Cholera, hepatitis, and typhoid are a few examples of diseases found in water in the developing world. Now, Stanford University researchers have discovered a new low-cost, high-speed filter that could be used to purify water in these areas. 

Yi Cui and Sarah Heilshorn, associate professors of materials science and engineering, partnered their teams together to make this discovery. Instead of trapping bacteria like traditional filters, these researchers made a filter that allows the bacteria to pass through with the water, but an electrical field kills the bacteria it passes. 

To make this filter, researchers dipped cotton cloth into a high-tech broth filled with carbon nanotubes and silver nanowires. Silver was used because it has chemical properties that kill bacteria, and has been used as a remedy in the past. Then the electrical field is run through the 2.5-inch thick nano-coated cotton, and as water passes through, bacteria is killed. With the electrical field sending 20 volts of electricity throughout the filter, their research shows that 98 percent of Escherichia coli was killed within seconds. 

"This provides a new water treatment method to kill pathogens," said Cui. "It can easily be used in remote areas where people don't have access to chemical treatments such as chlorine."

This type of filter also allows water to pass through more quickly, since it does not require small pore spaces to trap bacteria like traditional filters. This rids the need for pumps. According to Cui, their filter is 80,000 times faster than traditional trap filters. Also, the new filter avoids biofouling, which is when bacteria forms a film on the filter as it passes through. Biofouling is a problem in traditional traps, with the pores being so small, but the silver in the new filter kills any lingering bacteria. 

What makes the filter ideal for the developing world is that it requires a low amount of electricity and is also low in cost. The electrical current that aids in killing the bacteria is only a few milliamperes strong, and can be provided by a couple 12-volt car batteries or even a small solar panel. 

Less voltage is required because the nanomaterials are so small in size that they stick to the cotton easily, making a continuous surface on the cotton fibers. This helps electrons move more efficiently, making the filter "very conducting," thus less voltage is needed. 

As far as cost goes, Cui said he wanted to make the filter as inexpensive as possible, so that's why ordinary cotton was used, and the amount of silver used was so small that the cost was "negligible." In fact, Cui mentioned that he bought the cotton right from Walmart. 

With low cost and low electricity already achieved, the only thing left to do is try the filter out on all different types of bacteria and run tests on "successive" filters. 

"With one filter, we can kill 98 percent of the bacteria," said Cui. "For drinking water, you don't want any live bacteria in the water, so we will have to use multiple filter stages."

The study will be published in an upcoming issue of Nano Letters.