There are more than a billion people who lack clean and clear water, resulting in disease, most commonly, diarrhea which kills millions of children a year and stunts the growth of many more. Yi Cui of Stanford University has come up with a filter than could save the lives of many.

Traditionally, water filters function by forcing water through its pores to remove bacteria using power. For them to be effective, the filter should be changed regularly as the pores get blocked by bugs. Dr Cui’s filter, does not screen the bacteria out, it kills them.

He and his team have designed a filter mesh with carbon cylinders called nanotubes while placing silver wires across the top of a cotton cloth. The silver kills bacteria and he thought forcing the bugs close to the metal might lead to their destruction. He also thought that running an electric current might breakdown the membranes surrounding bacterial cells. Carbon as a conductor is cheaper than silver, however, the nanotubes provided extra electrical conductivity that was needed to help the process.

In making the water filters, the team dipped woven cotton strips dyed in ink containing nano tubes. They suspended methanol in the silver wires using pipettes on to the surface of the strips.

When the water filters dried Dr Cui connected them to a battery. He ran water containing E. coli, a common bacteria found in water, through them. Testing a few drops of the filtered water on an incubation plate leaving it to grow showed that the filter, operating at -20 volts, killed 89% of bacteria. At +20 volts, 77% of bacteria were killed. Running the water using 3 such filters killed 98% of bacteria.

Though silver is expensive the amount used was miniscule, so was the quantity of electricity needed to charge the filter, such as that supplied by a small solar pane. The water filters would last without letting up for a long time.

The next stage is finding out whether the filter can kill the thousands of dangerous bacteria found in polluted water. If this is so, then this water filter could make clean water accessible to millions.

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