Purpose: To make “contaminated” drinking water as clean as possible.

Procedural overview: Students can use common materials such as plastic bottles, sand and mesh to design, build and test their own water purification systems. Teachers can create “dirty” water for students to purify by adding a variety of substances such as coffee grounds, vinegar and copper sulfate to tap water. Student purification systems should remove solid and dissolved particles to improve water color and acidity.

Approximate class time: One 45-minute class period.

Supplies:

• Student handout
• Gloves
• Goggles
• Paper towels (both to filter and to clean up)
• Sinks
• Buckets (to put under the experiments)
• Duct tape
• Colorimetric pH measuring strips
• Scissors (sharp enough to cut plastic bottles)
• Disposable plastic pipettes
• Balances or scales
• Weigh paper
• Test tubes
• Test tube racks
• Wax pencils or other markers for writing on the test tubes
• Graduated cylinders
• Beakers
• Glass stirring rods
• Empty disposable plastic drinking bottles with sport-style squirt tops (if you cut off the bottom and invert the bottle, it makes a great funnel or column, and the squirt top can be adjusted up or down to control the flow)
• Empty disposable plastic bottles without squirt tops (these can still be cut and used for funnels or columns)
• Aquarium gravel or pebbles
• Clean sand
• Plastic wrap
• Cheese cloth (optional)
• Flexible tubing (optional)
• PVC plastic pipe or plastic tubes of various length (optional)
• Clear plastic bags in quart and gallon sizes (optional)
• A cheap bicycle pump or other hand-operated pumps and tubing (if students get bored watching things drip, they can apply pressure to speed up the process) (optional)
• Vinegar
• Baking soda (sodium bicarbonate)
• Copper sulfate pentahydrate (sold as root killer)
• Sodium carbonate (sold as washing soda or “pH up” for pools)
• Table salt (sodium chloride)
• Epsom salt (magnesium sulfate)
• Sidewalk ice melter (calcium chloride)
• At least 500 mL of dirty water for each student group to purify:
• At least 500 mL of dirty water for each student group to purify:
  • For particulate matter, consider adding tea, coffee, pepper, kitchen spices, flour, oatmeal, a little rice, craft glitter, etc.
  • To make the dirty water both blue and slightly acidic, dissolve a small amount of copper sulfate; to make it more acidic, add vinegar. You can test the water’s acidity with colorimetric pH measuring strips.
  • You could add vegetable oil to encourage students to include a step in which they must skim oil off the top of the water.

Directions for teachers:

Have students work in pairs or small groups.

Give students an assortment of supplies that can be used to build a water purification system. One example of a water purification system is a filtration column or a series of filtration columns. Filtration columns contain matrices of various compositions, such as sand, pebbles and cheese cloth, to remove smaller and smaller particles from the water.

Students’ purification systems should be able to remove solid and dissolved particles to improve water clarity, color and acidity. In this activity, students are not asked to remove microbes from water. Ask students to discuss what types of tests they would want to do to ensure the water is free of microbes and safe to drink. Note: Students should not drink the water.

To make water “dirty,” you can add a variety of common substances to tap water. Coffee grounds, pepper, and flour, for example, can be used to add particles to water. Make multi-sized solid particles the most obvious problem with the water. To make the water both slightly acidic and blue, you could add vinegar and copper sulfate. These chemicals will technically need to be removed by a chemical reaction, and you may need to prompt students about the necessity of performing such reactions. 

It is a good idea to filter the dirty water by first pouring it through pebbles, then sand, then paper towels, then finally filter paper. [Note that the more times a sample is run through the same or similar columns, the cleaner it gets. The sand works best if it is wet first so that it sticks together and does not fall through to cleaner water below.]

In addition to various filters to remove particles, students can add baking soda or detergent to neutralize the acid and precipitate the copper ions (as copper carbonate). In order to completely remove the copper precipitate, students will need to filter their solution using filter paper during the final step. Depending on the volume of solution, this step will take approximately 20 to 30 minutes.

Students may have good ideas for which suitable supplies are not available. They may suggest iodine crystals, boiling, boiling and distillation or other approaches. Students should discuss these alternative ideas but should be encouraged to do the best they can with the supplies that they have.

You can edit the student instructions depending on how advanced your students are, how much independent problem solving you would like them to do and the time and resources available.

General information and reminders for students:

Get ready to embrace your inner chemist and mechanical engineer! In the film The Martian, astronaut Mark Watney is marooned on Mars and must use his scientific creativity to find enough resources to stay alive. Among other challenges, Watney must purify wastewater to use as drinking water until he can be rescued. Now imagine that you are stuck on the Red Planet with an assortment of materials. Your life (or at least your grade) depends on how well you can use those materials to make dirty wastewater from your spacecraft as clean and clear as possible. And remember, you can’t always see everything you need to remove from a solution.

Create a stepwise plan to reclaim as much water as possible from your wastewater, then build a model of your purification system and test it.

Part 1: Initial observations

Make observations of the wastewater sample. Pour some of the water into a test tube.

1. Are there any visible particles suspended in the wastewater sample, or settling at the bottom of the sample?

2. Is the wastewater clear or cloudy? What color is it?

3. Measure the pH of the wastewater sample using colorimetric pH measuring strips. The test strips change colors to indicate how acidic (pH<7) or alkaline (pH>7) a sample is. Cut the test strips in half lengthwise, dip a strip into the sample for two seconds and then let it dry. Compare the strip’s color to the color code on the packaging to determine the sample’s pH.

Part 2: Filtration

The most apparent problem with the wastewater sample is the visible particles in it, so work on that problem first.

4. What general principles are used in water filtration?

5. Using the materials available to you, what design do you think will work best for water filtration?

6. Build and implement your water filtration design. Sketch your design or take a photo to show what it looks like.

7. Pour the wastewater through your filtration system. Are there any visible particles suspended in the filtered sample, or precipitating to the bottom of the water sample?

8. Is the water clear or cloudy? What color is it?

9. Use a colorimetric pH measuring strip to test the filtered water. What is the pH of the sample?

Part 3: Precipitation

10. The water’s blue color is caused by an ion contaminant that also produces blue colors in flames, minerals and metals. What is the contaminant? Do a little research if necessary.

11. If you add other specific ions to the wastewater, those ions could combine with the ion contaminant to make a salt that would precipitate to the bottom of the water, turning the water clear. Fill six test tubes about half full with filtered blue water. Weigh about 0.25 grams of each of the five salts listed below, and add each salt to one of the five test tubes. Keep the sixth tube unchanged for comparison. Cap and shake the tubes to mix, then let the tubes sit for at least two minutes and observe them for any changes.

Possible salts to add to wastewater:

• Epsom salt (magnesium sulfate, MgSO4)
• Detergent (sodium carbonate, Na2CO3)
• Table salt (sodium chloride, NaCl)
• Baking soda (sodium bicarbonate, NaHCO3)
• Sidewalk ice melter (calcium chloride, CaCl2)

12. Which powders and quantities best eliminated the blue color from the wastewater?

13. Test and record the water’s pH. How can you adjust the water’s pH to make it more drinkable?

14. Clean your equipment and your lab area.

Part 4: Analysis

15. After filtering and purifying the wastewater, is it drinkable? What other tests would you run to determine if the wastewater is safe to drink?

16. What have you learned about filtration for water purification?

17. What have you learned about chemical additives and precipitation for water purification?