MadSci Network: Earth Sciences |
Well, unless you're filtering golf balls out of a pond, let's just throw out the gravel all together. Most of the water filters that we use for everyday water purification work off the idea that you can't push a golf ball through a bed of gravel. There just isn't enough space in between the pieces of gravel through which the golf ball can travel. Another way of putting this is that the pores are too small to let the particle through, the "particle" being the golf ball and the spaces between are the "pores". The smaller the particle that you want to filter out of the water, the smaller the pores need to be in your filter. The materials you've provided as possibilities (even the gravel) will all work to some extent, and the one that has the smallest pore size is the one that will probably serve as the best filter. -Sand is a finer version of gravel, and is used in mass water treatment. It does a pretty good job but is very slow, bulky, and gets even heavier because it holds so much water. -Paper towels, if you fold them to be the same thickness as your sand or cotton beds will also work well. Coffee filters are a stiffer form of paper than the paper towels, but seem to hold back sand-sized particles pretty well while letting most of the coffee oils and flavors flow through. -My t-shirt (made of spun and woven cotton) serves me pretty well for draining pasta on a camping trip, but I wouldn't trust it to keep me from getting sick when I drink water from a stream. I'm not going to just blurt out the answer, I figure if you're truly curious, you'll give it the effort of an experiment (oh how I love experiments!) You can estimate pore space by measuring how much water your materials will displace- the more water displaced, the less pore space there is in your filter. OR... you can set up your own filters. Use some soda bottles with the bottoms cut off and half-fill them with the filter materials (half full so you can . Pour through whatever you want to filter (muddy puddle water, orange juice, or tap water with food coloring, flour, cornstarch, or active yeast). Just be sure that the following are consistent: bed length (the distance the water will have to travel through each of your filters), amount of fluid to be filtered (use at least a liter of each), and - if you're using something that could settle out in time, like the flower or mud, be sure you stir it well before pouring. Take note of the time it takes each to drip through the filter, and have a glass of the original solution handy to compare. My *unofficial* guess is that the sand will serve as the best filter- if your experiments show otherwise, write back, I'd love to hear about it!! –Sarah
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