10 questions about membranes and water

Water is important - people need it for drinking, agriculture and industry. What applications do you have in mind when it comes to membranes?

How do membranes work technically?

At Hereon, we are researching membranes made of polymers. You can think of them as a kind of cling film. These polymer membranes have tiny pores through which water can be pressed. Particles that are larger than the water molecules stick to the membrane. To do this, the water has to be pressed through the membrane using pumps, which of course costs energy. Polymer membranes are also used in seawater desalination, among other things.

The principle is still familiar from chemistry lessons...

Exactly, some people may still be familiar with osmosis and the semi-permeable membrane: In this process, two liquids with different salt concentrations are separated by a semi-permeable membrane that is permeable to water. The water tends to flow through the membrane into the liquid with the higher concentration in order to compensate for the difference in concentration. With membranes for water purification, it is exactly the opposite. The water is pumped through the membrane, leaving the salt or impurity behind. Experts refer to this as reverse osmosis because the water flows in the opposite direction to osmosis.

What other types of membranes are there?

Membranes are also made of metal or ceramic. Ceramic membranes are also used in water treatment. However, they have the disadvantage that they are brittle and difficult to process. One advantage, on the other hand, is that they can withstand higher temperatures.

What exactly are you researching and how are you improving membranes?

We are working on specifically modifying the polymers for our membranes in order to increase purification performance or reduce energy consumption. Take, for example, our membranes that remove heavy metals from water. We have coated the surface of the polymer membrane with molecules that specifically fish certain toxic heavy metals, such as lead and arsenic compounds, out of the water. Desirable metals such as magnesium can pass through the membrane unhindered. By specifically fishing out heavy metals, we can make the pores larger.

And that is an advantage?

The pores no longer have to be so small that heavy metals remain trapped because the membrane actively fishes them out, so to speak. Larger pores mean that the water flows more easily through the membrane. The pressure of the pumps can be reduced and this lowers energy consumption. We are also working on coatings that prevent residues from being deposited on the membranes.

How important are desalination plants today - in Germany and worldwide?

In Germany, seawater desalination is not a major issue because we are well supplied with surface and groundwater. The only major seawater desalination plant is operated on the island of Heligoland because a drinking water pipeline to the mainland would be too expensive. Desalination plants play a particularly important role in the Middle East and in California - especially in agriculture. In California, for example, a lot of water is needed for the production of almonds. The water consumption is enormous. More water is needed per kilo of almonds than for a kilo of melons. The growing world population and climate change will increase the importance of seawater desalination.

How does desalination work?

Using reverse osmosis - i.e. with the help of large pumps that press the water through the membranes. The largest seawater desalination plant in the world is located in Israel. It produces enough water per day to fill three million bathtubs. The membranes are wound into so-called spiral wound modules. If they were spread out, they would cover an area the size of 90 soccer pitches. That makes the dimensions clear. Although this system consumes a lot of electricity, it works quite efficiently: 1000 liters of water can be desalinated with three kilowatt hours of electricity - that is as much electricity as 50 60-watt light bulbs consume in one hour. This is a good figure for a reverse osmosis system.

Is there a waste product?

The problem is that the desalination process produces highly concentrated brine that has to be disposed of. In some regions, it is dumped in the sea. The effects of this on the marine environment have not yet been conclusively researched. However, there are now efforts to use the brine industrially and extract valuable lithium from it, which is needed for batteries.

Water purification: Why is it necessary to filter water with membranes at all?

Because membranes filter the water particularly well and remove many residues. Membrane processes are energetically advantageous, for example in contrast to distillation. Incidentally, membranes are not only used for water purification, but also for other liquids. Milk, for example, is treated using so-called nano- or ultrafiltration. Research into membranes will become increasingly important.

The Hereon laboratories also have membranes that are used for CO2 separation. And on the new Hereon research vessel CORIOLIS, membranes are intended to reduce oxygen in the exhaust air of the diesel engine and thus significantly reduce nitrogen oxide emissions.

Dr Volkan Filiz, Head of the Department of Microporous Polymers

Filiz studied physical chemistry and made his doctorate on the topic of "Functionalization of polymersomes for the encapsulation of hydrophobic and hydrophilic nanoparticles and active ingredients". He has been working at the Helmholtz-Zentrum Hereon since 2009.