Microplastic Transportation in a Typical Drinking Water Supply: From Raw Water to Household Water

Abstract

Microplastics in drinking water have attracted increasing global concerns due to their potential adverse impacts on human health. However, there needs to be more knowledge of the occurrence and distribution of microplastics in drinking water systems from water sources to household tap water. Herein, laser direct infrared spectroscopy is used to investigate the occurrence of microplastics in a typical drinking water plant with different water sources. Microplastic information is further used to understand microplastic fates during drinking water supply, including microplastic abundance, size, shape, and polymer type. Overall, the microplastic abundance in treated water ranges from 12.00 to 25.33 particles/L, higher than those in raw water (RW; 2.33–17.33 particles/L) and household water (HW; 8.00–19.67 particles/L), which shows that microplastics are not removed from RW. The main polymers in these microplastics are polyethylene terephthalate, polyvinyl chloride, polyethylene, and polypropylene. At the same time, the main microplastic shapes are fragments and fibers. Small-sized microplastics of 20–100 μm account for up to 76.74% and 79.30% of microplastics during the dry and wet seasons, respectively. Additionally, more microplastics are detected in RW from rivers than those from reservoirs and lakes, and the microplastic abundance in the wet season is higher than that in the dry season. As expected, the potential ecological risk of microplastics in all waters is the I level, which is the lowest level. Most importantly, the annual microplastic intake of an adult via drinking water is 5063–18,301 microplastics, less than that reported in previous studies. These results provide valuable data on the fates of microplastics in drinking water supply systems from water sources to HW and promote authorities to update the treatment technologies for drinking water in the future to remove microplastics efficiently.

1. Introduction

Since their artificial synthesis in the 1950s, plastics have been globally used because of their low cost and good performance [1]. According to the latest data in 2022, the global annual production of plastics is 400.3 million tons. However, only 54 million tons of plastic are recycled due to improper plastic waste management [2]. Moreover, it is estimated that 12 billion metric tons of plastic waste will be discharged into the environment in 2050 due to the rapid increase in global plastic consumption [3]. A lot of plastic waste in the environment will gradually degrade and break into small pieces under physicochemical and biological interactions, such as microplastics with sizes of <5 mm will be formed [4]. Consequently, microplastics have been detected in nearly all environmental compartments [5]. Undoubtedly, humans are also exposed to microplastics, which have been successfully detected in the human placenta [6,7], blood [8], and thrombi [9]. Hence, global concerns about the adverse impacts of microplastics on human health are rapidly increasing.

The annual intake of plastic particles per person was estimated to be 3.9–5.2 million [10], of which approximately 4000–90,000 microplastics were consumed via drinking water. Therefore, drinking water is one vital pathway through which humans are exposed to microplastics [11]. For example, Kosuth et al. studied 159 drinking water samples from 14 countries and found that 81% contained microplastics. In their study, the highest microplastic abundance with an average of 9.24 particles/L was detected in tap water from the USA, which was higher than that detected in developing nations (e.g., Cuba, Ecuador, India, Indonesia, and Uganda) [12]. Moreover, Mukotaka et al. collected tap water samples from five developed countries (i.e., Japan, the USA, France, Finland, and Germany). They reported that the microplastic abundance in tap water from these countries ranged from 1.9 particles/L to 225 particles/L (≥20 μm) [13]. Li et al. detected microplastics in tap water from China, and microplastic abundance ranged from 1.67 μg/L to 2.08 μg/L with a size of 58–255 nm [14]. In the Barcelona Metropolitan Area, the concentration of microplastic polymers ranged from 1 ng/L to 9 µg/L in household water (HW; 0.70–20 µm) [15]. In comparison, only 0.7 items/m³ were detected in tap water from the Oldenburg–East-Frisian water board [16], which was lower than previously reported. Therefore, the widespread detection of microplastics in tap water has raised concern about the microplastic removal efficiency of drinking water treatment plants (DWTPs), which ensure that the treated water (TW) consumed by people does not have microplastic abundances as high as the source water.

