Infrastructure Megaprojects: Water

Water stress is the resource challenge of this decade, and probably a few more to come. Much of the US has been in drought for the last five years, driving up food prices. Some estimates are that this drought has cost the US economy $150 billion dollars each year! Water tables are being drained faster than they can refill, and polluted beyond use. Some bodies of water are being diverted so much that they've become poisonous, or ceased to exist. We need new sources of water, and we need them now.

Unfortunately you can't just create water unless you have a lot of hydrogen lying around. But we're not lacking for water; we're lacking for potable water. We have all the water we need, if we can just clean it up a bit.

We need to build desalination plants to make the seawater drinkable. There are already several in the United States, and quite a few more around the world. This is not a new thing, it's just a question of scale.

How much water are we talking about? Looking at a couple sources, we can estimate that the US uses around 400 billion gallons per day, most of which goes to run power plants and irrigate crops. An average desalination plant (based on Australian installations) could do 60 million gallons per day, consume about 24 MW, and cost $1.8 billion. So to replace every source of fresh water in the US, we're talking about 6,700 desal plants, consuming 160 GW and costing $12 trillion.

Now, that's just an upper limit. There's no need to desalinate every last drop of water we use. Let's scale back a bit, and target 10%, which should be more than enough to relieve the water stress we're seeing. Now we're talking about 670 plants, 16 GW, and $1.2 trillion. That's eminently doable. And the system scales wonderfully. You can build it gradually over time. If you need more water later, you can build more plants.

Figure 3.5% salinity as a general average, so we're talking about having to find a home for 6 million tons of salt every year. That's enough to cause an environmental catastrophe if it's all in one place, so we need to plan for that. Luckily, the US presently produces 7-8 times that much salt in a year, so our economy could obviously absorb it.

Other resources are present in seawater. We'd be extracting 220,000 tons of potassium a year, about 1/5 our present production of potash. This has great possibility for fertilizers, though I can't speak as to the chemistry involved. We'd also get a comparable amount of magnesium, making us the world's fifth largest producer.

The total value of all those extracted solids comes several hundred million dollars a year. Trivial by comparison to the cost of the construction, but still, a nice offset to operating expenditures.

Of course, it's not just the coasts we're worried about; we also need a way to move the desalinated water from the sea to the midlands. We're talking about a huge aqueduct network. We already have quite a bit of experience building such things, but the scale would be unheard-of. Figure a trillion dollars to build the aqueduct network alone.

Now, we could do closed pipes, but I'm not sure that's what we want to do. Perhaps instead we should have open aqueducts, and let the water evaporate as it will. It will condense back out somewhere as rain, giving us the most efficient possible distribution method. Some combination of covered and uncovered aqueducts would probably be best.

Ultimately, the oceans will provide us with the only sustainable source of fresh water on the planet. We'll eventually have to start tapping it, and we're getting to that point.