[EDIT: A crucial consideration was pointed out in the comments. For all the designs I've looked at, it's cheaper to just get a heat exchanger and ventilation fans, and blow the air outside/pull it inside and eat the extra heating costs/throw on an extra layer of clothing, than it is to buy a CO2 stripper. There's still an application niche for poorly ventilated rooms without windows, but that describes a lot fewer occasions than my previous dreams of commercial use.]

So, I have finally completed building a CO2 stripper that removes CO2 from the air to (hopefully) improve cognition in environments with high CO2 levels. In California, the weather is pretty good so it's easy to just crack a window at any point during the year, but other areas get quite cold during the winter or quite warm during summer and it's infeasible to open a window unless you want to spend an awful lot of money on heating or cooling bills. It didn't work quite as well as the math indicated at first, but the whole thing is built, and basically functional. The rest of this post will be a reflection on the lessons learned while doing so.

1: In hardware, ideas are cheap, execution is expensive

So, the fundamental idea is extremely simple once you have some basic knowledge of chemistry. The goal is to get CO2 into some form that isn't the gas form, via some sort of chemical reaction.

Submarines and CO2 capture from flue gas use a reversible reaction with ethanolamines, where they absorb CO2 at high temperatures and release it at low temperatures. Reversible reactions are good for making waste, but heating up and cooling down large quantities of liquid takes an awful lot of energy. Submarines have nuclear reactors onboard, and flue gas is hot, but we don't necessarily have the energy required. Also ethanolamines are toxic and hard to get a hold of for a civilian and really stinky, being the major component of "submarine smell".

Adsorption onto zeolites is also plausible, but the issue is that it requires alternately exposing the zeolites to high air pressure and low air pressure, and high airflow is required. The combination of high pressure and high airflow means that again, you're using a lot of energy. The basic math is as follows: One human produces about 1 kg of CO2 in 24 hours. We can idealize a perfect CO2 stripper as a magic box that inhales air and spits it out at 0 ppm. If you want a steady-state concentration of 500 ppm for 2 people, then we can see how much air-flow is required to lock up 2 kg of CO2 in 24 hours. This comes out to about 100 cubic feet per minute. This is the bare minimum air flow for any CO2 stripper, but in this particular case, it corresponds to a 25 horsepower air compressor, which is 18 kilowatts. This is equivalent to running 5 electric dryers at once. So that one is out too, especially since we were assuming 100% efficiency at eliminating CO2.

What about irreversible reactions? Just lock the CO2 up as a solid waste? Well, to begin with, this is going to produce quite a waste stream, and consume quite a bit of chemicals, you'd better hope it's safe and that the feed chemical is cheap. The reaction used on space missions used lithium hydroxide. The basic idea is that lithium hydroxide makes a very basic solution. Carbon dioxide is slightly acidic, so it dissolves very fast into basic solutions. Then you get precipitation of lithium carbonate which is safe.

The problem is that lithium hydroxide is quite expensive. It was used on space missions because it's the most mass-efficient way of doing that sort of reaction and every gram counts in space missions, but we want the cheapest way of doing that reaction.

And then we hit upon the perfect solution. Calcium hydroxide. It's an extremely cheap bulk chemical, 15 bucks for a 50-pound sack of it at a hardware store. It's fairly mild as far as hydroxides go, being pH 12.4. So instead of giving you horrible chemical burns, it's safe to handle unless you're exposed to it for over an hour at a time without washing it off. It's the alkaline analogue of the difference between 1 M hydrochloric acid, and lemon juice. And when it reacts with CO2, it makes CaCO3, aka limestone, which is totally harmless. In fact, it's a common laboratory demonstration that breathing onto a solution of this stuff produces a white film/crust on the top, which is the CO2 in the breath locked up as solid limestone. It's the obvious choice if you're trying to remove CO2 via chemical means.

And in fact, in the SSC comment section, someone else independently had the exact same idea! Just lock up CO2 with calcium hydroxide!

The simplicity of an idea in the field of atoms instead of bits doesn't necessarily mean that anyone on earth has ever done it before, though, or will ever do it, and I'm not worried about anyone scooping the idea, because building novel hardware is hard enough to provide a natural barrier to entry unless it's a large company that's interested in the idea. Ideas are cheap, execution is expensive, in both time and money.

