4.7 Hydraulic retention time

Hydraulic retention time (HRT) is defined as the time that fermentation broth remains in a reactor. It is related to the working volume of the reactor and the influent flow rate. HRT affects a continuous hydrogen production. Hydrogenproducing bacteria are fast-growing bacteria, so they prefer short HRT, while the methanogens are slow-growing microorganisms, so they prefer long HRT [134]. Therefore, HRT can be used as controlling parameters to suppress the community of methanogens [102]. Jung et al. [134] reported that the HRT for treating liquidtype substrate is shorter than that of solid-type substrate because the times to hydrolyze substrate containing high solid are much longer.

#### 4.8 Hydrogen partial pressure

Hydrogen partial pressure affects hydrogenase activity because it is involved in reversibly oxidizing and reducing ferredoxin [102]. High accumulation of hydrogen partial pressure in the fermentation broth decreases the hydrogen production because the reaction tends to be reducing ferredoxin rather than oxidizing ferredoxin [135]. Hydrogen partial pressure can be reduced by biogas sparging [136], agitation, and reduction of headspace pressure using vacuum pump or enlarging the headspace volume.
