1 Introduction
Materials enable us to meet our daily demands like housing, transport, communication or packaging. With a growing world population as well as economic development, the demand for materials is predicted to more than double from 2015 to 2050 (UNEP 2017). A material group, which has become more and more essential in the 20th century, are plastics (Geyer et al. 2017). In 2015, the global production of plastics has been 322 million tonnes and is estimated to further grow within the next years. Plastics are versatile materials, which are used in many different application areas like packaging, building and construction, automotive, medical or electrical & electronic appliances. In Europe, the market is dominated by short life applications like packaging with 39.9% of total plastic material demand (PlasticsEurope 2016). Currently the main share of these plastics is made up by fossil-based resources like crude oil. Around 4% of the world’s annual oil production is used for plastics (Kreiger et al. 2014). Against the background of the finiteness of fossil resources as well as the challenges of global warming, the development of bio-based alternatives has been in the focus of research in recent years (BMBF 2014).
Bioplastics can be either bio-based and biodegradable (e.g. PLA—Polylactide) or bio-based and durable (e.g. Bio-PE—Bio-Polyetyhlene). The third type of bioplastic is fossil-based and biodegradable, e.g. PCL (Polycaprolactone) (Endres and Siebert-Raths 2011). Furthermore, it is important to distinguish between chemical novel bioplastics and so called drop-in bioplastics. Chemical novel bioplastics, (e.g. PHA—polyhydroxyalkanoate) have a unique chemical structure and therefore have unique properties, processing profiles as well as waste management demands. The drop-in types (e.g. Bio-PE) have the same chemical structure as their conventional counterparts and differ only in terms of feedstock’s (biomass instead of fossil resources). Therefore, their processing, utilization and disposal properties are identical to the conventional ones. In 2016, the production capacity of bioplastics amounts to merely 2.05 million tonnes and therefore constitutes currently just a small market share in comparison with conventional plastics. A growth of up to 9.20 million tonnes of bioplastics has been forecasted for 2021. The main share of bioplastics are bio-based plastics, which are based on biomass feedstock like sugar cane or corn (IfBB 2017). With a further evolving bioeconomy, the overall demand for biomass will increase as its usage in fuels, energy, heating and materials will broaden (Scarlat et al. 2015). However, given the limited biomass production capacities, resource efficiency and circular economy thinking are not only immanent for limited fossil-based product systems but also for bioeconomy ones like bioplastics. Defining what a ‘Circular Economy’ (CE) is, needs a multi-faceted approach. There exist different definitions for CE and there have been instances, where the understanding of CE (Geissdoerfer et al. 2017) and its different definitions have been extensively analyzed (Kirchherr et al. 2017). From these publications, it can be found out that defining a CE can be subjective and there are various possibilities for defining CE (Lieder and Rashid 2016). Out of all these definitions, the most famous definition of CE has been provided by Ellen MacArthur Foundation (Ellen MacArthur 2012), which reads: “CE is an industrial system that is restorative or regenerative by intention and design. It replaces the ‘end-of-life’ concept with restoration, shifts towards the use of renewable energy, eliminates the use of toxic chemicals, which impair reuse, and aims for the elimination of waste through the superior design of materials, products, systems, and, within this, business models”. While plastic production capacities are growing, the circular economy thinking has not been fully implemented yet (World Economic Forum 2016). This is also applicable for bioplastics. As highlighted by Geyer et al. (2017) in the timeframe of 1950 until 2015, about 8300 million tonnes of plastic have been produced. The main share of these plastics (4900 million tonnes) has been discarded (e.g. in landfill or litter into the environment), while 800 million tonnes were incinerated and just 600 million tonnes recycled. Another 2500 million tonnes are still in stock.
Life cycle of bioplastics and loop of different waste treatment options
It is also important to distinguish between pre- and post-consumer (bio-) plastic waste. Pre-consumer waste occurs in the polymer production and processing step (e.g. off-grade material or scrap) and is usually not subject to sorting as it is homogeneous (same plastic type). Post-consumer waste occurs after the use phase and can be contaminated e.g. with food waste, is subject to collection and sorting and therefore has usually a low homogeneity.
To develop optimal pathways for bioplastics within a circular economy, it is important to develop indicators, which take technical implications on material properties as well as environmental or sustainable (environmental, social and economic) impacts of different waste treatment options into account. This study aims to contribute to a stronger integration of circular economy thinking into the bioplastic development and thereby creating more sustainable bioplastic value chains. To do so, the study will review current approaches to identify optimal circular economy pathways (e.g. performance indicators) in the area of conventional plastics and adapt them to bioplastics. The focus will be set on the biodegradability as a unique aspect of waste treatment options of some bioplastic types.
2 Review of Circular Economy Approaches for (Bio-) Plastics
To review the circular economy approaches, a literature review has been done using established scientific search engines like web of science, science direct or springer link using a search word combination of ‘circular economy’ and ‘(bio-) plastic’ terms. While the topic has been discussed theoretically in many studies, few have developed or incorporated indicators. To assess the circularity, resource efficiency and environmental performance of bioplastics, it is essential to develop metrics and performance indicators for the corresponding products. Several approaches to assess the circularity of a product system have been developed so far. The cyclical use rate indicator for the reused and recycled material input (Ministry of the Environment, Japan 2003), the material circularity indicator to assess circularity at product and organization levels (Ellen MacArthur Foundation and Granta 2015) and the circular economy index to measure circularity based on the recycled material value (Di Maio and Rem 2015), to name a few. However, there is no standardized method for measuring the circularity of products (Linder et al. 2017) as of now. In addition, most of the above-mentioned studies are generic and are not product specific, which makes it difficult to interpret and apply them to plastics.