Recently, microplastics in different orders of magnitude have been reported in all processes of the drinking water treatment flow, including source water [16], TW [17], and network water [18]. These studies concentrate on the change in microplastic characteristics from drinking water sources to DWTPs [17,19,20,21,22]. For example, in the DWTPs in the Czech Republic, the microplastic abundance ranged from (1473 ± 34) to (3605 ± 497) particles/L in the raw water (RW) and (338 ± 76) to (628 ± 28) particles/L in the TW, showing that ~83% of microplastics were removed after treatment [17]. Similarly, Shen et al. also reported that 85–90% of microplastics were removed efficiently during the drinking water supply process in Changsha, China [22]. Among these studies investigating the change in microplastic abundance from water sources to consumed tap water [16,22,23,24,25], only Bäuerlein et al. have sampled RWs from different water bodies and found some differences in microplastic abundances [25]. Generally, the treatment processes in DWTPs aim to remove contaminants, such as turbidity, organic matter, and microorganisms, affording fresh drinkable water [26,27]. The water purification processes in DWTPs are not specially designed to remove microplastics, which are emerging global environmental pollutants; however, nontargeted microplastic removal often occurs after traditional sand filtration [27]. To date, little attention has been paid to the microplastic abundance in RW from different sources [17,21], but it is an essential factor affecting the removal of microplastics in DWTPs. Furthermore, microplastic transportation from the DWTP outlets to the actual drinking water consumption from residential taps via pipelines has rarely been considered.

Herein, it is hypothesized that the microplastic removal in DWTPs is related to the types of water sources and other factors, such as season. Therefore, three typical DWTPs with the same water treatment process were chosen, which provided drinking water to a central city with a population density of >1600 people/km² in 2022. To better understand the fates of microplastics in these DWTPs with different water sources, an advanced imaging approach was used to detect microplastics in RW, the corresponding TW from DWTPs, and HW from the pipeline network simultaneously. At the same time, the efficacy of microplastic removal through treatment processes was evaluated by examining changes in microplastic abundance. From a holistic perspective, the transmission of microplastics from source water to user terminals in different seasons suggested that traditional drinking water treatment processes exhibited unsatisfactory performance in removing microplastics. These findings provide valuable baseline data and scientific rationale for further in-depth research and comprehensive prevention and control measures against microplastic pollution in the drinking water supply chain. Such insights are crucial for enhancing the efficiency of existing treatment processes in DWTPs, aiming to effectively eliminate microplastics.

2. Materials and Methods

2.1. Sample Collection

RW samples were collected from a lake (RW1), a river (RW2), and a reservoir (RW3), which supplied water to three DWTPs. These three DWTPs had similar conventional drinking water treatment processes, including coagulation, sedimentation, filtration, and disinfection. The surface water (0–50 cm) was directly sampled from the water intakes using a stainless-steel bucket and stored in glass bottles on sites. TW samples were obtained from the outlets of DWTPs after a series of treatments. The HWs from the corresponding DWTPs were also directly sampled from the faucets in residential properties. The taps were opened several minutes before sampling freshwater. At the same time, ultrapure water was also sampled from each site and transferred into an empty glass bottle to prepare sampling blanks on the site. All water samples were collected during the dry (January 2021) and wet (July 2021) seasons. Three replicates were sampled for each water sample.

