Sewage treatment plants as a potential source of microplastics in the environment: A preliminary study in Central India

Sewage treatment plants (STPs) are one of the important technological structures for removing contaminants and other harmful impurities from wastewater which is generated from households, commercial establishments, and other public institutions. These entities play a vital role in reducing water pollution (SDG #6), maintaining public health (SDG #3), protecting aquatic ecosystems (SDG #14), and building up sustainable cities and communities (SDG #11), thus helping to achieve sustainable development goals (SDGs) (Obaideen et al. 2022). If maintained properly, the treated water (effluent) of STPs is clean enough to be discharged into surface water sources, and hence these plants help in the cycling of water for reuse (Delanka-Pedige et al. 2020). STPs are fundamentally designed for treating basic constituents of sewage, viz. organic impurities (represented in the form of biochemical oxygen demand, total nitrogen, total phosphorous), some amount of chemical impurities (represented in the form of chemical oxygen demand), suspended solids, and microbiological impurities (e.g. fecal coliforms) (NGT Order 2019). However, in the current scenario, STPs receive much more than conventional pollutants, for example, excreted/improperly disposed medical waste, pigments/dyes, plastics, and so on. Since STPs are not equipped to treat these waste materials, discharge of STPs' effluent in surface water sources becomes the source of these contaminants in the environment. It has been reported that STPs are one of the significant sources of pharmaceuticals and their metabolites (Vieno et al. 2007; Upadhyay et al. 2022), and microplastic particles (Singh et al. 2023a, 2024a) in the environment.

Microplastics (MPs) are small plastic items (1 μm–5 mm) which are generally originated through the breakdown of large-sized plastic pieces (69–81%) besides being originated from intentional manufacturing (15–31%) (Frias & Nash 2019; Singh et al. 2022). Plastic materials reach up to STPs through inappropriate dumping of plastic materials; domestic washing activities (Napper & Thompson 2016; Nolasco et al. 2022; Singh et al. 2024a); and use followed by wash-off of various products (e.g. toothpaste, facial scrubbers, shaving creams) having microplastic beads (Fendall & Sewell 2009). Bigger plastic items do get the chance of removal in STPs during the primary treatment processes, involving screening and sedimentation, and thus become part of the sludge; while small-sized items flow with the water without getting obstructed and finally become part of the effluent (Ziajahromi et al. 2017). According to an estimate, approximately 99% of the plastic particles present in sewage get transferred into the sewage sludge (Magnusson & Noren 2014; Murphy et al. 2016). Thus, sludge generated from the STPs contains considerably higher amounts of microplastic items. Global abundance of microplastics in sewage sludge has been reported to be in the range of 7.91–495 × 10³ particles/kg (Hayany et al. 2022). Sludge, having microplastic particles, is generally applied onto land, composted, or disposed of in dumpsites, thus further adding to the microplastic pool of the environment (Rolsky et al. 2020). Therefore, it becomes imperative to have an estimate of the contribution of STPs in releasing microplastic particles into the environment.

Research on microplastics in the Indian subcontinent is in its infancy, with the majority of the research being done in the water and sediments (Ashwini & Varghese 2019; Robin et al. 2020; Yaranal et al. 2021; Singh et al. 2025), soil (Singh et al. 2023b, 2024b, c), air (Pandey et al. 2022), and biotic organisms (Hariharan et al. 2021, 2022). Studies on the STPs' potential to contribute to the microplastic pool of the environment have started recently in selected pockets of the country (Parashar & Hait 2022; Patil et al. 2023). Moreover, studies in the Central Indian region are lacking. Therefore, this preliminary study has attempted to investigate the STPs of the central Indian region for their potential to act as a source of microplastics in the environment.

All the samples were stored at 4 °C till the analysis was initiated. Sieves (300 μm–5 mm) were stacked and inlet/outlet samples were passed through (Mason et al. 2016). Any plastic items having a size larger than 5 mm were discarded. The rest of the sieve content was transferred into separate sample containers. For analyzing the sludge, the sample was disrupted using ultrasonication. Organic matter was digested using 30% H₂O₂ in the presence of FeSO₄ solution (Singh et al. 2023b). The obtained solution was filtered and microplastic items were collected for microscopic and spectroscopic analysis. Physical characterization (size, shape, color) of microplastics was done using a stereomicroscope (Make: Carl Zeiss, Model: Stemi 305). Fourier transform infrared spectroscopy having ATR assembly (Make: Shimadzu, Model: IR Affinity-1S) was used for the chemical characterization of the obtained microplastic items. Significant precautions were taken during sample processing and analysis to minimize the background contamination of samples. Cotton lab coats and nylon gloves were worn all the time and all the instruments used were of metallic origin.

