Why is desalination so difficult?

Post by cyanydeez » Wed, Jul 05, 2023, 10:12 PM UTC

Robotbeat wrote: post #200

jerf wrote: post #126

Earlier quoted context omitted.

There are several practical problems dumping the brine back in the sea. If you dump it on a living ecosystem you tend to kill it. Living ecosystems are, unfortunately, concentrated right where we are desalinating and is cheap to dump. Compounding this problem is that water mixes much more slowly than your intuition suggests. It can stay a coherent mass of high-salt water way longer than you'd think, killing as it goe…

> If you dump it on a living ecosystem you tend to kill it. This is false and somewhat dishonest. This is simply a choice of not diluting it enough. There is nothing about the discharge from reverse osmosis that is any more fundamentally toxic than the natural process of evaporation. Proper dilution is essential, and treating the discharge as fundamentally toxic actually undermines the engineering to do this proper d…

Post by [deleted] » Wed, Jul 05, 2023, 10:13 PM UTC

eropple wrote: post #225

giantrobot wrote: post #220

Earlier quoted context omitted.

The brine waste from RO is still mostly water. In order to extract the salt you'd need to evaporate the water which still takes a lot more energy. You could use evaporative ponds to let the Sun do the work but that takes a lot of space. In either case you're spending a lot more money per pound than just digging the salt out of a mine.

Is that true when we take into account environmental externalities? I am not an expert in this field; I know that many forms of mining are capital-B Bad for the environment but I don't know how salt mines impact the area around them.

Post by Bost » Wed, Jul 05, 2023, 10:16 PM UTC

vladraz wrote: post #104

at_a_remove wrote: post #71

Earlier quoted context omitted.

I only have a BS in Physics but you're basically correct. But to make it even more simple and divorced from method: 1) There's a large difference in energy and entropy between seawater and drinkable "fresh" water. This represents a bare minimum expenditure, below which you can never go, lest you attempt to create a perpetual motion machine. 2) No matter how you do it: Well, now you have a bunch of previously dissolve…

A sensible thing to do is to turn the waste brine water into a resource. Since it's already been pumped up, pour it out into an evaporation pond to increase humidity in an area that could benefit from it, and then scoop up the salt to extract valuable minerals.

Post by 8n4vidtmkvmk » Wed, Jul 05, 2023, 10:23 PM UTC

Robotbeat wrote: post #199

rtkwe wrote: post #168

Earlier quoted context omitted.

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.

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.

Post by FredPret » Wed, Jul 05, 2023, 10:28 PM UTC

londons_explore wrote: post #218

daveslash wrote: post #216

Earlier quoted context omitted.

I know that generally speaking, disposing of waste in the ocean and expecting it to disperse enough to be harmless is foolish and wrong. But in the case of salt, It would seem to me that the ocean can handle that amount of salt. Course, I haven't done the math. But it would seem to me that the back in == salt taken out. We'd only be changing the net salinity by the amount of water subtracted. Without having done the…

There are potentially some good reasons to just spray the brine into the air. Seawater sprayed into the air becomes tiny salt crystals, which in turn help clouds to form, and cause increased rainfall. The rain produced has negligible levels of salt. In places with dry climates, this often can turn desert land into farmland across an area hundreds of kilometers wide.