MadSci Network: Earth Sciences
Query:

Re: What Materials Will Purify Water Best, and why?

Date: Thu Feb 17 15:51:25 2000
Posted By: Sarah Fretz,
Area of science: Earth Sciences
ID: 950652035.Es
Message:

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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