2.2. Sample Pretreatment

After mixing the water sample thoroughly and filtering with a 5 mm mesh, 1 L of water from each sample was filtered through a 20 μm stainless-steel membrane with a steady flow. For RW samples, the filters were further digested at 60 °C with 30% hydrogen peroxide (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) for 24–48 h until no significant suspended matter was observed [28]. The solution was further filtered through a 20 μm stainless-steel membrane. Afterward, the membrane was transferred into a 250 mL Erlenmeyer flask with potassium formate solution (McLean Biochemical Technology Co., Ltd., Shanghai, China) and ultrasonically treated for 30 min [29]. The floating particles on the solution surface were collected by adding more solution, as described in the study by Xu et al. [30]. This procedure was repeated three times, and the pooled solutions were filtered through a 20 μm stainless-steel membrane. Finally, the membrane was transferred into an ampere tube with 20 mL of an anhydrous ethanol solution (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China). After ultrasonic treatment for 30 min, the particles detached from the membranes and dispersed into the ethanol solution. The solution was concentrated to 0.5 mL under a nitrogen flow. However, TW and HW samples were directly conducted as described above, except for the digestion and flotation treatment.

2.3. Microplastic Analysis

All anhydrous ethanol solutions containing particles were dripped onto a highly reflective glass slide and naturally dried on an ultraclean bench. Then, the slide was transferred to an 8700 LD-IR platform for automatic detection and identification of microplastics. First, all particles were located with a fixed wavenumber of 1800 cm⁻¹. Second, the trans-reflection mode was used to obtain the spectrum in the wavenumber range of 900–1800 cm⁻¹. Finally, the spectra of all particles were mapped against the reference spectrum library to identify and confirm polymers and particles, and only those with a mapping rate >90% were confirmed to be microplastics. Meanwhile, additional information was also obtained, including the observed microplastics’ width, height, diameter, aspect ratio, eccentricity, and circularity [7].

2.4. Microplastic Pollution Assessment

The pollution load index (PLI) is often used to evaluate the degree of pollution in environments [31]. Similarly, PLI was also conducted to assess the status of microplastic pollution in the current drinking water supply system, which had been successfully used in the Changjiang Estuary [32] and the Qing River [33]. The formula is as follows:

CF_i = C_i/C₀

(1)

$${\mathrm{PLI}}_{\mathrm{i}}=\sqrt{\mathrm{CFi}}$$

(2)

$${\mathrm{PLI}}_{\mathrm{r}}=\sqrt[\mathrm{n}]{\mathrm{PLI}1\times \mathrm{PLI}2\times \dots \times \mathrm{PLIn}}$$

(3)

where C_i is the microplastic abundance in a sample; C₀ is defined as the background value, which was the lowest abundance of microplastics detected in the present investigation [34,35]; CF_i represents the pollution load factor of microplastics in a sample. Based on previous experiences [36,37], PLI is divided into four levels: I (low, 0–10), II (medium, 11–20), III (high, 21–30), and IV (extremely high, >30).

2.5. Quality Control

Experimental operators wore cotton lab coats, nitrile gloves, and masks during all operations to avoid background contamination during sampling and pretreatment. All tools were regularly rinsed three times with ultrapure water and dried in an oven at 100 °C. An aluminum foil was used to cover and wrap samples and instruments. At the same time, three blank samples and each batch of samples were analyzed with the same experimental procedure to detect background contamination.

2.6. Statistical Analysis

The microplastic abundance was expressed with mean ± standard deviation. The one-way analysis of variance and Kruskal–Wallis test were used to analyze multiple consecutive data sets, and data were adjusted using the Bonferroni correction. p < 0.05 was the threshold for statistical significance.