Extended framework for circular economy and bioplastics (adapted from Huysman et al. (2017) for bioplastics)
Option | Waste treatment option | Actual environmental benefit | Ideal environmental benefit |
|---|---|---|---|
Option Ia* | Re-use | Vα − R | V α |
Option Ib | Mechanical recycling | r × Vα − R | V α |
Option Ic* | Chemical recycling, Anaerobic digestion | r × Vα − R − T | V α |
Option II | Mechanical recycling | r × p × Vα − R | p × Vα |
Option III | Mechanical recycling | r × m × Vβ − R | m × Vβ |
Option IV | Incineration, Anaerobic digestion | E − I | E |
Option V* | Aerobic digestion | B − I | B |
3 Extended Framework for Circular Economy and Bioplastics
Bioplastics can offer additional waste treatment options in comparison to conventional plastics. While drop-in bioplastics can use the existing waste treatment structures of conventional plastics the chemical novel plastics might need modified structures or additional collection streams and separation processes. Biodegradable plastics can undergo aerobic (industrial/home composting) or anaerobic digestion (biogas plant) and thus provide further options of waste stream management. Therefore, it is important to develop circular economy performance indicators, which also include the biodegradable types of bioplastics. In Table 1, an advanced framework for covering waste treatment options of biodegradable plastics extended from Huysman et al. (2017) is presented. The extensions are highlighted with a “*”.
The framework from Huysman et al. (2017) has been extended for the waste treatment options aerobic and anaerobic digestion, which are unique to bioplastics. Furthermore, chemical recycling and re-use were added, as it is important to consider them in a circular economy. However, these waste treatment options are also suitable for conventional plastics. In extension to the framework, options Ia, Ic and V have been added. Following the Huysman et al. (2017) approach, the quotient of actual environmental benefit and ideal environmental benefit form the circular economy performance. An indicator with “1” as value is therefore an ideal circular economy option. The new proposed framework is set up for pre-consumer recycling. Post-consumer recycling has to take collection and sorting into account, which might differ for various waste treatment options. In addition, it is important to highlight that not all bioplastics are suitable for all pathways (e.g. Option Ib—anaerobic digestion is suitable for a few bioplastics like PHA but not for durable biobased plastics like Bio-PE, as PHA is biodegradable and resulting biogas can be fed back into the PHA value chain). For option Ia, the waste treatment option is re-use. The actual benefit is set up of Vα (avoided impact of virgin material) minus the environmental impact of the refurbishment process (R). Accordingly, the ideal environmental benefit is Vα. Option Ib is mechanical recycling, taking the recycling rate as well as the environmental impact of the recycling process into account. Option Ic for chemical recycling and anaerobic digestion also takes the impacts of the processing back to plastic grade into account as both waste treatment options generate intermediates like biogas or different bio-based oligo-or monomors, which can be used as feedstock for a new (re-)polymerization process. However, the resulting material has therefore the same properties as virgin material and no quality loss. Option II and option III consider mechanical recycling of lower quality, where either additional virgin material is needed or another secondary material is substituted. Option IV include incineration and anaerobic digestion. The actual environmental benefit is calculated with obtained amount of energy (electricity and heat) from direct incineration of plastic or biogas minus impact of incineration or biogas process. This means that option IV focuses on the energy usage of the reaction products of the anaerobic digestion process, while option Ic focuses on their usability as polymer feedstock. Option V is aerobic digestion with the actual benefit of the benefit from produced biomass as a digestion residue minus impact of aerobic digestion process (composting). The potential benefit and associated credit of the created biomass during aerobic digestion have to be further investigated. However potential benefits of biodegradability e.g. in connection with marine litter are not taken into account in this framework.
The highlighted formulas can be used to calculate the CPI for different waste treatment options and impact categories of bioplastics. Thereby more optimal pathways from technical and environmental perspective can be chosen. A challenge however is the incorporation of technical aspects. Huysman et al. (2017) use the quality factor to assess which option is, from a technical point of view, best suitable for the waste stream. However, Huysman’s proposed quality factor is solely based on compatibility (with current assumptions like binary compounds), just preliminary and needs further research. Beside other technical quality aspects, it is also important to assess the impact of a waste treatment option on the next waste treatment phase in further life cycles stages (cascade use). As different waste treatment options might have a different impact on this issue, such aspects also have to be considered. E.g. mechanical recycling of (bio-) plastics might cause the reduction of material quality in several life cycle stages, while chemical recycling has the opportunity to create recycled material without significant quality loss.
4 Conclusion and Outlook
To identify optimal circular economy pathways, it is important to develop performance indicators. For bioplastics, no performance indicators have been developed yet. As bioplastic value chains are currently established, it is important to provide this information to foster circular economy. As bioplastics provide additional waste treatment options like biodegradation, indicators have to be included also for aerobic and anaerobic digestion. The proposed framework provides indicators for bioplastics and considers these aspects for pre-consumer waste. Seven formulas enable to determine the circular performance for all available waste treatment options for bioplastics. When considering post-consumer (bio-) plastic waste, it is important to include the impact of collection and sorting processes as well. In addition, these might differ for various waste treatment options.
In the next step, this framework will be applied for different case studies on a product level, in cooperation with producers. Furthermore, the concept will be extended to post-consumer waste, by including collection and sorting procedures.
Acknowledgements
The authors thank hereby the German Federal Ministry of Education and Research as well as the project executing organisation within the German Aerospace Center (DLR) for the funding and support of the research project “New pathways, strategies, business and communication models for bioplastics as a building block of a sustainable economy” (BiNa) within this research has been conducted.