# Microbial and Enzymatic Biodegradation of Plastic Waste for a Circular Economy
## 1 Introduction
Of the plastic waste generated worldwide, 49% is accumulated in landfills, 22% is classified as mismanaged (which ultimately ends up in landfills), 19% is incenerated, and only 9% is recycled. These figures vary among the top economies, with the United States having the highest landfilling rate (73%) while mismanagement rates range between 4% in the US and 46% in India. Landfill is the most common destination for waste plastic based on its simplicity and low-cost, as no capital cost is incurred except for the land (Figure 2). Landfills are chosen in many regions due to poor recycling infrastructure. Plastics which are landfilled pose a risk of leakage into rivers, lakes, and the ocean, creating water pollution [19]. Conversely, incineration is a thermal process used to treat plastic waste and concurrently reduce its volume. In some cases, the heat generated during incineration is utilised to produce electricity. However, incineration produces toxic byproducts, which contribute to air pollution. For example, [PERSON] et al. reported that approximately 70.2 million tons (29% of the total) of plastic waste was burned without regulation worldwide in 2016, releasing almost a million tons of toxic aerosols into the atmosphere, with the majority occurring in developing regions [20; 21]. Recycling is an alternative option which can be more environmentally friendly compared with landfilling or incineration. Mechanical  
Figure 1: Plastic waste disposal management by region, globally, and in major economies. This image was reproduced from OECD (2023) [13] and processed by Our World in Data. Mismanagement refers to accumulation in landfill. recycling involves segregation, cleaning, and reuse of the plastics. Whilst this approach reduces demand for virgin materials, the plastic created with this method can be of lower quality, whereas chemical recycling handles mixed plastics, creates new plastics, and offers a closed recycling loop. Chemical recycling processes release hazardous byproducts and require high amounts of energy [11]. The development of biodegradable plastics and bioplastics has had a significant impact on the problem of increasing volumes of persistent plastics in the environment. Biodegradable plastics are defined as materials which are completely degraded to carbon dioxide and water by the action of naturally occurring microorganisms, such as bacteria, fungi, and algae, and as such are more easily broken down under natural conditions [22; 23]. While the terms biodegradable plastics and bioplastics have been conflated and are used interchangeably, there is a notable difference between the two. As the name suggests, biodegradable plastics are degraded by bio-organisms, whereas bioplastics are produced or derived from biomaterials and are not necessarily biodegradable. For instance, some fossil fuel-derived plastics are biodegradable, like polybutylene adipate terephthalate (PBAT) and polybutylene succinate (PBS) [24]. However, polylactic acid (PLA), a common type of bioplastic (derived from fermented products), is degradable only in industrial composite sites and is not rapidly degraded under natural conditions [23]. Conversely, an example of non-biodegradable bioplastics are PETs derived from renewable resources, which are targeted to replace PETs from non-renewable resources [25]. Figure 3 shows the production rate of biodegradable plastics in 2023 [2]. Among various types, polylactic acid (PLA) and polybutylene adipate terephthalate (PBAT) together make up almost 60% of the bioplastics produced, while polyhydroxyl alkanoate (PHA) and polybutylene succinate (PBS) production is less than 10%. Starch-based polymers and blended plastics are beginning to emerge with strategically improved mechanical properties. These types of plastic offer a promising alternative due to their enhanced biodegradability and potential for lower environmental impacts than conventional plastics [26; 27].