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Photobioreactor |
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Informations |
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Project:
Photobioreactor |
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Developed By:
Alexandre Dugas |
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Type of Project:
Hardware or software design |
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Category:
Applied science and technology |
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Class:
Intermediate |
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Age of Participant:
15 |
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School:
Collège Durocher |
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Teacher:
Patrick Mathieu |
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Has Won:
Gold medal for a biotechnology project, intermediate level |
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Project presented at the 1998 Montérégie
regional final of the Bell Science Fair
Selected for the 1998 Quebec final (Montreal) of the Bell Super Science
Fair
Selected for the 1998 Pan-Canadian Science Fair in Timmins, where the
participant won
a gold medal for a biotechnology project, intermediate level
Using a photobioreactor to produce oxygen
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Scenario |
Many people work every day in newly constructed,
hermetically sealed buildings. In a number of these office towers—easily
100 floors high—it is impossible to open a window to allow the
air to circulate. Although the buildings have modern ventilation systems,
these systems fail to adequately evacuate the carbon dioxide produced
by the people who occupy these offices.
As you know, carbon dioxide if harmful. A level of carbon dioxide that
exceeds 600 ppm (parts per million) is cause for concern. If the carbon
dioxide content in a room becomes too high (normal air content: 350 ppm),
building occupants can suffer migraines, an inability to concentrate
and possibly aggressiveness. A study by Dr. Fanger revealed that 42%
of problems involving employee dissatisfaction were related to poor ventilation.
The ideal solution would be to develop a ventilation system that could
transform carbon dioxide into oxygen, while operating as a closed circuit
in order to continue offering existing savings.
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Hypothesis |
Knowing that all green plants convert carbon dioxide
into oxygen through the process of photosynthesis, it might be possible
to regenerate air by designing a structure housing plants. But how do
we get around the fact that it would be practically impossible to provide
enough plants to satisfy the oxygen needs of an entire office? Moreover,
the cost of such a structure would be extremely high.
We have to turn our attention to microorganisms. Microscopic algae are
good candidates for our purposes because they can be concentrated in
a space as small as a cubic centimetre. They are also easier to control
than bacteria and pose no danger to humans.
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Solution |
I built a prototype of a photobioreactor in order
to demonstrate the feasibility of using microscopic algae to regenerate
the oxygen content of air.
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Operating the photobioreactor |
Principle:
The principle is as follows: Start by circulating air bubbles in a solution
of algae. The photosynthesis produced by the algae lowers the CO2 content
of the air being injected into the photobioreactor, increasing the concentration
of oxygen: :
6CO2 + 12H2O + light _ C6H12O6 + 6O2 + 6H2O
In order to sustain the growth of algae, phosphorus-rich nutrients are
regularly injected into the reactor. The effectiveness of production
depends on two conditions: the richness of the growth of the algae and
the distance covered by the air bubbles.
Algae and nutrients used
To start growing the algae, I used an aquarium containing two fish.
The fish produce excrement rich in phosphorus. After a few days, the
two fish are asphyxiated as a result of a lack of oxygen and the high
concentration of phosphorus, since the water is not being filtered. After
one month, the first algae—belonging to the group Ankistrodesmus—appear.
They multiply at an exponential rate. Since the Science Fair rules prohibit
the use of vertebrates in their booths, fertilizers composed of 10% nitrogen,
52% phosphorus and 10% soluble potash were used as alternatives to phosphorus.
Building the structure
Once the production of algae had begun, I had to design a container that
would favour oxygen production. This container—which will house
the photobioreactor—can be made of clear acrylic, a light material
that is easy to work with. The housing was built in the shape of a hexagonal
prism, which offers minimum surface area for maximum volume, thereby
minimizing material costs. (Leaks were sealed with silicone for aquariums
in order to avoid poisoning the algae.)
In this system, outside air is introduced through a pump. The bubbles
that are produced travel along a spiral staircase—also made of
acrylic—allowing the air to circulate slowly in the solution and
preventing the bubbles from reaching the surface too quickly.
Finally, light is needed in order for the algae to produce oxygen. The
photobioreactor is equipped with a 15-W neon light, which is placed in
a second hexagonal prism.
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Results obtained |
I obtained results over a 10-day period. These results
reveal the reactor’s ability to produce oxygen. I noticed that
the reactor worked more effectively when the algae were dispersed; they
tend to collect of the flat surfaces of the reactor. Also, contrary to
what I had initially anticipated, the oxygen enrichment process occurred,
not from the circulation of outside air, but from the constant recirculation
of air inside the reactor.
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Main problems encountered |
The main problem with this process is trying to
keep the algae suspended. Over time, they tend to form clumps. These
clumps hamper the bioreactor’s performance, since only a thin layer
of algae is left to contribute to the process of photosynthesis. A drill-activated
pump was therefore connected to the photobioreactor in an effort to stir
the water and keep the algae suspended. The water entry and exit points,
located at the base and the top of the reactor respectively, produced
a convection current.
Growth depletion was the second obstacle encountered. The reactor’s
effectiveness progressively decreased when the system operated on four
hours of rest for every 20 hours of activity. Its effectiveness was restored
when it operated on a cycle of 12 hours of rest for every 12 hours of
activity.
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Conclusion |
The algae provided less promising results than expected.
To improve overall performance, the flow of water would have to be much
slower in order to obtain an adequate decrease in the level of carbon
dioxide. An analysis of several varieties of algae would also have to
be done. In this case, only one variety of algae was tested. Thousands
of others with greater potential could be used. Maybe algae isn’t
the best solution after all. Maybe bacteria could be used instead?
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© 2002, Conseil de développement du loisir scientifique (CDLS). This
document is distributed by the Conseil de développement du loisir scientifique.
For more information, visit our Web site at www.cdls.qc.ca. |
The opinions expressed
in this section are those of the authors and do not necessarily
reflect the opinions of Merck Frosst or its employees. |
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Principle:
