Enhancing anaerobic digestion of polyhydroxybutyrate through combined pretreatment: Effects on solubilization, methane potential, and long-term continuous thermophilic digestion.

Combined mechanical and thermal-alkaline pretreatment of polyhydroxybutyrate (PHB), varying particle-size (100-2000 μm), NaOH-dosage (20-100 %), and treatment-duration (6-24 h), was evaluated to enhance solubilization and anaerobic digestion (AD). A response surface analysis optimized pretreatment, achieving ≥98 % PHB-solubilization, with thermal-alkaline pretreatment more effective at smaller particle sizes. In biochemical-methane-potential (BMP) tests, mechanical pretreatment alone was ineffective, while the combined pretreatment enhanced BMP 3.2-7.2 times. Thermophilic BMP outperformed mesophilic BMP, yielding 279-309  mL-CH₄/g-COD_fed. In 340-day continuous experiment, thermophilic AD of the pretreated PHB wastewater in CSTR was confirmed to be feasible at hydraulic retention time (HRT) of 24-30d, with 91-94 % COD removal and 276-290 mL-CH₄/g-COD_fed. At 20d HRT, crotonate accumulation caused severe methanogenic inhibition. Distinct bacterial communities and co-dominance of Methanosarcina and Methanothermobacter were underlying PHB and intermediate degradation and methanogenesis. These findings highlight the potential of thermophilic AD for valorizing PHB wastes following optimized pretreatment.