3. Results and Discussion

3.1. Microplastic Abundance in Waters

Microplastics were detected in all water samples collected in the dry season (Figure 1a). In RWs, microplastic abundance in RW1 (5.00 ± 1.00 particles/L) and RW2 (6.67 ± 1.16 particles/L) were significantly higher than in RW3 (2.33 ± 0.58 particles/L) (p < 0.05). However, the microplastic abundance in TWs was considerably higher than that in RWs (p < 0.05), and the microplastic abundance decreased from 8.00 ± 2.00 particles/L (HW3) to 12.33 ± 1.53 particles/L (HW2) (p < 0.05) from TWs to HWs. However, this is still higher than those in RWs (p < 0.05). Like the dry season, the microplastic abundance also increased significantly in TWs, ranging from 16.67 ± 2.89 particles/L (TW3) to 25.33 ± 4.04 particles/L (TW1), while the abundance decreased again in HW1 (19.67 ± 1.53 particles/L), HW2 (11.67 ± 0.58 particles/L), and HW3 (9.67 ± 1.53 particles/L) in the wet season (Figure 1b). Compared with other studies [17,19,38,39], the microplastic abundance detected in the water samples in this study was relatively low. The difference can be partly attributed to broad variations in the characteristics of water sources and non-unified sampling and analytical methods, quantified size ranges, and quality control [4]. For example, the number of microplastics in the 38 tap water samples collected in China by Tong et al. ranged from 0 to 1247 particles/L [39], which is higher than our results. This difference may be due to the low detection limit of 1.0 μm used by Tong et al. [39], which is 20 times higher than the detection limit used in our study. Therefore, it is irrational to compare data among different studies directly, and there is an urgent need to establish unified approaches for microplastic sampling and identification.

Moreover, more microplastics were detected in TWs than in HWs and RWs, implying that microplastics were not effectively removed after the conventional treatment process in DWTPs. Conversely, the treatment process introduced extra microplastics, increasing the microplastics in HWs. Wang et al. reported that the microplastic abundance slightly increased by 2.8–16.0% in the effluent after ozonation treatment [26], and Wu et al. also observed the fragmentation of microplastics during the drinking water treatment process via ozonation [40]. Moreover, Chu et al. reported an increase in the number of microplastics in a DWTP, which was attributed to the aging and rupturing of the filter membrane [18]. In DWTPs, microplastics undergo physical removal/fragmentation and chemical transformation, such as UV photolysis and chemical oxidation, leading to decomposition, rupture, and fragmentation [41,42]. In particular, Ding et al. conducted a further analysis based on the membrane aging mechanism and material properties, revealing that the release of microplastics in long-term operating DWTPs could be attributed to the aging of organic films resulting from chemical cleaning procedures [43]. During the processes of water purification or transportation, the wear and tear of plastic equipment, such as PVC, PE, and PA commonly utilized as components of plastic pipes, was identified as a potential source of microplastic contamination in drinking water [16]. Long-term exposure to plastic components in water supply pipeline systems could cause cracking and fragmentation in an oxidizing environment, which may become a possible source of microplastics released into drinking water [44,45]. However, microplastics presented in raw water undergo aging in the aquatic environment before entering DWTPs [46], and thus affect microplastic characteristics and could lead to the production of halogenated disinfection byproducts at different levels [47,48]. Oxidative dissolution and surface oxidation can also result in fragmentation, byproduct release, and changes in surface properties [49,50]. Chlorination, as the most commonly used disinfection method in DWTPs, could generate disinfection byproducts due to its reaction with organic substances [51]. Under simulated sunlight irradiation, microplastics might release precursor disinfection byproducts through hydrolysis and/or degradation processes, including dissolved organic carbon, bromide, and chlorinated trihalomethanes [52].

Surprisingly, these results differ from other reports [19,25]. For example, the microplastic abundance in TWs was, on average, 83% lower than that in RWs [17]. Similarly, approximately 82.1–88.6% of microplastics in the DWTP influent were removed by an advanced DWTP in China [26]. Generally, DWTPs do not have targeted processes for removing microplastics. However, microplastics may fragment into small particles after conventional treatment in DWTPs, increasing the microplastic abundance in the DWTP effluent. Unexpectedly, the microplastic abundance in HWs was lower than that in TWs, implying the nontargeted removal of microplastics from the pipeline during water distribution. The adsorption of the pipe scales might explain this finding. Chu et al. detected microplastics in water and pipe scales sampled from the DWTP and drinking water distribution system (DWDS), showing that pipe scales adsorb microplastics [18]. Therefore, the adsorption of the pipe scale plays a vital role in reducing microplastics from HWs during the distribution process [16]. However, once microplastics are re-released into the DWDS during pipe descaling, the safety of water quality from faucets will not be guaranteed. Regardless of the adsorption and release of microplastics, these results indicate a non-negligible human exposure to microplastics via directly drinking water. Certainly, the current data are minimal, and more research is needed to explain this finding.

