Sequential hydrolysis and fermentation of polyethylene terephthalate and dairy wastewater for the production of polyhydroxyalkanoate and glycolic acid by Cupriavidus necator

Polyethylene terephthalate (PET) waste and dairy wastewater pose major environmental challenges that require sustainable valorization strategies. In the current study, Cupriavidus necator was cultivated using a hydrolysate obtained from hydrothermal co -processing of PET waste and dairy wastewater for polyhydroxyalkanoate (PHA) production. Initially, raw dairy wastewater resulted in limited biomass accumulation (0.34 ± 0.02 g L −1 ) and low PHA titers (0.08 ± 0.01 g L −1 ). Hydrothermal treatment combined with preprocessing effectively depolymerized PET into a carbon-rich feedstock (7250 ± 388 mg L −1 COD). An 80:20 ( v /v hydrolysate: water) dilution strategy yielded 0.92 ± 0.06 g L −1 of PHA and 88.71% COD reduction. Evaluation of nitrogen sources revealed that ammonium sulfate outperformed nitrate-based alternatives, achieving a maximum PHA concentration of 6.24 ± 0.13 g L −1 (65.1% cellular content) and 98.28% COD removal at 2.0 g L −1 . Concurrently, glycolic acid co-production reached 1.34 ± 0.05 g L −1 , demonstrating efficient ethylene glycol metabolism. Structural characterization via FTIR and 1 H NMR identified the biopolymer as isotactic poly(3-hydroxybutyrate). The extracted polymer exhibited high crystallinity (CrI 30.7%) with a nanocrystalline domain size of ~16–19 nm measured using XRD. Thermal analysis by TGA confirmed high thermal stability, with a degradation onset at 229.8 °C and a maximum decomposition at 254.5 °C. These findings establish a viable platform for simultaneously remediating plastic waste and treating wastewater, while producing high-quality, thermally stable bioplastics.