Practicality test of CO2 scrubber with sodium hydroxide

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There have been some discussion about using sodium hydroxide solution to remove CO2 in place of CO2 scrubber. So I actually tested it out.

The setup was simple. A media reactor, with the center pipe down to the bottom removed, with an air stone going in, feeded by an air pump (Whisper 10). I found in my previous test that putting input air into water column would cause too much back pressure to skimmer. So this time both input and output are above water, just letting the air stone to drive air through the solution to scrub out CO2.
PXL_20210129_111001167.jpg
The solution was prepared by adding 400g sodium hydroxide powder into 1300ml water.
When the air pump runs, it forms quite a bit of foam on top, probably because of the characteristic of sodium hydroxide solution. That caused some issue when the skimmer turned off (like during feeding), the bubble overflowed from the input hole, caused some mess.
PXL_20210129_111119028.jpg

Then the most important part, the effectiveness.
955LC7jKE2uab6y.png
Here is the pH graph when I connected this reactor to the skimmer around dinner time. Light schedule is passed peak then. The pH soon started to drop in an increasing rate than it used to. By midnight, it dropped to 8.09. In the several nights before when the soda lime CO2 scrubber is used, the lowest it got is 8.21. At that point, I didn't believe it would slow down, so I unplugged the new reactor and plunged back in the typical one. pH flatlined right after. I think if I let it run overnight, the pH will drop below 8. I didn't want this trail become a stressful event for the tank. Also because of the overflow issue, I was not comfortable to leave it run overnight without oversight.

Although the test run is only lasted about 7 hours, I think it's enough to conclude that this design doesn't work. The effectiveness of it removing CO2 is far lower than soda lime. There are several possible reasons.
  1. the air pump output is not enough satisfy skimmer air input.
  2. the water volume is not deep enough, thus not enough contact time with the solution
  3. the bubble is too big, thus not enough contact surface area
At this point, furthering the design will probably require a closer to skimmer design, high reaction chamber with wood air stone, or a recirculating skimmer pump that drive and mix air into the solution. I think it will form significant foam head like skimmer would, but it can't actually be allowed to exist the reactor. With the highly corrosive sodium hydroxide, it won't be a easy thing to do and not much room for error.

So overall, I think the idea of using sodium hydroxide as cheap alternative of soda lime to scrub CO2 in reef tank setting is not very practical.
 

Cory

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There have been some discussion about using sodium hydroxide solution to remove CO2 in place of CO2 scrubber. So I actually tested it out.

The setup was simple. A media reactor, with the center pipe down to the bottom removed, with an air stone going in, feeded by an air pump (Whisper 10). I found in my previous test that putting input air into water column would cause too much back pressure to skimmer. So this time both input and output are above water, just letting the air stone to drive air through the solution to scrub out CO2.
PXL_20210129_111001167.jpg
The solution was prepared by adding 400g sodium hydroxide powder into 1300ml water.
When the air pump runs, it forms quite a bit of foam on top, probably because of the characteristic of sodium hydroxide solution. That caused some issue when the skimmer turned off (like during feeding), the bubble overflowed from the input hole, caused some mess.
PXL_20210129_111119028.jpg

Then the most important part, the effectiveness.
955LC7jKE2uab6y.png
Here is the pH graph when I connected this reactor to the skimmer around dinner time. Light schedule is passed peak then. The pH soon started to drop in an increasing rate than it used to. By midnight, it dropped to 8.09. In the several nights before when the soda lime CO2 scrubber is used, the lowest it got is 8.21. At that point, I didn't believe it would slow down, so I unplugged the new reactor and plunged back in the typical one. pH flatlined right after. I think if I let it run overnight, the pH will drop below 8. I didn't want this trail become a stressful event for the tank. Also because of the overflow issue, I was not comfortable to leave it run overnight without oversight.

Although the test run is only lasted about 7 hours, I think it's enough to conclude that this design doesn't work. The effectiveness of it removing CO2 is far lower than soda lime. There are several possible reasons.
  1. the air pump output is not enough satisfy skimmer air input.
  2. the water volume is not deep enough, thus not enough contact time with the solution
  3. the bubble is too big, thus not enough contact surface area
At this point, furthering the design will probably require a closer to skimmer design, high reaction chamber with wood air stone, or a recirculating skimmer pump that drive and mix air into the solution. I think it will form significant foam head like skimmer would, but it can't actually be allowed to exist the reactor. With the highly corrosive sodium hydroxide, it won't be a easy thing to do and not much room for error.

So overall, I think the idea of using sodium hydroxide as cheap alternative of soda lime to scrub CO2 in reef tank setting is not very practical.
Id be interested in seeing what happens if you used calcium hydroxide instead of sodium hydroxide. Thats dissapointing.
 
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Id be interested in seeing what happens if you used calcium hydroxide instead of sodium hydroxide. Thats dissapointing.
Calcium hydroxide is a lot less solvable, so it won't last very long I think. Not sure about the foam and reaction speed though.
 

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So do you have the airstone as your input? Or do you have input, output, and an airstone? Because you wrote that both input and output are above water
 
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So do you have the airstone as your input? Or do you have input, output, and an airstone? Because you wrote that both input and output are above water
The air stone line goes in from the input hole. It's a 3/8 quick connect buckhead, with 3/16 tubing going in.
 

