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Quiz & test questions Grade 6 Science

Bellmarsh Water Reclamation: Microbes, Oxygen, and the River

A 30-point grade 6 science quiz on how microbes treat wastewater, using a fictional plant record to test reasoning about oxygen, settling, and river health.

Objectives

  1. Students will explain how bacteria and oxygen work together to treat wastewater.
  2. Students will interpret a monthly plant record to identify what went wrong in one month.
  3. Students will propose what a plant should monitor and what result should trigger action.

Vocabulary

Aeration
Aeration means mixing air into water. At a wastewater plant, blowers bubble huge amounts of air through tanks so the helpful microbes have the oxygen they need to eat the waste.
Effluent
Effluent is the cleaned water that leaves a treatment plant at the end of the process. It is tested carefully before being released to a river, lake, or the ocean.
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.

Also: settling, sludge, discharge

Teacher overview

This quiz covers a short unit on biological wastewater treatment: what the microbes in an aeration tank do, what they need in order to do it, and what happens in a receiving river when treatment slips. It is worth 30 points across twelve items, mixing multiple choice with short written answers, and it includes a five-month plant record that several items depend on. No item asks for a calculation. One misconception the quiz probes directly is the belief that treatment is a matter of adding chemicals, and that a plant with a problem should simply add more disinfectant. Items 2 and 7 will separate students who understand that the plant is running a living process from students who picture treatment as filtering and dosing alone.

Bellmarsh Water Reclamation: Microbes, Oxygen, and the River — Grade 6 Science
Name: ____________________________ Date: ______________

Assessment

The Bellmarsh Water Reclamation Facility serves the fictional town of Bellmarsh and discharges treated water to the fictional Foxcote River. The record below was written for this quiz.

At Bellmarsh, wastewater first passes through screens and a settling tank that remove solids. The water then enters the aeration tank. Blowers push air through that tank. Bacteria and other microbes living in the tank feed on the dissolved waste in the water and use oxygen as they do it. The mixture then flows to a second settling tank, where the microbes clump together and sink, leaving clearer water above. That clearer water, called the effluent, is disinfected and sent to the river.

Biochemical oxygen demand, or BOD, measures how much oxygen microbes would use to break down the waste in a water sample. High BOD means a lot of material for microbes to consume.

Table 1. Bellmarsh plant record (fictional)

Month Oxygen in the aeration tank BOD entering the plant BOD in the effluent Settling test result
April 2.2 mg/L 220 mg/L 14 mg/L Good; sludge settled firmly
May 2.0 mg/L 235 mg/L 16 mg/L Good; sludge settled firmly
June 2.1 mg/L 240 mg/L 15 mg/L Good; sludge settled firmly
July 0.6 mg/L 310 mg/L 48 mg/L Poor; sludge floated
August 2.3 mg/L 245 mg/L 18 mg/L Good; sludge settled firmly

For reference, secondary treatment standards require a monthly average effluent BOD of 30 mg/L or less, and most fish need dissolved oxygen above 5 mg/L.

  1. (2 points) What is the main job of the microbes in the aeration tank?
    A. They filter solids out of the water.
    B. They feed on dissolved waste and break it down.
    C. They kill germs in the water.
    D. They add oxygen to the water.

  2. (2 points) Why do the blowers push air through the aeration tank?
    A. To cool the water before it reaches the river.
    B. To give the microbes the oxygen they need.
    C. To bubble the dirt out of the water.
    D. To kill the microbes once they have finished.

  3. (3 points) Explain what BOD measures. Then explain why water with a high BOD is a problem for fish in the Foxcote River.

  4. (2 points) According to Table 1, in which month did the plant send the most oxygen-demanding waste to the river?
    A. April
    B. June
    C. July
    D. August

  5. (3 points) Look at the aeration tank oxygen in July and compare it with the other four months. Explain how that difference could have caused the change in the effluent BOD.

  6. (2 points) The July settling test came back poor, with sludge floating instead of sinking. What does that result tell the operator?
    A. The effluent that month was cleaner than usual.
    B. The microbes in the tank were not in good condition.
    C. The river was running low.
    D. The screens at the front of the plant were clogged.

  7. (3 points) A resident says the plant should have added more disinfectant in July instead of fixing the blowers. Explain why more disinfectant would not have lowered the effluent BOD.

  8. (3 points) Explain why this plant depends on living things and not on filters alone. Name two things operators must supply to keep those living things working.

