DIY CO2 Scrubber

[Reefs and Geeks]

As I said, I've not used it on a reef, but it is widely used by scientists.

The soda lime that has so taken the reef community is mostly used in special environments where bulk liquids are not desirable, such as for a diver. That material is much more expensive.

I miss-read your comment as not using a scrubber as in not having used the common CO2 scrubber with solid media. This is a really interesting idea, and something I'm going to start looking into to see if I could come up with an effective and safe way of bubbling through a sodium hydroxide solution instead of the medical grade CO2 scrubbing media I currently use.

I'm assuming a wooden air stone would not be a good choice for this. Is there a common air diffuser that you'd recommend for this application?

Some challenges would be:

• Needing to pump the air through rather than just having the skimmer pull it through
• Monitoring the remaining life of the Sodium Hydroxide solution (just monitor pH of the tank as an indicator?)
• Making the setup safe from accidental spills into the tank or on the floor
• Making the setup safe from kids and pets, or at least having it kept out of their reach

I'm certainly going to look up how this is used in science to see what additional info I can pull from that. Thanks for the fun new project idea!

 

[homer1475]

I'm thinking a cheap air pump with a valve on it to adjust the incoming air to the match the air being pulled in from the skimmer. I already have an airline to my skimmer from the outside, so I'll affix that to the air pump, and the other end into the reactor.

I don't have your other 2 issues as my stand is oversized compared to the sump, and no children to worry about.

 

[William Morris]

Any report yet on how long the soda lime lasts in this setup? How often do you have to change it?

Don't know yet - just put online yesterday. Keep in mind I only have a 32G tank so it might not be all that expensive to run. In the article where I found this the guy said he is changing it out every 1-1/2 to 2 weeks on his 29G Biocube.
Here is a link to the article: DIY Scrubber

 

[BitReef]

Don't know yet - just put online yesterday. Keep in mind I only have a 32G tank so it might not be all that expensive to run. In the article where I found this the guy said he is changing it out every 1-1/2 to 2 weeks on his 29G Biocube.
Here is a link to the article: DIY Scrubber

Thanks! I have a 29G tank, so this may be something I build eventually. My tank is 3 months old, no ph problems yet. Just want to know what to do if its starts running low.

 

[William Morris]

Happy to report that my PH at the peak of the cycle has reached 8.16.
Previously it would not even get to 8.

 

[BitReef]

Thanks for the update. I’ll be interested to know how often you have to change the media. I like the idea of having a second scrubber filled and ready to swap in. I wonder how to keep the scrubber media fresh though.

 

[William Morris]

The media I am using turns purple as it's used which tells you when to change it.
My understanding is you have to watch it though as it will turn white again if you leave it in to long.
Mine has been running I think 4 days now and just a few of the pellets have turned purple.
Jorvet Soda Lime

 

[BranchingHammer]

Not to derail this thread, but I would like to discuss the "liquid CO2 reactor" further. I don't have any sodium hydroxide and wanted to try it with calcium hydroxide. I hooked up an air pump to a container with a valve to control the amount of air going into it. Without the air pump, there was not enough air entering the skimmer. The skimmer air line is attached to the top of the container, and draws air from the top of the container. I have the kalkwasser suspension vigorously bubbling with air.

Kalkwasser will work, but may be less effective because the pH is much lower than sodium hydroxide. hence the tendency to hold CO2 is lower.

@Randy Holmes-Farley, if I am correct in my chemistry knowledge, when CO2 is mixed with a calcium hydroxide solution, a calcium carbonate precipitate forms. When the precipitate forms, it shifts the value of the Ksp expression resulting in the dissolving of more calcium hydroxide. SO, if I make a saturated (with some undisolved kalk in the bottom) solution of kalkwasser, then, as the CO2 is consumed, more kalkwasser would be dissolved, resulting in a longer-lasting CO2-absorbing solution. So, even though the calcium hydroxide has a lower CO2-holding ability, the saturated solution with extra solid in it would be able to last a pretty long time?

Let me know if this makes sense...

 

[BranchingHammer]

Anyways, my pH as of right now is around 8.4 which is greater than the normal value, 8.2.
How high can my pH go before it's harmful?

 

[Randy Holmes-Farley]

Anyways, my pH as of right now is around 8.4 which is greater than the normal value, 8.2.
How high can my pH go before it's harmful?

I'd try to keep it below pH 8.55 or so. My tank ran up to the level at the daily peak for years.

The main drawback to higher pH in the 8.3 to 8.6 range is accelerating precipitation of calcium carbonte on heaters, pumps, etc.

 

[Adamantium]

I've never been a huge fan of soda lime, but did use it for a while. While I noticed a great increase in my PH, the cost was prohibitive to using it for me.

This is most certainly cheaper, just don't see any issue with getting the air to bubble through the sodium hydoxide if your using a reactor(very similar setup to a soda lime setup).

Can I ask if you used the recirculating method? I’ve implemented that, and my media lasts about a month and a half.

 

[BranchingHammer]

Thanks Randy! My pH is holding stable at around 8.4 or 8.5 during the peak of the day.

