Could desalination be used in the UK?

Desalination is the process of removing salt from seawater. Chris Sansom, is a Solar specialist at the University of Derby…

Chris Sansom - There are two principle ways in which we can use solar energy to desalinate sea water. The first is called CSP and the second is called RO. CSP stands for concentrating solar power and that's where we use the heat of the sun in hot and sunny parts of the world to basically evaporate sea water, which we can then distill again to produce pure H2O. So, in CSP, in concentrating solar power, you need a very hot, sunny environment. So typically around the tropics of the world, in the deserts of the world, where we use the heat of the sun. Not electrical power now, but the heat of the sun focused using very large mirrors to evaporate sea water. What we're doing there of course is we're just speeding up nature because seawater evaporates naturally in nature and falls again, as rain on the land and in rivers. So we are speeding that process up using CSP. The other way in which we can use solar energy, the power of the sun to desalinate sea water, is by using solar PV for example to produce electrical energy to power what we call the RO process, the reverse osmosis process. Now that's a very different process for evaporating seawater because that forces seawater through very fine membranes, a bit like forcing seawater through filter paper, that we all remember from school. You have to force it through the membrane. It's a very high pressure process, and requires a lot of electrical power to run the pumps that do that. But that is a perfectly feasible way of producing desalinated water using solar energy.

Chris Smith - But what's the energy footprint of doing this and how much water can you make this way?

Chris Sansom - If we consider the CSP first, then it's just one of those horrible facts of life that it requires a lot of energy to evaporate water. If you imagine boiling the kettle, then you can boil a kettle which may be one or two kilowatts. Boiling a kettle may be four minutes, five minutes or something like that. But you're actually talking here about boiling that kettle dry and that does take a lot of energy. That would take you about 20 minutes and would take you about one to two kilowatt hours in energy. So that is a lot of energy. In the RO process, It's a slightly different problem, but you will then be using a large amount of electrical energy in order to power those huge pumps to produce enough water to make it feasible. In the RO process, I think you could produce, for your kilowatt hour If I do a similar example to the CSP, for your kilowatt hour you will probably produce 200 liters of water rather than two liters of water. So maybe a hundred times more in the RO process. So it's feasible to use the RO process to desalinate water, but it's still a lot of energy to produce drinking water.

Chris Smith - And is the water arising from both processes equivalent and adequate as a drinking water source or is one better than the other?

Chris Sansom - They're certainly equivalent in the sense that they are both very close to pure H2O. As to drinking pure H2O, there are a number of opinions on this, as to whether this is a good idea or not. My own personal view is that you can drink small amounts of pure H2O but not n great quantities. Our bodies are not really adapted for pure H2O, we prefer minerals in our water. We've evolved that way and our cells require that.

Chris Smith - A dash of mineral water goes a long way, doesn't it? I suppose what we should talk about is what we do with the waste, because there are always, as we keep saying on this program, trade offs, and there are consequences. And what comes back from a desalination process is gonna be some very, very salty water you've got to get rid of, or a whole heap of salt that you've got to get rid of presumably

Chris Sansom - Yes, that is the biggest drawback and there is no simple solution to that. There are things that you can do with salt. You can use salt, it could be table salt. You can use salt as a preservative. You can use it in various industrial chemical processes, part of the chemical reactions. It also makes a very good thermal store. Molten salts make a very good storage medium for high temperature storage, thermal storage. But I have to say that in the large desalination plants that you find in the middle east, for example, you will notice around the plants, a lot of salt is just put on the land. What we can't do now is put it back into the oceans. It can't go back into the sea. Everybody agrees that but you will see salt just lying on the ground and just sitting there and buried in some cases.

Chris Smith - Yeah, I think there were some catastrophic stories of this extremely hypersaline solution going back into the ocean and it was poisoning the sea floor, because it was basically sticking to the seafloor because it was so heavy.

Chris Sansom - You're absolutely right and that was done in the early days of desalination and think as desalination has expanded in terms of volumes, we've realized that we cannot do that anymore. Yes, it is an environmental disaster, which is what we're trying to avoid.