Thanks for the detailed response!

I'm on the crackpot spectrum but I'll try not to waste your time. :-)

As I mentioned in a different sub-thread, I started looking at algae as part of a system to reprocess oceanic plastic trash into biomass, so my particular ideas and designs are probably not going to be directly applicable to CO2 sequestration.

> I'm bullish on algae but you may want to adjust your expectations. If you have a photobioreactor, with algae growing too densely to see through, in the sun, with goldfish, you're probably going to cook your goldfish.

In the context of an ocean-going plastic recycling system I hadn't thought of heat pollution before because I assumed I would have the thermal mass of the ocean in which to dissipate excess heat.

To handle it you could: reduce the infrared light reaching the tubes; chill the incoming air with e.g. Ranque-Hilsch vortex tube [1]; put radiator fins on the shade side; use heat-tolerant fish; hang evaporative coolers in between the tubes; rotate the tube assemblies.

> You're talking about engineering an ecosystem, and it is fancy technology. pH, temperature, and salinity all need to be copacetic. There's a reason "everyone" does aquaponics with talapia and vascular plants. Aquaponic systems usually run at pretty abysmal water qualities, but talapia will live in mud and vascular plants don't mind having lots of nutrients in their water. Algae will have a hard time growing at that high density you're imagining if they're being shaded by murky brown water. Of course these systems will have pretty big algae blooms, but usually these are biofilms growing attached to the walls of the system, close to the surface.

I cheat: I went to a pond and scooped up water and some of the bottom muck. Start with a viable ecosystem; in the tank it wobbles but then stabilizes; my experimental system ran for months with no adjustments.

At scale, I'd adopt a kind of evolutionary attitude, any tube that went bad would be flushed and replaced with a colony from a healthy tube. Combine that with a simple automated feedback system to help keep tubes in the optimal ranges and I think it would be pretty stable overall (depending on the failure rate of the tubes.)

If you have one seawater tank in your living room you're gonna have to paper and baby that thing. If you have thousands of simple robust ecosystems in tubes and some go bad you can just flush 'em. (And if you're worried about contaminates, chemical or biological, you can flush them through the MSO reactor. Those things are used to dispose of ordinance and chemical weapons. They really really break down molecules. Mad cow prions wouldn't make it through. Heck, you could toss the tube in there.)

Biofilms are fascinating! FWIW, in the experiment I ran the fish kept the inner surfaces of the tube pretty clear, I didn't have to do any scrubbing.

> If you feed the algae to other organisms, they're going to die and that carbon is going to turn into methane, which is 23 times more powerful a greenhouse gas than CO2.

I want to experiment with feeding the algae to compost worms. These prefer just-slightly-decayed plant matter so I am thinking they would gobble up the algae as it died. Basically, you'd drain half the water in a tube directly into one end of a worm bed. The bed filters the algae out of water which also picks up nutrients. Some of the outflow would be put back into the tubes directly; the rest would be distilled to make a high grade fertilizer and the excess water condensed and returned to the tubes.

(The worms also eat their own dead. Gruesome, but it speaks to your point about turning into methane.)

> There's also the matter of growing algae in tubes made of petroleum plastic. How much carbon went into mining & processing the petroleum, manufacturing your tubes, and transporting them out to where your facility is, 100 miles from the nearest Starbucks? How long will it take you to pay off that carbon debt? (If there's a biobased alternative I'd love to hear about it.)

Well, bioplastic.[2] I'm also looking at drying out the algae in sheets and using [some kind of] glue to bond them into a membrane, as opposed to going through a chemical process to convert them into plastic. Once you've got sheets and glue you can build your structures out of inflatables. [3]

> But there are solutions as well as problems. Aquaponic systems don't have to be directly coupled, for instance. And those biofilms are much denser than planktonic growth (ie tubes). My main "research" area (calling me a researcher would be too generous) is attached growth cultivation systems, for a lot of these same reasons. Pyrolysis can be used to turn the carbon into a biochar with very low bioavailability, solving the issue with methane.

I hadn't heard of "attached growth cultivation systems" specifically but they make a lot of sense. Just skimming the wikipedia article I think I can see a lot of low-hanging fruit in terms of efficiency gains.

Are you aware of the "Living Machine" stuff?

> Living Machine is a trademark and brand name for a patented form of ecological sewage treatment designed to mimic the cleansing functions of wetlands. ... the latest generation of the technology is based on fixed-film ecology and the ecological processes of a natural tidal wetland, one of nature’s most productive ecosystems.

~ https://en.wikipedia.org/wiki/Living_machine

To me biochar seems like a very exciting path for CO2 sequestration. Not so much on the ocean for obvious reasons, but if I had any land at all I would be trying it out.

> Just to clarify, bubbling doesn't maximize growth as much as keep the algae alive. Keeping high-density algae well mixed does ensure that everyone gets their turn to be in the sun.

Yeah, you explained it better than I did. Cheers. :-)

> But algae can't survive in an anoxic environment, and without aeration a tall column of water will quickly become anoxic. The algae on the bottom will rot and you're back to converting CO2 into methane (which, in most systems, would come right out of the top of the tube). Additionally, aeration provides CO2; no CO2 would mean no photosynthesis.

Well again, my ideas are developing in the context of a system that would have lots of CO2 already bubbling through a water column anyway. [4] If the bubbles are failing I've got bigger problems.

[1] https://en.wikipedia.org/wiki/Vortex_tube solid state; separates a presurized flow of air nito hot and cold portions. I suspect it has applications in separating gas mixtures.

[2] https://en.wikipedia.org/wiki/Bioplastic

[3] https://en.wikipedia.org/wiki/Tensairity

[4] https://en.wikipedia.org/wiki/Molten_salt_oxidation MSO converts plastic to a mixture of H2 and CO; run that through a water column and you get CO + H2O -> CO2 + H2; so now you have a bunch of hydrogen and carbon dioxide. Run that through a vortex tube to separate the gasses, send the H2 to fuel cells and the CO2 to the algae tubes... Where there used to be oceanic plastic trash there is now power and biomass.