Wastewater treatment plants as a source of plastics in the environment: a review of occurrence, methods for identification, quantification and fate

Elvis D. Okoffo *, Stacey O'Brien , Jake W. O'Brien , Benjamin J. Tscharke and Kevin V. Thomas
Queensland Alliance for Environmental Health Sciences (QAEHS), The University of Queensland, 20 Cornwall Street, Woolloongabba, QLD 4102, Australia. E-mail: e.okoffo@uq.edu.au; Tel: +61 7 3343 2443

Received 21st May 2019 , Accepted 9th September 2019

First published on 11th September 2019

Plastics accumulate in the natural environment due to their durability and low recycling volumes. Wastewater treatments plants (WWTPs) have been identified as important sources for the release of plastics into aquatic and terrestrial environments that may lead to further contamination. This review provides a comprehensive summary of current knowledge on plastic pollution from WWTPs. Specifically, this article presents the current status on the sources of plastics entering WWTPs via influent, the occurrence of plastics in WWTP influent, treated effluent and sewage sludge as well as the techniques used for sampling and analysing plastics in WWTP derived samples. The fate and transfer of plastics from WWTPs to aquatic and terrestrial ecosystems is also discussed. While various studies have reported the presence of plastics in WWTP samples, which have certainly improved our level of understanding on the fate of plastics within the WWTP treatment chain, many unanswered questions still remain. A major gap is the lack of standardized methods and robust analytical techniques for the sampling, identification and quantification of plastics including nano-sized plastics in WWTP derived samples, leading to the potential underestimation of total plastics. To aid comparison of data generated by different researchers, we advocate for the harmonisation of sampling approaches, extraction methods, analytical techniques and reporting units for plastics abundance. Future studies should focus on enhanced methods that can also include estimates of nano-sized plastics.

Water impact

Wastewater treatments plants (WWTPs) receive high amounts of plastic particles from the urban environment. These plastics are not eliminated during treatment and are released into receiving waters and soils through effluent discharges and the application of sewage sludge (biosolids) to land. We provide an overview of the current state of knowledge on plastic pollution from WWTPs assessing sources, occurrence, fate and transfer of plastics to freshwater and soil ecosystems.

1. Introduction

Plastics are synthetic polymers that are typically produced by the polymerization of monomers derived from the extraction of oil or gas.1 Globally, the rate of plastic production has shown a steady increase since 1950, reaching 348 million tonnes in 2017.2 It is estimated that about 12 million tonnes of uncontrolled plastic waste is released into the environment every year and is recognised as an important pollution issue.3 Due to the low degradability of plastic polymers, coupled with the high consumption and low recycling volumes, plastic debris have accumulated in the environment, becoming a ubiquitous pollutant.4 Once in the environment, plastic debris undergoes degradation and fragmentation following exposure to sunlight, wind, water and other environmental stressors5,6 into smaller particles, commonly called microplastics (MPs) (1–5000 μm).7–9 However, with plastics undergoing continuous breakdown and fragmentation, it is expected that microplastics will eventually become nano-sized plastics (NPs) (<1 μm) as suggested in recent reports.7,8,10

A number of environmental monitoring studies have reported on the occurrence of plastics in various environmental matrices including the marine environment,11,12 freshwater systems such as lakes and rivers,13 sediments,14,15 soil,16,17 dust and air.18 Plastics that enter the environment have the potential to effectively adsorb organic pollutants which may be released upon digestion by biota or through environmental degradation, leading to possible impacts on ecosystems.19,20 Similarly, plastics may contain additives such as UV stabilisers, plasticisers, flame-retardants and antioxidants which are used in their manufacturing process to enhance properties such as flexibility and durability.21 Certain chemical additives are toxic and can potentially leach from plastics that enter the environment.21 Plastic contamination has been reported to cause direct physical and chemical harm resulting in oxidative stress, mortality, physical damage, effects on reproduction, reduction of predatory performance, reduction in feeding rates among others, to a variety of exposed marine, freshwater and terrestrial organisms.1,4,22–27

Wastewater treatment plants (WWTPs) are known to be receptors for the cumulative loading of plastic particles derived from domestic wastewater, industrial effluents, stormwater and landfill,6 and have been identified as sources for the release of plastic into the environment.28–30 Through WWTPs treatment processes, most plastic particles (approximately 99%) are transferred from the liquid fraction of wastewater into sewage sludge, hence they are not eliminated (degraded) during the treatment process.6,31,32 Sewage sludge in some cases is treated (biosolids) and applied to agricultural lands as fertilizer.33,34 In Australia for example, 327[thin space (1/6-em)]000 tonnes of biosolids were produced in 2017 demonstrating an upward trend, with 75% applied for agricultural purposes.35 Similarly, approximately 50% of sludge is recycled for agricultural purposes in Europe and North America.36 The agricultural application of biosolids may therefore represent a significant source of plastic entering the environment. Even though only about 1% of plastics discharged from WWTPs are in the effluent, considering the amount of plastics entering WWTPs, this still substantially contributes to the amount of plastic entering freshwater environments and eventually the ocean.6,37

Although studies have identified WWTPs as an important source of plastics to receiving freshwater systems and soils, literature on plastics entering WWTPs and their fate are still lacking and remain largely unexplored.6,34,37–44 Compared to the marine environment, plastic analysis in WWTP derived samples is a relatively young and growing field, as evidenced by the number of papers published to date (Fig. S1 in the ESI). This has been attributed largely to the heterogeneous nature of plastics entering WWTPs and the general lack of standardized protocols to monitor plastics in WWTPs. Accordingly, this review article attempts to describe the current knowledge on plastic pollution in WWTPs and identify future areas of research. Specifically, the sources of plastics entering WWTPs, the analytical techniques used for sampling, sample handling, identification and quantification of plastics in WWTPs samples, occurrence and characteristics of plastics in WWTPs derived samples and fate of plastics in WWTPs are described.

2. Sources and transfer of plastics into WWTPs

Plastics entering WWTPs originate from diverse primary and secondary sources. Primary sources include manufactured plastic microbeads, pellets and fragments used in cleaning agents, personal care products (e.g. facial cleansers, body washes, cosmetics, and toothpastes) and industrial applications.27,37,45,46 For example, 100 ml of a body and facial scrub can contain an average of between 0.74 and 4.8 g of plastic microbeads (<100–1000 μm), respectively, with an estimated 15.2 mg being released into the sewage system per person per day.46 Similarly, a typical application (∼1.6 g) of a plastic containing toothpaste can contain up to 4000 fragments of polyethylene (PE) (100–600 μm).39 Bråte et al.,27 recently reported that one tube of toothpaste (100 ml) yielded approximately 100 mg (d.w) of extracted PE particles (50–590 μm i.e. 247.7 ± 95.1 μm). Secondary sources include smaller fragments of plastics that originate from the breakdown of larger plastics.6,47 These fragments can include polymer fibres released from synthetic textiles during the washing of fabrics, discharge from fibre manufacture, consumer products, household items, and wear and tear of plastic items.39,48 A single garment can produce thousands of microfibers (>1900) per wash, with the resulting effluents entering WWTPs through sewers.48 Although our knowledge around the relative importance of the various input sources is incomplete, primary sources are generally considered the dominant source of plastics to WWTPs.47,49

Multiple studies have tried to associate the number and fraction of plastics entering WWTPs with WWTP catchment specifics such as WWTP size and capacity, served population, proportion of industrial wastewater in influent, adjacent surrounding land use, seasonal variations, treatment technologies used, sewer system, combined sewer (sewer and stormwater) or sewer network separate to stormwater and wastewater sources (residential, commercial or industrial).6,28,29,37,38,47,50–54 Although these studies have improved our understanding of plastic pollution in WWTPs, an appropriate understanding of the drivers for plastics entering WWTPs is currently limited. For example, some studies have used the numbers or counts of plastic particles for such a correlation analysis between these parameters and between WWTPs. Comparing these units between WWTPs without normalising to population or flow may hinder comparisons as dilution and per capita plastics release into the sewer may vary significantly between locations.

The number of plastics entering WWTPs from household discharges might be directly affected by human activities, for instance within a served population and catchment area. Higher population density could possibly lead to higher consumption of personal care products that would translate to a higher mass load of personal care products related plastic particles being released into WWTPs. Browne et al.,48 suggested that more microfibers would be expected to enter WWTPs during winter, as people would wear more clothes during winter than in summer.55 Likewise, the expected number of fibres entering WWTPs may also be higher during weekends as people may be more likely to be doing laundry than during the week. It is therefore important to assess seasonal changes, monthly and within-day variances in the number of plastic particles entering WWTPs to provide a holistic understanding of the drivers for plastic in WWTPs.

