Question for you science buffs about water

[VALIS]

So I was watching a documentary on youtube about the issue of water availability and basically how humanity is a bit ed because we won't have enough drinking water. Apparently only 2% of the worlds water is fresh water and of that only 1% is available.

But my question is, why can't we convert sea water to serve human needs? I know that the salt is probably the issue but can't this sea water be converted or distilled into drinkable water for human consumption? And if not, why not?

[DisgruntledGoat]

http://en.wikipedia.org/wiki/Desalination

[Mangerman]

The problem with desalination is that it takes incredible amount of energy to desalinate seawater. And with the comming energy crisis, it'll be hard to fit desalination plants in.

[Simetrical]

The problem with water shortages isn't that we actually "run out" of water, it's that water can become more expensive. A lot of places rely on aquifers for their water, and aquifers can dry up if overused. Those places will have to switch to other water sources, like desalinization or reservoirs, which might be more expensive or less practical in a lot of places. This mostly affects poor countries that can't afford the more expensive technologies – Australians might not be happy about their water bills, but they're not going to see the kind of repercussions that might strike poor countries (like massive crop failure).

The problem with desalination is that it takes incredible amount of energy to desalinate seawater. And with the comming energy crisis, it'll be hard to fit desalination plants in.

The energy crisis only really affects small, mobile things like cars, which rely heavily on oil. Things like desalinization that rely on grid power will do just fine for many years to come by using coal, nuclear, solar, hydroelectric, geothermal, wind, etc. There's no crisis there. The oil crisis is possibly a nonissue also, given that we still have an awful lot of cheap oil left (especially if we open up more areas to drilling), and replacements like battery-powered cars are proceeding apace. There's no crisis if we get an economically feasible replacement to gasoline in place before oil gets too expensive.

[Theseus1234]

I was reading Time magazine and they had an article at one point about how scientists were trying to develop different filters to block out the salt and let the water through but with limited success.
It's a hard problem, whichever way you look at it.

[Sphere]

Also the 2% number is rather deceiving. All of that 2% comes from precipitation, almost all of which was at one point sea water before it evaporated into the atmosphere. So its not as if when we can use up that 2% and there will never be fresh water again. The only real water problems are in area's where aquifers that have taken thousands of years to build up are being tapped at a far faster rate then they are replenished.

[removeduser_4536284751384]

the water cycle means we'll never run out of water in places like northern europe

places that are short can take their excess from wetter areas. Dams can be built etc.

[Elfdude]

Before we run out of drinkable water we will have to have significantly gutted our civilization either by our own action or through catastrophe. Water is extremely simple, extremely stable and very abundant. The issue is most of it is salty, however through evaporation we could harness almost all of that water with little effort on our part assuming we have enough time and facilities.

For example you could use solar reflectors to focus their energy on a body of salt water and use a collection medium such as a giant plastic or glass dome to catch the evaporating fresh water and distribute it to storage. Such a device could even potentially harness fresh water from power generating water in steam turbines by recovery the evaporated water after it rises and powers the turbines. Creating both water and electricity.

You could alternatively use power to pump energy into the water to assist with the evaporation process such as in dubai however burning pure oil to drive enormous evaporators is a rather dirty and bad idea. Instead using renewable energies to pump the energy could allow us to set up water plants that are nearly completely independent of human involvement.

However production of water isn't an issue right now, it's more about moving it around equally that's the difficulty.

[Phier]

America's South West is in for a major wake up call as they are trying to create a 'midwestern' lifestyle in a desert and the aquifer is losing several feet a year (I recall it being something like 14 but that was several years ago, no idea of current rate).

What Simetrical said is 100% right. Sitting 30's mins from Lake Michigan, its hard to imagine a time when fresh water will be a major problem for quite a while for us, and then its the money, not the water itself. Just more expensive to treat and produce.

[chamaeleo]

Desalinization is costly, both energetically and cost-wise. I question the long-term prudence of inflating any society's population well beyond a region's natural carrying capacity, while solely dependent upon one such technology for drinking water.

places that are short can take their excess from wetter areas. Dams can be built etc.

IF you have access to wetter areas. Dams present many additional problems: evaporative loss, fluvial plain's deprivation of flood derived nutrients, saline accumulation, submersion of real estate, risk of catastrophic failure, etc.

America's South West is in for a major wake up call as they are trying to create a 'midwestern' lifestyle in a desert and the aquifer is losing several feet a year (I recall it being something like 14 but that was several years ago, no idea of current rate).

Referring to the Ogallala aquifer? Yeah, between overdraft and inefficient irrigation practices I'd say a lot of NE, north-central TX, and KS is headed towards ruination.

Still, there are big differences between states concerning water use. NM's prolly following the most sustainable course of any SW state. Ironically, our acequias are direct descendents of Egyptian, Moorish, and finally Spanish irrigation methods. Landowners get hefty incentives for xeriscaping and big penalties for waste. Even the biggest rednecks are down with conservation, as it's considered an integral part of our cultural heritage.

CA and AZ on the other hand...they'll reap what they sow, much sooner, hopefully beginning in Phoenix and Palm Springs.

