Oceans a Viable Source of Water? (Desalinization)

Factual Questions
1

With urban and suburban populations ever on the rise, it seems like our water resources are becoming seriously strained. Atlanta faced a serious shortage this year, owing partly to drought and poor water management, but also because Atlanta has surprisingly limited water resources.

Are desalinization plants a viable option? Could water be economically pumped inland from such plants? Is the cost prohibitive? Will it cease to be prohibitive in the future as water shortages worsen? Are there any other drawbacks to desalinization plants? What do they do with the resultant salt after ocean water is desalinized?

2

Desal plants are common throughout the world, and are the norm in places such as the middle east. I saw RO plants in Cancun in just about every hotel down there. They each have their own plant because not enough fresh water was available for large resorts without them. I didn’t ask, but I think they were taking lagoon water rather than sea water because of the lower salt content.

It is all a matter of economics. What water sources are available locally? If there is some mineral heavy water available, then that is generally easier to clean up than sea water. Brackish water is also preferable. Sea water is generally only used if no other cleanable source is available.

3

Here is a link (http://www.mwd.dst.ca.us/pa/docs/news/96-06/desalapp.htm) to a press release from the MWD of Southern California about desalinization. Note that it is four years old. You may also wish to search at the National Academies web site (the site itself looks partially screwed, but the search function appears to be working)/

To try and avoid turning this into a Great Debates thread (not that I have anything against GD, but I don’t think that that’s what you’re looking for), I merely mention that water consumption is as important as water supply. Some changes propsed are laughably useless, some are signficant, some would be worthwhile but, in the current climate of thought, are not politically feasible.

4

De-salinization plants exist in a number of places in the world today but as yet are considered impractical and are used for research into advancement of the technology itself.

The last article I read on the topic of claiming drinking water proposed that while the technology for de-salinization and hydrogen bonding existed they both required either too much power or had too many waste byproducts that needed to be disposed of.

From what I read the most promising technology under development is a reverse osmosis system similar to what is used onboard the shuttle to recycle waste water but on a much more grand scale. The power source would be fission (which is the reason this one is not in use today) running off the “waste” minerals extracted from the ocean water.

http://www.usbr.gov/water/desal.html and http://www.medrc.org.om/ both have some really good information on the topic.

zen101
D.F.A.

5

I bet even money that water conservation, meaning reduction in non-primary use and more efficient usage is the economical answer. Then there is also reuse – cleaning waste water.

As other posters have mentioned, desalinization is fairly expensive and energy intensive. That is, uneconomical for most of the US when one considers the alternatives, such as reducing usage and recycling waste waters. As for the salts, depends on the method of desalinization.

6

I currently live in metro Atlanta (specifically Gwinnett county), and the so-called water shortage hasn’t really affected us here, aside from the imposition of outdoor watering restrictions. The restrictions have been widely followed, and the county’s water usage has been reduced enough to not require further action for now.

However, Gwinnett gets its water from Lake Lanier, whereas Atlanta and points south (where water restrictions have been more severe) get their water from other sources. There’s also been discussion (and some adoption, particularly by larger businesses) of water recycling (i.e., gray water).

As far as desalination is concerned, it’s not really economically viable for a landlocked city like Atlanta – the water (either seawater or the desalinated stuff) would have to be piped from the coast. The initial costs would be quite high, as no such pipeline currently exists.

In fact, desalination’s not necessarily economical for coastal cities either. Back in the early 90s, during the last major Southern California drought, the city of Santa Barbara (where I was living at the time) spent about $40 million to build a desalination plant, which came online in the spring of '93, IIRC, just as the drought was ending.

They ran the plant for a couple weeks, then shut it down, presumably to be reactivated in a subsequent drought. In the meantime, the city has decided to join the California Aqueduct system, which will probably cost billions, since they’ll have to build at least 100 miles of new aqueduct.

I’ve also been told that Key West has a desalination plant, in addition to a pipeline from the mainland. I suspect that it, like Santa Barbara’s plant, is probably used as a backup to other sources.

7

3waygeek:

Welcome aboard, 3waygeek! Always good to see another Atlantan on the board.

