Anybody who’s ever filtered water knows that the filters get dirty. Supposedly clean water comes out one side, but the other side gets coated by whatever was in the water – dirt, bacteria, algae. Sooner rather than later the filter needs to be cleaned or changed. Commercial or public filtering of water isn’t much different, just bigger. Whether to take out impurities or to desalinate (remove sea water salt), large filtration systems are expensive to maintain. The filters – actually a membrane – in large installations get very dirty and therefore very inefficient. They also ‘crust over’ and the membrane must be exchanged.
To address this problem, researchers at the University of California Los Angeles (UCLA, USA) decided the best approach would be to make a filter that resisted getting clogged and dirty. They constructed a special membrane for the process of reverse osmosis.
In reverse osmosis (RO), water under pressure is forced through a membrane and the impurities are removed. Water molecules pass through the pores in the membrane but other molecules or larger material can’t go through. The problem, especially under pressure, is the build-up of material that clogs the pores.
The UCLA innovation was to develop a new membrane surface, one that in a sense has ‘hairs’ or ‘brushes’ that both repel impurities but also help to sweep it away from the membrane. The process of making the membrane is at least as important. Normally special membranes would be made in high temperatures, in a vacuum chamber, or have other (expensive) conditions. The new approach uses a three step process to create the membrane:
1. A conventional polyamide thin-film membrane is synthesized as a base.
2. The surface of the membrane is subjected to atmospheric pressure plasma (a gas with an electric charge). Because it’s applied at atmospheric pressure, there is no special chamber for the production. The charged surface, applied at specific points, is then ready to bond with material of an opposite charge.
3. At each activated site a ‘brush’ of polymer (plastic) material is applied, so that the surface might appear like it is coated with hairs.
The brush surface is in constant motion under the reverse osmosis process, and the moving ‘hairs’ make it very difficult for impurities to stick to the surface. In a clever addition, the research team led by Yoram Cohen, UCLA professor of chemical and biomolecular engineering, use material in the brushes that has a natural ability to create a charge in the surface of the membrane. This charge helps to further repel impurities from sticking.
Cohen’s team, in collaboration with the UCLA Water Technology Research (WaTeR) Center, is currently carrying out specific studies to test the performance of the new membrane’s fouling properties under field conditions.
“We work directly with industry and water agencies on everything that we’re doing here in water technology,” Cohen said. “The reason for this is simple: If we are to accelerate the transfer of knowledge technology from the university to the real world, where those solutions are needed, we have to make sure we address the real issues. This also provides our students with a tremendous opportunity to work with industry, government and local agencies.”
The new membrane appears to be easily modified to handle a wide variety of water conditions – also a big advantage. If the membrane technology proves effective in the field, the next step will be to figure out large scale production techniques. In theory, this new filtering process should be much less expensive and more effective than previous materials.