EHTEnvironmental Health and ToxicologyEnviron Health Toxicol2233-6567The Korean Society of Environmental Health and Toxicology10.5620/eht.e2018013eht-33-3-e2018013Review ArticleOccurrence of microplastics in municipal sewage treatment plants: a reviewKangHyun-Joong1ParkHee-Jin1KwonOh-Kyung2LeeWon-Seok3JeongDong-Hwan3JuByoung-Kyu3KwonJung-Hwan12
Division of Environmental Science and Ecological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea
O-Jeong Eco-Resilience Institute, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea
National Institute of Environmental Research, Environmental Research Complex, Hwangyeong-ro 42, Seo-gu, Incheon 22689, Republic of KoreaCorresponding author: Jung-Hwan Kwon Division of Environmental Science and Ecological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea E-mail: junghwankwon@korea.ac.kr
The National Institute of Environmental Research (NIER)
Municipal sewage treatment plants (STPs) are thought to be important point sources of microplastics in freshwater systems and many peer-reviewed articles have been published on this issue since mid-2010s. In this review, we summarize existing literature on the occurrence of microplastics in STPs and experimental methods used for isolation and identification of microplastics. The number concentrations of microplastics in STP influents were 15.1-640 L-1, whereas those in the STP effluents were highly variable and ranged from not detectable to 65 L-1. For most of cases, conventional STPs are removing microplastics very effectively. Fragments and fibers are dominant shapes of microplastics. Thermoplastics (polyethylene and polypropylene) and polyester are the predominant materials recovered. Although further research is needed, size distribution of microplastics in STPs is likely to follow a power law, implying that different studies using different size cutoffs may be compared after establishing a power law relationship.
Heavy use of plastic products inevitably ends up with smallsized plastic particles in the environment. Plastic particles less than 5 mm in size are called “microplastics” [1]. Recent studies revealed that microplastics are accumulating in the oceans [2-6] as well as in the terrestrial environments [7-12]. Because identified level of microplastics in the environment is much lower than estimated from mismanaged flows of plastic products [5,13,14], the actual level of microplastics including unidentified is suspected to be much higher than observed [4,13]. The increasing level of microplastics in the environment as well as biota has drawn great attention from researchers and general public with increasing evidences of adverse effects of microplastics [15-18].
The origins of microplastics are suspected to be engineered small plastic particles in products such as microbeads in cosmetics and other consumer products or breakage of bigger plastics into smaller particles via various weathering processes [19,20]. Due to long degradation half-life of plastics (often estimated over 100 years [21,22]), microplastics, once formed, may travel long distance and spread over the world like persistent organic pollutants [23-26]. Among many potential sources of microplastics to the environment, sewage treatment plants (STPs) are regarded as important point sources to the freshwater environments and released microplastics may ultimately reach to the oceans via river flows [19,20,27-41]. Thus, it is crucial to evaluate the contribution of STPs as sources of microplastics to the natural waterways.
In order to estimate the level of microplastics entering into and leaving from STPs, it is required to have reliable and reproducible experimental methods to count microplastic particles in sewage influent and effluent. Many researchers tried to isolate and quantify microplastics from wastewater influent and/or effluent [27,29,31-33,35-38,42-48]. In those studies, occurrence of microplastics is usually expressed in the units of number of plastic particles per volume of water [31-33,35-38,43-48]. The number concentration of microplastics in the STP influents was between 15 and 640 particles L-1 [32,35,37,38,44,46] and that in the effluents was much lower, but varied over 4 orders of magnitude [31-33,35-38,43-48]. Many different methods for sampling, isolation and identification of plastic particles from wastewater samples were tried [27,31-33,35-38,42-48]. Thus, it is unclear that the differences in number concentration of microplastics in wastewater is due to the difference in the level of plastic contamination or due to the difference in sampling and analytical methods used.
In this mini-review, we summarize existing peer-reviewed articles on microplastics in STPs. Because a few reviews and reports have been published in a broader context [19-20,39-41], we narrowed the scope to microplastics in STP influents and effluents. The reported variations in the number concentrations, types, and size distribution of microplastics in STP influents and effluents are compared with experimental methods used for isolation and identification of microplastics. Percent removal of microplastics are also assessed based on reported data. Finally, we propose future research needs on the refined assessment of the microplastics in STPs.
