4. Discussion

In the current study we found a large input of litter and microplastics to the sea via wastewater.  In accordance with the findings of Rist et al. (2020), we found that the number of items increased with decreasing size (Figure 6), while the largest size fraction contributed to the highest mass of plastic (Figure 12).  

Figure 12 The figure shows the distribution of litter in the wastewater effluent in percentage based on mass data for Sisimiut and Nuuk. Note: the contribution for the 1–5 mm fraction is considered as estimates, as the weight measurements for the individual particles are based on very few items and that weights were below detection limit for some items. The full scissor line indicates the fractions that will be removed by a preliminary 3mm screen. The dotted line indicates our hypothesized additional removal of a significant fraction of the 1–5 mm items, and even below by a preliminary 3 mm screen.

Extrapolating the results from the 4,000 PE sampled to the 56,696 inhabitants in Greenland (Statbank Greenland, 2023), approximately 2 tonnes of plastic litter year^-1 is discharged to the marine environment from local sources. Out of this, 59%, equivalent to 1.2 tonnes year^-1 comprises plastic or semisynthetic wet wipes. Due to sampling methods and the fact that data on meso- and larger sized microplastic items are based on very few items, the following numbers must be seen as rough estimates: Approximately 70%, equivalent to 1.4 tonnes year^-1 are items larger than 5 mm, while 29% equivalent to approximately 0.6 tonnes year^-1 are items between 1 and 5 mm, while 1% equivalent to 28 kg year^-1 are smaller than 1 mm and consist of 43% fibers. These approximations indicate that the largest proportion of litter enters the marine environment as macro plastic via wastewater.

The degree of littering through wastewater may vary across different regions in Greenland e.g. due to different infrastructural conditions.  Figure 11c indicates, however, that in the town of Aasiaat, wet wipes also constitute a significant source of litter to the sea from sewage. Thus, wet wipes do appear to be a general problem in Greenland, like it was shown in Britain (Marine Conservation Society, 2017). Another source of uncertainty arises due to the absence of data on the extent of littering through bucket toilets (also known as honey buckets) used in 20% of households in Greenland. We have initially assumed similar littering patterns as with sewers, but this may not be accurate. 

In Nuuk as well as Sisimiut, wet wipes made of plastic and semi-synthetic plastic materials were identified as the major contributor to the macro-plastics mass in the sewage let out to the marine environment. They constituted 82% of the macro-plastic mass, and 59% of the total identified plastic mass in the sewage. Of the 37 wet wipes that were found in the wastewater in the present study, only 3 were made from cellulose, while the rest were made from PET (31) or viscose (3). In consistency, the majority of wet wipes sampled from stores in Sisimiut were of PET or viscose, and even those declared to be of natural material (declared as natural fibers, or bamboo) turned out to be of a combination of biodegradable cellulose-based fibers and less degradable synthetic fibers as viscose. In accordance with our results, Munoz et al. (2018) demonstrated the presence of PET in all non-flushable wet wipes examined, while also identifying the presence of PET and other synthetic materials in a substantial number of flushable wet wipes. Similar to the conclusions of our trade survey, they concluded that commercially available wet wipes – even those labelled as flushable or natural - can be considered as a possible source of microplastic fibers in wastewater streams. Apart from contributing to plastic contamination, the content of synthetic material in wet wipes marked as natural may also increase their durability, i.e. decrease their rate of degradation, as do the diverse chemical additives designed to enhance their properties (Allison et al. 2023). Even wet wipes marked as natural, biodegradable or flushable may therefore last for long periods (Flury and Narayan, 2021; Allison, et al. 2023, Afshar et al. 2024), during which they can harm the environment e.g. by shading, leaving traces of chemicals, synthetic fibers, or being accidentally ingested by wildlife.

