Lesson One: Chlorination and Dechlorination

Grades: 9-12 (Science, Chemistry and Biology)
Time: 60 minutes
Space Requirement: Classroom with sink and ample counter space
Methodology: Hands-on and Cooperative Learning, Class Discussion

Materials not included in the kit:

For each group: Empty well-rinsed 2L plastic bottle, a watch or timer, tape and a marker

Materials included in the kit that will be used:

For each group: Total Chlorine test strips, 2mL of granular activated carbon, empty 5mL vial, plastic cup

Objectives: The students will gain a basic understanding of the role of water treatment facilities. Students will learn about the importance of eliminating biological activity in drinking water. Students will learn about chlorination as a means of disinfecting water. Students will learn about the concept of dechlorinating water and one of the ways this can be done.


  1. Before the class do a little bit of research on your local water treatment facility to find a few basic facts. For example: location of the facility, source of raw water, how much water is treated per day, per year, does it run 24 hours per day, etc.?
  2. Present any interesting facts you were able to find on the local treatment facility to the class.
  3. Introduce the kits to the students and explain what is going to be happening.
    a. Let them know that for the next several class periods they will be doing experiments and learning about different aspects of the water treatment process.
    b. There will be lessons on chlorination, ammonia and chloramine and the uses and problems associated with each, natural reduction and oxidation of iron and ammonia, and ways people can put bacteria to work for them, specifically in biological filters.
    c. There will be experiments and activities in many of the classes which build upon each other and culminate in them getting to filter a sample of simulated well water that they produced themselves.
    d. Mention any class speakers or related field trips you may be able to arrange.
  4. Distribute printed copies of the Lesson One Handout to the class. In addition to this handout you can distribute printed copies of the Test Procedures handout and the Glossary handout; these two are relevant to, and will be used during, each of the lessons in OWB.
  5. Read through the Lesson One Handout with the class until you reach the Lesson One activity and then have each student carefully read the instructions.
  6. The class will need to be split up into groups and assigned to work areas. Materials for up to six groups are provided. Each group would ideally have 3-5 students.
  7. Separate the materials listed for this activity from the rest of the kit and have someone from each group collect what they need.
  8. Each group will work through the dechlorination procedure.
  9. Have the students set their dechlorination bottles aside (be sure that they are labeled in some way to know which bottles belong to which group) and clean up.
  10. The plastic cups and 5mL vials will be reused in upcoming lessons, remind students not to throw these away and try to provide a safe place for them to store them until the next lesson.

Evaluation: Students can be evaluated based on their participation in both the activity and the discussion afterward.

For the Teacher:

Calibrating another container to be used as a beaker The container should be calibrated with 100mL, 200mL and 250mL marks using a graduated cylinder. You can do this by filling a graduated cylinder with 100mL of water then pouring this water into your other container and marking the water level with a marker. Pour this water out and fill the graduated cylinder with 200mL of water. Pour this water into your other container and mark the 200mL level. Repeat this once more for 250mL.

Related Links: 

What is Chlorination? Fact Sheet

What is the Purpose of Drinking Water Quality Guidelines/Regulations? Fact Sheet

Operation Water Biology

Lesson One

Chlorination and Dechlorination

More information about Guidelines for Canadian Drinking Water Quality can be found at: www.safewater.org/fact-sheets-1/2017/1/23/what-is-chlorination

The purpose of a water treatment plant is to take raw water from a well or fresh water source, remove all of the contaminants and make the water safe to drink. The contaminants that should be removed include bacteria, ammonia, phosphorus, nitrogen, dissolved organic material, iron, manganese, arsenic and many more. Health Canada is a government organization which sets guidelines on the maximum level of different contaminants that can be present in the treated drinking water. As long as the treatment plant takes out enough contaminants to meet these guidelines, the water is assumed to be safe to drink. There are guidelines for levels of iron, arsenic and nitrate because these things may have direct aesthetic or health consequences, as we will discuss later. In Canada, there are no guidelines for ammonia or phosphorus because it is not usually dangerous to drink water with naturally occurring levels of these chemicals. The guidelines were set by looking only at the direct effect each contaminant could have on someone drinking the water. We now have more information about the way that some contaminants interact with each other and other consequences of their presence. If you look at the whole picture it seems that the contaminants that do not have guidelines should have them and the guidelines that are already in place are not strict enough. The experiments and lessons in this kit will use iron, ammonia and chlorine as examples to explain why this is a problem and show how it can be solved.

In this lesson we will begin talking about chlorine. When most people think about chlorine the first thing that comes to mind is the smell of swimming pools. Although you probably cannot smell the chlorine in your tap water it is added to most municipal drinking water supplies as part of the treatment process.

More information on chlorination can be found at: www.safewater.org/fact-sheets-1/2017/1/23/what-is-chlorination

Much like in a swimming pool, chlorine is added to tap water to disinfect it. This is usually done as the last stage of water treatment after the water has been filtered in other ways. Chlorine can kill microorganisms like bacteria and viruses which could cause a wide range of diseases and health problems. Chlorine also reacts with, and removes, dissolved chemicals such as ammonia. Some of the chlorine gets used up every time it removes a contaminant from the water and these reactions produce small amounts of other chemicals called chlorine byproducts. These byproducts can sometimes be dangerous if they build up in large amounts. This can happen if a lot of chlorine is added to water that has not been filtered well and still has a lot of contaminants in it.

Chlorine is useful to treat water because it reacts with so many different things and killing microorganisms is very important but if you want to study chemical reactions in water the chlorine there can sometimes get in the way. In a future lesson we will want to use some water that does not have any chlorine in it so today we will begin the process of taking the chlorine out of some tap water. This process is called dechlorination.

This experiment will show how granular activated carbon (GAC) can remove gases such as chlorine from water. The GAC has a very large surface area and is full of tiny holes. This allows it to grab and hold gases in the same way that a sponge holds liquids.

Materials needed for this experiment are:

One total chlorine test strip
Plastic cup
2mL of Granular Activated Carbon (GAC)
Empty 5mL vial
Empty, well rinsed, 2L plastic bottle
Watch or timer
Tape and a marker

1. To know how much chlorine gets taken out of your water by the GAC you will need to know how much is in it at the beginning. To find this, fill a cup about halfway with tap water then follow the instructions in the total chlorine concentration test procedure from the test procedure sheet. Be sure to keep the cup. You can rinse it out and use it again in the next few experiments.

Total chlorine concentration of tap water:


2. Using the 5mL vial, measure out 2mL of GAC. When pouring GAC from one container to another you should do it over a clean piece of white paper so that any you spill can be poured back into its original container.

3. Put your 2mL of GAC into the empty 2L plastic bottle and then fill it up with tap water and put on the cap. The 5mL vial needs to be rinsed out right away to get the dust from the GAC out of it. This vial will be used in future lessons so it must be clean.

4. Swirl the water around so that the GAC spreads throughout the water. The GAC will settle to the bottom but you want it to be exposed to as much of the water as possible so hold the bottle upside down for a few seconds so that the GAC falls through the water then turn it right side up so it falls through again.

5. Label your bottle so it does not get mixed up with those of other groups.

6. You will open the bottle and test the chlorine concentration again in a few days after the GAC has had enough time to absorb the chlorine.