Key concepts
Chemistry
Coagulation
Water filtration

Introduction
It’s easy to take our clean water for granted, we’re so used to being able to drink from a fountain or faucet whenever we’re thirsty. But did you know more than a billion people in the world don’t have access to clean water? Part of the problem is that cleaning (or purifying) water is not an easy task. Not only do you need to remove dirt and other debris but you also have to get rid of all the invisible bacteria and microorganisms that can make people sick. In this activity we’ll explore a few methods for cleaning water, and learn about the steps involved in making sure you have safe water to drink when you're thirsty.

Background
Water purification is a complicated and expensive process. It can be broken down, however, into five basic steps: aeration, coagulation, sedimentation, filtration and disinfection. The first step, aeration, adds air to the water, simultaneously adding oxygen to the water as it allows other gases to escape. The second step, coagulation, pulls together the dirt and other solid particles in the water. As they’re pulled together, the dirt coagulates (forms bigger clumps), which are easier to filter out in the next steps of the process. The third step, sedimentation, allows gravity to work for us. It pulls the heavy clumps of dirt to the bottom of the water, separating it even further from the cleaner water at the surface and getting it ready for the next step—filtration. Filtration involves separating the dirt and other solid particles from the water. Finally (and perhaps most importantly) the fifth step is disinfection. We disinfect water by treating it with chemicals and/or heat to remove bacteria and other unsafe microorganisms that can make us sick.

You will be exploring several of these steps by testing a few water filtration systems of your own. And next time you drink water from a tap, remember how much work went into making sure that water is clean and safe for you to drink!

Materials

  • Two paper coffee filters
  • Three clear plastic water bottles
  • One cup uncooked rice or beans
  • Five clear plastic or glass cups
  • Two sheets of white paper
  • A pen or pencil
  • Four cups of water
  • One cup of loose dirt
  • One large plastic container with a lid (for mixing water and dirt)
  • Ruler
  • Scissors or an X-ACTO knife
  • An adult helper
  • Rubber band
  • Timer or clock
  • Measuring cup (at least a half cup)
  • Half teaspoon of alum
  • A spoon


Preparation

  • Combine your water and dirt in the large plastic container. Shake the container to ensure the dirt and water are well mixed.
  • Use your ruler and pen to evenly divide one sheet of paper into eight sections, four rows down and two columns across. In column 1, label each row as follows: row 1—“Control”; row 2—“Coffee Filter”; row 3—“Rice and Coffee Filter”; row 4—“Rice, Coffee Filter, Alum” (Note: if you use beans, write “Beans” instead of “Rice” for those labels.)
  • Have your adult helper carefully cut the bottoms off your water bottles, leaving the bottles and bottle mouths intact.


