Untreated water may look clear and clean to us, but may still contain microorganisms (bacteria, protozoa, algae, rotifers, worms, insect larva and others) that are invisible to our naked eye. Because some of these organisms can cause illness if ingested, it is important to remove or kill them from water before drinking it. Worldwide, the most common way to remove organisms is either to boil water, or to filter it.  In the United States, most of our drinking water is treated with chlorine.  However, because  some bacteria and viruses are not affected by chlorine, engineers have developed filters made of special materials with holes of exact shapes and sizes so that meant to filter out some materials and not others.  But there are still many countries that cannot afford these specialized filters or times of emergency (like the tsunami or hurricane Katrina) when they are not available.   

Understanding the principles of filtration can help save lives around the world by reducing cases of cholera and other illnesses from microorganisms in water.  In this activity students create several filters of varying types and sizes, modeling different problems of filtration that continue to challenge researchers and engineers alike, including how to prevent filter membranes from breaking and how to make filters with holes small enough to keep out bacteria and viruses, but large enough to let beneficial molecules through.

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