3. Sampling, pre-treatment and analytical methods

3.1. Sampling strategies for WWTPs samples

Sampling the right volumes/quantities of wastewater (influent and effluent) and sewage sludge for plastic analysis is a very important first step towards the accurate identification and quantification of plastic particles from WWTPs. Currently there are no standardized sampling methods for plastics in WWTP influent, effluent or sewage sludge but several different methods have been employed with various levels of success. For example, influent and effluent has been collected using auto samplers/automatic samplers,40,56via an in situ extraction pump coupled to filtration devices (e.g., stacked stainless steel mesh screens, mobile membrane pump, a suction pump),37,38,41,57,58 sampling containers/container collection followed by filtration (e.g., stainless steel bucket, glass jars)6,43 and surface filtration (skimming the water surface at the effluent discharge).39Table 4 summarises the methods used in various studies for sampling wastewater for plastics analysis. Among the above mentioned sampling methods, the collection of wastewater samples using mechanical screening techniques are the most common.59 This technique generally involves various customized in situ extraction pumps or collection devices coupled with filtration/separation devices composed of stainless steel mesh screens or sieves (Fig. 1–4).37,41,58,60 Usually, the pore size of the screens or sieves used during wastewater sampling influences the number and size of plastics that can be measured.31 In this regard, selecting an appropriate mesh size for sample collection in WWTPs should be done with the size distribution of plastics to be analysed in mind.32 In general, the pore sizes of screens/sieves used for wastewater sampling in the current studies (≥10 μm to 300 μm as shown in Table 4) means sampling procedures are currently not capturing/accounting for nano-sized plastics (<1 μm as defined in this review), which may lead to an underestimation of plastics to be reported. One key challenge in sampling nano-sized plastics in wastewater relates to the uneven mixing of particles and the depth at which they flow as this may affect sample homogeneity (see Fig. 1). Furthermore, using pore sizes of screens/sieves that are in the nano-size range could lead to clogging, lowering sampling volumes and making sampling laborious. Effectively sampling nano-sized plastics in wastewater is clearly challenging and standardized methods do not currently exist.
image file: c9ew00428a-f1.tif
Fig. 1 A schematic picture of a wastewater sampling method; beaker sampler and pump and filtering assembly. Reproduced from ref. 42 with permission from Elsevier, copyright 2019.

image file: c9ew00428a-f2.tif
Fig. 2 A custom-made mobile pumping device used for sampling treated wastewater samples. It consisted of a flexible polyvinylchloride hose with a weighted end-piece connected to a membrane pump (Jabsco EMG 590-8023, Xylem, Germany), a filter housing (polycarbonate) with polypropylene lid containing a 10 μm stainless steel cartridge filter (4 7/8′′, Wolftechnik, Germany) and a flowmeter (Gardena, Germany). Reproduced from ref. 37 with permission from Elsevier, copyright 2019.

image file: c9ew00428a-f3.tif
Fig. 3 A sample wastewater collection setup of four stack 8 in-diameter stainless steel sieves of various mesh sizes at 5000, 1000, 355, and 125 μm. Reproduced from ref. 60 with permission from Royal Society of Chemistry, copyright 2019.

image file: c9ew00428a-f4.tif
Fig. 4 A schematic diagram of wastewater sampling gear used for sampling plastics in WWTP. The ball valve controls the flow rate of sampling. When starting or ending the sampling, the hose is placed in the steel mesh screens or taken out. Meanwhile, the digital camera photographs the electromagnetic flowmeter to record the flow quickly. Reproduced from ref. 58 with permission from Elsevier, copyright 2019.

With regards to sampling strategies, samples are commonly collected using either grab sampling (a single sample or quantity is taken at a certain point in time6,41,42,61,62) or continuous sampling techniques when sampling is conducted over a period of time (usually 24 h at 2 h interval or 24 samples at 1 h interval or 24 h composite samples).40,42,52,59,60,63 Although grab sampling strategies are favoured due to their simplicity, continuous sampling can provide better temporal data by collecting a cross sectional sample for a given period, which is not the case for grab sampling.59,64,65

Sample collection via in situ extraction pump coupled with filtration has been used for effluent sampling since hundreds of litres of wastewater can be examined/sampled. However, this method is not suitable for influent sampling due to the high amounts of organic materials present which rapidly clog filters/sieves and limits the sampling volume (few millilitres or litres). In this regard, influent sample collection has been undertaken using containers or at times automatic-samplers.42 However, using sampling containers and automatic samplers for grab sampling might not be ideal, as only small volumes can be collected at a sampling event.28 The number of plastic particles in wastewater samples (influent and effluent) may be unevenly dispersed, therefore the frequency of sampling or the volume collected each time for analysis should be sufficiently high for the collected sample to be representative. Too low a sample volume reduces the chance of finding particles, reduces the power of a study and increases the margin of error. This is particularly important for effluent samples that typically contain a small number of plastic particles (<100 particles per L).

Sampling via in situ extraction pumps, containers and automatic processes might be influenced by several factors that could impact their accuracy, and the total reported plastics. These factors include: 1. the size of sampling tubes (in diameter) (see Fig. 1–3), 2. whether or not it has a filter attached (the filter may clog if in the nano size range), 3. whether it is able to sample nano-sized particles, 4. the velocity of the sample in the tube, 5. what type of plastic it is made of (see Fig. 2 and 3), 6. the type of sample bottle used, 7. where in the water column the inlet to the tube or sampling device is (where it sits during sampling) (see Fig. 1 and 2), 8. the suction force of pumps, 9. the depth of the inlet and the depth that the samples are acquired (taking into account low and high density plastics which could float and sink, respectively) (see Fig. 1), and 10. the flow and force of flow of the stream of wastewater. Sample collection by surface filtration has been observed to be effective for collecting cubic meters of wastewater, however, the technique has been reported to be only applicable at effluent discharge outfalls. In addition, air borne contamination cannot be eliminated with the technique as sampling is done in an open channel. It has also been reported that surface filtration can only capture plastics which float and not those with higher densities which naturally sink.28,66 To improve the technique, sampling should be undertaken at various depths and not just the surface. We recommend that researchers should detail the sampling regime using any of the above methods and strategies, how the sampling processes were setup and the flow proportions in which the samples were taken to the stream of flow of wastewater. If possible, studies should optimise and calibrate the sampling setup before use to mitigate potential contamination or improper sampling during in situ operations.

We suggest that future studies should sample wastewater using in situ extraction pumps coupled to particle size filtration/separation devices (e.g., stacked stainless steel mesh screens) as it is the most efficient and effective method for sampling larger volumes of samples and aids with particles size categorisation. However, the size of the sampling tubes, the size of filters/screens used (if nano-sized plastics are to be analysed), the plastic they are made of, where they sit in the water column during sampling and the proportion of flow of the stream of wastewater should be critically considered as potential impacts on analysis and the resulting data. We also suggest using continuous or composite sampling strategies as they provide data for a given period or sampling location, and can improve homogeneity and representativeness of acquired samples.

Due to potentially high quantities, poor mixing and larger plastic particles, sampling sewage sludge for plastics analysis can be a very challenging task.28,29,59,67,68 Commonly, sewage sludge samples for plastic analysis are collected directly using a glass jar/container/beaker (e.g., stainless steel bucket) and then stored in a refrigerator before analysis. Table 5 shows the methods used by various studies for sampling sewage sludge for plastic analysis. Sewage sludge samples are mostly collected randomly using grab sampling techniques. To overcome heterogeneity, most often several grab samples are collected from different parts of a given pile and composited or analysed separately. It should be noted that sludge samples have rarely been collected by a direct in situ filtration technique from WWTPs which may be attributed to the high organic matter in sewage sludge as compared to wastewater.28

3.2. Pre-treatment of WWTPs samples

Due to the high levels of interfering organic material that may be present in collected WWTP samples, multiple procedures have been applied to pre-treat samples prior to extracting plastics. These procedures have become necessary to reduce interferences during visual and spectroscopic identification of plastics.59,69 Nonetheless, the efficiency of removing organic matter and the effect of these procedures on polymers (with regards to the degradation and breaking of plastic particles or altering the size of particles) has not to date been properly documented.33

The most widely used pre-treatment method to remove organic materials in wastewater and sewage sludge samples is peroxidation using 30% hydrogen peroxide (H2O2).41,47,53,70 This method has been observed to remove most organic materials present in samples, with the majority of the exposed plastics remaining unchanged after visual and spectroscopic analysis. However, slight changes to particle size for polypropylene and polyethylene has been observed by Nuelle et al.,15 and McCormick et al.71 Wastewater samples treated with H2O2 (30%) for 7 days resulted in an organic material removal efficiency of 83% with unchanged FT-IR spectra for spiked plastics (polyethylene (PE), polystyrene (PS), polypropylene (PP), nylon-6, polyethylene terephthalate (PET), and polyvinyl chloride (PVC)) after analysis.72 Nonetheless, considering the duration of this procedure, it may not be appropriate when larger volumes of samples with higher amounts of organic materials are to be analysed.