METHODS FOR ISOLATION AND IDENTIFICATION
Table 1 summarizes recent peer-reviewed publications in which microplastics were identified in STP influents and/or effluents since mid-2010s [27,31-33,35-38,42-49]. As shown, researchers have used different methods of isolating, recovering, purifying, identifying, and counting methods.
Glass bottles or steel buckets were used for sampling STP influents that contain high concentration of microplastics and sampling volumes were from 0.1 to 30 L [32,35,37,38,44,46]. Larger volumes of STP effluents were required to isolate microplastics, ranging from 2 to 232,000 L [27,31-33,35,37,38,43,44,48]. Various sampling devices were used, including simple steel buckets [38,46], glass jars [32], commercial metal sieves [27,31,33,38,42,45], plankton nets [44], or custom-made pump-filter systems [35-37,43,44,47,48]. Pore size of filtering devices also highly varied. The smallest pore size was 0.7 μm [32] and the largest size cutoff was 300 μm [44]. Different size cutoffs inevitably lead to great variations in identified number concentration of microplastics.
In order to remove organic matters other than synthetic polymers, wet peroxide oxidation (WPO) method was predominantly used [31,33,42,43,45,46,48]. Reaction temperature and time varied depending on the concentration of organic matter. For some effluents, microplastics were isolated by simple filtration without any chemical treatment such as WPO [33,45,47,48]. Staining microplastics using fluorescent dyes such as Nile red are suggested for better detection of smaller microplastics [50-52].
Choosing an appropriate sampling volume is very crucial to obtain reliable number concentration of microplastics especially for analyzing influent samples in which concentration often exceeds 100 particles L–1 since identifying plastic particles under infrared spectroscopy is time-consuming. Performing preliminary tests would be helpful to decide an appropriate sampling volume for a given size cutoff. WPO is frequently used for isolating plastic particles from organic-rich water samples. Although it was proven to be reliable [42], this also requires long digestion time and needs further refinement.
OCCURRENCE OF MICROPLASTICS IN STP INFLUENTS AND EFFLUENTS
As summarized in Table 1, occurrence of microplastics in STP influents and effluents was expressed on the basis of the number concentration. Further details such as treatment methods of the investigated STPs, year of sampling campaign, sources of the influent are in the web-only Supplementary data file. Although it is difficult to directly compare literature values due to different size cutoffs, the reported number concentrations were not much different among STP influents [32,35,37,38,44,46], ranging between 15.1 and 640 L-1. It is not surprising that the greatest value was obtained when the influent was filtered through a 20 μm filter [37]. Unlike relative invariance among different STP influents, the reported number concentration in STP effluents varied from not detectable to 65 L-1. There is a general trend that the reported median number concentration increases with decreasing size cutoff (Figure 1).
The removal efficiency of STPs could be estimated when the number concentrations in both the influent and the effluent were reported although they are not based on the conservation of mass. It is not surprising that conventional STPs are very efficient for removing microplastics. The calculated removal efficiency was 98.3-99.9% [35,37,38,44,46] except for one study in the Netherlands [32], supporting that microplastics are easily removed during the conventional sewage treatment. Because microplastics are thought to be removed by settling to sewage sludge, recovering microplastics from STP sludge would be important to complete the mass balances of microplastics in STPs. This will help us understand the fate of microplastics entering STPs.
MATERIAL TYPES AND SHAPES OF MICROPLASTICS
The term “plastics” in the polymer industry often refers one of five forms of synthetic polymers, namely fibers, elastomers, plastics, adhesives, and coatings [53]. Plastics are further divided into thermoplastics and thermosets depending on the ease of reprocessing after molten plastics solidify into a shape [54]. However, the term “microplastics” is often used to include all types of anthropogenic polymers [1,55].
Figure 2 describes the relative abundance in percent of material-types of microplastics in STP influents and effluents. Representative thermoplastics (polyethylene (PE), polypropylene (PP), and polystyrene (PS)) and polyester are major materials. The relative abundance agrees with the reported production volumes [56]. PE is the most largely produced plastic material in the world and it has density lower than water [56]. The higher abundance of polyester is characteristic in STPs and is different from the material types of microplastics identified in the oceans and beaches, mainly PE, PP, and PS [55]. Because polyester is used as synthetic fibers in garments, sewage water may contain large amount of micro-sized polyester fibers from laundry effluents [28,57-59]. For example, more than 6 million microfibers may be released from a typical 5 kg polyester fabrics during domestic washing conditions [59].