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Is there a way to know if you passed all the air through the sodium hydroxide solution?

Could you plug the "inlet" and see if there is enough air passing through the solution?

Or just light a match in front of it and let the smoke run in?
 
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Is there a way to know if you passed all the air through the sodium hydroxide solution?

Could you plug the "inlet" and see if there is enough air passing through the solution?

Or just light a match in front of it and let the smoke run in?

Not all the air going into the skimmer run through the solution. Only the part that is pushed in by the air pump did. When I cover the gap on the input, leaving only air pump air going in, I can see the skimmer foam getting thinner. I didn't try a bigger air pump because the foam build up on top with the tested air pump is already high enough that might risk overflowing. The setup don't have enough fail safe for overflowing.
 

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While your attempt didn't work out, it doesn't mean a better functioning setup couldn't be made. Pushing all of the air going to the skimmer through the solution would be ideal of course. I don't know about your skimmer and air pump, but the air pump I have might be lucky to do 25% as much air as the skimmer does. so at best I'd only see 25% of possible gains from pumping the air through the solution. Perhaps the inlet of the air pump could also be pulled from the reaction chamber, causing the treated air that the skimmer doesn't use to recirculate through the solution, possibly further lowering c02 content.

I've thought about doing this myself as an experiment, but think I'll end up trying outside air first. I just got a c0@ monitor yesterday and have been checking the co2 levels in various rooms in my house over the day. It is crazy how high it can get just be standing near it for a few minutes or cooking with a gas stove. Perhaps just pulling more outside air will be enough. I'm using normal co2 absorbing media now, but does get expensive over time.
 

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Seems to be working here:

 
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Seems to be working here:


Maybe I'm approaching in from the wrong angle. The post also found that with hydroxide along was not able to bring ph above 8, which align with my attempt. But I haven't thought about using it to prolong CO2 scrubber, instead of replacing it.

I do wonder why. Maybe the rate of reaction with sodium hydroxide depends highly on CO2 concentration? Then with limit reaction time, it can only bring CO2 to so low that it's not enough to impact pH in the tank, but enough to save majority of the work from CO2 scrubber.
 

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Maybe I'm approaching in from the wrong angle. The post also found that with hydroxide along was not able to bring ph above 8, which align with my attempt. But I haven't thought about using it to prolong CO2 scrubber, instead of replacing it.

I do wonder why. Maybe the rate of reaction with sodium hydroxide depends highly on CO2 concentration? Then with limit reaction time, it can only bring CO2 to so low that it's not enough to impact pH in the tank, but enough to save majority of the work from CO2 scrubber.

I think the issue is contact time and surface area. CO2 does not instantly transfer into or out of water. That is why there is such a big daily pH swing in aquaria. If the transfer was fast, pH would be stable in reef tanks that are aerated.
 

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Is there a way to know if you passed all the air through the sodium hydroxide solution?

Could you plug the "inlet" and see if there is enough air passing through the solution?

Or just light a match in front of it and let the smoke run in?
Curious - what would be the problem with just dripping sodium hydroxide - in small amounts - into the tank water via a pump. I suppose there could be some precipitation.
 

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Curious - what would be the problem with just dripping sodium hydroxide - in small amounts - into the tank water via a pump. I suppose there could be some precipitation.

Nothing is wrong with it, but remember it is adding alkalinity.

It is the basis for my two ultra high pH two parts:


 

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Nothing is wrong with it, but remember it is adding alkalinity.

It is the basis for my two ultra high pH two parts:


Yes. thanks.
 

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I'm currently in the process of re evaluating my DIY liquid sodium hydroxide scrubber. My initial results showed substantial improvement of nighttime pH by a whole two points, but little difference during the day. My alk consumption went up quite a bit. My tank is a good test because its small, has lots of fast growing SPS, and trouble keeping pH any higher tha 8.15 parked next to an open window.

I'm now trying it again but changing some things. Since I'm using a straight airstone driven rig I don't have to deal with catching up to a skimmer. I'm then pumping it through a venturi in my tank that clouds my tank with bubbles for 5 hours before lights on. If this second test shows a good result I will run a side by side with a conventional solid pellet reactor. I simply can't believe pellets can keep up with pushing that same air flow throw liquid hydroxide.
 

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I'm currently in the process of re evaluating my DIY liquid sodium hydroxide scrubber. My initial results showed substantial improvement of nighttime pH by a whole two points, but little difference during the day. My alk consumption went up quite a bit. My tank is a good test because its small, has lots of fast growing SPS, and trouble keeping pH any higher tha 8.15 parked next to an open window.

I'm now trying it again but changing some things. Since I'm using a straight airstone driven rig I don't have to deal with catching up to a skimmer. I'm then pumping it through a venturi in my tank that clouds my tank with bubbles for 5 hours before lights on. If this second test shows a good result I will run a side by side with a conventional solid pellet reactor. I simply can't believe pellets can keep up with pushing that same air flow throw liquid hydroxide.
Keen to know the results. Cheers!
 

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