What Simetrical said is 100% right. Sitting 30's mins from Lake Michigan, its hard to imagine a time when fresh water will be a major problem for quite a while for us, and then its the money, not the water itself. Just more expensive to treat and produce.

Yeah...flooding, cryptosporidium and contamination are about all you've got to worry about for the moment. Lake Michigan water tastes and smells pretty bad, though. Like a fish took a dump in your glass.

[Phier]

Yeah...flooding, cryptosporidium and contamination are about all you've got to worry about for the moment. Lake Michigan water tastes and smells pretty bad, though. Like a fish took a dump in your glass.

I'm drinking a glass right now, check your pipes and your ice.

[chamaeleo]

I'm drinking a glass right now, check your pipes and your ice.

While I lived in Racine WI, I only noticed it in warm or hot tapwater. After moving out of state, subsequent visits found Lake Michigan water sadly lacking in the palatibility department; proving you can get used to just about anything through daily contact. For some reason, many lakeshore communities put their intakes just downflow of rivers, wastewater treatment plants, etc...which is what led to the Milwaukee crypto outbreak in 92. I still remember the tshirts emblazoned "Milwaukee: you cannot drink the water, but you CAN eat the people!" (Jeffrey Dahmer had just been captured).

Another potential problem with filtration systems is biofilms. You're basically exchanging one form of risk for another: removing chemicals, while giving bacteria a nice surface to grow upon.

[Phier]

While I lived in Racine WI, I only noticed it in warm or hot tapwater. After moving out of state, subsequent visits found Lake Michigan water sadly lacking in the palatibility department; proving you can get used to just about anything through daily contact. For some reason, many lakeshore communities put their intakes just downflow of rivers, wastewater treatment plants, etc...which is what led to the Milwaukee crypto outbreak in 92. I still remember the tshirts emblazoned "Milwaukee: you cannot drink the water, but you CAN eat the people!" (Jeffrey Dahmer had just been captured).

Another potential problem with filtration systems is biofilms. You're basically exchanging one form of risk for another: removing chemicals, while giving bacteria a nice surface to grow upon.

In the burbs here in Chicago, its damn good. I have a hard time drinking the water in a lot of places by comparison. The ONLY place I've been which is superior to LM water was in colorado near the springs. Thats from the mountain reservoirs and frankly awesome.

[xcorps]

Another potential problem with filtration systems is biofilms. You're basically exchanging one form of risk for another: removing chemicals, while giving bacteria a nice surface to grow upon.

UV sterilization takes care of that.

[Pra]

This is really a chemical engineering problem.

Separations is always one of the biggest and most energy consuming operations in any type of engineering operation. This is because thermodynamics favors the salty water state. It has greater entropy, and AFAIR the heat of mixing is zero because salt is an ionic compound.

Essentially all solutions boil down to filters or evaporation. Doing an [i]extremely simple form of matrix scaling[i] the energy load on both processes is high, and the cost equally so. You can harness solar power for the latter operation though, but it will require significant amount of mechanical engineering to ensure that the water vapor is collected properly. Also, you'll need a constant stream of cool dry air for maximum efficiency in order to 'take in' as much water vapor as possible.

If I was designing the evaporator, I'd have it as such

--->water vapor (sat water concentration
--->raw stream of salt water-->evaporator
--->brine


You could send the brine through another evaporator to recover more water, but the energy load becomes quite great, and the water starts behaving abnormally, and more non-newtonian once the salt concentration gets high. The water vapor can be collected and cooled to obtain drinkable water. It is possible to use sunlight as energy, but for the load desired, you'd need a lot of sunlight just to power the first evaporator. Our harnessing tools for sunlight, AFAIK, aren't that powerful, although I need to look at new models and haven't done so.



I really don't like the desalinator approach. I don't think filtration is the right way to go, but the operation is simple. A high pressure system forces water through a membrane, and the membrane is impermeable to salt. Technical issues make the operation more complex, but the rough system is essentially pressure-->pure water.

It'd be difficult also to use sunlight to power it, IMHO.

[MathiasOfAthens]

I actually like the taste of salt water.

[Simetrical]

Do you mean tap water with some table salt mixed in, or actual seawater? Seawater might taste fine, and it won't hurt you in moderate quantities, but it dehydrates you, so it's not suitable for drinking. It's also not useful for cleaning things, because it leaves salt behind when it evaporates. It works fine for cooling things, generating hydroelectric power, swimming, and various other things, but we still need lots of relatively pure water.

[xcorps]

You can desalinate water through a reverse osmosis watermaker fairly cheaply. One manufacturer makes a model that produces about 50,000 gallons a day off a 55 hp motor (460v).

[xcorps]

Figure that the average American uses 100 gallons a day, which is much higher than the world average. A single watermaker (of that particular model)can supply enough water for about 125 4 family homes every day for about the amount of electricity it takes to run 3 2000 sqft homes. Expensive? I guess it depends on need vs utility. And of course location, as you would need a constant supply of seawater.

The particular watermaker I'm referring to is a commericial watermaker designed for use by vessels at sea. I haven't researched what an RO watermaker designed for a landbased installation would look like.

[Turtules]

Zephyrhills for me in FL...soooooooooo good.