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Comment by nawitus | Hacker News Reader

One solution to producing water is desalination. You can effectively get unlimited amount of water, but you pay for it in energy. A quick Google search finds that there's a desalination plant under construction in Carlsbad, California.

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Given that by far most of the water in California is used for growing food, an interesting question would be: at what point is it cheaper to grow the food somewhere else where water is more abundant? Of course the amount of sunlight, temperature etc. would factor in too. But with enough energy, you can simulate the conditions of California anywhere.

MIT Technology Review wrote a great article on this: http://www.technologyreview.com/featuredstory/533446/desalin...

That is a good summary. A friend of mine is looking at putting a passive desalinator on the ocean floor off the coast of Monterey. Basically its a pipe with the permeable membrane surrounding the submerged collector. The pipe carries fresh water to the surface and as you pump out fresh water, the pressure of the ocean at depths of up to 1000 meters pushes in more water through the membranes, desalinating it in the process.

The challenge (other than just building the darn thing) is whether or not the current flow is sufficient to keep the brine levels down around the intake port.

I have no idea how practical that idea really is (he's pretty enthused of course) but I liked the notion of using the pressure difference between the ocean surface and at depths to do the reverse osmosis work for you.

How does the membrane stay clean of marine growth?

hiring a diver or two to do that is probably cheaper than the alternative ways to desalinate.

there is very little marine growth at 1000 meters depth. A bigger concern would be marine snow: http://en.wikipedia.org/wiki/Marine_snow

You can do it, but it turns out the work to pump water up to 1400 psi in a pressure vessel before it goes through a reverse osmosis membrane is about the same as the work to pump desalinated water up from the bottom of the ocean at 1000 meters. There are different inefficiencies in each situation, but I'd assume maintaining equipment 1km below the sea would trump them all.

I think he really means you bail the water from the top rather then pump it, leaving an airgap at the top that the pressure from below gradually fills with more water through the membrane. But then again unless you bail all the water, that leaves only a small pressure differential, and if you bail it all, it gets less efficient as it fills back up and the pressure differential gradually reduces and lifting it from the bottom takes a lot of energy :P. Yeah, I don't understand what he's getting at with the design.

As described in the paper the top 20 meters of the pipe to the surface are emptied (the paper refers to a 30 meter 'pump head' which I believe is the distance below the surface that the water has been pumped out of the pipe). The part on the surface of the ocean is what looks like a giant bucket of sand on the outside (it is supposed to be below the level at which coral grows). The pipe fills with fresh water at some rate (unspecified by the paper) and you can pump it out at the top when you need it. The part on the bottom of the ocean requires no maintenance.

Sounds like the MaHaLo Hawaii Deep Sea Water project[1]. They use that water in Ocean Vodka interestingly enough[2], which is where I found out about it. On the tour they indicated this project seems to be working out quite well.

[1] http://hawaiideepseawater.com/source [2] http://oceanvodka.com/our-vodka/ingredients/

Interesting that their "3000 feet" corresponds to the 1000 meters I read in the paper. I could not find anything on their site though that actually talked about the extraction process.

What does all that extra salt do to the chemistry of the water in that area?

Well depending on rate it can be a pollutant or not. Most desalinating plants have a mixing pond where the mix fresh seawater with partially desalinated water (which has a higher salt content) to produce an effluent that is saltier than the unprocessed water but not so salty that it causes issues. That same mixing could, in theory, be done by ocean currents if the water motion was enough.

A recent econtalk podcast (http://www.econtalk.org/archives/2015/03/david_zetland_o.htm...) discusses the feasibility of using desalination to cover California's water usage. Basically, one desal plant is a drop in the ocean compared with demand.

The relevant metric is price of desalinated water, not the number of plants.

There's also the problem of what to do with the high salinity waste water that is generated from the process. Simply dumping it back into the ocean can cause environmental problems.

What about putting the waste water in a salt pan?

What problems would it cause? As long as you had decent mixing, you'd barely change the local salinity.

I have a hard time understanding this would be a problem, considering the scale, i.e. how much water there is in an ocean.

Let's say we take the amount of water mentioned in a link in another comment, and desalinate 54 million cubic feet of clean water per day in a single plant. That is 204 400 cubic meters per day. It is a huge amount, but we're talking about operating this plant at depth of 1 km. 204 400 cubic meters is 0.0002 cubic kilometers. The California Current flows at around 0.25 m/s which means that the cubic kilometer of water around the desalination plant will be completely exchanged almost once per hour, 21.6 times in a day.

I would say that the salt that you take from desalinated water is an entirely insignificant problem when just left in the ocean, even locally. It's a drop in the ocean. Most likely the saltiness of water will vary a lot more due to other reasons (e.g. how much water evaporates locally).

Other aspects of access to more water in places like California are probably a much more relevant environmental problem. That water is mostly going to be used as irrigation water in agriculture. This will flush a huge amount of earth particles to the ocean. Literally, it will eventually wash away the agricultural land, unless it is used sparingly. When doing that, it puts additional earth material in streams and rivers.

And then there are the issues of fertilizers, pesticides etc for the ever increased volume of agricultural production.

> You can effectively get unlimited amount of water

For an effectively unlimited amount of energy. We do not yet live in a post-scarcity era when it comes to energy so that "unlimited" is quite hypothetical. In the same sense that there is unlimited amounts of space available for humans, <please insert spaceship here>.

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