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Current IssueJuly 23, 2014
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Ask The Expert: Science Fair Project #1: Getting the Dirt Out
10/01/2000 
 
Question: My 7th grade daughter has a school project which requires her to make a water filter that can clean 10 grams of dirt from 125 ml of water in 5 minutes. I don't have a clue how to do this. What would you recommend?

Lonnie Eiseman 

Answer: A simple way to accomplish this task would be to place a coffee filter in its support basket, line the bottom with one or two crushed charcoal briquettes, then cover the charcoal with a 1-2 inch layer of playground sand. (You many find that the charcoal isn't necessary.) Rinse this with clean tap water prior to testing, to wash any charcoal dust out. Add the water gently so as not to disrupt the layers. This filter will likely remove the dirt you add to the water, and the resulting water should appear clear, but this does not mean that it is safe to drink, as bacteria might still be found in the water. Many water utilities use a large version of this simple filter, but also add chlorine to disinfect microbiological organisms.

Another idea is to take a common household funnel and a paper towel to obtain 50-micron filtration and simply pour it through. A micron is 10-6 in size, which is very small. If you want to get more sophisticated, you could use filter paper, but too small a micron rating may inhibit the flow. A coffee filter may or may not be too tight to conform to the five-minute limitation. Again, a disinfection step may be necessary to ensure the water's safety and can be accomplished by a adding a few drops of chlorine. The chlorine can be removed prior to consumption by running the water through a second filtration step using the same funnel and paper towel with carbon added to it as the removal mechanism. If you have a fish tank, the type of carbon used in the filter unit will do nicely.

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