(401e) Continuous Co-Production of H2 and Biomass From Cyanothece 51142 Cultivated in a Two-Stage Chemostat

Continuous
Co-production of H₂ and Biomass from Cyanothece 51142 Cultivated
in a Two-Stage Chemostat

P.
Dechatiwongse, G.C. Maitland and K. Hellgardt

Imperial
College London, London, SW7 4ND, UK.

Abstract

          Due to a
rapid rise in global population, demands on food and energy have been projected
to increase significantly over the coming decades [1, 2].
At the same time, increasing concerns over global warming have emphasized the
need to transform our means of food production and energy usage into more
environmentally sustainable routes [3].

          Cyanobacteria,
also known as blue-green algae, are microorganisms, which have the ability to
photosynthetically derive their chemical energy from sunlight and CO₂.
Their biomass has long been utilised for human nutrition, with numerous
commercial scale production processes being demonstrated [4]. Cyanobacteria
could also serve as clean energy factories, as some of them are able to produce
molecular hydrogen (H₂), an energy carrier which has great potential
to provide clean power needed for transport, heating and electricity [5].

Cyanothece
51142, a unicellular nitrogen-fixing marine cyanobacterium, has been
reported for its excellent nutritional value in terms of biochemical
composition (50 ? 60% protein dry weight) [6] as well as its high production rate of H₂[7], thereby appearing to be a promising microbial platform
to answer mankind's expanding demand for food and clean energy.

In our study, we demonstrate a two-stage Chemostat cultivation
system, which enables the continuous co-production of H₂ and biomass
from Cyanothece 51142. The cyanobacterial culture in the primary tubular
growth bioreactor is continuously fed with an optimised rate of nutrient. Cells
from the growth reactor are then transferred into a flat-plate reactor. This
secondary bioreactor is equipped with a customised membrane system [8],
allowing the efficient separation of gaseous H₂ product from the
aqueous culture.

Results obtained during start-up and at steady state
conditions, including cyanobacterial growth, the corresponding H₂
and biomass production rates, nutrient uptake and the observed transients of
process parameters such as pH and pO₂, will be presented. These will
be used to define optimum operating conditions for the co-production process.

Figure 2: Results from a
two-stage Chemostat bioreactor system during start-up and steady state
conditions. The time starting to operate in Chemostat mode is presented as black
dot lines.

a) and b) Constant values of glycerol concentration
(carbon nutrient) and optical density (OD) in both tubular and flat-plate reactors,
indicating the optimised feeding rate of nutrient

c) Anaerobic conditions (pO₂ = 0) are
maintained throughout the experiment in the flat-plate reactor. The onset of H₂
production is observed as soon as the anaerobiosis is established.

d) In Chemostat mode, biomass is collected from the
flat-plate reactor at the same time as gaseous H₂ being extracted
though our design membrane system.

e) To facilitate cyanobacterial growth, pH of aqueous
culture in the tubular reactor is maintained by means of automated acid/base
pumps

References

1.         Bruinsma, J., The resource outlook to 2050: by
how much do land, water and crop yields need to increase by 2050?, 2009,
FAO.

2.         EIA, International energy outlook, 2011.

3.         EIA, Clean energy progress report 2011,
OECD/IEA: Paris.

4.         Spolaore, P., et al., Commercial
applications of microalgae. Journal of Bioscience and Bioengineering, 2006.
101(2): p. 87-96.

5.         Dunn, S., Hydrogen futures: toward a sustainable
energy system. International Journal of Hydrogen Energy, 2002. 27(3):
p. 235-264.

6.         Schneegurt, M.A., et al., Compositional
and toxicological evaluation of the diaztrophic cyanobacterium, Cyanothece sp
strain ATCC-51142. Aquaculture, 1995. 134(3-4): p. 339-349.

7.         Bandyopadhyay, A., et al., High rates of
photobiological H₂ production by a cyanobacterium under aerobic
conditions. Nat Commun, 2010. 1: p. 139.

8.         Tamburic, B., et al., Design of a novel
flat-plate photobioreactor system for green algal hydrogen production.
International Journal of Hydrogen Energy, 2011. 36(11): p. 6578-6591.