Biomedical Engineering Reference
In-Depth Information
9.5.4 Hybrid or combination growth systems
The choice of growth systems is not necessarily restricted to a single technology. Various
multiphase cultivation strategies have been considered. Richmond (1987) suggested
combining a tubular reactor connected to an open raceway to maximise biomass production
by optimising environmental variables. With this system the raceway cultures could be used
in the hot part of the day and photobioreactors used for high productivities during the periods
of the day with lower environmental stress. Multiphase cultivation strategies can be devised
that ensure maximum production of biomass in one stage and maximum induction and
accumulation of desired products in the other. This concept has been successfully applied to
outdoor cultures of Dunaliella for
-carotene production, as well as to photobioreactor
cultures of Haematococcus for astaxanthin production (Hu, 2004).
The approach for the next 5-10 years suggested by the International Network on
Biofixation of CO 2 and Greenhouse Gas Abatement with Microalgae (Von Harmelen and
Oonk, 2006) and Benemann (personal communication, 2010) is to produce microalgae in
conjunction with wastewater treatment. Benemann (personal communication, 2010), with a
background of decades in microalgal production including the US DOE Aquatic species
program (Sheehan et al ., 1998), expressed the opinion that considerable R&D is still
required for economic algal biofuels and other products, with the near-term opportunity
being in wastewater treatment, in particular municipal wastewaters. By adding carbon
dioxide to such wastewater treatment ponds, it would be possible to grow enough algae to
remove all nitrogen and phosphorus nutrients, thus achieving an improved level of treatment
of the wastewaters, while generating algal biomass, potentially with a high oil content. The
carbon dioxide required for the process could be provided by the organic matter in the
wastewater itself. In such scenarios algae wastewater treatment combined with biofuels
production, especially if municipal, agricultural and industrial wastes are used, has the
potential to reduce greenhouse gas emissions by perhaps 1%. Such systems could be further
developed to recycle available nutrients and waters, greatly increasing biofuel outputs.
Cost of production will obviously vary with the species and technology used, whether
algal growth is indoors using artificial illumination or outside using solar radiation, or a
combination of both. Zhang and co-workers (2001) made estimates of production costs for
the eustigmatophyte Nannochloropsis sp. in a flat-plate glass reactor. They determined that
a 2000 litre reactor was sufficient for industrial scale production of microalgae fed to rotifers
which in turn were fed to eight million Denise fish fingerlings annually.
9.5.5 Fermentation systems
There has been extensive research on fermentation technologies for growing microorganisms
such as bacteria, yeast and fungi for industrial and medical applications. Fermentation
technology is, therefore, much more mature and established at an industrial scale compared
with phototrophic production systems. However, the number of microalgae produced
heterotrophically remains very small. Technology development has, in general, not focused
on the particular requirements of microalgae and only a handful of microalgae have been
shown to grow effectively under heterotrophic conditions. These include species that can
produce bioproducts of interest, such as the chlorophytes Chlorella , Dunaliella and
Haematococcus species (Lee, 2004 ). Harel and Place ( 2004 ) considered that heterotrophic
microalgal production has high potential for aquaculture feeds and examined the industrial
potential including production issues.
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