For most Highlights readers, clean, safe drinking water is as convenient as the turn of a faucet. But in most of the world, this is not the case. Even in some areas where water distribution systems exist, the water contains disease-causing bacteria and microorganisms. However, several researchers and organizations are working to develop simple and effective water filters that can be produced and maintained locally in communities worldwide.
Porous Clay Pots
One such device looks more like something you’d plant a flower in than use to produce clean drinking water. A simple, porous clay pot placed in a 19-L (5-gal) plastic bucket with a spigot has the potential to save millions of lives each year, according to Vinka Craver, an assistant professor of civil and environmental engineering at the University of Rhode Island (URI; Kingston).
Working in collaboration with the nonprofit group Potters for Peace (Nicaragua) and colleagues at the University of Virginia (Charlottesville), Craver is testing the effectiveness of porous clay pot filters and working to ensure that they are accepted in local communities, according to a URI press release.
Potters for Peace began to distribute the filters in 1998, but research groups only recently began studying them to make sure they work properly, Craver explained. “I was the first to present in a scientific publication a mechanistic study of their effectiveness at removing bacteria,” she said.
|Local high school students distribute filters. Students help Vinka Craver’s study by acting as interpreters. They also are learning how to use GPS and test water for Escherichia coli, Craver said. Photo courtesy of Craver. Click for larger image.|
|This porous clay pot filter was first developed in 1992 by Fernando Mazariegos of the Central American Institute for Industrial Research and Technology in Guatemala, Craver said. Mazariegos called the device “Ecofiltro.” Potters for Peace and other nongovernmental organizations, such as the Ixtatan Foundation (North Garden, Va.), the organization with which Craver collaborates, have distributed these filters widely. |
The filters, which can be manufactured using local materials and labor, are made with a mix of clay and sawdust and can be impregnated with colloidal silver. When the clay is fired, the sawdust burns out, leaving a network of fine pores that filter out bacteria. A solution of colloidal silver can be painted onto the pots to kill bacteria.
The pots are household devices that filter between 2 and 2.5 L/h (0.53 and 0.66 gal/h). Even though that’s not much water, it’s plenty for drinking and hand washing, Craver said.
The clay filters remove between 97% and 99% of bacteria, Craver explained. The variation is due to the different properties of regional clays.
|The filters are tested for appropriate flow rate before distribution. Photo courtesy of Daniel Restivo. Click for larger image.|
| ||“The worst scenario, using very bad clay without colloidal silver, you get 97% removal,” Craver said. While the filtered water still may not meet U.S. drinking water standards, it helps make local tap water cleaner. The cost to make the pots varies with location, Craver said. The goal is to have the pots made by local craftsman using locally available materials. |
“Sometimes you have the clay in the backyard of the potter or the brick maker, and sometimes you need to go a couple of miles to collect it,” Craver said. The cost per pot varies between US$5 and US$15.
For the last 3 years, Craver has been studying the use of the filters by 70 families in San Mateo Ixtatan, a Mayan community in Guatemala, the URI release says. Between 35% and 40% of filter users see improved health, she said.
|The filters inside the kiln after firing. Photo courtesy of Vinka Craver. Click for larger image.|
Rapid Sand Filters
Another researcher is working to develop a different type of point-of-use filter system. James Amburgey, an assistant professor of civil and environmental engineering at the University of North Carolina (UNC; Charlotte), has created and is testing a rapid sand filter combined with a chemical pretreatment.
Simplicity is the primary objective of Amburgey’s rapid sand filter system. “The idea is to make it as simple as possible,” he said. “All that is needed is some PVC [polyvinyl chloride] pipe, sand, and inexpensive treatment chemicals.”
Ferric chloride and a pH buffer are added to the water, which is then passed through a rapid sand filter. The filter is essentially a piece of 75-mm-diameter (3-in.-diameter) PVC pipe about 0.3 m (1 ft) long filled with the same type of sand that would be found at a water treatment plant, Amburgey explained.
“One significant challenge with sand filters is in removing Cryptosporidium oocysts,” Amburgey said. “One ‘crypto’ is 5 µm in diameter, but the gaps between grains of sand are approximately 75 µm. So, we have to get the crypto to stick to the sand grains.”
This is where the chemical pretreatment comes into play. In its natural state, Cryptosporidium is negatively charged, as are sand grains, so they repel one another. The chemical pretreatment changes the Cryptosporidium surface charge to near neutral, which eliminates the natural electrostatic repulsion and causes it to be attracted to and stick to the sand grains through van der Waals forces, according to a UNC press release. The coagulant also makes the oocysts clump together, as well as stick to other suspended solids. The clumps then can be captured by the sand filter, Amburgey explained.
Chemical coagulation followed by filtration alone is not a new idea. What is new is that Amburgey’s pretreatment mixture doesn’t require optimization.
Coagulation conditions usually have to be optimized for each application, and this makes it difficult to implement such a system in the developing world, Amburgey said.
However, the target pH and coagulant dose that Amburgey has chosen should work universally, he said. “The real beauty of this method, and the only thing that I think is really innovative about it, is the applicability of the method to any type of water,” he said.
In research using a prototype of this system in his lab, Amburgey and his students have conducted preliminary tests on waters from local rivers, creeks, and wastewater treatment plants. Their results are typically greater than 99% removal for Cryptosporidium-size particles, a UNC press release says.
“We were looking at Cryptosporidium removal,” Amburgey said. “From the perspective of removing these 5-µm particles, [the rapid sand filters] performed pretty well.” The researchers compared their results to what point-of-use slow sand filters could achieve and found the chemical and rapid sand combination much more effective.
Additionally, the system is cheap to manufacture. Because the filter is simply a piece of pipe filled with sand, it costs only a few dollars. Also, instead of using filter sand, locally available materials, such as crushed granite, could be used for the filter media, Amburgey added.
|“I think I’ve created a set of conditions where you could treat anything from tap water to 50% wastewater effluent,” Amburgey said. |
While drinking 50% wastewater effluent after only a coagulation and filtration step is unheard of in the developed world, Amburgey explained that he went to that extreme because some source waters are that contaminated. “In the third world, if you’re living in a contaminated water area, you might not have any choice,” he said.
Selling the Idea
Developing these technologies is only the first step in making a difference, Craver said. The bigger challenge is explaining the need for their use, she noted.
“The first time you go to a community, they don’t realize that they are getting sick because of the contamination of water,” Craver said. The residents might acknowledge that the water is dirty but don’t see the direct connection between this and health.
|Undergraduate student Alice Wang conducts a test on the rapid sand filter created by James Amburgey. Photo courtesy of Mike Hermann. Click for larger image.|
“It’s not only a water problem,” Craver added. These communities “also have a minimum or nonexistent sanitation system,” she said. “The food sometimes is irrigated with untreated wastewater.”
Taking care of the drinking water side is important, but several studies have shown that food security and wastewater treatment also must be addressed to have a real effect on health, Craver explained.