Regional management options for floating marine litter in coastal waters from a life cycle assessment perspective

Plastic pollution in the marine environment stems from diverse sources, including poorly managed plastic waste along coasts and inland areas, lost fishing gear, and primary microplastics. Most originate from land-based sources, primarily transported by rivers and streams. According to The Economist (2018), almost 90% of the plastic debris found in the marine environment is transported by ten primary river systems, predominantly located in Asia (eight rivers) and Africa (the Nile and Niger). Macroplastics have been identified as the main contributor to marine plastic pollution, with land-based sources responsible for over 80% of total quantities (The Economist 2018; Lebreton et al. 2017). Various regional factors, such as population density, industrial activities, and tourism, influence the distribution of plastic debris in the oceans. Moreover, regional policies and regulations, including waste management strategies and initiatives to reduce plastic usage, can also impact the amount of plastic entering the marine environment.

The latest figures provided by Plastics Europe (2022) and the OECD (2022) indicate that global plastics production reached 390.7 million tons in 2021, driven by economic and demographic growth. The global community urgently needs to address marine plastics as plastics production is expected to triple by 2060, and an estimated 3% will eventually end up in the marine environment, corresponding to 11 million tons of plastics each year (Boucher and Billard 2019; UN Environment 2017).

In addition to land-based sources, ocean-based waste sources significantly contribute to marine pollution. Approximately 1.15 Mt/year of fishing gear, constituting 20% of the total, and 600 kt/year of shipping waste find their way into the sea due to accidents, bad weather conditions, and intentional dumping (Eunomia 2016; GESAMP 2019). The marine environment in Europe receives over 11,000 tons of abandoned, lost, and discarded fishing gear per year as well as 0.6 million tons of microplastics from the fishing industry (Boucher and Friot 2017; GESAMP 2019).

A comprehensive literature review conducted by Rochman et al. (2016) found that over 82% of the demonstrated impacts of marine debris were explicitly associated with plastic pollution, primarily observed at sub-organismal levels and attributed to plastic ropes, straws, and fragments larger than 1 mm.

The impacts of accumulated plastic debris in the marine environment include physical, chemical, and biological effects. Marine animals and organisms are entangled leading to injury, trapped or drowned, while ingestion can also procure physical injury, gut obstruction, or indigestible material, and toxic chemicals accumulation, ultimately leading to death (Law 2017; Smith et al. 2018). The accumulation of debris lead to the formation of new anthropogenic habitats colonized by microorganisms and microbiota and can destroy the foundational skeleton of coral reefs while the buoyancy of the debris facilitates rafting and transportation of marine organisms, including non-native and invasive species (Gall and Thompson 2015; Niaounakis 2017). Chemical impacts stem from the release of toxic substances from marine debris into the environment from where they are likely to enter the food chain and the adsorption of persistent organic pollutants (POPs) and other persistent, bio accumulative, and toxic substances (PBTs) on the surfaces provided by marine plastics ( Pawar et al. 2016; Rochman 2015). In the latter, ingestion of the plastics transfers the toxic substances into marine organisms, biologically affecting their molecules (Rochman et al. 2016; Teuten et al. 2009).

At end-of-life, plastic products accumulate as waste which need to be managed somehow. In 2016, it is estimated that around 260 million tons of mixed plastic waste were generated worldwide of which only 16% are recycled, mostly converted into recycled plastic resins that can be used to manufacture new plastic products while a small fraction of less than 1% is chemically converted into virgin feedstock in the form of plastic monomers (Hundertmark et al. 2018). The largest share however is disposed of at landfills with 104 million tons (or 40%) or incineration for energy recovery with 65 million tons (or 25%). In addition, a considerable share of around 49 million tons of mixed plastic waste (or 19%) remains unmanaged, either in uncontrolled dumping sites or as direct or indirect leachates to the environment (Hundertmark et al. 2018). Parts of this mismanaged waste fraction finally end up as marine plastic debris in the world’s oceans.

In Europe, recycling rates surpassed that of landfilling for the first time in 2016, following a constant increase over the last decade. Likewise, energy recovery increased from a share of around 29% in 2006 to a share of around 42% in 2020. However, landfilling is still the first or second option of plastic waste treatment in many countries in Europe (Plastics Europe 2022).