The average abundance of microplastics in RW2 (sampled from the river, 12.00 ± 1.16 particles/L) was significantly higher than in RW3 (sampled from the reservoir, 3.17 ± 1.17 particles/L) during two seasons (p < 0.05). Yang et al. reported that the microplastic abundance in sediments sampled from rivers was significantly higher than those sampled from lakes and reservoirs [53], which our study also investigated. This might be because these sampling sites are located in the lower reaches of the Yangtze River. Generally, microplastics are gradually accumulated or released along the river, probably resulting in a higher microplastic abundance downstream rather than upstream [54,55]. Moreover, the runoff [56] and local economic development [57] also increase the microplastic abundance downstream of basins. Importantly, RW3 sampled from the reservoir exhibits the lowest abundance of microplastics. This sampling site is located in the protection region for drinking water sources. It has few human disturbances, indicating that the current protection measures effectively prevent microplastic pollution in the closed water body.

Additionally, the average abundance of microplastics in all water samples collected during the wet season (10.67 ± 6.45 particles/L) was significantly higher than in samples collected during the dry season (15.33 ± 6.79 particles/L) (p < 0.05). Such seasonal variations in microplastic pollution have also been observed in several studies [54,58,59], and this can be attributed to the combined effect of regional precipitation [59]. During the wet season, more plastic debris and microplastics are discharged into water bodies with heavy rainfall and excessive freshwater inflows, leading to more severe microplastic contamination [60]. Furthermore, continuous rain during the wet season can even induce the release of microplastics from sediments, altering the microplastics’ distribution and causing exogenous microplastics to enter surface water [61]. For example, the microplastic abundance in freshwater sampled from Changsha in China in July (3998 ± 246 particles/L) was higher than that in April (2173 ± 112 particles/L), which may be due to the surrounding environment, weather conditions, and human activities [22]. Meanwhile, atmospheric deposition also contributes to the occurrence of microplastics [62]. Therefore, the impact of seasons and other known and unknown environmental factors on microplastic contamination in water should also be considered.

3.2. Polymer Compositions in Waters

Typical spectra and images of microplastics detected in drinking water supply systems can be seen in Figure 2. Despite the variation in microplastic composition, polyethylene terephthalate (PET), polyethylene (PE), polyvinyl chloride (PVC), and polypropylene (PP) were the main polymer compositions in all samples (Figure 3). These polymers have also been reported in the drinking water from a medium-sized Norwegian urban area [20]. These polymers are often the most abundant polymers in the environment, probably because materials like PE, PET, and PVC are the most frequently used packaging materials [26,63,64,65]. For example, PE is commonly produced to make plastic bags or bottles, food containers, and disposable packaging [66,67]; PET is mainly present in laundry wastewater discharged from garments and is also widely used in packaging such as mineral water bottles [17] because of its high resistance to chemicals, oxidation, and weathering [23]; PVC granules might be due to their use in municipal pipelines, especially in DWDSs [16,22,68]. While plastics are corrosion resistant, minor chipping and wear are unavoidable during transport for drinking water treatment [22], releasing microplastics into drinking water.

