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Quiz & test questions Grade 12 Science

Brockhurst Plant: Disinfection and Treatment Unit Test

A grade 12 unit test with multiple-choice, short-answer, and computation items on turbidity, chlorine dosing, removal efficiency, and detention time.

Objectives

  1. Students will identify the EPA turbidity and chlorine disinfectant limits.
  2. Students will calculate chlorine feed rate, demand, removal efficiency, and detention time.
  3. Students will explain why a chlorine residual is maintained across the distribution system.

Vocabulary

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.
Chlorine residual
Chlorine residual is the small amount of chlorine still left in the water after it leaves the treatment plant. It keeps protecting the water from germs all the way to your faucet.
Detention time
Detention time is how long water stays inside a tank or basin before flowing out. Water must stay long enough for each treatment step, like settling or disinfecting, to finish its job.
Disinfection
Disinfection is the step that kills or disables germs in water so it is safe to drink. Water plants usually disinfect with chlorine, ultraviolet light, or ozone.

Also: removal efficiency

Teacher overview

This 40-minute unit test is worth 25 points and checks whether students can apply the core numbers and processes of drinking-water disinfection. It mixes two multiple-choice items, two short-answer items, and four computation items built around a fictional plant's flow and readings. The computation items use the pounds formula, removal efficiency, and detention time, while the short-answer items check conceptual understanding of the finished-water standards.

Brockhurst Plant: Disinfection and Treatment Unit Test — Grade 12 Science
Name: ____________________________ Date: ______________

Assessment

Suppose the fictional Brockhurst Water Treatment Plant treats a steady flow of 5.5 MGD. Use these constants where needed: one gallon weighs 8.34 pounds, one MGD equals 694 gpm, and load (lbs/day) = concentration (mg/L) x flow (MGD) x 8.34.

  1. (2 points) EPA rules require conventional plants to keep finished-water turbidity at or below what level in 95 percent of monthly samples?

    • A) 0.3 NTU
    • B) 1 NTU
    • C) 5 NTU
    • D) 15 NTU

  2. (4 points) The plant feeds chlorine at a dose of 2.2 mg/L. Using the pounds formula and the 5.5 MGD flow, find the chlorine feed rate in pounds per day.

  3. (3 points) The plant measures a chlorine residual of 0.8 mg/L at the 2.2 mg/L dose. Find the chlorine demand, where demand = dose - residual.

  4. (2 points) The maximum residual disinfectant level (MRDL) for chlorine is:

    • A) 0.3 mg/L
    • B) 2.0 mg/L
    • C) 4.0 mg/L
    • D) 15 mg/L

  5. (4 points) Water entering the filters reads 9.5 NTU, and the filtered water reads 0.15 NTU. Find the turbidity removal efficiency across the filters, using removal efficiency (%) = (influent - effluent) / influent x 100.

  6. (2 points) Based on your answer to item 1, does the filtered water at 0.15 NTU meet the finished-water turbidity limit? State the margin.

  7. (4 points) The clearwell holds 300,000 gallons. Find the detention time in minutes at the 5.5 MGD flow. First convert the flow to gpm, then use detention time = volume (gal) / flow (gpm).

  8. (4 points) Explain why the plant maintains a chlorine residual all the way through the distribution system rather than only at the plant.

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: A) 0.3 NTU.
    Conventional filtration plants must keep finished-water turbidity at or below 0.3 NTU in 95 percent of monthly samples, so A is correct; 1 NTU is the never-exceed ceiling, not the routine limit.

  2. Answer: 100.9 lbs/day.
    2.2 mg/L x 5.5 MGD x 8.34 = 12.1 x 8.34 = 100.914, about 100.9 pounds per day.

  3. Answer: 1.4 mg/L.
    Chlorine demand = dose - residual = 2.2 mg/L - 0.8 mg/L = 1.4 mg/L.

  4. Answer: C) 4.0 mg/L.
    The chlorine MRDL is 4.0 mg/L, so C is correct; 2.0 mg/L falls inside the typical distribution residual range but is not the regulatory maximum.

  5. Answer: 98.4%.
    (9.5 - 0.15) / 9.5 x 100 = 9.35 / 9.5 x 100 = 98.42 percent, about 98.4 percent removal.

  6. Answer: yes, by 0.15 NTU.
    The filtered water at 0.15 NTU is below the 0.3 NTU limit from item 1, clearing it by 0.3 - 0.15 = 0.15 NTU.

  7. Answer: 78.6 min.
    Flow = 5.5 MGD x 694 = 3,817 gpm. Detention time = 300,000 gal / 3,817 gpm = 78.6 minutes.

  8. Answer: A residual guards the water against contamination on the way to homes.
    The distribution system is long and can pick up bacteria through leaks, repairs, or backflow. Keeping a measurable chlorine residual from the plant to the tap means any contamination entering the pipes still meets disinfectant, which protects customers far from the plant.

Differentiation

Support

Provide a formula card with the pounds formula, the removal-efficiency formula, and the flow conversion, and allow students to use it during the computation items so recall of the constants is not the barrier.

Extension

Ask students to find the chlorine dose that would raise the item 2 feed rate to exactly 150 pounds per day at the same plant flow, and to state whether the resulting residual could still fall within the typical distribution range.

ELL supports

Pre-teach "turbidity," "residual," "demand," and "detention time" with a labeled diagram of the treatment train, and allow the short-answer items to be completed with a sentence frame that names the hazard and the protection.

Extensions

Have students pull the reported finished-water turbidity and chlorine residual from a local Consumer Confidence Report and compare those real values with the Brockhurst figures on this test, noting how much margin the real plant keeps below each limit.