144
O. Pulz and K. Scheibenbogen
hydrogen of 7%. This is in agreement with the extended analyses of Rechenberg
[145] who calculated similar efficiencies in the sunny conditions of the Sahara
desert within his so-called Heliomites, i.e. tubular cone-shaped photobioreactors. Thus, photosynthetic systems for H2-production could compete with
photoelectric alternatives. However, due to the complexity of the system and
with view to the operational costs, industrial H2-production remains a longterm challenge.
Buchholz and Gerbsch [137] suggested for high value products immobilization of algae in transparent hollow micro-spheres by a polyelectric system
comprising sodium cellulose sulfate. By the defined porosity of membrane,
extracellular and intracellular products are trapped and enriched during downstream processing.
As already mentioned, self-immobilization of cells on planar surfaces led to
the idea to use inexpensive foil material as carrier. Pulz and Broneske [116]
investigated an ultrathin layer system in which adhesive forces between two
vertically arranged foils cause suspension layer thicknesses of 0.2-1 ram. While
growth in the suspension was insignificant, Chlorella dry matter concentrations
at the foil surface amounted to 40 g d.wt.m -2. By simple fluid-mechanical
harvesting methods, together with high up-scalability, this principle can be used
to yield very concentrated biomass suspensions, extracellular products like
exopolysaccharides [146], degradation of waste substances, and for intense
CO2/O2 gas exchange processes.
It is noted additionally that not all the cultivation systems are suited
to all species. In particular, the dinoflagellates, which are supposed to bear
a high potential for pharmaceutical substances, appear to be too fragile to be
cultivated in common systems. As was shown by Silva et al. [147], hydrodynamic stress derived from simple gas bubbling negatively affects the
growth of Dunaliella flagellate. Dialysis culture systems have been proposed for
shear-force sensitive organisms and are still being explored [12]. Numerous
examples are suggested to use permeable membranes for a sufficient diffusion of
nutrients. Membrane technology is an interesting means for the design of
separate chamber reactors for sensitive organisms or other special purposes
[148-150].
5.4 General Aspects of Phototrophic Mass Cultivation
Open pond technology is subject to the natural light/dark cycle, i.e. respiratory
metabolic processes will take place at night as reverse effect of photosynthesis.
The reduction of productivity depends on the cell concentration of the suspension [151]. For Spirulina an optimal areal density (OAD) of 50-75 g d.wt. m-2 is
given [152]. Tredici et al. [153] have examined the OAD in their VAP-reactors
with steady-state Spirulina cultures and determined respiratory losses of
18-30% with increasing biomass concentration. Due to the high surface/volume
ratio of the VAPs, the thesis of Richmond and Grobbelaar [154] seems to be
O. Pulz and K. Scheibenbogen
hydrogen of 7%. This is in agreement with the extended analyses of Rechenberg
[145] who calculated similar efficiencies in the sunny conditions of the Sahara
desert within his so-called Heliomites, i.e. tubular cone-shaped photobioreactors. Thus, photosynthetic systems for H2-production could compete with
photoelectric alternatives. However, due to the complexity of the system and
with view to the operational costs, industrial H2-production remains a longterm challenge.
Buchholz and Gerbsch [137] suggested for high value products immobilization of algae in transparent hollow micro-spheres by a polyelectric system
comprising sodium cellulose sulfate. By the defined porosity of membrane,
extracellular and intracellular products are trapped and enriched during downstream processing.
As already mentioned, self-immobilization of cells on planar surfaces led to
the idea to use inexpensive foil material as carrier. Pulz and Broneske [116]
investigated an ultrathin layer system in which adhesive forces between two
vertically arranged foils cause suspension layer thicknesses of 0.2-1 ram. While
growth in the suspension was insignificant, Chlorella dry matter concentrations
at the foil surface amounted to 40 g d.wt.m -2. By simple fluid-mechanical
harvesting methods, together with high up-scalability, this principle can be used
to yield very concentrated biomass suspensions, extracellular products like
exopolysaccharides [146], degradation of waste substances, and for intense
CO2/O2 gas exchange processes.
It is noted additionally that not all the cultivation systems are suited
to all species. In particular, the dinoflagellates, which are supposed to bear
a high potential for pharmaceutical substances, appear to be too fragile to be
cultivated in common systems. As was shown by Silva et al. [147], hydrodynamic stress derived from simple gas bubbling negatively affects the
growth of Dunaliella flagellate. Dialysis culture systems have been proposed for
shear-force sensitive organisms and are still being explored [12]. Numerous
examples are suggested to use permeable membranes for a sufficient diffusion of
nutrients. Membrane technology is an interesting means for the design of
separate chamber reactors for sensitive organisms or other special purposes
[148-150].
5.4 General Aspects of Phototrophic Mass Cultivation
Open pond technology is subject to the natural light/dark cycle, i.e. respiratory
metabolic processes will take place at night as reverse effect of photosynthesis.
The reduction of productivity depends on the cell concentration of the suspension [151]. For Spirulina an optimal areal density (OAD) of 50-75 g d.wt. m-2 is
given [152]. Tredici et al. [153] have examined the OAD in their VAP-reactors
with steady-state Spirulina cultures and determined respiratory losses of
18-30% with increasing biomass concentration. Due to the high surface/volume
ratio of the VAPs, the thesis of Richmond and Grobbelaar [154] seems to be