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Lesson plan Grade 7 Science

A Day at the Elmsworth Water Lab: Reading Sample Results

Students work a fictional technician's sample log, judge turbidity, chlorine, and pH results against published limits, and learn how operator licensing works.

Objectives

  1. Students will compare sample results against turbidity, chlorine, and pH limits.
  2. Students will calculate chlorine demand between the plant and a distribution sample point.
  3. Students will describe the education and licensing path for a water treatment operator.

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.
Distribution system
A distribution system is the network of pipes, pumps, valves, and storage tanks that carries treated water from a treatment plant to homes, schools, and businesses. Most of it is buried underground beneath streets.

Also: chlorine demand, licensure

Teacher overview

Every day, someone drives a route through a water system, collects samples at fixed points, and decides whether the results are acceptable. This lesson gives students one day of results from the fictional Elmsworth Water District and asks them to make those calls using published limits: finished water at or below 0.3 NTU of turbidity, a chlorine residual that systems usually hold between 0.2 and 2.0 mg/L in distribution, and a pH between 6.5 and 8.5. Students also learn what the job requires, since operators typically enter with a high school diploma, on-the-job training, and a state license earned by exam. Teaching note: students often think a treatment plant tests water once and the job is finished, so a single passing result settles the question. Chlorine keeps reacting as water travels, which is why the residual falls between the plant and the far end of the system.

Procedure

Hook (5 min): Ask students how anyone would know whether the water in the school's drinking fountain is safe this morning. Collect answers, then explain that in the fictional Elmsworth Water District a technician answers that question with a sample bottle, a meter, and a set of limits.

Reading the limits (10 min): Distribute the handout. Read the three limits aloud and have students underline them: turbidity at or below 0.3 NTU, chlorine residual usually held from 0.2 to 2.0 mg/L in the pipes, and pH between 6.5 and 8.5. Ask students to circle any value in the sample log that falls outside those ranges before doing any math.

Model chlorine demand (7 min): Show the class that chlorine demand equals the dose fed at the plant minus the residual measured later. Work one example that is not on the handout, such as a dose of 1.8 mg/L leaving a residual of 1.0 mg/L, which gives a demand of 0.8 mg/L. Ask for a thumbs up or down, and rework the example if more than five students signal down.

Group work (15 min): Students work in pairs through handout items 1 through 7. Circulate and watch for pairs comparing the residual to the 2.0 mg/L top of the range instead of the 0.2 mg/L bottom of the usual range, which is the most common error here.

Career discussion (8 min): Tell students that, according to the Bureau of Labor Statistics, water and wastewater treatment plant and system operators earned a median of $60,020 per year in May 2025 and that about 10,000 openings are projected each year, on average, from 2025 to 2035. Total operator employment is projected to decline 6 percent over that time, so all of those openings come from replacing workers who retire or move to other occupations. Explain that the usual path starts with a high school diploma and on-the-job training, that licenses come in classes tied to plant size and experience, and that engineers, chemists, and public health researchers work on the same water with different training. Students answer handout item 9.

Exit ticket (5 min): Students complete handout item 8 and hand it in. Look for answers that name a specific action, such as flushing the line, rather than a general call to add more chlorine everywhere.

A Day at the Elmsworth Water Lab: Reading Sample Results — Grade 7 Science
Name: ____________________________ Date: ______________

Student Handout

You are the technician on shift for the fictional Elmsworth Water District. Here is today's sample log.

Sample point Time Turbidity (NTU) Chlorine residual (mg/L) pH
Plant tap 7:00 0.08 1.4 7.6
Zone 1 hydrant 8:30 0.10 0.9 7.5
Zone 2 hydrant 10:00 0.12 0.4 7.4
Zone 3 hydrant 11:30 0.14 0.15 7.2
Riverside School 13:00 0.11 0.6 7.3

Limits to use: turbidity at or below 0.3 NTU; chlorine residual usually held between 0.2 and 2.0 mg/L in the distribution system; pH between 6.5 and 8.5.

Career note: operators usually enter this work with a high school diploma and on-the-job training, then earn a state license by passing an exam. Licenses come in classes tied to the size of the plant and the operator's experience, and an operator moves up by gaining hours and passing a higher exam.

  1. Which sample point has a chlorine residual below the 0.2 mg/L bottom of the usual range?
  2. Are all five turbidity results at or below the 0.3 NTU limit?
  3. How much higher is the Zone 3 hydrant's turbidity than the plant tap's, in NTU?
  4. What percent of the five samples have a chlorine residual of at least 0.2 mg/L?
  5. Every pH result today sits inside the 6.5 to 8.5 range. Which sample point is closest to the bottom of that range?
  6. The plant fed chlorine at 2.0 mg/L this morning, and the Zone 2 hydrant measured 0.4 mg/L. What is the chlorine demand between the plant and that hydrant?
  7. Your shift is 480 minutes long, and you spend 90 minutes running lab tests. What percent of the shift is that? Round to the nearest tenth of a percent.
  8. The Zone 3 residual is too low. Name one action the district could take, and explain what that action would do.
  9. Describe what a person usually needs in order to work as a licensed water treatment operator.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

These items key the numbered questions on the student handout, in the same order.

  1. Answer: The Zone 3 hydrant.
    Its residual of 0.15 mg/L is the only result below 0.2 mg/L. Every other sample in the Elmsworth log is 0.4 mg/L or higher.
  2. Answer: Yes.
    The highest reading is 0.14 NTU at the Zone 3 hydrant, which is well below the 0.3 NTU limit.
  3. Answer: 0.06 NTU.
    0.14 NTU minus 0.08 NTU equals 0.06 NTU.
  4. Answer: 80 percent.
    Four of the five samples are at or above 0.2 mg/L. Then 4 divided by 5 equals 0.80, or 80 percent.
  5. Answer: The Zone 3 hydrant.
    Its pH of 7.2 is the lowest of the five results, so it sits closest to the 6.5 bottom of the range.
  6. Answer: 1.6 mg/L.
    Chlorine demand equals dose minus residual, so 2.0 mg/L minus 0.4 mg/L equals 1.6 mg/L.
  7. Answer: 18.8 percent.
    90 minutes divided by 480 minutes equals 0.1875, which rounds to 18.8 percent.
  8. Answer: Accept any action that raises the residual at that point.
    Strong answers include flushing the Zone 3 line to pull fresher water into it, checking for a dead-end pipe where water sits too long, or raising the dose at the plant enough to hold a residual at the far end of the system. Accept answers that also mention resampling to confirm the result.
  9. Answer: A high school diploma, on-the-job training, and a state license earned by exam.
    Licenses come in classes tied to plant size and experience, and operators move up by gaining hours and passing higher exams. Other roles in the same field, such as engineering and public health research, follow degree paths instead.

Differentiation

Support

Give these students a version of the Elmsworth sample log with a blank column headed "pass or follow up" and have them mark each row before doing any arithmetic. Provide the chlorine demand formula on a card with one worked example.

Extension

Have students design a sampling route for the fictional Elmsworth district that would find a low residual sooner, naming where they would add a sample point and explaining what pattern in today's results led them there.

ELL supports

Preteach residual, turbidity, and demand with a labeled diagram showing chlorine dropping as water travels from the plant to the far end of the Elmsworth system. Allow students to answer item 8 orally or with a labeled sketch.

Extensions

Students can look up their own water system's annual water quality report and find the turbidity and disinfectant results, then compare the format with the Elmsworth sample log they worked from.