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Lesson plan Grade 11 Engineering & Design

Decomposing a Treatment Train: A Merrow River Design Lesson

A grade 11 engineering lesson where students break the problem of treating fictional Merrow River water into stages and size each stage against its own goal.

Objectives

  1. Students will break a full drinking-water treatment problem into four sequential stages, each with its own goal.
  2. Students will calculate a removal efficiency and use it to judge whether a treatment stage meets its target.

Vocabulary

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.
Sedimentation
Sedimentation is the step where water sits almost still in a big basin so the heavy clumps of dirt can sink to the bottom. The cleaner water on top then moves on to the filters.
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.
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.

Also: treatment train, removal efficiency

Teacher overview

Engineers rarely design a treatment plant as one giant problem; they break it into a sequence of stages, where each stage solves one piece and hands cleaner water to the next. This lesson uses a fictional river source, the Merrow River, to make that decomposition concrete: students specify what the finished water must achieve, then assign each requirement to the stage that handles it. The lesson is quantitative because students calculate a removal efficiency to judge whether a stage meets its goal. A common misconception is that filtration alone cleans the water; in practice, coagulation and sedimentation remove most of the load first, and the filters would clog almost immediately without them.

Procedure

Hook (5 min): Project a photo of cloudy river water beside a glass of tap water and ask students to list, in pairs, everything that must be removed or added to get from one to the other. Collect three or four answers on the board without sorting them yet.

Direct instruction (12 min): Introduce the conventional treatment train as four stages in order: coagulation, which clumps fine particles together; sedimentation, which lets those clumps settle out; filtration, which strains the remaining particles; and disinfection, which inactivates microbes. Explain that each stage targets a different part of the problem, which is exactly what the engineering practice of decomposition means. State the finished-water goal for turbidity: a conventional plant must keep finished water at or below 0.3 NTU in 95 percent of monthly samples.

Worked example (10 min): Present fictional Merrow River data. Raw water enters at 12 NTU of turbidity. After coagulation and sedimentation, the water measures 3 NTU. Model the removal-efficiency calculation on the board: removal efficiency = (influent − effluent) ÷ influent × 100 = (12 − 3) ÷ 12 × 100 = 75 percent. Point out that coagulation and sedimentation together removed three-quarters of the turbidity, which is why the filters that follow can reach the 0.3 NTU goal.

Guided practice (13 min): In pairs, students complete the stage-planning handout for Merrow River. For each of the four stages they write the sub-problem it solves and the measurable goal it targets, then they answer handout questions 1 and 2, calculating the removal efficiency first for the combined coagulation-and-sedimentation stage, which brings the raw water from 12 NTU down to 3 NTU, and then for the filtration stage, which brings the water from 3 NTU down to 0.2 NTU. Circulate and check that pairs are subtracting effluent from influent before dividing.

Independent practice (10 min): Each student answers handout questions 3 and 4: whether the finished 0.2 NTU water meets the 0.3 NTU goal, and which single stage they would strengthen first if the raw water arrived twice as cloudy, with a one-sentence justification.

Closure and exit ticket (5 min): Students turn in an index card naming the four stages in order and stating one reason the order cannot be rearranged. Scan for any card that places filtration before sedimentation, and address it with the whole class if it appears more than twice.

Decomposing a Treatment Train: A Merrow River Design Lesson — Grade 11 Engineering & Design
Name: ____________________________ Date: ______________

Student Handout

Merrow River stage-planning handout

Use these turbidity measurements from the fictional Merrow River throughout. The raw water enters the plant at 12 NTU. After coagulation and sedimentation it measures 3 NTU. After filtration it measures 0.2 NTU. The finished-water goal is a turbidity at or below 0.3 NTU. Removal efficiency = (influent − effluent) ÷ influent × 100.

For each of the four stages (coagulation, sedimentation, filtration, and disinfection), write the sub-problem it solves and the measurable goal it targets. Then answer the four questions below, showing your work on the two calculations.

  1. Calculate the removal efficiency of the combined coagulation-and-sedimentation stage, which brings the raw water from 12 NTU to 3 NTU.
  2. Calculate the removal efficiency of the filtration stage, which brings the water from 3 NTU to 0.2 NTU.
  3. Does the finished water at 0.2 NTU meet the 0.3 NTU turbidity goal? Answer yes or no, and give the comparison that supports your answer.
  4. If the raw water arrived twice as cloudy, which single stage would you strengthen first? Give a one-sentence justification.

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: 75 percent. The coagulation-and-sedimentation removal efficiency is (12 − 3) ÷ 12 × 100 = 75 percent, the worked example students reproduce.
  2. Answer: About 93 percent. The filtration stage brings water from 3 NTU to 0.2 NTU, so removal efficiency = (3 − 0.2) ÷ 3 × 100 = 2.8 ÷ 3 × 100 = 93.3 percent, about 93 percent.
  3. Answer: Yes. Finished water at 0.2 NTU is at or below the 0.3 NTU goal, so it meets the standard.
  4. Answer: Coagulation and sedimentation. Accept any answer that strengthens the early clarification stages first, since removing the bulk of the load before filtration is what keeps the filters from clogging; a justification pointing to protecting the filters or the disinfection step is the target reasoning.

Differentiation

Support

Provide the four stage names pre-printed on the handout so students match sub-problems and goals to given stages rather than recalling the sequence from memory, and give the removal-efficiency formula worked through once with a different pair of numbers.

Extension

Ask students to add a fifth consideration to the train for a source with a known chemical contaminant, name the added stage, and explain where in the sequence it belongs and why, referencing how its placement affects the stages around it.

ELL supports

Provide a labeled left-to-right diagram of the four stages with an icon for each, and a sentence frame for the exit ticket: "The order matters because ___ must happen before ___." Allow the guided-practice sub-problems to be recorded as short phrases rather than full sentences.