3. Mitigation technologies and best practices
This chapter documents and assesses available best practices and technologies that can be employed to mitigate the release of microplastics from textiles and tyres into the environment. The chapter follows a life-cycle approach, discussing options implementable at the design and manufacturing, use and end-of-life phases, as well as options for the end-of-pipe capture of microplastics.
The present chapter aims to provide a stocktake of knowledge and techniques currently available to mitigate the leakage of microfibres and TRWP into the environment. These include several mitigation best practices, actions and technologies implementable during different stages of the lifecycle of textile products and vehicle tyres, as outlined in Figure 3.1. The chapter is structured as follows: Sections 3.2 and 3.3 present and assess the available mitigation best practices and technologies that can be implemented throughout the lifecycle of textile products and tyres (design and manufacturing, use and end-of-life), while Section 3.4 documents and assesses options for the end-of-pipe capture of microplastics.
Source: Authors’ own elaboration
The sections below report and assess relevant best practices and mitigation technologies applicable throughout the lifecycle of textiles. Best practices and relevant mitigation technologies implementable at the design and manufacturing stage are discussed in Section 3.2.1, including also eco-design options for the detergent and washing machine industries and potential mitigation solutions for industrial emissions. Section 3.2.2 assesses mitigation actions implementable at the use stage, i.e. the uptake of best use practices and of mitigation technologies, while Section 3.2.3 outlines relevant end-of-life measures to prevent the leakage of textile waste into the environment as well as measures to extend the lifecycle of garments and reduce waste generation. Where knowledge is available, considerations on costs and potential trade-offs or synergies with other environmental objectives are also discussed.
3.2.1. Product design and manufacturing
Best practices and technologies for textile design and manufacturing
The textile design and manufacturing phase holds a large potential for microfibre mitigation, as it offers the opportunity to reduce overall microfibre release at source and to mitigate emissions into a variety of entry-pathways, including emissions into air occurring during wearing and everyday use (De Falco et al., 2020[1]). Several parameters in textile manufacturing influence the amounts of microfibres released during use, from the choice of fibre and yarn type, the fabric structure, the finishing treatments employed and the post-manufacturing processes. Table 3.1 presents and assesses a number of preferable parameters and processes for textile production in line with microfibre mitigation, as identified by available research.
Although the objective of this section is to assess and compare best practices aimed at minimising microfibre shedding, policy decisions will need to place this issue within a holistic approach taking into account considerations on the broader systemic environmental and climate issues associated with fast fashion. Decisions on manufacturing practices will also have to consider other areas for environmental impacts (e.g. climate impacts, land use, chemicals use and water pollution, resource use), social implications (e.g. jobs disruption and creation, labour rights protection) and risks for potential burden-shifting.
Research has identified several manufacturing processes in line with microplastics mitigation that are already widely employed in textile manufacturing, such as the use of continuous filaments, compact structures and laser cutting. For instance, the production of staple fibres has been decreasing in recent years in favour of continuous filaments, whose production costs less than staples (The Fiber Year, 2017[14]). However, trade-offs with desirable garment characteristics or with other environmental benefits may pose a barrier to their large-scale deployment. For instance, the use of continuous filaments and compact structures, in line with lower microfibre shedding, will affect the characteristics of the final product, in addition to also requiring higher chemical use and water consumption during manufacturing (e.g. in the case of woven fabrics which require sizing and desizing processes) (Shaker et al., 2016[15]). Similarly, a shift away from knitted and towards woven fabrics could not be easily achieved: knitted fabrics represent 57% of the world market (compared to 32% for woven fabrics) (The Fiber Year, 2017[14]) and are produced at significantly lower costs than woven ones (Shaker et al., 2016[15]). Further research could explore the relevance of increasing the use of knitted fabrics with more compact features, such as highly twisted yarns.
The application of finishing treatments that create protective coatings against microfibre shedding is a key mitigation solution, yet still in the developmental phases. A promising option in terms of efficiency, costs and implementation feasibility is a coating based on pectin, a natural polysaccharide that can be recovered from waste of the agricultural and food industries at a low cost. Washing tests of fabrics treated with pectin-based coatings showed a microfibre reduction effectiveness of about 90% (De Falco et al., 2018[16]). The coating is applied on fabrics with a treatment similar to padding, a process already commonly used in industrial finishing treatments.
Coatings based on biodegradable polymers (e.g. polylactic acid and polybutylene succinate-co-butylene adipate) also showed a promising mitigation effectiveness, although currently their application may be costly and challenging to scale up at industrial level (De Falco et al., 2019[7]). Coatings based on protein-based materials inspired by squids have also been proposed, yet the production costs for the raw material could be high and their effectiveness after repeated washing cycles remains unclear (Pena-Francesch and Demirel, 2019[17]). Overall, protective coatings offer a promising technological mitigation option implementable at industrial scale during the production of textiles, however further research and testing is required in order to assess its effectiveness, implementation costs, application feasibility on different types of fabrics, compatibility with textile manufacturing processes (e.g. dyeing) and durability.
As microfibre shedding tends to decrease in subsequent washing cycles1, prewashing of synthetic fabrics has been proposed as a potentially effective mitigation measure. The main benefit of the prewashing practice at industrial level is that it provides retailers and/or consumers with products with a lower tendency to shed microfibres. Where adequate wastewater treatment systems are in place, the highest quantities of microfibres released at the beginning of fabric lifetime would be more efficiently retained at the end of manufacturing. Further, the practice could be also useful to remove microfibres from manufacturing processes entrapped in the fabrics, as well as other chemical residuals (Cesa et al., 2020[18]; Cai et al., 2020[9]; Belzagui et al., 2019[19]).
Removing microfibres at the production stage2 can be a strategic and cost-effective option to tackle the issue of microfibre shedding as upstream as possible (before these are channelled into different environmental pathways) while also synergistically targeting industrial emissions of microfibres and other pollutants. However, prewashing may be associated with high implementation costs and a variable mitigation potential. Depending on the local context, the effective implementation of this practice may require the update of the current industrial wastewater treatment infrastructure with technologies able to retain microfibres with adequate efficiency. Thus, this mitigation best practice may be challenging and costly to implement in SMEs in emerging economies, where the majority of textile manufacturing takes place. In light of these challenges, it has been suggested that policy could also envision that the pre-washing of textile products manufactured abroad takes place in the importing country under controlled conditions. In sum, pre-washing has a high mitigation potential, however further tests are required to evaluate its implementation feasibility at different entry points (i.e. textile manufacturing, fabric manufacturing, or retail), as well as to further investigate whether the decrease in microfibre emissions occurs independently from the specific characteristics of different fabrics.
In general, as point-source emissions are easier to manage than diffuse ones, the design and manufacturing of textiles and garments generally offers large opportunities for cost-effective reductions in microfibre emissions. However, persisting knowledge gaps pose a barrier to the development and implementation of the identified mitigation best practices and technologies. Further research is currently required to more reliably assess the cost-effectiveness and implementation feasibility of the available mitigation options, as well as to evaluate the potential trade-offs with the preservation of desirable environmental benefits or garment characteristics.3
The lack of standardised methodologies and the lack of transparency along the textile value chain have been identified as key barriers to the investigation and implementation of best manufacturing practices. Firstly, the lack of common measurement standards for microfibre shedding renders test results difficult to compare and aggregate, limiting possibilities to draw conclusions on the manufacturing parameters which influence microfibre shedding during use. Secondly, the complex and geographically dispersed nature of textile and apparel value chains provides challenges to the adequate provision of product information further downstream (Niinimäki et al., 2020[20]). Knowledge and data gaps in the manufacturing history of fabrics (e.g. production steps, chemicals used, etc.) pose challenges to the evaluation of the effect of specific production processes on microfibre release and, therefore, to the identification of best design and manufacturing practices. For these reasons, collaboration between researches and textile industries could be beneficial and instrumental in accelerating research and industrial-scale deployment (see Chapter 5).
