Lesson plan Grade 11 Science
Hartwell's Discharge Permit: The Clean Water Act on the Sindle River
A grade 11 lesson where students analyze a fictional wastewater discharge permit, connecting the Clean Water Act and secondary treatment to dissolved oxygen in a river.
Objectives
- Students will distinguish point and nonpoint pollution sources and locate them in a discharge permit.
- Students will explain how the Clean Water Act's treatment limits protect dissolved oxygen and aquatic life in a receiving river.
Vocabulary
- NPDES permit
- An NPDES permit is a legal permission slip the government requires before any factory, treatment plant, or facility can discharge water into a river, lake, or stream. It spells out exactly what and how much can be released, and utilities must test and report their discharge to prove they are following the rules.
- Secondary treatment
- Secondary treatment uses billions of helpful microbes that eat the dissolved waste in the water. The plant keeps the microbes supplied with air, then settles them out, leaving much cleaner water.
- Biochemical oxygen demand (BOD)
- Biochemical oxygen demand, or BOD, measures how much oxygen tiny microbes will use up as they eat the waste in water. Dirtier water has a higher BOD.
- Eutrophication
- Eutrophication is what happens when a lake or bay gets overfed with nutrients: algae grows out of control, and when it dies and rots, the water loses the oxygen that fish need.
- 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: point source, nonpoint source
Teacher overview
Since the Clean Water Act of 1972, a treatment plant cannot legally discharge into a river without a permit that sets numeric limits on what it releases and requires it to monitor and report the results. This lesson has students read a fictional discharge permit for the Hartwell Water Reclamation Facility and trace how each limit protects the Sindle River downstream. The science underneath the law is dissolved oxygen: students should see that a permit limit on oxygen-demanding waste is really a limit on how much oxygen the discharge is allowed to pull from the river.
Procedure
Hook (7 min): Ask students what would happen to a river if a city simply piped its untreated sewage into it. Collect ideas, then name the two things that make the river unsafe and lifeless: pathogens and oxygen-demanding organic waste. Explain that the Hartwell facility used in today's handout is fictional but its permit mirrors a real one.
Direct instruction (16 min): Define a point source (a pipe or outfall you can point to) versus a nonpoint source (diffuse runoff), and explain that the Clean Water Act regulates point sources through NPDES permits. Introduce secondary treatment: biological processes that let microbes eat the organic waste before discharge, cutting biochemical oxygen demand (BOD) and total suspended solids to monthly-average limits of 30 mg/L, with at least 85 percent of each removed. Raw domestic sewage BOD typically runs about 100 to 300 mg/L, near 200 mg/L for medium-strength sewage. Explain biochemical oxygen demand as the oxygen microbes will pull from the river to finish decomposing whatever the plant fails to remove.
Guided practice (12 min): Project the permit table from the handout and work item 1 together, labeling the outfall as a point source and a neighboring farm field as a nonpoint source. Discuss why the law can put a hard number on the pipe but not on the field.
Independent practice (10 min): Students complete handout items 2 through 6, working in pairs.
Closure and formative check (5 min): Exit ticket: each student writes one sentence explaining why a BOD limit is really a dissolved-oxygen protection. Collect and check that at least 80 percent connect low BOD to preserving oxygen for fish before dismissing.
Student Handout
The Hartwell Water Reclamation Facility is a fictional plant. Its NPDES permit sets these monthly-average limits on what it may discharge into the Sindle River:
| Parameter | Permit limit (monthly average) |
|---|---|
| Biochemical oxygen demand (BOD) | 30 mg/L |
| Total suspended solids (TSS) | 30 mg/L |
| Nitrogen and phosphorus | limited, with monitoring required |
| Bacteria (E. coli) | limited, with monitoring required |
Hartwell's outfall pipe is a point source, but runoff from a nearby farm field is a nonpoint source. Explain the difference and why the permit can set a hard limit on the pipe but not on the field.
The permit limits BOD to 30 mg/L, while raw domestic sewage BOD typically runs about 100 to 300 mg/L, near 200 mg/L for medium-strength sewage. Explain why meeting the limit means the plant must remove the large majority of the incoming oxygen-demanding load, and name the treatment stage that does most of this work.
Explain why discharging water with high BOD lowers the dissolved oxygen in the Sindle River, and why that can harm fish and other aquatic life.
The permit also limits nitrogen and phosphorus. Explain how too much of these nutrients can damage the river even though they are not directly poisonous.
Hartwell proposes upgrading the plant to add a nutrient-removal stage. Evaluate this upgrade: what river impact would it reduce, and what trade-off comes with building and running it?
An NPDES permit and its monitoring data are public and enforceable. Explain how making that information public helps protect the river, and connect it to the Clean Water Act's stated goal of fishable and swimmable waters.
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: A point source is a single, identifiable discharge like Hartwell's outfall pipe, so its flow and concentration can be measured and capped. Farm runoff is nonpoint: it enters the river diffusely across the whole field after rain, so there is no single pipe to meter, which makes it far harder to regulate with a numeric limit.
Answer: To bring medium-strength sewage near 200 mg/L of BOD down to a 30 mg/L limit, the plant must remove (200 - 30) / 200 = 85 percent of the load, and stronger sewage near 300 mg/L needs 90 percent. Secondary standards also require at least 85 percent removal, so even weaker sewage near 100 mg/L must be treated to about 15 mg/L or less. The secondary (biological) treatment stage, where microbes consume the organic waste, does most of this removal.
Answer: Microbes in the river consume dissolved oxygen as they break down the discharged organic waste, so high-BOD water drives the river's oxygen down. Fish and other aquatic animals need dissolved oxygen above about 5 mg/L, so a sharp drop can stress or kill them.
Answer: Nitrogen and phosphorus act as fertilizer. In excess they trigger heavy algae growth, and when that algae dies and decomposes it consumes dissolved oxygen, a process called eutrophication that can suffocate aquatic life even though the nutrients themselves are not toxic.
Answer: A nutrient-removal stage would reduce the nitrogen and phosphorus reaching the river, lowering the risk of algae blooms and oxygen crashes downstream. The trade-off is the added construction cost and the extra energy and chemicals needed to run the new stage.
Answer: Public, enforceable permits let regulators, downstream communities, and watchdog groups check whether the plant is meeting its limits and act if it is not. That transparency supports the Clean Water Act's goal of making the nation's waters fishable and swimmable by holding dischargers accountable.
Differentiation
Support
Provide a labeled diagram of the river with the outfall, the farm field, and a dissolved-oxygen gauge, and give a short word bank (point source, BOD, dissolved oxygen, eutrophication) students can draw from when answering.
Extension
Have students research how a total maximum daily load (TMDL) addresses the nonpoint farm runoff that a single NPDES permit cannot, using only teacher-provided material, and write a short paragraph on why nonpoint pollution is the harder problem to solve.
ELL supports
Pre-teach "point source," "discharge," "oxygen-demanding," and "eutrophication" with simple visuals, including a pipe emptying into a river beside a rain-soaked field. Allow items 3 through 6 to be answered with a labeled sketch plus one sentence built from a provided frame.