Despite that we could not identify any beach stranded wet wipes during the two 'wet wipe beach surveys' in Sisimiut in June and July 2023, evidence of wet wipes in the sea exists. Figure 11b clearly shows how wet wipes accumulate on the seabed at the outlet point in Sisimiut, which is in a relatively closed bay with low current. Similarly, Figure 11c indicates the same issue in the Greenlandic town of Aasiaat where the wastewater outlet points to the open sea, where stronger currents are expected. It is thus hypothesized that the wet wipes deposit on the seafloor rather than being washed ashore. This is in contrast to the Marine Conservation Society's analysis of the Great British Beach Clean 2017 (MCS 2017) that determined that the presence of wet wipes along the UK coastline increased by 94% in 2017. The accumulation on the beach in UK was found to make a substantial 400% rise over the past decade (equivalent to 27.5 pieces of wet wipes per 100 meters of beach cleaned). Thus, local current patterns may also be responsible for the lack of wet wipes at the specific site surveyed in Sisimiut.

The results of this study showed that the most abundant shape of MP particles in Greenlandic sewage is fibers. This correlates well with the findings of Rist et al. (2020) who found fibers <300 µm to be the dominant MPs at 3 marine stations in fjord of Nuuk near the site investigated in the present study. In general, fibers are the most frequently documented MP type found in the marine environment (Cesa et al. 2017), including in the Arctic region (Table 1) as well as in wastewater (Table 3).

Table 3 Results reported in the literature on plastic microfibers in raw and treated wastewater. *** Only MP included, number estimated from bar chart.

Polymer type: PE (polyethylene), PP (polypropylene), PS (polystyrene), PES (polyester), PVC (polyvinylchloride), PC (polycarbonate), PMMA (polymethylmethacrylate), PA (polyamide), PUR (polyurethan), ABS (akrylonitril-butadien-styrene)

A dominance of white/transparent MP fibers was observed in the raw wastewater samples in our study, where a large fraction was proposed to be of viscose origin. This is in accordance with the findings of Yuan et al. (2021), who studied two Chinese wastewater treatment plants (WWTPs) and found transparent and white microplastic fibers to be the most abundant. As the fibers identified in our study were primarily of the same plastic polymer as the majority of the wet wipes (PET and viscose), it is reasonable to believe that a fraction of the fibers may be linked to the presence of wet wipes. Our research thereby indicates that wet wipes may play a substantial role in microplastic (fiber) pollution, contributing not only indirectly via the emission of the wet wipes themselves left for degradation in the marine environment, but also through the direct release of fibers from wet wipes during their passage in the sewer system. These findings align with the conclusions of Lee et al. (2021), who investigated the release of microfibers from wet wipes subjected to different impacts. They found that immersing wet wipes in water for one hour resulted in a greater release of MP fibers (1966 fibers per sheet) than subjecting them to physical abrasion. Thus, direct disposal of wet wipes in the sewage system will induce a significant release of fibers. In accordance, Briain et al. (2020), found that the disposal of wet wipes and sanitary towels into toilets represents an underestimated source of white microplastic fibers in the environment, based on fiber determination from intertidal sediment samplings and field observations of washed-up deposit of sewage-derived waste.

Main sources of MPs in wastewater commonly mentioned in literature are personal care products, laundry, surface runoff including tire wear and atmospheric deposition (Cesa et al. 2017; Liu et al. 2023b). Laundry is shown to be a major source with up to 7–800,000 fibers released from a single load of laundry (Kelly et al. 2019, Electrolux, 2022). The MPs identified in the Greenlandic wastewater in our study thus most likely constitute a mixture of fibers from wet-wipes and laundry, as well as other personal care products and atmospheric deposition as secondary sources. Road run-off is not a likely important contributor to piped sewage in Greenland since Greenland applies separate sewers for surface-runoff in ditches.

Wastewater treatment systems in general can be expected to retain large items efficiently, and items larger than 5 mm may be expected to be fully retained even in plants with preliminary treatment only.