To reduce the duration of the procedure but nevertheless sufficiently degrade organic matter, some studies have resorted to heating the suspended sample matrix with H2O2, which makes sample treatment time consuming and potentially dangerous.33,52,60,73 To reduce pre-treatment time, Sujathan et al.,73 used 30% H2O2 at 70 °C to degrade organic matter from sludge samples and quickly observed an improvement in reaction time to about 12 hours. However, the authors noted that plastic particles composed of poly(methyl methacrylate) (PMMA) might be affected (degraded). Similarly, Hurley et al.,33 observed the destruction of PA-6,6 particles, degradation of PS particles and reduction in PP particles size when H2O2 was used in combination with heating at 70 °C. The heating of samples in the presence of H2O2 is therefore not a recommended pre-treatment option. Hurley et al.,33 suggested that using lower temperatures could overcome the issue of plastic degradation, however the effect of low temperatures on reaction time should be evaluated. Mintenig et al.,37 used an enzymatic-oxidative procedure (purification started with addition of sodium dodecyl sulphate, then protease, lipase, and cellulose in that order) to remove organic materials in wastewater. This procedure took longer than 13 days to complete. The same study then used NaOH digestion to remove organic materials from sludge samples, suggesting that the same procedures could not be used to degrade organic materials in wastewater and sludge samples. Similarly, one study used isopropyl alcohol for 48 h at 60 °C to degrade organic materials in sludge samples collected for plastics analysis. However, the degradation efficiency of the procedure was not reported.74 A summary of the advantages and limitations of the various organic material removal techniques for WWTP derived samples have been provided in Table 1.

Table 1 Current organic matter removal methods used to improve the identification of plastics in WWTP samples
Digestion technique Advantages Limitations
Oxidative (H2O2 at ambient temperature) More than 80% of organic materials could be successfully removed with this technique.15,72 Many polymers are resistant to the technique, however, not all.15 Its effects on plastic particles degradation have been demonstrated to be minimal within an exposure of 48 h75 Takes days or weeks to degrade organic materials making its usage inappropriate for larger volumes of samples with higher amounts of organic materials.15,28,72,76 Usually organic materials are not completely removed based on the composition of the organic content in the samples.15 Causes discoloration of PET and PP.15,71 Visible changes for PA, PC, PET and PP15
Oxidative (H2O2 + heating above room temperature) (e.g. 60 °C and 70 °C33) Decreases the duration of exposure or reaction time from weeks to days.73,76 Approximately 80–87% of the organic content of sludge samples and 96–108% of soil organic material could be removed with this technique.33 This shows that higher temperatures could improve the removal efficiency of the treatment Time consuming and potentially dangerous procedure. Degradation/destruction of PA, nylon, PMMA and PS; reduction in PP size,33 colour change of PET and decrease in Raman spectroscopy peaks of PVC and PA33,73,77
A modified procedure using lower temperatures may overcome this issue, although the effect on reaction time must be assessed33
Fenton's reagent (H2O2 with ferrous iron catalyst at ambient temperature) Breakdown organic materials from complex environmental substrates within a short timeframe (few hours) without impacting plastics (PP, LDPE, HDPE, PS, PET, PA-6,6, PC, and PMMA).33,69 An organic material removal efficiency of 80–100% could be obtained for sludge and soil samples using this technique. It is a time efficient and cost effective technique which facilitates processing of large sample volumes compared to H2O2. For example, the reaction occurs more rapidly than traditional H2O2 oxidation, typically taking less than 1 h to process wastewater samples78 It is an exothermic reaction which can reach temperatures as high as 89 °C.79 This may disprove the benefit of using Fenton's reagent as some polymers may degrade at this temperature.33 Ice baths are therefore recommended to control temperatures to an optimum 40 degrees or below which also limit the occurrence of violent reactions improving safety conditions in the laboratory
NaOH + heating above room temperature (e.g. 1 M NaOH at 60 °C and 10 M NaOH at 60 °C33) Organic materials removal efficiency of >90% in biota-rich seawater and marine samples,80,81 with digestion efficiency increasing with the increase in the molarity and temperature NaOH resulted in an insufficient reduction in organic matter content in sludge and soil samples.33 The technique could only remove between 57 and 67% of organic material in sludge and 35–68% of soil organic matter
Causes degradation/destruction to PET, PE, PS, PA (nylon fibres), PC, PVC particles.15,33,82 NaOH is not recommended for organic matter removal in WWTP derived samples due to their unsuitability and degradation of multiple polymer types
KOH (e.g. 10% KOH solution at 60 °C33) KOH are effective digestion treatments (>90%) for biological samples,79,81,82 yet some doubts remain if this technique will also be adequate to digest organic materials in organic-rich samples83 Insufficient reduction of organic materials in sludge samples and soil samples (<70%),33 hence KOH is not appropriate for the removal of organic material in complex, organic-rich, environmental matrices such as WWTPs derived samples
KOH may cause discolouration of nylon, PE and uPVC, degradation of nylon, polyester, PE, PC, PET, PVC, LDPE, CA (cellulose acetate).79 It can also cause loss of PET and PVC and yellowing of PA.77 A decrease and increase in weight was observed for PC and PS following treatment with 10% KOH respectively33

A more successful technique for the degradation of organic materials in WWTPs derived samples has been the use of Fenton's reagent.32,44,56,60,69,84 Fenton's reagent is an oxidation process using H2O2 in the presence of a catalyst (Fe2+) and has been shown not to affect plastic particles visually and chemically using FTIR analysis.33,69 Compared with H2O2 pre-treatment, Fenton's reagent has been reported to be more time efficient and facilitates processing of large sample volumes.66 According to Ou and Zeng,59 the procedure can reduce exposure time from days or hours to several minutes and has now been widely applied to improve efficiency of WWTPs sample pre-treatment. In a more recent study to compare different organic matter treatment methods, Hurley et al.,33 identified Fenton's reagent as the optimal organic matter removal procedure for sludge samples, with the use of only H2O2, NaOH, or KOH in isolation showing signs of plastics degradation and insufficient reduction of organic matter content. We therefore recommend the use of Fenton's reagent for the removal of organic materials in WWTP derived samples as it degrades organic materials very well and does not affect polymer particles. Nonetheless, we suggest that studies should validate and record the efficiency and effects of Fenton's reagent on the removal of organic material and plastic particle before being used on samples to make sure they do not impact analysis.

Following organic matter removal, plastics in WWTPs samples are separated or extracted based on density separation using saturated salt solutions. This is achieved by mixing a salt solution of defined density with the sample which is then vigorously shaken and allowed to settle until two clear phases are formed.66 Usually, the low-density plastic particles floats to the surface with the high-density particles (e.g., clay) settling at the bottom. The plastic particles that float are then recovered through filtration.66 The most commonly used density separation technique for plastics in WWTP samples is floatation and separation using a saturated sodium chloride (NaCl) solution (density: 1.2 g cm−3). NaCl is inexpensive and eco-friendly that has been reported to work well for low-density polymers such as polypropylene (PP) (0.82–0.90 g cm−3), polyethylene (low and high density) (PE) (0.92–0.97 g cm−3), polystyrene (PS) (1.05–1.06 g cm−3), polyamide (PA) (1.13–1.15 g cm−3), poly(methyl) methacrylate (PMMA) (1.16–1.20 g cm−3) and polycarbonate (PC) (1.20–1.22 g cm−3), making it the preferred choice.15,28,30,43 However, a major shortfall of this technique is the inability to separate high-density polymers such as polyethylene terephthalate (PET) (1.31–1.43 g cm−3), polyoxymethylene (POM) (1.20–1.58 g cm−3) and polyvinyl chloride (PVC) (1.41–1.61 g cm−3), thereby resulting in an underestimation of the total plastics extracted and quantified. A study by Magni et al.,34 to separate plastic particles in WWTPs influent, effluent and sludge samples observed that high-density polymers such as PVC and POM cannot be separated with saturated NaCl solution. To overcome this and to separate and quantify both low and high-density polymer types, denser salt solutions including, zinc chloride (ZnCl; 1.5–1.7 g cm−3) and sodium iodide (NaI; 1.6–1.8 g cm−3) have been used in various studies.33,37,41,44,51,85 Nonetheless, the use of the above solutions have been reported to be more costly and could pollute the environment if not properly handled.66 It seems that the commonly used NaCl solution is not ideal for the separation of all polymer types. Therefore, moving forward, we suggest using denser salt solutions (e.g. NaI and ZnCl) that could extract both low and high-density polymers with high recovery rates. Table 2 summarizes the advantages, limitations and efficiency of the different density solutions adopted for separation of plastics in WWTPs originating samples.

