3.7 Pilot scale cultivation
The viability of using the most promising waste water, i.e. composting leachate,
was verified in pilot scale cultivations. Four flat panel photo bioreactors were purpose
built in two sizes (Fig. 3). Three had an effective volume of 170 l whereas the
volume of a thinner reactor was 80 l.
3. Algal cultivation for lipid production in waste water
23
Figure 3. Algae cultivated in photobioreactors in a temperature-controlled room in
the Department of Environmental Sciences, University of Helsinki.
The photobioreactors were robust in design. Filtered compressed air was pumped
through a perforated pipe to the reactor to provide mixing and to strip out excess
oxygen, resulting from photosynthesis. Bioreactors were illuminated with daylight
fluorescent tubes and installed with temperature control, as well as pH control
based on controlled CO2 input. This was a simple, one-way control which opened
the CO2 valve when the pH rose above a set value. In a full-scale operation this
type of control can be applied to adjust the flue gas flow to the cultivation process.
Composting leachate was diluted to 2.5% with tap water for reactor cultivation
of Selanastrum (Fig. 4). In a 17 day experiment, the algae gained a biomass of
1.2 g DW L-1, which was similar to that obtained in a control cultivation, in which a
defined inorganic medium was used as a substrate (data not shown). The nutrient
reduction was > 99% for ammonium, 83% for total nitrogen, 73% for phosphorus
from phosphate and 70% for total phosphorus. The experiments so far have
shown that composting leachates are suitable for algal cultivation also at this larger
scale. When reject water (2.5%) was used as the source for major nutrients, supplemented
with micronutrients (e.g. Mg), Selenastrum sp. produced 2.7 g DW L-1
biomass in a 35 day long experiment. Nutrients were reduced 91% for ammonium,
85% for total nitrogen, and