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Re: Aquatic Plants Digest V1 #90
Ho Doug,
> Have any of you done experiments with Trickle Filters and alleged CO2 drive
> off? (George didn't you do something?)
I'll include the result of an experiment I did.
BTW, did you ever get around to putting that CIS access software on
disk? I haven't seen it yet ...
> I'd appreciate some good solid chemistry if any of you have it. Does
> CO2 have a greater affinity for water then does O2? If it does then
> how can surface agitation or aeration decrease CO2?
The theorectical equilibirium value of CO2 in water is about 0.5 ppm.
There was a thread in sci.aquaria a few months ago that thrashed this
out.
Anything you do to agitate the water will promote diffusion and help
it reach equilibirium. Since our tanks are usually way above
equilibirium values due to CO2 injection and/or fish/plant/bacteria
respiration, any agitation tends to lower the amount of CO2 in the
water.
[From sometime in 1992]
CO2 Loss in Large Aquariums
by George and Karla Booth
We are in the process of setting up a new 90 gallon plant tank based
on concepts presented in Dupla's "The Optimum Aquarium". Before we
finalized the set up, we had the chance to conduct some experiments in
the bare tank. We ran a "CO2 loss" test recently and wish to share
the results with the AGA. We verified that the major culprit in CO2
loss is surface turbulence (no surprise here). However, counter to
our expectations, trickle filters DO NOT necessarily cause rapid CO2
loss.
The test tank was a "90 gallon" glass tank (48"x18"x24" tall, 79
gallons of water). It was set up with Dupla heating coils and a Dupla
"DuplaTherm" temperature monitor/controller to maintain the water
temperature at 79.0 +/- 0.1 degrees F. A Dupla Reactor "S" was used
to inject CO2 via the sump of a trickle filter.
CO2 concentrations and pH were measured with LaMotte test kits. Note
that the LaMotte CO2 test kit has a resolution of 1 ppm (mg/l) and an
error of about +/- 2 ppm. The LaMotte pH test kit has a resolution of
0.1 pH units and uses an "octet color comparator". The pH error is
about 0.05 pH units based on comparisons with a Sandpoint II pH/ORP
controller and comparisons with a pH/KH/CO2 table.
An AquaClear 802 powerhead was used to circulate water in the tank.
It was placed near the bottom in three tests to provide a gentle
circulation current with little surface turbulence. A 0.3 ft/sec
surface current was noted, giving a smooth surface pattern that looked
like "heat waves" rising off a highway in the summer. In a forth
test, the powerhead was placed at the surface and was adjusted to give
vigorous ripples without splashing.
The trickle filter used was an Amiracle "100 gallon" unit with a
bio-media capacity of 3.99 gallons. The media space is 16.125" long x
7.625" wide x 7.5" high. The media used was 238 Dupla BioKascade
bio-balls, with the internal slats arranged roughly horizontally to
allow the water to move through the media in a gentle, cascading
manner.
The filtered water is circulated by a Quiet One pump controlled by
ball valves to provide a 400 gallon per hour flow, turning over the
tank five times per hour. The trickle filter has two water returns.
One is directed across the bottom 1/3 of the tank, providing a flow at
what will eventually be the top of the gravel. The other return
utilizes a Magnum 330 water return fitting. To provide surface
turbulence, a Magnum diffuser was used to direct the return flow
across the surface, producing ripples equivalent to the powerhead when
placed at the surface. For tests without turbulence, the diffuser was
removed, allowing the water to be directed towards the bottom of the
tank.
The tank is bare except for the equipment mentioned - no gravel, no
livestock, no plants. Lighting is room ambient. The top is open.
Before we turned on any equipment, we filled the tank with tap water
and adjusted the water hardness by adding 3 tablespoons of calcium
carbonate (CaCO3) and 1 tablespoon of sodium bicarbonate (NaHCO3) to
achieve a GH of 3.5-4 degrees and KH of 7 degrees as measured by a
Tetra test kit. Note that GH has no bearing on the CO2 measurements,
but a KH of over 3.5 is needed to ensure accurate CO2 test kit
readings. A KH of 7 was selected to keep pH readings in the range of
the LaMotte test kit.
After letting the water equilibrate for one day we measured dissolved
CO2 at 2-3 ppm. We then set up a large powerhead to circulate the
water (Project RS-500, ~500 gph) and let it run for a day. The CO2
remained about 2-3 ppm. At the end of most of the tests, CO2 again
measured about 2-3 ppm, indicating that this was the equilibrium value
for the experimental conditions (note that the altitude was 5000 feet
above sea level).