Table 2 A summary of the density separation solutions adopted for separating plastics from WWTPs samples
Salt solution Advantages Limitations Separation efficiency
NaCl (1.2 g cm−3) It is commonly available, inexpensive compared to NaI, ZnCl, environmentally friendly/eco-friendly salt,15,86 and non-toxic to humans. NaCl is suitable for the separation of low-density polymers such as PP, PE, PS, PA, PMMA and PC Low recovery of higher density polymers such as PVC, PET and POM makes NaCl unsuitable for plastics separation, since it leads to underestimation of polymer types87 An extraction efficiency of 80–100% plastic particles could be obtained for low-density polymers.34,47,56,88 Smaller particles (<1 mm) can only be separated to a small degree of 40% (mass).30 Using NaCl requires three or more separation to improve efficiency compared to NaI and ZnCl which requires a single washing of samples87
NaI (1.6–1.8 g cm−3) Can be used to separate both low and high-density polymer types. NaI can be recycled for several extraction cycles, as long as it is not used with a cellulose filter87 It is expensive to use in comparison with NaCl and can pollute the environment if not handled properly or with care. NaI reacts with cellulose filters, turning them black and complicating visual identification87 Able to separate polymers with recovery rates of >99%15,85
ZnCl (1.5–1.7 g cm−3) Suitable for the separation of both low and high-density polymers from samples. ZnCl is recommended over NaI because it is less expensive.89 Subsequent recycling and reuse is possible It is hazardous compared to the other substances and must be disposed of properly in order to minimize environmental pollution. Cost of usage is high in comparison with NaCl High extraction efficiencies of 100% for particles (100 μm), >70% for particles (>1 μm) but low efficiencies (<30%) for particles with sizes 0.05 and 1.0 μm in biosolid and soil samples76

To gain a better understanding of the impacts of plastics in WWTP derived samples, we need to identify both nano- and microplastics. However, due to the complexities associated with sample processing, current studies have not been accounting for nano-sized plastics in WWTP-derived samples. The first challenge has always been to separate nano-sized plastics from the highly organic-rich complex samples. WWTP samples, such as those collected from influent, sand traps, fat traps, bar screens, or solid sludge, are amongst the hardest to process and analyse given the very wide variety of organic components within the samples that must be removed, reduced, or transformed, whilst preserving plastic particles. Current organic material removal techniques do not separate nano-sized plastics from all organic matter,76 and with organic matter (density of 1.0–1.4 g cm−3) and plastics having similar densities,76,83 nano-sized plastics cannot be readily fractionated or effectively separated during density separation.76 Usually, plastics separated by density separation methods are in the size range 40 to 5000 μm90 with little data available on the extraction of plastics in the lower micrometre (<40 μm) and nanometre size range.76 Wang et al.,76 reported low extraction efficiencies with ZnCl2 solution for nano- and microplastics in the range of 0.05 to 5 μm from biosolids and soils.

As an alternative, Fuller and Gautam91 developed a sequential extraction method for plastics (PE, PVC, PE, PS, PP and PET) in municipal waste and soil samples using a pressurized liquid extraction (PLE). Although, the technique reduced sample pre-treatment and plastic particles <30 μm could efficiently be extracted, information on particle size, shape and colour were lost as particles are dissolved. By using this technique, the high efficiency (85–94%) extraction of mass of plastics (including nano-sized plastics) in samples was obtained and is one of the most promising methods for plastic isolation from organic-rich environmental samples. In principle, this method should work for smaller particles in the lower micrometre and nanometre size range. However, the technique would further extract all organic materials that could obstruct plastic identification and possible quantification. Further investigation on the extraction potential of the technique and the possibility of identifying and quantifying the polymer extracts with thermo-analytical and chromatographic methods is needed.

3.3. Analytical methods

The most commonly used techniques for the identification and quantification of plastics in WWTPs derived samples are visual sorting/counting with a stereomicroscope/microscope,38,40,54 Fourier-transform infrared spectroscopy (FT-IR),6,41,52,57,92 Raman spectroscopy32,58,62,93 and scanning electron microscopy50 (Tables 4 and 5). Visual observation is used to sort and count plastics into various categories based upon size, morphology and colour. However, it is size dependent and can lead to over- or underestimation of the plastics present, usually, due to difficulties in distinguishing between synthetic and natural fibres.31,40 Small plastic particles characterised visually as plastics are often not confirmed by FT-IR and up to 70% error has been estimated.94 For example, only 50% of fibres initially identified as plastics in wastewater samples by visual observation were later confirmed as being synthetic fibres.37 To ensure consistency in results and for the reliable chemical determination of plastics, spectroscopic techniques are recommended to be used in conjunction with visual identification methods to assist visual observations. Generally, these techniques are strongly preferred over visual identification only.6,39,43

Although spectroscopic techniques provide insights into the types of plastic polymers in samples, they use polymer specific absorption patterns of irradiated infrared (IR) or laser light for identification32,37 that are only able to provide the number, size, colour and shape of individual plastics rather than total mass of plastics.95 Usually, a fraction of the identified particles from the total number are analysed and extrapolated to obtain the overall plastic content.37,53,93 Thus, these techniques make assumptions based on small subsamples that makes it difficult to accurately derive the total number of plastic residues in samples, for example, on a mass per sample basis. Although subsampling should be avoided, these techniques are so laborious that representative sub-sampling is often required.96 These techniques are also size dependent and in many cases, are not able to detect nano-sized plastics leading to underestimations. FTIR and Raman are limited to particles with physical sizes >20 μm and >1 μm, respectively and are not suitable for nano-sized plastics (<1 μm) identification. Compared with FT-IR, Raman techniques have been reported to show better spatial resolution and are therefore preferred for plastic particles >1 μm. However, only a few studies have reported attempts to use Raman techniques for plastic analysis in WWTP derived samples,32,44,58,62,93,97 with the observed particle sizes above the defined NP range in this review.

To use spectroscopic techniques, exhaustive and time-consuming procedures, typically involving organic matter removal and plastics separation are required, which are the major drawbacks to the techniques.32,37,52,53 Often only a part of a sample is scanned during analysis using spectroscopic techniques and are subject to particle size limitations. We suggest that ‘if possible’ researchers should scan multiple parts of samples before concluding on the polymer type. This is true since scanning a part of a plastic particle composed of different polymer materials could be misleading. However, we understand that this could be laborious and very challenging for smaller particles. Additionally, the spectra generated from a weathered plastic versus a virgin plastic can sometimes be different, confounding analysis.28,29 However, these techniques should be used since they yield important information for plastics research in regard to the sources, types and size of plastics in samples. A summary of the advantages and limitations to the various analytical techniques used for analysing plastics in WWTP samples are provided in Table 3.

Table 3 Summary of commonly used analytical techniques for analysis of plastics in WWTPs samples
Equipment Advantages Limitations Impacts on polymers
Visual method Microscopic counting Inexpensive; adequate for pre-sorting of samples for subsequent analyses (e.g. FTIR). It can be used directly to measure the size, characterize the morphology (shape, colour) and enumerate the count of plastics in samples.28,29,59,66,67 Large plastics in samples can be identified quickly by this technique, providing an overall picture of plastics abundance with low cost The technique is size dependent and open to bias due to the relatively low magnification factor of microscopes and is therefore discouraged when analysing particles <300 μm.28,96 Over-estimation or underestimation of plastic particles owing to misidentification. An estimated 70% error ratio of plastics could be observed in samples and this increases with decreasing particle size.94 For instance, it is difficult to distinguish between synthetic and natural fibres (such as textile fibres made of cotton) and even likely to duplicate or miss counts of plastics.28,31 The technique cannot be used to identify polymer types in samples and will require an advanced analytical method (listed below). It is a time-consuming procedure and difficult to automate, and items of similar colour to background paper might be overlooked or underestimated6,28 The technique is non-destructive to plastic particles
Spectroscopic method Raman spectroscopy Raman increases the accuracy of polymer type identification. It has higher sensitivity to non-polar functional groups and is insensitive towards disturbing signals of water and atmospheric CO2.30 The technique is suitable for small particles between 1 to 20 μm and above with better spatial resolution than FTIR.37 It can be used to analyse non-transparent and dark particles. It has the ability to perform fast chemical mapping which enables fast and automatic data processing. The technique produce visual images, which enables the determination of the dimension of particles It is difficult to identify tiny plastics (<1 μm) with this technique.28 Raman fluorescence would be interrupted by the presence of colour, additives, and microbiological, organic or inorganic impurities, hampering the identification of plastics in samples.30 Samples require careful purification to avoid undesirable modification prior to analysis,98 which is labour intensive and time consuming. Raman requires long processing time and can cause polymer heating and degradation.99 The data evaluation of the technique is very time-consuming and requires skilled personnel The technique is non-destructive to plastic particles
FTIR (micro-FTIR and ATR-FTIR) Polymer types of plastics could be identified quickly and directly by comparing the resulting spectra with those of known plastic polymers. They are well established, fast and quite reliable.30 Larger particle size of >500 μm can be analysed by ATR-FTIR, whiles smaller particle down to 20 μm can be analysed by microscopy coupled FTIR.87 It is widely independent of particle mass. Micro-FTIR allows correct counts of suspected plastics. Emerging automatic FTIR imaging such as FPA (focal plane array) makes it even possible for fast acquisition of several spectra within an area through one single measurements30 and shows better images37 ATR-FTIR analysis is labour-intensive as plastics need to be selected under a microscope and then analysed for the spectra of each particle,28 with only sub-samples from the total samples examined.6 Micro-FTIR is size-limited and plastic particles below 20 μm might not yield enough absorbance interpretable spectra which leads to uncertain extrapolation of plastics30 FTIR based techniques are non-destructive to plastic particles and allow the determination of particle size distribution, however focusing and pressing in ATR-FTIR can cause destruction to sample
Information about organic additives are usually not provided. Contaminations (organic and inorganic) and additives can overlap polymer bands which could create difficulty in data interpretation when using this technique. Regularly, non-transparent particles are difficult to be analysed by this technique. Commonly, reference spectra used for comparison always represent very clean and ideal samples not typically found in the environment and this could be impacting results.6 It is important to create a library of non-typical reference plastics taken from WWTPs, which allows a comparison to much more environmentally relevant samples.28 FTIR are expensive and require experienced personals for operation and data processing30
Thermo-analytical methods Pyrolysis GC/MS Enables fast identification of polymers in samples with high certainty by their characteristic decomposition products.100–102 It enables the quantitative estimation of the mass of plastics in samples with proper calibration irrespective of particle size and shape.101 Information about the chemical nature of polymer and contained organic plastic additives are specific and can be achieved simultaneously This technique works very well for single particles and can be applied for sample weights of 0.1–0.5 mg.103 A time consuming pre-selection (e.g. by microscope, filtering or flotation) of single particles is necessary before measurements.103 Particles to be analysed must be manually handled or transferred to the pyrolysis cups by tweezers which is a labour intensive process.101,104,105 It is dependent on particle mass (detection limit depends on polymer mass (μg level)) and are restricted by lower detection limits.101,105 Separating polymers in the analytical output of this technique can be difficult and challenging at times. The technique is prone to contaminations or even blockades, thus, cost of maintenance is very high103 The technique is destructive to plastic particle and information about size distribution is lost, hence measurements are not repeatable
TGA-DSC The technique allows for a quantitative estimates of polymers in environmental samples on mass related bases. The technique has the advantage of straightforward operation and only very small amount of samples are required (1 to 20 mg)70 Overlapping transition temperatures among polymers makes polymer identification and quantification more difficult70 The technique is destructive to plastic particles and unable to provide characterisation information
TED-GC-MS In contrast to the use of Py-GCMS, this technique allows the analyse of polymer particles in environmental samples without any pre-selection of samples by means of their characteristic decomposition products.100 Its enables the analysis of high sample masses (>20 mg), which assures the homogeneity of the sample and guarantees measurements of representative sample composition without a prior time consuming selection of polymer particles.100,103 The technique is suitable for the analysis of complex environmental samples100 The potential of this technique has yet to be fully exploited for analysing plastics obtained from WWTPs The technique is destructive to plastic particles and information on dimension, size and shape of particles cannot be determined. This could be solved by additional pre-sieving or pre-filtration
Chromatographic method Liquid chromatography The technique takes advantage of the different solubility of polymers.98,106 Appropriate solvents are used to dissolve different polymers and quantified with appropriate calibration Success depends on solubility which makes the technique unsuitable for broad application to analyse all polymer types The technique is destructive to plastic particles and does not deliver direct information regarding the size, number of particles, shape and colour of particles

