Consumption and inhibition dynamics of volatile tar compounds in syngas and co-digestion processes: Focus on acidogenesis and aceticlastic methanogenesis

Thermochemical processes enable utilization of recalcitrant biomass to produce syngas, which can be converted into biomethane. Integrating syngas biomethanation with waste treatment (syngas co-digestion) can enhance methane productivity. However, syngas contains impurities such as tar that negatively affect biomethanation. This study investigates the effects of volatile tar compounds (VTCs), including phenol, benzene, toluene, and styrene, on acidogenic glucose degradation and aceticlastic methanogenesis during syngas co-digestion. Mesophilic cultures were tested in two separate batch experiments using acetic acid and glucose as substrates, while exposed to VTCs at concentrations between 0.5 and 3 g L^-1. In the glucose experiment, BES (2-bromoethanesulfonic acid) was applied to inhibit methanogenesis. Gas production, glucose, volatile fatty acid (VFA), VTCs concentrations, and microbial community composition were monitored over time. A VTCs concentration of 3 g L^-1 was toxic to methanogenic archaea, while 2 g L^-1 delayed methane productivity. The best performance was observed at 1.5 g L^-1, where tar was effectively utilized, resulting in a 100.68 ± 3.44% increase in cumulative methane production compared to the control. VTCs did not affect glucose degradation, while they showed an inhibitory effect on homoacetogenic bacteria. High VTCs concentrations led to the accumulation of H₂, acetic acid, and butyric acid, while reducing propionic acid levels. The analysis of the microbial community revealed the presence of tar-degrading bacteria, confirmed methanogenic activity occurring during tar degradation, and indicated shifts in the microbial community involved in glucose conversion to VFAs.