Colorado Wastewater Treatment Operator Class
A Practice Exam Questions And Correct Answers
(Verified Answers) Plus Rationales 2026 Q&A |
Instant Download Pdf!!!105+Questions.
Passing Threshold (Practice): 90%
Colorado Wastewater Treatment Operator Class A Practice Exam
Topics: Activated Sludge, Advanced Treatment, Nutrient Removal, Process Control, Colorado
Discharge Permit System (CDPS), Safety, Calculations, Pumps/Motors, Anaerobic Digestion,
Solids Handling.
Section 1: Activated Sludge Process Control (Questions 1–20)
1. In a conventional activated sludge system, what is the primary purpose of returning
activated sludge (RAS) from the secondary clarifier to the aeration basin?
A) To increase dissolved oxygen levels
B) To maintain the required concentration of microorganisms
C) To remove scum and grease
D) To reduce the sludge volume index
Correct Answer: B
Rationale: RAS returns concentrated biomass to the aeration tank to sustain the mixed
liquor suspended solids (MLSS) concentration, ensuring sufficient microbial population to
treat incoming organic matter.
2. A sudden increase in the sludge volume index (SVI) from 120 to 220 mL/g typically
indicates:
A) Old, over-oxidized sludge
B) Filamentous bulking
,C) Hydraulic overloading
D) Low return activated sludge rate
Correct Answer: B
Rationale: High SVI (>150 mL/g) often signals filamentous bulking, where filamentous
bacteria hinder floc settling. Low SVI (<80 mL/g) suggests old sludge.
3. What is the target food-to-microorganism (F/M) ratio range for a conventional activated
sludge system treating municipal wastewater?
A) 0.01–0.05 lb BOD/lb MLVSS/day
B) 0.2–0.4 lb BOD/lb MLVSS/day
C) 0.8–1.2 lb BOD/lb MLVSS/day
D) 1.5–2.0 lb BOD/lb MLVSS/day
Correct Answer: B
Rationale: Conventional activated sludge operates best at F/M 0.2–0.4. Lower F/M indicates
old sludge (extended aeration), higher F/M indicates young sludge (high rate).
4. In Colorado, what is the typical minimum dissolved oxygen (DO) concentration to
maintain in an aeration basin to prevent bulking?
A) 0.5 mg/L
B) 1.0 mg/L
C) 2.0 mg/L
D) 4.0 mg/L
Correct Answer: C
Rationale: At least 2.0 mg/L DO is recommended to discourage filamentous organisms
(e.g., Sphaerotilus natans), especially in colder Colorado wastewater temperatures.
5. Calculate the mean cell residence time (MCRT) given: Aeration basin MLSS = 3,000
mg/L, basin volume = 0.5 MG, waste activated sludge (WAS) flow = 0.02 MGD, WAS SS =
8,000 mg/L, effluent SS = 10 mg/L. (Ignore effluent loss for approximation)
A) 5.3 days
B) 9.4 days
C) 12.5 days
D) 18.8 days
Correct Answer: B
Rationale: MCRT = (MLSS × Basin Vol) / (WAS SS × WAS flow).
(3,000 mg/L × 0.5 MG) = 1,500 lb (approx). (8,000 mg/L × 0.02 MGD) = 160 lb/day.
1,500/160 = 9.375 days.
6. Pin floc in the secondary clarifier effluent is most associated with:
A) Young sludge, low MCRT
B) Old sludge, high MCRT
, C) Low DO
D) High F/M
Correct Answer: B
Rationale: Old sludge (high MCRT, low F/M) becomes over-oxidized, producing tiny, slow-
settling flocs called pin floc.
7. A operator observes foam that is dark brown and greasy. This indicates:
A) Detergent foam
B) Old sludge foam
C) Nocardia foam
D) Start-up foam
Correct Answer: C
Rationale: Nocardia (filamentous) produces thick, brown, greasy foam. Detergent foam is
white and fluffy; old sludge foam is light tan.
8. In Colorado’s cold weather (wastewater temp <10°C), how should the operator adjust
MCRT?
A) Decrease MCRT by 20%
B) Increase MCRT by 30–50%
C) No change
D) Switch to step-feed only
Correct Answer: B
Rationale: Microbial activity slows in cold water. Increasing MCRT (longer sludge age)
maintains treatment by allowing more biomass to accumulate.
9. The primary goal of step-feed aeration is to:
A) Increase DO without blowers
B) Equalize organic load along the basin
C) Remove nitrogen
D) Thicken sludge
Correct Answer: B
Rationale: Step-feed distributes influent at multiple points, preventing oxygen demand
spikes at the head of the basin and improving shock load handling.
10. A high effluent nitrate (NO3-N) level of 15 mg/L in a non-nitrifying plant indicates:
A) Complete nitrification is occurring
B) Denitrification is complete
C) Incomplete denitrification only
D) Effluent ammonia is zero
A Practice Exam Questions And Correct Answers
(Verified Answers) Plus Rationales 2026 Q&A |
Instant Download Pdf!!!105+Questions.