While there is no standardized analytical method, novel alternatives for plastics analysis in WWTP samples include thermo-analytical methods such as pyrolysis-gas chromatography coupled with mass spectrometry (Pyr-GC/MS),88,101,102 TED-GC/MS100,103 and thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC).70 Pyr-GC/MS uses polymer specific decomposition products or marker compounds to identify polymers and enables quantitative estimation of the mass of plastics in samples with proper calibration irrespective of particle size and shape. It also has the advantage to be used to specifically identify polymer additives. TED-GC/MS uses a combination of thermal solid-phase extraction and thermal desorption and has been reported to provide quantitative estimates of the total mass of plastics in samples.103 To determine the amount of plastics in WWTPs effluents using TGA-DSC,70 observed that polyethylene (PE) in the samples represented 17–34% (81–257 mgPE meffluent−3) of the total extracted material. However, the authors concluded that the technique could only be used to measure PE and PP due to the overlapping transition temperatures of the other polymers. Pyr-GC/MS, TED-GC/MS, and TGA-DSC methods are destructive to plastic particles and unable to provide characterisation information.59 Thermo-analytical approaches are also restricted by lower detection limits (mass of particle in μg).

All plastics have the potential to become microplastics and even nano-sized plastics,11,107–109 as such there is a need to be able to quantify the total mass of plastics that may be potentially released into the environment via biosolids and discharges of effluents. Although destructive, we propose using thermo-analytical methods, specifically Pyr-GC/MS, for plastic analysis since polymer type and mass-related quantifications can be obtained. However, thermal methods are clearly not suitable if information on the size, number, shape and colour of particles should be required. In such cases non-destructive techniques such as FTIR and Raman can be used. Ideally samples could be analysed using both approaches since they provide complementary information. To investigate this complementarity further, there is a need to compare thermo-analytical methods and non-destructive techniques to have a more comprehensive understanding of plastic in WWTP derived samples.

