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Activity # 2
My Water is Gray Water

Grade Level: 5-8

Module: Hydroponics


Students will determine the average amount of water they use during a typical shower and the concentration of soap in that water. The students learn the need to recycle gray water to allow plants and animals (including humans) to survive in space.

Key Questions

  • How much water do we use in a typical shower?
  • What is in my gray water?
  • What is the concentration of soap in my gray water?
  • Besides soap, what else might show up in gray water?
  • Do longer showers use more water?

Time Frame:

1-2 weeks


For the class:
  • Stopwatch or timing device
  • Buckets
  • Balance (ideally, triple beam)--metric
  • Access to showers
For each student:
  • 1 bar of unused soap (can be brought in by students; if so, have a few extra bars on hand for those who forget)
  • "My Water is Gray" student worksheet (two pages; masters on the following two pages)
  • Empty 2 liter bottle

Getting Ready

Photocopy a "My Water is Gray"worksheet (two pages) for each student.

Gather all of the necessary materials. If you choose to have students bring in bars of soap, assign this before the day of the activity.

If your school has no showers for students to use, then this activity can be adapted with the "take 3 showers" part as a homework assignment.

Name: _______________________________________ Date: _________ Per._______

My Water is Gray Worksheet

Determine average shower time:

Length of shower

Shower 1:________min________sec = ____________sec
Shower 2:________min________sec = ____________sec
Shower 3:________min________sec = ____________sec

Total time for three showers = ____________ sec

Average shower time = total time for all three showers (in sec) ÷ 3
Average shower time = _____________sec
Average shower time = ________min ________ sec

Determining Average Amount of Soap Used Per Shower

Weight of unused soap ___________ g
Weight of soap after 3 showers ____________g
Weight of unused soap minus weight of soap after 3 showers = Amount of soap used
Total amount of soap used for three showers = ______________ g
Average amount of soap used per shower = total amount used for three showers ÷ 3
Average amount of soap used per shower = ______________ g

Determining Flow Rate of Shower

1. Turn shower on completely; place bucket under shower head, and time for 60 seconds.
2. Measure volume of water in bucket after one minute by pouring into two liter bottles. Round to the nearest quarter liter.

Flow Rate of Shower = _____________ liters per minute

Determining the Amount of Water Used During an Average Shower

Average amount of water used per shower = average shower time x flow rate

Average amount of water used = ________min x _______ liters/minute = ________liters

Determining Concentration of Soap in a Typical Shower

Soap concentration = amount of soap (in grams) ÷ volume of H2O (liters)

= ___________ grams per liter

Determining the Cost of a shower in Space

Use the following information to determine how much your shower would cost (this is a one time use of water -- not yet recycled).

1 liter water =1 kg

1 kg = 2.2 LB

Cost of sending 1 LB into space = $10,000

___________ = Cost of Shower

Classroom Activity

1. Discuss with the class the importance of using water efficiently in space. Ask the students what might be different about how we would use water in space?

2. Hand out a “My Water is Gray” student worksheet to each student.

3. Have the students weigh their unused bars of soap (in grams or convert) and record this measurement on the student worksheets.

4. Tell the students to go take three showers, using their bars of soap and timing how long each of the three showers lasts. Time starts as water is turned on and stops when water is turned off. (This step and the next one can be homework if necessary.)< p> 5. To measure the flow rate of the shower the students must measure the volume of water flowing in one minute. This can be done by putting a bucket under the spout and collecting water (with valve set a “normal” shower position) for one minute (60 seconds ). They can then measure the amount of water collected by seeing how many 2 liter bottles-full of water were collected.

6. Back in class, ask students to weigh their bars of soap again. Have them determine the average amount of soap used per shower by dividing the mass of soap used by three (record on student worksheet). They can also determine and record on their workshe ets:

a. the average amount of time per shower by dividing the total shower time by three.
b. the amount of water used in a typical shower by multiplying the flow rate by the average time for a shower.

7. Students will now know the amount of soap in one of their typical showers as well as the amount of water used. They can now determine and record on their worksheets the concentration of soap in their gray water (expressed in grams of soap per liter of water).

8. Students can determine how much their shower would cost in space using the following information:

1 liter water =1 kg 1 kg = 2.2 LB Cost of sending 1 LB into space = $10,000

Wrap-up Session

1. Ask the students what they feel they learned in this activity. Were they surprised by any of their results? Have your students recall the discussion they had at the beginning of this activity and ask them if they have any new ideas about the importance of using water efficiently in space? In light of the high cost of bringing water into space, ask students how their findings might relate to the challenge of growing plants in space.

2. Students can generate a list of all other substances that might appear in their waste water (shampoo, toothpaste, perfume, hair spray, etc.). Which of these might appear in an astronaut's gray water (hair spray, lipstick, cologne, etc. probably would n ot). Use the worksheet "What is in your Gray Water" or you make wish to make a transparency of this sheet.

3. Students can graph their results (Show sample "Bar Graph")

  • Soap concentrations used in the activity
  • Length of showers- boys vs. girls
  • Number of showers per bar of soap
  • Frequency of favorite brands brought to class by students
4. Discussion of averages and statistics
  • Why include measurements from three showers?
  • Why add the individual shower times and divide by three?
  • Is the average concentration a better measurement than would be calculated precisely for an individual shower? Discuss the precision of measurements versus the adequacy of measurements, i.e., what is "good" data?

Background for Teachers

Gray water is used water (water containing soaps, waste and other contaminants). In space, scientists hope to use gray water to grow plants. The major contaminant of water in space will be the soap that the astronauts use to cleanse themselves. A nutrient solution can be mixed with this gray water and then this solution can be fed to the plants. Plants take up water through their roots, use it for different chemical reactions (like photosynthesis) and then transpire it through their leaves. Transpiration occurs when water evaporates through tiny holes in the leaves. This transpired water is pure regardless of how pure or "gray" the nutrient water was. The plant itself provides the filtration step.


  • Graph 2-D data in various forms
  • Convert minutes to seconds, ounces to grams
  • Gray water - water that has been used and has contaminants
  • Contamination - introduction of an undesirable or harmful material
  • Concentration - the amount (by weight) of a stated volume
  • Flow rate - the volume of fluid passing a given point per unit period of time
  • Transpiration - release of water vapor through pores or membrane
  • Calculating quantity of water and soap used in a typical shower
  • Calculating the concentration of soap in their gray water
  • The importance of using gray water in space
  • Plants can help in recycling gray water

More Activity Ideas

See the Internet site for NASA Project STELLAR at http://stellar.arc.nasa.gov for the following related activities:
  • Activity #12 on Toxic Bio Assay Demonstration
  • Activity #13 on Toxic Bio Assay Laboratory


    Keywords: Hydroponics, Gray Water, Transpiration


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