Maxing CO2 in Low Techs

frothhelmet

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Co2 in a low tech | UK Aquatic Plant Society

What this thread means to me is that low tech tanks should do even better (get more CO2) in general when they have more opportunities for gas exchange with the atmosphere. Maybe we should try some wet/dry trickle filters on a lowtech and see the plant growth results...
 

BigTom

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Yeah we've discussed this a little bit in a couple of threads, but I'll be darned if I can find them. The general feeling from a few of us low-techers was that maintaining an equilibrium level of CO2 through having shallow tanks, lots of surface movement or wet/dry trickle filters was a better approach than - or at least a reasonable alternative to - the classic Walstad idea of minimal water changes and low flow in order to conserve CO2 produced from the substrate, with the added advantages that you can also discard the notion of avoiding water changes if desired, and the extremely stable CO2 levels mean the tanks are even more robust to certain forms of algae.

Furthermore, it means that O2 levels are similarly stable, which can only be a good thing for fish health (directly and by keeping the filter bacteria happy). Finally, shallow tanks and trickle filters also allow ample opportunity for emersed and riparian planting, which again help maintain a stable environment and good water quality.
 

a1Matt

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I don't know where those threads are lurking either.

I believe adding a wet\dry for this purpose has merit (it is on my list of things to try at some point in the future - probably via a sump).

The furthest I've taken it is having a high flow rate and making sure that my spray bar creates as much surface movement as possible.
 

BigTom

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And at the risk of self-promotion, I think Alastair and myself have demonstrated that the shallow method works pretty well -

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I certainly think it is for these tanks. A good trickle filter should give comparable results though, I would think, if paired with appropriate flow through the tank.

Yes, there's something very appealing to me about a trickle. Maybe I'm just a kid and would like the waterfall effect. :D

Would be ideal for growing flora In the top basket!
 
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I won't be posting pictures because my tank isn't scaped with such finesse but Tom's and Alistair's method works in a normally shaped tank too, at least in my 5f and a small 7g tank.
According to "scientists" :) , in still water it takes 6 years for oxygen to diffuse to a depth of 6 meters. So imagine if you've got a tank with no surface movement at all like in an old fashioned "Walstad" tank where this was not recommended at all...These tanks rely hugely on organics decomposition by bacteria for CO2 production which in turn is a massive oxygen consumer in order to do so. Plants do produce oxygen, but they consume it too and 99% of everything else that lives in the tanks consumes oxygen as well so you want as much as possible O2 in a low tech to keep it healthy.

I think that's why the large surface area combined with surface movement works great for Tom and Alistair. But it works in a normal shape tank as long as you do provide surface movement for gas diffusion.

On a side note, I've applied the same tactic over the years on a normal inert substrate planted low techs, loads of filtration and surface movement and no algae either. These are my observations but it works each time if you invest in heavy filtration and setup good circulation around the tank. However, it works way better with soil because the inert substrate doesn't seem to be able to store nutritients as well and plants show defficiencies despite algae never appearing.
 

foxfish

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I think we still remember the our fist planted tanks from years back, the concept of keeping the co2 in the tank rather than gassing it off.. it worked very well too.
Now we have a new angle but, I have yet to be convinced myself!
I have read Darrel's post & understand what he is saying but like yourself I don't really get it LOL
 

Andy Thurston

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I think we still remember the our fist planted tanks from years back, the concept of keeping the co2 in the tank rather than gassing it off.. it worked very well too.
Now we have a new angle but, I have yet to be convinced myself!
I have read Darrel's post & understand what he is saying but like yourself I don't really get it LOL

I think your bang on here foxfish. I dont think you will be convinced till you try it yourself and see
 

BigTom

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I used to run zero flow a la Walstad on my tank, and while plenty of plant species grew really well I did have trouble with some. Glosso, for example, only ever grew upwards and predictably died off in the end. I've now tried again and have glosso happily carpeting along the front. Although I've changed the lighting and substrate since the last attempt, light levels, water chemistry and water change rate seem to be broadly comparable to before, with the only major change has been the addition of flow and a lot of surface movement.