2: Only polymaths need apply

If you're trying to build a novel machine in your garage, and aren't working as part of an engineering team, you will either need an improbably wide range of knowledge, or the general ability to pick up whatever you need to learn. There's the basic knowledge of chemistry to spot that this is the obvious reaction to go for, but the full design requires:

Familiarity with wastewater aerators to know what to buy to prevent clogging with solids, knowledge on which materials won't react with your chemicals, the math of air flow in pipes, the ability to read fan pressure/airflow curves, the ability to go from "I want a circuit that does this" to building a novel electronic circuit on a breadboard without frying anything important, enough programming knowledge to write some basic arduino code, familiarity with hazardous waste disposal regulations in your state, familiarity with waste dewatering techniques, familiarity with which sort of pumps can pump sludge instead of pure water, some electrical engineering knowledge to work safely with 220V power without frying yourself or anyone else, knowledge of soundproofing, and especially the familiarity with everything at Home Depot that lets you home in on the most efficient and foolproof way of building a thing that does what you want. Probably some other stuff too that I consider obvious but others might not.

Now, most of this is pretty easy to pick up given enough starting mental firepower, and the sense of what to google for. Or just having lots of experience with building material things.

Having one of the relevant fields of knowledge manifests itself as knowing ahead of time which approaches will work and which will fail and what solutions past work in the area has already found.

For some of these, missing it will manifest as not knowing that there's an incoming bullet in a particular area, like not knowing that fine bubble aerators will promptly clog if there's lots of particulates in the water, or not suspecting that high air flow rates are incompatible with small pipe (I knew the latter one and it still almost got me until I idly decided to work out airflow velocity in the pipe and realized it was around 200 mph)

3: The planning fallacy is huge here.

So, it wound up costing a lot more than I thought and taking a lot longer than I thought. The mechanism of why the planning fallacy hits so hard here is tied in with the design process. What happens is that you start out with a sketchy outline of all the component parts (like, "I need something that automatically dispenses chemical powder"), and as it becomes time to build a part, you drill down further and further in fleshing out the details until eventually you've drilled down far enough for your design to Actually Work in reality. While you do this, you will inevitably come across parts that are a lot harder to do than you expected, which you were glossing over on the first pass. The shiny black box of "build a chemical dispenser" looks more tractable than "how the fuck do I build a motor mounting plate with my inadequate tools", which you didn't initially suspect you had to do because you weren't thinking at that level of detail. And also as you address the parts that are easy to do, all that is left is the parts that are hard or annoying or time-consuming to do, which can be somewhat demoralizing.

Same sort of thing goes with cost. You start out with "so here's the cost for the big parts and everything else that's left shouldn't cost that much" (black-box warning on "everything else"!), and then you go to Home Depot and pick up a bunch of 4-inch ABS pipe and black glue and all the 90 degree and T pieces you need for the aeration pipes and look at the cost and it's 100 bucks. Home Depot trips add up shockingly fast. There's also all the stuff you buy that you don't eventually end up using because the design evolves as you actually try to build it, like buying gears when you don't actually need gears, and all the stuff you didn't think you had to buy but it turns out that you do need it.

And sometimes you just get hit with some problem you didn't expect at all and now have to fix, like "my fan is making a screaming noise, what do"

4. Why is there a valley of death?

Universities and the government funds basic research. Then there's the private sector of business. The gap between the two, where you have to go from basic research to a business selling the new exciting thing is called the "valley of death". Now, you'd think this is what R&D is for. But a lot of R&D from a business seems to be focused on marginal improvements to existing things that already fall under the scope of what the existing business does, and not so much on building a novel thing that can be the seed of a new business. Building a novel thing requires a wide knowledge base, as discussed before, and inevitably takes a lot more money and time than expected. It's the sort of thing done by inventors in a garage as a project of love, not the sort of thing you get paid to do.

Further, crossing the valley of death requires both the technical capacity to build the thing, and the business skills to make a new business from scratch. If you have several people with different skills joined together, it can be bridged, but one flaw of doing it alone is that there are a lot more inventors with the ability to build the thing, than inventors with the ability to build the thing and also the ability or willingness to start a business that sells the thing. I'm in the former category. I can build it, but I hate building it and if I have to build all the machines myself to sell, I'd flatly reject it, and I really don't want to be responsible for running a business selling it, I'd have no idea how to run a business, and it'd eat too much time. My dream is to get a design good enough to sell, patent it, find someone willing to make a business out of it, and just receive a cut of profits without having to be involved in anything more regarding the production or selling of the machines, besides helping out with technical design work. Further, someone with just the business skills won't necessarily have the technical ability to come up with the machine in the first place, let alone build it. And there's also the lemon market problem of businesspeople identifying competent non-scam technical people with a viable design, and technical people finding competent non-scam businesspeople.

