3.2.1. Frequency of Microplastics
Sludge samples
Table 1 showed that the frequency of MPs in sludge varied from (23.05 to 43.7) × 103 MPs/kg during spring and summer, which is consistent with the results of studies in other countries such as Finland, China, America, Korea, Canada, Spain, Italy, and Mauritius (Carr et al., 2016; Gies et al., 2018; Jiang et al., 2020; Lee and Kim, 2018; Li et al., 2019; Li et al., 2018; Magni et al., 2019; Mintenig et al., 2017; Ragoobur et al., 2021; van den Berg et al., 2020). In the UK, the concentration of MPs varied from 37.7 to 286.5 MPs/gr of sludge (Harley-Nyang et al., 2022), which exceeds the results obtained in the present study. However, in the Netherlands, the frequency of MPs was reported to be between 0.2 and 0.45 MPs/g (Brandsma, 2013).
Table 1
Distribution MPs in 6 samples of sludge, wastewater and effluent in wastewater treatment plant of Qom city.
Sludge |
Sort | A | B | C | D | E | F | | A | B | C | D | E | F | |
Particles in 20 gr of dried sludge | | Abundance per 1 Kg (×103) | |
Fiber | 376 | 480 | 514 | 544 | 612 | 784 | | 18.8 | 24 | 25.7 | 27.5 | 30.6 | 39.2 | |
Sheet | 46 | 32 | 30 | 31 | 43 | 56 | | 2.3 | 1.6 | 1.5 | 1.65 | 2.15 | 2.8 | |
Fragment | 39 | 18 | 23 | 17 | 13 | 34 | | 1.95 | 0.9 | 1.15 | 0.85 | 0.65 | 1.7 | |
Total | 461 | 530 | 567 | 592 | 668 | 874 | | 23.05 | 26.5 | 28.35 | 29.6 | 33.4 | 43.7 | |
Wastewater |
| Particles in 3 liters | | Abundance per 1 Liter | |
Fiber | 1836 | 1902 | 1890 | 1971 | 2081 | 2130 | | 612 | 634 | 630 | 657 | 694 | 710 | |
Sheet | 123 | 128 | 106 | 112 | 58 | 135 | | 41 | 43 | 35 | 37 | 19 | 45 | |
Fragment | 57 | 43 | 84 | 53 | 44 | 43 | | 19 | 14 | 28 | 18 | 15 | 14 | |
Total | 2016 | 2073 | 2070 | 2136 | 2183 | 2308 | | 672 | 691 | 690 | 712 | 728 | 769 | |
Effluent |
| Particles in 3 liters | | Abundance per 1 Liter | |
Fiber | 112 | 130 | 140 | 138 | 142 | 150 | | 37 | 43 | 47 | 46 | 47 | 50 | |
Sheet | 19 | 12 | 7 | 12 | 15 | 15 | | 6 | 4 | 2 | 4 | 5 | 5 | |
Fragment | 5 | 3 | 5 | 3 | 3 | 9 | | 2 | 1 | 2 | 1 | 1 | 3 | |
Total | 136 | 145 | 152 | 153 | 160 | 174 | | 45 | 48 | 51 | 51 | 53 | 58 | |
Sample | A | B | C | | D | E | F | |
Removal efficiency (%) | 97.49 | 97.77 | 97.69 | | 97.61 | 97.57 | 97.48 | |
Samples taken on the following dates. A: 4th April; B:5th May; C: 6th June; D: 5th July; E: 6th August; F: 6th September. |
MPs in wastewater are transferred to sludge cake due to filtration and sedimentation units, because these units are most effective in removing MPs from wastewater (Jiang et al., 2020). The use of sludge with this level of pollution for fertilizing fields causes large amounts of MPs to enter the soil. Soil erosion, plowing and wind flow cause MPs to release the atmosphere (Pérez-Reverón et al., 2022). The presence of earthworms in agricultural soils causes the transmission of MPs. Also, MPs in soils with a higher degree of porosity can penetrate the groundwater due to infiltrating water flows (Yu et al., 2019).
Fibers were dominant in all samples. The frequency of sheets was more than fragments. The average frequency of MPs in summer and spring was 35.56×103 and 26×103, respectively, which indicated higher amounts of MPs in sludge in summer than in spring. However, this difference was not significant (p > 0.05). In the present study, the samples of the last two months of summer were taken while large-scale ceremonies were being held in the city, in which disposable plastic containers and cups were used to distribute food and drinks among people, the possible reason for the difference in frequency of MPs in spring and summer might be the mentioned subject.
Wastewater and effluent samples
The average concentration MPs detected in wastewater and effluent samples were 710 ± 34.67 and 51 ± 4.42 (MPs/L), respectively, which was similar to reports in South Korea, America, and China (Conley et al., 2019; Hidayaturrahman and Lee, 2019; Wang et al., 2020). Other studies in Europe, Asia, and America reported lower values (Bayo et al., 2020; Blair et al., 2019; Edo et al., 2020; Gies et al., 2018; Lares et al., 2018; Mason et al., 2016; Simon et al., 2019)[22, 36, 47, 55–58]. According to a study in Italy, the average levels of MPs in samples taken from the inlet and outlet of a wastewater treatment plant serving a population of 1.2 million people were 2.5 ± 0.3 MPs/L and 0.1 ± 0.4 MPs/L, respectively [41]. In the domestic wastewater of the rural area of Hangzhou, China, MPs values of the inlet wastewater were reported to be 430–2154 items/m3 (Wei et al., 2020).
