Several student-built water filtration devices

video showing how water is recycled on the International Space StationYoutube video

Engingeering design process diagram


Overview

In this activity, students are challenged to design and build a water filtration device using commonly available materials -- following the same design process used by the engineers and scientists who developed the International Space Station Water Recovery System for NASA. To meet this challenge, students use an iterative process as they build, test and measure the performance of the filtration device, analyze the data collected, and use this information to work toward an improved filtration design.

Although students will work in teams of two to three, they are encouraged to think of their entire class as a single design team working cooperatively and learning from the efforts of all members in order to produce the best water filtration device.

Students measure the effectiveness of their filtration device using pH test strips and a conductivity tester that is assembled from readily available materials and requires about a half-hour to construct. Detailed plans and a complete materials list are provided. 

Materials

Management

  • Begin collecting empty, matching 0.5 L bottles weeks or months in advance of this activity to save the cost of purchasing bottles.

  • Build (or have students build) and validate conductivity testers (if using) prior to engaging in this activity. Download Conductivity Tester Plan (PDF)

  • Be sure to use clean aquarium gravel and play sand (not yard gravel or beach sand, as these may add to the pollutants in the water).

  • Be sure to rinse and thoroughly dry activate carbon (if using). Un-rinsed carbon will turn the water black.

  • Consider assigning roles to team members, such as materials manager, science recorder, engineering recorder and engineering builder.

  • Note: Water filtered in this experiment SHOULD NOT be consumed by humans.
  • This Activity can be guided by competition of the student teams.  Be the judge for a qualitative assessment of the filtered water: hold a piece of white paper in back of the filtered water and examine for particulate matter and color.  The winning team has the clearest water bottle.

Background

Earth’s natural life support system provides the air we breathe, the water we drink and other conditions that support life. Sometimes our Earth resources become polluted and require cleanup.

One such cleanup project is going on now near NASA's Jet Propulsion Laboratory in Pasadena, California, through a groundwater cleanup project guided by the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) of 1980. Groundwater Cleanup Project Manager Steve Slaten said, “NASA is responsible for a large-scale groundwater cleanup that is a result of past waste disposal practices that go back to World War II, when the Army was operating JPL and developing rockets for the military. Liquid wastes -- everything from toilets to the analytical labs, chemicals, cleaning solvents and a component of rocket propellant called perchlorate -- are now in the deep ground water. It’s very important that we clean up this problem so that our neighbors have access to this resource.”

There's also a need for water filtration systems beyond Earth, like for astronauts on the International Space Station. For example, NASA's Marshall Space Flight Center is responsible for the design, construction and testing of a important system on the ISS that not only provides the crew with a comfortable environment, but also minimizes the number of resupply missions needed to keep the ISS and its crew functioning. The system, called the Environmental Control and Life Support System, performs several key functions:

  • Provides oxygen for metabolic consumption.
  • Provides potable water for consumption, food preparation, and hygiene uses.
  • Removes carbon dioxide from the cabin air.
  • Filters particulates and microorganisms from the cabin air.
  • Removes volatile organic trace gases from the cabin air.
  • Monitors and controls cabin air partial pressures of nitrogen, oxygen, carbon dioxide, methane, hydrogen, and water vapor.
  • Maintains total cabin pressure.
  • Maintains cabin temperature and humidity levels.
  • Distributes cabin air between connected modules.

The two main components of the Environmental Control and Life Support System on the ISS are the Water Recovery System and the Oxygen Generation System.

The Water Recovery System

The Water Recovery System provides clean water by reclaiming wastewater (including water from crew member urine, hand wash, and oral hygiene waters), cabin humidity condensate, and extravehicular activity (EVA) wastes. The recovered water must meet stringent standards before it can be used to support crew, EVA and payload activities.

The Water Recovery System is designed to recycle crew member urine and wastewater for reuse as clean water. By doing so, the system reduces the net mass of water and consumables that would need to be launched from Earth to support six crew members by 2,760 kg (6,000 lbs) per year.

The Water Recovery System consists of a Urine Processor Assembly and a Water Processor Assembly. A low-pressure vacuum distillation process is used to recover water from urine. The entire process occurs within a rotating distillation assembly that compensates for the absence of gravity and therefore aids in the separation of liquids and gases in space. Product water from the Urine Processor Assembly is combined with other waste waters and delivered to the Water Processor Assembly for treatment. The Water Processor Assembly removes free gas and solid materials (hair, lint, etc.) from the water before it goes through a series of multi-filtration beds for further purification. Any remaining organic contaminants and microorganisms are removed by a high-temperature catalytic reactor assembly. The purity of product water is checked by electrical conductivity sensors (the conductivity of water is increased by the presence of typical contaminants). Unacceptable water is reprocessed and clean water is sent to a storage tank ready for use by the crew.