  9. (2 points) If effluent BOD stays high for several weeks, what happens in the river below the discharge pipe?
    A. Dissolved oxygen in the river falls.
    B. Dissolved oxygen in the river rises.
    C. The river runs faster.
    D. Nothing changes until the next rain.

  10. (4 points) Name two measurements the plant should take every day. For each one, state the result that would tell the operator something is going wrong.

  11. (2 points) In your own words, what is effluent? Name the place in the plant where you would collect an effluent sample.

  12. (2 points) A restaurant dumps a large load of grease into the sewer one night. Explain why that is a problem for the microbes in the aeration tank.

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: B. The microbes feed on dissolved waste and break it down. Choice A describes the settling tanks and screens, and choice D reverses the process, since the microbes use oxygen rather than adding it.
  2. Answer: B. The blowers supply oxygen the microbes need to break down waste. Air does stir the tank, but stirring is not the reason the plant spends energy on blowers.
  3. Answer: BOD measures how much oxygen microbes would use to break down the waste in a sample.
    Award full credit when the student also explains that waste discharged to the river keeps river microbes consuming oxygen, which lowers the dissolved oxygen fish need, and that most fish need dissolved oxygen above 5 mg/L.
  4. Answer: C. July, when the effluent BOD reached 48 mg/L, far above the other months and above the 30 mg/L secondary treatment standard.
  5. Answer: Oxygen in the tank fell to 0.6 mg/L in July, compared with about 2 mg/L in the other months. Without enough oxygen the microbes could not break down the waste fast enough, so more of it passed through to the effluent and the BOD rose to 48 mg/L. Accept answers that also note the higher incoming load that month.
  6. Answer: B. The microbes were not in good condition. Sludge that floats instead of settling shows the community in the tank has changed, which is what happens when oxygen runs short.
  7. Answer: Disinfectant kills germs but does not remove dissolved waste. BOD is a measure of material left in the water for microbes to consume, so the only way to lower it is to let the microbes finish the job, which requires restoring oxygen to the tank.
  8. Answer: The microbes do the actual breaking down of dissolved waste, which no screen or settling tank can remove, so operators keep a living population healthy. Accept any two of these: oxygen from the blowers, a steady supply of food in the incoming wastewater, enough time in the tank, a stable temperature, and the return of settled sludge to keep the population large enough.
  9. Answer: A. Dissolved oxygen in the river falls, because microbes in the river use oxygen to break down the extra waste arriving in the effluent.
  10. Answer: Accept any two sensible measurements with matching triggers. Strong answers include dissolved oxygen in the aeration tank, where a reading well below about 2 mg/L signals trouble; effluent BOD or a same-day substitute, where a rise toward 30 mg/L signals trouble; a settling test, where floating sludge signals trouble; and incoming flow or load, where a sudden spike signals trouble.
  11. Answer: Effluent is the treated water leaving the plant. A student should name the point after the second settling tank or at the discharge pipe to the river.
  12. Answer: A sudden heavy load of grease is a large amount of food arriving at once, and it is hard for the microbes to break down. It can use up the oxygen in the tank quickly, coat the microbes so they cannot take in oxygen, and cause poor settling, all of which let more waste pass into the effluent.

Differentiation

Support

Provide a labeled flow diagram of the plant, from screens through the aeration tank and second settling tank to the discharge pipe, and let students mark the sampling points on it while they work. Reduce the quiz to items 1 through 7 and item 11, scaled to a smaller total. Provide a word bank containing oxygen, microbes, settling, effluent, and BOD, and allow students to answer item 8 with a labeled drawing.

Extension

Ask students to write the short note the fictional operator would send the town after July, explaining in plain language what happened, what was fixed, and what the plant will watch next. A second option is to explain what the plant should do differently if the incoming load rose every July, rather than only in one year.

ELL supports

Pre-teach "aeration," "effluent," "settling," "sludge," and "discharge" with the labeled flow diagram and a jar demonstration in which stirred soil settles once the stirring stops. Provide a bilingual glossary card and allow extra time. Accept short answers written in phrases rather than full sentences for items 3, 5, and 12, and allow oral responses for item 11.

Extensions

Have students find out where their own community's treated wastewater is discharged, and report the name of the receiving water body and one thing the utility measures before discharging. A second option is to compare the incoming BOD numbers in the fictional record with the usual range for raw sewage and write one sentence about what a value above that range might mean.