Prevention of industrial emissions
As outlined in Chapter 2, microfibres were found in relevant concentrations in industrial wastewater (Xu et al., 2018[21]) and in water and sediments sampled in textile industrial areas (Deng et al., 2020[22]). Given there is currently insufficient information to reliably quantify and characterise the release of microfibres occurring during manufacturing, there is a need for textile producers to collect data on industrial microfibre emissions in order to adequately inform mitigation action.
It has been suggested that in-line vacuum systems could be added to capture loose fibres via air filtration and exhaustion after processes such as brushing, sanding and raising (Carney Almroth et al., 2018[2]). The EU MERMAIDS project recommended to handle carefully mechanical finishings which generate many loose microfibres, such as napping (EU MERMAIDS, 2015[13]). Textile manufacturing facilities could be fitted with treatment systems of water and air dedicated to the removal of microfibres, although such systems have not yet been developed or presented. Implementation steps are expected to be long and potentially to bear high costs for the textile industry, given the existing technological barriers. Options for the improved treatment of industrial wastewater effluents are discussed in Section 3.4.1.
Best practices for the production of laundry detergents and washing machines
Detergent and washing machine manufacturers may also be important players in the mitigation of microfibre emissions. Available research has identified several entry points for the identification of best practices for microfibre pollution mitigation implementable during the production of detergents and washing machines, as summarised in Table 3.2.
The use of detergents and softeners can influence the microfibre shedding degree of clothing and also directly contribute to the release of intentionally-added microplastics. The development of products aligned with the prevention of fibre loss, such as detergents that are effective at low temperatures and during short laundry cycles, while also efficiently cleaning the fabric, can potentially contribute to microplastics pollution mitigation, although solutions may take time to be developed. It will be crucial that new detergents and additives do not contain intentionally-added microplastics or harmful substances available for release into the environment. A synergic action between innovation in detergent production and in the application of finishing treatments during textile manufacturing should also be taken into consideration to ensure that the laundry cycle does not provoke the detachment of coatings and that an overall microfibre reduction effect is achieved.
At the level of washing machines production, there is scope for adapting the design of products in order to help mitigate the generation and emission of microfibres during laundering. However, currently, there are no reports of washing machines available on the market with a tested effectiveness in mitigating microfibre emissions.4 Washing machines able to both maintain the correct functioning of the machine and mitigate microfibre releases into wastewaters (e.g. via built-in filtration systems) may take time to be developed and tested.
In both cases, the implementation of these mitigation actions may take time and could bear substantial costs for the relevant industries, from R&D to manufacturing, potentially also resulting in higher consumer prices. However, as for textile design and manufacturing, the development of solutions at the industry level can hold large potential for mitigation as well as for easier implementation and monitoring via adequate policy intervention.
3.2.2. Textile use stage
Best practices for maintenance and care
The use of garments contributes to microfibre pollution in two ways: it causes the release of microfibres into air (De Falco et al., 2020[1]; Dris et al., 2017[27]) and it causes fabric abrasion and tear that can lead to microfibre emissions during washing. Although further research is required in order to correlate practices aimed at improving fabric durability with microfibre release, several practices aligned with adequate textile care and improved durability of fabrics are expected to also minimise microfibre generation during laundering and drying, as summarised in Table 3.3. These include reducing the frequency of washing cycles, washing full loads and at low temperatures, preferring liquid to powder detergents, using fabric softeners (except for fabrics which can be damaged by the use of softeners, such as outdoor apparel) and avoiding tumble-drying.
Key barriers to implementation are a lack of consumer awareness and knowledge on the environmental consequences of microfibre pollution, as well as on the available mitigation measures. As reported in previous sections, scientific consensus over the relative effectiveness of these practices remains uncertain, also due to the absence of standardised methodologies to quantify microfibre release. Further research is required in order to gather more and conclusive data on the parameters that influence the release of microfibres during the laundering, drying and wearing of textiles. The good outcome of such investigations will also depend on the level of collaboration between relevant stakeholders and industrial sectors.
Further research is also needed to understand how the tendency of garments to emit microfibres changes over their lifetime, particularly to understand whether there is a threshold beyond which older garments start releasing higher amounts of microfibres. Several studies have found that the release of microfibres decreases during subsequent washing cycles (Cai et al., 2020[28]; Carney Almroth et al., 2018[2]; Cesa et al., 2020[18]; Belzagui et al., 2019[19]; De Falco et al., 2019[29]; Napper and Thompson, 2016[30]; Pirc et al., 2016[10]; Sillanpaa and Sainio, 2017[8]), however this trend has mainly been observed with new garments and might differ with clothes that have been worn. One study mechanically aged some garments and found that they released more microfibres than new ones (Hartline et al., 2016[24]). Tests conducted in real household conditions found relevant quantities of microfibres released, however these cannot give indications of potential trends over the lifecycle of garments (Galvão et al., 2020[31]; Lant et al., 2020[26]).
Capturing and filtration devices implementable at the level of washing machines
Several capturing and filtration devices have been developed to reduce microfibre release during washing processes. The majority of existing technologies are available to consumers on the market and include devices to be added to the drum of the washing machine (in-drum capturing devices), external filtration systems to be positioned at the end of the drainpipe (add-on external filters) and built-in filters. Selected examples are described in Table 3.4 according to their type, effectiveness (in terms of % of weight reduction of microfibres released) and key characteristics.
In general, several issues need to be taken into account when considering mitigation technological solutions implementable during the use phase of garments. Additional costs for the consumer are a primary concern, as also are the degree of additional maintenance required and the ease of use. In terms of user-friendliness, particular attention has been given to the need to find solutions which prevent mishandling, for instance via the rinsing of the filter in the sink and the dispersal of microfibres into household sewage. Additional concerns are the potential trade-offs that may arise with other environmental benefits, such as the energy efficiency of the laundering process, the environmental footprint of the lifecycle of filters (e.g. production, collection, reuse or recycling, disposal) as well as their durability and the need to ensure the adequate disposal of the retained microfibres (Herweyers et al., 2020[35]).
Key advantages of filtration devices are their commercialisation, availability for implementation and ongoing continued technological improvement. However, their use remains on a voluntary basis by the consumer, so it is difficult to control their uptake and assess their effectiveness in real-life conditions, especially where more delicate cleaning and maintenance operations are required. Furthermore, the majority of these devices have been conceived for use in household applications rather than at large scale (e.g. in industrial or commercial laundering facilities), although several models could be easily adapted to allow for larger water flows.5
Common barriers and issues to be addressed in order to allow the broader implementation of filtering technologies include:
The lack of standardised test methods to assess and compare the effectiveness of available devices. While some filters have been independently tested, a clear and reliable picture of their effectiveness and durability is not yet available. The dimensional range of the microfibres retained by filter devices needs to be investigated further. It is essential to develop standardised test methods to compare the effectiveness of filters and to assess their compatibility with washing machines and with textile laundering processes.
The need for further research on the potential trade-offs and synergies with other best practices and technologies for microfibre mitigation. For instance, although using the Guppyfriend bag can ensure better fabric care, it could potentially lead consumers to wash full loads less frequently.
The need for the provision of information on adequate maintenance practices. For some in-drum filters currently on the market, it is recommended that these are cleaned when fibres or entanglements are visible. Where maintenance or replacement of parts by the consumer is required, inadequate handling could potentially cause clogging or malfunctioning in the washing machine.
The lack of endorsement by the washing machine industry. As external filtration systems have not yet been endorsed by washing machine manufacturers, the compatibility of these devices with different types and brands of washing machines cannot be determined yet. Also, the impacts of these technological solutions on the normal functioning of the machine, notably in terms of energy/water consumption and cleaning effectiveness, are neither clear nor well documented.
