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Lesson plan Grade 8 Science

Marston Bay Water Plant: Chlorine Dose and Filter Loading

A 45-minute grade 8 lesson where students find chlorine demand and filter loading rate at a fictional plant and read loading rate as a proportional relationship.

Objectives

  1. Students will calculate chlorine demand from a dose and a measured residual.
  2. Students will calculate a filter loading rate and interpret it as a proportional relationship between flow and rate.

Vocabulary

Key words: chlorine demand, residual, filter loading rate, proportional

Teacher overview

A treatment plant adds chlorine to kill germs, but it has to add enough to leave a small amount in the pipes without going over the legal limit. This lesson uses the fictional Marston Bay Water Plant to practice two operator calculations: chlorine demand and filter loading rate. A common misconception is that the dose and the residual are the same thing; the lesson keeps them separate, since demand is exactly the gap between what is added and what is left.

Procedure

Hook (5 min): Ask students why tap water sometimes smells faintly of chlorine, and why a plant would want a little chlorine to remain all the way to the last house. Explain that operators add a dose, some chlorine gets used up, and the rest is the residual.

Direct instruction (12 min): Define chlorine demand as dose minus residual. Suppose the fictional plant doses 3.0 mg/L and later measures a 1.2 mg/L residual; then demand is 3.0 − 1.2 = 1.8 mg/L. State the rule that the residual must stay at or below the maximum of 4.0 mg/L. Next, define filter loading rate as flow divided by filter area, and note that conventional filters usually run between 2 and 6 gallons per minute per square foot. Model one loading-rate calculation.

Guided practice (12 min): Hand out the Student Handout. In pairs, students complete items 1 through 3 on chlorine demand and dose. Circulate and check that pairs subtract in the correct order for demand.

Independent practice (11 min): Students complete items 4 through 6 on filter loading rate on their own. Remind them to divide flow by area and to keep the unit "gallons per minute per square foot."

Closure and exit ticket (5 min): Students complete item 7 as an exit ticket, stating the constant that connects flow to loading rate on the plant's filter. Collect and scan for students who correctly identify the slope as one divided by the filter area.

Marston Bay Water Plant: Chlorine Dose and Filter Loading — Grade 8 Science
Name: ____________________________ Date: ______________

Student Handout

The Marston Bay Water Plant is a fictional plant. Answer each item and keep units attached.

  1. The plant adds a chlorine dose of 2.5 mg/L. After the water sits, the measured residual is 0.8 mg/L. Using chlorine demand = dose − residual, find the chlorine demand.
  2. The plant wants to leave a residual of 1.0 mg/L. If the demand stays 1.7 mg/L, what dose is needed? Use dose = demand + residual.
  3. The maximum residual allowed is 4.0 mg/L. The plant is leaving a 1.0 mg/L residual. Is that within the limit, and by how much margin?
  4. The plant's filter has a surface area of 300 square feet. Water flows onto it at 900 gallons per minute. Using loading rate = flow ÷ area, find the loading rate in gallons per minute per square foot.
  5. A conventional filter should run between 2 and 6 gallons per minute per square foot. Is the rate from item 4 within that range?
  6. If the flow doubles to 1,800 gallons per minute on the same 300-square-foot filter, what is the new loading rate? Is it still within the 2-to-6 range?
  7. Exit ticket: Loading rate equals flow divided by 300 on this filter. What is the constant of proportionality (the slope) that connects loading rate to flow?

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: 1.7 mg/L.
    Demand = dose − residual = 2.5 − 0.8 = 1.7 mg/L.
  2. Answer: 2.7 mg/L.
    Dose = demand + residual = 1.7 + 1.0 = 2.7 mg/L.
  3. Answer: Yes, with a 3.0 mg/L margin.
    A 1.0 mg/L residual is below the 4.0 mg/L limit, and 4.0 − 1.0 = 3.0 mg/L of margin.
  4. Answer: 3.0 gallons per minute per square foot.
    900 ÷ 300 = 3.0 gallons per minute per square foot.
  5. Answer: Yes.
    3.0 is between 2 and 6, so the filter is within the normal range.
  6. Answer: 6.0 gallons per minute per square foot, still within range.
    1,800 ÷ 300 = 6.0, which is at the top of the 2-to-6 range but still within it.
  7. Answer: One divided by 300.
    Loading rate = (1 ÷ 300) × flow, so the slope is one over the filter area, about 0.0033 per square foot.

Differentiation

Support

Provide a filled-in example (a different dose and residual) worked line by line before item 1, and a labeled diagram showing dose entering, chlorine being used up, and residual remaining. Allow calculator use for the division in items 4 through 6.

Extension

Ask students to graph loading rate against flow for the 300-square-foot filter at flows of 300, 600, 900, and 1,200 gallons per minute, then confirm that the slope matches the constant from item 7. Have them explain what would happen to the slope if the plant built a larger filter.

ELL supports

Pre-teach "dose," "residual," and "loading rate" with simple definitions and a picture. Provide sentence frames such as "Demand equals ___ minus ___." Allow students to answer with the setup and result rather than full sentences, and pair the words "flow" and "area" with arrows on a simple filter sketch.