**1. Introduction**

Single-use plastic consumption has been increasing for years due to its durability, light weight, and low cost [1]. The use of plastic has led to many technological advances, including high strength-to-weight ratio construction, automotive materials, and highly resistant packaging materials for food [2]. Approximately 9.2 billion tons of plastic have been produced worldwide, and the annual global production of plastic increased to 368 million tons in 2019 [3,4]. As estimated, the annual production of plastic waste is 34 million tons, and 93% of it is disposed of in landfills and oceans [5]. In 2015, 322 million tons of petroleum-based plastic were produced globally, compared with 1.7 million tons in 1950 [6]. Synthetic petroleum-based plastic leads to an increase in plastic waste, which contributes to adverse effects on the environment, such as ozone depletion, eco-toxicity, the release of carcinogens, global warming, and eutrophication [7]. Approximately 2.8 kg of CO<sup>2</sup> is released into the environment when 1 kg of plastic is burned [8]. Bioplastics emerged in response to environmental concerns about non-biodegradable plastics.

In the circular economy, bioplastics are expected to play an important role in achieving sustainable development goals, such as avoiding fossil fuels, introducing new degradation or recycling approaches, and reducing toxic chemicals during the manufacturing process. Biodegradable plastics derived from renewable biomass have become increasingly popular

**Citation:** Ahsan, W.A.; Hussain, A.; Lin, C.; Nguyen, M.K. Biodegradation of Different Types of Bioplastics through Composting—A Recent Trend in Green Recycling. *Catalysts* **2023**, *13*, 294. https:// doi.org/10.3390/catal13020294

Academic Editors: Zhilong Wang and Tao Pan

Received: 24 December 2022 Revised: 22 January 2023 Accepted: 25 January 2023 Published: 28 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and bioplastics are currently produced on a scale of 4 million tons annually [9]. Globally, bioplastic production is expected to increase from 2.11 million tons in 2019 to 2.42 million tons by 2024. A major market for bioplastics is the packaging industry, which accounts for nearly 40% of global production [10]. Although many reviews discuss bioplastics, few address the positive and negative impacts of bioplastics on the environment comprehensively and simultaneously [11]. Nonetheless, not all polymers that are derived from bio-based sources are biodegradable, and not all polymers that are derived from fossil sources are non-biodegradable [12]. In nature, bioplastics are primarily composed of renewable resources, such as cellulose, starch, sugar, etc. [13]. In fact, biodegradation rates differ among bioplastics, and biopolymer properties depend on external environmental factors, intrinsic biopolymer properties, and filler properties in blends and composites [14]. In addition to their original source, production processes also have a great deal to do with degradation [15]. Moreover, many reports show that bioplastic composites and films degrade slowly in normal water and soil environments [16]. Due to this, there are concerns about their disposal in landfills and in soils at the end of their useful lives. Thus, composting bioplastics becomes an important tool for their effective environmental management at end-of-life.

Composting is considered more environmentally friendly and cost-effective than recycling or incineration. Specific microorganisms, such as Pseudomonaceae, Comamonadaceae, Erythrobacteraceae, Streptomycetaceae, Caulobacteraceae families, and Enterobacteriaceae, and enzymes, such as N-acetyl-β-glucosaminidase, esterase, β-glucosidase, acid phosphatase, alkaline, and phosphohydrolase, are involved in the degradation and microbial decomposition of bioplastics [17,18]. Specifically, enzymatic decomposition has been regarded as a means of minimizing environmental pollution. Microbiological degradation of bioplastics, particularly microbial enzymatic catalysis, has drawn attention as a means of reducing the amount of pollution in the environment.

The process of composting involves decomposing organic matter and turning it into humus, which can be used to strengthen soil structure and its fertility rate [19,20]. Bioplastic waste is typically disposed of in landfills, followed by recycling, incineration, and composting [16,21]. In contrast, landfilling produces greenhouse gases and creates environmental concerns. Landfilling not only produces greenhouse gases but also occupies and contaminates future agricultural land [22,23]. Therefore, composting would be a more profitable and desirable method for disposing of bioplastic waste. As a cost-effective and safe waste management solution, composting technology is being adopted by several industries [24]. In the literature, industrial composting of bioplastics has been demonstrated to be one of the most desirable methods for managing the material's end of life [25].

Compostable polymers are being developed as environmentally friendly alternatives, especially if they can be recycled organically and derived from renewable resources. Using lifecycle assessment techniques, ASTM D7075 and ISO 14000 have developed standards to evaluate biobased products and their environmental performance [26,27]. However, only some of the biopolymers are listed as compostable materials by ASTM. In order for a polymer to be considered compostable, it must convert 90% of its carbon content to carbon dioxide in accordance with ASTM International (D5338). An ASTM International (D5338) polymer can only be considered compostable if 90% of its carbon content is converted into carbon dioxide. This prepared polymer undergoes three primary steps in order to become biodegradable: biodeterioration, fragmentation, and assimilation [28]. In addition, plant-based polymers, thermoplastic starch, polyhydroxyalkanoates (PHAs), and polylactic acid or polylactide (PLA) are commonly reported as biopolymers [29]. It is important to know that a number of factors affect the biodegradation rate of biopolymers in nature, such as their chemical structures, functional groups, crystallinities, and polymer chains [8]. Furthermore, temperature, oxygen, and pH content play a significant role in polymer biodegradation [30]. It has been reported that PLA degradation in the soil is much slower than in compost medium because compost has a higher moisture content and temperature range encouraging PLA hydrolysis and the assimilation of PLA by thermophilic microor-

ganisms. According to [31], Zn was used as catalyst in PLA depolymerization, but the problem with these catalysts was that they could not be recycled or re-used. However, ref. [32] reported that the degradation of PLA in soil takes much longer than in compost medium due to thermophilic bacteria which are able to hydrolyze and assimilate PLA with a higher temperature range and moisture content in the compost. After 47 days of composting, it was determined that the average rate of biodegradation for cellulose was 96.8 ± 6.7% [33] according to standard composting methods [34]. In addition, the composting of biobased polymers and the use of the compost in agriculture can result in significant emission and energy credits. Biobased polymers can be made even more sustainable through composting, which is an integral part of sustainable agriculture practices.

This review aims to gather information about the biodegradation of bioplastics in diverse environments and to discuss it to examine the compostability rate of different types of bioplastics through composting. Finally, this review concludes by discussing the composting technology in the biodegradation of bioplastics as well as classifications of different bioplastics according to the degradation rate through home and industrial composting.
