CO2 reactor renders bubble rate irrelevant?

[geoffrey]

Hi All,

I have changed from my previous limewood CO2 diffuser to a reactor with that spinning bio-ball. Notice that CO2 builds up within the container and the in rush of water from my pump would stir and mix the CO2 with the water via the spinning bio-ball. The rate the CO2 bubbles would only influence the size of the CO2 cavity within the reactor. Hence, wouldn't that make bubbles per second irrelevant as the amount of CO2 that goes into the tank would now depends on the stirring effect of the bio-ball?

Am I wrong?

[budak]

Aiyah, using bubbles per second to gauge CO2 level is like using rpm to see how fast the car is going. At a given CO2 bps rate, the KH level will determine (buffer) the CO2 concentration and corresponding pH level. Since a pH of 6.5-6.8 is recommended for optimal plant growth and fish health, a higher KH level means that more CO2 is required to achieve that desired pH. So measurement of KH and pH using proper test kits is the best way to know CO2 levels. Too high a KH and you need to inject more CO2 to get the proper pH, but also means that your fish will suffer. Too low KH will cause pH to fluctuate too much, which is also bad.

[geoffrey]

Budak,

You are right but I so used to using BPS to tell me I have enough CO2 that it hard for me to release that old paradigm. But then again, if the tank is CO2 limited, increasing BPS now is not going to help right? It only build up within the cavity, how much goes out depends on the stirring of the bio-ball? So, to get the right pH/KH/CO2 relationship will depends on the speed of the stirring, hence the flow rate of your pump. Right?

[bigjohnstud]

I personally feel that that the 'bps' rule serves merely as a guide where the simpler 'sense of sight' can be used to measure water parameters (compared with the hassle of using test kits) . What Budak commented on was the chemistry aspect of dissolving CO2 in water (ions exchange and its relation to pH and kH) while geoffrey had raised the mechanical aspect (rate and efficiency of dissolving CO2). I think the 2-3 bps guide is based on the assumption that the dissolution of CO2 is very efficient. Depending on whether a reactor or diffuser is used, one has to adjust the bps accordingly to make up for any loss of dissolution efficiency. The use of test kits as suggested by budak would of course reduce the error arising from guesstimation. Correct me if I strayed.

[bclee]

Bubbling rate does have some effect on the amount of CO2 dissolving into the water, although in the case of a reator, there is another factor: water flow.

More BPS means more CO2 in the chamber in contact of water. To a certain extent also mean more CO2 in contact with the water leading to more CO2 dissolving.

Experiment with your reactor, you will get a feel how the BPS and water flowrate influence your CO2 ppm. But be sure to be around when you experiment. Don't set your bubbling and go to work... because you may get a nasty shock when you come home... advice from my experience[].

BC

[PeterGwee]

Hi Geoffrey, what budak means is to ask you to test the water conditions to determine the CO2 level and don't bother much with the bubble rate (act as only reference point to start or know where to increase..eg..now I using 2 bubble per second..pH is 7.1 and KH is 2..I need to increase the bubble rate..so increase to three lor)...Most important is your pH and KH value..although I did not strictly follow it due to KH measuring alkalinity instead of availablity of carbonate in water (I set my CO2 rate against time needed for plants to bubble actively..kekekee..not recommended if you are not careful..and also plants must have settled in the tank before you do it..)[]

[vinz]

I think I know what Geoff is getting at here. With a CO2 reactor, amount of CO2 dissolve depends on the rate of dissolution of the reactor. Which in turns depends on the rate of spinning of the balls, surface area of contact between CO2 and water. The spinning of the balls depends on the amount of water in the reactor and the surface area of the CO2-water contact depends on the bps.

If there is too little water in the reactor, the ball(s) gets stuck, slows down or stops moving and the reactor becomes a fancy CO2 bell with only swirling water to dissolve the CO2.

If there is too little CO2 in the reactor... i.e. only a few small bubbles swirling around at the top, the surface area of exchange between water and CO2 is smaller.

So, yes, bps stills matter, but the relation is not linear or a smooth curve. You need to maintain a bps that maintains the largest possible surface area for CO2-water contact and yet allow the ball to spin at the right speed to ensure the level of CO2 you need. (Spin too fast -> too much CO2 dissolve, spin too slow -> not enough CO2 dissolve)

There are no hard numbers to my knowledge, so it's trial and error, plus pH and KH testing. But safe to say, never let the ball stop spinning.

[vinz]

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On 1/8/2003 9:43:32 AM

...
Too low KH will cause pH to fluctuate too much, which is also bad.
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To clarify a myth... quoted from Chuck Gadd's CO2 article.