In- and output concentrations of MPs from different sewage treatment plants in the Arctic and the Nordic region are shown in Table 3. Input concentrations (number of items) vary several size orders from 631 items m^-3 in Klettagarðar, Iceland, (Magnusson et al. 2016) to five million items m^-3 in Ny Ålesund, Svalbard (Granberg et al. 2019). This difference may be partly explained by the differences in size-fractions included: Magnusson et al. (2016) only included items larger than 0.3 mm, while Granberg et al. (2019) included items larger than 0.02 mm. In accordance with our results (Figure 12), the items considered as macro plastics have been shown to constitute the majority of plastic mass entering WWTPs (Rasmussen et al. 2021). The loads can, however, vary significantly: Magnusson et al. (2016), used identical cut-off size, but still found size-orders of difference among MP concentrations in raw wastewaters in Finland and Sweden compared to Iceland. This may be explained by the fact that Icelandic wastewater is known to be very dilute, due to mixing with rainwater and a general high-water consumption by industry in Iceland (more than 2,000 L capita^-1 day^-1 in 2015 all-inclusive according to Statistics Iceland). Dilution by runoff has by others been shown to impact concentrations of MPs in raw sewage concentration in combined sewer systems (Kittipongvises et al. 2022).

The removal rates of MP in percentage for WWTPs in the Arctic and Nordic region are stated in Table 3. Most wastewater treatment plants are successful in reducing the content of MPs significantly (Table 3), though among the listed investigations, the removal ranges between none and almost 100%, and the outlet concentrations vary from 8 items m^-3 in Ryaverket in Sweden (Magnusson et al. 2016) to 55,000 items m^-3 in Ny Ålesund, Svalbard (Granberg et al. 2019). Here again the numbers are expected to be impacted by the differences in the lower-end cut-off sizes. In a review reporting removal rates from a large number of different wastewater treatment plants, Gkatzioura et al. (2021) likewise concluded that data are extremely heterogeneous due to discrepancies in included size fractions, and are thus difficult to assess and compare. Their data showed removal rates from 72–99.9% and outlet concentrations ranging from 0.5 particles L^-1 to more than 50 particles L^-1 among plants employing secondary

In Conventional wastewater treatment PRELIMINARY TREATMENT involves removal of larger particles by screening and filtering, PRIMARY TREATMENT involves removal of solids by screening and/or sedimentation. The residual sludge contains nearly 50 % of the suspended solids within wastewater including a significant fraction of the organic matter. SECONDARY TREATMENT most often makes use of biological and/or chemical treatment, though effluents of secondary quality may also be obtained by mechanical means. Secondary treatment removes smaller biodegradable organics and suspended solids. In addition, it has a disinfection effect. TERTIARY TREATMENT is aimed for improved removal of phosphates and nitrates. Further disinfection is also obtained. In some treatment plants ADVANCED polishing steps may be engaged to reduce specific remaining components.

