Study of the biodegradation of pure and commercial PLA (polylactic acid) under anaerobic conditions
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Date
2026-07-07
Authors
Καπαδούκας, Ανδρέας Νικόλας
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Abstract
Plastic production has reached approximately 400 Mt annually, while recycling remains limited, leading to widespread environmental accumulation and fragmentation into micro- and nanoplastics with adverse ecological and human health impacts. In response, bio-based and biodegradable plastics have emerged as part of a circular economy strategy, although their sustainability depends on feedstock selection, material design, and appropriate end-of-life management.
Bioplastics include bio-based non-biodegradable polymers (e.g., bio-PE, bio-PET), bio-based biodegradable polymers such as polylactic acid (PLA) and polyhydroxyalkanoates (PHAs), and fossil-based biodegradable polymers like PBAT, PBS, and PVA. Although currently representing less than 2% of global plastic production, their share is expected to grow significantly. Among them, PLA is the most widely used due to its favorable mechanical properties and processability.
Anaerobic digestion (AD) is a promising end-of-life option for bioplastics, enabling simultaneous waste treatment, renewable energy production (biogas), and nutrient recovery (digestate). It is a four-stage microbial process (hydrolysis, acidogenesis, acetogenesis, methanogenesis) applied to various organic wastes and increasingly to bioplastics.
Despite being biodegradable, PLA exhibits slow degradation under mesophilic and natural conditions due to its moderate to high crystallinity and glass transition temperature, which limit hydrolysis. As a result, untreated PLA shows low methane yields and long degradation times. Thermophilic AD though, improves anaerobic digestion rates, methane yields and overall biodegradability, although extended digestion times and microbial acclimation are still required.
To enhance PLA biodegradability, two main strategies are employed, pretreatment and co-digestion. Pretreatment methods, including mechanical, thermal, chemical, and enzymatic techniques, aim to reduce polymer recalcitrance by increasing surface area, decreasing crystallinity, and promoting depolymerization. While effective, these methods must balance performance gains with energy input, chemical use, and operational complexity.
Co-digestion with substrates such as food waste, sewage sludge, or manure improves process stability by supplying nutrients and buffering capacity, usually leading to higher methane yields and improved biodegradation. However, incomplete degradation of PLA can result in residual fragments in digestate, posing risks of microplastic contamination in soils.
This work examines the anaerobic digestion performance of various PLA samples at thermophilic temperature conditions. Firstly, a thorough review of the theoretical background of anaerobic digestion was conducted, deriving information on the factors that affect the process, the appropriate equipment, the criteria for selecting raw materials, as well as how to set up the experiment and, by extension, the entire process, which are presented in the first section of this thesis report. The experimental procedure is then presented, including the experimental techniques used, the experiment’s set-up procedures, as well as the analysis and processing of the experimental data. Emphasis is placed on the proper pretreatment of the raw materials. The bioplastics were sourced from abroad (high purity PLA) as well as domestically from local companies (commercial PLA), and their structure and chemical identity were characterized using TGA, FT-IR, SEM-EDX, DSC, BET, Raman spectroscopy, 1H-NMR & 13C-NMR and VS analysis. Additionally, food waste was obtained from the university cafeteria, and its physicochemical analysis was performed in the laboratory, including carbohydrate, protein, and lipid content, along with elemental analysis and measurements of Chemical Oxygen Demand (COD), pH, as well as the total and volatile solids. Similar analysis was performed on the anaerobic inoculum which was sourced from a full‑scale mesophilic anaerobic digester located in Agrinio (Western Greece). After a physicochemical profile was established for all raw materials, the materials were loaded in predetermined ratios for the BMP and CSTR experiments which were performed under constant thermophilic conditions, while all data was monitored and recorded for subsequent analysis. Furthermore, mathematical modeling of the data using the Gompertz method quantified critical factors of the process, such as maximum methane yield, maximum methane production rate, and the lag phase at the beginning of the experiment.
Through the BMP experimental results, it was concluded that PLA mono-digestion under thermophilic conditions results in significantly higher PLA biodegradation (approximately 83.9-100% versus 2.6-10.9%) and methane yield (427.8-487.5 mL CH4 g-1 VS versus 12.0-55.8 mL CH4 g-1 VS) when compared to previous mesophilic BMP results performed under otherwise identical conditions. Additionally, PLA co-digestion with FW resulted in faster digestion when compared to mono-digestion and it was concluded that the co-digestion with FW has a neutral or enhancing synergic effect on the methane yield, signaling to the possibility of integrating bioplastics into existing digestion systems without causing a negative synergic effect. Furthermore, when comparing thermophilic and mesophilic co-digestion with FW, a biodegradation improvement of 28.8 to 34.2% was observed, as well as a methane yield improvement in the range of 40 to 51.3%.
CSTR experiments were conducted firstly on FW mono-digestion, in order to create a reliable reference state for the subsequent FW and PLA co-digestion, ensuring that any changes observed can be attributed to substate effects. Then, after the introduction of PLA, with a ratio of 15% PLA and 85% FW on a VS basis, a final methane yield of 430.7±9.7 NmL L-1 d-1 was observed, a value which is very close to the FW mono-digestion result of 428.9±8.3 NmL L-1 d-1, indicating that the addition of PLA leaves the overall methane productivity essentially unchanged and does not impair long-term process performance under thermophilic conditions.
These results indicate that thermophilic operation improves performance both in PLA mono-digestion and co-digestion with FW, though the choice between temperature conditions must include various parameters and is specific for each operation example. In addition, co-digestion with FW showed neutral to even enhanced synergic effects, which suggests that PLA does not impair digestion performance, when it is added to FW digesting systems, while it also improves digestion kinetics.
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Keywords
Polylactic acid, Anaerobic digestion, Bioplastics, Biodegradation, Thermophilic conditions