4. Occurrence and characteristics of plastics in WWTPs influents and effluents

Various studies have detected plastics in both WWTP influent and effluent samples. The number of plastic particles reported varied between 1 to >7000 particles per L and 0.0009 to 81 particles per L for influent and effluent samples, respectively (Table 4). The use of different sampling techniques, sample pre-treatment and analytical methods could be associated with the wide differences in the reported counts of plastic particles. Most of the plastic particles detected in influent samples were over 500 μm,32,40 with the majority of particles detected in effluent samples reported to be smaller than 500 μm.34,37,52,63,84,93 Nonetheless, some studies have identified plastic particles that were smaller than 100 μm in effluents samples.34,37,41,52,56,62,63,93 As far as we know, there are no studies reporting on the occurrence of nano-sized plastics in WWTP derived samples. Especially where the finest mesh cut (≥10 μm) was used during sampling (Table 4), with mostly millimetre-scale materials reported. The dilution of nano-sized plastics in wastewater and the general lack of appropriate sampling strategies for nano-scale materials in WWTPs could be attributed to the lack of evidence or data. However, looking at the environmental impacts of nano-sized plastics, it is important to include these in future WWTPs studies.
Table 4 Current studies on plastics pollution in wastewater treatment plants influent and effluent samples
Location Population served Treatment type WWTPs number Sampling process Sample volume (L) Pre-treatment Analytical equipment Minimum mesh sizes used (μm) Particle size range (μm) Influent (P/L) Effluent (P/L) Discharge (P per day) Retention efficiency (%) References
Wastewater treatment: P: primary treatment; S: secondary treatment; M: mechanical; B: biological; C: chemical; tertiary treatments = MPD: maturation pond; GrF: gravity filter; PF: post-filtration; GF: granular filter; BAF: biologically active filter; BF: biological filtration; AnMBR: anaerobic membrane bioreactor; RO: reverse osmosis; MBR: membrane bioreactor; OD: oxidation ditch; RSF: rapid sand filter, SBR: sequence batch; A2O reactor; anaerobic–anoxic–aerobic; H2O2: hydrogen peroxide; FeSO4: catalyst (Fe2+). Analytical method: FT-IR: Fourier transform infrared spectroscopy; OM: optical microscopy observation; FPA: focal plane array; Raman: Raman spectroscopy. P/day: plastic particles per day; P/L: plastic particles per litre. Labels: *: particle numbers include all textile fibre or plastic fibre/microfiber; **: particle numbers include plastics and some other small anthropogenic litter (in some cases other non-plastic fibres); ***: particle numbers include plastics and some microliter, ****: particles includes artificial fibres and synthetic plastic particles; MPFs: microplastics fibres; MPPs: microplastics particles; ML: total microliter; MP: microplastics. a Particles <500. b Particles >500. c Grab sampling. d 24 h sampling, ww: wet weather; dw: dry weather.
Netherlands P, S, T (MBR) 7 Container collection (glass jar) 2 OM >300–5000 68–910 51–81 43
Germany 7.0 × 103 2.1 × 105 P, S, T 12 Pump (mobile pumping device) 390–1000 Enzymatic maceration + H2O2 (35%) OM + ATR FTIR + FPA-micro-FTIR 10 20–7200 (>20) 0–0.05b 9 × 107–4 × 109 annual 97 37
USA 6.8 × 105 S 1 Container collection (stacked stainless steel mesh screens) 30% H2O2 + FeSO4 OM + micro-FTIR 125 >125 60
France P, S, B (biofilter) 1 Automatic sampler 0.05 OM 100 100–5000 260–320 (293) 14–50 (35) 83–95 40
USA 3.5 × 103 5.6 × 107 P, S, T (GF) 17 Pump (extraction pump + Tyler sieves) 5.00 × 102–2.10 × 104 30% H2O2 OM 125, 355 >125–>355 0.004–0.195 5.28 × 104–15 × 106 (4 × 106) 38*
(0.050 ± 0.024)
Scotland 6.5 × 105 S 1 Container collection(steel buckets and steel sieve) 30–50 OM + FTIR 65/11 >65 15.7 ± 5.20 0.25 ± 0.04 6.52 × 107 98.4 6
Finland 8.0 × 105 M, C, B, BAF 1 Pump (electric pumping device with filtering devices designed)/automated sampler 0.1–1000 OM + FTIR 20 20–>300 380–686.7c 0.7-14.2c 1.7 × 106–7.9 × 108 >99 42***
Australia 6.7 × 104–1.2 × 106 P, S, T(RO) 3 Pump (pumping device with stacked stainless steel mesh screens) 3–200 30% H2O2 OM + ATR-FTIR 25 25–>500 0.21–1.5 3.6 × 106–4.60 × 108 90 41*
Germany 1.1 × 104 2.1 × 105 P, S, T (PF, MPD) 5 Pump/container collection 1 30% H2O2 OM + FTIR+ 40/10 >40 0.01–0.38/80.4 2.79 × 105–2.62 × 106 115
USA S, T (GrF) 7 Assembled stack of sieves/surface filtration (skimming the surface water at effluent and sieving) 0.1–1.89 × 105–2.32 × 105 NaOCl (for some samples) OM + FTIR 20/45/125/180 20–400/100–200 1 0.00088 0.93 × 106 99.9 39
Denmark B, RSF 10 Auto sampler/container collection (glass bottles) 1–81.5 30% H2O2 + FeSO4 FPA-microFTIR 10 10–500 2223–10[thin space (1/6-em)]044 (7216) 19–447 (54) 98.3 56
USA S, T (GF, anMBR) 3 Container collection (plastic containers) 1–38 OM 20 20–4750 80–140 0.5–5.9 (incl. all textile fibres) 4.43 × 106–1.48 × 1010 95.6–99.4 54**
Finland T (MBR, discfilter, rapid sand filtration, dissolved air flotation) 4 Pump (filtering device with an electric pump)/a custom made filtering device with in situ fractionation was used/automated sampler 0.4–1000 OM + FTIR 20 20–>300 0.005–0.3 >95 63
Finland NR P, S, MBR 1 Container collection (stainless steel bucket attached to a metal wire) 4–30 30% H2O2 + FeSO4 OM + FTIR + microRaman 250 250–5000 57.6 1.05 1.00 × 107 98.3 32*
Sweden 1.2 × 104 M, B, C 1 Ruttner sampler/pump (a tube connected to a suction pump) 2–1000 OM + ATR-FTIR 300 ≥300 15.1 0.00825 >99.9 31*
Canada 1.3 × 106 P, S 1 ISCO glacier portable water sampler/container collection (glass jars/steel bucket) 1–30 Oil extraction followed by 30% H2O2 OM + FTIR 1–65 >1 31.1 ± 6.7 2.6 ± 1.4 after P 3.2 × 107 97.1–99.1 53*/**
9.7 × 107
0.5 ± 0.2 after S
USA 3.2 × 104 P, S 3 Container collection (glass jars with Teflon lined lids/galvanized steel containers with lids) 3.6–30 30% H2O2 + HCl OM + microATR-FTIR 43 60–>418 86–243 1–30 5.0 × 108 85.2–97.6 52*
1.0 × 109
1.8 × 105
Korea 6.77 × 104 B (A2O, SBR, media processes) 3 10–100 30% H2O2 + FeSO4 OM + ATR-FTIR 106 106–≥300 2.950–23.750 0.050–0.330 98 84
2.45 × 105
Germany 9.8 × 104 S 1 Centrifugal pump and a 10 mm cartridge filter (stainless steel) 40–200 H2O2 + NaClO Micro-Raman 10 ≤10–>500 1.2–7.9 (3.5) (MPPs) 62*
0.3–2.5 (1.1)(MPFs)
5.9 (ww)
3 (dw)
Italy 1.2 × 106 P, S, T 1 Container collection (steel bucket) 30 15% H2O2 OM + ATR micro-FTIR 63 10/63–5000 2.5 ± 0.3 0.4 ± 0.1 1.6 × 108 84 34*
Turkey 5.0 × 105–1.0 × 106 P, S 2 Automatic sampler (Endress + Hauser ASP-station 2000, vacuum system RPS20 model) 1.5–5 30% H2O2 + FeSO4 OM + micro-Raman 55 <100–5000 23.444–26.555 4.111–6.999 (4.49) 3.5 × 105–1.2 × 106 54–92 44
Australia T 2 Container collection (glass bottle with metal caps) 0.75 OM + ATR FTIR <1000 1.0 48
USA S, T 8 Container (stacked Tyler/stainless sieves) 30% H2O2 + FeSO4 OM 125 125–≥335 0.02–0.2 4.6 × 105–1.5 × 107 97
Finland 8.0 × 105 P, S, T (BF) 1 Pump (filtering device with an electric pump)/a custom made filtering device with in situ fractionation was used 0.3–285 OM 20 20–200 180 fibres 4.9 fibres 97 fibres 57****
98 synthetic particles
430 synthetic particles 8.6 synthetic particles
China P, S 1 A bucket made of stainless steel 10 30% H2O2 + FeSO4 OM + micro-Raman 47 20–5000 79.9 28.4 64.4 93
China 3.5 × 106 S 7 A multi-use pump connecting to an intelligent electromagnetic flowmeter through the hose/stainless steel mesh screens 2.98–348.71 30% H2O2 + FeSO4 OM + microRaman 43/63 43–>355 1.57–13.69 0.20–1.73 ∼6.5 × 108 79.3–97.8 58
China OD, MBR 1 MP trapper with stacked stainless steel sieves 1–200 30% H2O2 + FeSO4 OM + ATR FTIR 25 25–>500 0.28 0.05–0.13 3.5 × 106 MBR 97 (OD)–99.5 (MBR) by mass and 53.6 (OD)–82.2 (MBR) by number 112
6.5 × 106
China 2.4 × 106 P, S, T 1 Container collection (glass bottles) 30 30% H2O2 + FeSO4 OM + micro-FTIR 50 50–>500 12.03 ± 1.29 0.59 ± 0.22 0.59 ± 0.22 × 109 95% 92*
Finland 1.0 × 104 M, C, B, S 1 Pump (filtering device with an electric pump)/series of filter 2–220 OM + FTIR >20 >20–>300 3.0552 ML 116
0.4433 MP

Generally, the number of plastics in WWTPs influent has been reported to increase when a stormwater sewer system is connected to WWTP.28 For instance, plastics are known to be released from tyre and brake wear; consequently, road runoff that enter sewer systems could increase the number of plastics when sewers are coupled with domestic sewers.38,54,110 Similarly, discarded litter could degrade in the environment and enter WWTP influent through stormwater sewers. The number of plastics in the final effluent of WWTPs is also dependent on both the number in the influent as well as the treatment processes and technologies used at a WWTP.28 However, it should be noted that, some sewer pipes are made of PVC polymers and could potentially contribute to the number plastics in WWTPs treatment chain due to abrasion of the pipes.

Plastic particles discovered in WWTP influents and effluents have been characterized into different shape categories or types: spherical beads, microbeads, pellets, fibres, particles, flakes, films, fragments, foams, chips (paint), nurdles, foils, spheres, sheet, granules, lines and irregular shapes (for examples see Fig. 5).6,31,34,38–42,44,57,58,92,97,111 However, fibres were the most dominant proportion (by number or count) observed in wastewater (an average percentage of 65.4%) followed by irregular fragments (an average percentage of 42.6%).6,32,38,40,44,52–54,84,92,93,112 Mason et al.,38 observed that the most common plastic particles in 17 WWTP effluent samples were microfibres (59%), fragments (33%), films (5%) foams (2%) and pellets (1%). This indicates that plastics entering WWTPs are either mainly those used as synthetic fibres and fragmented secondary plastics113 or that current sampling and analysis techniques are insufficient to representatively sample and/or identify plastics in wastewater. It might also be that natural fibres such as cotton were included or counted as synthetic fibres during identification and quantification.28,42

image file: c9ew00428a-f5.tif
Fig. 5 Plastic particles encountered in WWTPs: (A) pellet, scale bar 500 μm; (B) fragment, scale bar 1 mm; (C) foam, scale bar 200 μm; (D) sheet, scale bar 500 μm; (E) granule, scale bar 500 μm; and (F) fibre, scale bar 1000 μm. Reproduced from ref. 53, with permission from Elsevier, copyright 2019.

The most common polymers found in influent and effluent samples have been PEST (∼30–90%), PE (∼6–60%), PET (∼5–40%), PA (∼5–35%), acrylate (∼4–31%), PP (∼4–27%), alkyds (∼4–25%), PS (∼4–25%), polyurethane (PU, ∼3–25%), polyvinyl alcohol (PAVAL, ∼3–20%), polylactide (PLA, ∼2–18%), PS-acrylic (∼2–14%) and polytetrafluoroethylene (PTFE, ∼2–8%).6,34,40,44,48,53,58,62,68,92,112 Because PEST, PET and PA are widely used in synthetic clothing, sewage may contain large quantities generated from laundry effluents. PE is the most produced plastic in the world and is used in personal care products such as facial and body scrubs, toothpaste, food packaging films and water bottles.32,33,37,39,114 Although the sources and pathways of plastics entering WWTPs are not fully understood yet, research on polymer identification and quantification in WWTP influent and effluent samples should be given to the above-mentioned plastic particles, since they are known to be originating from human daily activities.28

5. Fate of plastics in WWTPs

Although WWTPs are not specifically designed to remove plastic particles,43,63 they can treat wastewater through a number of different treatment steps; primary, secondary and tertiary treatment processes to purify wastewater before effluent or sewage sludge are released into the environment (Fig. 6).67 Currently, the number or count of plastic particles in WWTP influent and effluent samples has been used by various studies to estimate the removal efficiencies of treatment plants or stages of treatments.28,40,44,67 As noted by Ou and Zeng,59 plastic removal efficiencies in different treatment stages follows the order: pre-treatment > secondary treatment > tertiary treatment. Due to the variable treatment processes used by WWTPs and the different sampling/identification techniques used in various studies, the comparison between plastic removal efficiencies based on the current literature is difficult to make.59 Results from the most recent studies investigating plastics removal efficiencies from WWTPs is provided in Table 4.
image file: c9ew00428a-f6.tif
Fig. 6 Schematic of a typical municipal wastewater treatment facility indicating stages of processing. Tertiary treatment is optional and, if present, will vary between facilities. Coarse debris screening (identified here as part of the primary treatment) can also be considered to be preliminary treatment. Reproduced from ref. 38, with permission from Elsevier, copyright 2019.