Procedure

  • Measure one half cup of the dirty water and pour it into one of your plastic cups. Add one half teaspoon of alum to the water and stir gently for three to four minutes. What do you notice about the water as you stir?
  • Set the cup with alum and dirty water aside; let it sit undisturbed for at least 10 minutes.
  • Starting with Bottle 1, cover the mouth of the bottle with a coffee filter. Use a rubber band to secure it in place.
  • Turn the bottle upside down (so the bottle mouth faces downward). Hold the bottle over one of your clear cups.
  • While you hold the bottle, ask your adult helper to carefully pour one half cup of the dirty water into your bottle. Start your timer.
  • Hold the bottle over the cup until all of the water is in the cup. In Column 2 on your paper, next to the square that says “Coffee Filter” write how long it took for all of the water to get into the cup. What do you notice about the water in the cup? Does it look different than it did when you poured it in the bottle? What is left in the bottle?
  • Put the cup with water from Bottle 1 on your paper, in the square where you wrote the time. Use a measuring cup to measure how much water is in the cup. Write this amount next to where you wrote the time. Pour the water back into the cup and leave it on the paper. Did all of the water that you poured into the bottle make it into the cup? If not, where do you think it went?
  • Put Bottle 1 aside.
  • Prepare Bottle 2. Cover the mouth of the bottle with a coffee filter. Use a rubber band to secure it in place.
  • Turn the bottle upside down (bottle mouth downward). Hold the bottle over one of your clear cups.
  • Have your adult helper carefully measure and pour one half cup of rice into Bottle 2. Tap the sides of the bottle so that the rice settles at the bottle mouth.
  • While you hold the bottle, ask your adult helper to carefully pour one half cup of the dirty water into your bottle. Start your timer.
  • Hold the bottle over the cup until the bottle has no water left and all of the water is in the cup. In column 2 on your paper, next to the square that says “Rice and Coffee Filter,” write how long it took for all of the water to get into the cup. What do you notice about the water in the cup? Does it look different than it did when you poured it in the bottle? What is left in the bottle?
  • Put the cup with water from Bottle 2 on your paper, in the square where you wrote the time. Use a measuring cup to measure how much water is in the cup. Write this amount next to where you wrote the time. Pour the water back into the cup and leave it on the paper. Did all of the water that you poured into the bottle make it into the cup? If not, where do you think it went?
  • Put Bottle 2 aside.
  • Prepare Bottle 3. Cover the bottle mouth with a coffee filter. Use a rubber band to secure it in place.
  • Turn the bottle upside down (bottle mouth facing downward). Hold the bottle over one of your clear cups.
  • Have your adult helper carefully measure and pour one half cup of rice into Bottle 3. Tap the sides of the bottle so that the rice settles at the bottle mouth.
  • This time, have your adult helper carefully pour the water from the first steps into Bottle 3. Try to avoid disturbing any sediment that collected in the cup. Start your timer.
  • Hold the bottle over the cup until all of the water is in the cup. In column 2 on your paper, next to the square that says “Rice, Coffee Filter, Alum,” write how long it took for all of the water to get into the cup. What do you notice about the water in the cup? Does it look different than it did when you poured it in the bottle? What is left in the bottle?
  • Put the cup with water from Bottle 3 on your paper, in the square where you wrote the time. Use a measuring cup to measure how much water is in the cup. Write this amount next to where you wrote the time. Pour the water back into the cup and leave it on the paper. Did all of the water that you poured into the bottle make it into the cup? If not, where do you think it went?
  • Measure and pour one half cup of dirty water into your last clear cup. Place this cup on your paper, in the square next to the one that says “Control.”
  • You should now have four cups lined up on your paper. Look at the water in each cup as well as the time and measurements you wrote on your paper. Compare the results of each of your three tests, with the Control cup. Which water is the dirtiest? Which is the cleanest? Which filter system took the most time to filter the water? Which took the least? Which filter system absorbed the most water? Which allowed the most water to pass through it? (Note: Although your water may appear clean—it is not, and you should not drink it. Use it to water your yard or plants instead!)
  • Extra: Try this activity with other household materials such as cotton balls, paper towels, sponges or anything else you think might be a good filter. Did you find one that worked well? Why do you think it worked well—or not?
     

Observations and results
In this activity you tested several different methods for filtering dirty water. Your results may have varied slightly depending on the type of dirt you used in your dirty water. You should have found, however, the water was cleanest when you used the coffee filter, rice and alum all together. This filter did the best job of cleaning the water because it incorporated several different water purification steps. First you added alum to cause coagulation. You should have observed the water coagulating as you stirred the mixture during the first step. Next you allowed the water to sit undisturbed for at least 10 minutes. This resulted in sedimentation, where gravity pulled the coagulated substances to the bottom of the cup, leaving cleaner water at the surface. Finally you filtered the cleaner water through rice and a coffee filter, separating those coagulated chunks from the water.

You should have found the coffee filter alone did a fairly good job of filtering the water, however the extra steps you took with the alum yield better results. In addition you probably observed that you had the most water come through the coffee filter. Both the rice and alum absorb some of the water, so less makes it into your cup from the bottle. The coffee filter alone should have been the fastest filtering method of the three you tested, and have taken the least amount of time to empty the bottle.

More to explore
From Contaminated to Clean: How Filtering Can Clean Water, from Science Buddies
Which Filtration Material Leads to the Best Drinking Water?, from Science Buddies
How Dirt Cleans Water, from Scientific American
Clean Dirty Water with the Sun, from Scientific American
Science Activities for All Ages! from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies