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.
Cui isn’t sure just how the filter works, but he knows the two materials are better together. Silver has long been known to have antimicrobial properties, and carbon nanotubes are highly conductive. One guess is that very strong local electrical fields are formed at the tip of the silver nanowires, piercing the cell walls. When the electricity is off, the silver prevents bacteria from fouling the surface, a common problem with filters.
There have been no definitive studies of the effects of water-borne carbon nanotubes and silver nanowires on people and lower organisms; experiments with airborne carbon nanotubes have shown that their effect on mice lungs is similar to the effect of asbestos. But early tests on thousands of gallons of water suggest that the nanomaterials are not leaching into the water. The researchers will perform further tests to determine whether the nanomaterials remain enmeshed in the filter or are dislodged into the water over time.
“I believe there is tremendous potential for technological breakthroughs such as this to dramatically improve the options for low-cost water treatment,” says Kara Nelson, professor of civil and environmental engineering at the University of California, Berkeley. Now it’s important to take this proof-of-concept device to the next step, says Nelson, by improving the filter’s efficacy and demonstrating that it can work with a broad range of water-borne pathogens, including viruses and protozoa.
Chad Vecitis, professor of environmental engineering at Harvard University, says the most impressive aspect of the filter is its speed. Many university researchers are addressing the clean-water problem, but other low-power solutions take too long or are too complex. For example, some systems use a light-activated catalyst to kill bacteria in clear containers of water that are set out in the sun. This takes several hours, and it’s not easy to tell when the sterilization is done.