Just a small example, but I do think it makes a difference for some plants.
 

Tim Harrison

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A glosso carpet in a low-tech is pretty impressive Tom...if I had a big hat like yours I'd definitely take it off to you...

I can't quite get my head around the veeery slow rate of atmospheric CO2 diffusion in to water and plant demand...surely demand would soon outstrip supply...regardless of surface area : volume ratio and trickle filters etc...:confused:
 

dw1305

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Hi all,
I can't quite get my head around the veeery slow rate of atmospheric CO2 diffusion in to water and plant demand...surely demand would soon outstrip supply...regardless of surface area : volume ratio and trickle filters etc...
No it doesn't, CO2 is actually quite soluble in water, so even though there is only ~400ppm in the atmosphere if you have a large enough diffusion surface you can get the levels of CO2 fairly close to equilibrium values. The other great thing about a large gas exchange surface is that it is a negative feedback circuit, when the concentration gradient is steep, gas will diffuse in/out more quickly, as levels approach equilibrium rates slow.

The same applies to oxygen as CO2, but with the difference that you have a lot more in the atmosphere (22%), but O2 is a lot less soluble in water than CO2. I'll stick with dissolved oxygen, but by definition the same processes apply to all gases that have differing concentrations at the gas exchange surface.

If you use pollution as your measure of oxygen demand (so we are talking about BOD, Biochemical Oxygen Demand) you can ameliorate a huge bioload with a trickle filter. This is a waste water example, but landfill leachate and sewage are really just uber-polluted tank water <http://water.me.vccs.edu/courses/ENV149/trickling.htm>.
High-rate trickling filters have been used advantageously for pre-treatment of industrial wastes and unusually strong wastewaters. When so used they are called "roughing filters". With these filters the BOD loading is usually in excess of 110 pounds of BOD per 1000 cubic feet of filter medium. Generally, most organic wastes can be successfully treated by trickling filtration. Normally food processing, textile, fermentation and some pharmaceutical process wastes are amenable to trickling filtration.
If I'm not talking to the converted, I'd add plants into the equation. I think people should have access to this paper <http://www.feng.unimas.my/ujce/images/article/volume12013/paper%203encrypted.pdf>, and it has an interesting take on a free self-produced filter media (unless you are thinning on top like me).

Biological filtration capacity is a measure of oxygen exchange, and what applies to oxygen also applies to the other atmospheric gases. You can maximize gas diffusion in various ways, including having laminar flow <Feature Article: Water Flow is More Important for Corals Than Light, Part V — Advanced Aquarist | Aquarist Magazine and Blog> which is very effective, but technically more difficult than having a large gas exchange surface in the filter. If you have a wide shallow tank you have advantages in terms of area to volume ratio, but deep tanks can still be aerated by laminar flow, as long as you have enough water turn over.

cheers Darrel
 

Tim Harrison

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Good old air pollution...increasing the concentration of atmospheric CO2...there's always a silver lining...somewhere:D

Thanks for the papers...they are veeery interesting, as you say what goes for O2 goes for other gases too and especially CO2 because it's more water soluble than O2. And I also understand what you're saying about the parallels between the filters meeting BOD and therefore also being able to meet plant demand for CO2.

So for dissolved CO2 to remain constant the filter volume would have to be a function of aquarium volume and the density of the plant mass. So taking that in to account along with a realistic carbon affinity for a hypothetically typical macrophyte, etc do you reckon it'd be possible to arrive at a rough formula for calculating trickle filter size? Note I added an etc in there in case I've missed something crucial so it looks like I actually know what I'm talking about:shifty: :confused: I suppose realistically there are just too many unquantifiable variables to do it justice, but given our preference for densely planted tanks don't you think the filter would have to be quite large though?

Aside from that am I right in thinking water in equilibrium with air contains 0.5 mg/l CO2? If this is so, is it entirely possible - in soil substrate tanks at least with higher concs of CO2 due to decomposition - that trickle filters and tanks with large surface area:volume ratios are actually degassing CO2 to achieve equilibrium?
 
