H2-rich biogas recirculation prevents hydrogen supersaturation and enhances hydrogen production by Thermotoga neapolitana cf. capnolactica

This study focused on the supersaturation of hydrogen in the liquid phase (H2aq) and its inhibitory effect on dark fermentation by Thermotoga neapolitana cf. capnolactica by increasing the agitation (from 100 to 500 rpm) and recirculating H2-rich biogas (GaR). At low cell concentrations, both 500 rpm and GaR reduced the H2aq from 30.1 (±4.4) mL/L to the lowest values of 7.4 (±0.7) mL/L and 7.2 (±1.2) mL/L, respectively. However, at high cell concentrations (0.79 g CDW/L), the addition of GaR at 300 rpm was more efficient and increased the hydrogen production rate by 271%, compared to a 136% increase when raising the agitation to 500 rpm instead. While H2aq primarily affected the dark fermentation rate, GaR concomitantly increased the hydrogen yield up to 3.5 mol H2/mol glucose. Hence, H2aq supersaturation highly depends on the systems gas-liquid mass transfer and strongly inhibits dark fermentation.