Recycling marine litter has hardly played a role in research and in the public sphere up to now. Yet the potential for recycling is huge. In addition to known processes such as mechanical recycling, two other processes could play a major role in the future, especially for marine litter: thermochemical recycling and biotechnological upcycling - or even a combination of both.
It sounds paradoxical in view of the plastic waste problem, but in fact: so far, scientist paid little attention to the question of how marine litter can be recycled and at best brought back into the cycle. The reason: there is simply too little data on what kind of plastic the litter is made of, whether it is contaminated with pollutants and how much can be recycled at all. However, this data is important for researchers in order to identify suitable recycling processes.
In general, the most obvious approach is so-called material recovery, i.e. material recycling or mechanical recycling. In this process sorted types of plastic are first cleaned and then shredded thus producing recyclates, that can be converted into new plastics products – a new product is created with reused material instead of new one. Indeed, this method has a lot of potential and is currently the most used method to recycle post-consumer plastic waste streams. So-called thermoplastics such as polypropylene (PP), polyethylene (PE) or polyethylene terephthalate (PET) are suitable for mechanical recycling.
Unfortunately, you cannot choose the composition of marine litter. It consists of many different plastics and can therefore be described as very heterogeneous and random: no sample is like the other. In addition, material recycling is not feasible for complex material composites, heavily contaminated or partially degraded materials, which are also predominantly involved in marine litter.
Thermal treatment for energy recovery is a safe option here, with one problem: the material is taken out of the circular economy. You could also say, the material is lost. An increasingly popular alternative with regard to recycling is chemical recycling, in which the plastics are broken down into their chemical components by (thermo)chemical processes, such as gasification and pyrolysis, thus making an industrial reuse possible.
This is exactly where the current research of the authors of the recent study ‘Comparative Analysis of the Behaviour of Marine Litter in Thermochemical Waste Treatment Processes’, published in the journal Processes comes in. In this study, the Institutes of the RWTH Aachen University, the institute of Energy Raw Materials (TEER), Microbiology (iAMB) and Water Management (IWW) and the research team of everwave have investigated how plastic waste can be thermochemically recycled.
“This study is the first to analyse and evaluate chemical recycling (pyrolysis, gasification) and energy recovery (incineration) of marine litter”
Hee, J et al. Processes 2021
The marine litter was collected by volunteers on the coasts of Norderney and Sylt and made available to everwave for research purposes.* The great heterogeneity of the collected waste was combined into six different material groups: 3D plastics, films, metals, nets, rubber & elastomers and foamed plastics (picture below).
The materials shown in the picture were then shredded, mixed and examined within the three above-mentioned processes. And indeed: the results show that the mixed and weathered material mixtures can basically be treated by thermochemical processes and do not pose any major challenges to the state-of-the-art technologies.
The conclusion of the researches: “The results presented in this study demonstrate the principal possibility of thermochemical treatment of ML. [..] Pyrolysis as part of a chemical recycling process needs extensive pretreatment to remove impurities, for example, by crushing and sorting. The obtained condensable fractions present a valuable product for material recovery.”
Hee, J et al. Processes 2021
However, there is more to it than that! The study also presents an outlook on how the condensates produced by the pyrolysis process are biotechnologically, i.e. with microbes, converted and upcycled into valuable and environmentally friendly molecules. Unlike other biotechnological upcycling approaches, in which microbes can use the plastic monomers as a substrate converting those into new products, here they use the pyrolysis condensates as a substrate. It is also made clear that the biotechnological use of condensates poses many challenges; after all, it is still in its infancy. However, there are already versatile candidates among the microbes that could be able to meet these challenges. Once again, this shows what all-rounders microorganisms are.
"The valorisation of pyrolysis oils is challenging, especially pyrolysis oils from mixed plastic fractions such as marine litter. A valuable material is produced from a large number of different molecules or chemicals. In order for this not only to work in a rudimentary way, but to really contribute to the recycling of plastics, and thus to a reduction in the use of fossil raw materials, we are working across disciplinary boundaries."
- Mentioned Prof. Lars M. Blank, co-author of the study to everwave.
This combination of recycling and upcycling is an innovative approach. In this way, even plastic waste that is unsuitable for classic mechanical recycling can be reintegrated into a circular economy. The data collected in this study form a basis for the evaluation and application of possible treatment options for marine litter and offer starting points for further research to return marine litter to the circular economy in the future.
We look forward to hearing more from this new field of research!
The publication can be viewed and downloaded free of charge here : https://www.mdpi.com/2227-9717/9/1/13.
PS: part of the collected marine litter will also be used in the MIX-UP project for future experiments. Here, too, we are excited about news and will keep you posted!
*Many thanks to the ‘FÖJler’ (FÖJ: Voluntary Ecological Year) and volunteers of the Wadden Sea Visitor Centre Norderney and the Erlebniszentrum Naturgewalten Sylt for collecting and providing the marine litter!