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.