Passing Threshold (Practice): 90%
Colorado Wastewater Treatment Operator Class A Practice Exam
Topics: Activated Sludge, Advanced Treatment, Nutrient Removal, Process Control, Colorado
Discharge Permit System (CDPS), Safety, Calculations, Pumps/Motors, Anaerobic Digestion,
Solids Handling.
Section 1: Activated Sludge Process Control (Questions 1–20)
1. In a conventional activated sludge system, what is the primary purpose of returning
activated sludge (RAS) from the secondary clarifier to the aeration basin?
A) To increase dissolved oxygen levels
B) To maintain the required concentration of microorganisms
C) To remove scum and grease
D) To reduce the sludge volume index
Correct Answer: B
Rationale: RAS returns concentrated biomass to the aeration tank to sustain the mixed
liquor suspended solids (MLSS) concentration, ensuring sufficient microbial population to
treat incoming organic matter.
2. A sudden increase in the sludge volume index (SVI) from 120 to 220 mL/g typically
indicates:
A) Old, over-oxidized sludge
B) Filamentous bulking
,C) Hydraulic overloading
D) Low return activated sludge rate
Correct Answer: B
Rationale: High SVI (>150 mL/g) often signals filamentous bulking, where filamentous
bacteria hinder floc settling. Low SVI (<80 mL/g) suggests old sludge.
3. What is the target food-to-microorganism (F/M) ratio range for a conventional activated
sludge system treating municipal wastewater?
A) 0.01–0.05 lb BOD/lb MLVSS/day
B) 0.2–0.4 lb BOD/lb MLVSS/day
C) 0.8–1.2 lb BOD/lb MLVSS/day
D) 1.5–2.0 lb BOD/lb MLVSS/day
Correct Answer: B
Rationale: Conventional activated sludge operates best at F/M 0.2–0.4. Lower F/M indicates
old sludge (extended aeration), higher F/M indicates young sludge (high rate).
4. In Colorado, what is the typical minimum dissolved oxygen (DO) concentration to
maintain in an aeration basin to prevent bulking?
A) 0.5 mg/L
B) 1.0 mg/L
C) 2.0 mg/L
D) 4.0 mg/L
Correct Answer: C
Rationale: At least 2.0 mg/L DO is recommended to discourage filamentous organisms
(e.g., Sphaerotilus natans), especially in colder Colorado wastewater temperatures.
5. Calculate the mean cell residence time (MCRT) given: Aeration basin MLSS = 3,000
mg/L, basin volume = 0.5 MG, waste activated sludge (WAS) flow = 0.02 MGD, WAS SS =
8,000 mg/L, effluent SS = 10 mg/L. (Ignore effluent loss for approximation)
A) 5.3 days
B) 9.4 days
C) 12.5 days
D) 18.8 days
Correct Answer: B
Rationale: MCRT = (MLSS × Basin Vol) / (WAS SS × WAS flow).
(3,000 mg/L × 0.5 MG) = 1,500 lb (approx). (8,000 mg/L × 0.02 MGD) = 160 lb/day.
1,500/160 = 9.375 days.
6. Pin floc in the secondary clarifier effluent is most associated with:
A) Young sludge, low MCRT
B) Old sludge, high MCRT
, C) Low DO
D) High F/M
Correct Answer: B
Rationale: Old sludge (high MCRT, low F/M) becomes over-oxidized, producing tiny, slow-
settling flocs called pin floc.
7. A operator observes foam that is dark brown and greasy. This indicates:
A) Detergent foam
B) Old sludge foam
C) Nocardia foam
D) Start-up foam
Correct Answer: C
Rationale: Nocardia (filamentous) produces thick, brown, greasy foam. Detergent foam is
white and fluffy; old sludge foam is light tan.
8. In Colorado’s cold weather (wastewater temp <10°C), how should the operator adjust
MCRT?
A) Decrease MCRT by 20%
B) Increase MCRT by 30–50%
C) No change
D) Switch to step-feed only
Correct Answer: B
Rationale: Microbial activity slows in cold water. Increasing MCRT (longer sludge age)
maintains treatment by allowing more biomass to accumulate.
9. The primary goal of step-feed aeration is to:
A) Increase DO without blowers
B) Equalize organic load along the basin
C) Remove nitrogen
D) Thicken sludge
Correct Answer: B
Rationale: Step-feed distributes influent at multiple points, preventing oxygen demand
spikes at the head of the basin and improving shock load handling.
10. A high effluent nitrate (NO3-N) level of 15 mg/L in a non-nitrifying plant indicates:
A) Complete nitrification is occurring
B) Denitrification is complete
C) Incomplete denitrification only
D) Effluent ammonia is zero