The need for the adequate disposal of microfibres. It should be ensured that the disposal of microfibres, which are small in size and can be easily dispersed, is handled carefully. For instance, the PlanetCare and Indikon-1 filters use a cartridge that needs to be replaced after a certain number of washes and require customers to return the full cartridges so that these can be handled correctly. Both companies aim to reuse or recycle the used cartridges, although public information of how this aim is achieved is not yet available.
Drawing from these elements, an assessment of considerations relevant for the design of policies that mandate the adoption of filtering technologies for washing machines is included in Chapter 4.
3.2.3. Textile end-of-life stage
The current system of textile and fashion consumption is responsible for the production of more than 92 Mt of textile waste per year (Niinimäki et al., 2020[20]). The recovery of materials at the end of the life cycle of products is very low: it is estimated that 87% of the total fibre input in textile manufacturing is landfilled or incinerated and less than 1% of the materials used in textile manufacturing are recycled at the end of the lifecycle of products (EMF, 2017[46]).
Little information is currently available on the contribution of the end-of-life of textiles to overall microfibre releases. Since the mismanagement of plastic waste contributes to the emission of microplastics (see Section 1.2.2), it is likely that the inadequate management of textile waste contributes to the release of microfibres to both water and air. It is unclear to what extent recycling and reuse practices are aligned with microfibre mitigation objectives. For instance, fibre grinding, i.e. a recycling process where fibres are ground to be used in other applications such as construction, could require further evaluation with regards to microfibre release and the potential need for mitigation solutions at recycling facilities. With regards to garments made out of recycled fibres, available evidence does not provide a clear picture of trends in microfibre release in comparison with garments made of virgin fibres (Roos, Levenstam Arturin and Hanning, 2017[11]; De Falco et al., 2019[29]; Özkan and Gündoğdu, 2021[47]; De Falco et al., 2020[48]). With regards to reuse, further research is also required to ascertain whether and to what extent ageing garments have a higher tendency to shed fibres (see Section 3.2.2).
In general, further research is needed to adequately assess the impact of reuse and recycling practices on microfibre generation. Yet, given available knowledge, it is likely that the environmental benefits of reusing (or recycling) garments outweigh the potential additional microfibre leakage associated with the use of old (or recycled) garments. Reductions in textile production and waste generation, also achievable by extending the useful life of products and by keeping materials within the economy, can significantly reduce environmental impacts associated with the handling and transportation of textile waste and the demand for virgin materials.
Textile waste generation could be prevented or reduced via a number of measures, such as:
Reductions in pre-consumer textile waste generation. Between 10% and 30% of the fabric used in the manufacturing process is wasted. Additionally, incinerating unsold garments remains a common practice. Pre-production waste generation could be reduced by slowing down manufacturing rates and improving the accuracy of production via better communication between design and manufacturing (which are often in different geographical locations) (Niinimäki et al., 2020[20]). Regulatory measures can also be introduced to ban the destruction of unsold merchandise (see for instance (France, 2020[49])).
Extended lifetime of garments and reductions in post-consumer textile waste generation. A key barrier in the reduction of textile waste are practices associated with the concept of “fast fashion” (i.e. cheap manufacturing, massive production and continuous proposal of new, short-lived garments) which encourage fast disposal (Niinimäki et al., 2020[20]). Today, clothes are more and more underutilised: it is estimated that in the past 15 years, the average number of times a piece of garment is used before being thrown away has decreased by 36% (EMF, 2017[46]). Several options exist to extend the lifetime of clothing and textile materials, including creating markets for second-hand clothing. Efficient and dedicated textile collection systems are required to support reuse practices and to ensure that garments maintain their quality over their extended lifetime. Emerging business models such as product-service systems, supplier take-back schemes and sharing platforms, can play a large role in increasing the utilisation of garments and steering textile consumption towards higher sustainability (UNEP, 2020[50]).
Improved textile recycling. Several recycling practices exist for textiles and garments: conversion to cleaning and wiping rags, fibre recovery for use in new yarns, fibres re-spinning into new filaments and feedstock recycling (i.e. the polymer is broken down to its original monomers) (Piribauer and Bartl, 2019[51]). The presence of separate collection is a necessary prerequisite to enable recycling, especially to enable the larger uptake of higher value recycling opportunities. For instance, legislation in place in the EU obliges member states to collect textile waste separately by 2025 and ensure that waste collected separately is not incinerated or landfilled (EEA, 2019[52]). Measures aimed at encouraging eco-design could also facilitate end-of-life management and efficient recycling of garments, for instance by avoiding the use of multi-material textiles, which are more challenging to recycle efficiently.
The sections below report and assess relevant best practices and mitigation technologies applicable along the lifecycle of tyres. Section 3.3.1 discusses relevant mitigation options implementable during the design and manufacturing of tyres as well as of roads and vehicles. Section 3.3.2 assesses mitigation actions implementable during the use phase, i.e. the uptake of good practices for tyre use and maintenance and eco-driving practices, as well as broader actions aimed at reducing overall vehicle kilometres travelled. Section 3.3.3 focuses on the end-of-life phase and outlines relevant best practices for the maintenance of artificial sports turfs.
3.3.1. Product design and manufacturing
Mitigation technologies and best practices related to material design intend to reduce the tyre wear rate. This can be achieved either by optimising tyre tread and road pavement characteristics or by reducing vehicle weight (Table 3.5).
Optimisation of tyre tread characteristics and dimensions
Tyre characteristics, such as the dimension and the mechanical properties of the tread, influence the tyre wear rate. Mitigation measures may target an increase in stiffness ratio between tread and carcass. For instance, a wider tread and a low tread sea volume could result in decreased TRWP generation (Klüppel, 2014[56]). Wider tyres exert less pressure against the road surface and cause less abrasion, although this is partially offset by the larger contact area with the road surface. In general, wider tyres are expected to have slightly lower abrasion rates compared to narrow tyres (Pohrt, 2019[55]).
Efforts are ongoing to improve the eco-design of tyres in line with microplastics mitigation. These generally entail optimising design parameters to enhance resistance to abrasion, as well as replacing potentially hazardous chemicals employed during production in order to minimise the toxicity of emitted TRWP. Improvements in material design should not only respond to safety concerns but also ensure tyre durability resulting in longer tyre life, potentially reducing the resource requirements for tyre production. Tyres are designed to achieve a balance between safety and environmental performances, such as abrasion, braking, wet grip, rolling resistance and noise. With current technologies, these performance characteristics are variously antagonistic to each other. The development of innovative solutions in tyre design will be required in order to see significant reductions in the rate of tyre wear whilst preserving high standards in other performance areas. Policy interventions could be considered to incentivise or mandate the development of low abrasion tyre tread materials (see Chapter 4).
Tyre type and dimension are usually selected according to the vehicle type. Global car markets have witnessed a trend towards larger and wider tyres (which are generally in line with lower tyre tread wear), yet this came with concurrent increases in the average vehicle weight and power (which generally lead to higher tyre wear) (Li, 2018[57]). As the share of e-mobility in the vehicle fleet is expected to increase in the near future, tyre design may be adapted to minimise tyre wear. Certain recent innovations aim to reduce rolling resistance and increasing the vehicle mileage in order to reduce overall tyre tread wear (Continental, 2019[58]). The use of airless tyres, which cannot be operated with incorrect pressure conditions (see Section 3.3.2), may also contribute to reducing emissions in the future.
