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Lesson plan Grade 12 Science

Corrosion Control at Rothbury: pH, Alkalinity, and Lime Dosing

A 55-minute grade 12 lesson on corrosion control: students calculate lime feed rates and explain how raising pH and alkalinity keeps lead and copper out of tap water.

Objectives

  1. Students will explain how raising pH and alkalinity controls corrosion and limits lead and copper release.
  2. Students will calculate lime feed rates in pounds per day using the pounds formula.
  3. Students will evaluate whether treated-water pH falls within the secondary drinking-water range.

Vocabulary

Corrosion control
Treatment that keeps water from slowly eating away at the inside of pipes. It helps stop metals like lead from getting into drinking water.
Alkalinity
Alkalinity is water's ability to resist becoming acidic, acting like a buffer that keeps pH from swinging wildly when acids are added. Water with good alkalinity helps protect fish and other aquatic life from sudden pH changes.
pH
pH is a number that tells whether water is acidic, like lemon juice, or basic, like baking soda mixed in water. Pure water sits in the middle of the scale at a pH of 7.

Also: lime dose, protective scale

Teacher overview

Water that is soft, acidic, and low in alkalinity is corrosive: it slowly dissolves the inside of the pipes it flows through, which can pull lead and copper into drinking water. The fictional Rothbury treatment plant controls this by adding lime to raise pH and alkalinity, forming a thin protective coating inside the pipes. This lesson connects that chemistry to the pounds-formula math operators use to set a lime feed rate. A common misconception is that adding more chemical is always better; overdosing lime pushes pH above the range where the water stays stable and clear, so operators aim for a target range rather than a maximum dose.

Procedure

Hook (6 min): Ask students what protects the inside of an old metal pipe from the water moving through it. Introduce the fictional Rothbury system, where the raw water is corrosive, and name the real stakes: when corrosion control lapsed in Flint, Michigan, lead leached from pipes into homes. Frame the day's question as how a plant keeps water from attacking its own pipes.

Direct instruction (14 min): Explain corrosion in plain chemical terms. Low pH and low alkalinity make water aggressive toward metal, so it dissolves lead and copper from pipes and solder. Adding lime raises pH and alkalinity, which lets a stable mineral scale form on the pipe wall and shields the metal. Distinguish pH, a measure of acidity, from alkalinity, the water's buffering capacity that keeps pH from swinging. Note the secondary drinking-water pH range of 6.5 to 8.5, a non-enforceable guideline for taste and appearance that Rothbury uses as its target window; some systems deliberately run above 8.5 for corrosion control.

Worked example (10 min): Model the pounds formula with an example separate from the handout so students learn the method, not the answers. For a plant treating 2.0 MGD at a 10 mg/L lime dose, compute the feed rate: 10 mg/L x 2.0 MGD x 8.34 = 167 pounds per day. Walk through the units so students see how milligrams per liter, MGD, and the 8.34 factor combine into pounds per day.

Independent handout (18 min): Students complete the handout individually, calculating Rothbury's lime feed at two flows, judging whether raw and treated pH fall in range, and explaining the chemistry that ties corrosion control to lead and copper.

Exit ticket and formative check (7 min): Pose one question: Rothbury's treated water reads pH 8.0; is it within the 6.5 to 8.5 range? Collect a thumbs response and a one-line reason. The check is met when at least 80 percent answer yes and reference the range.

Corrosion Control at Rothbury: pH, Alkalinity, and Lime Dosing — Grade 12 Science
Name: ____________________________ Date: ______________

Student Handout

Use these values for the fictional Rothbury treatment plant. Show your work.

  • Normal plant flow: 3.2 MGD
  • Target lime dose: 12 mg/L
  • Pounds formula: pounds per day = dose (mg/L) x flow (MGD) x 8.34
  • Secondary pH range: 6.5 to 8.5
  • Lime is delivered in 50-pound bags

  1. Calculate the lime feed rate in pounds per day for a 12 mg/L dose at the normal flow of 3.2 MGD.

  2. During high demand the flow rises to 4.5 MGD at the same 12 mg/L dose. Calculate the new lime feed rate in pounds per day.

  3. Rothbury's raw water has a pH of 6.3. Is that within the secondary range of 6.5 to 8.5? Answer yes or no.

  4. After lime addition, the treated water has a pH of 7.8. Is that within the secondary range of 6.5 to 8.5? Answer yes or no.

  5. At the normal feed rate from question 1, how many 50-pound bags of lime does the plant use per day? Round to one decimal place.

  6. Explain, in terms of corrosion control, why keeping the treated-water pH and alkalinity up helps prevent lead and copper from entering tap water.

  7. Explain the difference between pH and alkalinity, and why an operator watches both when setting the lime dose.

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: 320 pounds per day.
    12 mg/L x 3.2 MGD x 8.34 = 38.4 x 8.34 = 320.3, which rounds to 320 pounds per day.

  2. Answer: 450 pounds per day.
    12 mg/L x 4.5 MGD x 8.34 = 54 x 8.34 = 450.4, which rounds to 450 pounds per day.

  3. Answer: No.
    A pH of 6.3 is below the lower end of the 6.5 to 8.5 secondary range, so the raw water is slightly outside the range and corrosive.

  4. Answer: Yes.
    A pH of 7.8 falls between 6.5 and 8.5, so the treated water is within the secondary range.

  5. Answer: 6.4 bags.
    320 pounds per day / 50 pounds per bag = 6.4 bags per day.

  6. Answer: Higher pH and alkalinity let a protective scale form and stop metal from dissolving.
    Corrosive water dissolves lead and copper from pipes and solder. Raising pH and alkalinity lets a stable mineral coating build on the pipe wall, which separates the water from the metal, so far less lead and copper reach the tap.

  7. Answer: pH measures acidity; alkalinity is the buffering that keeps pH steady. pH tells how acidic or basic the water is at one moment, while alkalinity is the water's capacity to resist a change in pH. An operator watches both because water can sit at a good pH yet still swing if alkalinity is too low to hold it, which would let corrosion return.

Differentiation

Support

Provide the pounds formula on a printed card with the units labeled, and set up question 1 as 12 x 3.2 x 8.34 so students complete only the arithmetic. Give a two-column chart contrasting corrosive and protected pipe for questions 6 and 7.

Extension

Ask students to find the lime dose, in mg/L, that would push Rothbury's finished water to a pH near 8.5, and to explain why an operator would stop short of the top of the range rather than dose to the maximum.

ELL supports

Pre-teach "corrosion," "alkalinity," and "scale" with paired images of a bare pipe and a coated pipe. Provide sentence frames for questions 6 and 7 and allow labeled diagrams in place of full paragraphs.

Extensions

Have students read the corrosion-control section of a local utility's water quality report and note what treatment the utility lists for controlling lead and copper, then compare it with Rothbury's lime approach in a short paragraph.