At Home for Earth Day: Make a Water Filter
Use common materials from your home and yard to make a basic water filter, and watch it work on muddy water!
Filtration is the process of separating solids from fluids using a a filter—a medium that only the fluid can pass through, leaving the solids “trapped” behind. Natural and human-made filters are all around us: paper filters keep coffee grounds out of freshly brewed coffee; HEPA filters capture dust particles in our vacuum cleaners; our kidneys even act as filters to remove harmful materials from our blood! In nature, dirt is naturally filtered out of water as the water moves through sand, soil, gravel. This is the process that we will be demonstrating today.
Materials (per filter—we made 2)
Three cups or glasses
Two empty plastic water bottles
One rubber band
Gauze or cheesecloth
Scissors
Cup of sand
Cup of pebbles or gravel
Cup of muddy water (mix this yourself out of dirt and water)
Procedure
Ask an adult to carefully cut off the bottom third of the plastic bottle, and dispose of the cap
Set aside the base of the bottle for now
Use a rubber band to secure a piece of gauze around the mouth on the top portion of the bottle
Turn the gauzed piece of bottle upside-down (it’s a good idea to ask a friend to hold it in place for you!) and pour in your sand and pebbles one-at-a-time, creating layers (we made one filter of sand-over-pebbles and the other pebbles-over-sand, to compare their efficiency)
Insert the tip of your filter (the part covered with gauze) into the empty base of the water bottle that you set aside earlier, so that your filter is essentially standing upright in an empty cup
Carefully pour your cup of muddy water into the top of the filter
Wait…
Wait…
Wait…
When the water has had time to flow through both layers of the filter (powered, of course, by gravity), it will drip into the empty bottom. This is your filtered water. (Careful: this water is cleanER, but it’s not clean enough to drink!)
Compare the appearance of the filtered water at the bottom of your setup with the muddy water that went into the top. The lighter color of the filtered water tells you that the filter worked: some of the dirt became trapped in the filter materials, while the water was able to flow through. Manufactured filters generally get finer and finer as the water moves through—so that each layer of the filter can catch particles that the last layer missed. Our pebbles-over-sand filter was more closely aligned with this design—although we didn’t notice much difference in the filtration capabilities, compared with the sand-over-pebbles model.
Some people use a design similar to this one to create very effective filters: effective enough to create clean drinking water. In parts of the world where water pollution is high, this can help people survive by having clean enough water to drink. “Real” filters use more finely-grained materials than just sand and pebbles—including, often, activated charcoal.
Think Like a Scientist—Variations on This Project
Make more filters, using other natural or manufactured materials instead of, or in addition to, your sand and pebbles. You might use grass clippings, popcorn kernels, a coffee filter, cotton balls, or even different sized pebbles or more finely grained sand. How does the grain size of the filter material impact the quality of the filtered water?
Set up several single-layer filters (one of just sand, one of just gravel, etc.), add water to all of them at the same time, and measure how long it takes the water to get through each filter. Do you see a relationship between how long the water took to get through the filter and how clean the water is on the other end? What is the relationship between the size of the filter particles and the time it takes water to flow through the filter?
Instead of dirt, try adding other contaminants to your water: food coloring, cooking oil, baking soda…Which ones can your setup most effectively filter out?
