Set up before lesson:
Cut recycled water bottles in half and discard cap.
Cut a square of panty hose/nylons and use a small elastic band to secure over the spout of the water bottle.
Ideally, make one filter per student.
Mix a cup of soil with water in several large tubs.
Where do we get our drinking water? [Tap.] Where does this water come from? [Reservoirs.] It is cleaned to be safe.
Indigenous people and others living off the land would get their water from natural water sources such as fast flowing streams. This is usually OK, if water is taken well away from toilets (most common source of dangerous microorganisms).
We will filter muddy water through some different materials to see how well they can clean it.
Show students the filter, how to pour and show them the materials selection (depending on age group):
e.g. coffee filter, moss, kleenex, wood chips, cotton balls, washed sand, washed gravel
e.g. Kindergarten: washed sand, cotton balls, filter paper and tissue
For older students, discuss setting up a control, where the filter has no material added.
Show students the worksheet, where they can record the colour of the emerging water (see worksheet attachments).
Encourage students to try one filter material at a time, before showing them how to layer filter materials, or stack up the filter units, to make the water cleaner.
Free play will happen naturally, but encourage note-taking so that students know what works best.
If students need encouragement: ask them to try different filter combinations that are all natural/one kind repeated to see if this cleans the water better.
Class discussion of results:
Ask how good each material was for filtering water when used alone, and make a class chart. (The results will vary widely, so use the chart to show the best that each material did.) Discuss how the materials filter the water: the dirt particles are trapped in the tiny spaces between the filter material. The water flows through, along with anything that can fit through the spaces. Different materials have different sized spaces, so are effective at filtering different sized dirt particles. Materials that clean the water better have smaller spaces in them, but the water takes longer to pass through them.
Then students will be keen to tell how their different combinations of materials worked.
Connect to water treatment plants:
For younger students they can be told that water treatment filtering systems that clean our water also use sand and gravel. They use several filters and use other membranes as well.
For older students discuss how sand and gravel are arranged in water treatment filtering systems, and that filtering is just one step that our water treatment facilities use. They use gravel and sand to remove larger particles, and meshes to remove smaller particles. Here is the multistep process:
1. aeration to remove gases from water
2. coagulation to clump together dirt (floc)
3. sedimentation allows floc to fall to bottom of settling beds
4. filtration to remove particles (several filters including meshes, sand and gravel)
5. disinfection with chlorine to kill remaining pathogens (ozone and UV treatment do not persist)