134
o. Pulz and K. Scheibenbogen
seas) at reasonable cost, the investment efforts will be relatively low even at
scaling up to 100 ha [16].
Natural and artificial ponds are generally used for the cultivation of fast
growing, naturally occurring or extremophilic species, as otherwise certain
contaminants will dominate. The cultivation of the extremely halophilic and
highly light-tolerant green alga Dunaliella salina for/~-carotene production in
the Hutt-Lagoon in Western Australia is a good example. In the shallow unlined
ponds of more than 50 ha the algae are harvested continuously through a widely
branched tubing system and the nutrient-enriched sea water is added through
pumps. Evaporation regulates the temperature of the suspension and produces
hypercritical salt concentrations so that the growth of other species is inhibited.
Due to the lack of any stirring mechanisms the material transport for CO2
supply and biomass localization is minimized, which is the cause of relatively
low productivities not exceeding 1 g d.wt. m- 2 d- 1 [69, 77].
At optimal temperatures obtained by utilizing the cooling water of power
stations, Spirulina was also cultivated successfully in Europe. The yields amount
to 24 tonnes dry biomass per ha shallow lagoons per 6 months, which corresponds to a growth rate of 1.3 g d.wt. m-2 d-i [78].
In pools of the described dimension, the problem of stirring is of fundamental importance because large amounts of energy are required for the prevention of concentration gradients and algal sedimentation. Floating bodies with
impellers which were pulled over the surface of the ponds [23] or raceway ponds
were energy-saving solutions. The latter are characterized by parallel, loop-like
channels of a length of several kilometers; several paddle wheels combined with
aerating units produce an economic motion of the algal suspension and a uniform nutrient supply. The production of biomass in a plant of several hectares
thus has a relatively low operating cost [16].
Weissman et al. [40] have investigated the influence of thorough mixing of
the suspension on algal production in a demonstration plant, but did not
observe any significant increase of productivity with the suspension velocities of
1-30cms -1 reached. As already mentioned, Grobbelaar [41] pointed to
a synergistic relation between light/dark cycles and mechanisms of material
transport which was verfied by his experiments with highly turbulent suspension
flows. The energy input through stirring in order to maintain the necessary
average lighting of the algal cells within the range of 10-i to 10-2 seconds is
rather expensive in larger plants. The simultaneously occurring increased shear
forces have a counterproductive effect to numerous algal species [79].
The growth of biomass in raceway ponds is also dependent on the prevailing
regional climate. Mean growth rates of 15-25 g d.wt. m -2 d-1 are recorded in
tropical regions [23] and in California [16], in South Europe up to
28 g d.wt. m -2 d-1 [80], in Central Europe and Japan mean values of only
12-15 g d.wt.m -2 d-1 [59, 72, 81], related to Chlorophyceae and suspension
depths of approximately 20 cm. At an average radiation of the surface
of the ponds of 16.7MJm-2d -1 (PAR) a theoretical productivity of
130 g d.wt. m -2 d-1 is calculated on the basis of a photosynthetic efficiency of
o. Pulz and K. Scheibenbogen
seas) at reasonable cost, the investment efforts will be relatively low even at
scaling up to 100 ha [16].
Natural and artificial ponds are generally used for the cultivation of fast
growing, naturally occurring or extremophilic species, as otherwise certain
contaminants will dominate. The cultivation of the extremely halophilic and
highly light-tolerant green alga Dunaliella salina for/~-carotene production in
the Hutt-Lagoon in Western Australia is a good example. In the shallow unlined
ponds of more than 50 ha the algae are harvested continuously through a widely
branched tubing system and the nutrient-enriched sea water is added through
pumps. Evaporation regulates the temperature of the suspension and produces
hypercritical salt concentrations so that the growth of other species is inhibited.
Due to the lack of any stirring mechanisms the material transport for CO2
supply and biomass localization is minimized, which is the cause of relatively
low productivities not exceeding 1 g d.wt. m- 2 d- 1 [69, 77].
At optimal temperatures obtained by utilizing the cooling water of power
stations, Spirulina was also cultivated successfully in Europe. The yields amount
to 24 tonnes dry biomass per ha shallow lagoons per 6 months, which corresponds to a growth rate of 1.3 g d.wt. m-2 d-i [78].
In pools of the described dimension, the problem of stirring is of fundamental importance because large amounts of energy are required for the prevention of concentration gradients and algal sedimentation. Floating bodies with
impellers which were pulled over the surface of the ponds [23] or raceway ponds
were energy-saving solutions. The latter are characterized by parallel, loop-like
channels of a length of several kilometers; several paddle wheels combined with
aerating units produce an economic motion of the algal suspension and a uniform nutrient supply. The production of biomass in a plant of several hectares
thus has a relatively low operating cost [16].
Weissman et al. [40] have investigated the influence of thorough mixing of
the suspension on algal production in a demonstration plant, but did not
observe any significant increase of productivity with the suspension velocities of
1-30cms -1 reached. As already mentioned, Grobbelaar [41] pointed to
a synergistic relation between light/dark cycles and mechanisms of material
transport which was verfied by his experiments with highly turbulent suspension
flows. The energy input through stirring in order to maintain the necessary
average lighting of the algal cells within the range of 10-i to 10-2 seconds is
rather expensive in larger plants. The simultaneously occurring increased shear
forces have a counterproductive effect to numerous algal species [79].
The growth of biomass in raceway ponds is also dependent on the prevailing
regional climate. Mean growth rates of 15-25 g d.wt. m -2 d-1 are recorded in
tropical regions [23] and in California [16], in South Europe up to
28 g d.wt. m -2 d-1 [80], in Central Europe and Japan mean values of only
12-15 g d.wt.m -2 d-1 [59, 72, 81], related to Chlorophyceae and suspension
depths of approximately 20 cm. At an average radiation of the surface
of the ponds of 16.7MJm-2d -1 (PAR) a theoretical productivity of
130 g d.wt. m -2 d-1 is calculated on the basis of a photosynthetic efficiency of