Figure 3 describes morphology of microplastics in STPs. Fibers forms, mainly from synthetic fibers for fabrics, are the most dominant followed by flakes/fragments. This suggests that microplastics entering STPs are mainly those used as synthetic fibers and fragmented secondary microplastics. Less than 10% are films, pellets, and foams. Because STPs in the United States, Europe, and Australia were studied, further investigation in other regions would provide different abundance patterns owing to different culture and life-styles.
STP AS SOURCES OF MICROPLASTCS TO FRESHWATER SYSTEMS
As summarized in Table 1, majority of studies on the occurrence of microplastics was from the United States, Europe and Australia. Because plastics are also massively used in the other geographic regions, it is expected that microplastics are widespread in other nations as well. If the volumetric flow rate and the population size are counted, it is possible to estimate the microplastics load per capita to the freshwater systems [27,33,37,38]. Murphy et al. estimated daily discharge of 6.5×107 microplastic particles per day from a secondary STP treating 2.6×105 m3 d-1 and the population equivalent to 650,000 [38]. Large amount of microplastics daily discharge from STPs in spite of high removal efficiency, which denotes the requirement of further investigation on the contribution of STPs as point sources of microplastics.
Although no peer-reviewed publications were found yet for Asian countries, a few technical reports were accessible. In Korea, 0-2.2 particles m-3 were detected in river water [60], which were similar to the level in the United Kingdom and Austria [61]. In one Korean STP, the number concentrations in the influent and the effluent were 1.3-4.6×103 L-1 and 0.007-0.022 L-1, respectively, indicating more than 99.99% removal [60]. In Japan, microplastic fibers were detected in STP influent and primary sludge by a research group in National Institute of Environmental Studies [62]. However, no number concentration was reported in this study [62].
STPs are regarded as one of the most important sources of microplastics in public waterways and a few studies quantitatively estimated the microplastic load via STPs [27,35,38]. However, it is still not clear whether STPs contribute predominantly compared to other routes of entrance (e.g., direct input to rivers and lakes, stormwater runoff, dry and wet deposition from air, etc.). Contributions by different routes of entrance warrant further investigations. In addition, published results about microplastics in STPs are mainly from developed countries where most of sewer is treated though STPs. However, percentage of sewage treatment is much lower in developing countries [63]. Thus, microplastic input to freshwater systems might be greater in developing countries because STPs are found to be able to remove microplastics very efficiently [35,37,38,44,46,60]. Another aspect to be considered is that the annual plastic consumption in developing countries is lower than developed countries. Further studies on the occurrence of microplastics in various geographic regions, especially in developing nations, would provide better estimates of global microplastic load to freshwater environment.
SIZE DISTRIBUTION OF MICROPLASTICS
As shown in Table 1, researchers have used different size cutoffs for isolation of microplastic particles. Except for primary microplastics that are engineered to small sizes, microplastics are thought to be formed via various weathering processes from bigger plastic products. Although our understanding of these fragmentation processes is very limited, it may be assumed that the fragmentation of plastic particles to smaller ones is a scale-independent process within the size range of microplastics predominantly found (20 μm - 5 mm). Thus, the size distribution of microplastics should follow the power law [64]. Laboratory fragmentation study by Song et al. [65] provides a good support for this hypothesis. However, the size distribution of microplastics isolated from environmental samples did not always satisfy the power law relationship [4,45,66,67]. In a recent study using a novel Nile red staining method, more small-sized microplastics were found and the size distribution followed the power law [52].
Only a few studies reported size distribution of microplastics in STPs using size fractionation [31,37]. Figure 4 describes particle size distribution, log (△N/△dp) versus dp, where N denotes number concentration (particles L-1) and dp is the median size in μm for the number concentration of microplastics reported by Ziajahromi et al. [31] (A) and Talvitie et al. [37] (B). The median values of reported dp were used to draw the size distribution. As shown, the particle size distribution in general follows the power law although the slopes were obtained from only 3 points. Interestingly, the slopes in Figure 4 did not vary much (-2.68 ~ -1.92 in Ziajajromi et al. [31] and -2.39 ~ -1.08 in Talvitie et al. [37]) among different STPs in two studies. It is also noteworthy that the slopes of the size distribution curve are smaller than those obtained in batch tests by Song et al. (-4.57 ~ -2.74) [65]. This might be because of the enhanced aggregation and removal of smaller plastic particles in the real environment that do not occur in a batch test.