Optimisation of road design and road surface characteristics
The design of road infrastructure and the characteristics of traffic impact tyre wear by influencing the conditions in which vehicles are operated. Road design characteristics (e.g. curves, hills) can be optimised so as to mitigate TRWP generation. For instance, tyre wear abrasion that occurs at curves may be reduced by increasing the roadway inclination in curves (Klüppel, 2014[56]). Another influencing factor is the extent to which road features lead to frequent and large speed changes, for instance due to the presence of traffic lights (Andersson-Sköld et al., 2020[53]). Further, the choice of road markings (and of the application method) can also mitigate the rate of wear (Andersson-Sköld et al., 2020[53]). Not only different types of road markings are wear-resistant to different degrees, but they also may directly contribute to microplastics pollution. Lastly, as damaged road pavements may lead to higher tyre wear, adequate road surface maintenance could substantially contribute to TRWP mitigation. Key stakeholders involved in the partial or full implementation of optimised road pavement and infrastructure characteristics are road authorities and municipalities as well as the construction industry.
The structure of the road pavement also affects tyre and road wear. Coarser textures are expected to cause higher road wear compared to smoother surfaces and asphalt roads generally cause a lower wear rate than concrete pavements (Pant and Harrison, 2013[59]). The roughness of the pavement micro-texture is the main driver of wear in the road surface, while the macro-texture has a minor influence (Andersson-Sköld et al., 2020[53]). In general, there is a trade-off between improved resistance to road wear and safety: the micro-texture may be adapted to reduce tyre wear, however this might lead to reduced friction and thereby safety.
Several studies are ongoing to address this conflict and test innovative pavements optimised for lower tyre and road wear. For example, the Danish Road Directorate has been working on developing a road pavement that reduces the rolling resistance between vehicles and road pavements, to explore solutions to reduce road transport GHG emissions. Tested pavements have shown a reduction in rolling resistance, resulting in reduced fuel consumption, without significant compromises on safety requirements and durability (Pettinari, Lund-Jensen and Schmidt, 2016[60]).
Although the implementation of such road pavements is associated with higher costs, it also leads to a lower noise level and it may benefit from higher acceptability by municipalities, road authorities and the public (Pettinari, Lund-Jensen and Schmidt, 2016[60]). This technology is not yet state-of-the-art and further research and development is needed, in particular to improve durability, before implementation will be possible. Once innovative road pavements are available, it may be advisable to apply these in areas with particularly high tyre wear abrasion rates, for instance on congested roads or on high-speed motorways. For the time being, given the well-known relationship between the state of road surfaces and rolling resistance, adequate road maintenance to preserve smooth and even surfaces can be an effective strategy to reduce the production of TRWP (ETRMA, 2018[61]).
Reductions in vehicle weight
As vehicle weight increases, so does the frictional force between tyres and road surfaces and therefore the generation of TRWP. Yet, trends in the composition of the vehicle fleet show a tendency towards a higher proportion of larger and heavier vehicles (Andersson-Sköld et al., 2020[53]). Additionally, electric vehicles also tend to be heavier than their traditional counterparts, mainly due to the weight of batteries (Timmers and Achten, 2016[62]). As a result, total emissions of TRWP are projected to increase at a higher rate than the increase in traffic (Andersson-Sköld et al., 2020[53]). The higher torque (rotational force) of electric cars compared to their traditional counterparts may also lead to increased tyre wear during acceleration (Soret, Guevara and Baldasano, 2014[63]).
Reductions in vehicle weight may be achieved by the application of advanced lightweight materials in cars (Serrenho, Norman and Allwood, 2017[64]). Aluminium alloys are commonly used as replacement materials, as they provide similar performance properties as steel with lower weight (Hirsch, 2011[65]). However, these advanced materials generally require a greater amount of energy to manufacture and recycle and further research and development is required to enable their larger uptake in vehicle production (Raabe, Tasan and Olivetti, 2019[66]). More broadly, measures aimed at encouraging or incentivising the uptake of lighter vehicles as well as reducing overall volumes of road traffic (as discussed in Section 3.2.2) may significantly contribute to reductions in air pollution and in GHG emissions, while also generating co-benefits in terms of TRWP pollution mitigation (see Chapter 4 for a discussion of relevant policy instruments).
3.3.2. Tyre use stage
Mitigation actions implementable during the use phase include the optimisation of vehicle operation parameters such as tyre pressure, wheel alignment, vehicle load, vehicle speed, driving conditions, driving behaviour and reductions in total transport volumes (Table 3.6). The advantages and disadvantages of each mitigation action are discussed below. Importantly, several mitigation best practices and technologies implementable during the use phase generate numerous synergies with other relevant benefits and environmental policy objectives.
Optimising vehicle maintenance: tyre pressure and wheel alignment
Maintaining optimal pressure in vehicle tyres throughout their use can optimise performance as well as minimise TRWP generation. If the tyre pressure is too low, internal heat generation occurs, which increases wear (Li et al., 2011[69]). Over-inflation, on the other hand, leads to uneven tyre tread wear, which can reduce the lifespan of a tyre. In OECD countries, the share of the vehicle fleet operating with suboptimal tyre pressure could be significant. For instance, it was calculated that in Sweden one in seven cars has at least one tyre with an air pressure which is 30% too low, causing higher tyre wear (Andersson-Sköld et al., 2020[53]).
Tyre pressure monitoring systems (TPMS) have been introduced on passenger cars in several OECD countries and provide one technological solution to the problem. These are electronic systems designed to monitor tyre pressure and to report real-time information to drivers when the tyre requires to be inflated. Verschoor and de Valk (2017[72]) estimated that equipping all Dutch cars with a TPMS would lead to a 14% reduction in tyre wear. Since 2014, newly registered cars in the EU are fitted with TPMSs (EC, 2010[70]). Still, as regular pressure tests are not always performed, older vehicles registered in the EU before November 2014 may operate under non-optimal tyre pressure conditions. Where measures for TPMS have been introduced, it is expected that the number of cars operating with sub-optimal tyre pressure will decrease as older cars without pressure monitoring system are removed from the market.
Incorrect wheel alignment may increase tyre wear rates by up to 10% (Verschoor and de Valk, 2017[72]). The share of the vehicle fleet operating with incorrect wheel alignment can also be significant: in Germany, for instance, 15% of vehicles operate with incorrect wheel alignment (Kraftfahrtbundesamt, 2019[73]). Tests on wheel alignment are generally done during the change of tyres, e.g. from summer to winter tyres. In countries or regions where seasonal changes of tyres are not required, alignment checks occur less frequently. Yet, regular testing and wheel realignment is a relevant option that can increase the lifetime of tyres (without adaptation of infrastructure) and also reduce tyre wear.
Eco-driving and traffic flow management: vehicle speed and driving behaviour
Driving behaviour is one major parameter influencing the tyre wear rate. Higher speeds, fast acceleration and fast retardation and high cornering speeds in particular are associated with increased tyre wear (Pohrt, 2019[55]). Changes of direction, congestion and high-speed roads are also generally associated with higher TRWP generation (Andersson-Sköld et al., 2020[53]).
Significant reductions in TRWP can be achieved by encouraging drivers to adopt eco-driving practices, i.e. maintaining lower and constant speeds. Since tyre wear rate increases by a factor of four relative to increases in speed, speed reductions hold a high mitigation potential (Pohrt, 2019[55]). Examples of specific measures include the introduction and effective enforcement of local speed limits of cornering roads as well as of motorways, measures to improve driver awareness, the use of advanced driver-assistance systems, such as cruise control and adaptive distance control and general traffic management measures.
Speed reduction will be relevant either to reduce or better enforce existing speed limits or to introduce (nationwide or local) measures where these are not present (e.g. in Germany). Implementation costs would be relatively low as only limited infrastructure would be required, however public acceptability is likely to be the main implementation barrier to the introduction or adaptation of speed limits. For this reason, it will be important to share with the public and policymakers scientific evidence on the adverse consequences of speeding in terms of potential safety and on the environmental gains which can be achieved via speed limits.