The Oxygen Generation System

The Oxygen Generation System produces oxygen for breathing air for the crew and laboratory animals, as well as for replacement of oxygen lost due to experiment use, airlock depressurization, module leakage and carbon dioxide venting. The system consists mainly of the Oxygen Generation Assembly and a Power Supply Module.

The heart of the Oxygen Generation Assembly is the cell stack, which electrolyzes, or breaks apart, water provided by the Water Recovery System, yielding oxygen and hydrogen as byproducts. The oxygen is delivered to the cabin atmosphere while the hydrogen is vented overboard. The Power Supply Module provides the power needed by the Oxygen Generation Assembly to electrolyze the water.

The Oxygen Generation System is designed to generate oxygen at a selectable rate and is capable of operating both continuously and cyclically. It will provide from 2.3 to 9 kg (5 to 20 lbs) of oxygen per day during continuous operation and a normal rate of 5.4 kg (12 lbs) of oxygen per day during cyclic operation.

The Oxygen Generation System will accommodate the future addition of a Carbon Dioxide Reduction Assembly. Once deployed, the Carbon Dioxide Reduction Assembly will cause hydrogen produced by the Oxygen Generation Assembly to react with carbon dioxide removed from the cabin atmosphere to produce water and methane. This water will be available for processing and reuse, thereby reducing the amount of water to be resupplied to the Space Station from the ground.

Procedures

The student-built filtration device is made from two 0.5-liter water bottles with the bottoms cut off. The bottles will be stacked so as to allow the wastewater to filter through filter media in the top bottle and collect in the bottom bottle. The challenge is for students to determine which filter media they should use to get the purest filtered water. Clearly communicate to students that the water filtration devices they are about to make will remove some impurities, but they will NOT make the water safe to drink.

  1. Students should research the definition of pure water, as it is defined for safe human consumption.

  2. Show students the simulated waste water and allow them to use a wafting technique to detect the odor of the compound. 

  3. Describe to students the filter media available and have them research any materials with which they are unfamiliar (usually zeolite and activated carbon, if using).

  4. Discuss the meaning of pH (acidity or alkalinity of the solution), the ideal pH of drinking water, and measure the pH of the tap water in your school. For contrast, consider having students measure the pH of other common liquids such as vinegar, soda pop, and baking soda dissolved in water.

  5. If using conductivity testers, discuss their use at this time. Explain to students that a conductivity tester is creating a circuit through the water in order to measure the conductivity of the water. Conductivity is a standard method used to measure the purity of water, specifically the quantity of inorganic contaminants. Completely pure water will not conduct electrical current. Thus, the smaller the amount of current that flows through the treated wastewater, the lower the concentration of inorganic contaminants. The water recovered and purified by the Water Recovery System on the ISS has an average conductivity of approximately 1 µmho/cm, most of which is a result of residual iodine added to the water for its biocidal properties.

  6. Discuss with students how to use the conductivity tester:

    • Do not allow the two exposed metal ends (the wire coming from the battery snap connector and the multimeter lead) to touch. This could cause the battery to overheat or the multimeter to malfunction.

    • The distance leads are from the bottom of the container and the distance leads are from each other will affect the current readings.

    • Make sure all students agree on a consistent distance for measurement, e.g., place leads 1 cm from the bottom of 100 mL beakers and keep them on opposite sides against the glass.

    • Gently swish the fluid in the container immediately before taking a reading, then count to 10 and record the mA value.

    • Clean and dry the leads between every test. 

    • Students should briefly practice using the conductivity testers in known solutions (tap water, saline solution, etc.) to ascertain facility with the devices and verify device functionality. 

Student procedure (students should work in groups of 2-3):

  1. Remove the labels from two 0.5-liter water bottles. Discard one screw cap. Securely affix the other screw cap to the bottom bottle.

  2. Use scissors to remove the bottoms from both bottles.

  3. Secure cheesecloth (folded as necessary to retain filter media in bottle) around the neck of the open top bottle with rubber band.

  4. Nest the two bottles. 

  5. Fill the top bottle to within 1.5 inches of its top with filter media of various types and layers. Document the amounts and sequence of filter media used.

  6. Measure and record the pH and conductivity of the simulated wastewater.

  7. Slowly pour 200 milliliters of simulated wastewater through each student-built water-filtration device.

  8. Measure and record the pH and conductivity of the filtered water.

  9. Compare the results (color, odor, pH, conductivity) among student groups. Discuss the filter media used and results achieved.

  10. Allow each group to design and build a better filter, based on the class data.

Discussion

  • Which filter media were most effective at filtering the water?

  • How might you further improve upon the water filter design?

Assessment

Assessment for this activity varies based on materials available and testing methods used. If filtered water is more pure than the simulated wastewater, then the student-built filtration device was somewhat effective. The degree to which purity is possibly attained is dependent on materials available, so, in most classes, encourage students to use data collected in the course of the water analysis to decide which filters are most effective. 

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