Effects of agitation speed and gas recirculation on hydrogen supersaturation and dark fermentation by Thermotoga neapolitana

Thermotoga neapolitana is capable to produce bio-hydrogen at high yields in hyperthermophilic dark fermentation from a large variety of substrates including organic wastes. However, in dark fermentation hydrogen concentration is one of the main inhibitors affecting the hydrogen yield and production rate. In this study, the effect of agitation speed and recirculation of the produced biogas on dissolved hydrogen concentration and its impact on hydrogen production by Thermotoga neapolitana were investigated. In 2 L batch bioassays using 1% inoculum (v/v), an increase of the agitation speed from 300 to 500 rpm led to a decrease of the dissolved hydrogen concentration from 17.0 ± 2.3 to 7.4 ± 0.7 mL/L. As a result, the hydrogen production rate and the glucose consumption rate increased from 54 ± 13 to 112 ± 22 mL H2/L/h and from 3.6 ± 0.4 to 4.7 ± 0.3 mmol glucose/L/h, respectively. In a second stage, the biomass concentration was increased to 0.8 g initial cell dry weight /L (equal to 200% inoculum v/v), applying agitation exclusively (low agitation “LA” - 300 rpm; high agitation “HA” - 500 rpm) and agitation combined with gas recirculation “GaR” (“LA+GaR” - 300 rpm; “HA+GaR” - 500 rpm). Gas recirculation increased the hydrogen production rate (ml H2/L/h) from 235 ± 35 (LA) and 535 ± 35 (HA) to 850 ± 71 (LA+GaR) and 813 ± 18 (HA+GaR). Concomitantly, the hydrogen yield (mol H2/ mol glucose) was increased from 3.0 ± 0.0 (LA) and 3.2 ± 0.1 (HA) to 3.5 ± 0.2 (LA+GaR) and 3.3 ± 0.1 (HA+GaR), respectively. At the same time, the fermentation time was reduced from 11 (LA) and 6 h (HA) to 4 h (LA+GaR; HA+GaR). The previous results confirm that the dissolved hydrogen plays a crucial role in dark fermentation hydrogen production even under hyperthermophilic stirred conditions. At higher biomass concentrations, the increase of the agitation speed increased the hydrogen yield and production rate by approximately 9 and 128% (HA/LA). In comparison, the application of the biogas recirculation increased the hydrogen yield and production rate by approximately 17 and 262% (LA+GaR/LA). The increase of agitation speed had a little effect when gas recirculation was applied, as indicated by the similar results achieved under LA+GaR and HA+GaR conditions. The present study confirms that gas recirculation is a simple and effective method to eliminate hydrogen supersaturation, considerably accelerating the hydrogen production and enhancing the yield.