where I is the current, Eap the applied voltage, Dt(s) the time
increment for n data points measured during the batch cycle, and
Rex is an external resistance which was 10 V by Call and Logan
[162]. Using Eq. (18), the efficiency was raised from 23% with a gas
diffusion membrane and 53% with a Nafion membrane to 76% in
the membraneless reactor [162]. Under these conditions a
hydrogen production rate of 3.12 m3 H2/(m3 reactor day) [162].
However, the methane production rates also increased to an
average of 3.5% methane in the production gas [162]. To control the
methanogensis in these reactors, strategies involving intermittent
draining and air exposure or in situ air-sparging have been
proposed [162]. However, these strategies will result in more
complex systems with significantly increased operations and
maintenance requirements, translating into more expensive
systems. In addition to methane suppression, continuous operation,
decreasing the pH, operating under carbon limited conditions,
increasing the microorganisms tolerance to impurities, and
examining other feedstocks are all issues to be addressed.
3.1.2.5. Multi-stage integrated process. Multi-stage hydrogen production
has been implemented to maximize the hydrogen
production from the feed [156]. Initially, the process consisted of
two stages, dark fermentation followed by photo fermentation
[156], but three or even four stages have since been proposed in
different configurations (Fig. 7) [9]. In this process, the biomass
material is first fed to a dark fermentation reactor where the
bacteria decompose the feedstock to hydrogen and an organic acid
rich effluent. Since the effluent has organic acids in it, this
eliminates the challenge of developing a supply of organic acids
for the photo-fermentative process. Since the photo-fermentative
process uses primarily infrared light, the sunlight is first filtered
through a direct photolysis reactorwhere the visible light is utilized,
but the infrared light is not [9]. The forth stage is the use of a
microbial electrolysis cellswhich produces hydrogen, not electricity