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September 2nd, 2010

Missouri S&T researchers question need for silver additive to ceramic water filter

Published on September 2nd, 2010 | By: pwray@ceramics.org

Pot-shaped filters, like those above, are used to remove bacteria from water in Guatemala. The filters are typically made out of native sawdust and clay, and imported colloidal silver. (Credit: Missouri S&T.)

Ceramic pot filters are crucial for accessing clean water in third-world countries and underdeveloped regions of emerging nations. For a variety of reasons, much of the research regarding the manufacturing and effectiveness of these filters has occurred in Guatemala.

The attractiveness of these filters in countries like Guatemala is that almost all of the raw materials are easily available from abundant indigenous sources, with one exception: colloidal silver (silver nitrate). Levels of E. coli and coloform bacteria are typically used as representative indicators to measure general water quality, and while the ceramic material filters out most of the E. coli and coloform in the water, silver is added to as an additional anti-bacterial agent to kill what is not caught in the filters. In a story that appeared in our Bulletin magazine last year, a researcher reported that untreated filters caught 97% of the bacteria, while filters painted with the colloidal silver stopped 99%.

Everyone agrees that the amount of colloidal silver required per pot is very small. But, even though the per-pot cost is minimal, this additive must be imported and moved to remote regions. Ceramic filter technology is also competing, in a sense, with other indigenous filtering approaches, such as biosand filter applications.

Heinley exams E. coli growing from water samples.

Now, new research suggests that the use of the silver additive may not be all that important and the benefits may not justify the effort to obtain the material. According to a press release, Missouri S&T graduate student Nicole Heinley traveled to Guatemala to conduct research on the ceramic pot filters used to remove bacteria from water.

Heinley collected contaminated water samples from a river in the city of Antigua and studied the structure of the ceramic pot filters available locally. She found that the filters not lined with silver removed a higher than expected rate of E. coli — an average of 99.499 % — high enough that she calls into question whether the benefits of silver justify the logistical and expense problems involved.

“[Colloidal silver] is the only material that has to be imported to manufacture the filters,” Heinley says. “The remaining materials – sawdust and clay – are available locally.”

A paper on these findings written by Heinley and Curt Elmore, associate professor of geological engineering at Missouri S&T, will soon be published in the Journal of Water Science and Technology.

“Perhaps the greatest drawback to using the silver is maintaining the imported supply,” notes Elmore.  He said that developing a household water treatment that does not rely on imported components would be appealing even in a developed area, let alone a developing one.

Moving from lab-based conclusions to real manufacturing often leads to unexpected problems and Heinley warns that, “additional, long-term studies of filters without silver should be undertaken in order to further investigate the issue.”

Below is a two-part video explaining the making and function of ceramic/colloidal filters:

 


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