium (Ca), Chromium (Cr), Magnesium (Mg), Chloride (Cl), Iron (Fe), Aluminium (Al), Silicone (Si), Phosphorous (P), Sodium (Na) and Sulphur (S) were between the KeV ranges 0.5–4.

Figure 10(b) shows elemental analysis peaks of high intensity between 0 and 4 KeV which are indexed to the elements C, O, P, Na, Mg, and Cl. The most intense peak occurs in the range 2.5–3.0 KeV peak and is caused by Cl followed by the peaks attributed to Na and P between 1 and 2 KeV. The O and C peaks were noticeable between 0 and 1 KeV. The polymer linked to the elements detectable in the effluent of WWTW B is PAM. The influent and effluent at WWTW A are composed of elements that can be linked to the presence of microplastics, with heavy metals prevailing in the analysed samples which contain PE and PAM polymers.

Figure 10(c) presents the elemental analysis of PET particles identified in the influent of WWTW B. The most intense peaks were identified in the range 0–4 KeV indicating the abundance of C, O, Na, Si, S, Cl, and P in the microplastics. Other elements identified at lower peaks include Fe, Mg, and Ca. The presence of C, O, and Cl provides evidence of polymer presence in the samples and associated metal (see Figure 10(c)) for reference.

Figure 10(d) presents the elemental peaks in the effluent sample for WWTW B, with the highest peaks occurring between 0 and 4 KeV. The highest peaks recorded are Cl between 2.8 and 3 KeV, Na between 1 and 1.5 KeV, Si between 1 and 2 KeV and O and C between 0 and 1 KeV. Other elemental peaks recorded include Mg, Al, P, and S. The elemental composition is associated with PE particles observed in the effluent of WWTW B.

SEM imaging was used to generate high-resolution micrographs which were used to differentiate the microplastics from organic materials. EDX was used to determine elements present in the microplastics. Woo et al. (2021) further explained that elements such as Al, Ca, Mg, Na and Si are regarded as components of colourants, plasticisers and flame retardants in which microplastics are manufactured. In the current study, heavy metals related to microplastic additives were observed on polymers such as PAM, PET and PE. The analysis of samples containing PE, PAM, and PET showed a high ratio of C and O which can be linked to fibres and fragments observed in the current study. The presence of heavy metals in microplastics is associated with toxic effects on aquatic species. In a study by Banaee et al. (2019) cyprinus carpio species were exposed to Cd and Cl leading to immune disorders and blood flow malfunctioning, indicating the toxic effects of the presence of toxic metals and microplastics in water.

The summary of elemental composition at WWTW A influent is presented in Table 3 showed that the particles of PE were dominated by C (73.46%) followed by O (15.9%). Other elements constitute 11%, mostly sodium was abundant at 4% as compared to the other remaining elements such as Fe, Mg, Si, P, S, Cl, K, and Ca.

The elemental composition for effluent is presented in Table 4 for PAM particles identified on the effluent of WWTW A. The elements dominating the effluent sample at WWTW A were C (58%), O (26.9%), Cl (8.73%) and Na (8.66%) (see Table 4) for reference. Other elements such as Fe, Mg, Si, P, S, Cl, K, Ca and Na were detectable in smaller percentages between ranges of 0.04 and 5%. Although some metal elements were detectable in smaller amounts, microplastic particles of PAM may contain contaminants that can be distributed into surface water.

Table 5 presents the elemental analysis for the influent sample for WWTW B, with PET particles dominating the samples as identified from the reference spectrum. The elemental analysis of the particles indicated high percentages of C and O at 52.26% and 19.92% respectively. The influent for WWTW B also contained elements such as Cl (8.73%) and Na (8.6%) in smaller percentages as compared to C and O. Other elements identified ranged between 0.18 and 2.21%, with Al in the smallest abundance.

According to Table 6, the effluent at WWTW B was predominantly composed of elements such as C (58.31%), O (22.01%), and Cl (6.38%). Other elements such as Mg, Al, Si, P, and S were prevalent in smaller percentages. The particles were dominated by PE as identified from the reference spectrum. The influent and effluent of WWTW B both contained high percentages of C and O elements, indicating that the particles comprised similar elemental compositions but different polymers identified, i.e. PE, PAM and PET.

According to Zhang et al. (2022) when plastic waste is transformed, aromatic oxygenates are formed, this is attributed to the fact that during plastic manufacturing, oxygenated additives are added. The high percentages of oxygen obtained in the elemental analysis of particles conducted in the current study point out that polymers such as PE, PAM and PET are made out of additives rich in oxygen. For example, the transformation of 0.5 g of PS resulted in 0.36 g of aromatic oxygenates produced as stated by Zhang et al. (2022). Therefore the presence of carbon in higher percentages in samples containing polymers in Tables 3–6 indicates polymers made with carbon linkages. Other compounds identified were metals, which form bases in the manufacturing and production of microplastics.

The identification and quantification of microplastics in wastewater treatment plants by spectroscopic and microscopic techniques were carried out in Johannesburg East, South Africa. The study considered the physical and chemical characteristics of microplastics and briefly quantification in terms of concentrations. The concentrations of microplastics in WWTW A and B varied across seasons. The seasonal changes showed that from summer to autumn concentrations are slightly low as compared to winter and spring.

The shapes, colours, and sizes of microplastics are not different across WWTWs, the physical properties of microplastics in WWTW A and B were similar across four seasons. The polymers identified include acrylic, acrylonitrile butadiene styrene, cellulose, polycarbonyl, PAM, PE, PET, poly(ethyl cyanoacrylate), PS, poly(methyl methacrylate) and polycarbonate. The chemical composition of polymers was dominated by trace metals and organic compounds such as hydroxyl compounds, Alcohol, saturated aliphatic, polymeric –OH stretches which confirms the presence of plastic particles in the samples. The elemental composition was dominated by non-metallic elements such as S, Si, O, C and Cl occurring together with metallic elements such as Mg, Al, P, and non-metals such as S and Si.

Due to high concentrations of microplastics escaping from WWTW A and B, the receiving water bodies are affected and the water quality regime is changed by the chemical compositions of microplastics. The seasonal variations in microplastic concentrations point out that there are changes in the patterns of human habits which play a role in the abundance of microplastics in wastewater. The abundance of fibres indicates domestic sources such as laundry machines, therefore, is important to equip them with filters to avoid high accumulation in wastewater. Due to the amount of microplastics that pass through the wastewater treatment works, there is an urgent need to replace the existing wastewater treatment technologies with the new emerging technologies that are tailored for the efficient removal of microplastics from wastewater.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.