Although measures aimed at reducing speeds are primarily driven by safety concerns, a reduction of tyre wear rates can be achieved as a co-benefit, for instance as a result of smoother driving with continuous traffic flow instead of stop-and-go traffic. These measures also generate significant synergies with environmental objectives, such as reduced fuel consumption and lower emissions of CO2 and air pollutants (e.g. NOx and particulate matter). The adoption of eco-driving practices and smart road management can reduce fuel consumption up to an average of 6.3%, with consequent reductions in CO2 emissions (Wang and Boggio-Marzet, 2018[74]). Further evidence is required in order to quantify potential reductions in tyre wear emissions achievable via improved driver awareness and traffic management.
Reducing transport volumes
Several options exist to mitigate TRWP emissions via overall reductions in traffic volumes. These are not unique to TRWP generation, rather have been discussed extensively in order to respond to needs to mitigate GHG emissions and air and noise pollution, limit sealed surfaces in urban areas in order to allow rainwater to seep away and prevent floods, decrease the amount of urban space occupied by road traffic and reduce congestion.
In order to decrease transport volumes, overall higher accessibility can be delivered by increasing the role of transport modes such as public transport, cycling and walking (which generate less TRWP per capita); ultimately increasing TRWP mitigation potential while also improving other well-being (e.g. health, equity) goals (OECD, 2019[75]). The promotion of modal shifts requires adequate infrastructural investments for sustainable transport modes, for instance via the construction of advanced railroad systems to increase ridership and capacity of public transportation systems. Urban areas can also be designed in compact ways so as to reduce dependence on private vehicle travel. In general, creating proximity between people and places is key to avoiding unnecessary trips or unnecessarily long distances and to increasing the scope for active and public transport (OECD, 2019[75]).
A reduction in total traffic and total mobility should be considered a long-term goal that requires parallel adaptations of transportation systems and urban forms. Notable drawbacks of traffic reduction measures include the need for public acceptability as well as potentially high implementation costs and the potential for distributional effects. Again, these can be greatly reduced if policy and investments are refocused on the enhancement of accessibility. To the extent that traffic reduction also reduces local air pollutants and GHG emissions, policies that seek to reduce TRWP via a reduction in vehicle-kilometres travelled also have environmental health and climate mitigation co-benefits. Furthermore, by incentivising active travel and enhancing accessibility, particularly for vulnerable population, policies that generate “avoid” and “shift” effects can also bring additional health benefits as well as contribute to more equitable access to services and opportunities (OECD, 2019[75]).
A number of cities around the world have implemented traffic reduction schemes aimed at improving air quality, quality of life and safety. For example, Strasbourg, Nurnberg, Copenhagen, Vienna and Ghent have successfully implemented traffic calming measures (EC, 2004[76]). Other options may include the re-allocation and re-design of streets, parking and road pricing, expansions and upgrades of public transport and active mode services, or incentives for the uptake of shared services. Such measures usually reduce vehicle traffic while promoting other modes of transport as cycling and public transport (Titos et al., 2015[77]). Although these measures are not typically implemented in order to mitigate microplastics, reductions in TRWP will occur as a co-benefit.
3.3.3. Tyre end-of-life stage
As discussed in Chapter 2, End-of-Life Tyres are commonly employed in material recovery applications, including for the production of rubber granulate used as infill in artificial sport pitches or in moulded surfaces used in playgrounds and outdoor facilities. The use of rubber granulate as infill material for sport pitches offers improved durability and resistance to varying weather conditions, good shock absorbance and safety characteristics, low costs and a lower need for virgin materials (Magnusson et al., 2016[78]). Yet, several studies have indicated that sport pitches may constitute a significant source of microplastics pollution (see Section 2.3.2).6
Guidelines have been developed to support the prevention of rubber granulate leakage in the design and operation of artificial turf pitches (Fidra, 2020[79]; EuRIC, 2020[80]; CEN/TR, 2020[81]). Selected options, presented in Table 3.7, include several low-cost, high-potential mitigation actions. Examples include the installation of infrastructure which prevents the emission of rubber granulate particles (e.g. side paved areas around the pitch, cattle grids and brushing stations located near the pitch entrance, drainage and filtration systems for runoff) and the routine maintenance of the pitch (EuRIC, 2020[80]; Eunomia, 2018[82]). The lack of awareness among owners and operators as well as the lack of regulatory or financial incentives may be posing barriers to a larger uptake of the identified mitigation best practices and technologies.
3.3.4. Comparison of mitigation options for tyre-based microplastics
Potential mitigation best practices and technologies for tyre-based microplastics are evaluated in Table 3.8 according to three criteria: mitigation potential, implementation efforts required and costs and the societal impact. In general, progress in tyre and pavement design in line with lower wear rates offers a high mitigation potential, although the development of optimised tyres and road pavements without compromises in safety, noise and durability may take some time. During the use phase, reduced vehicle speed, adapted driving behaviour and adequate maintenance of vehicle and tyres also have a high TRWP reduction potential. At the same time, since complete prevention of TRWP generation and emission cannot be achieved by optimized materials, end-of-pipe options may be important solutions to retain the emitted TRWP before these enter the environment. There, case-by-case designs may be required for the installation or adaptation of stormwater and road runoff treatment options, as discussed in Section 3.4.2.
End-of-pipe options, such as (potentially separate) wastewater and stormwater collection and treatment, constitute a last barrier to pollutants present in contaminated water sources and play a critical role in preserving water quality. The performance of end-of-pipe technologies in removing microplastics is only of recent interest and not yet fully understood. Existing treatment processes for wastewaters generally retain the majority of microplastics initially in the wastewater effluents. However, in an attempt to improve their overall performance in retaining a range of water pollutants (including microplastics), there is interest to explore ways to enhance the treatment efficiency further. Additionally, substantial quantities of microplastics enter the environment via diffuse pathways (i.e. dry and wet deposition, road and stormwater runoff). In this sense, end-of-pipe options such as improved stormwater management, generally driven by the need to manage increased runoff rates caused by urbanisation, can also contribute to the preservation of water quality.
The next sections explore existing facilities and technologies that can be adopted to enhance the capture of microplastics carried in domestic and industrial wastewaters (Section 3.4.1) and in road dust and stormwater runoff (Section 3.4.2).
3.4.1. Wastewater treatment and sewage sludge management
Treating municipal wastewaters in a wastewater treatment plant (WWTP) is the norm in OECD countries. WWTPs purify used water resources from pollutants originating from human activities and rainwater runoff before these are reintroduced into the water cycle, preventing the spread of pollutants and bacteria hazardous to human health and the environment.
Each WWTP involves selected combinations of chemical, physical and biological processes taking place simultaneously or interacting, in order to achieve a final effluent which is in line with existing regulations for water reuse or release into the environment. Depending on the stringency of the regulations in place and other location-specific characteristics (e.g. availability of space, capacity of treatment, types and concentrations of pollutants and features of the receiving water body such as the dilution capacity and sensitive uses), WWTP may be designed to have unique combinations of preliminary, primary, secondary and tertiary (and potentially additional) treatment stages. When required, old WWTPs are retrofitted (via the addition and/or replacement of some treatment units) to enable compliance with more stringent standards.
Although several knowledge gaps remain (Box 3.1), some indicative conclusions can be drawn from available data on the fate of microplastics during WWT. Table 3.9 describes the main objectives to be attained during each stage of the treatment process in a conventional setup and outlines the expected microplastics stage removal rate. Conventional WWTPs can achieve microplastics retention rates of 80-95% (by number), likely mostly attained in preliminary and primary treatment steps (e.g. screening, removal of grit and grease).7 WWTPs with tertiary treatment8 show only marginal higher efficiency at retaining microplastics compared to plants with only secondary treatment, although this might vary depending on the specific technologies in place (Talvitie, 2018[83]; Nikiema, Mateo-Sagasta and Saad, 2019[84]). The effectiveness of the process in terms of microplastics removal seems to be more affected by the size rather than type of the particle, with smaller particles being more difficult to remove (Lv et al., 2019[85]; Talvitie et al., 2017[86]).
