This extreme sensitivity to even moderate FFA content has proven
a major hindrance for commercial production of biodiesel from
inexpensive feedstocks like used vegetable oils, tallow, and other
waste oils which have high FFAs and water content [53].
Therefore, based on the available technologies, the combination
of both acid and base catalysts (two-step reaction) has been proposed
to produce biodiesel from lipids with high FFAs content
[54]. This study proved that the high FFA content (10.8%) of S. bijugatus
lipids could be reduced to less than 0.6% to obtain 91% yield
of biodiesel by employing sequential stages of esterification and
transesterification.