Ceramic filter makes water treatment easy
Silver-impregnated ceramic filters prove to be effective for point-of-use water treatment.
The technology sounds simple: fire a ceramic pot, perhaps coat it with a fine layer of silver, and let the water percolate through. In regions where water carries millions of microbes, this relatively inexpensive treatment method has its attractions. For about a decade, the nonprofit organization Potters for Peace has been teaching communities to manufacture their ceramic water filters, which retail for $5–15. Although they have been used widely from Nicaragua to Thailand, no scientific data have been published to prove their efficacy or to show how these ceramic filters work.
Now, in ES&T (DOI: 10.1021/es071268u), researchers from the University of Virginia report that clay water filters from Mexico and the U.S. can remove more than 98% of the test organism E. coli. With an added layer of silver, the filters remove all E. coli. "Without the silver—just the ceramic filter—it seems like it works well, but adding the silver further improves the performance," says coauthor James Smith.
"Until this report, there have been very little data on how [the filters] work and what the specific mechanisms are," says Kara Nelson, an environmental engineer who studies a variety of low-cost point-of-use water treatments at the University of California Berkeley. "Without an understanding of the fundamental mechanisms behind [a technology], we have to take a black-box approach," she says. "That's not efficient. There are so many different pathogens, so many environmental conditions, so many different things that could be in water."
Lead author Vinka Oyanedel-Craver and Smith examined the ceramic filter materials in the lab and found that commercial pottery material from the U.S. was the most successful at removing E. coli. The authors hypothesize that the industrial material has many more and much smaller pores than the coarser clays from Mexico. In addition, the pathways between the pores where water flows are more interconnected, allowing water to speed through more quickly, whereas the smaller pore sizes block E. coli from getting through. Added silver—purchased by local pottery manufacturers as a relatively inexpensive slurry, facilitated by Potters for Peace—could actually be killing the remaining E. coli through direct toxic contact, the authors posit.
The results suggest that more sieving at on-the-ground manufacturing sites could lead to finer-grained ceramic filters and more effective water treatment. Although the filters' $5–15 cost may be insurmountable for some households, it is a worthwhile investment for several years' worth of filtration, the authors argue. The next step is to test how these filters "behave under real-world, long-term use," Smith says.
A major (PDF size: 204 KB) push on the household treatment front came last year from the Bill and Melinda Gates Foundation, which stepped in with $17 million and resources to further study simple point-of-use water treatments. Their end goal: determine which devices to mass produce and how best to promote them.
But researchers and policy makers alike emphasize that household water treatments may be appropriate only under certain circumstances. "There's no single answer to the water supply and sanitation problem," says Peter Gleick, director of the nonprofit Pacific Institute. "Point-of-use systems can be effective, and can be a critical component, but they must not be allowed to derail the larger efforts to get cheap water for everyone."