Plastic removal is mainly achieved via skimming and settling, sedimentation, bar screening, flotation, grease and grit removal, coagulation–flocculation, aeration and clarification, biofilm/activated sludge, chemical oxidation, membrane separation, chlorination, biological treatment, disinfection and filtration processes.32,39,63,67 It should be noted that none of these treatments steps are particularly designed for eliminating plastics, although they remove plastics by trapping them in sludge.59 Carr et al.,39 suggested that primary treatment using skimming and settling removed majority of plastic fragments and other fibrous residues that entered WWTPs, with fewer amounts of plastics observed in tertiary effluents. Similarly, Murphy et al.,6 reported that grease and grit removed approximately 45% of plastics that entered a WWTP, with sedimentation removing about ∼34%. It is reported that about 70% to 98% of plastics were eliminated following primary treatment.6,39,42 Secondary treatment is reported to further decrease plastics to about <20% in wastewater effluent,6,42,54 whereas tertiary treatment is reported to further reduce the number of plastics in effluents to about <2% (of influent number/relative to influent) (Fig. 7).6,39,54,57

image file: c9ew00428a-f7.tif
Fig. 7 Schematic showing plastic particle size flow and plastic removal in a typical WWTP (primary, secondary and tertiary). The particle size flow are based on estimated data reported.40–42

Mostly, WWTPs that used tertiary treatment processes had lower numbers of plastic particles in the final effluent discharged compared to those that used primary or secondary treatment processes only (Table 4). However, there have been cases where the number of plastics in the final effluent of WWTPs did not decrease when tertiary treatment processes or advanced treatment processes were used.37–39,41 For example, effluents from reverse osmosis41 or microfiltration54 treatment still contained 0.21 (10[thin space (1/6-em)]000[thin space (1/6-em)]000 particles discharged per day) and 0.25 plastic particles per L, respectively. Generally, the number of plastics removed during tertiary treatment depends on the type of removal technology applied, such as the disc filter (DF), rapid sand filtration (RSF), dissolved air flotation (DAF), membrane bioreactor (MBR),63 granular filtration,38 gravity filters39 reverse osmosis (RS)41 and membrane microfiltration. Membrane related technologies have been reported to show the best removal efficiencies63 with 99.9% of plastics removed by a membrane bioreactor (reduction from 6.9 to 0.005 particles per L). The study revealed 95% removal by DAF (from 2.0 to 0.1 particle per L), 40–98.5% by disc filter treatment (from 0.5 to 2.0 to 0.03–0.3 particle per L) and 97% by rapid sand filter (from 0.7 to 0.02 particle per L).

Primary treatment has been reported to remove large plastics and fibres better than fragments,31,39,41,57 whereas secondary treatment has been shown to remove more fragments than fibres (Fig. 7). For instance, the size of fibres in raw wastewater have been observed to have decreased following primary treatment.40 Talvitie et al.,57 Talvitie et al.,42 and Ziajahromi et al.,41 observed a decrease in the numbers of plastic fragments following secondary treatment while fibre particles increased. Overall, effluents from WWTPs using or undergoing tertiary treatment have been reported to discharge a higher relative number of fibre particles to fragments.54,59 It should be noted that smaller plastics may escape treatment processes6 with fibres more easily retained.31 According to one study, large plastic particles (≥300 μm) were captured in the pre-treatment stages while smaller particles (100–300 μm) were removed in the second and tertiary treatment stages.42 However, the study revealed that the smallest plastic particles (20–100 μm) were able to bypass all the treatment stages (including tertiary treatment), hence were discharged with the effluent. A practical solution is to use novel tools such as filters to remove fibres from washing machines at the source or to use hand- and machine-washing bag that acts as a microfiber filter between synthetic clothing and the drain.117,118 However, we understand that there are stark differences in the design of US and European washing machines with those in the US typically lacking filters that require maintenance.

Due to the low numbers of plastic particles in tertiary treatment effluent, larger sampling volumes are required when assessing plastic removal efficiencies than for pre-treatment and secondary treatment processes. However, since the same volumes of samples for influent and effluent are usually not used, the results from most removal efficiency studies could be providing us with the wrong information or data. For instance, if studies express removal efficiencies based on count or numbers of plastic particles per L, then it might be biased if they are extrapolating from smaller volumes for influent compared to effluents due to the non-homogenous mixing of plastic particles.

5.1. Plastic retention in sewage sludge

WWTPs have been shown to be effective in removing plastics from raw wastewater by capturing them in sewage sludge. Sewage sludge generated in WWTPs are reported to contain approximately 99% of plastic particles entering the works.6,31,32,53,84 There are limited studies on the occurrence, transformation and fate of plastics in sewage sludge (Fig. S1 in the ESI). Table 5 summarizes studies reporting plastics in sewage sludge samples. The reported number of plastics in sewage sludge samples ranged from 510 to 76[thin space (1/6-em)]300 particles per kg for wet weight (ww) and from 1000 to 240[thin space (1/6-em)]300 particles per kg for dry weight (dw). The average size of the plastics detected in sewage sludge were found to be larger than that detected in wastewater6 suggesting that larger plastics are more effectively removed by WWTPs than smaller ones. Often only larger size classes of plastics (typically >100 μm) are detected and quantified in sewage sludge samples. Approximately 50% of plastic particles identified in sewage sludge samples were between 100–500 μm, over 20% were between 10–100 μm, over 10% were between 500–1000 μm and around 10% were between 1000–5000 μm.34 Similarly, 81% of plastic particles in sewage sludge samples were below 1000 μm when the detection limit was 50 μm, with the remaining 19% being between 1000 and 5000 μm.51 The same study then identified 34% of plastic particles in the smallest class analysed (50–125 μm), however, these particles could not be chemically characterised or confirmed due to the size limitations of the FT-IR used. This shows the analytical bias in plastics analysis when using visual identification and spectroscopic methods, as one can imagine that smaller plastics (nano-sized plastics) will not show up easily or be analysed.
Table 5 Summary of studies in which plastic particles were identified in sewage sludge
Location Sampling point Sampling method Organic matter removal Analytical method Smallest mesh size (μm) Particle per kg Particle size range (μm) Reference
FT-IR: Fourier transform infrared spectroscopy; OM: optical microscopic observation; SEM: scanning electron microscopy; DSC: a differential scanning calorimeter; Raman: Raman spectroscopy; ww: wet weight; dw: dry weight; H2O2: hydrogen peroxide; FeSO4: catalyst (Fe2+), *: particle numbers include fibre and in some studies other anthropogenic litter (paint chips, rayon, cotton and other non-plastic fibres); **: based on reported estimates from ref. 28.
Italy Recycled activated sludge Glass beaker 15% H2O2 OM + ATR microFTIR 63 113[thin space (1/6-em)]000 (dw) 10/63–5000 34*
Korea Sludge cake Shovel 30% H2O2 + FeSO4 OM + ATR-FTIR 106 1620–13[thin space (1/6-em)]275(dw) 106–≥300 84
Canada Primary sludge secondary sludge Glass jar 30% H2O2 OM + FTIR 1–65 14[thin space (1/6-em)]900 (ww) >1 53*
4400 (ww)
Sweden Slightly dewatered sludge OM + ATR-FTIR 300 16[thin space (1/6-em)]700 (dw) ≥300–5000 31
720 (ww)
United States Returned activated sludge NaOCl OM + FTIR 20/45/125 50** <400/<5000 39
1000 (dw)**
Primary tank skimming
Scotland Grit Stainless steel bucket and sieve OM + FTIR 65 1440** >65 6
Grease skimming 7868**
Sludge cake
Netherlands Sludge Glass jars OM + FTIR 510–760 (ww) >300–5000 43
Finland Activated sludge Stainless steel bucket/stainless steel cup OM + FTIR + Raman 250 23[thin space (1/6-em)]000 (dw) 250–>5000 32*
170[thin space (1/6-em)]900 (dw)
Digested sludge
27[thin space (1/6-em)]300 (dw)
MBR sludge
China Sewage sludge 30% H2O2 OM + microFTIR+SEM 37 1565–56[thin space (1/6-em)]386 (dw) 37–5000 47
Ireland Sewage sludge OM + ATR-FTIR + SEM 45/250 4196–15[thin space (1/6-em)]385 (dw) 250–4000 50
Germany Sewage sludge Shovel (stored in PVC containers) NaOH OM + FTIR + ATR-FTIR + FPA-micro-FTIR 10 1000–24[thin space (1/6-em)]000 (dw) 20–500 37
Spain Anaerobic sludge Glass containers Isopropyl alcohol OM + FTIR + DSC 75 75–850 74
Primary clarifier
Finland Excess sludge+ Metallic beaker/hand collection OM + FTIR 20 76[thin space (1/6-em)]300 (ww) 20–>300 42
186[thin space (1/6-em)]700 (dw)
Raw sludge
Dry sludge
Norway Sewage sludge Glass jars using metal or wooden spoon 30% H2O2 + FeSO4 OM + FTIR 1701–19[thin space (1/6-em)]837 (dw) 54–4987 51
China Sewage sludge A bucket made of stainless steel H2O2 OM + micro-Raman 47 240[thin space (1/6-em)]300 (dw) 20–5000 93