The difference in the values of MPs in different studies can be attributed to the type of wastewater treatment system and its efficiency in removing particles (Harley-Nyang et al., 2022). Sampling season, amount of rainfall, factors related to the studied area, such as population, culture, development and industries also cause fluctuations of MPs in wastewater and consequently in the concentration of MPs in sludge, wastewater, and effluent (Lee and Kim, 2018; Li et al., 2018). In Italy, it was reported that municipal wastewater is diluted with underground water up to 30%, which influenced the concentration of detected MPs (Magni et al., 2019). In some countries, like the city of Qom, household waste and surface runoff are collected along with the wastewater network. The samples examined in the present study were taken from domestic sewage, and there was no industrial sewage. As a result, most identified MPs had a secondary origin (from street washing and domestic discharges). These discharges can include washing synthetic clothes and larger plastic materials like plastic bags, packaging, bottles, and pipes that degrade and produce MPs in various ways (Andrady, 2017; Yang et al., 2021).
In the city of Qom, 52,000 m3/day of effluent enters the urban green space, and it is estimated to enter 2652×106 MPs into the environment daily by effluent, which can introduce the air as a result of soil erosion or affect other parts of the ecosystem (Reddy and Nair, 2022). Our observations were similar to the results detected in US, so that the release of 500 to 1000 million MPs/M3 from the three treatment plants was reported (Conley et al., 2019). However, based on the removal efficiencies reported in the studies, treating all wastewater sufficiently can reduce the global MPs load by 90% in water environments (Reddy and Nair, 2022).
In the present study, the concentration of MPs had an ascending trend from the first month of sampling to the last month, and the maximum concentration of MPs in wastewater was 679 MPs/L, and in effluent was 58 MPs/L. samples were for the last summer month (September). The values of the MPs were different between the months, but the difference was not significant (p > 0.05). Table 1 shows the trend of frequency changes of MPs during the sampling period.
There was a significant difference between wastewater and effluent (p < 0.001). Figure 2 shows the average concentration comparison of wastewater and effluent MPs during sampling. Also, the average MPs removal efficiency was 97.6%. In other studies, the impact of the various unit of treatment on the final MPs content in the effluent was significant. The treatment efficiency in the removal of MPs from the wastewater was stated 97% by Magni et al. (2019and was reported 90% by Bayo et al. (2020. Table 1 provides more details on the frequency of MPs in all 6 samples of wastewater and effluent.
Table 1. Distribution MPs in 6 samples of sludge, wastewater and effluent in wastewater treatment plant of Qom city.
Figure 2. Comparison of number of microplastics in wastewater and effluent samples in this study.
The shape, color, size and type of polymer of microplastic
In Tables S3-S5, the data for MPs identified in the samples is provided. The fibers were the most frequent in all samples. In sludge, wastewater, and effluent, fibers were 88%, 92%, and 88%, respectively. The frequency of fragments in all three samples was 3%. Figure 3b indicates the lower frequency of fragments than sheets and fibers in all three samples. In most studies on MPs content in sludge and wastewater, fibers were introduced as dominant MPs that correspond to the results of the present study (Blair et al., 2019; Edo et al., 2020; Gündoğdu et al., 2018; Jiang et al., 2020; Li et al., 2018; Liu et al., 2019; Magni et al., 2019; Ragoobur et al., 2021; Talvitie et al., 2017). A study in Bangladesh also found that microfibers had a frequency of more than 75% in the sludge samples taken from the textile industry (Hossain et al., 2023). Microfibers can be attributed to the wastewater caused by the washing of synthetic clothing and fabrics in homes and industries (Rochman, 2018). A synthetic cloth can produce more than 1900 fibers per wash with a washing machine (Browne et al., 2011). The reason for the presence of more fibers in the sludge was more contact surface compared to the other MPs because it is easily eliminated by the activated sludge system (Jiang et al., 2020). Fragments and sheets are directly produced from the MPs in the manufacture of cosmetics and other similar products or are produced by the breaking and fragmentation of macroplastics (Ben-David et al., 2021).
The reports related to the shape of MPs are very important because they can specify the origin of MPs and wastewater characteristics. However, the treatment processes may also affect the shape of MPs, which require more studies (Collivignarelli et al., 2021). The color, size, and shape of MPs are changed under the influence of weathering and physico-chemical changes, and even due to chemical changes, objects with more dangerous chemical characteristics can be derived from them (Liu et al., 2020).
In this study in sample of wastewater and effluent, fibers were also dominant, which supported the results of previous studies (Blair et al., 2019; Gündoğdu et al., 2018; Talvitie et al., 2017).
