Fig. 3b and c presented the changes of the six acids concentration
and the specific uptake rate of each. The order for VFA utilization
was firstly acetic acid, which was followed by n-butyric acid,
propionic acid, iso-butyric acid, n-valeric acid and iso-valeric acid.
It indicated that even-number acids were easier to use than oddnumber
ones, straight-chain acids were easier to use than
branch-containing acids.