Nanotech Filter Delivers Drinking Water

Nov. 20, 2006 — Turn on the tap, and out comes clean, fresh water for drinking, cooking and bathing. But not everyone is so lucky.

Fresh water is an increasingly limited resource, so finding a way to purify polluted and saline waters is gaining interest.

A new technology from the University of California, Los Angeles could offer an efficient and comparatively inexpensive means for accomplishing just that.

The technique augments a 50-year-old method known as reverse osmosis but it uses a membrane made with nanotechnology. In early laboratory tests, the method has performed as effectively as current technology for desalinating water, but it's more energy efficient and potentially much less expensive.

If the new membrane indeed proves effective and affordable, it could help address a problem that is only expected to grow.

According to a United Nations' World Water Development Report, more than 50 percent of the world will face water stress or shortage by 2025. In California alone, demand for water could increase by 40 percent in that same time frame.

In conventional reverse osmosis methods, salty or polluted water is forced under high pressure through a semi-permeable membrane. Water molecules pass through the membrane, but salt and other particles, including bacteria and minerals, do not. The result is clean water.

The problem is that, over time, the impurities tend to build up on the membrane, forming a film that decreases the filter's efficiency. This means that pressure on the water must be cranked up, which demands more energy, or the membrane has to be cleaned with harsh chemicals that eventually ruin it.

The UCLA membrane, developed by civil and environmental engineer Eric Hoek and his team, is designed to quickly shuttle water molecules while at the same time repel all other particles.

It's a composite material made from a thin, polymer film that is imbedded with nano-sized, water-loving particles. The particles have tiny tunnels, so "not only does water love to be in contact with the material," said Hoek, "it shoots right through the tunnels very efficiently."

The researchers also gave the nanoparticles a negative charge. That's because impurities in water, including salt, organic matter and bacteria, also have negative charges.

Try to put two negatively charged objects together and they will repel each other just like the same two polar ends of magnets.

Laboratory tests showed that the new membranes demanded 50 percent less energy than conventional membranes. Such energy efficiency could reduce the total expense of desalinated water by as much as 25 percent.

In addition, the new membrane can be manufactured using existing techniques, which could ease its transition into the commercial market.

"It seems to be a practical modification of the existing membrane that offers significant advantages, including more flow, better quality and less maintenance," said Tom Pankratz, a desalination consultant and editor of the Water Desalination Report, published by Global Water Intelligence in Oxford, England.

UCLA's recently licensed the technology to the new startup, NanoH2O, which will develop the patent-pending nanocomposite membrane. Hoek expects that by this time next year, the company will be undergoing field trials of the membrane and that it could be on the market in two years.