1 Introduction

Water pollution is a widespread problem nowadays. It is related to the presence of chemical, physical or biological components, or other factors that interfere with the beneficial use or the natural functioning of ecosystems of a given water body (Schweitzer and Noblet, 2017). Among water bodies, oceans represent the ultimate receptacle of all pollutant substances, which reach them through rivers, runoff, atmospheric deposition, and direct discharges (Landrigan et al., 2020). Covering more than 70% of the earth’s surface and holding 97% of the world’s water, oceans provide many important ecosystem and economic services, which are threatened by increasing pollution. There are various sources of pollution, most of which come from human land-based activities. Hence, oceans contain a complex mixture of toxic metals, manufactured chemicals, petroleum, urban and industrial waste, pesticides, fertilizers, pharmaceutical chemicals, agricultural runoff, sewage, and plastics.

Plastics are a wide family of petrochemical materials broadly used for many applications thanks to their favourable properties, i.e., lightness, flexibility, resistance, stability, and durability. These properties have made plastic materials an incomparable success. Industrial plastic production has therefore increased exponentially, from 1.5 million tonnes (Mt) in 1950 to nearly 370 Mt in 2019 (Plastics Europe, 2008, 2019, 2020), as shown in Fig. 1. Unfortunately, with the growth of production of these materials, plastic pollution is also increasing because of mismanagement of its end of life. Eight megatonnes of plastic was released into the environment in 2010 and 90 Mt are estimated to be released every year by 2030, in a business-as-usual scenario (Lebreton et al., 2019). Thanks to its longevity and resistance to decomposition, plastic is destined to last for decades in the environment, where an estimated 60% of all plastics ever made are found (Geyer et al., 2017). Unsurprisingly, by 2050, plastics are predicted to outweigh the fish in the ocean, which is the aquatic environment most affected by plastic pollution (World Economic Forum, 2016).

Fig. 1
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Trend of plastic production since 1950 (Plastics Europe, 2008, 2019, 2020)

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Even though plastic pollution has existed for decades, it has aroused world attention only in the last few years. This is clear from the exponentially growing number of scientific publications, which have increased from just a few between 1991 and 2008 to over 1700 between 2015 and 2019 (Freeman et al., 2020).

Plastics pollute the aquatic environment in many forms: from plastic debris along riverside and coastlines, to limitless accumulations in the open oceans, called garbage patches. And besides the most evident big plastic debris, there are also microscopic plastic fragments everywhere; these are known as microplastics. These particles significantly contribute to plastic pollution; although almost invisible, they are very abundant.

In fact, microplastics represent over 90% of the total number of particles identified in the oceans, even though they account only for 13% of the total weight of ocean plastic litter (Eriksen et al., 2014). Moreover, according to a recent study, only 1% of plastics float on the ocean surface and 94% reach the deep sea, where it is estimated that there are over 14 Mt of microplastics (Lebreton et al., 2019). In fact, many microplastics disappear from the ocean surface layer because of various phenomena, like settling, ingestion, aggregation, stranding, or even further degradation (Lebreton et al., 2019). This means that the problem of microplastic pollution is much more serious than meets the eye, and it is urgent to find a solution.

The gravity of microplastic pollution is exacerbated by the health hazard posed by microplastics. In fact, they seem to be a threat to every ecosystem and every living being, from smallest organisms to human beings. However, the assessment of the toxicological impacts of microplastics is challenging, because of their heterogeneity and great complexity. Once ingested, microplastics can cause both physical and chemical toxic effects (Wu et al., 2019; Landrigan et al., 2020; Wang et al., 2017).

Among the advanced technologies, Membrane BioReactor (MBR) seems to be the most efficient one. It combines membrane filtration and biological processes to treat the primary effluent, which contains both suspended solids and dissolved organic matter. Hence, MBR is suitable to replace secondary clarifiers in conventional activated sludge systems (Talvitie et al., 2017). It shows high MP-removal efficiency, between 79 and 99.9% (Bayo et al., 2020; Lares et al., 2018; Talvitie et al., 2017), but it suffers from rapid clogging and significant technical barrier in size-based treatment plants (Hou et al., 2021).

