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Are we missing a much more simple CO2 solution?

Discussion in 'CO2 Enrichment' started by jeremy v, Sep 16, 2008.

  1. jeremy v

    jeremy v Guru Class Expert

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    Hello,

    I have been reading a lot lately about people now using fine CO2 mist (mazzei, etc.) to increase plant growth versus having the CO2 completely dissolved in the water. It has been said that the plants can grow faster and the aquarium dissolved oxygen levels will be higher with a mazzei type of setup compared to a reactor that has a 100% dissolve rate. The higher dissolved oxygen levels indicate increased photosynthesis occurring when all other factors involved in plant growth remain the same. It has been said that these benefits are also achieved while using less CO2 than with a highly efficient reactor.

    I have never used a mazzei setup to test any of this for myself, but it has brought something to my mind that I can't stop thinking about.

    My question is that if you can get a boost in plant growth just by blowing a fine mist of CO2 bubbles around versus fully dissolving the CO2 in the water, why couldn't you achieve most of the same benefits by just blowing around a fine mist of air in the tank instead of pure CO2? Air already has a much higher concentration of CO2 in it (380ppm or so) than the fish in our tank water could ever handle in a dissolved state. This would completely eliminate the need for CO2 tanks, regulators, solenoids, etc. You would mist the tank with micro air bubbles and the plants would get their CO2 from those bubbles directly. Then the tank would also never have fish kills from CO2 accidents, the aquarium would never have too little oxygen for the fish at night, drop checkers/pH probes, etc. would become unnecessary, you wouldn't have to worry about CO2 outgassing in your filter designs at all, etc. Just run something like a mazzei with a high enough pressure so it can create a super fine mist of microbubbles and then run air through it.

    It seems to me like the only problem with airstones and venturi's on powerheads is that they make bubbles that are too large to ever make it down to the plants to stick to them and give the plants their CO2. As a result, they instead just end up outgassing the tank's dissolved CO2 (anything over natural equilibrium with air) and don't benefit the plants at all by having the bubbles stick to the leaves to give the plant atmospheric CO2.

    Tom has also talked about the boundary layer on the plant's leaves as well, and mentioned how the boundary layer on terrestrial plants is much lower than on aquatic plants. Wouldn't the boundary layer also be lessened substantially then with a bubble hitting a leaf directly and creating an air/leaf interface between the bubble and leaf instead of a water/leaf interface?

    Is there something that I am missing having to do with understanding how CO2 interacts with the plants to make these questions null and void?

    I don't necessarily think that this setup would get you all the growth of a CO2 only mazzei since each bubble in the tank would have less CO2 to give the plant when stuck to it, but it might possibly easily get you something in-between a CO2 injected tank and a natural "walstad" type of tank growth level. I am thinking somewhere around Excel levels of growth without needing Excel at all.

    I just don't have any way to try this right now with my current setups in order to test this for myself, so I thought I would throw it all to you guys and ask for your two cents/experience.

    Have a good one, Jeremy
     
  2. Carissa

    Carissa Guru Class Expert

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    So basically you are saying that a bubble of air with 380ppm of co2 in it is better than water that has significantly less dissolved co2.

    One issue I can see is that since it's not pure co2, once the tiny amount (percentage wise) of co2 is extracted, the bubble theoretically sits there in a useless state, unless it is moved away somehow. I'm not sure what happens then, if these bubbles would be dissolved into the water or otherwise be dissipated?

    Interesting thought and I would like to know the answer.
     
  3. VaughnH

    VaughnH Lifetime Charter Member
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    That is a very interesting idea! But, I think I see the flaw in it. Let's assume that the aquarium needs one bubble per second of CO2 in order to get enough CO2 to the plants. So, if the bubble had only 50% CO2 in it, the tank would need 2 bbs of that gas mix. But, in reality air bubbles have only .04% CO2 in them, so the tank would need 1/.0004 or 2500 bubbles per second! That is a lot of bubbles, Bubbles!!:p
     
  4. jeremy v

    jeremy v Guru Class Expert

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    What originally got me thinking about it was the combination of the posts I have been reading on mazzei misting techniques, along with the ongoing thread (of Tom's) about growing carpets of HC emersed before flooding the tank with water after the carpet is filled in.

    http://www.barrreport.com/articles/3361-new-methoid-make-nice-rug-hc-before-you-add-water.html

    All the aquatic plants that can handle being out of the water will grow much faster in air from the extra CO2, so I would think you would get the same benefit by getting lots of micro-bubbles to stick to the leaves underwater. I would imagine it being sort of a happy medium in growth rate between standard immersed and emersed growth levels for any one plant.

    You are right Carissa, once the bubbles were stripped of their CO2 by the plant it wouldn't benefit the plant to have that bubble on its' leaf anymore. If the current and micro-bubbles were kept relatively continuous during the day though, the plants could constantly shed and then replace CO2 exhausted bubbles with newer ones.