The slope in the size-distribution of microplastics is important for the practical purpose of comparing experimental data using different size cutoffs. It is allowed to compare reported concentration data with different size cutoffs, if the size-distribution of microplastics in STPs or in other environmental media follows the power law with a specific exponent. It is also important to know the size distribution over which the power law is applicable. Studies on the size distribution of microplastics in environmental media warrants further investigation to estimate the current number and mass of microplastics in the environment.
CONCLUDING REMARKS AND FUTURE RESEARCH NEEDS
Recent investigations on microplastics in STPs show that microplastics are ubiquitously found all over the world. Although the occurrence of microplastics was investigated only in limited regions, secondary microplastics and synthetic fibers originated from garments are major source of microplastics in STPs [28,46,49]. Conventional STPs under typical operation conditions are found to be removing microplastics from their influents. However, the occurrence of microplastics in STPs in other geographic regions are needed to study for a better global estimation of microplastics load to freshwater systems. Further studies on the size distribution of microplastics in STPs are also needed to understand the fate of microplastics in STPs and to compare results from different studies using different size cutoffs.
The authors have no conflicts of interest associated with the material presented in this paper.
This work was supported by a grant from the National Institute of Environment Research (NIER), funded by the Ministry of Environment (MOE) of the Republic of Korea (NIER-2018-04-02-038).
REFERENCESGESAMPSources, fate and effects of microplastics in the marine environment:
A global assessmentKershawPJ2015GESAMP9096ThompsonRCOlsenYMitchellRPDavisARowlandSJJohnAWGLost at sea: where is all the plastic?20043045672838BarnesDKAGalganiFThompsonRCBarlazMAccumulation and fragmentation of plastic debris in global environments200936419851998CózarAEchevarriaFGonzález-GordilloJIIrigoienXÚbedaBHernández-LeónSPlastic debris in the open ocean201411181023910244JambeckJRGeyerRWilcoxCSieglerTRPerrymanMAndradyAPlastic waste inputs from land into the ocean20153476223768771MooreCJSynthetic polymers in the marine environment: A rapidly increasing, long-term threat20081082131139BlairRMWaldronSPhoenixVGauchotte-LindsayCMicro- and nanoplastic pollution of freshwater and wastewater treatment systems20171-21930NgE-LLwangaEHEldridgeSMJohnstonPHuH-WGeissenVAn overview of microplastic and nanoplastic pollution in agroecosystems201862713771388de Souza MachadoAAKloasWZarflCHempelSRilligMCMicroplastics as an emerging threat to terrestrial ecosystems201824414051416EriksenMMasonSWilsonSBoxCZellersAEdwardsWMicroplastic pollution in the surface waters of the Laurentian Great Lakes2013771-2177182KleinSWorchEKnepperTPOccurrence and spatial distribution of microplastics in river shore sediments of the Rhine-Main area in Germany2015491060706076WangWNdunguAWLiZWangJMicroplastics pollution in inland freshwaters of China: A case study in urban surface waters of Wuhan, China201757513691374JangYCLeeJHongSMokJYKimKSLeeYJEstimation of annual flow and stock of marine debris in South Korea for management purposes2014861-2505511KimMHyunSKwonJ-HEstimation of the environmental load of high- and low-density polyethylene from South Korea using a mass balance approach2015693367373de SáLCLuísLGGuilherminoLEffects of microplastics on juveniles of the common goby (Pomatoschistus microps): Confusion with prey, reduction of the predatory performance and efficiency, and possible influence of developmental conditions2015196359362LeeK-WShimWJKwonOYKangJ-HSize-dependent effects of micro polystyrene particles in the marine copepod Tigriopus japonicus201347191127811283LönnstedtOMEklövPEnvironmentally relevant concentrations of microplastic particles influence larval fish ecology2016352629012131216JeongC-BWonE-JKangH-MLeeM-CHwangD-SHwangU-KMicroplastic size-dependent toxicity, oxidative stress induction, and p-JNK and p-p38 activation in the monogonont rotifer (Brachionus koreanus)2016501688498857AutaHSEmenikeCUFauziahSHDistribution and importance of microplastics in the marine environment: A review of the sources, fate, effects, and potential solutions2017102165176RezaniaSParkJDinMFMTaibSMTalaiekhozaniAYadavKKMicroplastics pollution in different aquatic environments and biota: A review of recent studies2018133191208ArthamTSudhakarMVenkatesanRMadhavan NairCMurtyKDobleMBiofouling and stability of synthetic polymers in sea water2009637884890Restrepo-FlórezJ-MBassiAThompsonMRMicrobial degradation and deterioration of polyethylene-A review2014888390BarnesDKAWaltersAGonçalvesLMacroplastics at sea around Antarctica2010702250252Ivar do SulJASpenglerACostaMFHere, there and everywhere.