However, with the usual treatment steps, MPs removed from wastewater are gathered in sewage sludge, like other intercepted solids. The sewage sludge is commonly used as a soil conditioner in landfills and cultures (Ngo et al., 2019; Sun et al., 2019); this means that MPs removed from wastewater are not completely eliminated. Sooner or later, they return to WWTP through leachate, or they enter the natural water environment by stormwater runoff or they even end up in cultured products for human consumption. Hence, if wastewater treatments are optimized or improved with advanced technologies to ensure the complete removal of MPs from wastewater, the polluted sewage sludge would then have to be properly disposed of, e.g., through burning, to avoid further soil and water contamination (Sun et al., 2019). Otherwise, it would be necessary to develop MP-targeted treatment technologies for WWTPs aimed at their separation from both wastewater and sewage sludge, ensuring water safety and sludge usability. Furthermore, according to recent studies, the presence of MPs in wastewater affects the efficiency of treatments themselves, inhibiting the microbiological activity involved in the nitrogen cycle and responsible for the removal of nutrients in activated sludge systems (He et al., 4.1.2 Personal Care Products

The release of microplastics from personal care products is the only one that can be considered intentional: in this case, a product containing specifically added microplastics is intentionally poured into water (Boucher & Friot, 2017).

In 2012, Plastic Soup Foundation started the “Beat the Micro Bead” campaign against microbeads, raising awareness of microbeads and microplastics in companies, governments, and people (“Global Impact”). At first, this led many multinational companies to promise to withdraw microbeads from cosmetic products and then led many governments to ban microbeads from products. In 2014, microplastics and microbeads were banned from rinse-off cosmetic products certified by the EU Ecolabel, and even though there is still not a European-wide ban, various European countries have declared national legislations since then (Anagnosti et al., 2021; Eu Ecolabel rinse-off cosmetic products, 2014). Withal, cosmetics containing microbeads are still in the European market, in a significantly higher number than that supposed (Anagnosti et al., 2021). In 2015, Obama signed a bill against microbeads in the USA, the so-called Microbead-Free Waters Act (“Microbead-Free, 2015,” 2015). Since then, 15 states and 448 brands from 119 different manufacturers have taken action to ban and remove microplastics from personal care products (“Global Impact”). However, in some cases, legislation is limited to specific products, e.g., rinse-off cosmetics for exfoliating and cleansing purposes, leading to a limited reduction of emissions in this way (Anagnosti et al., 2021). Moreover, the reformulation of some products, e.g., leave-on cosmetics, is not easy, immediate, and cost-effective, meaning that traditional products will remain on the market still for a long time to come, kee** on polluting.

4.1.3 Marine Coatings

Marine coatings are applied to all parts of vessels and marine infrastructures for protection and include solid coatings, anticorrosive paint, or antifouling paint. They are usually made of several types of plastics and, consequently, primary microplastics are released during building, maintenance, repair, or use of boats. The key activities that seem to lead to the release of microplastics are surface pre-treatment, coating application, and equipment cleaning (Boucher & Friot, 2017).

Pinovo supplies various tools engineered with innovative and patented technology for dust-free abrasive vacuum blasting of surfaces, preventing the release of microplastics in the environment and allowing their collecting and recycling in this way, being in addition safer for operators (“Pinovo”).

4.1.4 Road Markings

Road markings are applied during the manufacturing of road infrastructure and its maintenance. They include different types of markings, mainly paints, thermoplastics, preformed polymer tape, and epoxy resins that is all fossil-derived materials. Loss of microplastics results from weathering or abrasion by vehicles (Boucher & Friot, 2017).

4.1.5 City Dust

The term city dust refers to nine different sources occurring in urban environments that are grouped together because their individual contribution is small. It includes losses from the abrasion of common objects and infrastructure as well as from blasting of abrasives and intentional pouring (Boucher & Friot, 2017).