There are further issues such as designing the new invention such that it is robust and keeps working for a while (not a property that prototypes generally have), and designing it such that it is easy to build and maintain (also not usually a property associated with garage prototypes).

I've heard that there's a company in the UK that takes garage prototypes and updates the design for robustness, easy constructibility, and cost, which seems like an important part of closing the valley.

5. Building alone vs building as part of a team.

In a certain sense, I was blessed on this project, because I had complete control over the entire design. I had to contend with no meetings, and no unexpected changes to parts of the design that were already locked in, and no team decisions that were dumb and couldn't possibly work. It's the dream for anyone who dislikes group projects in engineering. All failures are attributable to me alone, as well as all successes. Then again, having someone else to work on the project with me definitely would have sped it up and I could rely on their knowledge of things I was ignorant of, relaxing the polymath requirement. Maybe there's an optimal design team size? I guess it'd depend on how parallelizable the work is, as well as how decision-making-quality scales with group size.

6. Final diagnosis and where to go from here.

So, it was over-time and over-budget and didn't work as well as I had hoped, but it does indeed work. Planning fallacy is a huge obstacle here, and I now certainly see why there's a valley of death for this sort of work.

In order to make a version that's practical for domestic use, I'd have to redo the design to be a rain-column design, primarily because it only requires high airflow, instead of the combination of high airflow and high pressure, which requires buying an expensive fan from China and the expensive electronic components which provide the appropriate power to operate the fan. A rain column design could use a much cheaper and simpler fan that operates from a wall outlet.

Further, in order for others interested in CO2 reduction to have one of their own, I'd have to team up with someone who could make a small business in assembling and selling these things, preferably involving someone who is not me building the relevant thing. PM me if interested.

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37 comments, sorted by Click to highlight new comments since: Today at 10:29 PM

It's good that you built it, but it seems to me that now you have a prototype, before you start investing in a patent or business to sell scaled-up versions, it'd make more sense to invest $100 in a CO2 air sensor and a Raspberry Pi with a switch to randomly turn it on/off: to verify that it decreases CO2 as much and as long as expected, whether you can tell when CO2 levels have been lowered, and whether this has any measurable behavioral effects. The value of such information is very high: there is no point in scaling up a design which isn't working at its basic task of lowering CO2 levels, and commercialization will be difficult if it does nothing observable (especially given our questions about how and whether CO2 does anything) and, perhaps more importantly from a marketing perspective, if the user can't feel it doing something.

I have the relevant air sensor, it'd be really hard to blind it because it makes noise, and the behavioral effects thing is a good idea, thank you.

It's not currently with me.

I think the next thing to do is build the 2.0 design, because it should perform better and will also be present with me, then test the empirical CO2 reduction and behavioral effects (although, again, blinding will be difficult), and reevaluate at that point.

I have the relevant air sensor, it'd be really hard to blind it because it makes noise, and the behavioral effects thing is a good idea, thank you.

Just randomizing would be useful; obviously, your air sensor doesn't care in the least if it is 'blinded' or not. And if it's placed in a room you don't go into, that may be enough. As well, maybe you can modify it to have a flap or door or obstruction which opens or closes, greatly changing the rate of CO2 absorption, and randomize that; or if you have someone willing to help, they can come in every n time units to replace the filler or not, giving you both blinded & randomized comparisons between high-CO2-removal vs low-CO2-removal conditions based on whether they pulled out the used filler or not, since the fan presumably still makes the same noise regardless of whether it has brand-new filler removing CO2 at maximum rates or expired tired filler removing only a little CO2. (Remember, experiments work fine comparing 100% removal rates to, say, 10% removal rates; it doesn't have to be exactly 'on'/'off', that's just a bit more statistically-efficient because it has a slightly larger effect size, and you have to remember the estimate is a bit lower than the 'true' estimate because the 'off' condition has 10% of the benefits of the 'on'.)

Verifying that the thing scrubs CO2 at the expected rate is definitely a good idea. Verifying the behavioural effects is much harder - you’d need to avoid unblinding, and ideally have several different people with varying levels of age, fitness etc, and then you’d get affected by weather, unless your house is very well sealed...

