A water filter under development at Stanford University removes bacteria from water quickly and without clogging–and could lead to a simple and inexpensive method of cleaning water for the developing world. The device, which uses a piece of cotton treated with nanomaterial inks, kills bacteria with electrical fields but uses just 20 percent of the power required by pressure-driven filters.
At least a billion people have access only to water contaminated by pathogens or pollution. “There is a huge need for an extremely robust, low-cost filter material that does not require a lot of power,” says Mark Shannon, who directs a center of advanced materials for water purification at the University of Illinois at Urbana-Champaign. “Most places that need this the most do not have electricity at all, or at most a couple of hours a day,” says Shannon, who is not involved with the research.
The filter developed by the Stanford researchers tries to improve on other “point-of-use” systems for removing bacteria outside of centralized water treatment facilities. There are two major chemical methods: adding chlorine to the water to kill the bacteria, or adding iron, which causes the bacteria to clump so it’s easily removed. Chemical methods are difficult because they require training and a continual supply of the chemicals.
Filtration, in contrast, is attractive because it’s simple. But most point-of-use filtration methods move bacteria from water by excluding the organisms by size. Such filters clog over time, and they work very slowly unless energy-intensive pumping pushes the water through. The Stanford filter, which is driven by gravity, has pores large enough to allow for a high flow rate–about 100,000 liters per hour. It uses electrical pulses to inactivate bacteria by poking holes in their cell walls. The research was led by Stanford materials science and engineering professors Yi Cui and Sarah Heilshorn.
To make the filter, researchers dip a piece of cotton batting in a water-based carbon-nanotube ink, let it dry, then dip it in an alcohol-based silver-nanowire ink and let it dry again. Cui and others have used similar dipping methods to make paper-nanotube battery electrodes and nanotube textiles. The long, narrow nanotubes and nanowires get enmeshed in the fibers.
So far, the researchers have been testing the filters by stuffing them into a glass funnel mounted over a beaker. The filter is connected to electrical wiring to provide a voltage as water is poured through the funnel. Cui says it could be powered with car batteries or solar panels.
Cui’s group has tested the filter against high concentrations of E. coli. In these preliminary tests, described online in the journal Nano Letters, the filter inactivated about 98 percent of the bacteria. Even a single bacterium can make you sick, so that’s not good enough for use in the field, but Cui hopes to improve the filters.