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Lesson plan Grade 12 Careers & CTE

Wrentmere Corrosion Control: A Water Quality Specialist Lesson

Students take the role of a corrosion control specialist, calculating orthophosphate feed, reporting pH to meter precision, and ranking lead tap samples.

Objectives

  1. Students will calculate an orthophosphate feed rate in pounds per day and convert it to gallons per day of stock solution.
  2. Students will report an averaged pH result at an accuracy matched to the instrument's resolution.
  3. Students will rank a set of tap samples and identify the ninetieth percentile lead result used for compliance.

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.
Orthophosphate
A chemical that utilities add to water in very small amounts. It coats the inside of pipes so lead cannot easily get into the 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.
Action level
A trigger number in lead and copper testing. If too many homes test above this level, the utility must take steps like improving treatment and telling the public.

Also: ninetieth percentile, stock solution

Teacher overview

In corrosion control the chemistry a specialist adjusts at the treatment plant is intended to change what happens miles away, inside the pipes of a customer's building. In this lesson students take the role of a water quality specialist at the fictional Wrentmere Water Treatment Plant, setting an orthophosphate feed, reporting a pH result honestly, and working through the ranked tap samples that decide whether a system is above or below the lead action level. The misconception the lesson addresses is the belief that lead in drinking water comes from the source water, when in nearly all cases the lead enters after treatment, from service lines and solder inside the distribution system and the building. Students who hold that belief will look for a treatment barrier that removes lead rather than a chemical that keeps the pipe wall from releasing it.

Procedure

Hook (5 min). Project a photograph or diagram of a cross-sectioned lead service line with scale on its interior wall. Ask students where the lead in a tap sample comes from and record their answers without correcting them. Tell them the plant they are about to work at, the fictional Wrentmere Water Treatment Plant, treats 3.6 MGD and has water that meets every standard when it leaves the building.

Corrosion control briefing (12 min). Explain that a corrosion control specialist manages three variables: pH, alkalinity, and a corrosion inhibitor, most often orthophosphate, which forms a low-solubility coating on the interior of the pipe. Return to the hook and correct the misconception directly, naming the service line, the solder, and the brass fixture as the sources. Introduce the compliance side of the job: samples are collected at customer taps, ranked from lowest to highest, and the ninetieth percentile result is compared against the action level. Note that the action level is 15 ppb and drops to 10 ppb in 2027 under the Lead and Copper Rule Improvements, which also require systems to replace nearly all lead service lines within ten years.

Modeled calculation (10 min). Work one feed calculation aloud at a flow the students will not reuse, so the model is instruction rather than a keyed answer. Show the pounds formula written with every unit label attached, then show the second step that divides pounds per day by the pounds of chemical in each gallon of stock solution to get a pump setting in gallons per day. Emphasize that the flow must be entered in million gallons per day and ask two students to state the flow units aloud before you continue.

Student handout (20 min). Distribute the handout and have students work individually or in pairs. Circulate and check the unit labels in items 1 and 2 rather than the final numbers, since a missing label is the reliable early sign of the error that will appear in item 8.

Formative check and debrief (8 min). Collect answers to item 6 as an exit ticket. If fewer than three quarters of the class correctly identifies that the ranked ninetieth percentile result of 11 ppb would exceed a 10 ppb action level, reteach the ranking procedure with a smaller sample set before moving on. Close by asking what a specialist would change at the plant if the ninetieth percentile rose over two consecutive monitoring periods.

Wrentmere Corrosion Control: A Water Quality Specialist Lesson — Grade 12 Careers & CTE
Name: ____________________________ Date: ______________

Student Handout

You are the water quality specialist at the fictional Wrentmere Water Treatment Plant, which treats 3.6 MGD. Use 8.34 pounds per gallon for the weight of water. Show your work and label every unit.

