Washing Air: Wet Scrubber Pollutant Recovery Method

One of the biggest challenges environmental engineers face is devising new techniques to prevent industrial air pollution. Engineers are creative in designing modern pollutant recovery methods and industrial technologies that clean up and prevent air pollution. Scrubbers are designed for use at power plants and facilities that emit sulfur dioxides and hydrogen sulfide gases. Engineers also design cleaning products that scrub stains from your tub and shower, and non-toxic technologies that clean your oven.

Figure 1. How a wet scrubber cleans polluted air.CopyrightCopyright Natalie Mach, Graduate Fellow, Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder, 2004.

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.

A basic understanding of air pollution.

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 (PDF)). 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.

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 (PDF) (attachment) for more details.
• Test the activity at least once before presenting it as a class demonstration.

Figure 3. Experiment setup: A model wet scrubber. Left to right, bottle 1, 2, 3.CopyrightCopyright Sharon Perez, Graduate Fellow, Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder, 2003.

Figure 4. Label the tubes for set up of the wet scrubber model. Place the "in" tubes below the water line and the "out" tubes above the water line. Use duct tape to completely seal the connections.CopyrightCopyright Sharon Perez, Graduate Fellow, Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder, 2003.

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.)

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.

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

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.

Foresman, Scott. Science Insights – Exploring Matter and Energy. Reading, MA: Addison Wesley, 1999.

Markle, Sandra. The Kids' Earth Handbook. Atheneum, NY: John Wiley & Sons, Inc, 1991.