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Quiz & test questions Grade 10 Science

Northaven Plant Barriers: A Pathogen and Disinfection Quiz

A 50-point grade 10 quiz on waterborne pathogens, treatment barriers, contact time, log removal, and the criteria a plant must meet before declaring water safe.

Objectives

  1. Students will identify which waterborne pathogens resist chlorine and explain what that means for treatment design.
  2. Students will calculate contact time, required clearwell volume, and log removal from plant data.
  3. Students will state the criteria a utility should meet before declaring treated water safe.

Vocabulary

Multi-barrier approach
The multi-barrier approach is the idea that drinking water is protected by several layers of defense instead of just one, from protecting the source, to treating the water, to keeping it clean all the way through the pipes to the tap. If one barrier fails, the others still help keep water safe.
Turbidity
Turbidity is a measure of how cloudy or murky water looks because of tiny particles floating in it. Clear water has low turbidity, and muddy water has high turbidity.

Also: contact time, log removal, indicator organism, residual

Teacher overview

This quiz covers the pathogens a surface water plant is built to stop, the barriers that stop them, and the arithmetic operators use to prove the barriers are working. It is worth 50 points and mixes multiple choice, short answer, and computation, with the computation items drawn from a single fictional plant so students carry one scenario through. Item 11 and item 12 ask for judgment rather than recall, and they are weighted accordingly. The misconception these items target is the belief that chlorine handles every organism, which shows up as answering every contamination question with a higher dose and overlooking filtration as the barrier that stops Cryptosporidium. Allow calculators.

Northaven Plant Barriers: A Pathogen and Disinfection Quiz — Grade 10 Science
Name: ____________________________ Date: ______________

Assessment

Suppose the fictional Northaven treatment plant draws from a river, runs conventional filtration, and disinfects with chlorine before the water enters a clearwell. The plant treats 3.0 MGD. Use 694 gallons per minute per MGD, and use contact time value equals residual in mg/L times contact time in minutes.

  1. (2 points) Which organism is the most resistant to chlorine at the doses a drinking water plant normally uses?
    A. E. coli bacteria
    B. Giardia cysts
    C. Cryptosporidium oocysts
    D. Rotavirus
  2. (2 points) A distribution sample comes back positive for total coliform bacteria. What does that result most directly indicate?
    A. Customers in that area are already sick
    B. The sample contains raw sewage
    C. A pathway exists for contamination to enter the system
    D. The chlorine residual is above 4.0 mg/L
  3. (3 points) Name three barriers in a multi-barrier treatment approach, and state for each one what it removes or inactivates.
  4. (4 points) The plant holds a free chlorine residual of 1.2 mg/L, and water spends 45 minutes in the clearwell. Calculate the contact time value in mg/L-min.
  5. (4 points) The plant must reach a contact time value of 90 mg/L-min while holding the same residual of 1.2 mg/L. Calculate the contact time in minutes that this requires.
  6. (5 points) The plant treats 3.0 MGD. Using 694 gallons per minute per MGD, calculate the clearwell volume in gallons that would give 75 minutes of contact time at that flow.
  7. (5 points) A raw water sample contains 30,000 organisms per liter, and the filtered water contains 3 organisms per liter. Calculate the log removal the filters achieved.
  8. (4 points) The plant reports finished water turbidity of 0.12 NTU against a standard of 0.3 NTU. Calculate the percent by which the plant's result sits below the standard.
  9. (4 points) A turbidimeter cannot detect a single organism. Explain why plants still monitor turbidity continuously as evidence that pathogen removal is working.
  10. (5 points) A main break drops pressure in one zone to 8 psi. Explain how low pressure can allow contamination into a distribution system, and state the minimum pressure most states require.
  11. (6 points) After heavy storms the plant's raw water carries more Cryptosporidium. The plant can add ultraviolet disinfection, extend chlorine contact time, or improve filter performance. Choose the option that best addresses this organism, and justify your choice using what you know about how the organism responds to each barrier.
  12. (6 points) A contamination event has been corrected and the plant wants to declare the water safe again. State two quantitative criteria and one qualitative criterion the plant should meet first, and explain briefly why each one belongs on the list.