    If this didn't work for growing plants like I am hypothesizing, I could see this as possibly being a viable technique for adjusting newly purchased plants that were grown emersed to the immersed conditions of the aquarium. That way they wouldn't have to replace their existing leaves (with new ones suited to submerged life) so quickly, because they could have an intermediate "happy medium" phase while transitioning.

    As always though, none if this means anything if it doesn't actually work, haha.

    Have a good one, Jeremy
     
  5. jeremy v

    jeremy v Guru Class Expert

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    Vaughn,

    I think I see where I went wrong. Your explanation makes perfect sense. I approached it from another way when initially thinking about it, and that screwed me up. I made the mistake of assuming that 380ppm of CO2 in air was measured just like with water, so that 380ppm of CO2 would be 380mg/L of air. Ppm's only convert directly to mg/L in water though, and I forgot that. In air, 380ppm is actually a much larger volume, so it converts down to only 0.736mg of CO2 in 1L of air (0.736mg/L). If my new calcs are right, that means that I would need to push about 41 Liters of air through a 1L aquarium to get the equivalent of 30ppm of CO2 in the water, haha. You are right, that probably wouldn't work very well for the plants or fish, haha.

    Have a good one, Jeremy

    P.S.- I think this was one of those cases of having enough understanding to get me into trouble and not enough to get me back out, haha.
     
  6. tedr108

    tedr108 Lifetime Charter Member
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    Jeremy,

    I started a similar type of thread a number of months ago. My question was: OK, CO2 dissolves in water much more easily than O2. So if we "needle wheel" plain ol' air bubbles into the tank, wouldn't the CO2 in the air bubbles be pulled out and dissolved in the water fairly easily? If so, what CO2 ppm would we end up with, as opposed to the standard "no-air-bubble-needle-wheeling" tank? I didn't think we'd end up with 30ppm, but I thought maybe 10 or 15ppm, which would sure help the plants out. My guess would be that the turbulence would probably knock just as much CO2 out of the water as was dissolving into it. Who knows, it might even reduce the CO2 ppm... :)

    Anyway, this question goes unanswered until I get a good way to measure CO2.
     
  7. Tom Barr

    Tom Barr Founder
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    1.The diffusion of CO2 is 10,000X faster in air than in water, so it rarely ever limits plant growth.
    2. Boundary layers have large influences
    3. Air might degas more of the CO2 than we might want and require a lot more misting than many may want.
    4. We add CO2 anyway.
    5. Yes, below water falls, you often see more plants that grow well.
    But those are also places where there's less flow/deeper permanent stable water and where the debris accumulates.

    You could also add N2 gas as a control(careful, this will blow out the O2 as well).


    Regards,
    Tom Barr
     
  8. Tom Barr

    Tom Barr Founder
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    Also, you do not get 380ppm of CO2 in tap water sitting there for 48 hours do you?
    Gas ppm's vs the dissolved gas partial pressures in water are not the same.
    Even though the air is 20% O2, do we get 200,000ppm of O2?
    No, we get maybe 7-8ppm O2.

    To get 25ppm CO2 dissolved into water using air with CO2, it would need about 1800-2000ppm of CO2 in the air.

    So if we burned every bit of carbon we could on the earth based on known reserves, then yep, we'd just have bubble the air in.

    But slowly the oceans would remove the CO2 and back to 400ppm of so after a peak, but that will take millions of years to get below 400ppm(no matter what at this point).



    Regards,
    Tom Barr
     
  9. jeremy v

    jeremy v Guru Class Expert

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    Thanks Tom,

    That makes total sense, I just had to see things from different directions in order to see the error in the logic. I figured there had to be something off in my thinking or everyone would have been just misting air into their tanks a long time ago.

    On a sort of related note, that leads me to another question. I have been trying to better understand how plants work, and how to optimize their growth and I came upon something I wanted to ask.

    It is my current understanding that plants photosynthesize during the day (due to light) and that requires CO2 and gives off O2 as a byproduct. Then there is also plant respiration that goes on constantly day and night, and that requires oxygen and gives off carbon dioxide as a by-product. From what I understand, the act of respiration only accounts for about 25% of the plant's total activity, so that's why a plant is a net producer of oxygen and a net consumer of CO2.

    Based on that understanding, I was wondering if a plant needing O2 and giving off CO2 during respiration at night, would be boosted in its' growth by aerating the tank at night to increase the oxygen levels in the tank? I am thinking that if boosting the CO2 during the day gives the plants more of what they want and increases growth, then wouldn't keeping the oxygen levels as high as possible at night do the same thing since oxygen is what the plants want at that time? I would think it would only be 25% as effective as the CO2 injection during the day though, since respiration is a smaller part of the plant's activities. I realize that the difference between aerating and not will only change the ppms of oxygen a few ppm at most, but with CO2 that can be a significant change in growth levels sometimes.

