Worksheet Grade 11 Science
Fenwick Hollow's Bacteria: Nitrification, Denitrification, and Removal
Grade 11 students trace how two bacterial processes strip nitrogen from wastewater at a fictional plant and calculate removal efficiency and pounds removed per day.
Objectives
- Students will describe nitrification and denitrification as sequential bacterial processes that convert ammonia to harmless nitrogen gas.
- Students will calculate nitrogen removal efficiency from influent and effluent concentrations.
- Students will explain how a breakdown in one biological stage affects overall nitrogen removal.
Vocabulary
Key words: nitrification, denitrification, ammonia-nitrogen, aerobic, anoxic
Teacher overview
Fenwick Hollow (a fictional wastewater treatment plant used for this worksheet) removes nitrogen from wastewater using two bacterial stages working in sequence: an aerobic zone where bacteria oxidize ammonia into nitrate, and an anoxic zone where a different set of bacteria convert that nitrate into harmless nitrogen gas, which bubbles out into the atmosphere. This worksheet asks students to track nitrogen through both stages and calculate how completely the plant removes it before discharge. A common misconception worth addressing directly: students often assume "biological treatment" is a single step, when nitrification and denitrification actually require opposite conditions (oxygen present versus oxygen absent) and depend on two different groups of bacteria, so plants give each process its own zone or alternate the two conditions over time.
Student Worksheet
Fenwick Hollow's plant (a fictional facility used for this worksheet) treats an average flow of 1.8 MGD. Two sets of measurements from a recent monitoring report appear below.
Ammonia-nitrogen (measures how completely nitrification is working):
- Influent ammonia-N: 24 mg/L
- Effluent ammonia-N: 0.8 mg/L
Total nitrogen (measures the combined effect of nitrification and denitrification together):
- Influent total nitrogen: 30 mg/L
- Effluent total nitrogen: 3.0 mg/L
- Explain what nitrification does to ammonia. Name the general conditions (oxygen present or absent) the bacteria that carry it out require.
- Explain what denitrification does to the nitrate that nitrification produces. Name the general conditions these bacteria require. Then state what happens to the nitrogen once denitrification is complete.
- Explain why removing nitrogen from wastewater before it reaches a stream matters for that stream's ecosystem. Consider what excess nitrogen can cause once it reaches a body of water.
- Using the removal-efficiency formula (Removal % = (influent − effluent) ÷ influent × 100), calculate the ammonia-nitrogen removal efficiency.
- Calculate the total nitrogen removal efficiency using the same formula.
- Using the pounds formula (Loading in lbs/day = concentration in mg/L × flow in MGD × 8.34), calculate how many pounds of total nitrogen Fenwick Hollow removes each day. Use the difference between the influent and effluent total nitrogen concentrations as your concentration value.
- Suppose the plant's flow rose to 2.4 MGD on a rainy day. The pounds of nitrogen removed per day (your answer to item 6) stayed exactly the same. Assume the influent total nitrogen stays at 30 mg/L. Would the effluent's total nitrogen concentration go up, go down, or stay the same compared to the original 3.0 mg/L? Use the pounds formula to explain your reasoning.
- Suppose a chemical spill killed the bacteria in the anoxic zone. Denitrification stops, but the aerobic zone's nitrifying bacteria keep working normally. Predict what happens to the effluent's nitrate-nitrogen level. Explain why total nitrogen removal efficiency would fall even though ammonia removal stays high.
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: Nitrification oxidizes ammonia first into nitrite and then into nitrate. The bacteria that carry it out are aerobic, meaning they require dissolved oxygen to be present.
- Answer: Denitrification reduces nitrate into nitrogen gas (N2). The bacteria that carry it out work under anoxic conditions, meaning dissolved oxygen is largely absent, and once nitrogen gas forms, it escapes harmlessly into the atmosphere rather than remaining in the water.
- Answer: Excess nitrogen reaching a stream acts as a fertilizer for algae, and a large algae population can later die and decompose in a process that consumes dissolved oxygen, leaving too little for fish and other aquatic life. Removing nitrogen before discharge reduces this risk to the receiving stream's ecosystem.
- Answer: 96.7 percent. (24 − 0.8) ÷ 24 × 100 = 23.2 ÷ 24 × 100 = 96.67%, which rounds to 96.7%.
- Answer: 90 percent. (30 − 3.0) ÷ 30 × 100 = 27 ÷ 30 × 100 = 90%.
- Answer: 405.3 lbs/day. Concentration removed = 30 − 3.0 = 27 mg/L. 27 mg/L × 1.8 MGD × 8.34 = 48.6 × 8.34 = 405.324 pounds, which rounds to 405.3 lbs/day.
- Answer: It would go up, to about 9.8 mg/L. Since lbs/day removed = (concentration removed) × flow × 8.34, holding lbs/day removed constant while flow increases means the concentration removed must decrease: 405.3 ÷ (2.4 × 8.34) = 405.3 ÷ 20.016 ≈ 20.25 mg/L removed, so the effluent concentration becomes about 30 − 20.25 = 9.75 mg/L, roughly 9.8 mg/L, well above the original 3.0 mg/L.
- Answer: Nitrate-nitrogen in the effluent would rise, because without denitrification the nitrate that nitrification produces has nowhere else to go and accumulates instead of converting to nitrogen gas. Total nitrogen removal efficiency would fall even though ammonia removal stays high, because total nitrogen counts both ammonia and nitrate, and the nitrate portion is no longer being removed.
Differentiation
Support (below grade level)
Provide a two-step flow diagram (Ammonia → [aerobic bacteria] → Nitrate → [anoxic bacteria] → Nitrogen gas) for students to reference while answering items 1 and 2, and work through item 4's removal-efficiency calculation together before releasing item 5.
Extension (above grade level)
Ask students to reason out (using only the vocabulary and process described on this worksheet, not outside sources) why nitrification and denitrification cannot happen in the same tank at the same time, and to propose one plant design feature (beyond separate tanks) that could keep the two bacterial populations from interfering with each other.
ELL supports
Pre-teach "nitrification," "denitrification," "aerobic," and "anoxic" with a simple diagram using color to distinguish the oxygen-present zone from the oxygen-absent zone. Allow items 1, 2, and 3 to be answered with a labeled diagram plus short phrases rather than full paragraphs.