How portable can this scrubber be? If you’re somewhere cold and not getting enough air at night and it’s your house, you could install a heat recovery ventilator. There is evidently a big market for portable air conditioners, despite their inefficiency; the description of this thing (water, air, pumps out sludge) sounds a lot like a washing machine.

Verifying the behavioural effects is much harder

Not really. There's scads of behavioral measures you can collect passively.

you’d need to avoid unblinding,

No you don't, and blinding is easy if you think about it for a few seconds, see the comment I left well before yours.

and ideally have several different people with varying levels of age, fitness etc,

No, you don't, you are letting perfect be the enemy of better

and then you’d get affected by weather, unless your house is very well sealed...

This is a feature, not a bug.

If the whole reason you didn't want to open the window was the energy put in to heating/ cooling the air, why not use a heat exchanger? I reackon it cold be done using a desktop fan, a stack of thin aluminium plates, and a few pieces of cardboard or plastic to block air flow.

Yup, this turned out to be a crucial consideration that makes the whole project look a lot less worthwhile. If ventilation at a bad temperature is available, it's cheaper to just get a heat exchanger and ventilate away and eat the increased heating costs during winter than to do a CO2 stripper.

There's still a remaining use case for rooms without windows that aren't amenable to just feeding an air duct outside, but that's a lot more niche than my original expectations. Gonna edit the original post now.

I was very confused about your proposed setup after reading the wikipedia article on heat exchangers, since I couldn't figure out what thermal masses you proposed exchanging heat between. But I found this article which resolved my confusion.

Ventilation has the advantage that it dumps all pollutants, not just CO2. In fact, the premise that CO2 affects cognition is false.

Your statement that "the premise that CO2 affects cognition is false" seems not obviously correct. Is this the current expert consensus? How can the rest of us evaluate it?

Well, who are the experts? Submarines routinely have CO2 levels much higher than even Berkeley group homes. Naval researchers do experiments with higher levels still, showing little effect. There seems to be an illegible LW consensus to the opposite, probably from people pretending to read this post. People praise Gwern for his quantity, but they don't actually read him.

Again, most research is about ventilation and is thus confounded by other pollutants. I don't usually speak up about this because most discussion of this doesn't depend on the CO2 hypothesis.

I'm afraid I still don't understand what the basis is for your claim that "the premise that CO2 affects cognition is false".

I understand why you consider it not clear that CO2 does affect cognition: experiments yield results in different directions, and people survive on submarines. But that, at least so far as you've described it, seems to fall far short of justifying the flat statement that "the premise is false". What am I missing?

You asked for an expert consensus and I gave it to you. Naval researchers are the experts.

No, "experiments yield results in different directions" is not an accurate summary. Experiments with large interventions trump experiments with small interventions.

But, it's true, I left out the most convincing evidence, which is back of the envelope calculations with gross anatomy.

Note that there could be significant variation among humans on this axis and submariners are selected on 'low response to CO2'. I think the illegible LW consensus is mostly people who are on the other end of this axis.

When people say that ventilation helps them, I believe them. They might even be far on an axis of response to pollution. But how would they know that the particular pollutant they respond to is CO2? They should be cautious in assigning blame and trying specific interventions. Gwern points out that one of the studies that most impressed Paul about CO2 actually found larger effects from mold, which is a big problem in the foggy slums of Berkeley. In theory there are ways to isolate human pollution from house pollution, such as varying the number of roommates, but I doubt people are careful enough to disentangle that and CO2 isn't even the only human pollutant. [Added: but submarines are equally subject to all human pollutants, so that should limit the possibilities to the short list of what they scrub.]

Are submariners selected on that axis? I'm skeptical. In any event, the naval studies don't restrict to submariners.

Pro tip: When you come up with an idea like this, or want to further refine it, go to your local universities engineering departments, and pitch this as an undergraduate capstone project. This is exactly the kind of project and scope that their looking for. Be prepared to put down a couple grand to fund it, and make sure you discuss who owns the resulting IP. This will give you basically free labour from a team of 3-8 people with more expertise and equipment at their disposal than you could ever hope to have individually. Them and their professors will be much better equipped to deal with all the BS that comes with bridging the gap between ideas and reality.

Have you done this?