Other than these popular polymers, some other polymers were also successfully identified in water samples, including polyamide (PA) and poly(methyl methacrylate) (PMMA) (Figure 3). PA, a thermoplastic polymer, is often used in numerous dairy products, such as toothbrushes, gloves, and stockings [69]. PMMA is added to personal care products and utilized in industrial manufacturing [60]. Similarly, Shen et al. also detected these two polymers in the water taken in the Xiang Jiang River for supplying drinking water [22]. Unlike these insoluble polymers, polyacrylamide (PAM) has been detected in TWs and HWs. PAM is often used as a flocculating agent during the coagulation treatment process. This study did not count PAM in microplastics because it is water-soluble; however, in previous studies, PAM is regarded as microplastics in some drinking water [17,26]. Polytetrafluoroethylene (PTFE) was also detected in TW1 after passing through the DWTPs but not in RW1, indicating that the treatment process and pipeline contribute microplastics into the water [27]. At the same time, large amounts of chlorine-containing disinfectants are also widely used in DWTPs. Water pipes are unavoidably exposed to oxidizing environments for long periods, releasing microplastics from pipelines into the water due to mechanical wear, surface oxidation, polymer chain breakage, and crack extension [44,70,71]. Ultraviolet-induced photodegradation [72] and abrasion of plastic equipment during water transport [39] may also increase the number of microplastics in drinking water. Therefore, it is urgent to update the conventional treatment process in DWTPs to reduce the release of microplastics during water treatments and remove microplastics from drinking water to protect human health.

3.3. Microplastic Size and Shape in Water

Herein, microplastics were divided into three groups according to the particle size: 20–100 μm, 100–200 μm, and 200–500 μm. Overall, the 20–100 μm microplastics had the highest abundance among all water samples (p < 0.05), consistent with many previous studies [16,17,25]. For example, the size of most microplastics in drinking water at different supply chain stages was 50–150 μm [16]. At the same time, the microplastic abundance decreased with an increase in the particle size, which was similar to the results of [17,39,64]. The percentage of small-sized microplastics slightly increased from the RWs (68.94%) to the TWs (81.13%), implying that these large microplastics were further broken into smaller particles during water treatments in DWTPs (Figure 4). Additionally, Mintenig et al. found that the coagulation and sedimentation process increased the proportion of tiny microplastics (<50 μm), indicating that large particles could be easily removed from the water [16]. However, small-sized microplastics are often challenging to capture because of limitations in detection methods, which might lead to an inevitable underestimation of observed microplastic pollution [17,27]. The potentially harmful effects of small-sized microplastics on human health are more significant than those of large-sized microplastics [73]. Therefore, in the future, more attention should be paid to understanding the potential health risks of small-sized microplastics in drinking water.

These microplastics were further classified into four categories, i.e., fiber, fragment, film, and subspherical. Overall, the fragment shape of microplastics was predominant in all water samples, followed by fiber-shaped and film-shaped microplastics. However, the subspherical particles were rarely detected (0–13.79%, 4.11%) and not even found at several sample sites (RW3 and TW1), indicating that the majority of irregular-shaped microplastics in environments are formed due to the breakdown and abrasion of plastics under various environmental stressors (Figure 5). Namely, large plastic wastes were susceptible to natural weathering, physical stresses, and biological degradation, fragmenting into tiny plastic particles gradually [74,75]. Fibers are typically released from the laundry wastewater during washing [17] and are not effectively removed in wastewater treatment plants [76].

3.4. Pollution Load Index and Exposure Assessment of Microplastics in Water

As shown in

Table 1. Pollution load index (PLI) of microplastics in drinking water supply systems sampled in wet and dry seasons, including raw water (RW), treated water (TW), and household water (HW).

Sample	Dry Season		Wet Season
	PLI Score	PLI Level	PLI Score	PLI Level
RW1	1.57	I	1.91	I
RW2	1.82	I	2.40	I
RW3	1.07	I	1.14	I
TW1	2.44	I	2.89	I
TW2	3.44	I	2.74	I
TW3	2.93	I	2.35	I
HW1	2.08	I	2.56	I
HW2	2.48	I	1.97	I
HW3	1.98	I	1.79	I

, the orders of the pollution load index (PLI) were TW, HW, and RW. Moreover, TW2 exhibited the highest PLI score of 3.44 during the dry season but decreased to 2.74 during the wet season. This index depended mainly on the microplastic abundance rather than their chemical composition [77]. Regardless, the status of microplastic contamination in all samples was at level I, and the source water from the river suffered more serious microplastic pollution than that from the lake and reservoir.

