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Quiz & test questions Grade 10 Science

Brightwater Utility Operations: A Grade 10 Unit Test

A 30-minute grade 10 unit test on the fictional Brightwater Utility, mixing multiple-choice, short-answer, and computation items on disinfection, turbidity, and pressure.

Objectives

  1. Students will calculate a chlorine feed rate using the pounds formula.
  2. Students will calculate turbidity removal efficiency and distribution pressure.
  3. Students will explain the purpose of a chlorine residual and the disinfectant limit.

Vocabulary

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.
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: removal efficiency, pounds formula, distribution pressure

Teacher overview

This 30-minute unit test (25 points total) checks whether students can apply the core numbers of drinking-water operations: disinfection, turbidity removal, and distribution pressure. It uses the fictional Brightwater Utility and mixes two multiple-choice items, four computation items, and two short-answer items, drawing on the chlorine limit, the turbidity standard, and the pressure formula operators use every day.

Brightwater Utility Operations: A Grade 10 Unit Test — Grade 10 Science
Name: ____________________________ Date: ______________

Assessment

Brightwater Utility (a fictional utility) runs a small treatment plant and distribution system. Use these facts: the maximum residual disinfectant level (MRDL) for chlorine is 4.0 mg/L, and systems keep a residual of 0.2 to 2.0 mg/L in distribution; finished-water turbidity must stay at or below 0.3 NTU in 95 percent of monthly samples; pressure (psi) = height of water (ft) x 0.433; chlorine feed (lbs/day) = dose (mg/L) x flow (MGD) x 8.34; removal efficiency (%) = (influent − effluent) divided by influent x 100.

  1. (2 points) What is the main purpose of adding chlorine at a drinking-water plant?
    • A) To raise the water pressure
    • B) To kill disease-causing organisms
    • C) To remove cloudiness
    • D) To soften the water
  2. (2 points) A finished-water sample at Brightwater reads 0.5 NTU. Compared with the standard of 0.3 NTU or less, this reading is:
    • A) below the standard
    • B) above the 0.3 NTU benchmark
    • C) exactly at the standard
    • D) impossible to measure
  3. (4 points) Brightwater treats 2.0 MGD and doses chlorine at 2.5 mg/L. Using chlorine feed (lbs/day) = dose x flow x 8.34, calculate the chlorine feed rate in pounds per day.
  4. (4 points) The raw water enters with a turbidity of 12 NTU and leaves the filters at 0.2 NTU. Calculate the turbidity removal efficiency as a percent. Round to the nearest tenth.
  5. (3 points) A home sits 90 ft below the tank's water surface. Using pressure (psi) = height (ft) x 0.433, calculate the pressure at the home, and state whether it is above the 20 psi minimum.
  6. (3 points) An operator suggests raising the chlorine dose well above what is needed, to be extra safe. Using the MRDL of 4.0 mg/L, explain one benefit and one drawback of that change.
  7. (3 points) Brightwater doses chlorine at 2.5 mg/L and measures a residual of 1.8 mg/L leaving the plant. Using chlorine demand = dose − residual, calculate the chlorine demand in mg/L.
  8. (4 points) Explain why Brightwater works to keep a chlorine residual of at least 0.2 mg/L all the way to the last home on the line, 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: B) To kill disease-causing organisms. Chlorine is a disinfectant; it inactivates pathogens, which is the central goal of disinfection, not pressure, clarity, or hardness.
  2. Answer: B) above the 0.3 NTU benchmark. A reading of 0.5 NTU is higher than the 0.3 NTU standard, so the sample is above the benchmark.
  3. Answer: 41.7 lbs/day. Chlorine feed = 2.5 mg/L x 2.0 MGD x 8.34 = 41.7 pounds per day.
  4. Answer: 98.3%. Removal efficiency = (12 − 0.2) divided by 12 x 100 = 11.8 divided by 12 x 100 = 98.33, which rounds to 98.3 percent.
  5. Answer: 39 psi, above the minimum. Pressure = 90 ft x 0.433 = 38.97, about 39 psi, which is above the 20 psi minimum (though below the typical 40 psi target).
  6. Answer: Accept any answer naming a real benefit and a real drawback. A benefit is stronger protection against pathogens and a lasting residual; a drawback is that too much chlorine can push the residual toward the 4.0 mg/L MRDL, worsen taste and odor, and form more disinfection byproducts.
  7. Answer: 0.7 mg/L. Chlorine demand = dose − residual = 2.5 − 1.8 = 0.7 mg/L.
  8. Answer: A residual all the way to the last tap means chlorine is still present to inactivate any organisms that enter through a leak, a repair, or backflow between the plant and the home, so keeping at least 0.2 mg/L protects the water during its whole trip, not just at the plant.

Differentiation

Support

Provide a formula reference card with the pounds formula, the pressure formula, and the removal-efficiency formula already written out, and allow a calculator. Give partial credit for correct setup even when the final arithmetic slips.

Extension

Add a challenge item: if Brightwater's flow rises to 3.0 MGD but the operator must keep the same 41.7 pounds per day of chlorine feed, ask students to solve for the new dose in mg/L and explain what that lower dose means for the residual.

ELL supports

Pair "residual," "turbidity," "removal efficiency," and "distribution" with short definitions and a simple plant-to-home diagram. Provide sentence frames for items 6 and 8, such as "A higher chlorine dose helps because ___, but it can cause ___."