As their name suggests, most existing water purifying filters clean the water by physically trapping or filtering out bacteria.
Stanford researchers have now developed a new kind of water purifying filter that isn’t really a filter at all. Instead of trapping bacteria, the new filter actually lets them pass right through. But, by the time they emerge from the filter they have been killed by an electrical field running through it. Not only is the new filter more than 80,000 times faster than existing filters, it is also low-cost, has no moving parts and uses very little power, which should make it particularly attractive for use in the developing world where it is needed most.
The key to the new filter is coating the filter fabric – ordinary cotton – with nanotubes and silver nanowires. When an electric field is passed through the highly conductive “nano-coated” cotton, it kills almost all the bacteria passing through it. 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.
“This really provides a new water treatment method to kill pathogens,” said Yi Cui, an associate professor of materials science and engineering at Stanford whose research team is also responsible for using nanomaterials to build batteries from paper. “It can easily be used in remote areas where people don’t have access to chemical treatments such as chlorine.”
Speeding things up
Filters that physically trap bacteria must have pore spaces small enough to keep the pathogens from slipping through, but that restricts the filters’ flow rate. Since the new filter doesn’t trap bacteria, it can have much larger pores, allowing water to speed through at a faster rate – about 80,000 times faster. The larger pore spaces in Cui’s filter also keep it from getting clogged, which is a problem with filters that physically pull bacteria out of the water.