Comment by carapace | Hacker News Reader

Comment by carapace | Hacker News Reader

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Bubblers and tubes of algae in the sun. The bubbles stir the algae to maximize growth. Add a few goldfish to help cycle nutrients[1]. You can get the algae so thick that a test stick (plastic spoon) can't be seen beyond a centimeter deep.[2]

At this stage you can remove half of the algae each day and they'll regenerate within twenty-four hours.[3] The extracted algae biomass (mostly composed of atoms that used to be air or water) can be "upcycled" by feeding to higher organisms (fish, earthworms, etc.) or processed industrially for e.g. plastics, fuel, foodstuffs, whatever.

Ah, here we go, a few people are doing it already: http://www.iflscience.com/environment/urban-algae-farm-gobbl...

The important thing is it's cheap as heck and doesn't require fancy technology.

[1] Goldfish eat algae. Few in the tank makes it a simple aquaculture system.

[2] I looked for images of "tubes of algae in the sun" and most of them were transparent. If you can see through the tube you're doing it wrong.

[3] If conditions are optimal, otherwise you might have to be happy with 35-40%.

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.

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.

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.

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.)

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.

PS

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. 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.

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.

Thank you for your detailed response as well, it's always heartening to see people chipping at these problems. FWIW I've met plenty of crackpots and you are not one of them. You become a crackpot when you can't tell the difference between what is true and what you want to be true, not when your ideas are "out there".

Regarding your experiment with the fish, I suspect this is a difference in your environment, as you alluded to. I'm out here in the desert, and it is sunny and it is hot, and if you're maintaining a low evaporation rate there's no where for that heat to go. Algae don't usually mind though. If you deploy a tube out here it'll only be so long until it gets warped and won't accept a pig (basically a plug that moves along a pipeline & cleans the sides), so I'm very interested in this idea of having the fish clean the tube. But I suspect that the same technique would go belly-up for me, pun fully intended.

I completely agree with your evolutionary approach and with using ecosystems that are already functioning. I did the desert equivalent and found my attached growth culture under a rock.

I was not aware of Living Machine but thanks for pointing it out. I think a lot about a similar concept called Algae Turf Scrubbers (http://algalturfscrubber.com/) (also trademarked), which mimic coral reefs.

You clearly have put a lot of thought into this so I hope I'm not rehashing ideas you've already dismissed, but here it goes.

You may want to consider using fungi. They are the matchless experts in breaking down toxic material, and my inkling is that they will be the ones to crack the problem of digesting plastics. Some organisms have started to do this already. Fungi have the ability to learn and adapt as individuals, so they're more friendly to this sort of engineering than most complex lifeforms. Microalgae are of course fantastic because they evolve on human timescales.

Keep in mind that plastics aren't really pure hydrocarbons. It would be very nice if they were. But they'll come with paints and stabilizers and such, and you'll end up with heavy metals and other contamination (I won't pretend to understand the intricacies here). If this ends up in your biomass then it cannot be used for agriculture. Dirty biomass is usually fine for fuel production, however. I'm sure you've thought of this though and that I would see your reasoning if I understood MSO reactors better. What happens to this waste when it leaves your reactor is another big question.

Gas separation is hard. You might consider separating the nitrogen and calling that good. The more conventional approach would be to use a pressure swing absorber but I'm taking the impression you want to use your vortex tube for cooling as well as separation. I've never tried pumping pure CO2 through an algae culture, but I believe it will become toxic at some concentration.

Rather than feeding it to worms, you might consider an anaerobic digester. You'll still produce fertilizer but you'll also be able to capture methane, which you can blend with your H2 and send to a generator instead of a fuel cell. While they have the same problems with shading as aquaponic systems, AD effluents are a very good fertilization ratio for algae. (I'm remembering a lecture here so I don't have a source to cite & don't recall the ratio, I'm afraid.) Another advantage is that they can accept wet biomass, and harvesting is often the most expensive part of the process. The methane could be blended with hydrogen and fed into an off-the-shelf natural gas generator. But I imagine that you did choose fuel cells for a reason. My personal view is that developing technologies are expensive in terms of liability, and that you can only pick a few of them. So if I were you I wouldn't put fuel cells and a novel approach to gas separation in the same design. Of course methane can be converted to hydrogen through steam reformation, or it can also be used in a fuel cell. Maybe you can even put them in the same fuel cell, I don't understand this technology whatsoever.

I've also got some more open ended questions you might find interesting to think about.

Are you planning to do this on a barge or derrick? How will your process effect the chemistry of the ocean around you? How will that effect the marine life around you?

If you're concentrating metal contamination and taking water in from the ocean, can you recovery dissolved metals? Perhaps you can combine this with desalination?

Is there a way your project can help with ocean acidification?

This talk seems to paper over a lot of problems with this system (its one of those "CO2 to methane" systems), as a TED talk does, but it's an interesting idea & related to your problem space.

https://www.youtube.com/watch?v=X-HE4Hfa-OY

You're probably aware but some people are producing hydrogen from algae. Could enhance your yield.

https://en.wikipedia.org/wiki/Biological_hydrogen_production

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