Reading passage Grade 11 English Language Arts
Keeping a Chlorine Residual at Marden Creek: Water Chemistry
A grade 11 informational passage on chlorine demand and residual, using a fictional plant and a sampling table students interpret to judge safety.
Objectives
- Students will explain what a chlorine residual is and why systems maintain one throughout distribution.
- Students will integrate a sampling table with the passage to describe how residual changes with distance.
- Students will apply the dose, demand, and residual relationship to a new situation.
Vocabulary
- Chlorine residual
- Chlorine residual is the small amount of chlorine still left in the water after it leaves the treatment plant. It keeps protecting the water from germs all the way to your faucet.
- Disinfection
- Disinfection is the step that kills or disables germs in water so it is safe to drink. Water plants usually disinfect with chlorine, ultraviolet light, or ozone.
- Distribution system
- A distribution system is the network of pipes, pumps, valves, and storage tanks that carries treated water from a treatment plant to homes, schools, and businesses. Most of it is buried underground beneath streets.
Also: chlorine demand
Teacher overview
Disinfection does not end when treated water leaves the plant; it has to keep working through miles of pipe on the way to a faucet. This original passage explains how the fictional Marden Creek Water Treatment Plant maintains a chlorine residual so the water stays protected all the way to the system's far ends, and it gives the simple relationship operators use to reason about it. A sampling table lets students track how the residual falls with distance and judge whether the far end stayed safe, so the reading connects a chemistry idea to real monitoring data.
Passage
Keeping a Chlorine Residual at Marden Creek
When the fictional Marden Creek Water Treatment Plant adds chlorine to kill germs, not all of that chlorine is used up at once. Some reacts immediately with material in the water, and some stays behind, ready to keep disinfecting. The chlorine that remains is called the residual. Operators want a residual to last all the way through the distribution system, so that water sitting in a pipe far from the plant is still protected against germs that might enter through a leak or repair.
The plant reasons about this with one relationship. The chlorine demand is the amount of chlorine used up reacting with material in the water, and it equals the dose the plant adds minus the residual that is left. In plain form, dose minus residual equals demand. If the plant adds a dose of 2.0 milligrams per liter and measures a residual of 1.2 milligrams per liter as the water leaves, then 0.8 milligrams per liter of chlorine was consumed meeting the demand right at the plant.
Chlorine keeps reacting as water travels. The longer water sits in the pipes and the more material it meets, the more the residual falls. That is why operators sample the residual at points across the system, not just at the plant. Federal rules cap how much disinfectant may be present, setting a maximum residual for chlorine of 4.0 milligrams per liter, and systems usually run a residual somewhere between 0.2 and 2.0 milligrams per liter in the distribution pipes. The goal is a residual high enough to stay detectable at the far ends, yet not higher than it needs to be.
The table below shows one day's residual readings at four points in the Marden Creek system, from the plant to the end of the line.
| Sampling point | Distance from plant (miles) | Chlorine residual (mg/L) |
|---|---|---|
| At the plant | 0 | 1.2 |
| Oak District | 2 | 0.9 |
| Riverside | 5 | 0.6 |
| End of the line | 8 | 0.3 |
The pattern is steady: the residual drops as the water travels, from 1.2 milligrams per liter at the plant to 0.3 milligrams per liter at the end of the line. Even the farthest point still holds a detectable residual, so the whole system stayed protected that day. If a far-end reading ever fell toward zero, the operator would know the water there was no longer guarded and would raise the dose at the plant so a residual could survive the trip. Although the chemistry cannot be seen at the tap, the residual is what keeps water protected all the way to the last house on the line.
Questions
According to the passage, what is a chlorine residual?
State the relationship the passage gives among chlorine dose, residual, and demand.
Using the table, describe what happens to the chlorine residual as the water moves farther from the plant.
The passage says the goal is a residual high enough to stay detectable at the far ends. Using the table and the passage together, did the end of the line stay protected that day? Explain how you know.
The passage uses the term "demand." Using context, what does chlorine demand mean here?
Why does the author include the sampling-point table rather than only describing the residual in sentences? Explain what the table lets a reader see.
Based on the passage, if the plant kept the same dose but the pipes were older and dirtier, with more material for chlorine to react with, what would the passage predict about the far-end residual, and what might the operator do?
Writing task. In one paragraph, explain the relationship among the dose the plant adds, the demand the water places on it, and the residual measured downstream. Use at least two figures from the table to support your explanation.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Answer: A chlorine residual is the chlorine that remains in the water after some is used up reacting with material in it, staying behind ready to keep disinfecting as the water travels.
Answer: Dose minus residual equals demand; the chlorine demand is the amount used up reacting with material in the water, equal to the dose the plant adds minus the residual left over.
Answer: The residual falls steadily with distance, from 1.2 milligrams per liter at the plant to 0.9 at Oak District, 0.6 at Riverside, and 0.3 milligrams per liter at the end of the line.
Answer: Yes. The end of the line still read 0.3 milligrams per liter, a detectable residual within the usual 0.2 to 2.0 range, so the passage's goal of a residual reaching the far ends was met and the whole system stayed protected that day.
Answer: Chlorine demand is the amount of chlorine consumed reacting with material in the water, which the passage defines as the dose added minus the residual that remains.
Answer: The table lets a reader see the residual actually falling point by point and confirm that even the farthest reading stayed detectable, giving evidence for the passage's claim that the system stayed protected, which prose alone would only assert.
Answer: The passage would predict a lower far-end residual, because more material means more chlorine consumed as demand along the way. To keep a residual reaching the far ends, the operator would likely raise the dose at the plant.
Writing task exemplar. At Marden Creek the plant adds a chlorine dose, some of which is consumed meeting the water's demand while the rest remains as a residual that keeps disinfecting downstream. The passage's example shows a dose of 2.0 milligrams per liter leaving a residual of 1.2 milligrams per liter at the plant, meaning 0.8 milligrams per liter met the demand there. As the water travels, more chlorine reacts, so the residual falls to 0.6 milligrams per liter at Riverside and 0.3 milligrams per liter at the end of the line. Because that final reading is still detectable, the dose was high enough to meet the demand along the whole route and keep the far end protected.
Differentiation
Support
Provide the dose-minus-residual relationship on a reference card with the plant example worked out, for students to use on questions 2 and 7. Allow question 1 to be answered by locating and restating the sentence that defines the residual.
Extension
Ask students to explain, using only the passage, why a system would not simply add a very large chlorine dose to guarantee a strong residual everywhere, drawing on the passage's note that federal rules cap the maximum residual.
ELL supports
Pre-teach "residual," "demand," and "disinfection" with a simple drawing: a full cup of chlorine, part poured out as "used up" and the rest labeled "residual." Allow students to answer the literal questions (1, 3, 5) with short phrases before writing full sentences.