Associating a certain microplastics removal rate to specific parameters, such as the type of treatment process employed, can be challenging. This is for several reasons, including:
The quality and availability of the data. There are concerns related to the quality of some available data on microplastics’ removal in WWTPs. The majority of available studies build on one-time measures and replications are scarce. This is an issue since there could be a wide temporal and spatial variation in influent and effluent wastewater quality from one or different countries and studies.
The lack of standardised methods. The lack of consistency in analytical protocols for sampling, processing and analysis limits the comparability of available studies. This poses challenges to the accurate identification of factors and treatment stages which influence the microplastics removal rate (Weis, 2020[88]). There is an important need to develop methods for the sampling and analysis of TRWP in wastewaters, for which very little information is available and no studies have been carried out so far (Andersson-Sköld et al., 2020[53]; Baensch-Baltruschat et al., 2020[89]).
The general complexity of the issue. During wastewater treatment, numerous interactions may occur between various chemical and physical processes, microorganisms and other factors. These could impair or otherwise support the removal of microplastics by modifying the surface of microplastics or causing their further breakdown into smaller particles that are more difficult to remove (Lv et al., 2019[85]; Ruan et al., 2019[90]; Nikiema et al., 2020[87]).
Note: Harmonization and standardization of analytical techniques is required, including for the mesh size used for sampling, which defines the lower size limit for microplastics’ detection. This is further discussed in Chapters 4 and 5.
Opportunities to improve existing municipal WWTPs in OECD countries
In OECD countries, the infrastructure in place can be considered largely effective at preventing the release of microplastics present in the wastewater influent to surface waters. Table 3.10 presents selected cases to illustrate how primary, secondary and tertiary treatment stages influence the microplastic removal rate in a number of OECD countries (and China). As illustrated, conventional secondary treatment systems remove between 86% and 99.8%n of microplastics in raw wastewater.
According to Murphy et al. (2016[91]), the highest microplastic removal is achieved during skimming (especially for lighter microplastics), while other authors (see for instance (Talvitie et al., 2017[92]; WHO, 2019[93])) also emphasize the important role played by filtration or gravity settling processes for the removal of heavier microplastics. The overall performance in terms of microplastics removal is mainly determined by the removal performance achieved during the primary treatment stage.
In response to more stringent regulations on water quality, several countries have seen WWTPs being retrofitted with additional treatment units in recent years. Although these advancements primarily aim to ensure that nutrient or heavy metal levels in treated effluents are within water quality standards, they may offer potential co-benefits with higher cumulative MP retention efficiencies. Given the large volumes of wastewaters treated and the possible high quantities of microplastics entering the environment via the wastewater pathway, there is scope for exploring the effectiveness of available technologies in removing microplastics to potentially inform the design of end-of-pipe capture solutions for microplastics and other micropollutants contained in wastewaters (Talvitie et al., 2017[86]).
Different tertiary technologies may offer varying microplastics mitigation potential. If processes such as Biological aerated filter (BAF) and Rapid sand filters yield inconsistent or limited results for microplastics removal (Bayo, López and Olmos, 2020[96]; OECD, 2020[100]), filtering disks, dissolved air flotation and membrane-based systems (i.e. membrane bioreactor (MBR); reverse osmosis (typically only employed for specific reuse options); membrane filtration) offer the potential for effective treatment of both microplastics and nutrients and heavy metals. With most tertiary treatment processes, there are issues reported with risk of by-passing of microfibres, due to their longitudinal shape, hence resulting in their escaping into the environment (Bayo, López and Olmos, 2020[96]). The removal of microplastics through advanced filtration may vary depending on the surface characteristics and size of microplastics: for instance, MBR has been found to be particularly effective at retaining micro fragments and MPs < 1 mm, but less so for microfibres and larger particles (Bayo, López and Olmos, 2020[96]; Lv et al., 2019[85]; Talvitie et al., 2015[97]). Furthermore, membrane defects and fouling can negatively affect their performance.
Table 3.10 illustrated examples of the use of MBR technology for microplastic removal. Membrane bioreactor (MBR) units are a well-established membrane-based technology among the most effective treatment options for microplastics. MBR offers the benefit of combining biological treatment (secondary) and membrane filtration (tertiary) in a single step. The membrane (1.0-0.01 μm) is selected based on its effectiveness at removing targeted contaminants and its durability based on the operating conditions. Currently, MBR is used for municipal or industrial wastewater treatment to enhance removal of nitrogen or hardly biodegradable compounds. Membrane technologies are costly to implement and maintain and are currently only employed where the objective is to enable water reuse in scarce areas, retrofit inefficient plants, minimise effluents in small vulnerable water bodies, or where compact installations are required. Typically, MBR is 38-53% more expensive and 25-50% more energy-intensive than use of conventional activated sludge process (Bertanza et al., 2017[101]).
Overall, risk assessments and cost-benefit analyses will need to be carried out in order to evaluate costs against the advantages offered (e.g. removal of higher levels of suspended solids or nutrient). Conventional wastewater secondary treatment processes, such as activated sludge, seem to be more cost-effective to implement than tertiary treatment processes with regards to removal of microplastics. Beyond this, knowledge remains limited and to some extent insufficient to derive conclusions on the best technology to retain microplastics.
Industrial wastewater treatment
As discussed in Section 2.2.2, the textile industry is an important potential source of microfibres. Although microplastics is not a targeted contaminant for removal during industrial wastewater treatment,9 currently employed technologies at industrial WWTPs, where in place, may be fairly effective at retaining microplastics, with certain industrial WWTPs exhibiting microfibre removal efficiencies >85%.
Table 3.11 presents the treatment performance achieved by typical textile-based industrial WWTP in China, the world’s largest producer of fabrics. In general, large microfibres are more easily removed than small microfibres (i.e. < 50µm) (Zhou, Zhou and Ma, 2020[102]). Air flotation appears to be a suitable technology for removal of low-densities microfibres. However, removal of microfibres is mostly achieved in membrane-based processes such as MBR or Reverse Osmosis (i.e. during Step 2 and Step 3, respectively). On the other hand, the several pigments found in the influent wastewater responded differently to the wastewater treatment process and the reasons for this behaviour remain unclear (Xu et al., 2018[21]; Zhou, Zhou and Ma, 2020[102]).
The range of contaminants typically targeted by WWT for industrial effluents from textile manufacturing/dyeing plants is exemplified in Table 3.12. An improved understanding of the quantities and fate of microfibres emitted during textile manufacturing processes is required in order to inform future decisions on the optimisation of industrial wastewater treatment to retain microfibres emitted during the manufacturing process. As industrial and commercial laundry facilities are also potential hotspots for microfibres in OECD countries, ad-hoc wastewater treatment may also be an effective option to mitigate microfibre pollution closer to the source of emission. For instance, a study conducted in Sweden found that wastewater treatment adjacent to industrial laundries reduced microfibre concentrations by 65-97% (Swedish EPA, 2018[103]). In general, whether further investments in WWTP upgrades are cost-effective will vary depending on the types of contaminants emitted into industrial effluents and the local conditions (e.g. the type of WWT infrastructure in place, other micropollutants present in sewage).
Management of sewage sludge
Sewage sludge is the by-product of wastewater treatment, i.e. the residual mixture of solids and water retained from the influent wastewater. As microplastics are captured by the wastewater treatment process, these are transferred into the sludge fraction. The situation varies according to the types of MPs present in the influent and the type of WWT infrastructure. Typically, 69-99% in number of the microplastics initially in the influent wastewater are transferred to the sludge fractions produced at different stages of the wastewater treatment process (including the preliminary stages). It has been estimated that, in Swedish WTTPs, only 40-60% of microplastics originally in the wastewater influent are transferred to the anaerobically digested sludge, although it remains unclear to what extent treatment may lead to MP degradation to a size not detectable by commonly employed analytical methods (Tumlin and Bertholds, 2020[104]). As illustrated in Table 3.13, microfibres typically represent 63-80% of microplastics in sludge, but, especially when the WWTP also receives stormwater, other types of microplastics can also be present.
