Debate kit Grade 10 Science
Should Cardington Switch to Chloramine Disinfection?
A grade 10 science debate kit on converting a city's distribution system from free chlorine to chloramine, with two-sided evidence on byproducts, residual, and risk.
Objectives
- Students will weigh the byproduct and residual trade-offs between free chlorine and chloramine.
- Students will build an argument on the resolution supported by named evidence.
- Students will rebut the opposing team's strongest point with evidence rather than assertion.
Vocabulary
- Disinfection byproduct (DBP)
- A disinfection byproduct is a chemical that can form when disinfectants like chlorine react with natural organic matter in water. Treatment plants work to control disinfection byproducts because some of them can be harmful if people are exposed to too much over a long time.
Also: chloramine, free chlorine, residual, nitrification
Teacher overview
This is a two-team structured debate that runs across two class periods, with one period for preparation and one for the rounds. Students argue whether the fictional city of Cardington should convert its distribution system from free chlorine to chloramine. The question is genuinely contested among water chemists, because chloramine lowers one set of health risks while raising another, and the trade-off depends on the pipes a particular city happens to own.
Resolution & Background
Resolution: The fictional Cardington Water Department should convert its distribution system from free chlorine to chloramine.
Background: Utilities that treat river or lake water must keep a disinfectant residual in their pipes, and most other utilities do as well, so that bacteria cannot regrow between the plant and the tap. Cardington now uses free chlorine. Free chlorine is a strong, fast disinfectant. It also reacts with natural organic matter from decayed leaves and soil in the source water. Those reactions form disinfection byproducts, a family of regulated compounds that includes trihalomethanes. Chloramine is made by adding ammonia to chlorinated water. It is a weaker disinfectant, yet it lasts much longer in a pipe and forms far less of that byproduct family.
Cardington's system is a hard case. The fictional city has long mains that reach a hilltop tank at the edge of town, and the water sitting in that tank sometimes loses its residual before it reaches the last customers. The city also has an unknown number of old lead service lines still in the ground. Nationally, EPA estimates that about 4 million lead service lines remain in service. Changing a disinfectant changes the water chemistry that keeps those pipes coated and stable.
Roles: Two teams of four argue the resolution. One student serves as moderator and keeps time. Two students serve as judges, score with the rubric below, and ask one question of each team after the rebuttals.
Evidence File
Case for converting to chloramine:
- Chloramine reacts with natural organic matter far more slowly than free chlorine does, so a system that switches usually forms much less of the trihalomethane family of byproducts. (Source: standard drinking-water treatment practice described in state operator training materials.)
- Chloramine holds a residual much longer than free chlorine. Systems commonly aim for a residual between 0.2 and 2.0 mg/L throughout the distribution system, and the fictional Cardington system cannot hold that range at its hilltop tank today. (Source: EPA Stage 1 Disinfectants and Disinfection Byproducts Rule.)
- Customers often report less of the chlorine taste and smell after a switch, because free chlorine is the more volatile of the two. (Source: standard drinking-water treatment practice described in state operator training materials.)
- A fictional Cardington engineering memo written for this debate estimates that adding carbon filters to strip organic matter before chlorination would cost several times what a chloramine conversion would cost. (Illustrative estimate created for this debate, not a documented figure.)
Case against converting to chloramine:
- Chloramine is the weaker disinfectant. A plant needs a longer contact time to inactivate the same pathogens, so an operator error that shortens contact time matters more. (Source: standard drinking-water treatment practice described in state operator training materials.)
- Chloramine carries ammonia into the pipes. In warm tanks and dead-end mains, bacteria convert that ammonia to nitrite. The reaction eats the residual and can drop the pH, a problem called nitrification. (Source: standard drinking-water treatment practice described in state operator training materials.)
- Changing the disinfectant changes the chemistry at the pipe wall. In a city with old lead service lines, that shift can disturb the protective scale and release lead. EPA estimates that about 4 million lead service lines remain in US systems. (Source: EPA, 2025 update to the Drinking Water Infrastructure Needs Survey and Assessment.)
- Chloramine must be removed before water is used in kidney dialysis and before it goes into a fish tank, and it does not evaporate out of a bucket the way free chlorine does. Clinics and pet owners must change their equipment. (Source: standard drinking-water treatment practice described in state operator training materials.)
- A residents' group invented for this debate argues that the fictional city should spend first on flushing and tank turnover, on the grounds that a stagnant tank is the real cause of its residual problem. (Illustrative testimony created for this debate.)
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Rubric
| Criterion | Emerging | Developing | Proficient | Advanced |
|---|---|---|---|---|
| Use of chemistry evidence | Asserts a position with no evidence from the file | Names one piece of evidence without explaining the chemistry behind it | Names at least two pieces of evidence and explains the chemistry each depends on | Names multiple pieces of evidence, explains the chemistry, and shows which piece carries the most weight |
| Handling of trade-offs | Treats the resolution as having one obvious answer | Mentions a trade-off but does not weigh it | States the trade-off between byproducts and residual strength and weighs it against a stated criterion | Weighs several trade-offs against prioritized criteria and names the condition that would change the team's answer |
| Rebuttal quality | Does not respond to the other team | Responds to a point without using evidence | Rebuts a specific opposing point using evidence from the file | Rebuts several opposing points and explains why the team's evidence outweighs them |
| Organization and delivery | Hard to follow; no visible structure | Structure present but transitions unclear | Clear claim, evidence, and conclusion in each speech | Clear structure, confident delivery, and direct answers to the judges' questions |
Differentiation
Support
Give each team a one-page card listing the single strongest point on their side and the sentence frame "Our opponents claim ___, but the evidence shows ___." Allow the first speech to be read from a written script.
Extension
Ask students to identify what measurement Cardington should collect before deciding, and to explain which side's argument that measurement would test. Have them write a short paragraph describing how the utility would report the result to residents.
ELL supports
Pre-teach "residual," "byproduct," "nitrification," and "service line" with a labeled diagram of water traveling from the plant through a main to a tank and a house. Allow students to deliver rebuttals from written notes and to use their home language while preparing with a partner.