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I am repeating myself but I think Diana Walstad's original idea of having no surface movement was to preserve the CO2 produced in the substrate by decomposition of organics via oxygen consumation. This bacteria is aerobic! She also said that the CO2 amount produced is higher than the one via surface movement diffusion and that's what many folks don't understand. However she first advised to eliminate surface movement in order to prevent diffusion of CO2 in air, but she recently advised she's changed her mind, and advised for surface movement and means of oxygenation explaining that it's the oxygen those tanks are having problem with. It's way less solubale in water than CO2 but it's essential for decomposition of organics, so low oxygen, low CO2 at the end of the day. Without enough oxygen, there's not enough CO2 production, plants suffer, the substrate goes anaerobic, that leads to even less oxygen, bad bacteria flourishes, fish die. :) She said that especially at the start, when the soil is full of organics, this can lead to a dead tank pretty fast. Imagine also an old Walstad tank where organics have been piling up in the substrate for months and years, same thing. Oxygen is the key and I agree with Darrel on that as he pointed it out many times.
In a high tech that's not an issue because one scrubs the tanks clean several times a week as per the advise here as well :)
 
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Aside from that am I right in thinking water in equilibrium with air contains 0.5 mg/l CO2? If this is so, is it entirely possible - in soil substrate tanks at least with higher concs of CO2 due to decomposition - that trickle filters and tanks with large surface area:volume ratios are actually degassing CO2 to achieve equilibrium?

From what I understand equilibrium refers to the saturation of gasses at surface area but levels of gasses below that area are totally different story. So basically in a Walstad tank there will be low CO2 concentrations at the surface(equilibrium), but the deeper it goes, the higher concentrations of CO2, the highest being near the substrate where the decomposition occurs which is constant as long as there's enough O2. So plants always have a chance to consume CO2 on time.
However, the O2 is the opposite story, the highest concentrations will be near the surface and rapidly decrease the deeper the tank. If one has no surface movement, then no oxygen enters via equilibrium(as it takes 6 years for 6 meters in still water) Tom and Alistairs tanks are shallow with big surface area and surface movement, therefore oxygen gets down to that substrate where the aerobic bacteria consums it in high rates in order to decompose organics and produce CO2. You need aerobic bacteria decomposition, not anaerobic, So it's the O2 you need worrying about. Temperature is also a key factor to O2. And obviously, everything in a tank competes for oxygen, from decomposition bacteria, to nitrogen bacteria to fish and plants when they don't photosynthesise.

Obviously in a lightly stocked tank, well planted with careful feeding one may get away with no surface movement if the soil is not rich in organics as plants produce O2 as well, but the more O2, the better.
 

Tim Harrison

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For me mucho surface movement, flow and distribution, and regular and substantial water changes are key. Degassing from trickle filters is something else altogether, and high surface area to volume ratio will also accelerate the process.
 
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For me mucho surface movement, flow and distribution, and regular and substantial water changes are key. Degassing from trickle filters is something else altogether, and high surface area to volume ratio will also accelerate the process.

But if you call it oxygenation instead of degassing it takes a different meaning.
CO2 gets produced 24 hours a day in a Walstad tank in the substrate and via fish respiration. CO2 requirement by plants is majorly needed during the light period of 7-8 hrs a day max. So even with some diffusion, there'll be enough left for the plants if you promote faster CO2 production than it can be lost by diffusion.
Oxygen comsumation on another hand is a 24/7 requirement and is vital not just for fish, but for plants methabolism and CO2 production by bacteria. Oxygen in a non-surface movement tank gets produced just during the light period by plants for 7-8 hours a day. So no back up there. Oxygen is less soluble in water and harder to retain in water. When you run low on oxygen CO2 production slows down as well, plants growth and methabolism slow down, so why pointing out CO2 degassing as an issue when you wouldn't be promoting plant health, fish health, tank stability and CO2 production in the first place via oxygenation.
So personally, I don't see why a shallow tank would be a disadvantage as it promotes better oxygenation, and in turn more CO2 and better plant health. I don't see how trickle filters can be a disadvantage but I think the means of directing the flow to reach all around the tank is as important and trickle filters are a bit tricky that way :)
 

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