4.1.6 Tyres

During use, the outer parts of the tyres become eroded. The formed particles consist of a matrix of synthetic polymers in a mix with natural rubber and additives (Boucher & Friot, 2017). Tyre dust is then spread by the wind in the atmosphere, accounting for up to 50% of air particulate emissions (“The Tyre Collective”), or washed off the road by the rain. In any case, after a certain time, they can reach aquatic systems.

Clearly, prevention can be performed by rethinking the design of the products. However, the wear of tyres is probably unavoidable; hence, the improvement of interception technologies also plays a significant role in tackling microplastics pollution.

The Tyre Collective (“The Tyre Collective”) aims to mitigate the emissions of microplastics from tyres by capturing them at the source. It consists of a device combined with the tyre and located close to where the tyre meets the road. When rubber particles are released from the tyre, they are electrically charged, and the device takes advantage of this charge to capture them. The particles can be easily collected and reused for other applications. The device is still under development and, according to the most recent tests, it is able to capture 60% of all airborne particles.

4.1.7 Synthetic Textiles

Primary microplastics originate from the wearing and washing of synthetic textiles, through abrasion and shedding of fibres. During wearing, microplastics are released mostly into the atmosphere but they can be washed out by rain, or reach rivers or sewage water with road runoff, ending up in the ocean in this way. During washing, they are discharged directly into sewage water and potentially end up in the ocean again. The release of microfibres can be reduced by modifying the productive process and develo** novel finishing treatments for fabrics but most likely it cannot be totally avoided (de Falco et al., 2019).

For this reason, many devices are being developed to intercept released microfibres at the outlet of washing machines, preventing them from reaching sewage and hence WWTPs. To date, most devices are meant to be placed outside of the household appliance, on the water discharge pipe, and they take advantage of a filtering medium to catch microfibres. Various systems differ from each other in the filtering medium material and its mesh size, which determine different efficiencies, as well as in the cleaning mode, as outlined in Table 2. Other develo** devices, instead, are designed to be integrated in the washing machine, which will then be sold equipped with this innovative feature. Finally, some washing accessories are available to reduce the production of microfibres and intercept those released. These systems are easily inserted into the drum during the washing; they are cheaper than the previous filters, but they are also less efficient.

Table 2 Main washing machine microfibres filters, their efficiency, cleaning frequency, and main features, divided into (a) external filters on the washing machine’s discharge hose, (b) internal filters, (c) in-drum filtering devices
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4.2 Secondary Microplastics

Secondary microplastics derive from the degradation of bigger plastic debris that ends up in the ocean mostly because of mismanaged plastic waste. Thus, implementing better waste management is one of the main actions that could help reduce microplastics pollution. However, tonnes of plastic debris already pollute many aquatic environments, from the open oceans to the rivers and beaches, so their removal is also important. Without the removal of macroplastic debris, the level of microplastics in the ocean could double by 2050 as a consequence of the degradation of the already-accumulated plastic waste, as outlined by the recent research of Lebreton et al. (2019).

Much effort has already been made to collect macroplastic litter from the aquatic environment; in general, it can be distinguished between removal from the open sea and interception in rivers and coastlines. In fact, according to a recent study (Lebreton et al., 2019), nearly 67% of all the buoyant macroplastic released into the marine environment since the 1950s is still stored by the world’s shoreline, as the debris is stranded, settled, and buried, or captured and resurfaced.

4.2.1 Ocean Clean-Up Technologies

The removal of macroscopic plastic waste is easier than that of microscopic waste because of its bigger size and also because it is concentrated in specific ocean areas, known as Ocean Gyres, where it is pushed by ocean currents forming the so-called Garbage Patches. Apart from focused expeditions, there are only a few solutions dedicated to a systematic clean-up of garbage patches.

