Hands-on Activity Washing Air:
Wet Scrubber Pollutant Recovery Method

Learning Objectives

After this activity, students should be able to:

• Describe and explain how a wet scrubber works as a pollutant recovery method.
• Give examples indicating when the use of a wet scrubber is appropriate.
• Describe how engineers create technology to help industry clean up their air pollution.
• Use their knowledge of percentages to understand technological efficiency.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

International Technology and Engineering Educators Association - Technology

Materials List

For the class demonstration:

• Duct tape
• 1/2 cup chalk dust or fireplace ash
• 3 clear, glass bottles (such as large olive oil or liquor bottles)
• 3 rubber corks with 2 holes drilled in each (the corks must fit snuggly into the bottle openings; available at hardware stores, sometimes predrilled)
• Various pipe connectors, as needed, to transition the narrow tubing to the wider vacuum cleaner nozzle (see Figure 2)
• 5 barb connectors (to connect plastic tubing to corks; the connector must fit snuggly into the holes in the corks; available at hardware stores; see Figure 3)
• 5 ft. clear plastic tubing (the tubing must fit snuggly over the barb connectors; available at hardware stores; see Figure 3)
• Water
• Paper towels
• Wet/dry shop vacuum cleaner

Worksheets and Attachments

Pre-Req Knowledge

A basic understanding of air pollution.

Introduction/Motivation

One of the biggest challenges environmental engineers face is devising new techniques to prevent additional air pollution. Industry is a large contributor to air pollution. Have you ever seen a big factory releasing dark smoke into the air? How does it smell? Sometimes, it does not smell so great! The first type of technology (or pollutant recovery method) we are going to examine is a wet scrubber. A wet scrubber uses water to clean pollutants out of the air.

Scrubbers are used at coal burning power plants, asphalt/concrete plants, and a variety of other facilities that emit sulfur dioxides, hydrogen sulfide and gases with high water solubility. Inside a wet scrubber, solid particles and gases are trapped as they pass through a fine water mist (refer to Figure 1 or the attached How a Wet Scrubber Cleans Polluted Air Diagram). Sometimes the mist is injected with limestone powder to help extract the dirt particles. Wet scrubbers are often used for corrosive, acidic or basic gas streams. The resulting pollutant-laden wastewater is processed into sludge, which is used to make bricks.

How well do you think this works? Today, we are going to model the wet scrubber technology in class.

Procedure

Before the Activity

• Gather materials and set up the apparatus as shown in Figures 3 and 4. Refer to the Wet Scrubber Model Photo Collection (attachment) for more details.
• Test the activity at least once before presenting it as a class demonstration.

With the Students

1. As a class, brainstorm different ways that water could be used to clean the air. List ideas on the board.
2. Put chalk dust or fireplace ash (representing pollution) in the first bottle.
3. Fill bottle 2 and bottle 3 both 1/3 full of water.
4. Make sure the "in" tube in each bottle is submerged in the water, while the "out" tube is not.
5. Connect the "out" tube of bottle 3 to the shop vacuum cleaner. Use the other pipe connectors (see Figure 2) to make a tight connection from the narrow tube to the larger vacuum cleaner tube. Use duct tape to make all the connections airtight.
6. When ready to start, shake bottle 1 to create the polluted air and turn on the vacuum cleaner at a low setting.
7. What should happen: Polluted/dirty air in bottle 1 is suctioned into the water in bottle 2. The bottle 2 water cleans the particulates from the air while the cleaner air is suctioned on into bottle 3. Dust may collect in the air above the water in bottle 2. This is why there is a third bottle. The air is cleaned a second time in bottle 3 before being suctioned out by the vacuum cleaner as clean air released into the environment.
8. Have students record their observations. Tell them to write down anything that strikes them as important.
9. Discuss the student observations. Explain that a wet scrubber collects not only particulate matter but also captures waste gases. Discuss that the white plume you see coming from a smokestack of a factory may really be (unpolluted) steam coming from a water scrubber.
10. In conclusion, ask the students the following questions, and discuss as a class:

• Why does the water in the wet-scrubber change color? (Answer: Visible air pollution [particulate matter] moves from the air into the water.)
• What does the scrubber filter out of the air? What does it not filter out? (Answer: It filters out fine particulate matter and waste gases. It does not filter out heavy particles of dirt and sludge.)
• Suggest ways to dispose of the pollutants that are now trapped in the water. (Possible answer: The resulting pollutant-laden wastewater is processed into sludge, which is used to make bricks.)
• Does the wet scrubber remove all of the particulates? (Answer: The average efficiency of a wet scrubber is about 95%. That means that if there are 100 particulates, 95 of them are caught and six of them make it back into the air.)
• Is this a good or bad efficiency rate? (Opinions may vary. 95% still means that 5% of the particulates are returning to the atmosphere.)

Assessment

Pre-Activity Assessment

Brainstorming: As a class, have the students engage in open discussion. Remind students that in brainstorming, no idea or suggestion is "silly." All ideas should be respectfully heard. Take an uncritical position, encourage wild ideas and discourage criticism of ideas. Have them raise their hands to respond. Write their ideas on the board. Ask the students:

• How can water be used to clean the air?

Activity Embedded Assessment

Observations: Ask students to pay close attention and record their observations of the experiment, explaining to them that this is what real scientists and engineers do. Instruct them to record anything that seems important.

Post-Activity Assessment

Observations: Discuss student observations.

Drawing/Journaling: Depending on the students' age, have them draw a picture or write in their own words a description of how a wet scrubber works to clean polluted air. Ask for volunteers to share their descriptions with the class.

Safety Issues

• Only draw air through the tubing using a vacuum cleaner; using your mouth may cause accidental inhaling of the contaminated air.

Troubleshooting Tips

The corks, tubing and connectors can be purchased at a hardware store. It is possible to find the corks with predrilled holes.

Make sure that all the connections are tightly sealed with duct tape.

If you are using fireplace ash, replenish it frequently because only the light "floaty" particulates are pulled through the scrubber by the vacuum; the heavier charcoal pieces are not useful for the demonstration.

It is strongly recommended that you practice this demonstration at least once before presenting it to the students.

Activity Extensions

A wet scrubber's resulting pollutant-laden wastewater is sometimes processed into sludge, which is used to make bricks. How does this work? Have students investigate this process on the Internet (search: sludge dryers, dewatering sludge) and report back to class. Is this a successful pollutant recovery process?

Find local examples of wet scrubbers and arrange a field trip. What pollutants does this wet scrubber remove? What do they do with the wet scrubber wastewater?

Activity Scaling

• While the demonstration is appropriate for any age level, younger students may have difficulty understanding some of the ways the scrubber functions. Tailor the explanation and discussion to fit the comprehension level of your students.
• For upper grades, add a math component. Give students a number of particulates, and have them calculate how many are cleaned from the air using a wet scrubber with 95% efficiency. For example, if 100,000 particulates went through this wet scrubber, about 100,000 x 0.95 = 95,000 would be removed, and 5,000 would remain.

References

Copyright

Contributors

Supporting Program

Acknowledgements

The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0338326. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.