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Lesson plan Grade 8 Science

Cliffmere Water Plant: Modeling the Multi-Barrier Process

A 55-minute grade 8 science lesson where students track turbidity through a fictional treatment plant and solve a linear equation for the chlorine dose.

Objectives

  1. Students will describe how coagulation, filtration, and disinfection each reduce a different contaminant.
  2. Students will solve a linear equation to find the chlorine dose that produces a target residual.

Vocabulary

Turbidity
Turbidity is a measure of how cloudy or murky water looks because of tiny particles floating in it. Clear water has low turbidity, and muddy water has high turbidity.
Coagulation
Coagulation is the first cleaning step at a drinking water plant. Operators quickly mix in a chemical that makes tiny bits of dirt stop pushing each other away so they can start sticking together.
Filtration
Filtration is the step where water passes through layers of sand, gravel, or other materials that trap the small particles still left in it. It works like a very fine strainer.
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.
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.

Teacher overview

A water treatment plant does not clean water in one step; it uses a series of barriers, each aimed at a different problem. This lesson follows raw water through the fictional Cliffmere Water Plant as coagulation and settling, then filtration, then disinfection each do part of the job. Students track turbidity falling stage by stage and solve a linear equation for the chlorine dose. A common misconception this lesson addresses is that adding more chlorine always means more chlorine left in the water; students learn that the water first exerts a demand, and only the dose beyond that demand remains as residual.

Procedure

Hook (5 min): Show, or describe, a jar of cloudy water and a glass of clear tap water. Ask what has to happen to turn one into the other, and list student ideas on the board to revisit at the end.

Direct instruction (12 min): Walk through the fictional Cliffmere Water Plant's three barriers. Coagulation and settling clump fine particles so they sink; filtration strains out what remains; disinfection adds chlorine to kill germs. Introduce turbidity as a cloudiness measure in NTU, and note that finished water must reach 0.3 NTU in nearly all samples. Then introduce the chlorine idea: residual = dose − demand, where demand is the chlorine the water uses up.

Guided practice (13 min): As a class, track the plant's turbidity: suppose raw water enters at 12 NTU, drops to 4 NTU after coagulation and settling, then to 0.2 NTU after filtration. Have students compute the percent removed by the first step together. Then model solving residual = dose − demand for a sample demand.

Independent practice (20 min): Students complete the Student Handout, finding percent turbidity removed and solving for chlorine dose. Circulate and check that students subtract in the right order and rearrange the equation to isolate the dose.

Closure and exit ticket (5 min): As a formative check, students answer on a card: "If the water has a chlorine demand of 2.0 mg/L and the plant wants a 1.0 mg/L residual, what dose is needed?" Confirm that at least 80 percent set up dose = demand + residual and answer 3.0 mg/L before dismissal.

Cliffmere Water Plant: Modeling the Multi-Barrier Process — Grade 8 Science
Name: ____________________________ Date: ______________

Student Handout

Suppose the fictional Cliffmere Water Plant treats water that enters at a turbidity of 12 NTU. Use residual = dose − demand for the chlorine items, where every value is in mg/L.

  1. After coagulation and settling, turbidity drops from 12 NTU to 4 NTU. What percent of the turbidity was removed in that step?
  2. After filtration, the turbidity is 0.2 NTU. Does the finished water meet the 0.3 NTU standard?
  3. Overall, turbidity fell from 12 NTU to 0.2 NTU. What percent was removed from raw water to finished water?
  4. The water has a chlorine demand of 1.8 mg/L, and operators want a residual of 1.0 mg/L. Write and solve residual = dose − demand for the dose.
  5. The maximum residual disinfectant level for chlorine is 4.0 mg/L. Is the 1.0 mg/L residual in item 4 within that limit, and why does the limit apply to the residual rather than the dose?
  6. Suppose the demand rose to 2.5 mg/L and the plant still wanted a 1.0 mg/L residual. What dose is needed now?
  7. Explain why the plant uses three barriers in a row instead of relying on chlorine alone.

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: 67 %.
    (12 − 4) ÷ 12 × 100 = 8 ÷ 12 × 100 = 66.7 percent, about 67 percent removed.
  2. Answer: Yes.
    The finished turbidity of 0.2 NTU is at or below the 0.3 NTU standard, so it meets the limit.
  3. Answer: 98 %.
    (12 − 0.2) ÷ 12 × 100 = 11.8 ÷ 12 × 100 = 98.3 percent, about 98 percent removed overall.
  4. Answer: 2.8 mg/L.
    1.0 = dose − 1.8, so dose = 1.0 + 1.8 = 2.8 mg/L.
  5. Answer: Yes.
    The MRDL limits the chlorine left in the water, and the 1.0 mg/L residual is below 4.0 mg/L. The limit does not measure the 2.8 mg/L dose, because 1.8 mg/L of that dose is used up by demand before it reaches customers.
  6. Answer: 3.5 mg/L.
    1.0 = dose − 2.5, so dose = 1.0 + 2.5 = 3.5 mg/L.
  7. Answer: Each barrier removes something the others miss.
    Coagulation and filtration take out particles that chlorine cannot, and clear water lets disinfection work better; relying on one barrier would leave contaminants that a later barrier is meant to catch. Multiple barriers also protect the supply if one step underperforms.

Differentiation

Support

Provide the percent-removed formula worked on one example and give the chlorine equation already rearranged to dose = demand + residual, so students substitute rather than rearrange. Allow calculators throughout.

Extension

Ask students to find the finished-water turbidity that would be exactly 95 percent below the raw 12 NTU, and to explain whether that value would still meet the 0.3 NTU standard, justifying with their calculation.

ELL supports

Pre-teach "turbidity," "coagulation," "filtration," and "disinfection" with a left-to-right diagram of the three barriers labeled with a simple icon each. Provide sentence frames for item 7, such as "The plant uses ___ barriers because ___."