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Worksheet Grade 9 Math

Dunmere Clarifier Testing: Removal Efficiency and Oxygen

A grade 9 water-testing worksheet at the fictional Dunmere plant, calculating turbidity removal efficiency and checking dissolved oxygen against a fish limit.

Objectives

  1. Students will calculate removal efficiency as a percent from influent and effluent readings.
  2. Students will compare dissolved-oxygen readings against a fish-health benchmark.
  3. Students will report a measured value at a precision that matches the instrument.

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.
Dissolved oxygen
Dissolved oxygen is the oxygen gas mixed into water that fish and other underwater animals breathe. Cold, moving water holds more oxygen than warm, still water.

Also: removal efficiency, NTU, precision

Teacher overview

The fictional Dunmere treatment plant tests its water at every step and reports the results to the state. Two habits from a real lab drive this worksheet: measuring how much of a pollutant a step removes, stated as a percent, and matching the number of reported digits to what the meter can actually read. Students calculate removal efficiency, check dissolved oxygen against a fish-health limit, and clean up a reading that claims false precision.

Dunmere Clarifier Testing: Removal Efficiency and Oxygen — Grade 9 Math
Name: ____________________________ Date: ______________

Student Worksheet

Dunmere (a fictional plant) records readings across its process. Use these facts:

  • Removal efficiency, in percent, = (influent minus effluent) divided by influent, times 100.
  • Fish need dissolved oxygen above 5 mg/L.
  • The finished-water turbidity goal is 0.3 NTU or below in 95 percent of samples.
  1. The clarifier influent reads 45 NTU and the settled water reads 9 NTU. Find the removal efficiency as a percent.
  2. A filtered sample reads 0.2 NTU. Is it at or below the 0.3 NTU finished-water goal?
  3. A later batch reads 60 NTU influent and 12 NTU settled. Find the removal efficiency as a percent.
  4. The plant target is at least 85 percent removal. Does the item-3 result meet the target?
  5. Starting from 60 NTU influent, find the highest settled turbidity that still gives exactly 85 percent removal.
  6. A creek sample below the outfall reads 6.4 mg/L dissolved oxygen. Is that above the 5 mg/L fish limit?
  7. A downstream sample reads 4.2 mg/L. Is it above the 5 mg/L limit, and if not, by how much is it short?
  8. Three dissolved-oxygen readings are 6.4, 5.8, and 6.0 mg/L. Find the mean.
  9. The meter reads to the nearest 0.1 mg/L. A student writes the item-8 mean as 6.06667 mg/L. Give a better way to report it and say why.
  10. Explain why the plant tracks removal as a percent instead of just reporting the settled-water number.

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: 80 %. (45 minus 9) divided by 45, times 100 = 36 divided by 45, times 100 = 80 percent.
  2. Answer: Yes. 0.2 NTU is at or below the 0.3 NTU goal.
  3. Answer: 80 %. (60 minus 12) divided by 60, times 100 = 48 divided by 60, times 100 = 80 percent.
  4. Answer: No. 80 percent is below the 85 percent target.
  5. Answer: 9 NTU. Solve 85 = (60 minus x) divided by 60, times 100. Then 0.85 = (60 minus x) divided by 60, so 60 minus x = 51 and x = 9 NTU.
  6. Answer: Yes. 6.4 mg/L is above the 5 mg/L fish limit.
  7. Answer: No, short by 0.8 mg/L. 4.2 is below 5; 5 minus 4.2 = 0.8 mg/L short.
  8. Answer: 6.07 mg/L. (6.4 plus 5.8 plus 6.0) divided by 3 = 18.2 divided by 3 = 6.0667, about 6.07 mg/L.
  9. Answer: 6.1 mg/L. The meter reads to the nearest 0.1 mg/L, so report only one decimal place; extra digits claim precision the meter does not have.
  10. Answer: A percent shows how much of the incoming load was removed, not just what is left. Two plants can leave the same settled number yet remove very different shares if their influent differs.

Differentiation

Support

Provide the removal-efficiency formula on a reference card and one fully worked example using numbers not on this worksheet. For items 1, 3, and 5, print the subtraction step already set up so students focus on the division and the percent.

Extension

Ask students to find the settled turbidity needed to reach 90 percent removal from a fictional 75 NTU influent, then write a general equation for the settled turbidity that meets any target percent from a 75 NTU start.

ELL supports

Pre-teach "influent," "effluent," "removal," and "precision" with two labeled jars, one cloudy and one clear, and a meter face showing one decimal place. Provide a sentence frame for item 10: "A percent tells us how much was ___, not just how much is ___."