Touted for its versatility, durability, convenience, and cost-effectiveness, plastic has become an indispensable part of our daily lives. It is used across various sectors, including packaging, construction, and healthcare. However, this reliance on plastic comes at a significant cost—approximately 360 million tonnes of plastic waste are generated annually, with a major portion either improperly managed or unrecycled. As global awareness of environmental challenges grows, addressing plastic waste is imperative, calling for a transformation from a linear ‘take-make-dispose’ model to a circular economy.
At the core of combating plastic waste is recycling, an essential process for producing new products or packaging from existing plastic waste. Effective recycling systems maximize recycling rates, retain material value, and strive to reduce the carbon footprint. Recycling is generally classified into two main categories: mechanical and chemical recycling. However, adopting a single approach is often insufficient. Instead, integrating multiple technologies in what is known as ‘cascade recycling’ can effectively retain materials in circulation, optimizing their economic and environmental value.
Mechanical recycling is the most established form of recycling. It converts plastic waste into recyclates without significantly altering the material’s chemical structure. This method is cost-effective and environmentally beneficial; studies indicate that mechanical recycling can save an estimated 1.4 to 2.1 tonnes of carbon dioxide per tonne of plastic recycled by reducing the need for virgin materials. Under ideal conditions, materials like PET bottles can be mechanically recycled back into new bottles or high-value textiles.
In a broader context, a closed-loop supply chain aims to reuse or recycle materials, thereby mitigating waste. Closed-loop recycling—where recycled materials are used to create products of similar quality—remains the ideal choice. However, barriers such as inadequate waste segregation, collection, and sorting hinder its effectiveness. Notably, about 2.7 billion people worldwide still lack organized waste collection services, and many less-developed countries exhibit minimal efforts at source segregation. Furthermore, the presence of mixed plastic types and organic contamination compromises the quality of available plastic feedstock, undermining closed-loop recycling initiatives.
When high quality is not available, mechanical recycling may lead to ‘downcycling’, a process where recycled materials are transformed into lower-quality products. These might include garden furniture, basic construction materials, and certain textiles. While downcycling does not match closed-loop recycling economically and environmentally, it still provides benefits over producing new virgin plastic or sending plastic to landfills. The extended life cycle of downcycled products aids in preserving the carbon emitted during manufacturing, preventing it from entering the atmosphere immediately.
Nevertheless, both closed-loop and open-loop recycling face challenges in scalability, primarily due to market limitations for downcycled products. Additionally, mechanical recycling cannot be performed indefinitely; as plastic deteriorates through multiple cycles, the quality eventually falls below acceptable thresholds for many applications.
Turning towards chemical recycling offers potential solutions for more complex recycling challenges. Through processes such as pyrolysis, gasification, and depolymerisation, chemical recycling technologies can break down degraded plastics back into raw materials. These materials can then serve as substitutes for fossil feedstock, generating plastic of a quality comparable to virgin material, which is crucial for demanding applications like food or medical packaging. While chemical recycling presents advantages—a higher potential recovery of specific plastic types—it often has a higher carbon footprint and operational costs than closed-loop and open-loop recycling.
Given the complexity of plastic recycling, it becomes evident that no single recycling method is ideal. Rather, a series of complementary technologies should work together to enhance material circularity. This is where the concept of cascade recycling plays a vital role. Cascade recycling maintains that mechanical recycling should be the first step, followed by downcycling, and only then transitioning to chemical recycling when necessary. For example, the lifecycle of a PET beverage bottle illustrates this process. The bottle is mechanically recycled into a new beverage bottle as often as possible, and once its quality is compromised, it is downcycled into lower-grade applications before considering chemical recycling to restore its virgin quality.
As it stands, fewer than 10% of the world’s plastic waste is currently recycled, indicating that substantial investments are imperative to scale recycling efforts and create an effective cascade encompassing closed-loop, open-loop, and chemical recycling. However, innovative technologies are emerging rapidly, showcasing potential advancements in scaling, reliability, and efficiency in plastic recycling. By harnessing renewable energy and adopting heat-integrated designs, we can pave the way for improved recycling processes.
In conclusion, the journey towards a circular economy for plastics is gradual, marked by each step in the cascade recycling process. The path begins with vigorous mechanical recycling, transitions through open-loop downcycling, and ultimately focuses on chemical recycling when other options are exhausted. By repeating this cycle, we will significantly enhance recycling rates and foster a more sustainable approach to plastic waste management.
