Sandra Kentish

Climate change threatens long-term water security in many parts of the world – including Australia where drought remains a significant threat. Producing potable water from seawater by desalination is high on the list of large scale and long-term solutions. Associate Professor Sandra Kentish, of the University of Melbourne’s Department of Chemical and Bio-Molecular Engineering, leads a team working to improve a desalination technology known as reverse osmosis which removes the salt from seawater by forcing it through semi-permeable plastic membranes. Dr Shane Huntington, of the University’s School of Physics, interviewed Associate Professor Kentish on her research recently for the fortnightly audio podcast Melbourne University Up Close. This is an edited version of the interview. The full interview and the transcript are available here.

SH: Tell us about the desalination process.

SK: Well, the most obvious way is heating salt water and collecting the vapour. That is, if you like, nature’s way of doing it – evaporating water from the oceans, which comes down as rain. It’s quite pure water, certainly good enough to drink, but not as pure as you can get with reverse osmosis which uses pressure to remove the salt from the water.

Commercial reverse osmosis installations are taking over desalination around the world primarily because they use less energy than thermal methods – however, reverse osmosis still consumes a significant amount of energy.

SH: Why specifically is it called ‘reverse osmosis’?

SK: It’s a fundamental law of nature that a salt water solution doesn’t like to become more salty. In a natural process called osmosis fresh water will cross a permeable barrier to dilute salt water. In reverse osmosis, we do the opposite – make salty water more salty by pulling water out of it. You’ve got to overcome some quite fundamental forces to do that.

SH: And that uses a lot of energy. Is this a key area of research?

SK: Yes, there’s a lot of work aimed at improving the actual membranes so that they permeate water more readily but still retain salt. Part of that is related to the fact that the membranes eventually get covered in slime. If you can create a membrane that doesn’t get fouled, then you can reduce the energy demand because it passes the water more easily.

Chlorine would stop the slime growth but today’s membranes aren’t very tolerant of chlorine-based chemicals, so another area of research is to develop chlorine-tolerant membranes.

A third area looks at removal of particular impurities – there’s concern for instance about arsenic and boron levels in water. The World Health Organisation is tightening the allowable levels of various impurities in drinking water.

SH: Desalination sounds like a very good technology – so why the controversy?

SK: Firstly, it does require a lot of energy – up to 10 times the energy needed to recycle wastewater, such as from sewage.

We are also left with a very strong salt solution – considerably above the salt content of the original sea water. There’s dispute over what you do with that concentrated brine. Returning it to the ocean could have impacts on marine flora and fauna.

One option is to not return it. You evaporate it to dryness in a salt pan and store pure salt as a by-product.

SH: I have this image of the membranes as something akin to a fish-tank filter where the water is pumped through and the salt is left behind.

SK: Yes, essentially that’s the way they work. They look a bit like sheets of paper. The paper is full of holes too small to see with the naked eye. The holes don’t really offer much resistance to either water or salt flow, but coating the paper is a very thin layer of plastic – about .1 of a micron thick – that selectively allows the passage of water and stops the passage of salt. In commercial desalination plants you want thousands and thousands of square meters of this material installed in spiral modules in containers with plumbing to carry salty water in, pure water out, and salt water away.

SH: How do you maintain the membranes in a large facility?

SK: They’re cleaned monthly or twice monthly, but they do have an ultimate life. You’d hopefully get two or three years, and we can stretch that to five years. Then you install new modules – new rolls of membrane material.

SH: Are membranes being used for reclaiming potable water from both seawater and wastewater, or are they completely different worlds?

SK: Both. You can buy from a manufacturer today a membrane that is designed specifically for sewage or brackish water applications or for seawater. In the sewage recycling and water recycling areas you actually use a range of membranes. You start with ‘larger’ micro pores to get rid of larger impurities, then step down the size of holes to an ultrafiltration or a nanofiltration membrane.

The pore size gets selectively smaller as you move through those operations. Reverse osmosis is literally your final polishing stage. It uses the membrane with the smallest pore size and therefore the largest energy costs because you have to apply the most pressure to pass the water through.

SH: If I gave you a glass of water from reclaimed sewage and a glass of water from a desal plant, would you be able to tell them apart?

SK: I wouldn’t be able to tell which one was which if they had both passed through the same sort of reverse osmosis at the end. I could tell them apart from, say, rainwater, because they would be purer than rainwater.