The principal VFA produced was acetic acid (Table 2). The
highest acetic, butyric, isovaleric, acetone and ethanol production was obtained at 24 h and can be related to the
highest H2 production. For the case of the propionic acid, the
highest production was achieved with an HRT of 6 h. This
increase in the propionic acid is correlated with the decrease
in the H2 production because H2 is consumed for propionic
production [28]. It has been reported that if the process is
primarily oriented to acetic and butyric acid production
pathways, H2 production can be maximized [5].