Desalination: What is the future of water

[chrispenycate]

Perhaps we should be following the 'Oath of Fealty' scenario and towing in the icebergs breaking off the Antarctic shelf to corners of civilisation where they would be useful?

Lot of water in one of those lumps. Not a long term solution, and I can't think of a low energy propulsion method; sails would have to be so enormous.

[skeptical]

Just throwing some numbers at you.

A typical suburban dweller in the wealthy West uses about 200 litres per day per person for drinking, washing, laundry, flushing toilets, sprinkling lawns etc. Thus a city of one million people needs 200 million litres per day. And that is not taking into account industrial use.

The minimum food requirement for an adult person requires crops grown on 1000 square metres of land, assuming no animal protein in the diet. Crops need about 10 mm of water per day to grow vigorously. Some of this may come, of course from rain, but the rest must come from irrigation. That represents a total of 10 cubic metres per day water. Assuming half comes from rain, we must still allow for 5,000 litres per person per day of irrigation water.

For a million people that means 5 billion litres of water per day for irrigation, even assuming everyone is vegan!

[Ursa major]

Originally Posted by skeptical

Crops need about 10 mm of water per day to grow vigorously.

Is that right?

That's 3650mm per annum, 3.65m or 143.7 inches (i.e. just short of twelve feet.)


It seems rather a lot.

[Drachir]

Originally Posted by skeptical

Drachir's reference clearly shows the potential of reverse osmosis desalination. However, it is worth remembering that it supplies only 10% of the needs of an urban population of 4,500,000 people. That is : a plant costing $US 140 million supplies 450,000 city dwelling people with their water.

Since agricultural irrigation requires many times that amount per person, this is a clear indication of the fact that, while desalination is great for supplying safe drinking water, it is NOT the answer for total fresh water supply.

I agree that it is not the answer currently, bit it is a start. A decade ago the costs would have been much greater, so perhaps in the future the cost may be lowered further. Of course, we could always begin to use water more efficiently. I know that is a startling thought but too often the approach to water use is to look for other sources instead of using what we have more carefully.

[skeptical]

Ursa
You are correct.

However, remember my earlier comments about the wasteful nature of most irrigation. To maximise crop growth, normally a hell of a lot of water is used.

[Wiglaf]

Originally Posted by skeptical

Just throwing some numbers at you.

A typical suburban dweller in the wealthy West uses about 200 litres per day per person for drinking, washing, laundry, flushing toilets, sprinkling lawns etc. Thus a city of one million people needs 200 million litres per day. And that is not taking into account industrial use.

The minimum food requirement for an adult person requires crops grown on 1000 square metres of land, assuming no animal protein in the diet. Crops need about 10 mm of water per day to grow vigorously. Some of this may come, of course from rain, but the rest must come from irrigation. That represents a total of 10 cubic metres per day water. Assuming half comes from rain, we must still allow for 5,000 litres per person per day of irrigation water.

For a million people that means 5 billion litres of water per day for irrigation, even assuming everyone is vegan!

Annual withdrawls from water resources per capita in North America is 1,664 cubic meters per year. The second highest is Asia at 644 cu meters. The world average is 623 cu meters.
You are right about the world being more wasteful for crops(in another post); the US and China use between 1/3 and 1/2 the water for the same output as India and Brazil.
The industrialized countries use more water but poor countries have more wasteful agriculture.

[Urlik]

a better solution (excuse the pun) is to grow crop hydroponically.
this would also help keep nitrates from poluting rivers.

[zachariah]

Geothermal - blast a giant hole in the bottom of the sea floor and build a...er...thing...to collect the freshly boiled water. The heat will provide electricity too. I'll collect my second Nobel now thanks.