The characteristic absorbance bands for PE can be seen at 2,914; 2,847; 1,470; and 718 cm⁻¹ (Figure 5(a)). In the case of PP, the distinctive bands are present at 2,956; 2,921; 2,875; 2,840; and 1,377 cm⁻¹ (Figure 5(b)). Apart from these peaks, a broad peak is also present at 3,423 cm⁻¹ which might indicate the presence of the hydroxyl (O–H) group due to the material degradation processes taking place in the sewage water (Figure 5(b)). Similarly, a peak at 1,032 cm⁻¹ may also refer to the material breakdown process (Formela et al. 2016). The presence of PVC was determined through the occurrence of absorbance bands at 2,958; 2,890; 1,425; 1,339; 1,240; 961; 844 and 610 cm⁻¹ (Figure 5(c)). Similar to PP, the peaks present at 1,643 and 1,032 cm⁻¹ refer to the PVC degradation processes (Formela et al. 2016; Nolasco et al. 2022). Bands for PET were found at 3,432; 2,969; 2,908; 1,960; 1,730; 1,577; 1,504; 1,453; 1,410; 1,240; 1,096; 972; 872; 848; 795; and 712 cm⁻¹ (Figure 5(d)). PVAc was confirmed through the bands present at 2,925; 1,725; 1,450; 1,370; 1,228, and 1,010 cm⁻¹ (Figure 5(e)). These bands show the presence of a –CH symmetric stretch (2,925 cm⁻¹), –C = O stretch (1,725 cm⁻¹), –CH₂ bending (1,450 cm⁻¹), and a –C–O stretch (1,228 and 1,010 cm⁻¹) (Acik et al. 2018). In Figure 5(f), the characteristic peaks are present at 3,280 cm⁻¹ (aromatic C–H stretching), 2,918 and 2,848 cm⁻¹ (aliphatic C–H stretching), 1,637 cm⁻¹ (C = O stretching), and 1,408 cm⁻¹ (C–N stretching), which confirm the presence of PA (Ghaemy & Barghamadi 2009). The plastic identity of PS was established through two saturated C–H stretching peaks present at 2,923 and 2,850 cm⁻¹. Peaks at 1,492; 1,452; and 756 cm⁻¹ are also notable for the PS (Figure 5(g)).

PES was confirmed by peaks at 3,334; 2,900; 1,712 cm⁻¹ (C = O vibration), 1,244; 1,101; 1,053; and 1,020 cm⁻¹ (O = C—O—C) (Bhattacharya & Chaudhari 2014) (Figure 5(h)). The PAC was identified by the peaks at 1,733; 1,645; and 1,069 cm⁻¹ (Figure 5(i)). The characteristic peaks of the PU were found at 3,323; 2,918; 2,853; 1,726; 1,532; 1,219; and 1,071 cm⁻¹ (Asefnejad et al. 2011; Boulaouche et al. 2019) (Figure 5(j)). Similarly, PVS was identified by the peaks at 2,924; 2,855; 1,726; and 1,639 cm⁻¹ (Ozcan & Kandirmaz 2018) (Figure 5(k)). Epoxy resin is different from other plastic items found in the sewage samples as it is a thermoset plastic contrary to the thermoplastic nature of other plastic items. However, it also produces microplastics if not disposed of properly. The FTIR spectrum of epoxy resin was identified through the peaks present at 2,929; 1,506, and 911 cm⁻¹ (Sahmetlioglu et al. 2006) (Figure 5(m)).