Small plastic fragments and pellets on beaches of Fernando de Noronha (Equatorial Western Atlantic200958812291244ThompsonRCSwanSHMooreCJvom SaalFSOut plastic age200936419731976RohmannRMicroplastics are not important for the cycling and bioaccumulation of organic pollutants in the oceans—but should microplastics be considered POPs themselves?2017133460465CarrSALiuJTesoroAGTransport and fate of microplastic particles in wastewater treatment plants201691174182CesaFSTurraABaruque-RamosJSynthetic fibers as microplastics in the marine environment: A review from textile perspective with a focus on domestic washings201759811161129KalčíkováGAličBSkalarTBundschuhMGotvajnAŽWastewater treatment plant effluents as source of cosmetic polyethylene microbeads to freshwater20171882531LaseeSMauricioJThompsonWAKarnjanapiboonwongAKasumbaJSubbiahSMicroplastics in a freshwater environment receiving treated wastewater effluent2017133528532ZiajahromiSNealePARintoulLLeuschFDLWastewater treatment plants as a pathway for microplastics: Development of a new approach to sample wastewater-based microplstics20171129399LeslieHABrandsmaSHvan VelzenMJMVethaakADMicroplastics en route: Field measurements in the Dutch river delta and Amsterdam canals, wastewater treatment plants, North Sea sediments and biota2017101133142MasonSAGarneauDSuttonRChuYEhmannKBarnesJMicroplastic pollution is widely detected in US municipal wastewater treatment plant effluent201621810451054PrataJCMicroplastics in wastewater: State of the knowledge on sources, fate and solutions20181291262265TalvitieJHeinonenMPääkkönenJ-PVahteraEMikolaASetäläOVahalaRDo wastewater treatment plants act as a potential point source of microplastics? Preliminary study in the coastal Gulf of Finland, Baltic Sea201572914951504TalvitieJMikolaAKoistinenASetäläOSolutions to microplastic pollution – Removal of microplastics from wastewater effluent with advanced wastewater treatment technologies2017123401407TalvitieJMikolaASetäläOHeinonenMKoistinenAHow well is microliter purified from wastewater? – A detailed study on the stepwise removal of microliter in a tertiary level wastewater treatment plant2017109164172MurphyFEwinsCCarbonnierFQuinnBWastewater treatment works (WwTW) as a source of microplastics in the aquatic environment20165011158005808Eerkes-MedranoDThompsonRCAldridgeDCMicroplastics in freshwater systems: A review of the emerging threats, identification of knowledge gaps and prioritization of research needs2015756382DrisRImhofHSanchezWGasperiJGalganiFTassinBEnvironmental Chemistry, CSIRO Publishing201532LassenCHansenSFMagnussonKHartmannNBRehne JensenPNielsenTGCopenhagen KDanish Environmental Protection Agency2015206DyachenkoAMitchellJArsemNExtraction and identification of microplastic particles from secondary wastewater treatment plant (WWTP) effluent2017914121418MintenigSMInt-VeenILöderMGJPrimpkeSGerdtsGIdentification of microplastic in effluents of waste water treatment plants using focal plane array-based micro-Fourier-transform infrared imaging2017108365372MagnussonKNorénF2014Swedish Environmental Research Institute19SuttonRMasonSAStanekSKWillis-NortonEWrenIFBoxCMicroplastic contamination in the San Francisco Bay, California, USA2016109230235LaresMNcibiMCSillanpääMOccurrence, identification and removal of microplastic particles and fibers in conventional activated sludge process and advanced MBR technology2018133236246DrisRGasperiJRocherVSaadMRenaultNTassinBMicroplastic contamination in an urban area: a case study in Greater Paris2015125592599VermaireJCPomeroyCHerczeghSMHaggartOMurphyMMicroplastic abundance and distribution in the open water and sediment of the Ottawa River, Canada, and its tributaries20172301314BayoJOlmosSLópez-CastellanosJAlcoleaAMicroplastics