Polyhydroxyalkanoates (PHA) polymers are emerging within biobased biodegradable plastic products. To build a circular economy, effective recycling routes should be established for these and other end-of-life bioplastics. This study presents the first steps of a potential PHA recycling route by fermenting hydrolyzed PHA-based bioplastics (Tianan ENMATTM Y1000P; PHBV (poly(3-hydroxybutyrate-co-3-hydroxyvalerate)) into carboxylates acetate and butyrate. First, three different hydrolysis pretreatment methods under acid, base, and neutral pH conditions were tested. The highest 10% (from 158.8 g COD/L to 16.3 g COD/L) of hydrolysate yield was obtained with the alkaline pretreatment. After filtration to remove the remaining solid materials, 4 g COD/L of the hydrolyzed PHA was used as the substrate with the addition of microbial nutrients for mixed culture fermentation. Due to microbial conversion, 1.71 g/L acetate and 1.20 g/L butyrate were produced. An apparent complete bioconversion from intermediates such as 3-hydroxybutyrate (3-HB) and/or crotonate into carboxylates was found. The overall yields of the combined processes were calculated as 0.07 g acetate/g PHA and 0.049 g butyrate/g PHA. These produced carboxylates can theoretically be used to reproduce PHA or serve many other applications as part of the so-called carboxylate platform.

Biobased biodegradable plastic recycling with an overall efficiency of 90% was achieved via an open-culture process enriched with species related to Clostridium tyrobutyricum. Mixing bioplastics enhances hydrolysis and expands VFA product spectrum.

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.

Polyhydroxyalkanoates (PHAs) are a promising class of biobased and biodegradable polymers; however, their inherent brittleness and limited processability hinder widespread application. The glycerol trilevulinate (GT) bioplasticizer has been proposed to enhance the mechanical properties of PHAs without compromising their biodegradability. This study investigates the effect of GT at different concentrations on the anaerobic fermentation of poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) to volatile fatty acids (VFA), focusing on microbial conversion efficiency. These VFA are platform chemicals serving various applications, including resynthesis of PHAs. PHBV/GT blends were characterized in terms of their thermal and morphological properties. The results show that the increased GT percentage reduces the crystallinity and melting temperature, thereby enhancing polymer flexibility. Anaerobic fermentation experiments were conducted using (i) prehydrolyzed PHBV/GT fermentation at fixed 10 wt % GT content and (ii) direct fermentation of PHBV/GT solid particles at different GT concentrations. The findings show that GT-plasticized PHBV was successfully converted into VFA, with acetate and n-butyrate being the predominant fermentation products. While low concentrations of hydrolysate (4-10 g SCOD/L) were efficiently converted, higher concentrations (20 g SCOD/L) did not lead to fermentation, suggesting potential microbial toxicity. The different contents of GT did not affect the microbial conversion process. These findings support the use of GT as plasticizer for PHA-based materials and development of renewable, biodegradable, and microbial recyclable plastic products.