Among the seven STPs studied for the presence of microplastic items in Central India, it was found that there is substantial variation in the abundance as well as shape/color of the microplastics. Where the highest amount of microplastics in inlet samples was found in STP #6 (viz. 35.5 items/L), the lowest was found in STP #3 (viz. 3.0 items/L). This variation largely depends upon the size of the catchment area, socio-cultural and behavioral practices of the inhabitants, variation in commercial activities, the extent of institutional and public facilities, adjacent land-use pattern, and the influx of tourists in the catchment area from where the sewage is being received in any particular STP (Sperling 2007; Patil et al. 2023; Rodriguez-Alcantara et al. 2024). Similarly, the concentration of microplastics was also found to vary in outlet samples which depend upon various primary, secondary, and tertiary treatment processes employed (Talvitie et al. 2017). Moreover, sampling approaches and differences in the daily mass load of microplastics among different STPs also affect the variation in outlet samples (Patil et al. 2023). A study carried out in 200 MLD STP, based on SBR technology, in Nagpur, India reported the presence of microplastics in the inlet and outlet as 1,860 ± 265 items/L and 148 ± 51 items/L, respectively, resulting in 91.4% removal (Patil et al. 2023). A similar type of study in a moving-bed biofilm reactor (MBBR)-based STP system located in Bihar, India reported the presence of microplastics in raw sewage as 64.3 ± 4.9 items/L and 47.7 ± 4.7 items/L in two different sampling periods. Moreover, the amount of microplastics post-secondary sedimentation was found as 24.3 ± 2.2 items/L and 28 ± 2.1 items/L, which led to the percentage removal of 41.5 and 37.8% in two different sampling periods (Parashar & Hait 2022). Influent microplastic concentration in a similar range was also reported by Yang et al. (2022) in Chinese sewage. A mean abundance of 70, 73, and 98 items/L was found in the industrial, urban, and rural area's influent, respectively (Yang et al. 2022). Post-treatment in the wastewater treatment plant, the microplastic concentration was obtained as 4, 8, and 16 items/L in the industrial, urban, and rural area effluent, respectively. The technologies adopted in the studied Chinese wastewater treatment plants were the membrane treatment process, anaerobic–anoxic–oxic process, cyclic activated sludge system, and oxidation ditch system and/or combination thereof. Another study carried out in Denmark reported the median microplastic concentration in influent and effluent, obtained from 10 wastewater treatment plants having biological nitrogen and phosphorous removal based on activated sludge technologies. With a concentration of 250 μg/L (or 7,216 items/L) in influent and 4.2 μg/L (or 54 items/L) in effluent, the study reported the microplastics' removal efficiency as 98.3% (Simon et al. 2018). Owing to the variabilities involved in the units of reporting, technologies adopted, treatment capacity of plants, population served, and other related factors, it becomes challenging to compare the microplastics' concentration among various studies (Okoffo et al. 2019).

Another notable point is that sludge samples in the present study showed considerably higher concentrations of microplastics ranging from 16 to 389 items/kg, which implies that, STPs are able to filter out microplastic items while sewage undergoes various treatment processes, increasing microplastics in sludge (Singh et al. 2023a; Singh 2024). Parashar & Hait (2022) reported the number of microplastics in sludge as 1.1 ± 0.3 and 1.4 ± 0.7 items/g, which is higher than the present study. Studies carried out in China have reported even higher concentrations of microplastics in sludge as 10.1 items/g (Yang et al. 2021) and 22.7 ± 12.1 × 10³ items/g (Li et al. 2018). The presence of a higher number of microplastic items in sludge indicates that treatment processes employed in STPs tend to remove these particles from the liquid stream. Studies have reported that primary, secondary, and tertiary treatment processes of STPs help to remove up to 50–97% of the microplastic items depending upon the technology employed and other relevant factors (Talvitie et al. 2017; Sun et al. 2019).

It was also interesting to note that outlet microplastic items were mostly white/colorless while inlet and sludge items had varied colors and shapes (Figure 4). This change may be attributed to various oxidative treatment processes which cause bleaching to some extent, resulting in loss of colour. On the other hand, since inlet and sludge samples do not pass through all the treatment units, these items were found to retain different colours.

The studied STPs resulted in varying amounts of different polymer types, such as PE, PP, PVC, PET, PS, PU, PA, PES, and other fibers. This shows the variety in usage of various plastic items in catchment areas of STPs. Figure 6 shows that STP #4–7 have resulted in a higher amount of fibers, such as PES, PA, CA, PVS, and PAC. This can be corroborated by the presence of several textile industries in and around Indore city resulting in the presence of textile fibers (Singh et al. 2024a, c). Textile industries are one of the biggest microfiber generators throughout the chain of textile life cycle (Singh et al. 2024a). Moreover, textile fibers being light in weight can be easily blown away by wind resulting in their deposition in far-off areas. Domestic washing activities also result in a significant amount of textile microfibers which ultimately become part of the sewage. It has been reported that PES, PA, and polyacrylamide textile materials are some of the sources of microplastics from households (Hernandez et al. 2017). Other studies from the Indian subcontinent have also reported the occurrence of PE, PP, PES, PET, PVC, PA, PU, and rayon (Parashar & Hait 2022; Patil et al. 2023). Murphy et al. (2016) reported the presence of PES, PE, and PP in municipal effluent of the secondary wastewater treatment plants serving the population equivalent of approximately 650,000 in Scotland. Similarly, PES, PE, and polyacrylates were reported by Simon et al. (2018) in the wastewater treatment plants of Denmark.