Wastewater sludge is most commonly incinerated or employed in agriculture as fertiliser. In several OECD countries, land application of sludge plays a key role in enhancing soil health through enrichment with the organic matter and nutrients. However, there are concerns that, in the long term, regular application of large volumes of sludge may result in the pollution of soil with contaminants not targeted by sludge treatment, including microplastics. Generally, sludge fractions undergo thickening and dewatering to reduce the water content, and, when intended for land application, they also undergo stabilisation to reduce the risks associated with pathogens and odours from biodegradation of organic matter. These processes may cause melting or shearing in microplastics, but typically no effective removal of the particles.
Nutrient recovery is a promising option to recycle back nutrients into agriculture without potentially re-applying hazardous organic and inorganic pollutants to land. Currently, only phosphorus recovery from digested sludge is carried out in some places to recover dry struvite, which is used as a slow-release fertilizer. This enables the recovery of 40% of the total phosphorus content, which corresponds to 90% of the soluble phosphate ion, although nitrogen and organic matter are not recovered (Koga, 2019[105]). Nutrient recovery options have a large commercial potential, however currently financial sustainability is not always achieved (Koga, 2019[105]).
In general, further research is required to assess the concentrations and hazards of microplastics in sewage sludge and evaluate possible end-of-pipe mitigation options. For the time being, a low-cost strategic way of approaching the issue may be to avoid land application for sludge fractions richer in MPs, where the adequate infrastructure for incineration is present (taking account of potential GHG emissions and other environmental impacts). Sludge microplastics contamination varies widely according to the phase of the process from where it was obtained (Table 3.13). Given the majority of microplastics are retained during the preliminary and primary stages, sludge generated during skimming or primary sedimentation, for instance, will be richer in microplastics (typically 5-10 times) than sludge from biological treatments (secondary sludge). Indeed, in most OECD countries the fate of these sludge fractions richer in microplastics is generally incineration or landfilling. In order to evaluate further options for the management of microplastics in wastewater sludge, further research is required to assess the fate of microplastics during conditioning and treatment (e.g. in digesters) as well as to evaluate the potential risks for terrestrial environments posed by sludge application on agricultural land.
3.4.2. Management and treatment of stormwater runoff and road dust
As discussed in Chapters 1 and 2, stormwater runoff collects pollutants originating from several sources, including a range of microplastics deposited on roads and washed off by precipitation events. Although the lack of data on the concentrations of microplastics in stormwater remains a challenge, stormwater runoff is believed to be the major entry pathway into the environment for TRWP (Parker-Jurd et al., 2019[109]). Despite the knowledge gaps, several existing measures to manage stormwater can contribute to minimising microplastics runoff into water bodies. Additionally, nature-based solutions (e.g. green roofs, permeable surfaces) can avoid that rainwater and runoff further contaminate sewage.
In light of emerging evidence on the contribution of tyre and road wear to non-exhaust particulate emissions and microplastics pollution, potential mitigation measures for airborne TRWP have also recently gained policy attention (OECD, 2020[54]). Targeting pollutants present in air and road dust may prove to be a cost-effective solution to prevent stormwater contamination with TRWP. Measures such as improving urban cleaning services and installing and adequately maintaining meshes, booms or separators on drains to retain and remove microplastics can help reducing solids which would otherwise be carried away with the water (Prata, 2018[110]).
The sections below discuss selected options to i) manage road dust and collect TRWP and other road traffic-related pollutants and ii) manage and treat stormwater runoff.
Street sweeping and direct particle collection
Street sweeping can provide an effective end-of-pipe measure to collect coarse particles contained in road dust, although the practice is less effective with fine dust (OECD, 2020[54]). Different techniques are available, such as removal by air, vacuum, or mechanical action of a broom (Calvillo, Williams and Brooks, 2015[111]), which will deliver different levels of performance. The collected particulate material is transferred and retained in the waste storage tank of the street sweeping machine. Although street sweeping does not require additional infrastructure, the collected street dust requires safe disposal after collection.
Further research is required in order to allow for a reliable assessment of the effectiveness of road sweeping at retaining TRWP (Andersson-Sköld et al., 2020[53]). Current knowledge indicates that the cleaning efficiency is likely to depend on the type of pavement, the type of sweeping machine and the precipitation conditions. Available research has identified a number of best practices to optimise the efficiency of street sweeping:
Coordinating street sweeping with weather conditions to the pavement properties can increase its effectiveness (Andersson-Sköld et al., 2020[53]). For instance, road sweeping prior to strong rain events may remove TRWP before these are flushed away with the road runoff (ETRMA, 2018[61]).
It may be more cost-efficient to prioritise street sweeping for high-traffic roads that are not equipped with efficient runoff water collection systems. Also, street sweeping is less effective on porous asphalt than on non-porous asphalt.
Adapting the sweeping method to the weather and road conditions may also optimise the process. For instance, tests from the city of Stuttgart have shown that using street cleaning machines in wet mode (combining sweeping and water flushing) can provide a higher level of TRWP removal than other systems (ETRMA, 2018[61]). Conversely, with dry conditions, dry vacuum sweeping may be most effective at retaining road dust particles.
Filter techniques also exist for the treatment of road runoff, such as gully pot filters and underground sedimentation facilities (see next Section), which both generally bear small footprint requirements. In the case of gully pots, which are already widely employed in several countries to retain sediment in road runoff and minimise problems with sediment deposition downstream, available knowledge indicates that they can offer a low-cost and effective solution to retain microplastics coming from road transport activity, yet they require regular maintenance and cleaning in order to be effective (NIVA, 2018[112]). In general, further research is required to assess the adequacy and effectiveness of different techniques at retaining TRWP and other microplastics from road runoff. There are also efforts to develop technological solutions to capture emitted particles directly at the point of emission from the vehicle tyre, although innovative technologies still require further research, development and evaluation (Smithers, 2020[113]).
Improving stormwater management and treatment
Stormwater runoff not intended to be directed to a municipal WWTP should generally be treated before release into the environment for the removal of conventional pollutants such as heavy metals, oils and other organic pollutants (including PAHs), nutrients, pathogens and solids (Liu et al., 2019[114]; Strassler and Strellec, 1999[115]). Several technologies can serve this purpose, including wetlands, retention and detention ponds and infiltration systems. These can be found in urban and non-urban areas and are designed to remove particulate material and, to some extent, dissolved contaminants.
Table 3.14 presents the advantages, disadvantages, potential co-benefits and costs of common stormwater treatment technologies discussed in this section. Although the purpose here is to evaluate options that might be well-suited for microplastics removal, it is important to note that considerations on the implementation of these solutions are generally driven by a diverse set of concerns that include water quality preservation but also flood protection, climate change mitigation and adaptation, or habitat creation.
Detention/retention ponds
Sedimentation ponds are artificial basins commonly employed in OECD countries to manage stormwater runoff and remove particulate material via sedimentation processes. Two main types exist: detention and retention ponds. In detention ponds, runoff is captured and detained for a period of time and then clean water is released gradually, providing water quantity and peak flow regulation and limited water quality control. In retention ponds, the system maintains a permanent pool: the captured runoff water is retained until it is released or replaced by the following runoff water. Retention systems can provide both quantity and quality control for water runoff (Strassler and Strellec, 1999[115]).
