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.
First Nations 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:
The 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.
Connect to water treatment plants:
Quick discussion: The city cleans our water with sand, gravel and other membranes. They use several filters.
Longer discussion: One step that our water treatment facilities use is filtration. They use gravel and sand to remove larger particles, and meshes to remove smaller particles. This is just part of a 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)