    I have only ever heard of aerating a tank at night for the purpose of keeping oxygen levels higher for the fish, but I have never heard whether doing that actually helped plant growth as a side benefit.

    Not that I would want to do it because of the fire hazard of having an oxygen tank sitting around the house, but would injecting O2 at night to supersaturate the water increase plant growth at all?

    Also, if a plant is photosynthesizing and respiring at the same time during the day and both those processes are giving off what the other process wants as a byproduct, why doesn't the plant just circulate all that stuff internally instead of letting it go and then having to go and find it again within the water for its' other processes?

    Plant biology is one of the fields of study that I feel I have the least comprehension of, so bear with me if these questions seem silly to you guys.

    Have a good one, Jeremy
     
  10. jeremy v

    jeremy v Guru Class Expert

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    Ted,

    Since your question involved the CO2 actually dissolving into the water instead of the air bubbles resting on the plant leaves, that is a little different of an issue then what I was wondering/confused about. I think I can answer your question.

    The faster you run air bubbles through the tank water, the closer the dissolved levels of CO2 in the tank water will be to equilibrium concentration with air at all times. I have heard that the equilibrium concentration is about 0.75ppm, and I have also heard 2-3ppm. I haven't tried to do the calcs myself as of yet to know which is correct, but they are both pretty low, so I don't know if it matters too much.

    I think of it like having a large container filled with pure water. Now I have in my hand a glass of water with 15ppm of nitrate in it. Now I begin dumping glasses of that nitrate water into my pure water tank. After every glass of nitrate water I put in, the tank gets fully mixed and then a scoop of water gets picked up from the tank and dumped out. Over a long period of time (involving many glasses of nitrate water being dumped in the tank and then many glasses of fully mixed tank water being scooped out and removed from the tank), the entire volume of water in the tank will eventually end up all having a nitrate concentration of 15ppm. It will never be more than that.

    Now if I start with a tank that has 100ppm of nitrate in it and begin the same process with dumping in glasses of 15ppm nitrate water and then taking out a glass of tank water and throwing it away, eventually the entire tank would drop in concentration to that same 15ppm of water and never less.

    The CO2 works the same way between water and air. If your tank has less than 2-3ppm of CO2 to begin with, aeration will increase the water's concentration, and if the tank has more than 2-3ppm of CO2 in it, aeration will lessen the CO2 in the tank. The faster you pump air through the tank of water the faster the water will balance out at that 2-3ppm equilibrium level. Pumping more air into the tank is just the same as repeating the the two water tank tests with much larger glasses of 15ppm nitrate water. The whole tank would just all become 15ppm nitrate water faster, but it would still never end up anything other than 15ppm nitrate water.

    I hope that makes sense.

    Have a good one, Jeremy
     
  11. VaughnH

    VaughnH Lifetime Charter Member
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    That is a very good mind experiment! I like it.
     
  12. tedr108

    tedr108 Lifetime Charter Member
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    Excellent explanation, Jeremy ... thank you.
     
  13. Carissa

    Carissa Guru Class Expert

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    Ok just checking my understanding..... this wouldn't work because a million parts of air shall we say are much bigger in volume than a million parts of water because air is far less dense, so you would have to be driving a ridiculous amount of air through the tank to get close to beneficial effects?
     
  14. Tom Barr

    Tom Barr Founder
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    Valid common question.
    One that gets most folks if you asked at the AGA convention say...........
    I do not think more than a few folks might even be able to answer it well.
    Simple logically question as well.

    Those are often the hardest, but you can gain a lot from them and pursuing the answer.

    Folks had long thought they knew everything about CO2.
    Yet I keep finding new stuff to apply.

    Do not feel shy about not knowing anything.
    Ask without reserve.


    Yep, the % can and does vary, say at cooler temps, at less light illumination times, you can drive the % of O2 to CO2 up to 100% for respiration- this is the light compensation point.

    Plant is just hanging on.

    You can drive it more the other way by adding more light/longer times(up to a point) and say have only 10%.

    You can also drive the O2 to toxic levels that kills fish, plants, algae etc.
    Not typically in our tanks, but does happen in nature at times.

    No, the amount of O2 needed and the levels can be very low for plants, they need far less than critters.


    Generally high O2 is bad due to photorespiration for plants.
    So actually the opposite:cool:

    Plants do recycle and circulate O2 and CO2.
    CO2 from the root zones goes up, O2 comes down to the roots.

    Regards,
    Tom Barr
     
  15. jeremy v

    jeremy v Guru Class Expert

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    Tom,

    Oh, this is getting really interesting now. Okay, I think this is all starting to make some sense to me, but like usual the more I learn the more questions I end up having:) . It is kind of funny how growing in wisdom actually makes you feel humble, because you end up realizing how little you actually know, haha.

