Wastewater Operator Class III
Licensing Exam Questions And Answers
Plus Rationales | Qs & Ans 2026 | Instant Pdf
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The Class III exam typically covers advanced process control,
complex troubleshooting, solids handling, advanced laboratory
analysis, plant optimization, safety management, and regulatory
compliance for larger or more sophisticated facilities.
1. An activated sludge plant is experiencing high effluent nitrate
and low effluent ammonia. The DO in the aeration basin is 2.5 mg/L.
What does this indicate?
A) Denitrification is complete
B) Full nitrification is occurring
C) The F/M ratio is too high
D) The sludge age is too low
Answer: B
Rationale: Low effluent ammonia with high nitrate indicates complete
nitrification (ammonia converted to nitrate). DO > 2.0 mg/L supports
nitrifying bacteria.
,2. A Class III operator notices that the secondary clarifier has a
sludge blanket depth of 5 feet (normal 2 ft). Mixed liquor DO is 0.3
mg/L. What is the most likely cause?
A) Excessive wasting
B) Low RAS rate combined with low DO causing poor flocculation
C) High recirculation rate
D) Chlorine residual too high
Answer: B
Rationale: Low DO (0.3 mg/L) leads to poor floc formation and
incomplete settling. Insufficient RAS exacerbates blanket depth. Increase
aeration and RAS.
3. Which of the following best describes the purpose of anoxic
zones in activated sludge?
A) To increase BOD removal
B) To facilitate denitrification by providing low DO and nitrate presence
C) To add chlorine for disinfection
D) To settle solids
Answer: B
Rationale: Anoxic zones have DO < 0.5 mg/L but nitrate present,
allowing facultative bacteria to use nitrate as electron acceptor
(denitrification), reducing nitrogen.
,4. A plant operates a 5-stage Bardenpho process. What is the
primary goal?
A) Phosphorus removal only
B) Biological nitrogen and phosphorus removal (BNR)
C) Methane production
D) Sludge thickening
Answer: B
Rationale: The Bardenpho process includes anaerobic, anoxic, and
aerobic zones for enhanced biological nitrogen (denitrification) and
phosphorus removal (PAOs).
5. A sudden increase in effluent TSS occurs after a heavy rain. The
secondary clarifier SVI is 110 mL/g. What is the most likely cause?
A) Hydraulic overload causing short-circuiting and solids washout
B) Filamentous bulking
C) Low F/M ratio
D) Excessive polymer
Answer: A
Rationale: SVI of 110 is normal; heavy rain increases flow, raising
overflow rate and weir loading, leading to solids carryover. Check flow
equalization.
, 6. An anaerobic digester’s volatile acids/alkalinity ratio is 0.65. Gas
production has dropped. What action should be taken?
A) Increase feed rate
B) Reduce or stop feeding, add alkalinity (lime or bicarbonate), and
increase mixing
C) Increase temperature
D) Add polymer
Answer: B
Rationale: Ratio > 0.4 indicates souring. Stop feeding to allow recovery,
add alkalinity, and maintain mixing to restore methanogens.
7. What is the maximum recommended hydrogen sulfide
concentration for 8-hour worker exposure (OSHA PEL)?
A) 1 ppm
B) 10 ppm
C) 50 ppm
D) 100 ppm
Answer: B
Rationale: OSHA permissible exposure limit (PEL) for H₂S is 10 ppm as
an 8-hour time-weighted average. IDLH is 100 ppm.
Licensing Exam Questions And Answers
Plus Rationales | Qs & Ans 2026 | Instant Pdf
Download
The Class III exam typically covers advanced process control,
complex troubleshooting, solids handling, advanced laboratory
analysis, plant optimization, safety management, and regulatory
compliance for larger or more sophisticated facilities.
1. An activated sludge plant is experiencing high effluent nitrate
and low effluent ammonia. The DO in the aeration basin is 2.5 mg/L.
What does this indicate?
A) Denitrification is complete
B) Full nitrification is occurring
C) The F/M ratio is too high
D) The sludge age is too low
Answer: B
Rationale: Low effluent ammonia with high nitrate indicates complete
nitrification (ammonia converted to nitrate). DO > 2.0 mg/L supports
nitrifying bacteria.
,2. A Class III operator notices that the secondary clarifier has a
sludge blanket depth of 5 feet (normal 2 ft). Mixed liquor DO is 0.3
mg/L. What is the most likely cause?
A) Excessive wasting
B) Low RAS rate combined with low DO causing poor flocculation
C) High recirculation rate
D) Chlorine residual too high
Answer: B
Rationale: Low DO (0.3 mg/L) leads to poor floc formation and
incomplete settling. Insufficient RAS exacerbates blanket depth. Increase
aeration and RAS.
3. Which of the following best describes the purpose of anoxic
zones in activated sludge?
A) To increase BOD removal
B) To facilitate denitrification by providing low DO and nitrate presence
C) To add chlorine for disinfection
D) To settle solids
Answer: B
Rationale: Anoxic zones have DO < 0.5 mg/L but nitrate present,
allowing facultative bacteria to use nitrate as electron acceptor
(denitrification), reducing nitrogen.
,4. A plant operates a 5-stage Bardenpho process. What is the
primary goal?
A) Phosphorus removal only
B) Biological nitrogen and phosphorus removal (BNR)
C) Methane production
D) Sludge thickening
Answer: B
Rationale: The Bardenpho process includes anaerobic, anoxic, and
aerobic zones for enhanced biological nitrogen (denitrification) and
phosphorus removal (PAOs).
5. A sudden increase in effluent TSS occurs after a heavy rain. The
secondary clarifier SVI is 110 mL/g. What is the most likely cause?
A) Hydraulic overload causing short-circuiting and solids washout
B) Filamentous bulking
C) Low F/M ratio
D) Excessive polymer
Answer: A
Rationale: SVI of 110 is normal; heavy rain increases flow, raising
overflow rate and weir loading, leading to solids carryover. Check flow
equalization.
, 6. An anaerobic digester’s volatile acids/alkalinity ratio is 0.65. Gas
production has dropped. What action should be taken?
A) Increase feed rate
B) Reduce or stop feeding, add alkalinity (lime or bicarbonate), and
increase mixing
C) Increase temperature
D) Add polymer
Answer: B
Rationale: Ratio > 0.4 indicates souring. Stop feeding to allow recovery,
add alkalinity, and maintain mixing to restore methanogens.
7. What is the maximum recommended hydrogen sulfide
concentration for 8-hour worker exposure (OSHA PEL)?
A) 1 ppm
B) 10 ppm
C) 50 ppm
D) 100 ppm
Answer: B
Rationale: OSHA permissible exposure limit (PEL) for H₂S is 10 ppm as
an 8-hour time-weighted average. IDLH is 100 ppm.
