Water for Schools
Menu

Quiz & test questions Grade 12 Engineering & Design

Redmarsh Plant Operations Exam: Operator Math and Unit Conversions

A grade 12 engineering unit test on plant operations math, mixing multiple-choice, short-answer, and computation items set at a fictional treatment plant.

Objectives

  1. Students will apply standard operator conversions and the pounds formula to plant calculations.
  2. Students will interpret detention time, pressure, removal efficiency, and loading rate results.

Vocabulary

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.

Also: pounds formula, loading rate, removal efficiency, head

Teacher overview

This 40-minute unit test (30 points total) checks whether students can apply operator math to a treatment plant. The items are set at the fictional Redmarsh regional water plant, which runs at 4.0 MGD. The test mixes three multiple-choice items, two short-answer items, and five computation items, drawing on conversions, the pounds formula, detention time, pressure, removal efficiency, and filter loading rate.

Redmarsh Plant Operations Exam: Operator Math and Unit Conversions — Grade 12 Engineering & Design
Name: ____________________________ Date: ______________

Assessment

  1. (2 points) One gallon of water weighs about how much?
    • A) 7.48 lb
    • B) 8.34 lb
    • C) 0.433 lb
    • D) 2.31 lb
  2. (2 points) One MGD equals about how many gallons per minute?
    • A) 448
    • B) 694
    • C) 7.48
    • D) 325,851
  3. (4 points) The fictional Redmarsh plant doses chlorine at 2.0 mg/L while treating 4.0 MGD. Using the pounds formula (pounds per day = concentration x flow x 8.34), find the chlorine feed in pounds per day.
  4. (4 points) A clearwell at the plant holds 250,000 gallons. At a flow of 4.0 MGD (4,000,000 gallons per day), find the detention time in hours. Use detention time = volume / flow.
  5. (3 points) A hydrant gauge reads 62 psi. Convert this pressure to feet of head, using 1 psi = 2.31 feet.
  6. (4 points) A sedimentation step lowers solids from 40 mg/L to 6 mg/L. Find the removal efficiency, using (influent - effluent) / influent x 100.
  7. (3 points) Distribution systems usually operate in what pressure range?
    • A) 0 to 10 psi
    • B) 40 to 80 psi
    • C) 200 to 300 psi
    • D) up to 500 psi
  8. (3 points) A filter is running at a loading rate of 7 gpm per square foot, but conventional filters are designed for 2 to 6 gpm per square foot. In one or two sentences, explain what an operator should conclude and do.
  9. (3 points) A rectangular basin is 60 feet long, 20 feet wide, and 10 feet deep. Find its volume in gallons, using 1 cubic foot = 7.48 gallons.
  10. (2 points) In one or two sentences, explain why an operator must carry units through every step of a plant calculation.

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) 8.34 lb.
    One gallon of water weighs 8.34 pounds, the standard constant used in the pounds formula.
  2. Answer: B) 694.
    One MGD equals about 694 gallons per minute by the standard operator conversion.
  3. Answer: 66.72 pounds per day.
    Feed = 2.0 mg/L x 4.0 MGD x 8.34 = 66.72 pounds per day.
  4. Answer: 1.5 hours.
    Detention time = 250,000 gallons / 4,000,000 gallons per day = 0.0625 day; 0.0625 x 24 = 1.5 hours.
  5. Answer: 143.22 feet.
    62 psi x 2.31 feet per psi = 143.22 feet of head.
  6. Answer: 85 percent.
    Removal efficiency = (40 - 6) / 40 x 100 = 34 / 40 x 100 = 85 percent.
  7. Answer: B) 40 to 80 psi.
    Distribution systems typically operate between 40 and 80 psi, with most states requiring at least 20 psi at the connection.
  8. Answer: The filter is overloaded.
    The loading rate of 7 gpm per square foot is above the 2-to-6 design range, so the filter is overloaded. The operator should reduce the flow through it, or bring another filter online, to avoid pushing solids through.
  9. Answer: 89,760 gallons.
    Volume = 60 x 20 x 10 = 12,000 cubic feet; 12,000 x 7.48 = 89,760 gallons.
  10. Answer: Units prevent errors.
    Accept any sound explanation: tracking units confirms each step is set up correctly and catches mistakes, such as mixing MGD with gallons per minute, that would otherwise produce a wrong feed rate or detention time.

Differentiation

Support

Provide a conversion and formula reference sheet during the test, and allow a calculator for every computation item. Reduce the multiple-choice distractors from four options to three where a shorter test is appropriate.

Extension

Ask students who finish early to rework item 4 for a flow of 3.0 MGD and explain how the longer detention time would affect settling, then check whether the item 8 filter would return to the design range if its flow were cut by one-third.

ELL supports

Provide a glossary card with "detention time," "head," "removal efficiency," and "loading rate," each with a small labeled diagram. Provide sentence frames for the short-answer items 8 and 10.