New ways to desalinate seawater

Fundamental to human life is a supply of fresh water. And in many countries, that's in short supply. One approach is the "desalination" of seawater. But this is a very energy hungry process. So are there better ways of doing it? Noreddine Ghaffour at KAUST in Saudi Arabia is working on some of them. Chris Smith dropped by to his office, admiring the view over the university campus...

Noreddine - So here we are in a very special place, which is KAUST, and you can see it's quite green but this is through irrigation. There is not rain.

Chris - If I just go outside to the walls of this campus, it's pretty much desert, isn't it?

Noreddine - Yes, it is but, this is the winter season. If you come to us in the summer then it is much, much, dryer than this one.

Chris - I mean, this whole campus has got fountains, it's go water features and, as you say, very, very green - lots of trees and plants. Where's the water for that coming from?

Noreddine - This water comes from a desalination plant. Desalination plant means we are making freshwater from seawater and here, as you can see, we are in front of red sea water.

Chris - The Red Sea. I mean it should be clear, it's not red seawater, it is the Red Sea we're looking at. It's very salty?

Noreddine - It's very salty.

Chris - How do you deal with the rhyme of the Ancient Mariner "water, water everywhere and not a drop to drink." How are you solving this?

Noreddine - Yes. That is the main issue here and, from that concept, water desalination technologies have been developed. So the reverse osmosis concept is using a membrane. It's a semi-permeable membrane but very fine, very small pores to the point where there is no pore, and we try to push seawater from one side of the membrane.  But this membrane is very smart and allows only fresh water to pass through its pores or the structure of the membrane.

Chris - So, the water molecules can slip through but the salt particles are kept on the other side - they can't get through?

Noreddine - That's the situation.  But, in order to do this, we need to overcome what we call the osmotic pressure. What is osmotic pressure? Each seawater has it's own osmotic pressure depending on the concentration of salt. For example, her in the Red Sea, the osmotic pressure is 30 bars. So we need pressure to pressurise seawater much above 30 bars in order to get this fresh water from the other side.

Chris - So, in other words, if you've got a very, very concentrated strong salt solution that's trying to pull water into it. So you've got to push the water really hard through the membrane and keep that push on, otherwise the water would just come back into the salt?

Noreddine - That's what makes this process very energy intensive. Just to give you an example, to produce 1 cubic meter of fresh water from the Red Sea, we need 4 kilowatt hours. It's something like 50 elephants pulling something.

Chris - Tell us something about the new things that you're trying to develop here to surmount some of these problems.

Noreddine - What we are developing now is to be as close as possible to nature. For example, our body is full of membranes and we call those forward osmosis membranes. So by nature, if you put that membrane and you put two liquids from both sides of the membrane. One liquid is fresh water, the other liquid is salty water. You do nothing - no pressure, no temperature, nothing. By nature, the liquid of the low concentration starts to move towards the salty place. This is by nature, we don't push anything.

Chris - That's just straightforward osmosis isn't it?

Noreddine - Yes. So what we have as a concept. In any place we need water, we have a lot of waste water. We are using that water then we throw it. So we put waste water in one side of the membrane and on the other side we put our sea water, So according to what I explained, what will happen is the dirty water, which is the wastewater, will start moving towards the seawater but this membrane only allows clean water to pass.  All the dirty bacteria and this suspended salt will not pass. And what do you think we get? We are diluting seawater because of that process. Diluting seawater makes our feed for reverse osmosis very energy efficient, why?

Chris - I get it. Because you're now getting proportionally much less salt.

Noreddine - And this will reduce the energy consumption significantly. So if you have diluted seawater, which is something equivalent to brackish water, let's say, then the energy required for reverse osmosis is much, much, less. Half or even maybe less.

Chris - Other techniques that you're developing?

Noreddine - Absorption desalination. It's thermal based, but it's a new process. We are using an absorbent like silica gel it's a sand. If you put everything under vacuum environment, the silica gel or the absorbent will absorb water vapour from the sea. Seawater flows into an evaporator at it's ambient temperature, ambient pressure, nothing to do. And that evaporator is connected to beds filled with absorbents, as an example, silica gel. By nature the silica gel with create a vacuum because gel by nature will absorb means we will suck water vapour from that seawater. And  then when the absorbents are saturated with vapour, we heat the silica gel using solar energy, because we need 50-60 degrees centrigrade and when the vapour is released from the silica gel, it goes into a condenser to condense it and with this we are producing very high quality water.

Chris - In a nutshell then. The water's pulled out of the ocean, goes into a chamber where it just naturally evaporates because connected to that chamber is a very dry bed of silica, which is sand effectively. And that's pulling vapour out of the air and pulling the pressure down so more water wants to evaporate but the only things that's going to evaporate is freshwater.  You're then using energy from the sun to bake that water saturated silica, drive off the water it's absorbed, releasing water that's fresh, clean, that you can condense and use, but then you recycle the silica back to start the process again. So, when I do the energy calculations for this, how much energy are you saving?

Nodreddine - With this we are saving more than 50% of the energy compared to reverse osmosis.