Preventive measures are imperative to mitigate the entry of plastic waste into the marine environment. In their “Action plan for the Circular Economy”, the European Commission implemented a waste hierarchy in which the prevention of products and materials from becoming waste for as long as possible was set as the key step to achieve the goal of a circular economy (European Commission 2015). Waste that cannot be avoided should be turned into a resource, first through reuse, followed by recycling or as a last option with the help of energy recovery. All these options are favoured over landfilling, which shall be avoided in any case.

Plastics waste is one of the main focus of the Action Plan (European Commission 2015) consolidated by the adoption of the EU Plastics Strategy in 2018, identifying a pioneering set of measures explicitly tailored to address the challenges posed by plastics, hence a trajectory toward realizing a transformative concept called the ‘new plastics economy’ (European Commission 2019; Gionfra et al. 2020). Embedded within the Plastics Strategy is the aspiration for the comprehensive integration of principles encompassing ‘the design and production of plastics and plastic products in full respect of reuse, repair and recycling’ while concurrently advocating for the development and propagation of more environmentally sustainable materials (Penca 2018).

Most of the measures are expected to positively affect the marine environment as they decrease the creation of plastic litter needed to be disposed of. In this respect, they can be seen as reinforcing the link between existing legislations, Marine Strategy Framework Directive (Directive 2008/56/EC), which requires the monitoring and reduction of marine litter, and Waste Directive (Directive 2008/98/EC), which sets out the principles of waste management, including the waste management hierarchy. Widely supported by scientific findings, the waste management hierarchy implies that the higher the hierarchy, the more cost-effective the measures. Efficient production and consumption models are commendable, as they reduce the marine debris problem closer to the source (Penca 2018).

Despite explicit prioritization within the EU waste hierarchy, the relatively limited efforts to in curbing waste generation and in promoting material reuse resulted in the Commission taking a strong commitment to tackle single-use plastics introducing Directive 2019/904 (Baldassarre and Saveyn 2023; Kiessling et al. 2023; Milanović et al. 2023). The Directive covers some measures, including banning certain items commonly found on beaches, such as straws, cutlery and food packaging, and introducing extended producer responsibility measures, mandatory labeling, and product design requirements (Penca 2018). As only certain single-use plastics fall within the scope of the Directive, it was essential to assess the potential effectiveness of these measures in reducing plastic litter in the environment and to identify possible shortcomings in the legislation (Kiessling et al. 2023).

The second Circular Economy Action Plan, introduced in 2020, is a proactive response to this context, recognizing the imperative to foster innovative business models and reassess established approaches within value chains. The European Commission undertook key initiatives aligned with this objective, exemplified by creating the Circular Plastics Alliance. In addition, a concerted effort is being made to invigorate new business paradigms by integrating targets related to reuse and refilling strategies in the proposal for a Regulation on packaging and packaging waste (European Commission 2022). Concurrently, the Commission is driving focused actions aimed at prolonging product lifecycles. This includes concrete measures within the realm of plastics, mandating recycled material content and addressing microplastics, as well as biologically and biodegradably sourced plastics.

This is particularly relevant in the case of marine plastics, due to the fact that an instantaneous halt to inflow of plastics into the oceans will not address the already accumulated marine plastic waste expected to endure for generations (Gerke et al. 2016). Therefore, alongside proactive prevention endeavours, it is equally crucial to develop environmentally sustainable downstream solutions for managing the plastic waste already accumulated in the marine ecosystem, continuously improving the pathways for further utilization or valorisation. It can be generally observed that many existing initiatives that aim to collect accumulated marine debris from the environment do not consider an appropriate strategy for the treatment of collected material downstream. Thus, in many cases, the collected material was disposed of in landfills. However, to enable effective downstream handling of marine litter, identification and classification methods have been developed (Armitage et al. 2022; Cocking et al. 2022; Owens et al. 2022; UN ESCAP 2022).

For those approaches which do consider downstream pathways for collected marine debris apart from disposal, mechanical recycling, reuse, energy recovery options, and to a lesser degree chemical recycling have been adopted (Amesho et al. 2023). Mechanical recycling is well-established technique for single polymer waste with good while different types of plastic waste are used as an additive in asphalt to provide unique properties such as hardness, viscosity temperature susceptibility. In particular, HDPE-modified binder in asphalt mix has enhanced the resistance against permanent deformation in the pavement industry (Hahladakis et al. 2020; Rahman et al. 2020). Chemical recycling of marine PET plastic is enhanced due to depolymerization induced by the marine environment while fuel production from marine plastics recently analyzed by several researchers indicate good environmental performances though facing yield challenges related to various operating conditions, reactor types and catalyst (Hussein et al. 2021; Peña-Rodriguez et al. 2021). Advanced incinerators coupled with combined heat and power plants have proved to be environmentally friendly and have low impacts on human health. Moreover, Refuse Derive Fuel (RDF) from plastic waste is compatible with EU waste-to-energy strategy and Paris Agreement (Hahladakis et al. 2020). The main challenges of these recycling methods are analyzed on the basis of economy, efficiency, and environment. Among all these methods, gasification and pyrolysis are proven to be the most efficient and environmentally friendly (Salahuddin et al. 2023; Solis and Silveira 2020; Xayachak et al. 2023).