and tertiary treatment. They also found removal rates ranging from 25–99% already in the primary treatment step of different plants. The three plants showing lowest removal rates in their study (i.e. 70–80% removal) all applied secondary treatment, but the one plant showing 80–90% removal applied tertiary treatment, while several plants applying only secondary treatment removed above 90%. No clear link between treatment method and removal rate obtained could be observed, though plants employing membrane processes seem to consistently exhibit high-end removal rates, as also observed in the review by Zhang et al. (2022). Zhang et al. (2022), however, also noted that the deposition of pollutants and MPs on the surface of the membrane can greatly reduce the permeability, creating membrane contamination and reducing purification efficiency. Poerio et al. (2019) specifically reviewed literature on membrane processes for plastic removal and found that among membrane technologies, membrane bioreactors (MBRs) are the most efficient, supposedly due to their biodegradation ability. This is in accordance with Vuori and Ollijainen (2022), who made a cost-effectiveness analysis of removal of microplastics from wastewater, and also recommended MBR technology. Carr et al. (2016) found that conventional wastewater treatment processes remove MPs effectively, and that in particular skimming and settling processes in primary tanks remove the major fraction of MPs. They conclude that effluents from either secondary or tertiary wastewater treatment facilities contribute only minimally to the microplastic loads in oceans and surface water environments. High removal may also be obtained by more simple wastewater treatment systems. In an investigation of three Australian wastewater treatment plants with advanced secondary treatment (Ziajahromi et al. 2021), it was shown that most microplastics (69–79%) were retained during the initial screening and grit removal process (i.e. the preliminary treatment). In accordance Rasmussen et al. (2021) found that most plastic (73%) was removed in the initial bar screening (20 mm and 2 mm bars) in Ryaverket in Sweden, and furthermore that the bar screens retained plastics smaller than the screen size and in total retained 50% of all incoming MPs (Rasmussen et al. 2021). As an example from the Arctic region, a recently installed treatment plant in the small settlement of Ny Ålesund, Svalbard, engaging tertiary treatment was shown to be successful in retainment of > 99% of the incoming MPs (Granberg et al. 2019). Only very few studies exist on the fate of MPs in non-conventional wastewater treatment systems such as constructed wetlands, which are used in the Canadian Arctic (Kadlec and Johnson, 2023) and Nordic region (e.g. Postila and Heiderscheidt, 2020). Büngener et al. (2023) found that the MP concentration increased by 92% during intense rain and 43% in low precipitation periods, respectively, due to atmospheric deposition in a horizontal flow treatment wetland, while Bydalek et al. (2023) observed 95% removal in a similar treatment system. Further evidence is therefore needed before choosing constructed wetlands to mitigate plastic pollution from sewage in Greenland or beyond.

Removal of MPs from more pristine waters such as sea- or lake water was shown to be significantly more challenging and with lower removal percentages compared to wastewater (Badola et al. 2022). This is likely due to the mix of MPs with a cellulosic matrix composed of toilet paper fibers, food waste, and other sewer solids causing floc formation and thus effective removal via skimming and settling processes at the preliminary, primary and secondary treatment stages (Carr, 2017). Therefore, the very limited removal (0–30%) of MPs in Icelandic wastewater observed in both Klettagarðar and Hafnarfjörður (Magnusson et al., 2016), can be speculated to be due to the very dilute Icelandic wastewater, which does not allow the formation of flocs. The low removal in Iceland can therefore not necessarily be extrapolated to situations with more concentrated wastewater like that in Greenland where wastewater has been shown to be of medium to concentrated quality (Jensen et al. 2013).

Implementing wastewater treatment in small, remote, Arctic communities is practically and economically challenging. First, is the issue of scaling: The cost capita^-1 of some commercial wastewater treatment technologies double when the feed person equivalents (PE) is reduced by a size order (Vuori and Ollikainen, 2022). Second, building and construction projects in Arctic locations is more costly compared to similar projects in Europe due to among others remoteness from supplies and low activity during long winter seasons. Third, specialized personnel with skills to operate and manage advanced technical systems are in high demand in such locations with few people and many technical installations to care for. Fourth, particularly relevant for Greenland, to reduce the need for technical repair and maintenance and possibly save some on the construction costs, the municipalities of Greenland have implemented sewer systems, which are primarily gravity-driven. This implies that the wastewater is not collected in one single sewer outlet. Instead, the wastewater is discharged via several minor outlets spread along the coastline of the towns. An intention to treat the water further adds to the complexity and cost by either having to extend sewer lines and introduce pumping stations to collect the sewage or installing multiple treatment facilities. Fifth, the most significant cost of commercial wastewater treatment plants (WWTPs) was shown to be attributed to the abatement of natural organic matter and nutrients (Vuori and Ollikainen, 2022), which has been the focus of wastewater treatment in densely populated areas. Finally, many wastewater treatment processes, both physical, chemical, and biological ones are more efficient at higher temperatures because longer residence times and thus larger reactor sizes must be expected in cold regions.