3.5. Estimated Intake of Microplastics by Adults and Possible Adverse Effects

Furthermore, the amount of microplastics ingested by a human via drinking water was estimated using the Exposure Factors Handbook of Chinese Population from the Ministry of Environmental Protection [78]. The results showed that approximately 5063–18,301 microplastics are ingested by an adult per year (

Table 2. Estimated intake of microplastics by adults of different genders and age groups through drinking water annually in a certain city in China.

	Drinking Water Intake (mL/d)			Annual Microplastic Intake (Particles)
Age (Years Old)	Male	Female	Mean	Min	Max	Mean
18~44	2465	2161	2315	6310	17,698	9861
45~59	2549	2148	2348	6272	18,301	10,001
60~78	2418	2021	2220	5901	17,360	9456
>80	2112	1734	1898	5063	15,163	8085
mean	2475	2124	2300	6202	17,769	9797

). Combined with the reports of [23,38,79], it was concluded that drinking water was one of the main routes of microplastic intake by humans [10,39]. Consequently, more and more studies have reported the presence of microplastics in human tissues [80,81], metabolites [7,82], blood [8], and breast milk [83]. Meanwhile, an increasing number of toxicological studies in vivo and in vitro using experimental evidence have suggested that microplastics may lead to systemic adverse effects in the digestive, respiratory, cardiovascular, nervous, reproductive, and immune systems, which are related to oxidative stress, inflammatory response, metabolic disorders, induction of cell apoptosis, and even genotoxicity [84]. The digestive tract was usually considered the main accumulation site and toxic target for microplastic ingestion. Yan et al. reported a positive correlation between the concentration of microplastics in feces and the severity of inflammatory bowel disease [85]. Several in vivo studies have demonstrated that mice exposed to PE [86,87] and PVC microplastics [88,89] exhibited changes in gut microbiota composition, as well as disturbances in metabolic homeostasis in liver or bile acids. It is worth noting that the toxic effects of microplastics may be related to their particle size, and nanoplastics are more likely to enter intestinal epithelial cells and cause toxic effects than microplastics [90,91]. Undoubtedly, these findings promote a growing concern about the potential toxicity of microplastic pollution on human health [11], and more research is needed to assess the potential health risks of microplastic exposure via drinking water.

4. Conclusions

Herein, microplastics were detected in all samples collected from three water bodies in two seasons. PET, PVC, PE, and PP were the most abundant polymers, and small-sized particles had the highest abundance. Overall, the microplastic abundance was higher in TWs than RWs, which was beyond expectation. Furthermore, the current conventional treatment in DWTPs exhibited dissatisfactory performance in removing microplastics from the source water and instead introduced extra microplastics. This phenomenon may be related to various factors during the treatment of tap water, such as the pollution of the water source, aging of pipelines, etc., which lead to an increase in the content of microplastics. Additionally, the decrease in microplastic abundance in HWs might be explained by pipeline adsorption, resulting in some microplastics being intercepted during transportation during transmission. Meanwhile, the average abundance of microplastics in the wet season was higher than in the dry season, showing the variation in microplastic abundance between seasons. This might be due to the enhanced erosion effect of rainwater during the wet season, which leads to more microplastics being brought into the water body. Most importantly, the status of microplastic pollution was level I in all samples, showing neglected risks from microplastics. Furthermore, it was estimated that 5063–18,301 microplastics were ingested by an adult annually. This information provides valuable data for assessing future human health risks and encourages authorities to update the treatment processes in DWTPs.