The recycling of DFG such as fishing nets, fragments, and the remaining plastic debris followed by the production of RDF to enhance the calorific value of the material for later energy recovery are prevalent practices. The RDF treatment option was introduced as part of a practical approach toward the management of accumulated marine debris in South Korea. It was observed that RDF from marine debris shows a higher calorific value than land debris due to the high plastic content and the increased carbon and hydrogen content as a result of the residence time in sea water. In Germany, as in many other European countries, RDF production from residual waste streams gained more attention in recent years, as it shows benefits over direct waste incineration in many cases and thus represents a useful supplement to a sustainable waste management strategy (Abeliotis et al. 2012; Cherubini et al. 2009).

For the above-mentioned downstream treatment of collected marine debris, an evaluation of the performance of proposed waste management strategies to identify associated benefits and drawbacks is often missing, particularly in view of environmental impacts. Scagnetti and Lorenz (2022) developed a theoretical framework for incorporating plastic leakage from marine plastic packaging into LCA models, specifically focusing on marine plastic pollution, arguing that the current practice of excluding plastic leakage from LCA models underestimates the environmental impact of plastic packaging. Moreover, process-related data on currently established and developed technologies are limited, as well as their environmental performance using robust methodologies such as Life Cycle Assessment (LCA) (Liu et al. 2022; Schneider et al. 2018, 2023; Veksha et al. 2022). Schneider’s 2023 study, for instance, focused on addressing the environmental challenges associated with derelict fishing gear (DFG) as a prominent marine litter category, using LCA to evaluate various waste management options adapted to the European context. The study demonstrated the positive environmental outcomes of mechanical recycling and energy recovery compared to the detrimental effects of syngas production and landfill disposal. Similarly, Veksha’s investigation delved into a lab-scale recycling process for plastic marine litter. The LCA emphasized environmental benefits by avoiding raw material extraction, with multi-walled carbon nanotubes (MWCNTs) synthesis leading in benefits, followed by pyrolysis oil. The research proposes PET sorting followed by thermochemical treatment for oil and MWCNTs as a viable and scalable approach for effective waste management (Veksha et al. 2022). These studies underscore the importance of comprehensive assessments to guide effective waste management solutions for marine litter, bridging the gap between technological innovation, environmental considerations, and real-world applicability.

However, a comprehensive and in-depth waste management concept specifically tailored to the unique properties of marine plastic waste, distinct from non-marine plastic waste, is still lacking (Cañado et al. 2022). Hence, this study aims to enhance the understanding of the environmental aspects related to managing marine plastic debris from a downstream perspective. Specifically, it focuses on identifying the challenges and opportunities related to the treatment of plastic debris accumulated in the German Bight within the North Sea. To address these issues, a Waste Management Strategic Plan (WMSP) is designed and evaluated using the Life Cycle Assessment (LCA) methodology. This comprehensive approach provides practical strategies for managing marine plastic pollution and promoting sustainable practices.

Table 5 and Fig. 4 summarize the environmental impacts of the studied scenarios evaluated in the Life Cycle Impact Assessment phase. The CML-IA baseline method (Leiden University 2013), version 3.08 June 2022, the USEtox consensus model (Rosenbaum et al. 2008), version 2.12 recommended only, and the Cumulative Energy Demand method version − 1.11 (Althaus et al. 2007) were applied to the LCI data with the aid of the SimaPro ver. 9.4. These include the following impact categories: Global Warming Potential (GWP100, excluding biogenic carbon), Acidification Potential (AP), Photochemical Ozone Creation Potential (POCP), Human Toxicity Potential (HTP, cancer and non-cancer), Cumulative Energy Demand (renewable CEDr and non-renewable CEDnr), and Marine Aquatic Ecotoxicity Potential (MAETP).