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Myth: A Low KH results in a larger pH swing when adding CO2.
Many people are under the mistaken impression that a low KH results in large pH swings when adding CO2, while raising the KH will result in smaller pH swings. This is not the case. The KH will move the start and end pH values, but the pH swing will be the same for a given level of CO2. You can see this in the chart below, or using the calculation:

Case 1: Assume a KH of 15 degrees, and a starting CO2 level of 4.5ppm, which would result in a pH of 8.0. If we then add CO2, to increase the CO2 level to 28ppm, that would drop the pH down to 7.2, for a pH shift of .8.
Case 2: Assume a KH of just 1.5 degrees, and a starting CO2 level of 4.5ppm, which would result in a pH of 7.0. If we then add CO2, to increase the CO2 level to 28ppm, that would drop the pH down to 6.2, for a pH shift of .8, the exact same as in case 1.


One possible explanation for this myth is that many copies of this pH chart skip some of the higher pH values, for example, jumping from pH 7.4 to a pH of 8.0. If the reader didn't pay careful attention, they might mis-interpret the size of the pH swing. I specifically made sure to include all pH values, between 6 and 8, in steps of .2.

This relationship will break down at extremely low KH levels (below 1 degree), when there isn't enough carbonate to completely buffer the acids present. In that case, the pH can drop quickly and dramatically. But if the KH is 1 degree or higher, then the size of the pH swing when injecting CO2 will be determined only by the amount of CO2 dissolved in the water.
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To be on the safe-side, I think better to start at 2KH... we don't really know how accurate the test kits are.

[Allen_1971]

Geoffrey,

There are 2 things to note here... bps communicates to us the rate at which CO2 is being injected. This is useful because it helps us to calibrate our system in case we are over/under injecting. So no using a reactor does not remove the need for a bubble counter.

Secondly, the question of a reactors efficiency comes into play. I'm probably not wrong to say that a reactors efficiency is directly proportional to the amount of surface area in contact between the CO2 and water. i.e. if you have more surface area, you get better efficiency. For the taiwanese reactors, my main gripe is that their optimum efficiency is roughly 1-1.5bps. At this level, the water flow from your filter is probably able to smash the bubbles into smaller bits and thus increase the surface contact. At anything higher, CO2 starts to accumulate in the top of the reactor... this effectively changes your reactor into no more than a glorified inverted bell. The bio balls do help somewhat by spinning and catching some of the air, but since it spins below the water line, it can only catch small bubbles which are somehow pushed below the water line. Worse still, the bio balls start to get stuck (not spin) after sometime...

So basically my answer is, bps is not irrelevant... and your question should be is your reactor still efficient at high bps? and my answer is no.

pushing the point one step further... what does this mean to you? Say your tank at 1bps was having about 10ppm of CO2 and you wanted to increase your CO2 to 2 bps... you might find that due to the inefficiency of your reactor at 2bps that your CO2 level may not increase at all... worse still, you could even have a drop in CO2... though this last point depends heavily on how much your efficiency is compormised.


Vinz,

I agree totally wif what was posted by chuck... which is one of the reasons I recommend that people simply target a 0.6ph (0.8 is a bit too much for my taste)drop from their ph with no CO2 injection. If we assume ambient dissolved CO2 is roughly 5ppm, then this will give us roughly 15-20ppm CO2 dissolved - Regardsless of our KH.

[bclee]

Vinz, I somewhat disagree with the Chuck Gadd's article.

At higher KH, we need to inject more CO2 to achieve the same ppm. Although the 23.5ppm change at both KH 15 & KH 1.5 causes same pH change of 0.8, it takes more CO2 to achieve the 23.5ppm change at KH 15 compared to KH 1.5.

Why?

At KH=0, 0.1% of CO2 dissociates in the water.

CO2 + H20 <---> H+ + HCO3-

The H+ will cause the pH to drop.

But with the presence of KH,

CO3-- + H2O <---> HCO3- + OH-

Some H+ from CO2 dissolving in water will neutralise the KH buffering. Therefore, more CO2 is needed to dissolve in the water to achieve the ppm and some will react with the KH.

Correct?

KH is call a alkaline buffer for a reason. A buffer will cause solution to maintain a pH value. It will take more acid to change the pH compared to a solution will lower or no buffer. The acid in this case is the weak carbonic acid formed by 0.1% of the CO2 dissolved in water.