    I have only been looking into photosynthesis and respiration and seeing them as two unique processes within a plant, and when they are at work they are always doing the same thing. Photosynthesis produces energy for the plant, and respiration requires it, and that's why a plant will slowly whither away if it doesn't get enough light that is above its' "Light Compensation Point" in order to keep the plant in a net gain of energy.

    You used the word photorespiration, and now I am confused. I looked it up and tried to understand the meaning of that a bit, and now it is seeming like a third process "photorespiration" occurs, but only when the light levels are high?

    It is seems now like photorespiration is a reaction that occurs just because of the fact that the rubisco enzyme can actually accept CO2 or O2, not that it is really a process the plants needs or even wants. Is that correct? Does the plant gain anything at all from photorespiration?

    Rubisco accepts a CO2 molecule during photosynthesis, it goes through the Calvin cycle within the plant, and then ends up back where it started as a free and once again usable rubisco enzyme, and the result of those three steps is a net energy gain for the plant. That cycle then just continues over and over during photosynthesis.

    Now, if light or oxygen levels are high, a kink in the system occurs. A rubisco enzyme can (unintentionally?) pick up an oxygen molecule instead of a CO2, and if that happens the plant actually has to expend energy to shed itself of that oxygen molecule before that same rubisco enzyme can be used to try to grab a CO2 again? In other words, photorespiration would be the equivalent of someone spending energy to go fishing and being hindered in their pursuit by catching lots of trash from the bottom of a lake on their hook instead of fish?

    Does that mean that photorespiration is occurring when plants are pearling, and/or is it a factor at all if the plant isn't pearling? Pearling would then indicate that tank conditions (circulation, oxygen levels, light levels, etc.) are causing the plant to get behind in its' ability to have the waste products of photosynthesis pulled away from the plant leaves and now photosynthesis efficiency is being lessened (possibly dramatically) because of photorespiration taking place and clogging rubisco receptors (with O2 instead of CO2) and lowering efficient CO2 uptake?

    If so, wouldn't that mean that pearling is something that is actually hindering growth instead of an indication of it? To put it another way, you would need rapid photosynthesis occurring to even see pearling to begin with, so pearling is still an indicator of good nutrient levels and tank conditions overall, but once you do see pearling it is actually an indication that the plant is beginning to lose efficiency due to photorespiration now coming into play?

    Wow, this is getting more interesting the farther I dive into this stuff. I feel like the more little tidbits of this stuff I can grasp the better I can "read" my tank and make small adjustments to optimize everything.

    The stuff I read seemed to indicate that photorespiration can actually cut the plant's efficiency of photosynthesis by as much as 50% when it is a factor. That would indicate to me that there is possibly a lot of plant growth to be gained (under the same light levels) if the by-products of photosynthesis (elevated oxygen levels in the tank during the day) could be dealt with somehow and brought down from supersaturated levels. Is that a correct statement? Is there any way to do that without losing the needed dissolved CO2 in the process?

    Wouldn't that be a side benefit to having a rich aerobic substrate with lots of bacterial action, because the bacteria could use up a lot of that extra O2 in the aquarium and even add CO2 to the tank water as a side benefit. The bacteria would be helping to increase CO2 and also to reduce the effects of photorespiration at the same time? Or alternately instead of having a rich substrate, wouldn't that be a definite benefit gained from having a large bio-filter in the aquarium?

    Have a good one, Jeremy

    P.S.- That got me thinking heavily about mazzei's again. If the mazzei's are actually providing the CO2 to the plants in a micro-bubble gas state by sticking CO2 bubbles to the plant leaves instead of relying on actually dissolving the CO2 in the water, couldn't you then heavily aerate the tank water (in the sump for instance) in order to drive off the supersaturated oxygen in the tank water? You would only risk losing the dissolved CO2 in the water, never the micro-bubbles of CO2, so the aeration wouldn't hurt the available CO2 the plant receives as long as the micro-bubbles are numerous enough, and also able to stick to every one of the plants in the tank to provide each of them their needed CO2. That would take some fine tuning of flow patterns and stuff to distribute all the CO2 mist evenly, but it would allow you to minimize the energy loss (to the plant) associated with photorespiration causing the rubisco enzymes to process an O2 molecule when it really wanted to find a CO2 one instead. I would think that would give you a huge possible increase in plant growth since photorespiration can hinder photosynthesis tremendously if the conditions are right for it to occur. Would that work?
     
  16. Carissa

    Carissa Guru Class Expert

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    ^ one point further to the above....if excessive O2 at the leaves is not a good thing, wouldn't increased circulation also benefit the plants by removing the pearling?
     
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