Worksheet Grade 6 Science
Quillfield's Treatment Wetland: Cleaning Water with Plants
A grade 6 worksheet where students use inlet and outlet readings to calculate how well a fictional constructed wetland removes waste and nitrogen from water.
Objectives
- Students will calculate removal efficiency for waste and nitrogen using inlet and outlet readings.
- Students will explain how wetland plants and microbes use nitrogen as they clean the water.
- Students will judge whether the wetland's outlet water meets a stated discharge limit.
Vocabulary
Key words: constructed wetland, removal efficiency, nitrogen, microbes
Teacher overview
A constructed wetland is a shallow, planted pond built to clean water. In the fictional town of Quillfield, cattails and reeds grow in gravel, and tiny microbes live on their roots. As water moves through slowly, the microbes break down waste and the plants take up nitrogen to grow. That is the key idea of this worksheet: nitrogen is both a pollutant, because too much of it downstream feeds algae, and a resource, because the plants and microbes use it as food. Students compare inlet and outlet readings using the removal-efficiency formula, which is (inlet minus outlet) divided by inlet, times 100. For comparison, state rules ask many treated-water discharges to reach 30 mg/L or less for waste strength. A common error to watch for: students divide by the outlet number instead of the inlet, so remind them the inlet is always the starting amount.
Student Worksheet
The fictional Quillfield wetland takes in dirty water at the inlet and releases cleaner water at the outlet. All readings are in milligrams per liter (mg/L). Use these readings for every question.
| Measurement | Inlet (mg/L) | Outlet (mg/L) |
|---|---|---|
| Waste strength (BOD) | 60 | 15 |
| Nitrogen | 40 | 10 |
| Solids (TSS) | 50 | 20 |
Removal efficiency (%) = (inlet − outlet) ÷ inlet × 100.
- What is the removal efficiency for waste strength (BOD)?
- What is the removal efficiency for nitrogen?
- What is the removal efficiency for solids (TSS)?
- The state asks the outlet water to have a waste strength of 30 mg/L or less. Does the outlet BOD of 15 mg/L meet that limit? By how many mg/L is it under?
- Based on your percents from items 1, 2, and 3, which measurement did the wetland clean up the most? Which did it clean up the least?
- Explain how the plants and microbes work together to lower the nitrogen in the water.
- Suppose a rainy week raised the inlet nitrogen to 50 mg/L while the outlet stayed at 10 mg/L. What would the new nitrogen removal efficiency be?
- Using the inlet and outlet nitrogen readings from the table, write the ratio of nitrogen removed to nitrogen coming in, in simplest form.
- The wetland uses no electric pumps and adds no chemicals. Explain one way that makes it a low-impact method for cleaning the town's water compared with a concrete plant.
- If Quillfield sends more water than the wetland can handle, the outlet readings rise. Explain why treating too much water at once lowers the removal efficiency.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
- Answer: 75%.
(60 − 15) ÷ 60 × 100 = 45 ÷ 60 × 100 = 75 percent. - Answer: 75%.
(40 − 10) ÷ 40 × 100 = 30 ÷ 40 × 100 = 75 percent. - Answer: 60%.
(50 − 20) ÷ 50 × 100 = 30 ÷ 50 × 100 = 60 percent. - Answer: Yes; 15 mg/L under the limit.
The outlet BOD is 15 mg/L, which is below the 30 mg/L limit. 30 − 15 = 15 mg/L under. - Answer: Most: BOD and nitrogen (tied at 75%). Least: solids at 60%.
Compare the three percents from items 1, 2, and 3. BOD and nitrogen are both 75 percent, higher than the 60 percent for solids. - Answer: The microbes on the roots break down waste and change some of the nitrogen into a gas that escapes to the air, and the plants take up nitrogen through their roots to build stems and leaves. Both processes lower how much nitrogen stays in the water.
- Answer: 80%.
(50 − 10) ÷ 50 × 100 = 40 ÷ 50 × 100 = 80 percent. - Answer: 3 to 4.
Nitrogen removed is 40 − 10 = 30, and nitrogen coming in is 40. 30 to 40 both divide by 10 to give 3 to 4. - Answer: Answers will vary. Accept clear points such as: it uses no electricity for pumps, it adds no chemicals, and it works using natural plants and microbes, so it uses fewer resources and makes less pollution than a concrete plant.
- Answer: When too much water flows through at once, it moves faster and spends less time in the wetland. The microbes and plants have less time to remove waste and nitrogen, so more stays in the water and the outlet readings rise, which lowers the removal efficiency.
Differentiation
Support
Work item 1 through with students as a model, writing each step of the formula on the board. Give them a version of the table with the "inlet minus outlet" subtraction already filled in so they can focus on the division and the percent.
Extension
Ask students to design a target for the wetland: choose a nitrogen removal efficiency of at least 85 percent, then, keeping the inlet at 40 mg/L, calculate what the outlet reading would have to be to reach that target.
ELL supports
Pre-teach "inlet," "outlet," "removal efficiency," and "nitrogen" with a simple left-to-right diagram of water flowing into and out of a planted pond. Allow students to answer items 6, 9, and 10 with short phrases or a labeled sketch.
Extensions
Have students look up whether any park, roadside, or school near them has a rain garden or planted basin that slows and cleans runoff, and write a short description of how it works using the words from this worksheet.