 

[ReefBeta]

My DIY involves any cheap sodium hydroxide dissolved in water. The air is bubbled through it to remove CO2 from the air. I never needed to use a scrubber on my reef, but I have used that in a lab to remove toxic gases. I know a few folks use them on reefs.

When using sodium hydroxide, is there any easy way to tell when the solution exhausted, so that I know it's time to replace?

 

[Reefinmike]

@Brett S Check out Randy's comments- really interesting concept. I did some calculations using BRS's scrubber media accounting for the water weight, tax etc. Their pure hydroxide ends up costing ~$35.50/kg. a 50lb sack of lye from amazon runs about $13.60 per kg pure hydroxide... this could make co2 scrubbing significantly cheaper and in my case where I dose Lye; I wouldn't have to be worried about product purity or sodium rising in the system...

I'm amazed by what I learn every day in this hobby... year after year.

 

[Randy Holmes-Farley]

@Randy Holmes-Farley, if I am correct in my chemistry knowledge, when CO2 is mixed with a calcium hydroxide solution, a calcium carbonate precipitate forms. When the precipitate forms, it shifts the value of the Ksp expression resulting in the dissolving of more calcium hydroxide. SO, if I make a saturated (with some undisolved kalk in the bottom) solution of kalkwasser, then, as the CO2 is consumed, more kalkwasser would be dissolved, resulting in a longer-lasting CO2-absorbing solution. So, even though the calcium hydroxide has a lower CO2-holding ability, the saturated solution with extra solid in it would be able to last a pretty long time?

Let me know if this makes sense...

Yes, the precipitation of calcium carbonate makes it last longer, although you may need to keep it mixed somehow to dissolve more calcium hydroxide.

The pH might be 1-2 pH units higher in the sodium hydroxide solution, making the trapping of CO2 take less time and be more complete in an equivalent air/liquid contact time.

Both work, but I think soluble sodium hydroxide may work better.

 

[Randy Holmes-Farley]

When using sodium hydroxide, is there any easy way to tell when the solution exhausted, so that I know it's time to replace?

I've not tried to evaluate this, but a very high pH indicator dye might work. Maybe indigo carmine.

pH measurement with a meter would work, but is more work.

Over time, you may get a handle on how much solid precipitated sodium carbonate settling to the bottom indicates that it is nearly depleted.

 

[ReefBeta]

I've not tried to evaluate this, but a very high pH indicator dye might work. Maybe indigo carmine.

pH measurement with a meter would work, but is more work.

Over time, you may get a handle on how much solid precipitated sodium carbonate settling to the bottom indicates that it is nearly depleted.

Sodium carbonate is soda ash, right? The one we used in 2 part solution. Isn't it very dissolvable? I didn't thought it would be much precipitation. Unless we're using fully saturated sodium hydroxide solution?

 

[Randy Holmes-Farley]

Sodium carbonate is soda ash, right? The one we used in 2 part solution. Isn't it very dissolvable? I didn't thought it would be much precipitation. Unless we're using fully saturated sodium hydroxide solution?

Yes. But sodium carbonate is not very soluble compared to sodium hydroxide (especially when sodium is unusually high at the start), but much more soluble than calcium carbonate.

That is why some folks want to use the DIY two part using sodium hydroxide: much lower volume dosed.

 

[ReefBeta]

Yes. But sodium carbonate is not very soluble compared to sodium hydroxide (especially when sodium is unusually high at the start), but much more soluble than calcium carbonate.

That is why some folks want to use the DIY two part using sodium hydroxide: much lower volume dosed.

I did some calculations. The overall reaction is 2NaOH + CO2 = Na2CO3 + H2O. The molecular weight of NaOH is 40, Na2CO3 is 106. So For each 100g of Na2CO3 will come from 75g of NaOH.

Solubility of Na2CO3 around room temperature is 18~20% weight. So it's like 250g of it to 1L of water. That equals to 187.5g of NaOH. So if we dissolve more than that, it will Na2CO3 will start to precipitate when NaOH close to deplete. Given some margin of error, we can make 200~250g sodium hydroxide per liter of water, and change it when precipitation start to appear. Sounds about right?

 

[ReefBeta]

Just tried out a simple setup with just water:

There are couple problem.

First problem is the back pressure. The water height in the reactor is about 7". That enough to stop the skimmer from being able to pull air completely. The skimmer was running at 60%. When I turn it up to 100% it is able to pull air through. So the back pressure is a big factor and highly depends on the skimmer's performance and water height of the reactor. Even after adjusted accordingly, it still need to make sure every time to refill it's back to the same height, or it will need to be readjusted again. Also evaporation might be a problem, if one refill can last long enough time.

Second problem is the bubble size. It's big chunk raising up in water column. I imagine that won't be very efficient for the reaction to take place. Will it be efficient enough to make an impact to tank pH? To increase contact area, it could run through a air stone to produce finer bubble. But that will put even more back pressure to the skimmer.

I imagine the actual reactor will need to incorporate an air pump to dry air through the solution to relief the back pressure to the skimmer. It's not as simple as it initially sounds.