Batch and semi-batch anaerobic digestion of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) bioplastic: New kinetic, structural, microbiological and digestate phytotoxicity insights

This study investigated the bioconversion of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) in batch and semi-batch anaerobic digestion systems, focusing not only on methane production and microbial community dynamics, but also on the structural changes that occur during degradation and the potential use of the resulting digestate as a soil enhancer. Both systems operated under mesophilic conditions (37 ± 2 °C) and stable pH (7.9 ± 0.2). The batch system achieved a methane yield of 550.5 ± 78.79 NmL CH₄/g VS added over 50 days, with a typical sigmoidal methane production pattern. A carbon mass balance analysis indicated a 96.09 % recovery, with 47.62 % of the carbon converted to methane. SEM, FTIR and XRD analyses of the partially degraded material showed that the anaerobic biodegradation of PHBH was characterized by surface erosion and weight loss, with minimal changes in crystallinity. Conversely, the adaptation of the microbial community to 93 days of continuous PHBH feeding allowed the achievement of a stable methane yield of 562.34 ± 44.97 NmL CH₄/g VS added, along with a corresponding volumetric methane production rate of 281.17 ± 22.48 NmL CH₄/L-d. Microbial community analysis, at pseudo-steady state, revealed the dominance of Methanosaeta, Anaerolineaceae, and Thermovirga in driving the anaerobic digestion of PHBH via acetoclastic methanogenesis. Despite high methane production efficiency, digestate toxicity tests using perennial ryegrass indicated phytotoxic effects on seed germination, highlighting the need for further investigation to characterize inhibitory compounds and develop mitigation strategies.