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Classroom activity Grade 9 Careers & CTE

Tanbury Water Lab: A Grade 9 Bench Technician Rotation

A grade 9 careers rotation through four benches of a fictional water laboratory, where student teams run turbidity, chlorine feed, reporting, and duplicate-check math.

Objectives

  1. Students will carry units through a multi-step chlorine feed calculation and report the result in pounds per day.
  2. Students will compare a month of turbidity results against the 95 percent requirement in the federal rule.
  3. Students will report a laboratory average at the accuracy the test kit supports.

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.
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.

Also: chlorine demand, duplicate sample, relative percent difference, resolution

Teacher overview

The fictional Tanbury Regional Water Laboratory runs the daily samples for a small drinking water system, and its bench technicians repeat the same four jobs every morning. This rotation moves groups of three or four students through four benches, ten minutes each: turbidity, chlorine feed, reporting accuracy, and the duplicate check. Every bench hands the group a set of readings and asks for the number a technician would enter in the log, so the rotation follows the order of work in a working laboratory rather than treating the four calculations as unrelated exercises. The common misconception this rotation corrects is that a laboratory result is whatever the instrument displays; what gets reported depends on the resolution of the instrument, on the duplicate that confirms it, and on the rule the number is checked against.

Activity Directions

  1. Before class, print the four bench cards and set out four tables labeled Bench 1 through Bench 4, each with its card and a calculator. Print one Student Handout per student. Setup takes about ten minutes.
  2. Open by naming the job. A water quality laboratory analyst at a plant such as the fictional Tanbury system tests samples, records results, and flags anything that crosses a limit. Tell students they will hold that job for four rounds.
  3. Sort the class into four groups of three or four and send one group to each bench. Every student keeps a handout and answers every question, though the group works the readings together.
  4. Set the timer for ten minutes per round. Groups work only the questions belonging to their current bench, then rotate one bench clockwise when the timer sounds.
  5. Bench 1 covers turbidity. The card lists five daily readings from the fictional Tanbury plant and last month's sample count, then asks whether the month met the federal requirement of 0.3 NTU or less in 95 percent of samples.
  6. Bench 2 covers the chlorine feed. The card gives a dose, a residual, and the plant's flow, and asks for the chlorine demand, the feed rate in pounds per day, and the same flow written in gallons per minute.
  7. Bench 3 covers reporting accuracy. The card gives three chlorine readings from a kit that resolves to the nearest 0.1 mg/L, along with the long decimal a calculator returns for their mean.
  8. Bench 4 covers the duplicate check, in which one sample is split in two and both halves are analyzed. The card gives both results and the formula for relative percent difference.
  9. End the rotation once every group has worked all four benches, which takes forty minutes including movement between tables.
  10. Close with a ten-minute debrief on two questions. Ask first why a technician holding a calculator reading of 0.8666667 mg/L still writes 0.9 mg/L in the log. Ask second what a laboratory learns from splitting a sample that it cannot learn from a single result.
Tanbury Water Lab: A Grade 9 Bench Technician Rotation — Grade 9 Careers & CTE
Name: ____________________________ Date: ______________

Student Handout

Work only the questions for the bench you are standing at, and show the numbers you used. Every reading below comes from the fictional Tanbury Regional Water Laboratory.

Bench 1: Turbidity

  1. Bench 1 logs five finished-water turbidity readings at the fictional Tanbury plant: 0.08, 0.11, 0.09, 0.14, and 0.13 NTU. What is the mean of the five readings, rounded to two decimal places?
  2. Federal rules require finished-water turbidity at or below 0.3 NTU in 95 percent of a month's samples. Bench 1 ran 40 samples last month, and 3 of them read above 0.30 NTU. What percent of the 40 samples were at or below 0.30 NTU?
  3. The result in Question 2 is 92.5 percent, and the rule requires at least 95 percent. Did the fictional Tanbury plant meet the turbidity requirement last month?

Bench 2: Chlorine feed

  1. Bench 2 records a chlorine dose of 2.4 mg/L at the fictional Tanbury plant and a residual of 0.9 mg/L after contact. Chlorine demand equals dose minus residual. What is the chlorine demand?
  2. The fictional plant treats 3.5 MGD. Pounds per day equals dose in mg/L times flow in MGD times 8.34. How many pounds of chlorine per day does the 2.4 mg/L dose require, rounded to the nearest pound?
  3. One million gallons per day equals 694 gallons per minute. What is the fictional plant's flow of 3.5 MGD in gallons per minute, rounded to the nearest ten?

Bench 3: Reporting accuracy

  1. The chlorine kit at Bench 3 resolves to the nearest 0.1 mg/L. Three readings of one sample give 0.9, 0.8, and 0.9 mg/L, and the calculator returns a mean of 0.8666667 mg/L. What value belongs in the log?

Bench 4: Duplicate check

  1. Bench 4 splits one sample into two halves. The first half reads 1.8 mg/L and the duplicate reads 1.6 mg/L. Relative percent difference equals the difference between the two results, divided by their mean, times 100. What is the relative percent difference, rounded to the nearest whole percent?

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: 0.11 NTU.
    The five readings from the fictional Tanbury plant total 0.55 NTU, and 0.55 divided by 5 is 0.11.
  2. Answer: 92.5 percent.
    Of the 40 samples, 40 minus 3 leaves 37 at or below 0.30 NTU, and 37 divided by 40 is 0.925.
  3. Answer: No.
    At 92.5 percent the month falls short of the 95 percent the rule requires, so the plant did not meet it.
  4. Answer: 1.5 mg/L.
    Chlorine demand is the dose of 2.4 mg/L minus the residual of 0.9 mg/L.
  5. Answer: 70 lbs/day.
    The product of 2.4 and 3.5 is 8.4, and 8.4 times 8.34 is 70.056, which rounds to 70 pounds per day.
  6. Answer: 2,430 gpm.
    The product of 3.5 and 694 is 2,429 gallons per minute, which rounds to 2,430.
  7. Answer: 0.9 mg/L.
    The kit resolves to the nearest 0.1 mg/L, so the mean of 0.8666667 mg/L is logged as 0.9 mg/L. Extra decimals would claim accuracy the kit cannot deliver.
  8. Answer: 12 percent.
    The difference is 0.2 mg/L and the mean of 1.8 and 1.6 is 1.7, so 0.2 divided by 1.7 is 0.1176, or about 12 percent.

Differentiation

Support

Give these groups a bench card with the formula already written out and one substitution shown, so the fictional plant's chlorine question reads as 2.4 times 3.5 times 8.34 rather than as a formula to assemble. Allow twelve minutes per round instead of ten.

Extension

Ask these students to explain what the fictional Tanbury laboratory should do when a duplicate check returns a relative percent difference near 40 percent, and which of the two results, if either, may be entered in the log.

ELL supports

Pre-teach turbidity, residual, demand, and duplicate with a labeled sketch of one sample bottle split into two halves. Provide a sentence frame for the debrief: "The technician writes ___ in the log because the kit reads to the nearest ___."

Extensions

Invite a laboratory analyst from a nearby utility or from a community college water technology program to walk the class through one day of bench sheets. Students who finish the rotation early can build a fifth bench card for a pH meter that resolves to 0.01 units, writing three readings and the value the fictional Tanbury laboratory should report from them.