The results indicate that removal efficiency of STPs for microplastics varies depending on the treatment technology used, sewage load received, and quality of the influent. In the studied seven STPs of Central India, five employed the sequencing batch reactor (SBR) technology, while two employed upflow anaerobic sludge blanket (UASB) and attached growth biological bed reactor (AGBBR) technologies, respectively. While SBR and AGBBR are aerobic technologies, UASB is the anaerobic one which maintains a high concentration of biomass through microbial aggregates. The results indicate that STP #6 resulted in considerably higher removal (91.5%) of microplastic items than other STPs (Figure 4). Likewise, STP #3 showed the least removal (16.7%) of microplastics. However, since the number of samples collected was few and only inlet/outlet samples were under consideration in this preliminary study, drawing any conclusion about the microplastics' removal efficiency of STPs would be inappropriate. It was also one of the limitations of the study that flow of the liquid/mass stream was not measured, which could have led to the adoption of a mass balance approach for understanding the removal efficiencies of STPs. Nevertheless, the noticeable fact is that even after removal through various processes of STPs, a sizable fraction of microplastics remains in the treated effluent.

Considering this, it is essential to have better management practices for minimizing the microplastics' contamination of environmental matrices. Efficient solid (plastic) waste management is one of the key solutions which can prevent unwanted disposal of plastic items in the sewage (Sahoo et al. 2022; Singh & Biswas 2023). However, some of the microplastic items are bound to be part of the sewage owing to domestic washing activities, wash-off of cosmetic items having microplastic particles, and dispersal through wind movement (Hernandez et al. 2017; Long et al. 2022; Singh et al. 2024a). Therefore, advanced techniques are required for the improved removal of microplastics in the STPs. Removal of microplastics through adsorption on a suitable medium and/or entanglement in the filtration medium are some of the physical techniques which might help in microplastics' entrapment. Similarly, chemical (advanced oxidation process, metal-organic frameworks) and biological (algal, fungal, bacterial, and constructed wetlands) techniques also hold significant potential to trap these items (Singh 2024). Nevertheless, proper sludge disposal and treatment are crucial as all the entrapped microplastic items during the sewage treatment process eventually accumulated in the sludge. Conventionally, sludge in the STPs undergo thickening, stabilization, and dewatering processes after which it is either sent to waste dumpsites, or used for composting, or many of the times directly applied on the land (Yang et al. 2015). These sludge treatment processes offer limited scope for microplastics' removal, thus leading to the ultimate fate of environmental contamination (Mahon et al. 2017). Recently, pyrolysis (at a temperature higher than 450 °C) has been identified as a promising technology to control microplastics in sludge (>99%), with the additional advantage that the resultant solid product (biochar) can be applied to soil for enhancing soil fertility (Ni et al. 2020). Emphasis needs to be put on the development of similar technologies that can completely degrade the microplastics while posing minimal side-effects on other components of the environment.

This study investigated the potential of STPs as one of the sources of microplastics in the environmental components. A total of seven STPs of different operational capacities were studied in two cities, namely Bhopal and Indore, in Central India. The presence of microplastics was found in the outlet of all the studied STPs (2–13.5 items/L), indicating that discharge of this effluent is the possible source of microplastics in the receiving environmental matrix. Moreover, the considerable presence of microplastics in the sludge (16–389 items/kg) collected from STPs poses an additional negative impact on the soil and groundwater quality. Hence, there is an urgent need to develop and incorporate microplastics' targeted removal technologies in conventional STPs. Moreover, effective sludge treatment is also a crucial aspect to look into in order to avoid soil and groundwater contamination.

The authors are thankful to the municipal authorities of Bhopal and Indore for giving permission to collect the samples from sewage treatment plants. The authors also acknowledge the support received from the officers and staff present at the sites. Mr Surendra Singh Mehra is deeply thanked for the support provided during STPs' sampling in Bhopal. Also, S.S. is extremely grateful to Dr Swapnali Barman (NERC, NIH Guwahati, India) for helping in the preparation of the location map of STPs.

The authors are thankful to the Indian Council of Medical Research (ICMR), New Delhi for the financial support (project grant number ICMR-NIREH/BPL/IMP-PJ-44/2021–22/469: Principal Investigator – Surya Singh, ICMR – NIREH, Bhopal).

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

The authors declare there is no conflict.