• Plastic funnel - 1
• Gauze pad - 1
• Straws - 4
• Coffee filters - 2
• Small bag of stones
• Small bag of sand
• Small bag of gravel
• Paper towel - 1
• Masking tape
• Paper for blueprint
• Plastic 2 oz. cups - 2
• Craft sticks - 2
• Elastics - 2
• Dirty water (water containing some soil) – 1 cup
• Scissors
• Plastic drinking cups - 2

1. Introduce the challenge to the students: their goal is to filter the dirty water using the materials provided.
2. Divide the students into groups.
3. Provide each group with a paper on which to design their filtration system.
4. Each group must draw and label the design of their filtration system before they receive their materials plus tape and scissors.
5. Assemble-- and test-- the filtering systems.

• How many steps are needed to filter the dirty water?
• Compare the efficiency of large gravel versus small sand granules, which relates to the filtering of water found in nature.
• Compare the rate of flow between the media.

1. Discuss the importance of water (and water quality) in our daily lives.
2. Discuss the filtration process (the use of large stones versus small ones, like found in sand).

Water filtration is a method of cleaning water by passing it through a filter (permeable substrate). Permeability is the measure of how easily a liquid can pass through a substance.

This activity explores the various filter media available. Once water is dirty or polluted, it can be very complicated to clean-- many steps may be necessary to filter dirty water! The rocks and sand formed by erosion perform a very important function: they help to clean our water supply. Sand and gravel make good filters of water because they are permeable materials (they have lots of pores), and they trap some of the dirt particles.

When comparing the rate of flow between media, it is generally found that the tighter the spaces in the filter, the slower the flow of water. These small spaces cause tiny dirt particles to be trapped in the filter media. A slower flow of water through the system produces cleaner water!

In the environment, erosion is responsible for the movement and redistribution of sand and gravel, which helps clean our natural water supply. A more complex version of sand and gravel filters are recreated in water treatment plants to remove fine bits of floc, algae, and silt, in one step of the purification process.

Wetlands are also very important in cleaning water: they are very effective at cleaning water and acting as large filters to help decrease the amount of water pollution.