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Could wastewater plants simply heat up water past 500C to decompose all chemicals and output clean water? (fedia.io)
submitted 1 year ago by to c/nostupidquestions@lemmy.world
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Our waterways are becoming more and more polluted due to PFAS, plastics, medicines, drugs, and new chemicals made by companies that just hand over the responsibility of cleaning to plants paid for by public moneys. Detecting the different chemicals and filtering them out if getting harder and harder. Could the simple solution of heating up past a point where even PFAS/forever chemicals decomposes (400C for PFAS, 500C to be more sure about other stuff) be alright?

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[–] 3 points 1 year ago* (last edited 1 year ago) (5 children)

You realise water boils at 100°C, right?

Edit: yes, I know it boils a different temperatures, but we're talking about 500°C for a practical use case at scale here..

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  • [–] 4 points 1 year ago

    Bit pendantic but I think its interesting: no, water doesn't always boil at 100 °C. It can boil anywhere between -50 °C and 317 °C, depending on pressure.

    On top of Mt. Everest you cannot cook potatoes because the water boils at 71 °C. On the other hand, with enough pressure water does not boil at all, instead becoming a supercritical fluid - a different phase from gas or liquid.

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  • [–] 3 points 1 year ago (1 child)

    At standard pressure. high pressures can make it liquid. I can't find charts that go high enough with a simple search but it looks like you need to get to 4000-5000psi. industry does go that high for some operations. It needs special design to toeit safely though.

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  • [–] 3 points 1 year ago (1 child)

    Right.. Have you considered that a basic order-of-magnitude estimate of scale of water, energy, and pressure requirements make the idea wildly infeasible in practice?

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  • [–] [S] 0 points 1 year ago (1 child)

    There's no need to be condescending. You seem to have misunderstood the question. I'm not trying to keep water liquid at 500C and decompose other particles at that temperature. The state of the water isn't mentioned anywhere in my post, just the temperature.

    FYI, lava is 800-1000C and regularly comes in contact with water. The resulting vapor has a temperature way higher than 100C.

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  • [–] 2 points 1 year ago

    Fair enough, sorry. It's just that your question (and some of your answers) don't seem to be accounting for dealing with the volume change of steam, and how that would be managed.

    Also the fact that if you're evaporating the water off anyway, why not just let it escape and concentrate the chemicals, and then deal with them that way? I'd guess most of them would not be in the vapour anyway? (unless they're volatile, in which case they'd probably boil off even earlier)

    Re: Lava contact. I don't think the resulting water vapour is much more than 100°C? The phase change takes a lot of energy to phase change, and is still at about 100°C after that, and then the steam would escape very quickly, and be displaced by more water, so it would not have much chance to heat up more. The lava-water interface would always be at about 100°C, give or take a few tens of degrees for the Leidenfrost effect, maybe? I might be wrong here, but I can't see how it would get MUCH hotter than 100°C (assuming normal surface pressure).

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