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Lesson plan Grade 7 Science

Blackwood Springs: Tracking Chlorine Demand in Distribution

A grade 7 lesson calculating chlorine demand at two points in a fictional plant's distribution system, checked against the real federal maximum residual limit.

Objectives

  1. Students will calculate chlorine demand from dosage and residual measurements at two points in a distribution system.
  2. Students will explain why water utilities maintain a chlorine residual throughout the distribution system.
  3. Students will check a plant's chlorine dose and residual readings against the typical target range and the federal maximum residual disinfectant level.

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.
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.
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.

Also: chlorine dose, chlorine demand

Teacher overview

Tap water sometimes carries a faint chlorine smell, strongest close to the treatment plant, because utilities add chlorine to disinfect water and then keep a small residual moving through the pipes to protect it all the way to the tap. This lesson uses a fictional plant, Blackwood Springs, and two sample points in its distribution system to teach the chlorine-demand formula: demand equals dose minus residual. A common misconception is that chlorine simply "runs out" evenly over time; in fact, chlorine reacts with organic matter and pipe biofilm along the way, so demand builds up gradually with distance from the plant, which is why residual is lowest at the farthest points in the system.

Procedure

Hook (5 min): Ask students whether they have ever noticed a chlorine smell in tap water, and whether it seemed stronger or weaker depending on where they were. Record responses without comment.

Direct instruction (12 min): Explain that chlorination disinfects water by killing pathogens, and that as chlorinated water travels through pipes, it reacts with organic matter and biofilm on pipe walls, consuming some of the chlorine. This consumed amount is called demand, and what remains is called residual: Demand = Dose − Residual. Introduce the federal maximum residual disinfectant level (MRDL) of 4.0 mg/L and the typical in-distribution target range of 0.2 to 2.0 mg/L. Present Blackwood Springs' fictional data: the plant doses water with 3.0 mg/L of chlorine at the start of the system; a mid-town sample point shows a residual of 1.8 mg/L; a far-edge-of-town sample point shows a residual of 0.6 mg/L.

Guided practice (12 min): In pairs, students calculate the chlorine demand at the mid-town point (3.0 − 1.8 = 1.2 mg/L) and the far-edge point (3.0 − 0.6 = 2.4 mg/L), and compare both residuals (1.8 mg/L and 0.6 mg/L) to the typical 0.2–2.0 mg/L target range and to the 4.0 mg/L MRDL. Ask pairs to explain why demand is larger at the far-edge point than at the mid-town point.

Independent practice (8 min): Each student answers two items on the handout: (1) if a fourth sample point even farther from the plant showed a residual of 0.5 mg/L, what would the demand be at that point; and (2) is 3.0 mg/L an acceptable starting dose given the 4.0 mg/L MRDL, and by how much room does it stay under that limit.

Closure / exit ticket (8 min): Students answer one question on an index card: "A fifth sample point shows a residual of 0.1 mg/L. Is that within the typical 0.2–2.0 mg/L target range? What might a reading that low mean for water safety that far from the plant?" Collect and scan for correct comparison to the range before dismissal.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 1.2 mg/L.
    Mid-town demand: 3.0 mg/L dose − 1.8 mg/L residual = 1.2 mg/L of demand.
  2. Answer: 2.4 mg/L.
    Far-edge demand: 3.0 mg/L dose − 0.6 mg/L residual = 2.4 mg/L of demand, larger than the mid-town figure because the water has traveled farther and reacted with more organic matter and biofilm along the way.
  3. Answer: 2.5 mg/L.
    Independent practice item 1: 3.0 mg/L dose − 0.5 mg/L residual = 2.5 mg/L of demand at the fourth point.
  4. Answer: yes.
    Independent practice item 2: 3.0 mg/L is below the 4.0 mg/L MRDL, with 4.0 − 3.0 = 1.0 mg/L of room remaining before the limit would be reached.
  5. Exit ticket answer: no.
    0.1 mg/L is below the typical 0.2 mg/L minimum of the target range. A residual that low that far from the plant could mean the water has too little disinfectant protection left, which utilities treat as a signal to investigate and possibly add a booster chlorination station in that part of the system.

Differentiation

Support (below grade level)

Provide the chlorine-demand formula as a labeled arrow rule ("dose minus residual equals demand") on a printed strip, and complete the mid-town calculation together as a class before pairs attempt the far-edge calculation independently. Allow a number line marked in tenths of a mg/L to support the subtraction.

Extension (above grade level)

Ask students to calculate what residual Blackwood Springs would need at the far edge of town to bring demand there down to 2.0 mg/L, assuming the dose stays at 3.0 mg/L, and to discuss one real reason a utility might add a booster chlorination station partway through a large distribution system.

ELL supports

Pre-teach "dose," "residual," and "demand" with a simple diagram of water leaving the plant at full strength and gradually losing chlorine as it travels through pipes toward homes farther away. Allow the guided-practice explanation of why demand is larger at the far-edge point to be given orally rather than in writing.