After the initial tests, the heating coils, trickle filter and CO2
injection were set up. The large powerhead was replaced with the
AquaClear 802. For the first test ("trickle, turbulence"), the
trickle filter was run with the Magnum diffuser producing surface
turbulence and with the powerhead running at the bottom. For the
second test ("powerhead, turbulence"), the filter was turned off and
the powerhead was placed near the surface. For the third test
("trickle, quiet"), the trickle filter was run without the diffuser
and with the powerhead running at the bottom. For the last test
("powerhead, quiet"), just the powerhead was used, running at the
bottom. In all tests, the trickle filter and reactor were used to get
the CO2 level up to the point were the test started. At that time,
the CO2 was turned off and the reactor allowed to clear of residual
CO2 before actually starting the test.
Due to some difficulty in getting the CO2 to the same starting point
for each of the tests (actually, a lack of patience on our part), the
CO2 readings were normalized for the table below. To normalize the
readings, the raw data was plotted with the CO2 concentration on a log
scale. A best-fit line was drawn by eye through the data points. The
numbers in the table below were then read from the plotted lines at
hourly intervals. Just CO2 data in the range of 10-33 ppm is shown,
since we consider that range to be the most relevant for planted
tanks. The raw data is shown at the end of this note.
CO2 concentration (ppm)
-----------------------
trickle, powerhead, trickle, powerhead,
Time (hrs) turbulence turbulence quiet quiet
-------------------------------------------------------------
0 33 33 33 33
1 21 24 28 28.5
2 13.5 17 24 25
3 - 12.5 20 21.5
4 - - 18 19
5 - - 14.5 16
6 - - 12.5 14
7 - - 10.5 12
8 - - - 10.5
At KH=7, the following table relates CO2 to pH:
CO2 (ppm) pH
-------------
42 6.7
33 6.8
26 6.9
21 7.0
17 7.1
13 7.2
10.5 7.3
8 7.4
What surprised us was the fact the trickle filter itself was not
responsible for much CO2 loss (compare the last two tests). It should
be noted that air was not pumped into the media chamber during the
tests. We suspect that any out-gassing of CO2 by the media will
quickly produce a concentration of CO2 in the chamber such that it
reaches equilibrium with the CO2 in the water.
In our other trickle-filtered tank, we have noted very high CO2 loss
(we go through a 10 pound tank in 6-7 weeks). We now suspect that the
loss is caused by the Ehiem canister filter spray-bar return. We plan
to run further experiments on that tank to verify this conjecture.
Although some authorities recommend pumping air into the media chamber
of a trickle filter, we have found no evidence of a need for this.
Thriving plants will provide plenty of oxygen for the aerobic bacteria
colonies during the day and we have noticed no problems at night when
the plants are at rest. We ran a long term test on another tank using
a Sandpoint II pH/ORP controller and found no difference in ORP with
the air pump on or off. We also noted that less CO2 was used with
the air pump off (longer intervals between CO2 bottle refills).
From the table, it would appear that a 10 pound CO2 tank will last
about 5 months when we finally get the new tank set up. We will have
a KH of about 5 and will regulate the pH to be 6.8 +/- 0.5. This is a
CO2 concentration swing of 5 mg/l (27 mg/l to 22 mg/l) times 300
liters and should occur within 1.25 hours for a usage of 29 grams per
day. Of course, the usage by the plants will increase this by some
amount, but that's another experiment!
CO2 test raw data
-----------------
CO2 concentration in ppm and measured pH ()
Clock trickle, powerhead, trickle, powerhead,
Time turbulence turbulence quiet quiet
-------------------------------------------------------------
6:00 pm - - 23 (7.0) 47 (6.65)
7:00 pm - - 20 37 (6.75)
7:30 pm - 33 (6.8) - -
8:00 pm 27 (6.9) 27 (6.9) 17 31 (6.8)
8:30 pm 20 (7.0) 24 (7.0) - -
9:00 pm 17 (7.1) 19 (7.0) 14 27 (6.9)
10:00 pm 11 (7.3) 14 (7.2) 10 23 (7.0)
11:00 pm 8 (7.4) 11 (7.3) 10 21 (7.0)
12:00 am - - 9 17 (7.1)
3:00 am - - - 15
8:00 am 2 2 - 8 (7.4)
12:00 pm - - - 5