Photobioreactors
13 5
the sunlight of approximately 18 %. The actually calculated average conversion
of 1-2% in large plants implies that raceway ponds are a limited cultivation
system because of the self-shadowing and deficiencies in the material transport.
In this connection, Benemann [72] refers to mixotrophic algal growth during
the treatment of highly organic loads of sewage. The peak values up to
100 g d.wt. m- 2 d- x biomass or above 40 g d.wt. m- 2 d- 1 as the annual average
support the thesis of an expressed light limitation. Furthermore, he refers to
a number of contamination problems caused by zooplankton, fungi, or heavy
metals in industrialized areas; it is true that productivity could be increased by
covering the ponds, but not with respect to the contamination prevention [82].
In the inclined surface plant of an active surface of 900 m 2, developed by
Setlik et al. [83] in the Czech town Trebon, the algal suspension flows over
terraces of a defined inclination so that layer thicknesses below 1 cm and
turbulent flow prevent any shadowed volumes. With the example of Scenedesmus the authors achieved peak productivities of 19 g d.wt. m- 2 d- 1. Pulz [59]
reports a Chlorella productivity of 24 g d.wt. m- 2 d- 1 which was achieved in
a smaller plant of this type of a total volume of 5 m3; this exceeds the productivity of raceway ponds by 100%. This superiority with regard to the suspension
volume used is expressed even more clearly by the ratio of 0.95 g d.wt. 1-1 d- a
and approx. 0.01 g d.wt. 1-1 d-1 for raceway ponds, so that the constructive and
energetic additional expenditures seem to have been worthwhile. The fundamental influence of the illuminated surface with regard to the total volume of the
suspension (O/V-ratio) is obvious. Inclined surface systems with a small dark
volume of collecting pond with thin suspension layers and a corresponding
O/V-ratio of 20-100 m-1 (as against 3-10 m-1 in raceway ponds) fulfil the
requirements, although the investment and energy costs are increased drastically. Nevertheless, a larger plant (5000 m 2) of this type was established in
Rupite, Bulgaria, for the successful production of Chlorella with peak values of
37gd.wt.l-ld -~ during summer [72, 84]; it was established in a modified
arrangement in sunny regions of the Third World (e.g. the Peruvian-German
algal project [85]).
5.2 Closed and Semi-Closed Outdoor Photobioreactors
Attempts to make the cultivation conditions independent of environment were
associated with an immense increase in investment and operating costs at
corresponding scale-up, as against the open systems. This was justified only in
the production of high-value products (e.g. radioactively labelled substances
[-16]). However, closed reactors have a number of advantages:
- low CO2 losses,
- reduced risk of contamination
-
temperature regulation,
- controllable hydrodynamics,
13 5
the sunlight of approximately 18 %. The actually calculated average conversion
of 1-2% in large plants implies that raceway ponds are a limited cultivation
system because of the self-shadowing and deficiencies in the material transport.
In this connection, Benemann [72] refers to mixotrophic algal growth during
the treatment of highly organic loads of sewage. The peak values up to
100 g d.wt. m- 2 d- x biomass or above 40 g d.wt. m- 2 d- 1 as the annual average
support the thesis of an expressed light limitation. Furthermore, he refers to
a number of contamination problems caused by zooplankton, fungi, or heavy
metals in industrialized areas; it is true that productivity could be increased by
covering the ponds, but not with respect to the contamination prevention [82].
In the inclined surface plant of an active surface of 900 m 2, developed by
Setlik et al. [83] in the Czech town Trebon, the algal suspension flows over
terraces of a defined inclination so that layer thicknesses below 1 cm and
turbulent flow prevent any shadowed volumes. With the example of Scenedesmus the authors achieved peak productivities of 19 g d.wt. m- 2 d- 1. Pulz [59]
reports a Chlorella productivity of 24 g d.wt. m- 2 d- 1 which was achieved in
a smaller plant of this type of a total volume of 5 m3; this exceeds the productivity of raceway ponds by 100%. This superiority with regard to the suspension
volume used is expressed even more clearly by the ratio of 0.95 g d.wt. 1-1 d- a
and approx. 0.01 g d.wt. 1-1 d-1 for raceway ponds, so that the constructive and
energetic additional expenditures seem to have been worthwhile. The fundamental influence of the illuminated surface with regard to the total volume of the
suspension (O/V-ratio) is obvious. Inclined surface systems with a small dark
volume of collecting pond with thin suspension layers and a corresponding
O/V-ratio of 20-100 m-1 (as against 3-10 m-1 in raceway ponds) fulfil the
requirements, although the investment and energy costs are increased drastically. Nevertheless, a larger plant (5000 m 2) of this type was established in
Rupite, Bulgaria, for the successful production of Chlorella with peak values of
37gd.wt.l-ld -~ during summer [72, 84]; it was established in a modified
arrangement in sunny regions of the Third World (e.g. the Peruvian-German
algal project [85]).
5.2 Closed and Semi-Closed Outdoor Photobioreactors
Attempts to make the cultivation conditions independent of environment were
associated with an immense increase in investment and operating costs at
corresponding scale-up, as against the open systems. This was justified only in
the production of high-value products (e.g. radioactively labelled substances
[-16]). However, closed reactors have a number of advantages:
- low CO2 losses,
- reduced risk of contamination
-
temperature regulation,
- controllable hydrodynamics,