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Free WTO Practice Questions

10 free, exam-style Water Treatment Operator Certification (Class I) (WTO) practice questions with answers and explanations. No signup required. Work through them below, then take the full free WTO practice test to study every exam domain.

These 10 free WTO questions are organized by exam domain, so you can see how each part of the Water Treatment Operator Certification (Class I) blueprint is tested. Reveal the answer and explanation under each question.

Domain 1: Treatment Processes

Question 1

An operator needs to maintain a free chlorine residual of 0.5 mg/L. Laboratory testing shows the chlorine demand of the water is 1.8 mg/L. What chlorine dose should the operator apply?

  1. 0.5 mg/L
  2. 1.3 mg/L
  3. 1.8 mg/L
  4. 2.3 mg/L
Show answer & explanation

Correct answer: D - 2.3 mg/L

Question 2

The required CT for 0.5-log Giardia inactivation at the current pH and temperature is 30 mg·min/L. The free chlorine residual leaving the contact basin is 0.5 mg/L. What is the minimum contact time needed?

  1. 15 minutes
  2. 30 minutes
  3. 45 minutes
  4. 60 minutes
Show answer & explanation

Correct answer: D - 60 minutes

Question 3

The breakpoint on a breakpoint chlorination curve is the point where:

  1. Combined chlorine residual reaches its maximum
  2. Chloramines are destroyed and free chlorine residual first appears
  3. Total chlorine residual equals zero
  4. The chlorine dose equals the chlorine demand
Show answer & explanation

Correct answer: B - Chloramines are destroyed and free chlorine residual first appears

Question 4

During a filter run, the effluent turbidity has been steady at 0.06 NTU for 36 hours but suddenly rises to 0.4 NTU. This is MOST likely:

  1. A calibration error on the turbidimeter
  2. Turbidity breakthrough indicating the filter needs backwashing
  3. Initial ripening turbidity from a recent backwash cycle that will stabilize
  4. Evidence that the disinfection system has failed
Show answer & explanation

Correct answer: B - Turbidity breakthrough indicating the filter needs backwashing

Domain 2: Water Quality Characteristics

Question 5

A routine distribution system sample tests positive for total coliforms but negative for E. coli. This result indicates:

  1. Recent fecal contamination requiring immediate public notification
  2. Possible environmental contamination but no fecal source confirmed
  3. The sample was improperly collected and must be discarded
  4. An automatic MCL violation has occurred under the RTCR
Show answer & explanation

Correct answer: B - Possible environmental contamination but no fecal source confirmed

Domain 3: Treatment Equipment and Instrumentation

Question 6

A centrifugal raw water pump is making a sound like gravel or marbles passing through it. What is the MOST likely cause?

  1. Worn bearings
  2. Loose impeller
  3. Cavitation
  4. Air trapped in the casing
Show answer & explanation

Correct answer: C - Cavitation

Question 7

A pump delivers 350 gpm against 120 feet of head. The pump efficiency is 80% and the motor efficiency is 92%. What is the approximate Motor Horsepower required?

  1. 8.5 HP
  2. 10.6 HP
  3. 13.3 HP
  4. 14.4 HP
Show answer & explanation

Correct answer: D - 14.4 HP

Domain 4: Source Water and Supply

Question 8

During fall reservoir turnover, an operator should be MOST concerned about:

  1. Decreased raw water flow to the treatment plant intake
  2. Release of iron, manganese, and taste-and-odor compounds from the bottom
  3. Increased algae growth at the reservoir surface layer
  4. Freezing of the intake structures and raw water lines
Show answer & explanation

Correct answer: B - Release of iron, manganese, and taste-and-odor compounds from the bottom

Domain 5: Safety and Regulatory Compliance

Question 9

When testing the atmosphere of a confined space before entry, which parameter should be tested FIRST?

  1. Carbon monoxide
  2. Hydrogen sulfide
  3. Oxygen
  4. Lower explosive limit
Show answer & explanation

Correct answer: C - Oxygen

Question 10

According to LOTO procedures, who may remove a lockout device?

  1. Any qualified maintenance worker
  2. The supervisor only
  3. Only the person who applied it
  4. The equipment manufacturer
Show answer & explanation

Correct answer: C - Only the person who applied it

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