Mesophilic fermentation explorations for anaerobic carboxylates production from commercial bioplastic products: PHA-based cups & PLA-based lids

Biodegradable plastic polymers such as polyhydroxyalkanoates (PHA) and polylactic acid (PLA) are increasingly applied in commercial products, but their end-of-life (EoL) processing remains inefficient in terms of carbon and energy recovery. This study investigated mesophilic (35℃) open-culture fermentation to convert PHA and PLA raw materials and commercial bioplastic products into carboxylates. A gas-lift anaerobic filter bioreactor was inoculated with a mixed culture sourced from rumen liquid and lab-scale open-culture fermenters, and operated over 200 days under controlled pH (5.9) and hydraulic retention times (2-18 days). Hydrolysates obtained from hydrothermally pretreated (150℃, 15 h) PHA (10 g/L) and PLA (1.4 g/L) pellets were continuously fermented, yielding up to 6.6 g/L acetate and 4.8 g/L n-butyrate. Subsequent co-fermentation with shredded commercial bioplastics (PHA-based cups and PLA-based lids) further increased acetate and n-butyrate to 7.2 g/L and 5.5 g/L, respectively. Approximately 35 % of the PHA-based cups were converted into carboxylates, while PLA-based lids showed negligible degradation despite an operation time of 157 days. Full-length 16S sRNA sequencing revealed that Clostridium tyrobutyricum was likely a dominant species during hydrolysate fermentation of PHA and PLA pellets, while a broader microbial consortium contributed to solid bioplastic product conversion. These findings indicate that mesophilic open-culture fermentation offers a viable route for the conversion of real PHA-based products into carboxylates, whereas PLA-based products may remain more recalcitrant under the same conditions.