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Worksheet Grade 7 Science

Calderwick Water Works: Percent Turbidity Removed at Each Stage

A grade 7 science worksheet: students track cloudiness in NTU through a treatment plant, find the percent removed per stage, and compare finished water to the limit.

Objectives

  1. Students will calculate the percent of turbidity removed across a treatment stage.
  2. Students will compare finished-water turbidity to a regulatory limit.
  3. Students will explain why lower turbidity supports safe disinfection.

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.
Sedimentation
Sedimentation is the step where water sits almost still in a big basin so the heavy clumps of dirt can sink to the bottom. The cleaner water on top then moves on to the filters.
Filtration
Filtration is the step where water passes through layers of sand, gravel, or other materials that trap the small particles still left in it. It works like a very fine strainer.

Also: NTU, removal efficiency

Teacher overview

Turbidity is the cloudiness of water, measured in nephelometric turbidity units (NTU). Cloudy water hides particles that can shield germs from disinfection, so treatment plants must lower turbidity before the water is sent out. The EPA rule for conventional filtration plants requires finished water at or below 0.3 NTU in 95 percent of monthly samples. This worksheet follows the fictional Calderwick Water Works as river water moves through two cleaning stages. Students use the removal-efficiency idea, that percent removed equals the drop divided by the starting amount times 100, to track how clear the water becomes.

Calderwick Water Works: Percent Turbidity Removed at Each Stage — Grade 7 Science
Name: ____________________________ Date: ______________

Student Worksheet

Suppose the fictional Calderwick Water Works treats river water in two stages. All turbidity is in NTU. The finished-water limit is 0.3 NTU.

  • Raw river water: 40 NTU
  • After sedimentation: 8 NTU
  • After filtration: 0.2 NTU
  1. Find the percent of turbidity removed across sedimentation, as the water went from 40 NTU to 8 NTU.
  2. Find the percent removed across filtration, as the water went from 8 NTU to 0.2 NTU. Round to the nearest tenth of a percent.
  3. Find the overall percent removed from the raw water (40 NTU) to the finished water (0.2 NTU). Round to the nearest tenth of a percent.
  4. The finished water is 0.2 NTU. Does it meet the 0.3 NTU limit? By how much is it above or below the limit?
  5. During a storm the raw water jumps to 60 NTU. If sedimentation still removes 80 percent, what is the turbidity after sedimentation?
  6. After that storm, filtration removes 97.5 percent of the turbidity from item 5. What is the finished turbidity?
  7. Compare your item 6 result to the 0.3 NTU limit. Does the storm-day water still meet the limit? How much margin is left?
  8. On a normal day, which stage removed the larger percent of turbidity, sedimentation or filtration? Use your item 1 and item 2 answers.
  9. Explain why lowering turbidity helps the disinfection step work better.
  10. A different plant removes only 95 percent of turbidity overall from the same 40 NTU raw water. What is its finished turbidity, and does it meet the 0.3 NTU limit?

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 percent.
    The drop is 40 minus 8, which is 32 NTU. 32 divided by 40 equals 0.80, or 80 percent removed.
  2. Answer: 97.5 percent.
    The drop is 8 minus 0.2, which is 7.8 NTU. 7.8 divided by 8 equals 0.975, or 97.5 percent removed.
  3. Answer: 99.5 percent.
    The drop is 40 minus 0.2, which is 39.8 NTU. 39.8 divided by 40 equals 0.995, or 99.5 percent removed.
  4. Answer: 0.1 NTU below.
    The finished water is 0.2 NTU and the limit is 0.3 NTU, so it meets the limit. 0.3 minus 0.2 equals 0.1 NTU of margin below the limit.
  5. Answer: 12 NTU.
    Removing 80 percent leaves 20 percent behind. 60 NTU times 0.20 equals 12 NTU after sedimentation.
  6. Answer: 0.3 NTU.
    Removing 97.5 percent leaves 2.5 percent behind. 12 NTU times 0.025 equals 0.3 NTU of finished turbidity.
  7. Answer: Yes, but with no margin.
    The storm-day finished water is 0.3 NTU, which is exactly at the 0.3 NTU limit, so it just meets the standard with no room to spare.
  8. Answer: Filtration.
    Filtration removed 97.5 percent while sedimentation removed 80 percent, so filtration took out the larger percent of turbidity.
  9. Answer: Particles can shield germs.
    Cloudy water holds particles that germs can hide behind, so removing turbidity first lets the disinfectant reach and kill more of the germs.
  10. Answer: 2 NTU, does not meet the limit.
    Removing 95 percent leaves 5 percent behind. 40 NTU times 0.05 equals 2 NTU, which is well above the 0.3 NTU limit, so the water fails the standard.

Differentiation

Support

Work item 1 together as a class so students see the removal steps: find the drop, divide by the starting amount, then multiply by 100. Provide a printed reference strip with the removal-efficiency formula and the 0.3 NTU limit.

Extension

Ask students to find what percent the filtration stage would need to remove on the storm day (from 12 NTU) to leave the finished turbidity comfortably below the 0.3 NTU limit instead of landing right at it, and to explain why an operator wants that margin.

ELL supports

Pre-teach "turbidity," "sedimentation," "filtration," and "removed" using two jars, one cloudy and one clear, as a visual. Allow students to answer items 7 and 9 with a short phrase or a labeled sketch rather than a full sentence.