For Scenario 1, CEDr and CEDnr have negative values which means energy resources are conserved. The baseline scenario however has marginal savings of about 220 MJ on non-renewable energy resources while it contributes to renewable energy resource depletion significantly (Table 5). On the contrary, savings linked to Scenario 1 are significant, and in spite of the much lower quantity of material treated in the DFG Recycling Plant, the savings related to CEDnr are more compared to the treatment of the MML fraction at the MBT + CHP plant (Fig. 4(a)). However, the impact on CEDr from the electricity consumption at the DFG Recycling Plant outweighs the savings of produced plastic granulates.

Analyzing the contribution of the various process stages involved in the treatment of MML, energy recovery from RDF at the CHP plant contributes the highest to savings of both renewable and non-renewable energy resources, while electricity, diesel, and natural gas consumed during MBT processing stages contributes the most to CED (Fig. 5).

Major differences can be noted between the baseline scenario and the proposed WMSP for GWP, with Scenario 1 contributing 10 orders of magnitude less than Scenario 0. The main contribution of scenario 1 is from emissions during combustion of RDF at the CHP plant, while to a lesser degree, there are savings resulting from the recycling of DFG at the recycling plant due to avoided emissions by substituting virgin plastics with recycled granulates. Previous studies report results one order of magnitude lesser, likely due to lower emissions and non-consideration of emissions from fossil fuels used to initiate combustion in the fluidized bed combustor (Consonni et al. 2005b).

Despite the lower quantity treated, energy recovery from RDF produced from MML contributes more to POCP than recycling of DFG. In the treatment of MML, aluminum recovery at the MBT plant and energy recovery at the CHP plant lead to the highest savings in POCP, followed by the recovery of recyclates leading to an overall impact of − 0.036 kg C₂H₄ eq per ton of MML treated (Fig. 5). These findings are more or less in line with those of similar LCA studies, even though values can occur in one magnitude higher or lower (Abeliotis et al. 2012; Consonni et al. 2005b). At the DFG Recycling Plant, a total of about 0.5 kg C₂H₄ eq/t DFG treated is avoided, attributed to the production of plastic granulates. The output of the plant clearly compensates the impacts associated with the operation of the plant (Fig. 6).

Separate treatment of the different fractions of the collected marine litter mitigates Acidification Potential (AP) by around 0.53 kg SO₂ eq/t treated waste, whereas scenario 0 contributes 3.3E + 09 kg SO₂ eq/t treated waste. Recycling of DFG at the Recycling Plant reduces the most the overall impact of Scenario 1 while in the case of MML treated at the MBT plant, the recovery of aluminum mitigates AP the most. However, high contributions to AP are linked to transportation as well as to emissions of SO₂ and NO₂ emissions from the use of fossil fuels. Overall, the treatment of MML is associated to savings in AP of approximately 0.3 kg SO₂ eq/t MML, in line with previous studies (Abeliotis et al. 2012; Consonni et al. 2005b). At the DFG Recycling Plant, contribution to AP is around − 2 kg SO₂ eq/t DFG.

Scenario 1 mitigates MAETP by 2.65E + 05 kg 1,4-DCB eq/t treated waste material. This reduction can be traced to the energy recovery at the CHP plant as well as to the recovery of aluminum at the MBT plant, from the avoided use of fossil energy. Recycling of DFG at the Recycling Plant contributes to MAETP by 4.4E + 05 kg 1,4-DCB eq due to high energy consumption.

From Fig. 4f) it can be seen that HTP cancer-related impacts have a minor relevance compared to non-cancer-related impacts. Scenario 1 mitigates cancer-related impacts by 3.17E-10 cases due to the recycling of DFG at the Recycling Plant, where the production of HDPE granulates and to a lower extent, PP granulates outweighs contributions from MBT plant energy consumptions and emissions. Likewise, HTPnc impacts from the recycling of DFG are lower compared to that of the treatment of MML. The HTP results obtained for the treatment process at the MBT plant are in the same order of magnitude with findings from other studies, applying the same impact assessment method (Consonni et al. 2005b).

The developed WMSP shows some benefits as well as some drawbacks as regards environmental performance. In addition to its lower contributions to impact categories such as resource depletion, acidification and marine ecotoxicity, electricity and district heat is produced as well as plastic granulates at the DFG Recycling Plant. A sensitivity analysis was carried out varying the composition of the input materials to the waste treatment systems, in order to capture the relevance of spatial and temporal effects on the composition of marine litter. Five different variations of material compositions are considered. For the first two variations (V1 and V2), changes in the ratio between the collected DFG fraction and MML fraction were undertaken. For the other three variations, V3, V4, and V5, the ratio of the two fractions were maintained, whereas the proportions of the different materials in the MML fraction were changed.