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: C.
    Cryptosporidium oocysts have a thick wall that resists chlorine at drinking water doses, which is why plants rely on filtration and ultraviolet light for that organism. Giardia cysts are more resistant than bacteria but far less resistant than Cryptosporidium, and E. coli and rotavirus are readily inactivated by chlorine.
  2. Answer: C.
    Total coliform bacteria are indicator organisms rather than pathogens. Their presence signals that a route into the system exists, through a break, a cross connection, or a loss of residual, and it triggers follow-up sampling. It does not by itself show illness or sewage.
  3. Answer: Source protection, coagulation and sedimentation, filtration, disinfection, and a maintained residual.
    Award one point each for any three barriers correctly paired with what they do. Source protection keeps contamination out of the raw water. Coagulation and sedimentation clump and settle particles including many organisms. Filtration strains out the remaining particles, and it is the main barrier against Cryptosporidium. Disinfection inactivates bacteria and viruses, and a residual in the distribution system guards against later contamination.
  4. Answer: 54 mg/L-min.
    Multiply residual by contact time: 1.2 mg/L x 45 minutes = 54 mg/L-min.
  5. Answer: 75 minutes.
    Divide the required value by the residual: 90 / 1.2 = 75 minutes. Students who multiply will report 108 and should check which quantity the formula asks them to isolate.
  6. Answer: 156,150 gallons.
    Convert flow first: 3.0 MGD x 694 gallons per minute per MGD = 2,082 gallons per minute. Then multiply by the required time: 2,082 x 75 minutes = 156,150 gallons of clearwell volume.
  7. Answer: 4.0 logs.
    Divide the two counts and take the base ten logarithm: 30,000 / 3 = 10,000, and the logarithm of 10,000 is 4. The filters removed 99.99 percent of the organisms, which is the same statement in different form.
  8. Answer: 60 percent.
    Subtract and divide by the standard: 0.3 minus 0.12 = 0.18, and 0.18 / 0.3 = 0.60, so the finished water sits 60 percent below the standard.
  9. Answer: Turbidity is a continuous surrogate for particle removal.
    Organisms such as Cryptosporidium are particles, and the same filter that captures clay and silt captures them. A microbiological test takes a day or more and covers one sample, while a turbidimeter reports every few seconds on every gallon leaving the filter. A rise in turbidity means the filter is passing particles, so it is the earliest available warning that the pathogen barrier has weakened. Full credit requires both the surrogate idea and the speed of the measurement.
  10. Answer: Low pressure allows backflow, and most states require at least 20 psi.
    Positive pressure inside the pipe keeps water moving outward through any crack, joint, or cross connection. When pressure falls toward zero, that protection disappears and water can be drawn inward from a flooded trench, a submerged hose, or a customer's plumbing. Soil around a break is often contaminated, so the break itself becomes the entry point. Most states set 20 psi as the minimum pressure the system must maintain everywhere.
  11. Answer: Improve filter performance, with ultraviolet disinfection as a strong second choice.
    Cryptosporidium resists chlorine, so extending contact time buys very little against this organism and would raise disinfection byproducts at the same time. Filtration physically removes the oocysts, which makes filter performance the barrier that matters most after a storm, and small reductions in filter effluent turbidity translate into measurable gains in removal. Award full credit for a defense of ultraviolet disinfection that states the organism is inactivated by ultraviolet light at doses that chlorine cannot match. Award partial credit at most for extending chlorine contact time.
  12. Answer: Two measured criteria plus one judgment criterion.
    Acceptable quantitative criteria include a detectable chlorine residual restored throughout the affected zone, finished water turbidity back at or below 0.3 NTU, a required number of consecutive negative bacteriological samples taken on separate days, and system pressure restored above the 20 psi minimum. An acceptable qualitative criterion names something that cannot be read off an instrument, such as confirmation that the source of the contamination has been found and repaired, that hydrants and dead ends have been flushed, or that the state agency concurs. Award four points for the two measured criteria with reasons and two points for the qualitative criterion with its reason.

Differentiation

Support

Give students a formula card for the contact time relationship and the flow conversion, and provide item 7 with the division already performed so the task narrows to taking the logarithm. Allow students to answer item 3 as a labeled flow diagram of the fictional Northaven plant rather than in sentences.

Extension

Have students calculate the clearwell volume the fictional Northaven plant would need if flow rose to 4.5 MGD while the required contact time value stayed at 90 mg/L-min and the residual dropped to 1.0 mg/L. Ask them to state one operating change that would meet the requirement without building a larger clearwell.

ELL supports

Pre-teach "residual," "barrier," and "indicator organism" with a diagram showing chlorine remaining in the water after the reactions finish. Provide a word bank for item 3 that lists the barrier names, so students supply the function rather than recalling both halves, and offer sentence frames for items 9 and 10.

Extensions

Ask students to convert item 12 into a one-page checklist that the fictional Northaven plant could hand to an operator during a contamination event. The checklist should show each criterion, the measurement or confirmation that satisfies it, and the person responsible, and it should end with the statement the plant would release only after every line is complete.