Worksheet Grade 10 Science
Copper Bend Lake: Nutrient Loading and Biological Nutrient Removal
A grade 10 worksheet where students calculate dissolved oxygen loss and phosphorus removal at a fictional lake facing nutrient pollution and algal blooms.
Objectives
- Students will calculate percent change in dissolved oxygen following a nutrient-driven algal bloom.
- Students will apply a stated removal rate to calculate a treated wastewater phosphorus concentration.
- Students will compare two engineering strategies for reducing nutrient pollution reaching a lake.
Vocabulary
- 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.
- Biological nutrient removal (BNR)
- Biological nutrient removal uses specially managed bacteria to strip extra nitrogen and phosphorus out of wastewater before it is released. Removing these nutrients helps prevent algal blooms in the rivers and lakes that receive the treated water.
Also: environmental biotechnology
Teacher overview
Copper Bend Lake (a fictional lake used for this worksheet) receives nutrient runoff and treated wastewater from its watershed, conditions that can trigger algal blooms when nitrogen and phosphorus levels rise. This worksheet has students calculate how a bloom affects dissolved oxygen, then evaluate a proposed engineering solution: upgrading a wastewater plant with biological nutrient removal, a process that uses living microorganisms to remove nutrients before they reach the lake. A common misconception is assuming more algae always means healthier water; the worksheet is built to show that a bloom's real damage often happens after the algae die and decompose, consuming oxygen in the process.
Student Worksheet
- Explain what causes an algal bloom in a lake like Copper Bend when excess nitrogen and phosphorus enter the water. What happens to the lake's dissolved oxygen (DO) when a large bloom dies and decomposes?
- Suppose Copper Bend Lake measured 9.2 mg/L of dissolved oxygen before a nutrient-driven bloom and 3.8 mg/L after the bloom decomposed. Most fish need dissolved oxygen above 5 mg/L to survive. Which reading puts fish at risk, and why?
- Calculate the percent decrease in dissolved oxygen between the before and after readings in item 2, using Percent decrease = (before − after) ÷ before × 100.
- A wastewater treatment plant upstream of Copper Bend Lake discharges effluent with a phosphorus concentration of 4.0 mg/L, a hypothetical value used for this worksheet. Engineers propose a biological nutrient removal (BNR) upgrade. The upgrade would create conditions where bacteria take up phosphorus and convert nitrogen compounds into harmless nitrogen gas before the water is released. If the upgrade removes 90 percent of the phosphorus, what will the new effluent concentration be?
- Using your answer to item 4, calculate how many mg/L of phosphorus the upgraded plant removes compared to the current process.
- Biological nutrient removal relies on living microorganisms, not just chemicals. In your own words, explain why using bacteria to remove nutrients counts as a form of environmental biotechnology.
- Suppose Copper Bend Lake's watershed also includes farms. Their runoff reaches the lake without passing through the treatment plant at all. Explain why upgrading a single wastewater plant might reduce, but not eliminate, the lake's risk of future algal blooms.
- Engineers are also considering restoring wetlands along the stream that feeds the lake. Wetland plants and soil microbes naturally absorb nitrogen and phosphorus before it reaches open water. Compare this wetland approach with the BNR plant upgrade. Name one advantage of each.
- Challenge. Copper Bend Lake's healthy baseline DO is 9.2 mg/L. Suppose each severe bloom reduces DO by about the percentage you calculated in item 3. The lake recovers partway to 7.0 mg/L before a second bloom of similar severity strikes later the same summer. Estimate the DO level after that second bloom. Show your calculation.
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: Excess nitrogen and phosphorus act as fertilizer for algae, causing rapid, dense growth (a bloom). When the algae die, bacteria decomposing them consume large amounts of dissolved oxygen, which can leave too little oxygen for fish and other aquatic life.
- Answer: The after-bloom reading (3.8 mg/L) puts fish at risk, because it falls below the 5 mg/L threshold most fish need to survive; the before-bloom reading (9.2 mg/L) is well above it.
- Answer: about 58.7 percent.
(9.2 − 3.8) ÷ 9.2 × 100 = 5.4 ÷ 9.2 × 100 = 58.70, which rounds to 58.7 percent. - Answer: 0.4 mg/L.
Effluent = influent × (1 − removal rate) = 4.0 × (1 − 0.90) = 4.0 × 0.10 = 0.4 mg/L. - Answer: 3.6 mg/L.
4.0 mg/L − 0.4 mg/L = 3.6 mg/L, meaning 3.6 mg of phosphorus removed from each liter of effluent. - Answer: It is a form of environmental biotechnology because it deliberately uses living organisms, in this case bacteria, to carry out a treatment process, removing nutrients, that would otherwise require purely chemical or mechanical methods.
- Answer: Farm runoff can still carry nitrogen and phosphorus directly into the lake or its tributaries without passing through the treatment plant at all, so the plant's upgrade only addresses the nutrients that flow through it, not every source in the watershed.
- Answer: Answers will vary. A wetland can treat runoff from many sources across the watershed and provide wildlife habitat, but it works slowly and depends on available land; the BNR plant upgrade treats a concentrated, measurable flow reliably but only the wastewater that passes through that one plant.
- Answer: about 2.9 mg/L.
7.0 mg/L × (1 − 0.587) = 7.0 × 0.413 = 2.89, which rounds to about 2.9 mg/L.
Differentiation
Support (below grade level)
Provide the percent-decrease and removal-percent formulas as labeled reference cards, and work item 3 as a fully modeled example before students attempt item 9 independently.
Extension (above grade level)
Ask students to work backward from item 4: using only the removal information given (not outside sources), what starting phosphorus concentration would bring the final effluent to exactly 0.2 mg/L at the same 90 percent removal rate, and show the algebra that solves for it.
ELL supports
Pre-teach "nutrient," "bloom," "dissolved oxygen," and "biotechnology" with a simple sequence diagram: nutrients enter the lake, algae grow, algae die, oxygen drops. Allow students to answer items 1, 6, and 7 with short phrases or labeled diagrams rather than full paragraphs.