I've been on the side actually doing the projects, have worked at companies that farm out the investigation of experimental ideas to engineering students, and acted as a point of contact for those students. Come summer I'll be a project sponsor.

Would love more info on how this works

I really can't help too much there, the way these capstone projects work is not only dependent on the university but also the department. I have a detailed understanding of who to talk to and what the process looks like for my own department and university, but not for others. It's very much the kind of thing where you need to go and fire off some emails or walk into an office. As some examples, here's the UofT ECE website for industrial project sponsorship, and here's the general UofC website for the same kind of thing. Note that in both cases they don't really tell you anything except to just email them. However, do look around the UofT site as it gives you some idea of what these projects are like and what these students are expected to accomplish.

Also, I suspect that it might be harder to get larger departments/ more reputable universities to help you with your problem because they tend to be much more flush with cash and the students work is in higher demand.

You say it works but not as well as hoped. It would be interesting to know more about that.

E.g., how effective is it, in the end, at removing from the air? (Less so than you hoped?) How big and power-hungry and noisy is it? (More so than you hoped?) How much did it end up costing to make? (More than you hoped?)

Adsorption onto zeolites is also plausible, but the issue is that it requires alternately exposing the zeolites to high air pressure and low air pressure, and high airflow is required. We can idealize a perfect CO2 stripper as a magic box that inhales air and spits it out at 0 ppm. If you want a steady-state concentration of 500 ppm for 2 people, then we can see how much air-flow is required to lock up 2 kg of CO2 in 24 hours. This comes out to about 100 cubic feet per minute. This is the bare minimum air flow for any CO2 stripper, but in this particular case, it corresponds to a 25 horsepower air compressor, which is 18 kilowatts.

I'm not sure this is as much of a barrier as it sounds, at least if you have access to a window you can vent to. Imagine if I had a membrane that let only CO2 through, and a 50-micron vacuum pump (typically 1/2 HP). I could pump CO2 out of the room about as fast as the CO2 would diffuse across the membrane. In this setup, the amount of gas pulled out would be pretty tiny, because you'd be pumping only the CO2, but the effective amount of air being processed would be quite large.

So if you have a vacuum swing adsorption machine, with some zeolites in an array of mixing chambers, that can be alternately mixed with the air in the room and then evacuated outside, what matters is the equilibrium adsorption of each component of air, and the speed of adsorption / desorption. The air mixing can be done with a simple fan (providing the effective high volumes of air processing).

That said, there are other major problems with zeolites. The main one I've been struggling with is that zeolites really love to adsorb water vapor, nearly as much as CO2, and there’s a lot more water vapor than CO2 in the air. Competition for adsorption sites isn’t well-understood, but one study shows that water vapor in the air seriously decreases the amount of CO2 adsorbed. This also means that the system described above would function as a dehumidifier as much as a CO2 pump.

[epistemic status: very uncertain; writing as though I were more certain because I think it's more fun / engaging]

I want to nominate this as a thoughtful reflection on a project, but it matters to me whether the stripper worked. Diffractor, did you test it as Gwern suggested, and did it remove CO2 from a room?

It is currently disassembled in my garage, will be fully tested when the 2.0 version is built, and the 2.0 version has had construction stalled for this year because I've been working on other projects. The 1.0 version did remove CO2 from a room as measured by a CO2 meter, but the size and volume made it not worthwhile.

One human produces about 1 kg of CO2 in 24 hours. We can idealize a perfect CO2 stripper as a magic box that inhales air and spits it out at 0 ppm. If you want a steady-state concentration of 500 ppm for 2 people, then we can see how much air-flow is required to lock up 2 kg of CO2 in 24 hours. This comes out to about 100 cubic feet per minute. This is the bare minimum air flow for any CO2 stripper

Wait, why does stripping one person's CO2 require more airflow than breathing (0.3 cubic feet per minute)?

Because the air you breathe out has much more CO2 by volume than the air you're processing (it diffuses quickly). See my comment elsewhere for a potential way around this, though.

Ah, silly me. Thanks!

The planning fallacy for garage projects is an interesting problem, because it doesn't lend itself immediately to a reference class unless you have done a lot of projects before.

Still, next time you want to tackle a garage invention that you can just predict it will take as long as this one. It will be interesting to see how the difference between projects compares to the planning fallacy impact on a single project; in very big projects, the size of the project completely dominates any considerations of field or technology.