and microfibers in the sludge of a municipal wastewater treatment plant2016115812821ShimWJSongYKHongSHJangMIdentification and quantification of microplastics using Nile Red staining20161131-2469476ColeMA novel method for preparing microplastic fibers2016634519Erni-CassolaGGibsonMIThompsonRCChristie-OlezaJALost, but found with Nile Red: A novel method for detecting and quantifying small microplastics (1 mm to 20 μm) in environmental samples201751231364113648CarraherCEJr2nd edCRC PressBoca Raton, FL2010BakerAMMMeadJThermoplasticsHarperCAMcGraw-HillNew York, NY2000ShimWJHongSHEoSMarine microplastics: abundance, distribution, and compositionZengEY2018Elsevier IncPlastic EuropePlastics – the Facts 2017: An analysis of European plastics production, demand and waste dataHernandezENowackBMitranoDMPolyester textiles as a source of microplastics from households: A mechanistic study to understand microfiber release during washing2017511270367046NapperIEThompsonRCRelease of synthetic microplastic plastic fibres from domestic washing machines: Effects of fabric type and washing conditions20161121-23945De FalcoFGulloMPGentileGDi PaceECoccaMGelabertLEvaluation of microplastic release caused by textile washing processes of synthetic fabrics2018236916925LeeJHParkCHHuhIALeeSHLeeYSLeeSKLeeSHStudies on the investigation method of microplastic in the freshwater2016RP2016282SadriSSThompsonRCOn the quantity and composition of floating plastic debris entering and leaving the Tamar Estuary, Southwest England20148115560http://www.nies.go.jp/risk_health/chemsympo/2017/h29_youshi/sympo20180216-006-ogawa_fumiaki.pdf (In Japanese) (Accessed on July 20, 2018)European Union, Eurostat – Water statistics. url: http://ec.europa.eu/eurostat/statistics-explained/index.php/Water_statistics (Accessed on July 11, 2018)NewmanMEJPower laws, Pareto distributions and Zipf’s law200546323351SongYKHongSHJangMHanGMJungSWShimWJCombined effects of UV exposure duration and mechanical abrasion on microplastic fragmentation by polymer type201751843684376ChaeD-HKimI-SKimS-KSongYKShimWJAbundance and distribution characteristics of microplastics in surface seawaters of the Incheon/Kyeonggi coastal region2015693269278PengGZhuBYangDSuLShiHLiDMicroplastics in sediments of the Changjiang estuary, China2017225283290Figures and Table
Relationship between the number concentration of microplastics in STP effluents and size cutoff. Mean values from literature are shown with error bars representing standard deviations.
Relative abundance in percent of material-types of microplastics identified in STP influents and effluents. Minimum, 25 percentile, median, 75 percentile, and maximum values from ref 27, 32, 34, 36, and 39 are presented in the box plot. (Abbreviations: PE=polyethylene, PP=polypropylene, PS=polystyrene, PET=polyethylene terephthalate, PA=polyacrylate, PU=polyurethane, PVC=polyvinyl chloride, PVA=polyvinyl alcohol, PPO=polyphenylene oxide, ABS=acrylonitrile butadiene styrene, SAN=styrene acrylonitrile, EVA=ethylene-vinyl acetate, PLA=polylactic acid, PLE=polyaryl ether).
Relative abundance in percent of shapes of microplastics identified in STP influents and effluents. Minimum, 25 percentile, median, 75 percentile, and maximum values from ref 28, 29, 31, 33, 34 are presented in the box plot.
Microplastic particle size distribution in (A) primary and secondary effluents from three wastewater treatment plants (WWTP) by Ziajahromi et al. [31] and in (B) influent and effluents after mechanical treatment, chemical and biological treatment, and final effluent by Talvitie et al. [37]. Dashed lines are best-fit using linear regression.
Summary of studies in which microplastics were identified in STP influents and effluents.