4 ms·

It takes less energy but produces a concentrated liquid waste stream that must be disposed of. I've heard that this brine is toxic. Does this make disposal an

by dfxm12 3y ago

It takes less energy but produces a concentrated liquid waste stream that must be disposed of. I've heard that this brine is toxic. Does this make disposal an issue? Is the toxicity true or hyperbole? I mean, do we know how bad it is, and if we can do anything safely with it? It seems like "salt" is useful in a lot of contexts, including industrial, so can we do something with the brine besides disposing it somewhere?

function_seven 3y ago
It’s toxic only because of concentration. AKAIK, there aren’t any compounds in the brine that weren’t present in the seawater to begin with. The solution is dilution, but I’m sure it’s easier for me to type that than it is to achieve in a desalination plant. But, why not really-long-pipe-with-small-holes-along-the-length? That seems to me like a simple mechanism to send the brine back into the ocean without causing a local disaster on the sea floor. Is there maintenance required that makes it more expensive than I realize?
- Timshel 3y ago
  I believe I had read just dilute it until concentration is ok then release. But might be more tricky than that ^^ : https://www.sciencedirect.com/science/article/abs/pii/S0048969719334655 https://www.sciencedirect.com/science/article/abs/pii/S00489...
  - dylan604 3y ago
    dilute it with what? the clean water you just removed the salt from?
    - function_seven 3y ago
      No, the salt water you're about to discharge it into. Have a pump that draws in 10L of ocean water for every 1L of brine you need to dispose of, mix 'em up, and discharge the 11L of only-slightly-saltier water back into the sea. Not sure when it makes more sense to do that vs. having a leach-field type of brine discharge. They both ultimately do the same thing, but one requires more mechanicals, the other requires more piping and "passive" infrastructure.
    - VintageCool 3y ago
      Or a large amount of regular saltwater.
    - toast0 3y ago
      Sewage treatment outflows maybe? Wouldn't be enough in a community where all the freshwater is coming from desalination, because not all of the freshwater goes into the sewage system, but it might be workable in communities where desalination is augmenting other sources of freshwater.
- kibwen 3y ago
  > But, why not really-long-pipe-with-small-holes-along-the-length? The OP mentions that this is common in practice, although it's easier to tell in the video that this is what is being described.
emodendroket 3y ago
It’s full of contaminants from the process that make it unsuitable for a lot of uses, as I understand.
xenadu02 3y ago
No, fresh water just enters the water cycle. It will eventually evaporate or end up in a river and back into the ocean where it will be reunited with the salt. The overall salt concentration of the ocean would not be changed unless we sequestered the fresh water permanently. Even then it would take a tremendous effort to make even the tiniest difference in salinity.
- rtkwe 3y ago
  It will change the salinity on the short term though at the release location and the amount a large plant will be discharging is enough to alter the local salinity so long as the plant remains operational which will negatively affect sea life in that area.
  - Robotbeat 3y ago
    Only if the discharge is not diluted sufficiently. There is nothing about reverse osmosis that is fundamentally more toxic or harmful than the typical evaporation that takes place naturally in the ocean. And it’s pretty dishonest to claim otherwise. If there’s a problem with too high salinity of discharge, that’s an engineering problem that should be fixed with greater dilution.
    - xenadu02 3y ago
      I almost addressed this point in my original post... you need a diffusion system that runs the brine out into the ocean and mixes it with seawater to a certain concentration... as long as it isn't super concentrated or at a very different temperature it won't create a brine pool and the salt will diffuse harmlessly.
    - strken 3y ago
      If you're drawing water from a bay and not putting all that water back in, then the salinity of the bay must depend on the rate it mixes with the outside ocean, since you're removing water from the system. I assume that in practice the amount of water taken by a desal plan is tiny and most bays have high tidal inflow and outflow, but it's obvious that more than just dilution should be considered.
tivert 3y ago
> I've heard that this brine is toxic. Does this make disposal an issue? Is the toxicity true or hyperbole? I mean, do we know how bad it is, and if we can do anything safely with it? It seems like "salt" is useful in a lot of contexts, including industrial, so can we do something with the brine besides disposing it somewhere? Too much salt can kill stuff (e.g. people, plants), so I suppose that makes it "toxic." Maybe there's a tiny amount old industrial pollution from anywhere an everywhere that concentrated in there, too. However, if you're desalinating seawater, what's the problem with just dumping the brine back in the sea? Unless you introduced new stuff into it during the desalination process, you wouldn't be making anything worse.
- grogenaut 3y ago
  Because the brine is hyper-salty compared to ocean water and takes a while to mix back in, essentially creating a new ecosystem, brackish, where the outlets are. What's wrong with pumping 10% CO2 into your office constantly from a compressed gas plant extracting oxygen and argon next door?
  - tivert 3y ago
    > Because the brine is hyper-salty compared to ocean water and takes a while to mix back in, essentially creating a new ecosystem, brackish, where the outlets are. How large would those brackish areas near the outlets be? It seems to be that would be a big problem in an enclosed bay, but much less so on a shore facing open ocean. Could they run pipes out a few kilometers with small, regular holes (maybe some modification of oil pipeline technology) to spread the discharge out and mitigate the concentration problem? Could they make the waste output less concentrated? Maybe by either running the desalination process less (would that also increase energy efficiency?) or by pre-mixing the waste with some un-desalinated intake water?
    - rtkwe 3y ago
      The pipe with many holes is one of the solutions used today it's just imperfect and requires a lot of pipe to make sure the waste output isn't too concentrated in a single area.
mr_toad 3y ago
Even normal seawater has toxic concentrations of salt: “seawater's sodium concentration is above the kidney's maximum concentrating ability. Eventually the blood's sodium concentration rises to toxic levels, removing water from cells and interfering with nerve conduction, ultimately producing a fatal seizure and cardiac arrhythmia.” https://en.wikipedia.org/wiki/Salt_poisoning https://en.wikipedia.org/wiki/Salt_poisoning

by dfxm12 3y ago

function_seven 3y ago

It’s toxic only because of concentration. AKAIK, there aren’t any compounds in the brine that weren’t present in the seawater to begin with. The solution is dilution, but I’m sure it’s easier for me to type that than it is to achieve in a desalination plant. But, why not really-long-pipe-with-small-holes-along-the-length? That seems to me like a simple mechanism to send the brine back into the ocean without causing a local disaster on the sea floor. Is there maintenance required that makes it more expensive than I realize?