Figure 3a shows that black, white, blue, red, and green colors were found in all samples. The green color was the least frequent in all samples. Also, white color was dominant in sludge (33%), and black color was the most frequent in wastewater (37%) and effluent (39%). In China, the results of a study showed that gray color was the most frequent (41.3–54.5%) in sludge MPs. While in other studies on sludge in the same country, white color was dominant with 38.0-70.4%, which was consistent with the results of the present study (Li et al., 2018). In England, sludge MPs were mainly green, blue and red, and transparent and white colors were only 2.5% and 0.5% (Harley-Nyang et al., 2022). There is a possibility that transparent and white colors are not well distinguished due to the white surface of the filters, especially in smaller sizes (Liu et al., 2019). Carr reported blue color and Murphy determined red color as the dominant color of MPs in wastewater (Carr et al., 2016; Murphy et al., 2016).
According to the data presented in Tables S3-S5, most fragments and sheets were smaller than 500 µm. MPs with a size of 250–500 µm were the most frequent in wastewater and sludge with 24% and 27%, respectively. But in the effluent samples, the size of 100–250 µm was dominant with 33%. Similar results were obtained in other studies on sludge and wastewater (Carr et al., 2016; Ding et al., 2020; Gies et al., 2018; Gündoğdu et al., 2018; Lares et al., 2018; Magni et al., 2019), but it is possible that MPs smaller than 100 µm may not be accurately reported as they are difficult to visually detect and separate (Harley-Nyang et al., 2022). In the UK, similar to Nanjing, China, the majority of detected MPs were in the range of 25–100 µm (Horton et al., 2021; Yuan et al., 2022). In China, the average size of MPs was reported to be 203.8 µm, which was lower than the average size in the present study, and fibers in larger sizes and fragments in smaller sizes are predominated (Liu et al., 2019). Contrary to other reports in China and India, MPs with sizes > 0.5 and > 1.5 mm were predominant, respectively (Lv et al., 2019; Raju et al., 2020).
The difference in the size of MPs in the present study can be attributed to the mechanical forces caused by mixing or pumping wastewater in treatment plants, which can convert MPs into smaller pieces, and on the other hand, increase their number in the wastewater (Enfrin et al., 2020). On the other hand, according to the results of previous studies, MPs with a size larger than 500 µm are usually removed by treatment plants more easily than MPs with a size less than 100 µm (Freeman et al., 2020; Menéndez-Manjón et al., 2022).
Figure 3c shows that MPs were found in all size groups, and in all three samples, the size group 1000 < L < 5000 µm was the least abundant. Figure 4 shows some MPs identifying the present study.
Figure 3d shows the type of polymers and their frequency. Polyethylene terephthalate (PET) was dominant in sludge, wastewater, and effluent samples, Also, polypropylene (PP) was more frequent in effluent and sludge than wastewater, and the lowest frequency was related to polyamide (PA). but other polymers such as poly Ethylene (PE), polystyrene (PS), and polyester were also observed in this study. Other studies also obtained similar results (Lv et al., 2019; Yang et al., 2019). In some studies, PE was predominant (Carr et al., 2016; Gündoğdu et al., 2018). However, in Blair's report, PP polymer was dominant (Blair et al., 2019). This evidence suggests different sources of MPs in the urban wastewater network. PET and polyester MPs mainly originate from textiles and clothing washing operations, while PE and PP are released from personal and household care products (Lv et al., 2019).
Figure 3. Frequency percentage of: (a) colors, (b) shapes, (c) size and (d) polymer of microplastics detected in this study.
Figure 4. Photographs of MPs observed in samples. a, b: Sheets, c, d, g: Fibers and e, f: Fragment.
SEM results:
SEM was used to obtain clear images of the surface properties of different MPs (Fig. 5). The images showed signs of fragmentation and flaking and physical and chemical decomposition. Figure 5a shows the irregular surface of a MP in the sludge. Numerous, deep and relatively sharp pits that increase its surface area enhance its ability to carry microbial and chemical objects, and its relatively sharp edges indicate that it was recently derived from larger particles and has little decay (Klein and Fischer, 2019). Figure 5b is the surface of a MP in the wastewater sample, which has deep pits with rounded edges, and erosion and corrosion can be clearly seen in it. Figure 5c shows a fragment in sludge sample that has a relatively smooth surface and shallow pits. Figure 5d shows a sheet in the wastewater sample, which has a relatively smooth surface and many objects are attached to it.
Overall, SEM images showed evidence of erosion and deep and shallow pits on the surface of MPs, similar to the results in other studies (Li et al., 2018; Mahon et al., 2017). pits on the surface of MPs in wastewater may be attributed to impact and friction caused by its dynamics, and fractures may be caused by treatment processes (Klein and Fischer, 2019). These surface features cause the transport of other environmental particles by MPs, which increases their toxicity and environmental hazards (Cai et al., 2017).
Figure 5. SEM images of selected MPs. (a) Fragment, (b) Fragment, (c) Fragment, (d) Sheet.