Mahon et al.,50 found that the number of plastics isolated from sewage sludge ranges from 4196–15[thin space (1/6-em)]400 particles per kg dry sludge. Similarly, Mintenig et al.,37 detected plastic particles in all sewage sludge samples analysed, and the numbers ranged from 1000 particles per kg to 24[thin space (1/6-em)]000 particles per kg. Generally, the number of plastics reported in sewage sludge were higher than that in wastewater. Plastic particles in sewage sludge samples have been categorized into fibres, films, lines, fragments, glitters, spheres, flakes, shaft, irregular shapes and beads.31,32,34,47,50,51 Fibres (average percentage range 60–85%) and fragments were the dominant types of plastics detected by numbers.32,47,93 The most commonly detected plastics in sewage sludge by chemical composition have been polyolefin (PE and PP), PS, PEST, PET, acrylic, polyamide, polycarbonate, acrylonitrile–butadiene and ethyl acrylate.32,34,47,50

6. WWTPs as a source of plastics to the environment

6.1. Occurrence of plastics in the freshwater environment from WWTP effluent

WWTPs act as barriers, as well as entrance routes for plastic to freshwater systems such as rivers and lakes which eventually get into the marine environment. It should be noted that some WWTPs discharge effluent directly into the marine environment. Although WWTPs have been reported to filter plastic particles out of the wastewater, they may be a significant source of plastics in freshwater systems or oceans given the large volumes of effluent waters that are discharged.6,29,37,39,41,42 WWTPs in some countries may not have the capacity to treat all of the wastewater that may enter the influent and as a result, some untreated influent wastewater may be discharged into receiving waters. Similarly, during wet weather a WWTP may become inundated and may not be able to treat all of the incoming influent, hence discharging influent into receiving waters. Until recently, the distribution of plastics in the freshwater systems was unknown. However, over the last few years, due to the availability of sampling and analytical techniques, studies have been identifying plastics in freshwater systems across the globe, with municipal effluents discharged from WWTPs suspected to be the significant contributor and source.6,39,41,57Table 4 summarizes the estimated counts or numbers of plastic particles that are released into receiving water per day from WWTPs. An estimated 65 million plastic particles were reported to be released into receiving waters each day from a WWTP serving a population of around 650[thin space (1/6-em)]000.6 Cheung and Fok119 estimated the release of 210 trillion microbeads into the aquatic environment in mainland China every year and mentioned WWTP effluent to be the source of more than 80% of these particles. Similarly, Magni et al.,34 estimated that about 160 million plastic particles are released daily into freshwaters from selected WWTP in Northern Italy. An estimated 900[thin space (1/6-em)]000 plastic particles were being released each day into receiving waters from a WWTP treating 1.06 × 106 m3 of wastewater per day.39

A higher number of plastic particles for instance, were observed in freshwater samples taken downstream, relative to upstream of WWTPs in the North Shore Channel in Chicago71 and in the Raritan river in New Jersey in the USA.120 High amounts of plastics have also been reported downstream to WWTPs.31,48 However, it is not clear whether WWTP discharges contribute a greater quantity of plastics into freshwater and the oceans than other sources, such as direct input to rivers and lakes, stormwater runoff, road runoff or dry and wet season deposition from air.

6.2. Presence of plastics in soil through the application of biosolids

The application of biosolids to land is common practice in Australia and in some European and North American countries35,36 as it contributes positively to the recycling of nutrients, improving soil properties, increasing fertility and sustaining soil productivity. However, the use of biosolids for agricultural fertilisation could be a potential source of plastics in receiving agricultural soils with unknown consequences for soil ecosystems, food security and human and animal health. It is estimated that about 63[thin space (1/6-em)]000 to 430[thin space (1/6-em)]000 tonnes of plastics may be entering the agroecosystems annually in Europe and 44[thin space (1/6-em)]000 to 300[thin space (1/6-em)]000 tonnes annually in North America through the application of treated sewage sludge (biosolids).36 Similarly, it is estimated that between 2800 and 19[thin space (1/6-em)]000 tonnes of plastics may be applied to agroecosystems each year through biosolid applications in Australia.121 It is also estimated that over 500 billion plastic particles may be released into the environment via the application of sewage sludge each year in Norway.51 Following release from biosolids into soils, plastics and associated chemicals could cause subsequent contamination to receiving water bodies through runoffs. Many questions therefore remain with respect to the amounts, fate, behaviour, uptake, and potential implications of plastics transported or released from biosolids into agricultural soil.

7. Conclusions and future perspectives

Plastics are clearly a pollutant of emerging concern in wastewater treatment plants with their presence reported in WWTP derived samples globally. Generally, WWTPs receive high amounts of plastics from the urban environment and have been identified as a source of plastic release into the environment. There is in general little current understanding about the sources, occurrence, identification and fate of plastics in WWTPs. The key conclusions from this review are:

(1) There is a lack of standardized protocols for the analysis of plastics in WWTP-derived samples. Studies have used a combination of different methods for sample collection, sample pre-treatment and identification, which makes the comparison of studies challenging.

(2) As far as we are aware, current sampling and analytical strategies are not able to account for nano-sized plastic particles (<1 μm as defined in this review) in WWTPs derived samples leading to underestimation of total amount of plastics. Current studies have reported and observed plastic particles which are greater >10 μm. This has been attributed to the deficiencies in sampling equipment with regards to the pore sizes of mesh screens/sieves used during sampling, the pore sizes of filters used during the filtration of samples in the laboratory and the lack of analytical instruments for plastic particles <10 μm even if or when sampled.

(3) Collection of wastewater samples via in situ pumping coupled with separation device (such as stack of sieves or screens) seems to be effective for sampling larger volumes of effluent wastewater and has become more common in recent years based on current studies.

(4) Characterising plastics with FTIR techniques is the most common option used to chemically identify plastic particles in WWTPs samples and provides information on the polymer types, sizes and numbers of particles in samples.

(5) Plastic particles have been detected in both wastewater and sewage sludge samples in WWTPs. Fibres and fragments were the dominant types of plastics (by numbers) in terms of shape classification. The most common polymers reported were polyester, polypropylene, polyamide, polyethylene, polyethylene terephthalate and polystyrene.

(6) Primary and tertiary treatment processes were observed to filter high numbers of plastic particles from WWTP liquid fractions. The use of a tertiary filtration technology at some WWTPs was found to be effective in reducing the number of plastics discharged through the final effluent.

(7) Most of the plastic particles filtered from the wastewater fraction are retained in the waste sludge generated in WWTPs. The land application of treated sewage sludge (biosolids) could therefore be an important route for plastics emissions to the soil environment. However, given the large volumes of effluent waters that are discharged, final effluent may be a significant source of plastics in freshwater systems as well.

(8) It is unknown whether plastics identified in WWTPs have undergone exposure to degradation processes such as thermal, mechanical and biological interactions during treatment. Transformative effects such as variation or impact to surface morphology compared to the original polymer remain unknown.

Based on the current state of knowledge on plastics pollution in WWTPs, the following areas should be incorporated or addressed in future investigations for effective understanding of plastics pollution in WWTPs:

(1) Due to the significant impact various sampling and identification strategies have on the levels and types of plastic particles observed in WWTPs derived samples, there is an urgent need for standardizing sampling and robust analytical methods that could capture and analyse both micro and nano-sized plastics in WWTP-derived samples to improve the estimation of total plastic.

(2) The use of thermo-analytical methods such as Pyr-GC/MS and TED-GC/MS, to include both micro- and nano-sized plastic particles with the potential for a more robust estimation of the total mass of plastic in samples. It should however be noted that these techniques cannot be used if particle size, shape, colour, number information is of interest.

(3) In the absence of thermo-analytical techniques, for plastic particles in the size range of 1 μm to 20 μm, we propose Raman spectroscopy as it is the only technique currently suitable for these sizes with improved spatial resolution.

(4) As it appears hugely challenging to stop the use of plastic materials in industrial and domestic applications and products, measures on reducing the input of plastics into WWTPs across the globe should be of prime importance. Future efforts could be dedicated to separate, reduce and prevent plastics in wastewater at the household level and industrial levels. Likewise, the use of alternative manufacturing products instead of plastic microbeads in most personal care products could be a good step.

(5) Although difficult, a regulatory level or acceptable levels of plastic particles in WWTPs final effluent and biosolids needs to be established in the future to help limit the number of plastic particles that could be released into freshwater systems, the marine and terrestrial environment.

Conflicts of interest

There are no conflicts of interest to declare.


Mr Elvis Dartey Okoffo is supported by a Research Training Scholarship awarded by the Queensland Alliance for Environmental Health Sciences and The University of Queensland. The Queensland Alliance for Environmental Health Sciences, The University of Queensland, gratefully acknowledges the financial support of the Queensland Department of Health.


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Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ew00428a

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