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Low-cost, high-speed filter purifies water using electrified nanostructures

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Stanford engineers have developed a water filter that uses a conductive "nano-coated" cotton to purify water more than 80,000 times faster than existing filters. The affordable, high-speed filter could be implemented to filter water in the developing world and fight waterborne diseases such as cholera, typhoid and hepatitis.

The device is constructed by dipping a plain cotton into a high-tech broth of silver nanowires and carbon nanotubes. The photo above shows a scanning electron microscope image of the silver nanowires. The large fibers are cotton.

Unlike traditional water-purification systems, which have tiny spaces to trap bacteria, the new filter allows the pathogens to flow through with the water. But, as they pass through, an electrical field running through the conductive fabric kills them. According to the Stanford release:

In lab tests, over 98 percent of Escherichia coli bacteria that were exposed to 20 volts of electricity in the filter for several seconds were killed. Multiple layers of fabric were used to make the filter 2.5 inches thick ... Since the new filter doesn't trap bacteria, it can have much larger pores, allowing water to speed through at a more rapid rate.

Comparatively large pores help prevent the filters from getting clogged, which can be a problem with purifiers that actually strip bacteria from the water. Another advantage over conventional filters is that this device has a low electricity requirement: a stationary bicycle, small solar power panel or car batteries can power the new filter.

Because the primary material in filter is ordinary cotton fabric - and a minimal amount of silver is used for the nanowires - engineers were able to keep production costs relatively low.

Photo by Yi Cui

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