Clay ceramic filters work in a fashion similar to the desalination technology described in the previous section. Basically, water flows through clay that contains a lot of really tiny holes, which are big enough to let water molecules though, but too small for bacteria, dirt, and other bad stuff [source: Doulton USA]. The first such device was developed by a British potter, Henry Doulton, back in the early 1800s for purifying water drawn from the Thames, which was so contaminated with raw sewage that cholera and typhoid were continual dangers [source: Brodrick].
Since Doulton, other inventors have made improvements to his basic concept, such as adding silver coatings to kill bacteria, so that today's ceramic filters do an even better job of getting rid of dangerous pathogens. The really revolutionary development, though, is that humanitarian non-governmental organizations have set up factories to make and give away large numbers of inexpensive ceramic filters in the developing world.
A 2006 study found that Cambodians who used the simple filters, which are portable and require no energy to run, reduced the incidence of diarrheal disease by 46 percent, and E.coli contamination in their water by 95 percent from 2003 rates [source: Resource Development International – Cambodia ]
One drawback with these ceramic filters is the speed of filtration. The water seeps out the clay filter at a rate of just 2 liters (2.11 quarts) per hour. But the process needs to be slow in to give the silver solution time to kill pathogens. The filter also does not remove harmful chemicals like arsenic.