Every few years, you hear about some new revolution in the process, but nothing much after. Why is that? Is it very hard to scale up or…?
Also, is it going to become more viable as fresh water is becoming more scarce?
Thank you in advance.
Thank you all. So it was basically what I thought. It’s not hard to do, but it is not really feasible due to many factors.
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Essentially, desal turns energy into water. You’re right that mass media has this tendency to write stories about breakthroughs, but they’re either tiny incremental improvements, or baloney.
In places that have almost no other water, like Israel, it makes sense. But if you have sources of water that cost half as much, it’s often cheaper to save a gallon of cheap water than it is to generate a gallon of desal water. So in places like Santa Barbara or northern San Diego County, it’s more like a last resort, the last water source they draws upon.
Desal is, at its heart, forcing water through a very fine filter, a filter so fine that it lets little more than water molecules though. The sort of pressure you need to force water through the filter is like pumping the water to a tank on a 1500’ tower. Which is doable, but at the scale of a city of 100,000 people it would be crazy expensive. At least compared to other cheaper sources of water.
There are consumable costs to an RO desal plant too, such as the filters themselves. They have to be replaced after a while and they’re not cheap. It’s great if those costs come down but they’re marginal compared just to the energy required to filter every single gallon of water. And that cost is really set by the global energy market.
Distillation and filtering are both energy expensive. So is electrolysis. If it weren’t we could produce hydrogen gas cheaply, run hydrogen fuel cells to generate electricity, and generate water as a waste product. There’s cheaper sources of hydrogen though, and a fuel cell bus emits just a tiny trickle of water from its exhaust. So if you try to use electrolysis to generate hydrogen to run a hydrogen fuel cell to power your electrolysis, it’s not going to give you free energy. It takes as much energy to break those bonds as is released when you form them.
Right now, in most parts of the world, there isn’t an actual shortage of fresh water. The problem is managing and distributing the water that *is* available there, and managing the usage. So it’s generally cheaper to buy & transport water, or reduce usage, than to build and operate a desalination plant. There are some exceptions, like some newly built cities in the Middle East.
It’s not hard or difficult. It’s just that clean water is really really cheap under usual circumstances. So using electricity to make clean water usually ends up costing more than the water is worth.
It is not hard, it is just very expensive because of how energy intensive it is. When it comes to anything, but especially water, cost is king, in regards to mass adiption.
If the water cost is double to irrigate crops, that is a big problem.
Desalination is primarily done through high pressure filtration, no by phase change. It is more energy efficient, but still extremely demanding.
Also, the super salty brine needs to be disposed of, and can’t just be dumped into one spot in the ocean or it creates a dead spot. You gotta pump it out into the open water in a current…
Basically, it’s expensive. So you don’t do it unless you absolutely must.
I operate in the climate space but am not an engineer or expert in water resources. So remember that as I give my understanding. It is *very* expensive. It uses a lot of energy. And it changes the salivation of ocean water around it which probably is not a good thing. The salt must go somewhere. And it can mess with groundwater. It is a solution of last resort.
The primary and least sexy way to begin the fix is conservation in a serious way. No more golf courses. No more home lawns bigger than x. Make wicked sure agriculture is using water in a sensible way. This is very hard but crucial – tackle the water rights monster and make it realistic and sensible. And a culture change. If your clothes aren’t dirty you wear them multiple days before washing. No washing hair everyday. Maybe a shower every few days with quick ‘army’ cleanups everyday. I know this sounds weird or far fetched but it is these type of mind shift that will help us manage in days ahead.
It takes a lot of pressure and therefore energy to pump salt water through salt-removal membranes.
I wouldn’t say it is hard more than it’s energy intensive.
Even using the Sun’s heat–a solar still– which is free, takes quite a bit of energy: I guess it depends on scale as well.
To provide potable water for a medium sized city needs a lot of energy. Then, what to do with the salty sludge leftover.
Do you return this to the ocean? It will make the dump site toxic. Do you store all of it somewhere on land? Again, it’ll make that area toxic to plants and you don’t want rain bringing that salt to the water table, contaminating it.
It’s not hard, in the sense that it’s not difficult to do in principle. It’s just energy-intensive and therefore expensive, and it requires facilities that are themselves expensive.
It isn’t so much that it’s hard to do, but it’s hard to do cheaply enough to make it economically feasible. Most methods take a lot of energy, others are relatively slow.
In every case though, desalination on a large scale leaves you with all the salt and minerals that you remove from the water. It’s difficult to find somewhere inexpensive to dump those without it screwing up the local ecosystem.
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