Based on the above reviewed literature, and in due consideration of the listed challenges, selecting a simple preliminary method based on screening to remove the macro plastic and reduce the microplastic could be a balanced way of mitigating plastic to the marine environment from sewage in Greenland.

A major part of the plastic litter in the sewer in our study was wet wipes. Disposal of wet wipes into sewer systems not only pose implications for the marine environment, but also raises significant practical concerns, as indicated by figure 11a in that the flushing of larger items like wet wipes can lead to clogging of the sewer system (Durukan and Karadagli, 2019). The Facebook post (Figure 11a) urged citizens to stop flushing wet wipes. From the continued occurrence of wet wipes items during that last sampling occasion, the Facebook post, however, must be concluded to have had limited impact on citizens acting. Via dialogue with Qeqertalik municipality, it was confirmed that in Aasiaat, they regularly also suffer clogging of pumps due to wet wipes in the sewage.

At the Fluids Engineering Division Summer Meetings (FEDSM), clogging of sewer pumps gained high attention in recent years, because it is a growing nuisance worldwide (Jensen, 2017; Mitchell et al. 2019; Müller et al. 2022; Beck et al. 2021). Müller et al. (2022) tested the effect of pump speed variation on clogging of sewage pumps and found that speed influences on the clogging of pumps. Some pumps improve their ability for pumping wipes in sewage water with increasing speed. Among the tested pumps, the hydraulic pumps with vortex impeller showed a significantly better capability transporting fibrous contaminated fluid with higher speed (Müller et al. 2022). Furthermore, Beck et al. (2021) showed that pumps behave very differently, and that some retain their hydraulic performance despite large amounts of wet wipes, but at high energy costs. Durukan and Karadagli (2019) hypothesized, based on their investigations of tensile properties of different types of wet wipes, that flushable wet wipes containing synthetic fibers i.e. regenerated-cellulose fibers, seem to be the key reason for operational problems in sewer systems. Mitchell et al. (2019) found that profound differences in the clogging effect of the nonwoven wet wipes could be observed. Wet wipes labelled as "flushable" had different clogging effects, depending on whether they complied with industry flushability guidelines or not.

Several authors commented on the necessity of taking steps to enforce legislation to combat the growing problem of wet wipes in the environment in general (e.g., Mitchell et al., 2019; Badola et al., 2022; Vuori and Ollikainen, 2022). Mitchell et al. (2019) concludes that the main part of the clogging-problem can only be solved if users of non-flushable wipes change their disposal behavior. The authors also point out. that steps must be taken to ensure the compliance of flushable nonwoven wipes with industry guidelines, and that wastewater system operators have to educate their clients on what belongs in the toilet, and wipe manufacturers and retailers have to ensure the reliability of the term "flushable". Vuori and Ollikainen (2022) recommend that in addition to wastewater treatment, policies targeting companies using microplastics in their products are necessary to solve the problem ultimately. Lam et al. (2018) provided a comprehensive analysis of plastics and microplastic legislation worldwide. They showed how levies, taxes, bans as well as voluntary campaigns all are strategies used to reduce plastic consumption and thus emission of plastic litter. In addition, efforts were put into the increased recovery and recycling, for which in some cases the measures were successful, while in others not (Lam et al. 2018).

To reduce the environmental load of MPs, comprehensive legislation to limit the inclusion of microplastics in cosmetics is operational in an increasing number of countries (Lam et al. 2018). Reduction of microfibers from laundry of synthetic clothes is inherently difficult and the only way is to cease the use of these completely. Lam et al. (2018) suggests that a reduction could be obtained by encouraging the usage of longer length of fibric yarn, and the use of liquid detergent rather than powder form. A tax could apply for fibric materials with shorter lengths of yarns, and for detergents which generate the release of more microfibers (Lam et al. 2018). These measures could effectively reduce the origins of pollution according to the authors. They finally suggest that to achieve better cooperation at the global level, an institutional setting needs to be devised with a multilateral agency or initiative, to integrate national efforts and promote the global policy agenda e.g. under the frame of UN.