Come to think of it, does anyone know if there is a maker community somewhere that records its budgets and timelines? Maybe that could be used as a reference class for garage projects.

I'm curious about what you think the price of the device would be if a company would sit down and mass produce it. If you had a kickstarter with 10,000 minimum orders, what would the price for the device be? What would be the running costs?

I think there is a fatal problem here apart from cost and complexity. A fatal problem that makes this device potentially DEADLY. Sorry for shouting.

Consider air. Add humans. Humans convert O2 and water and food into CO2 and H2O. Your machine takes out the CO2.

What happens is that the O2 levels will inexorably fall, until you basically have pure-ish Nitrogen. If a human or other mammal breathes pure Nitrogen they will not feel suffocation. Suffocation detection is triggered by CO2 levels which your device removes. What happens is that you would feel OK and then suddenly faint and subsequently die.

Here is a link to a euthanasia device that implements this mechanism as a way to euthanize yourself.


Person in a room: - 35 g of O2/hr from room
Person in a room with a CO2 stripper: -35 g of O2/hr from room

How does the presence of a CO2 stripper do anything at all to the oxygen amount in the air?

I just skimmed the article, but given that you already willing to make suck huge investment in capital and space, I just got a feeling it's easier if you just have a grow room or an algae bio-reactor running at night. the immediate benefit would be the hardware are available in consumer form and most of the design parts have been attempted.

This way the on going cost would only be marginally higher [back of envelope calculation] and you would reap other benefit beside. [food or algae oil]

Back of the envelope: a person exhales about as much carbon as they eat, and a plant removes carbon from the air only by increasing its size, so to remove one person's CO2 exhalations, you would need to grow as much plant matter as they ate. That's not impossible, but at that point you're looking at something more like a greenhouse than like a grow room.

yeah, I dropped the ball on that one, the grow box is out but I got a feeling a grow room is still possible but this is splitting hair.

I am wondering if I also dropped the ball on the algae claim

[EDIT: I see numbers as high as 4 g/L/day quoted for algae growth rates, I updated the reasoning accordingly]

The numbers don't quite add up on an algae bioreactor for personal use. The stated growth rate for chlorella algae is 0.6 g/L/day, and there are about 4 liters in a gallon, so 100 gallons of algae solution is 400 liters is 240 g of algae grown per day, and since about 2/3ds of new biomass comes from CO2 via the 6CO2+6H2O->C6H12O6 reaction, that's 160 g of CO2 locked up per day, or... about 1/6 of a person worth of CO2 in a 24 hour period. [EDIT: 1 person worth of CO2 in a 24 hour period, looks more plausible]

Plants are inefficient at locking up CO2 relative to chemical reactions!

Also you wouldn't be able to just have the algae as a giant vat, because light has to penetrate in, so the resulting reactor to lock up 1/6 [EDIT: 1] of a person worth of CO2 would be substantially larger than the footprint of 2 55-gallon drums.

Also, a paper on extremely high-density algal photobioreactors quotes algal concentration by volume as being as high as 6% under optimal conditions. The dry mass is about 1/8 of the wet mass of algae, so that's 0.75% concentration by weight percent. If the algal inventory in your reactor is 9 kg dry mass (you'd need to waste about 3 kg/day of dry weight or 24 kg/day of wet weight, to keep up with 2 people worth of CO2, or a third of the algae each day), that's 1200 kg of water in your reactor. Since a gallon is about 4 kg of water, that's... 300 gallons, or 6 55-gallon drums, footprint 4 ft x 6 ft x 4 ft high, at a bare minimum (probably 3x that volume in practice), so we get the same general sort of result from a different direction.

I'd be quite surprised if you could do that in under a thousand dollars.

I vaguely remember the 4 g/L/day but on further inspection, I now realise that we can't ever reach that efficiency. If we use the 1 g/L/day but human use 1kg/day (they use less in sleep don't they?) divided by 2/3 it would be about 1.5 m3 (still quite big) and we have to account for the actual foot print which would be much higher (2x higher? 3 m3?).

I am tempted to argue that we don't have to match the CO2 production rate so perfectly since the CO2 should naturally diffuse a bit, but let's leave some margin of error on our side.

for the total volume, can't we just continuously filter out the algae to retain the optimal condition while barely increasing total volume? and since we only use it at night, the total volume need would be less. If we automatically drain it to a hidden larger tank or press it straigth to oil in the basement we maybe saving effort here but increasing volume.