[9]. This cell utilizes the same organic acids, but does not require
light. Therefore, it can operate during the night or other times of low
light [9]. The effluent from the first stage contains ammonia, which
inhibits the second stage, so some dilution and neutralization to
adjust the pH to 7 is required prior to feeding it to the second stage
[156].
Integration of multiple processes produces significant challenges
for the reactor engineering, system design, process control,
and operation and maintenance. The challenges with the coproduction
of hydrogen and oxygen from photolytic hydrogen
production include:
Photosynthetic and respiration capacity ratio. Green algae and
cyanobacteria become anaerobic when their photosynthesis/
respiration (P/R) capacity ratio is 1 or less. Under such anaerobic
conditions, photosynthetic water oxidation produces H2 instead
of starch, and the oxygen evolved by photosynthesis is consumed
by respiration, to produce CO2. Currently, this process is achieved
by nutrient deprivation, with the drawback that the resulting P/
R 1 ratio is achieved by partially decreasing the quantum yield
of photosynthesis. Alternative mechanisms to bring the P/R ratio
to 1 need to be investigated, particularly those methods that
focus on achieving a P/R ratio of 1 without changing the quantum
yield of photosynthesis. Two further issues will need to be
investigated under these conditions: (1) rate limitations due to
the non-dissipation of the proton gradient and (2) the ability of
the culture to take up a variety of exogenous carbon sources
under the resulting anaerobic conditions [9].
Co-culture balance. To extend the adsorption spectrum of the H2-
photoproducing cultures to the infrared (700–900 nm), the
possibility of co-cultivating oxygenic photosynthetic organisms
with anoxygenic photosynthetic bacteria that absorb light in the
visible (400–600 nm), thus potentially competing with green
algae for these latter wavelengths. Strategies need to be devised
to either maintain the appropriate biomass ratio of the two
organisms as suspensions in the same cultures. The competition