Photobioreactors
13 3
interactions with exponential decrease along the boundary normal [66]. Only
the conversion processes with the assimilation pigments of the microorganisms
are used as energy receptors for the development of biomass, i.e. the radiation
energy is lost largely as heat, fluorescence etc.
The large energy density at the boundary surface contrasts with the insufficient supply to the cells at only a few mm layer thickness. Despite a few
commercial applications a significant break-through has not occurred. This is
mainly due to the large space requirements for open plants and the expensive
cultivation technique for closed plants, which allows only for a relatively low
biomass concentration of 1-2 g dry matter per litre down to 0.4 g in open
systems. Mainly extremophilic species of low contamination risk like osmotolerant DunalieUa are cultivated; the requirements of G.M.P.-complying production
mostly being no issue [67, 68].
The biological and technical potential of algal biotechnology has been
described in detail in several reviews [69-71]. Nearly all the commercial plants
are based on open pond technologies, their construction and productivity is
known from detailed descriptions [5, 72]. These systems, however, seem to have
reached their limits. The gap between the theoretical biological potential of
microalgal biomass and the productivities actually achieved may be reduced by
developing closed cultivation systems [73]. It is rather difficult to compare the
open pond technology with closed systems and indoor photobioreactors because of the strong variations of the prevailing boundary conditions. The
general review suggests that open systems are predominating in mass cultivation
because of cost considerations, and photobioreactors become more and more
interesting for the preparation of valuable substances and for special applications [13, 16, 40, 74]. Emphasis of the present review is laid on approaches to
minimize light limitations within the photobioreactors, with a short glimpse of
the latest developments of small-scale laboratory applications and immobilization processes previously described by Lee [75] and Brouers et al. [76],
respectively. The results of the 1st and 2nd European Workshops on Microalgal
Biotechnology, Germany, the 6th European Congress on Biotechnology, Italy,
and the 7th International Conference on Applied Algology, South Africa, will be
respected, too.
5 Cultivation Techniques
5.1 Open Cultivation Systems
Open cultivation systems comprise natural or artificial ponds, raceway ponds,
and so-called inclined surface systems. They represent the classical processes of
the production of algal biomass. All of them require large areas. If appropriate
areas are available in regions of balanced and sunny climate (e.g. waste land near
13 3
interactions with exponential decrease along the boundary normal [66]. Only
the conversion processes with the assimilation pigments of the microorganisms
are used as energy receptors for the development of biomass, i.e. the radiation
energy is lost largely as heat, fluorescence etc.
The large energy density at the boundary surface contrasts with the insufficient supply to the cells at only a few mm layer thickness. Despite a few
commercial applications a significant break-through has not occurred. This is
mainly due to the large space requirements for open plants and the expensive
cultivation technique for closed plants, which allows only for a relatively low
biomass concentration of 1-2 g dry matter per litre down to 0.4 g in open
systems. Mainly extremophilic species of low contamination risk like osmotolerant DunalieUa are cultivated; the requirements of G.M.P.-complying production
mostly being no issue [67, 68].
The biological and technical potential of algal biotechnology has been
described in detail in several reviews [69-71]. Nearly all the commercial plants
are based on open pond technologies, their construction and productivity is
known from detailed descriptions [5, 72]. These systems, however, seem to have
reached their limits. The gap between the theoretical biological potential of
microalgal biomass and the productivities actually achieved may be reduced by
developing closed cultivation systems [73]. It is rather difficult to compare the
open pond technology with closed systems and indoor photobioreactors because of the strong variations of the prevailing boundary conditions. The
general review suggests that open systems are predominating in mass cultivation
because of cost considerations, and photobioreactors become more and more
interesting for the preparation of valuable substances and for special applications [13, 16, 40, 74]. Emphasis of the present review is laid on approaches to
minimize light limitations within the photobioreactors, with a short glimpse of
the latest developments of small-scale laboratory applications and immobilization processes previously described by Lee [75] and Brouers et al. [76],
respectively. The results of the 1st and 2nd European Workshops on Microalgal
Biotechnology, Germany, the 6th European Congress on Biotechnology, Italy,
and the 7th International Conference on Applied Algology, South Africa, will be
respected, too.
5 Cultivation Techniques
5.1 Open Cultivation Systems
Open cultivation systems comprise natural or artificial ponds, raceway ponds,
and so-called inclined surface systems. They represent the classical processes of
the production of algal biomass. All of them require large areas. If appropriate
areas are available in regions of balanced and sunny climate (e.g. waste land near