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Separating algae from water from a raceway pond - Page 2

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Posted by Michael B on December 13, 2009, 1:57 pm

Anaerobic means without oxygen. So if it's in an airtight container,
the bacteria are forced into a condition of anaerobic metabolism,
and they will make methane. Rather like the "swamp gas" you
can read about. Those swamps are more often in warm climates,
the warm water holds less or no oxygen, the bacteria function in
an anaerobic metabolism. Like in a septic tank.
But in the raceway you describe and diagram, oxygen exposure
would be enhaned by the paddlewheel, the shallow depth, the
overall water exchanges. Not saying you couldn't do it, but that
from the descriptions, I would be expecting aerobic metabolism,
without methane, without hydrogen sulfide.

Posted by Curbie on December 13, 2009, 6:08 pm


You and I are on the exact same page up to this point.

Here is where I haven't been clear enough; there are two processes at
play here, 1) the growing and collecting of algae for bio-mass in a
raceway pond, and 2) the anaerobic digestion of the bio-mass in a
separate digester. In this post I am focusing on the growth and
collection process and the digester hasn't been depicted out here.

"The notion is algae grown in, and collected from a raceway pond, the
bio-mass (algae) is THEN composted in an anaerobic digester to produce
compost and methane."

What I'm mainly looking for is feedback on filtering the algae via the
paddle-wheel and concentration channel notion pictured in the image in
an effort not to convolute or complicate any step of the process.


Posted by Morris Dovey on December 13, 2009, 6:38 pm

Curbie wrote:

I don't have any expertise here - and you've stirred my curiosity enough
to prompt questions...

Is it possible to configure a raceway such that a single transit of the
raceway would provide sufficient algae to justify algae/water separation
at the output end of the raceway?

Would re-use of the recovered water enhance algae growth in subsequent
"cycles" (I'm assuming that there would be water and heat conservation

Could the raceway be "featured" in such a way as to induce non-aerobic
churning/turbulance throughout its length?

Is there an actual process advantage to separating the growth and
digestion stages, or is this separation primarily for convenience?

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by Curbie on December 13, 2009, 7:47 pm


The algae is just driven around the raceway like cars round a
racetrack, if I understand the concept behind your use the phrase
"single transit" as a single trip around the raceway, my answer would
be probably, but keep in mind that the primary function of the
paddle-wheel is to maintain algae at a higher exponential growth rate
and to help prevent smelly anaerobic digestion
in the raceway pond. Collecting all the algae from the pond in one
circuit would greatly impede further growth.

Growth and collection happens all season, it's a continuous process as
long as the temperatures are within the algae's growth range.

The temperature of the pond is primarily dictated by indigenous
algae's preferred maximum growth rate temperature and ambient
temperatures. The algae growth process takes place in the warmer
months (just solar heat gain), so I don't have a use for low
temperature heat at that point in time.

In the plan I'm looking at, the concentrated algae is moved to an
anaerobic digester where the water is drained of to equalize the
pressure between the channel and the raceway so the gates will open

Maybe, but the reason the growth raceway and a separate digester is
that they are two different process. I don't want digestion in the
raceway where the gases produced by anaerobic digestion (CH4 + C02)
are produced where they can't be captured, and algae won't grow in a

The separation is necessary to my understanding.


Posted by Jim Wilkins on December 14, 2009, 3:48 pm

I wonder if you could combine circulation, aeration and separation by
raising the water on one side of a barrier with a solar-powered
Archmedian screw and dumping it on a cloth filter belt on the other.
One motor could drive both through a non-reversing drive such as a
worm gear. A brass worm with a plastic wheel doesn't need lubrication
or protection from the weather. Figure about 50% efficiency for it to
size the motor and solar cell.

Hardwood boiled in wax should work if you can't find a large enough
plastic disk. Make one brass and one steel worm (or use large wood
screws), notch the steel one so it cuts and run it against the wheel
blank to cut the teeth. You'll have to experiment with the diameter to
make the cutter fall into the previous grooves.

The cloth could be long enough that the algae is fairly dry when a
scraper under the far end roller removes most of it. I've used a strip
from the unworn edge of an old cotton bedsheet to filter rainwater off
the roof and it lasted all summer. Fully dried algae flaked off
easily. The water in the cistern under it stayed clear.

The Archmedian screw could be coiled clear plastic tubing that you
flush with a hose when it clogs. If water alone isn't enough you can
force a swab through with water or air pressure, meaning it needs a
hose coupler on one end.

The stream at UNH was fairly steep and fast flowing but most of the
algae attached itself to the rocks anyway, not much ran free with the
water. It sticks to my PVC gutters and downspouts. You could try black
polyethylene sheet, Home Depot sells stainless steel staples to hold


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