Sedimentation ponds are considered as one of the most effective stormwater management installation for removing particulate material and potentially microplastics, because the long residence time allows particles to settle. Sedimentation ponds are designed to remove particles > 63 µm with removal efficiencies > 50% (Boogaard et al., 2017[117]). However, retention is expected to vary widely due to highly variable particulate loads (NIVA, 2018[112]). To ensure performance, the system requires regular removal and appropriate disposal of retained sediments (Liu et al., 2019[118]).
Infiltration systems
Infiltration systems are a sedimentation technique designed to aid water infiltrate into the ground, while the soil, organic matter, or a membrane serves as filtering media to remove sediments from stormwater runoff. Variants include:
Infiltration basins, designed to drain their accumulated water within 3 days;
Porous pavement systems made of porous asphalt or porous concrete, which typically reduce runoff formation by 45% compared to a fully asphalted area; and
Infiltration trenches or wells, which have limited capacity and thus are often used in combination with detention or retention ponds.
These options have the merit of enriching groundwater, although adequate monitoring is required to ensure that the process does not result in groundwater contamination. Infiltration systems are not appropriate in all locations; for instance, sufficient levels of clay are needed in soil to allow the removal of dissolved pollutants in the runoff water (Strassler and Strellec, 1999[115]). As the infiltration systems are prone to clogging, it is crucial that accumulated sediments are removed from the pond bottom at least yearly and that soil compaction is prevented. Pavement systems must undergo periodic vacuuming or jet-washing to remove sediment from the pores and be protected from excessive equipment traffic.
Wetlands
Wetlands are a commonly employed nature-based solution for stormwater management and treatment (Ziajahromi et al., 2020[119]). Wetlands are known for their ability to improve water quality via natural processes involving wetland vegetation, soils and their associated microbial assemblages to filter water as it passes through the system. Benefits of wetlands include removal of nutrients and pharmaceutical residues and the prevention of unwanted releases of untreated water (Coalition Clean Baltic, 2017[120]). For conventional contaminants, removal occurs primarily via degradation and uptake by microbes and plants or their assimilation and absorption into organic and inorganic sediments.
Wetlands may prove effective at retaining microplastics present in stormwater, although research on the topic is limited. Different wetland variants exist:
Constructed wetlands (CWs) are engineered and managed wetland systems designed to mimic natural wetlands. Available investigations of the performance of CWs in removing microplastics reported removal efficiencies over 99.7% for microplastics with a size exceeding 20 µm (Coalition Clean Baltic, 2017[120]; Liu et al., 2019[114]).
Floating wetlands (FWs) are also manmade ecosystems. They employ small artificial platforms that allow plants to grow on floating mats in open water where their roots spread through the floating mats and down into the water. In a recently published study, between 15% and 38% of microplastics in the sediments accumulated in a FW were found to be synthetic rubber-carbon filled particles, most likely derived from vehicle tyres (Ziajahromi et al., 2020[119]). Further research is required to adequately assess the removal effectiveness of FWs and to allow for comparisons with other options. Furthermore, as FWs can be built from plastic materials, there may be a risk of potential contribution to microplastics pollution via the degradation of the plastic construction materials (Ziajahromi et al., 2020[119]).
Comparison of stormwater treatment options
Although further research is required to fill the persisting data and knowledge gaps on the contribution of stormwater treatment infrastructure to microplastics pollution mitigation, available data suggests that wetlands and retention ponds may be highly effective at removing microplastics from water. It is likely that careful management of retained sediments is necessary to ensure the effective control of microplastics.
Wetlands are generally cost-effective because of the low investment and maintenance costs. The costs to set-up a wetland system in the United States are USD 379-11,016 (average USD 3,441) per m3/d treated or USD 86 per m2 (Hunter et al., 2018[121]), although these are highly variable in different locations. The operation and maintenance cost is typically USD 3.5-40 per m3/d treated. Normally, wetlands have indefinite lifespans and are expected to be permanent landscape. The opportunity cost of land removed from other uses (e.g. agricultural production) is not negligible: it could represent between 50% and 70% of the total implementation costs. Construction costs per volume of runoff treated for wetlands are 25% higher than for retention and detention ponds, mainly due to the plant selection and sediment forebay requirements. Infiltration basins can be significantly more expensive, with 1.5 to 4 times higher costs for installations of equivalent size (Strassler and Strellec, 1999[115]). Also, annual operation and maintenance costs represent a significant percentage of the capital expenditure: 2%-6% for retention basins and constructed wetlands, 1% or less for detention ponds and 1-20% for infiltration trenches or ponds. These costs will vary based on a number of location-specific parameters.
Stormwater management infrastructure described above offers multiple co-benefits such as enhanced availability of water (which contributes to sustainable basin management), rainwater flow management, water quantity control, as well as increases in wildlife habitat, property values and recreational opportunities. In turn, all stormwater management options cannot be easily implemented everywhere. Soil characteristics, volumes of runoff and traffic conditions will determine the most suitable solution. There may also be a case for prioritising the installation of stormwater infrastructure near pollution hotspots, such as road sections with high traffic volume, although the cost-effectiveness of this approach needs to be assessed further (Gehrke, Dresen and Blömer, 2020[122]).
The decentralisation of stormwater and road runoff treatment may also contribute to reducing the pressure on combined sewer systems and the potential for combined sewer overflows, a substantial source of diffuse pollutants, including microplastics (as discussed in Chapter 1). Adequate stormwater runoff is necessary to prevent important floods in highly populated and paved urban areas, especially as pressures from diffuse sources of water pollution intensify. Furthermore, the development of flood management strategies may provide a useful entry-point to also improve microplastics capture as a co-benefit. Conversely, solutions such as constructed wetlands could also provide a treatment solution for excess loads occurring during heavy rain events (Meyer et al., 2013[123]).
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Notes
← 1. The trend was observed in 100% polyester fabrics (Pirc et al., 2016[10]; Sillanpaa and Sainio, 2017[8]; De Falco et al., 2019[29]; Cai et al., 2020[28]; Napper and Thompson, 2016[30]; Carney Almroth et al., 2018[2]), in 100% acrylic fabrics (Cesa et al., 2020[18]; Napper and Thompson, 2016[30]), in 100% polyamide fabrics (Cesa et al., 2020[18]) and in blends of polyester/elastane and acrylic/polyamide (Belzagui et al., 2019[19]).
← 2. It has been suggested that the removal of microfibres could also be carried out using dry methods. Provided that the microfibres are disposed of in a safe way, dry methods could be most cost-effective than preliminary washing, as they enable the collection of microfibres before these are dispersed into sewage and/or air (Roos, Levenstam Arturin and Hanning, 2017[11]).
← 3. A relevant initiative is a project financed by the Swedish Environmental Protection Agency to identify, prevent, and reduce microplastics pollution from textile industries and wastewater treatment plants through pilot projects in coastal areas in China (Swedish EPA, 2021[124]).
← 4. According to the producers, parent companies Grundig AG - Arcelik A.Ş. have been working on developing a new washing machine with a built-in microplastic filter able to filter out 99.9% of microfibers released into water, although no further information is available on this product (Arcelik, 2018[125]).
← 5. Information gathered from conversation with experts held during the Workshop on Microplastics from Tyre Wear (17-20 May 2020) and during following meetings.
← 6. More recently, concerns have also emerged with regards to the potential for moulded granule surfaces in playing fields and other outdoor sport facilities to also release microplastics when not properly maintained, however this is still an emerging area of research.
← 7. The removal efficiency can be obtained on a percent mass basis (indicated by m) or on a percent number basis (n). The latter is adopted throughout the section, except where otherwise specified.
← 8. The OECD defines tertiary treatment as treatment additional to secondary that removes nutrients such as phosphorus and nitrogen and practically all suspended and organic matter from waste water.
← 9. The range of contaminants typically targeted by WWT for industrial effluents from textile manufacturing/dyeing plants is exemplified in Table 3.12.