Timshel 3y ago
I believe I had read just dilute it until concentration is ok then release. But might be more tricky than that ^^ : https://www.sciencedirect.com/science/article/abs/pii/S0048969719334655 https://www.sciencedirect.com/science/article/abs/pii/S00489...
- dylan604 3y ago
  dilute it with what? the clean water you just removed the salt from?
  - function_seven 3y ago
    No, the salt water you're about to discharge it into. Have a pump that draws in 10L of ocean water for every 1L of brine you need to dispose of, mix 'em up, and discharge the 11L of only-slightly-saltier water back into the sea. Not sure when it makes more sense to do that vs. having a leach-field type of brine discharge. They both ultimately do the same thing, but one requires more mechanicals, the other requires more piping and "passive" infrastructure.
  - VintageCool 3y ago
    Or a large amount of regular saltwater.
  - toast0 3y ago
    Sewage treatment outflows maybe? Wouldn't be enough in a community where all the freshwater is coming from desalination, because not all of the freshwater goes into the sewage system, but it might be workable in communities where desalination is augmenting other sources of freshwater.
kibwen 3y ago
> But, why not really-long-pipe-with-small-holes-along-the-length? The OP mentions that this is common in practice, although it's easier to tell in the video that this is what is being described.

emodendroket 3y ago

It’s full of contaminants from the process that make it unsuitable for a lot of uses, as I understand.

xenadu02 3y ago

No, fresh water just enters the water cycle. It will eventually evaporate or end up in a river and back into the ocean where it will be reunited with the salt. The overall salt concentration of the ocean would not be changed unless we sequestered the fresh water permanently. Even then it would take a tremendous effort to make even the tiniest difference in salinity.

rtkwe 3y ago
It will change the salinity on the short term though at the release location and the amount a large plant will be discharging is enough to alter the local salinity so long as the plant remains operational which will negatively affect sea life in that area.
- Robotbeat 3y ago
  Only if the discharge is not diluted sufficiently. There is nothing about reverse osmosis that is fundamentally more toxic or harmful than the typical evaporation that takes place naturally in the ocean. And it’s pretty dishonest to claim otherwise. If there’s a problem with too high salinity of discharge, that’s an engineering problem that should be fixed with greater dilution.
  - xenadu02 3y ago
    I almost addressed this point in my original post... you need a diffusion system that runs the brine out into the ocean and mixes it with seawater to a certain concentration... as long as it isn't super concentrated or at a very different temperature it won't create a brine pool and the salt will diffuse harmlessly.
  - strken 3y ago
    If you're drawing water from a bay and not putting all that water back in, then the salinity of the bay must depend on the rate it mixes with the outside ocean, since you're removing water from the system. I assume that in practice the amount of water taken by a desal plan is tiny and most bays have high tidal inflow and outflow, but it's obvious that more than just dilution should be considered.

tivert 3y ago

> I've heard that this brine is toxic. Does this make disposal an issue? Is the toxicity true or hyperbole? I mean, do we know how bad it is, and if we can do anything safely with it? It seems like "salt" is useful in a lot of contexts, including industrial, so can we do something with the brine besides disposing it somewhere? Too much salt can kill stuff (e.g. people, plants), so I suppose that makes it "toxic." Maybe there's a tiny amount old industrial pollution from anywhere an everywhere that concentrated in there, too. However, if you're desalinating seawater, what's the problem with just dumping the brine back in the sea? Unless you introduced new stuff into it during the desalination process, you wouldn't be making anything worse.

grogenaut 3y ago
Because the brine is hyper-salty compared to ocean water and takes a while to mix back in, essentially creating a new ecosystem, brackish, where the outlets are. What's wrong with pumping 10% CO2 into your office constantly from a compressed gas plant extracting oxygen and argon next door?
- tivert 3y ago
  > Because the brine is hyper-salty compared to ocean water and takes a while to mix back in, essentially creating a new ecosystem, brackish, where the outlets are. How large would those brackish areas near the outlets be? It seems to be that would be a big problem in an enclosed bay, but much less so on a shore facing open ocean. Could they run pipes out a few kilometers with small, regular holes (maybe some modification of oil pipeline technology) to spread the discharge out and mitigate the concentration problem? Could they make the waste output less concentrated? Maybe by either running the desalination process less (would that also increase energy efficiency?) or by pre-mixing the waste with some un-desalinated intake water?
  - rtkwe 3y ago
    The pipe with many holes is one of the solutions used today it's just imperfect and requires a lot of pipe to make sure the waste output isn't too concentrated in a single area.

mr_toad 3y ago

Even normal seawater has toxic concentrations of salt: “seawater's sodium concentration is above the kidney's maximum concentrating ability. Eventually the blood's sodium concentration rises to toxic levels, removing water from cells and interfering with nerve conduction, ultimately producing a fatal seizure and cardiac arrhythmia.” https://en.wikipedia.org/wiki/Salt_poisoning https://en.wikipedia.org/wiki/Salt_poisoning