So What Exactly Goes On At A Desal Plant?

It’s just physics class 101, actually
BY: GUNGA DIN

Desalination plants are expensive and fairly complicated to build, even though the process of taking out the salt is pretty simple.

First you have to have a supply of salt water flowing into your desal plant, and that is the first hurdle to overcome. There are tough political and physical considerations.

The political problems include NIMBYs, the agitators who are always quick to say, “not in my neighborhood.” And, obviously, the salt water intake has to be in somebody’s neighborhood. Of course the large plant itself will then be fairly close to that.

Then there’s the fish huggers who have a legitimate point that the intake is going to be near the pipe that disgorges brine, a by-product of desalinated water containing high concentrations of salt and, when released back into a natural body of water, can cause damage to marine life. That’s because brine, which is usually denser than the water into which it’s released, settles atop low lying sediment at the bottomm where it depletes surrounding waters of oxygen

The physics part of this activity demand that you don’t just stick a pipe in the ocean. The waves and currents will batter it to pieces, and the water drawn out will have the maximum salt content there is. If you’re going to buy and use energy to take the salt out, why not start with some of the salt already out? If they can find water that is only 50% salty, then half their work is already done. So they drill down into the water table near the ocean, hoping to find brackish water. Brackish means water that has been contaminated with salt.

So now all those hurdles have somehow been overcome, usually through the expenditure of lots of money on buying land, sinking wells and fighting court battles.

The water is flowing into your desal plant that looks kind of like a giant still. Pipes and tanks abound. The brackish water is pushed through membranes, powered by a fossil fuel: electricity. That’s expensive and damaging to the environment. Another cost is what’s called the consumables: the membranes that must stand up to water being pushed into it at one half to one and a half horsepower. These need to be replaced often. Water flows through the little bitty holes in the membranes that are smaller than the width of a single hair on your head. That membrane says no-can-pass to the salt, which piles up and can not get into the fresh water tank. That water is now the brine that has to be gotten rid of without damaging sea life.

And no, you can’t use the brine for irrigation, that’s the current problem in San Quintin, the large agricultural area south of here. If they water the crops with brackish water, most crops will be harvested OK, but then after the plants suck up what they want, and evaporation takes care of the rest of the water, you’re left with more brine! After each crop rotation, more brine piles up until the earth is unusable. Any life at all can only take so much brine. It’s salt, and in concentration, salt kills.

So that’s how desal plants work. Get used to coexisting with them because only a sliver of the world’s water is fresh to begin with, and after subtracting gluttons like glaciers and ice caps, all that’s left is a drop in the proverbial bucket.

There are currently about 1440 large commercial desalinization plants worldwide with another 240 under construction. Desal is here to stay.