Batch and Continuous Anaerobic Digestion of Poly(3-Hydroxybutyrate-Co-3-Hydroxyhexanoate) (Phbh) Bioplastic: New Kinetic, Structural, Microbiological and Digestate Phytotoxicity Insights

Abstract

This study investigated the bioconversion of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) in batch and continuous anaerobic digestion systems, focusing not only on methane production and microbial community dynamics, but also on the mechanism of degradation and the potential of the resulting digestate as a soil amendment. 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 production pattern. Carbon mass balance analysis indicated a 96.09% recovery, with 47.62% of carbon converted to methane. SEM, FTIR and XRD analyses of the partially degraded material showed that the biodegradation of PHBH was characterized by surface erosion and weight loss, with minimal changes in crystallinity. Conversely, the adaptation of the microbial community in continuous culture enabled 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 indicated phytotoxic effects on seed germination, suggesting further investigation into potential inhibitory compounds and their mitigation.

Keywords: Anaerobic digestion, biodegradation, Bioplastic, Continuous reactor, Digestate phytotoxicity, Microbial analysis