BC

[PeterGwee]

Ermm...Allen..I don't think using just pH to judge the level of CO2 in tank water is correct although it seems true in theory. For my tap water, the pH is about 8.0 which is similar to what vinz is getting as well (KH is zero though). That means by your assumption, the level of CO2 should be around 15-20ppm the moment my pH is 7.4???

[bclee]

I think the KH is not exactly 0. Could be 0.something.

Note:
KH=0.5, CO2=0ppm, pH slighly >8

BC

[bclee]

Take this case.

KH=0.5, CO2=0.2ppm, pH will be 8.0.

Let the water stand for some time, CO2 in atmosphere dissolve in water.

KH=0.5, CO2=1.5ppm, pH will be 7.0.

Hope this explain the pH=8 tap water mystery.

BC

[PeterGwee]

Bclee...I did an experiment before..I inject air bubbles into the tap water using an air pump to see if the pH would change...no changes in pH were found though after 10-15 minutes...

Wonder if Amano's dropchecker solution is any different from JBL or Sera type...he did say that his solution is used to measure the level of acidity in water which would not be affected by KH value of the water.

[vinz]

BC,

Regarding Chuck's article. He does not go by amount of CO2 inject but the ppm of CO2 dissolved in the water. Quoting his article again, the last line: "But if the KH is 1 degree or higher, then the size of the pH swing when injecting CO2 will be determined only by the amount of CO2 dissolved in the water."

So, errr... you aren't really disagreeing, just coming from a different angle?

[bclee]

Maybe you can let the water stand for 1/2 a day or so. CO2 level in the atmosphere is very low. It will take quite some time for the CO2 level to rise.

BC

[bclee]

The statement was "Low KH will cause large pH swing when adding CO2". Chuck Gadd do not think that this statement is true.

But I think this statement to some extent is true. Low KH will allow more CO2 to dissolve compared to higher KH while keeping the CO2 injection constant. More CO2 dissolving leads to more ppm in water and causing larger pH swing.

If you are going to achieve 28ppm of CO2 at KH=15 as per his case study, you definitely having to increase the CO2 bubbling or the like. But he is treating injecting CO2 at KH=15 the same as KH=1.5.

BC

[Allen_1971]

Peter...

in my earlier post i mentioned this line "If we assume ambient dissolved CO2 is roughly 5ppm"... that is my context for the recommendation for a 0.6ph drop. Choose any chart and any kh you will, it will result in the same level of CO2. This can even be proven mathematically if you care to do so.

As for why your tapwater is 8, many water treatments use processes which outgas any dissolved gasses in your water... as such its not surprising to have KH0 and PH8 water... as BC said, leave it to stand... I think 3-4 hours is necessary for equilibrium... then measure again.

[Allen_1971]

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On 1/8/2003 2:06:04 PM

But I think this statement to some extent is true. Low KH will allow more CO2 to dissolve compared to higher KH while keeping the CO2 injection constant. More CO2 dissolving leads to more ppm in water and causing larger pH swing.

If you are going to achieve 28ppm of CO2 at KH=15 as per his case study, you definitely having to increase the CO2 bubbling or the like. But he is treating injecting CO2 at KH=15 the same as KH=1.5.

BC
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BC,

I think the point vinz is trying to tackle is this... Chuck doesn't talk about injection of CO2 in terms of bps he talks about it in terms of ppm. Yes you may need more bps to get your higher KH tank at the same ppm. but for an increase of 15 ppm from (say 5 to 20ppm) with an arbitrary KH, you should find that the amount of PH swing is constant.

For example,

if my KH is X and my PH is Y and my CO2 is 5ppm at the start. Then I start injecting CO2 at Z bps to achieve CO2 of 20ppm. Assume I measure the PH and find that it is now Y-0.7.

Now I add some more buffers... so that my KH is now X+A, and my PH is Y+B for a CO2 of 5ppm at the start. Then I start injecting CO2 at Z+C bps to achieve a CO2 level of 20ppm. Now lets measure the PH, and you should find that your PH is Y+B-0.7!

So Chucks statement is true. Perhaps his statement on the top "A Low KH results in a larger pH swing when adding CO2" is a bit misleading. The myth he is trying to debunk is what I mentioned above, and what he gave examples of in his webby.

p/s I tried to think of a one line "title" which would be more accurate, and really its not that easy...

[bclee]

Allen, I read Vinz's post. He seemed to say that it is not true that low KH causes pH to fluctuate.

And, I am trying to say that low KH it takes lesser acid/CO2 for pH to swing... which also may mean that it is "easier" for pH to change at lower KH, contrary to what Chuck Gadd's article claim.

BC