I’m not sure what you mean by this. Doesn’t Atlanta gets its water from the Chattahoochee River? How is that a “different source” from Lake Lanier?

spoke-, I know some Mideastern OPEC countries use desalination as their main means of getting drinking water; they’re wealthy enough to afford the extravagance.

I also know, although I’m not studied up on this, that the leftover salt and minerals (called “brine”) are dumped back in the sea. I’d be very interested, and concerned, to know what effect that has on sea life in the area.

I’d guess whatever effect would not be good.

8

My dad worked for a natural gas pipeline company (originally Michigan-Wisconsin Pipeline, now Enron I think) that always had contigency plans available so that they could switch from pumping gas to pumping desalinized water from the Gulf of Mexico. They would have just been the transport, somebody else would have done the desalizination.

9

Five wrote:

Yikes! That’s the sort of thing I feared. I would imagine that the radical increase in salinity would be death to fish, at least in the localized area where the brine is dumped. Seems like it would make more sense to bury the salt, or find some pre-existing salt flats in the desert and put it there.

3waygeek, welcome to the board. Five is right. Lake Lanier is part of the Chattahoochee River system. It was formed by damming the river. (Look on the map, and you’ll see the Chattahoochee flowing into and out of the lake.) When those of us who live downstream take water out of the river, we are relying on the same ultimate water source as you Gwinnetians.

10

That’s the sort of thing I’m wondering about. What is the route of the pipes you’re talking about? Do you know? I am assuming (despite the company name) that you’re not talking about pumping water up to Michigan/Wisconsin, since they would seem to have plenty of water already. What area would your dad’s company have serviced, if you know?

11

Cecil on desalination
As to the “waste” salt left over from desalination, isn’t salt an economically viable product in its own right? Why don’t they sell it?

12

[Hijack]spoke-:

(Underlining mine.)

For those of you playing at home, spoke- is using the proper adjective used to describe residents of Gwinnett County, one of Atlanta’s northeastern suburbs.

Obvious, you say. Yes, but what you won’t know is the idiosyncratic way this word is pronounced.

It’s “gwuh-NEESH-uns.”

13

Chronos: There are too many readily available resources for procuring salt that is more or less mineral free. The process of desalinization does not produce a powder or produces a “slurry” which is mainly sediment laden very salty water. Again it is a matter of expense to extract the salts from this briny slop. Not too commercially viable when there are loads of salt to be had and harvested in naturally ocurring salt flats where the salt crystalizes at a natural rate and the other sediment settles below it.

If we has some sort of salt shortage this would become somewhat of an issue, but it is just too common of a mineral. As it is now the slurry has to be removed at a cost to the de-sal plants’ overhead.

zen101
D.F.A.

14

“which is mainly sediment laden very salty water”

This is the brine. They often dump it back into the ocean, much to the chagrin of the fish.

15

Seawater can contain as many as 80 different dissolved salts, although the most common are gypsum (CaSO[sub]4[/sub])and halite (NaCl, or table salt). Under natural evaporative conditions, the first salt to precipitate will be gypsum, followed by halite, then a number of other minor (and poisonous) salts, like sylvite (KCl). If you wanted to market the salts, you should be able to separate them through a simple stepwise distillation. To precipitate gypsum, you’d have to reduce the seawater’s volume to about 19% of the original volume; halite precipitates when the seawater has reached about 10% of its original volume. After halite precipitates, the remaining water full of potassium salts would still have to be disposed of. For the reversed osmosis method, though, or any sort of electrolytic method, it’s not clear to me that you would be able to separate the salts during the desal process.

So, it should be possible to do under certain circumstances. But as zen101 just pointed out, there are so many other more commerically viable sources of salt that it’s probably not with the effort.

handy said:

Depending on the volume of salts being dumped back in and the initial salinity of the water, the most significant effects will be on benthic communities (organisms living on the sea floor), because concentrated brines are more dense than regular sea water and will sink. At least the fish can swim away… If the dumping went on long enough, the existing sea floor communities would eventually be replaced by more salt-tolerant life.