hepatitis, and typhoid are a few examples of diseases found in water
in the developing world. Now, Stanford
University researchers have discovered a new
filter that could be used to purify water in these areas.
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
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.
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."
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.
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
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.
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.
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.
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."
study will be published in an upcoming issue of Nano
quote: The reference dose, published by the United States Environmental Protection Agency in 1991, which recommends the estimated daily exposure which is unlikely to incur a appreciable risk of deleterious effects during a lifetime, is 5 µg/kg/d; meaning 5 microgram of silver per kilo of weight per person each day – about 1 liter of 10 ppm colloidal silver per month for a 66 kg person.