World without Water: The Dangerous Misuse of Our Most Valuab

[jamest]

There will come a time when water is worth more than gold and oil. Britain will become rich again!!

On a serious note, just how difficult/expensive is it to purify sea water? I've sometimes wondered why that doesn't happen, but this is the first effort I've made to find out why. Surely, piping purified sea water inland would solve all problems in places like California?

[JimC]

There will come a time when water is worth more than gold and oil. Britain will become rich again!!

On a serious note, just how difficult/expensive is it to purify sea water? I've sometimes wondered why that doesn't happen, but this is the first effort I've made to find out why. Surely, piping purified sea water inland would solve all problems in places like California?

It is quite doable, and there are desalination plants around the world. However, whichever method used (evaporative or forced reverse osmosis), they are expensive to build, and they also consume a lot of energy. Solar evaporation can certainly work without expensive energy inputs, particularly in a dry, sunny region on the coast, but requires a large area to provide significant quantities of potable water.

[jamest]

You'd think somewhere rich, like Cali, would be pioneers in the endeavour.

[JimC]

You'd think somewhere rich, like Cali, would be pioneers in the endeavour.

I'm sure they have some, from what I remember reading. However, unless you are powering them with strictly renewable resources (or perhaps nuclear), the carbon emissions involved only add to the climatic problems we face in the future.

[mistermack]

Solar evaporation can certainly work without expensive energy inputs, particularly in a dry, sunny region on the coast, but requires a large area to provide significant quantities of potable water.

How does that work? I can see that it's easy enough to evaporate salt water, but how do they condense it?

[JimC]

Solar evaporation can certainly work without expensive energy inputs, particularly in a dry, sunny region on the coast, but requires a large area to provide significant quantities of potable water.

How does that work? I can see that it's easy enough to evaporate salt water, but how do they condense it?

You can do it simply on the inside roof of a greenhouse-like structure, but it is not nearly as efficient as something a lot colder would be. They could pump cold seawater from a deep pipe to cool a surface for condensation, then return it, I suppose...

The one in my state (a controversial construction) works instead on forcing seawater through pipes with osmotic membrane linings. The fresh water is forced out, and the remaining brine is pumped back into the ocean. Still takes a lot of energy.

[mistermack]

Yeh, the reverse-osmosis seems to be the way that most of them are going at the moment.
It's viable for large cities, where the demand is high, and the users don't use vast amounts.
But it's not economic for agriculture.

It can be more economic, if they are extracting groundwater which is salty, but much less salty than seawater.

[rainbow]

Solar evaporation can certainly work without expensive energy inputs, particularly in a dry, sunny region on the coast, but requires a large area to provide significant quantities of potable water.

How does that work? I can see that it's easy enough to evaporate salt water, but how do they condense it?

Evaporation is the most expensive way to treat water.
Most of the water that is discharged is lightly contaminated.
You take out the poo and the condoms, filter it, add ozone, put it through activated carbon and then back into the taps.

Not rocket science.

[mistermack]

Solar evaporation can certainly work without expensive energy inputs, particularly in a dry, sunny region on the coast, but requires a large area to provide significant quantities of potable water.

How does that work? I can see that it's easy enough to evaporate salt water, but how do they condense it?

Evaporation is the most expensive way to treat water.
Most of the water that is discharged is lightly contaminated.
You take out the poo and the condoms, filter it, add ozone, put it through activated carbon and then back into the taps.

Not rocket science.

In this country, it's compressed air that bubbles through the tanks, feeding the microbes that kill the harmful bacteria, and eat the shit. ( I think ). I've never heard of Ozone being used, but I might be behind the times.
They also drizzle it over rocks, which are coated with bacteria that eat the nasties.
That's another way of providing a big surface area for the air to get at the water.
Maybe they don't use it much in hot dry countries, where evaporation might spoil the party.

Over here they put it back in the rivers, although it's safe to drink, by the time it leaves the plant.
We ain't short of water so we don't need to drink it.

I have heard that man made reed beds are also ace at cleaning contaminated water. It's the same principle, they are full of bacteria that feed on the nasty stuff.

