Paddlewheels mix the water column very well
for the first few meters downstream of the
Green House with Submerged pipes
| But from there on, until the water reaches
the paddlewheel again, the flow is laminar
and the algal filaments which are on top
have a tendency to stay on top and those
on the bottom have a tendency to stay on
the bottom. Only a slow "rolling over"
effect caused by friction with the bottom
of the basin and a similar effect seen at
the sides of the basin succeed in continuing
at a much slower rate the "in light-out
of light" regime for the algal filaments.
.. faster in shallow basins and slower in
One can continue turbulence far downstream
of the paddlewheel by putting baffles across
the water channels so as to thrust the water
upward, and then a little farther along the
channel another baffle to push the water
flow downward toward the bottom of the basin.
Under poor meteorological conditions I obtained
36 grams of equivalent dry Spirulina per
square meter per day.
The flat baffles
I used were difficult
install and more
fragile than desired,
I later used sand-filled
PVC plastic pipe
set across the channels
and under water,
the surface and near
This gave about the same effect, but any
baffle system resists the flow of water and
hence requires more power to be applied to
the paddlewheels. Doubling or tripling the
production certainly justifies using a little
more "push " .
Another method of enhancing paddlewheel
mixing is to put very low raised bars or
ridges (herringbone pattern ) on the basin
bottom. Water flowing over them will create
vortexes that will mix the culture top-to-bottom.
A third way of imparting vertical mixing
to horizontal flow is to make an imprinted
pattern on the bottom material (whether it
be of plastic or cement) where the imprint
consists of an endless chain of long, fairly
wide, oval grooves connected by venturi sections.
A film of water will be sent toward the surface
at the entrance to the venturi and downward
into the groove after the venturi. The effect
in trials at 10-cm per second was only partial,
and interesting work remains to be done to
maximize the effect and minimize clogging
along the bottom pattern.
Concerning temperature at night: Spirulina
is able to tolerate nighttime temperatures
near freezing - they grow in Lake Titicaca
at 12,506 feet (3800 meters) elevation. This
actually is an advantage as there is less
nighttime conversion of daytime carbohydrate
photosynthate into excreted polysaccharides,
so respiration biomass loss is minimized.
But cold nights also can be a disadvantage
as cold filaments are especially vulnerable
to photolysis when flooded with bright and
powerful sunlight at dawn. The advantage
can be kept and the disadvantage eliminated
if just before dawn the water can be heated
to around 20°C and the mixing can be raised
to maximum - that is, a turbulence sufficient
to raise and lower the algal filaments rapidly
in the water column - in and out of the sunlight.
This way, maximum biomass is conserved and
maximum photosynthesis can begin immediately.
Otherwise there is very little photosynthesis
until around 10:30 AM when the water has
been sufficiently warmed naturally by sunlight
( and you have lost alI these hours of biomass
production). As Spirulina is moderately thermophilic,
temperature - up to 40°C - means greater
yield. Low at night, high during the day
but with maximum turbulence during the day
is what is desired.
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