The analyzed composition variations show different results depending on specific impact categories (Fig. 7). In general, for the CED and GWP, a diversion of plastic debris such as derelict fishing gear to recycling reduces associated environmental loads. However, for toxicity-related categories as well as for POCP and AP, treatment at the MBT plant in combination with an efficient metal recovery and energy recovery from RDF can be an adequate solution for the treatment of such waste material. Hence, an increase in the recovery of plastic litter contributes to a higher environmental performance of both recycling and waste-to-energy processes Table 6.

Major drawbacks of the WMSP are related to the significant contributions of stack emissions of the waste-to-energy process at the CHP plant to overall impacts. Thus, a key factor in improving the environmental performance of the treatment pathway for non-recyclable marine litter is the reduction of emissions during the energy recovery process downstream. Analysis on changes in material composition of collected marine litter indicates that an increase in the recovery of recyclable materials such as derelict fishing gear or aluminium and their subsequent recycling has a positive effect on the environmental performance of the waste strategy. In addition, an increase in plastic content in the mixed marine litter fraction delivered to the MBT plant leads to a higher calorific value of the produced RDF which again increases the energy output at the CHP plant. Hence, pre-treatment at MBT plant should be tailored to the specific spatial and temporal context in order to enrich the energy recovery fraction with high calorific material while recyclable materials such as aluminium are made available for further utilization.

The main aim of this study is to inform decision-making on the comprehensive management of marine litter investigating low environmental impact choices. The main obstacles and opportunities associated to the treatment of marine debris were considered, and a regional waste management strategy was developed and further evaluated according to the LCA methodology, highlighting strategic insights in addressing concerns of marine litter and in particular of marine plastics Table 8.

Findings of this study show that no single treatment method is appropriate rather a combination of different treatment pathways should be designed considering the composition and properties of accumulated marine litter in a specific area. In general, the diversion of useful materials from waste-to-energy to recycling improves the environmental performance. From state-of-the-art reviews, future assessments of WMSPs including different solutions for the valorisation of marine litter as a resource should consider alternative treatment options such as plastic-to-fuel technologies Table 9.

As part of this study, an LCA study was performed to analyze environmental impacts linked to the management of accumulated marine plastic litter. However, the change in mechanical and chemical properties of such debris typically from degradation mechanisms in the marine environment was not sufficiently considered due to lack of necessary data. The specific features of marine plastics can likely affect the outcome of treatment processes. Thus, a decrease of mechanical properties could lead to a lower quality of recycling products, or chemical pollutants commonly associated with marine plastic litter could result in an increase of toxic pollutants during plastic-to-fuel or waste-to-energy processes. Therefore, laboratory studies are needed to quantify the influence of the marine environment on plastics.

In designing the WMSP, a collection system is envisaged which incorporates the necessary pre-treatment stages as well as the main waste treatment processes. This can be in the form of a floating collection structure or onshore facility for collecting the marine debris from the sea surface and where sorting, shredding, and cleaning take place followed by treatment in compact treatment units that transforms marine litter to valuable products, for example, into fuel in the form of oil or syngas. Further research and assessment of a WMSP including the collection system will provide a more comprehensive picture and information for decision-making. However, the choice of the collection facility should be aligned with existing guidance on reducing impacts on biodiversity of ocean plastic patches (IMO 2018).

In addition, there is a need to adequately address marine plastic litter in Life Cycle Assessment in order to quantify associated environmental impacts of this pollution problem. Currently, there is a lack for appropriate impact assessment models (Sonnemann and Valdivia 2017). Therefore, more research in this field is needed to integrate marine plastic pollution in existing impact categories such as human- and eco-toxicity or resource demand or to develop new methodologies, considering for example the residence time of plastics in the environment or fate and exposure. Effect factors need to be developed that enable the quantification of impacts of plastic litter on marine biodiversity and human health (Bertling 2017).

Finally, it should be mentioned that downstream solutions for already accumulated marine plastic litter including collection and the utilization of it can only serve as a supplement to a complete waste management strategy. Preventive measures including a change in consumer behaviour and an improvement of recycling rates and plastic waste management are of highest priority to stop plastics from entering the marine environment.