[laklak]

Reverse osmosis works well but as mentioned requires a lot of energy to run the pumps. A small system used on cruising yachts costs a minimum of $5000 and will produce under 4 gallons an hour. The most efficient utilize a direct-drive pump on one of the main engines, as they require a fairly high pressure on the salt side of the membrane but you're looking at >$10,000 for one of those. Maintenance is a continual process and quite expensive. I'm considering a manually operated emergency system that would produce a couple of gallons a day, but even those are $3000 and up and would only be useful in an extended lifeboat situation. They're experimenting with towed systems, where the "pump" is towed behind the boat, but the jury is still out on those.

From wiki

The high pressure pump supplies the pressure needed to push water through the membrane, even as the membrane rejects the passage of salt through it. Typical pressures for brackish water range from 225 to 376 psi (15.5 to 26 bar, or 1.6 to 2.6 MPa). In the case of seawater, they range from 800 to 1,180 psi (55 to 81.5 bar or 6 to 8 MPa). This requires a large amount of energy. Where energy recovery is used, part of the high pressure pump's work is done by the energy recovery device, reducing the system energy inputs.

Very high pressures.

You can get a home system to purify drinking water for less than a grand, but they aren't sturdy enough for commercial or marine use. That might be a possibility, though - require every home to have a small system for drinking water and pump non-potable water for other uses?

[Seth]

There will come a time when water is worth more than gold and oil. Britain will become rich again!!

On a serious note, just how difficult/expensive is it to purify sea water? I've sometimes wondered why that doesn't happen, but this is the first effort I've made to find out why. Surely, piping purified sea water inland would solve all problems in places like California?

Maybe drinking water, but 80 percent of all fresh water is used for crop irrigation, not domestic or industrial use.

[mistermack]

You could maybe have a system where a windmill directly drives the reverse osmosis pump, and store fresh water on windy days.

I've always thought that you could use Mangrove bushes in a saltwater evaporation scheme.
They have an ability to use saltwater, and fresh water evaporates from their leaves. The plant does the work for you, and you just need to run the moist air through a condenser. It would need to be done on a very big scale, but it could be economic.
If it was combined with shrimp or oyster farming around the roots, it could be a winner in suitable areas.
You just build a waterproof base, and clear plastic roof.

[Seth]

You could maybe have a system where a windmill directly drives the reverse osmosis pump, and store fresh water on windy days.

I've always thought that you could use Mangrove bushes in a saltwater evaporation scheme.
They have an ability to use saltwater, and fresh water evaporates from their leaves. The plant does the work for you, and you just need to run the moist air through a condenser. It would need to be done on a very big scale, but it could be economic.
If it was combined with shrimp or oyster farming around the roots, it could be a winner in suitable areas.
You just build a waterproof base, and clear plastic roof.

That's an interesting idea, but I imagine the scale necessary to produce useful amounts of fresh water would preclude this as a large-scale solution to fresh water shortage.

[jamest]

You'd think somewhere rich, like Cali, would be pioneers in the endeavour.

I'm sure they have some, from what I remember reading. However, unless you are powering them with strictly renewable resources (or perhaps nuclear), the carbon emissions involved only add to the climatic problems we face in the future.

California has a long coast by the Pacific. Ideal for utilising both wind and wave power. I guess nobody wants to foot the bill.

[Calilasseia]

Actually, the evaporation problem can also be solved by taking advantage of existing freeze drying technology. Which relies on the fact that water, when the ambient air pressure above it is reduced, undergoes interesting phase changes. First, the drop in temperature (courtesy of the Gas Laws) causes the water to freeze, and become ice. As the pressure drops further, water then moves into an interesting part of the phase diagram, where it undergoes sublimation, and moves straight to the gas phase from the solid, without pausing to occupy the intermediate liquid phase. As it does this, it leaves solutes behind it. Which is a neat way of making yourself not only potable water without the hideous energy costs of heat, but making yourself some saleable sea salt into the bargain. Food freeze drying operations have been using this process for decades, to produce nice, bacteriologically sterile food. I can't recall if it also destroys viruses, but I suspect the sudden water removal will have a fairly interesting impact upon the structural integrity of many viruses.

So, set up some big, gas tight tanks, charge them with sea water, connect them to vacuum pumps, and then extract the air from above the water, and watch the Gas Laws do the work for you. Provide some means of removing the solids from the bottom, and hey presto. Power the vacuum pumps with some nice solar panels, and job done.