The Ocean Cleanup

The Ocean Cleanup is a non-profit organization that aims to remove plastic debris from the Garbage Patches, and it is actually the only one of this kind to date. It has developed a system that concentrates the plastic taking advantage of the natural oceanic forces, wind, waves, and currents (“Cleaning up the Ocean Garbage Patches”). The device consists of a long floater that provides buoyancy to the entire system and a skirt that hangs beneath it and prevents debris from esca** underneath. A speed difference between the system and the plastics is ensured through active propulsion to allow their catch and retainment. Thanks to the U shape of the floater, plastic debris is collected into a retention zone at its far end, from which it can be taken once the system is full. Collected plastic is then sorted and recycled. Since it is meant to stay deployed for long periods, the system is designed to withstand the forces of the ocean. Extensive measures are implemented to ensure the safety of the system and of vessels eventually passing through, even though no heavily trafficked ship** routes traverse the garbage patch, so the chances of a crossing vessel are minimal. The first trial campaign has been carried out in the Great Pacific Garbage Patch in 2019 and the collected trash has been processed and successfully recycled (“Cleaning up the Ocean Garbage Patches”). In 2021, the organization reached the proof of technology. Now, an upscaled, scalable, and fully operational version is under development.

Project Kaisei—Ocean Voyages Institute

Ocean Voyages Institute is a non-profit organization founded in 1979 with the mission of teaching maritime arts and science and preserving the world’s oceans. In 2009, it launched Project Kaisei, which focuses on major ocean clean-up in particular in the North Pacific Garbage Patch, promoting clean-ups through periodic vessel expeditions. Three expeditions have been organized so far, in 2009, 2010, and 2012 (“Project Kaisei”).

4.2.2 Plastics Interception Technologies

Concerning the interception of plastic litter in rivers and coastlines, many different devices have been developed to date, for local customized applications or with scalability and adaptability purposes, but, in general, they can all refer to three main technologies and their combination: boats, barriers, and receptacles. They are mainly intended to be deployed in rivers, which have been identified as the main source of ocean plastic pollution, as they carry waste from the hinterland to the open sea (Lebreton et al., 2017; Meijer et al., 2021; Schmidt et al., 2017).

Boats are usually employed to gather floating plastic waste from well-known hotspots along rivers and they are often equipped with booms for this purpose. Floating barriers are also frequently deployed in rivers to intercept floating litter and prevent it from reaching the open sea. They are usually low-tech and low-cost technologies that can be settled also in poor countries. Receptacles are instead mainly used in closer areas, such as marinas and harbours, which are other relevant pollutant areas due to the many occurring human activities. However, receptacles can also be installed in rivers to gather the plastic waste intercepted by the booms. The employment of boats for litter collection often implies a huge consumption of fuel to perform every mission, contributing to another form of environmental pollution; instead, most of the boom technologies obstruct the river flow, thus not fitting for navigable rivers.

The existent solutions for ridding rivers and oceans of plastic have been recently inventoried by other authors and are not analysed in this work (Helinski et al., 2021; Schmaltz et al., 2020). Table 3 only summarizes some of the most innovative devices that are engineered to overcome the main flaws of these technologies.

Table 3 Main features of plastic interception devices, according to their type of technology and preferable placement
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5 Conclusion

This paper provides an overview of the existing and develo** solutions to tackle the problem of microplastics environmental pollution. Given the complexity of the problem, information is categorized by spot—removal after spread or interception at source—and by origin—primary or secondary microplastics. In fact, microplastics pollution has a multiplicity of implications and hence it must be faced from several sides. Certainly, dispersed microplastics must be urgently removed from the environment to limit their harmfulness. Then, as a long-term goal, their further production and emission must be prevented, intercepting them before their release and withdrawing potential sources. Finally, the removal of scattered plastic waste must be paired with the enhancement of its collection and disposal, to progressively reduce the mismanaged fraction. Thus, a single solution would be insufficient to solve microplastic pollution, but every solution contributes to tackling it and herein an overview of the main relevant ones has been presented.