Purpose
Bio-based biodegradable plastics led by polylactic acid (PLA) are becoming increasingly popular as a sustainable alternative to traditional plastics. Although bioplastics are designed to break down easily, many do not fully degrade in the natural environment as intended. Anaerobic digestion (AD) is a promising solution for decomposing bioplastics alongside food waste, turning them into biogas for energy and digestate, rich in nutrients, that can be used as fertilizer. However, studies have shown that bioplastics, particularly PLA, does not degrade fully in AD systems. The byproducts left over from AD raise serious problems, especially if residual microplastics could still be present in the digestate and affect the quality of the soil and water. While existing research predominantly focuses on enhancing methane production and biodegradation efficiency during AD, the quality of digestate after the digestion process has been overlooked. This significant research gap was highlighted in this review, emphasizing the need for comprehensive studies that evaluate digestate composition alongside biogas production.
What Did We Do?
A systematic review of peer-reviewed studies was conducted using databases such as Scopus, ScienceDirect and Google Scholar to assess research on bioplastic degradation in anaerobic digestion. The literature search was performed using the keywords ‘bioplastic degradation’, ‘pretreatment methods,’ ‘Anaerobic digestion,’ ‘biodegradation,’ ‘biogas production’ and ‘digestate quality’. Search filters were applied to prioritize recent studies (2010-present), peer-reviewed journal articles, and experimental studies analyzing bioplastic degradation and digestate quality. The initial search yielded 172 papers, which were then screened for relevance based on their focus on bioplastic degradation, biogas production, and digestate analysis. After filtering out studies that were not directly related, 42 papers were selected for detailed analysis. A significant portion of the literature examined the effectiveness of different pretreatment methods in improving bioplastic degradation. These methods included but not limited to thermal pretreatment, where the plastics are exposed to elevated temperatures to increase its hydrolysis potential; alkaline pretreatment, which involves chemical treatments to enhance polymer degradation; and thermo-alkaline pretreatment, a combination of heat and chemical treatment to increase its susceptibility to decomposition. This allowed us to assess the extent to which bioplastic degradation has been addressed and the incomplete degradation persisting, highlighting the need for more comprehensive studies into the digestate quality.
What Have We Learned?
Studies consistently showed that bioplastic, especially PLA, degradation in AD remains incomplete in most cases, leading to concerns about the accumulation of microplastics residues in digestate. While pretreatment methods have been effective, with thermo-alkaline pretreatment yielding the highest methane outputs across most studies. The variability in methane yields across different pretreatment conditions suggests that degradation efficiency is highly dependent on factors such as temperature, retention time, microbial communities, and chemical additives. However, very few studies have explicitly analyzed whether residual bioplastic particles persist in the digestate post-AD. Given that AD is promoted as a promising solution for sustainable plastic waste solution, failing to assess digestate composition may lead to unintended environmental consequences. The implications of these findings are significant, particularly for large-scale implementation. If AD-derived digestate is to be used in agriculture or soil restoration, it must be free of persistent microplastics. Without comprehensive digestate analysis, the environmental benefits of AD for bioplastic waste management remain uncertain.
Future Plans
We are currently conducting an experimental study to evaluate the degradation of PLA in AD under different pretreatment conditions – thermal, alkaline, and thermo-alkaline treatments – to enhance PLA degradation and improve methane yields. More importantly, we aim to go beyond methane production by analyzing the resulting digestate for microplastic residues and overall chemical composition. Future studies will involve optimizing pretreatment strategies to minimize microplastic residues and investigating the long-term impacts of digestate when applied to soil systems.
Authors
Presenting & corresponding author
Nadia Bawa Fio Bekoe, Graduate Research Assistant, Biosystems and Agricultural Engineering Department, Oklahoma State University, nbekoe@okstate.edu
Additional author
Douglas W. Hamilton, PhD, P.E., Associate Professor and Extension Waste Management Specialist, Biosystems and Agricultural Engineering, Oklahoma State University
Acknowledgements
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- South Central Sun Grant Program Fellowship
- Livestock & Poultry Environmental Learning Community (LPELC) Professional Development Grants
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