Abstract:
Most dark fermentation (DF) studies had resorted to above-ambient temperatures to
maximize hydrogen yield, without due consideration of the net energy gain. In this study,
literature data on fermentative hydrogen production from glucose, sucrose, and organic
wastes were compiled to evaluate the benefit of higher fermentation temperatures in
terms of net energy gain. This evaluation showed that the improvement in hydrogen yield
at higher temperatures is not justified as the net energy gain not only declined with
increase of temperature, but also was mostly negative when the fermentation temperature
exceeded 25 C. To maximize the net energy gain of DF, the following two options for
recovering additional energy from the end products and to determine the optimal
fermentation temperature were evaluated: methane production via anaerobic digestion
(AD); and direct electricity production via microbial fuel cells (MFC). Based on net energy
gain, it is concluded that DF has to be operated at near-ambient temperatures for the net
energy gain to be positive; and DF þ MFC can result in higher net energy gain at any
temperature than DF or DF þ AD.