  1. You set the orthophosphate dose at 1.2 mg/L. Calculate the feed in pounds per day, using loading (lbs/day) = dose (mg/L) x flow (MGD) x 8.34. Round to one decimal place.
  2. The orthophosphate stock solution contains 3.0 pounds of orthophosphate per gallon. Calculate the setting for the feed pump in gallons of stock solution per day. Round to one decimal place.
  3. Your bench pH meter resolves to the nearest 0.01 pH unit. Four readings on today's finished water are 7.82, 7.86, 7.79, and 7.85. Calculate the mean and report it at an accuracy the meter supports.
  4. The secondary standard for drinking water pH is a range of 6.5 to 8.5. Is your mean from item 3 inside that range? Answer yes or no.
  5. Ten first-draw tap samples came back at 8, 2, 11, 5, 18, 3, 9, 6, 4, and 7 ppb of lead. To find the ninetieth percentile with ten samples, rank the results from lowest to highest and take the ninth value. What is the ninetieth percentile result?
  6. The lead action level is 15 ppb today and drops to 10 ppb in 2027. Would the result from item 5 exceed the action level that takes effect in 2027? Answer yes or no.
  7. Wrentmere's raw water alkalinity is 42 mg/L as calcium carbonate, and you want to raise it to 70 mg/L to stabilize pH in the distribution system. How many mg/L of alkalinity must you add?
  8. Calculate the pounds per day of alkalinity that increase represents at the plant's flow, using the same pounds formula from item 1. Round to one decimal place.

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: 36.0 lbs/day. Feed = 1.2 mg/L x 3.6 MGD x 8.34 lb/gal. First, 1.2 x 3.6 = 4.32. Then 4.32 x 8.34 = 36.0288, which rounds to 36.0 pounds per day.
  2. Answer: 12.0 gallons per day. Pump setting = 36.0 pounds per day / 3.0 pounds per gallon = 12.0 gallons of stock solution per day.
  3. Answer: 7.83 pH units. The sum is 7.82 + 7.86 + 7.79 + 7.85 = 31.32, and 31.32 / 4 = 7.83. Reporting 7.8300 or 7.83000 would claim precision the meter does not deliver, and rounding to 7.8 would discard precision it does.
  4. Answer: Yes. A mean of 7.83 sits inside the 6.5 to 8.5 range, in the upper half of it.
  5. Answer: 11 ppb. Ranked lowest to highest, the ten results are 2, 3, 4, 5, 6, 7, 8, 9, 11, 18. The ninth value in that ranking is 11 ppb. Students who take the highest value will report 18 ppb, and students who take the mean will report 7.3 ppb, neither of which is the compliance figure.
  6. Answer: Yes. A ninetieth percentile of 11 ppb is below the 15 ppb action level in effect now but above the 10 ppb level that applies from 2027, so the system would be required to act under the new level.
  7. Answer: 28 mg/L. The increase = 70 mg/L target minus 42 mg/L raw water alkalinity = 28 mg/L.
  8. Answer: 840.7 lbs/day. Pounds per day = 28 mg/L x 3.6 MGD x 8.34 lb/gal. First, 28 x 3.6 = 100.8. Then 100.8 x 8.34 = 840.672, which rounds to 840.7 pounds per day.

Differentiation

Support

Give students a formula card with the pounds formula and a worked example that uses a different flow, and provide the ranked list in item 5 already sorted so the percentile procedure is the only new step. Pair students for items 7 and 8, and accept a correct item 7 as evidence of understanding even if item 8 is incomplete. A labeled diagram of a service line with the lead sources marked helps students connect the chemistry to the compliance samples.

Extension

Ask students to determine what orthophosphate dose would be needed if the plant flow rose to peak summer demand, then research how a state defines optimal corrosion control treatment and what a system must do after exceeding an action level. Students can also calculate what the ninetieth percentile would become if the two highest results were resampled and came back lower.

ELL supports

Pre-teach dose, feed, stock solution, alkalinity, percentile, and action level with a bilingual glossary that pairs each term with its unit. Provide the four pH readings in item 3 in a vertical column rather than a sentence, and offer sentence frames for the yes-or-no justifications in items 4 and 6. Allow students to answer with labeled calculations rather than prose.

Extensions

Have students find their own water system's most recent consumer confidence report, locate the lead ninetieth percentile result and the number of sites sampled, and compare that figure against the action level taking effect in 2027. Students can also map the service line inventory their utility has published and describe how a specialist would prioritize replacement work across neighborhoods.