Multi-faceted analysis of thermophilic anaerobic biodegradation of poly (lactic acid)-based material

Currently, the production of bio-based polymeric materials, of which poly(lactic acid) (PLA) is the most popular, has been increasing, causing the growth of PLA waste in municipal waste. Thus, it is necessary to develop sustainable methods for treating it. Methane production, resulting from anaerobic digestion (AD), is a potential end-of-life scenario for PLA waste that needs to be investigated. To obtain high efficiency of AD, thermophilic fermentation was applied, and to overcome low rates of biodegradation, hydrothermal (HT) and alkaline (A) pretreatments were used. For a deep insight into the process, differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), and microscopic and microbial analyses (based on 16S rDNA) were applied. For both untreated (PLA) and pretreated (PLA(HT), PLA(A)) samples a high maximal methane production (MP) of 453 L/kg volatile solids (VS) was obtained, almost 100 % of the theoretical methane yield from PLA. The use of pretreatment allowed shortening of the time for obtaining maximal MP, especially the hydrothermal pretreatment, which shortened the overall time of MP 1.3-fold, and methane was produced at an almost 10 % higher rate (8.35 vs 7.79 L/(kg VS center dot d)). However, DSC and microscopic analyses revealed that, in all cases, methane was intensively produced i) after the reduction of the molecular mass of the PLA material and ii) also when PLA pieces were not visible. This should be considered when designing the operational time for the AD process. Parallel to the gradual biodegradation of PLA, the abundances of Firmicutes, Thermotogae, and Euryarcheota increased. With PLA(HT), Syntrophobacteraceae, Thermoanaerobacteraceae, and methanogens were identified as potential key thermophilic PLA biodegraders.

Automated summary

Currently, there is a growing amount of poly(lactic acid) (PLA) waste in municipal waste due to the increasing production of bio-based polymeric materials. This study investigates the potential use of anaerobic digestion (AD) to treat PLA waste, focusing on the effects of thermophilic fermentation and hydrothermal and alkaline pretreatments. The findings show that pretreatments significantly increase methane production and shorten the overall time of maximal methane production. However, it is important to consider that methane production is not necessarily linked to the visible presence of PLA pieces.