The Greenlandic action-plan to reduce consumption of plastics was published in 2021 (Naalakkersuisut, 2021). One of the actions is establishment of knowledge on simple wastewater treatment methods to reduce the emission of microplastics to the environment.

The decision of not treating wastewater in Greenland is based on the conclusion of a consultant report made in 2005 for the Danish EPA (COWI, 2005), when Greenlandic environmental policy was still under Danish legislation. The report concluded that the recipients (exclusively the sea) were in general unimpacted by wastewaters, and that treatment for removal of organic matter and nutrients was thus not needed. The relevance of treatment was mentioned as a potential future possibility for wastewater discharge to recipients with low water exchange, and where local eutrophication was observed. At that time, plastic as well as many types of chemicals were not in focus in Greenland, and thus not assessed in the report. An up-to-date evaluation of the same issue may result in another conclusion. 

As a result of the Greenlandic action-plan to reduce consumption of plastics, the "Act on use of plastic bags and single-use plastics (SUP)" (Naalakkersuisut, 2022) was enforced. In the act a number of SUP items is prohibited in Greenland (§4). The list of banned items is identical to the list in the EU Directive on SUP (EU, 2019), and does thus not take its offset in the specific Greenlandic context, where most abundant plastic items identified in nature were linked to fishing, hunting and other outdoor activities (Strietman et al. 2021; Mallory et al. 2021). Our investigation, however, revealed the presence of three cotton buds in the wastewater, made from cardboard and cotton rather than plastic. That the sticks of the cotton buds were made of cellulose and not plastic may reflect a direct effect of the Greenland action plan on SUP, indicating a shift towards more eco-friendly alternatives for the products that have been banned. More evidence is, however, needed to draw a safe conclusion.

The EU directive includes requirement for labelling of wet wipes, extended producer responsibility and awareness raising measures. The labelling and extended producer responsibility are not adopted in the Greenlandic act, but, since almost all products are imported via EU, the EU directive would still entail for these imported products through EU. This is in accordance with the observations made in our survey of products in retail in this project, where all products were marked as non-flushable (except one product solely made of cellulose). Due to the apparent lack of impact of the labelling, additional measures need to be taken to prevent wet wipes and related MPs and other items from entering the sea via sewage. The requirement for awareness raising measures is not mentioned in the legislation (Naalakkersuisut, 2022), but is so in the Action Plan (Naalakersuisut, 2021).

EU's REACH legislation has recently been adopted to include the banning sale of both microplastics themselves and products to which they have been intentionally added. For cosmetics containing microbeads (small plastic beads used for exfoliation) and loose glitter made of plastic, the ban took effect in mid-October 2023. While for other cosmetics, there will be a transition period of between four and 12 years, depending on the complexity of the product and availability of suitable alternatives (EU, 2023). As this legislation is implemented in EU, it is considered that the legislation will also automatically function to reduce microplastics in sewage in Greenland for the same reason as mentioned above.

The findings of this study highlight a significant contribution of micro- and macro-plastic discharged by untreated wastewater in Greenland to the marine environment in the Arctic. The main contribution by mass is from plastic items larger than 25 mm, and only 1% is smaller than 1 mm. Among the large items, wet wipes are highly dominating, constituting 59% of the emitted plastic by mass. On top of that our findings suggest that a fraction of the micro-plastic is directly related to the presence of wet wipes. Thus, eliminating wet wipes from the sewage could drastically reduce the emission of plastic from sewage in Greenland. Apart from constituting an environmental threat, wet wipes are also of significant nuisance to the operation of the sewage systems in Greenland. Therefore, measures to exclude wet wipes from entering the sewage system could be prioritized above measures to treat the wastewater to remove them. We suggest the following measures be taken in prioritized order: