Basic Wastewater Operator Practice Exam
(2025/2026) aligned with ABC / entry-level
municipal wastewater operator standards.
1. The primary purpose of primary clarification in wastewater
treatment is to:
A. Remove dissolved organic matter.
B. Convert ammonia to nitrate.
C. Remove settleable and floatable solids.
D. Disinfect the final effluent.
Correct Answer: C. Remove settleable and floatable solids.
Explanation: Primary clarifiers use gravity settling (sedimentation) to
remove settleable solids (sludge) and skimming to remove floatable
solids (scum) before biological treatment.
2. An Imhoff cone is most commonly used in a wastewater plant
laboratory to measure:
A. pH
B. Dissolved Oxygen (DO)
C. Settleable Solids
D. Biochemical Oxygen Demand (BOD)
Correct Answer: C. Settleable Solids
Explanation: An Imhoff cone is a graduated, cone-shaped container
used for the volumetric determination of settleable solids in a one-liter
sample.
3. Which of the following is the most accurate indicator that an
activated sludge process is experiencing a healthy, well-oxidized
nitrifying population?
,A. A sharp decrease in Dissolved Oxygen (DO).
B. The presence of a rising sludge blanket in the clarifier.
C. A decrease in effluent ammonia and an increase in effluent nitrate.
D. An increase in mixed liquor turbidity.
Correct Answer: C. A decrease in effluent ammonia and an increase
in effluent nitrate.
Explanation: Nitrification is the two-step biological process where
ammonia (NH3) is converted to nitrite (NO2-) and then to nitrate (NO3-).
A successful process shows low ammonia and higher nitrate in the final
effluent.
4. The most probable cause of billowing, greasy, tan to brown foam
on an activated sludge basin is:
A. Low pH
B. Nutrient deficiency (often phosphorus or nitrogen)
C. High Dissolved Oxygen (DO)
D. Old, over-oxidized sludge
Correct Answer: D. Old, over-oxidized sludge (Nocardia foam).
Explanation: This type of foam is typically caused by filamentous
bacteria like Nocardia or Microthrix parvicella, which thrive in systems
with long sludge ages, often from over-aeration or old sludge.
5. The formula for calculating Mean Cell Residence Time (MCRT) is:
A. (Tank Volume, MG x MLSS, mg/L) / (WAS flow, MGD x WAS SS, mg/L +
Effluent flow, MGD x Effluent SS, mg/L)
B. (Flow, MGD) / (Tank Volume, MG)
C. (BOD Loading, lbs/day) / (MLVSS, lbs)
D. (WAS flow, MGD) / (Influent flow, MGD)
Correct Answer: A.
Explanation: MCRT (also called Sludge Age) is the average time a
microorganism stays in the system. It’s calculated as the total mass of
,solids in the aeration system divided by the total mass of solids leaving
the system daily (via waste and effluent).
6. The main purpose of the chlorine contact chamber after secondary
treatment is to:
A. Provide enough time for chlorine to react with and kill pathogenic
organisms.
B. Allow solids to settle.
C. Reduce the Biological Oxygen Demand (BOD).
D. Provide equalization of flow.
Correct Answer: A.
Explanation: Disinfection relies on Contact Time (T) and Concentration
(C). The contact chamber ensures wastewater is exposed to chlorine for
a sufficient duration (as mandated by permit) to achieve proper
disinfection (CT value).
7. A 10-foot deep circular clarifier has a diameter of 60 feet. What is
its approximate volume in gallons? (Use π = 3.14, 1 ft³ ≈ 7.48 gal)
A. 42,390 gallons
B. 169,000 gallons
Correct Answer: B. 169,000 gallons
Explanation: Volume (ft³) = π × r² × h = 3.14 × (30 ft)² × 10 ft = 3.14 × 900
× 10 = 28,260 ft³. Gallons = 28,260 ft³ × 7.48 gal/ft³ = ~211,000
gallons (Closest provided answer is B, though exact calc yields ~211k).
8. The most likely hazard associated with entering a wastewater wet
well is:
A. High noise levels
B. Oxygen deficiency and/or presence of toxic gases (H₂S, CH₄)
C. Extreme heat
D. Electrical shock from submerged pumps
, Correct Answer: B.
Explanation: Confined spaces in wastewater collection can have
atmospheres deficient in oxygen and/or enriched with Hydrogen Sulfide
(H₂S), Methane (CH₄), and Carbon Dioxide (CO₂). A proper confined
space entry permit and atmospheric testing are required.
9. Rotating Biological Contactors (RBCs) achieve treatment by:
A. Passing wastewater through a stationary fixed film.
B. Providing a surface for microbial growth that rotates in and out of the
wastewater.
C. Using fine bubble diffusers for aeration.
D. Rapidly mixing chemicals with the flow.
Correct Answer: B.
Explanation: RBCs are a fixed-film treatment process. Discs coated
with microbial biomass rotate, alternately exposing the microbes to the
wastewater (for food) and the air (for oxygen).
10. The primary gas produced in a properly operating anaerobic
digester is:
A. Hydrogen Sulfide (H₂S)
B. Oxygen (O₂)
C. Methane (CH₄)
D. Nitrogen (N₂)
Correct Answer: C. Methane (CH₄)
Explanation: Anaerobic digestion breaks down organic matter in the
absence of oxygen, producing a biogas that is typically 60-70% methane
(CH₄) and 30-40% carbon dioxide (CO₂), with trace amounts of H₂S.
11. A condition where activated sludge solids do not settle well,
resulting in a cloudy supernatant with pin floc, is called:
A. Rising Sludge
B. Bulking Sludge
(2025/2026) aligned with ABC / entry-level
municipal wastewater operator standards.
1. The primary purpose of primary clarification in wastewater
treatment is to:
A. Remove dissolved organic matter.
B. Convert ammonia to nitrate.
C. Remove settleable and floatable solids.
D. Disinfect the final effluent.
Correct Answer: C. Remove settleable and floatable solids.
Explanation: Primary clarifiers use gravity settling (sedimentation) to
remove settleable solids (sludge) and skimming to remove floatable
solids (scum) before biological treatment.
2. An Imhoff cone is most commonly used in a wastewater plant
laboratory to measure:
A. pH
B. Dissolved Oxygen (DO)
C. Settleable Solids
D. Biochemical Oxygen Demand (BOD)
Correct Answer: C. Settleable Solids
Explanation: An Imhoff cone is a graduated, cone-shaped container
used for the volumetric determination of settleable solids in a one-liter
sample.
3. Which of the following is the most accurate indicator that an
activated sludge process is experiencing a healthy, well-oxidized
nitrifying population?
,A. A sharp decrease in Dissolved Oxygen (DO).
B. The presence of a rising sludge blanket in the clarifier.
C. A decrease in effluent ammonia and an increase in effluent nitrate.
D. An increase in mixed liquor turbidity.
Correct Answer: C. A decrease in effluent ammonia and an increase
in effluent nitrate.
Explanation: Nitrification is the two-step biological process where
ammonia (NH3) is converted to nitrite (NO2-) and then to nitrate (NO3-).
A successful process shows low ammonia and higher nitrate in the final
effluent.
4. The most probable cause of billowing, greasy, tan to brown foam
on an activated sludge basin is:
A. Low pH
B. Nutrient deficiency (often phosphorus or nitrogen)
C. High Dissolved Oxygen (DO)
D. Old, over-oxidized sludge
Correct Answer: D. Old, over-oxidized sludge (Nocardia foam).
Explanation: This type of foam is typically caused by filamentous
bacteria like Nocardia or Microthrix parvicella, which thrive in systems
with long sludge ages, often from over-aeration or old sludge.
5. The formula for calculating Mean Cell Residence Time (MCRT) is:
A. (Tank Volume, MG x MLSS, mg/L) / (WAS flow, MGD x WAS SS, mg/L +
Effluent flow, MGD x Effluent SS, mg/L)
B. (Flow, MGD) / (Tank Volume, MG)
C. (BOD Loading, lbs/day) / (MLVSS, lbs)
D. (WAS flow, MGD) / (Influent flow, MGD)
Correct Answer: A.
Explanation: MCRT (also called Sludge Age) is the average time a
microorganism stays in the system. It’s calculated as the total mass of
,solids in the aeration system divided by the total mass of solids leaving
the system daily (via waste and effluent).
6. The main purpose of the chlorine contact chamber after secondary
treatment is to:
A. Provide enough time for chlorine to react with and kill pathogenic
organisms.
B. Allow solids to settle.
C. Reduce the Biological Oxygen Demand (BOD).
D. Provide equalization of flow.
Correct Answer: A.
Explanation: Disinfection relies on Contact Time (T) and Concentration
(C). The contact chamber ensures wastewater is exposed to chlorine for
a sufficient duration (as mandated by permit) to achieve proper
disinfection (CT value).
7. A 10-foot deep circular clarifier has a diameter of 60 feet. What is
its approximate volume in gallons? (Use π = 3.14, 1 ft³ ≈ 7.48 gal)
A. 42,390 gallons
B. 169,000 gallons
Correct Answer: B. 169,000 gallons
Explanation: Volume (ft³) = π × r² × h = 3.14 × (30 ft)² × 10 ft = 3.14 × 900
× 10 = 28,260 ft³. Gallons = 28,260 ft³ × 7.48 gal/ft³ = ~211,000
gallons (Closest provided answer is B, though exact calc yields ~211k).
8. The most likely hazard associated with entering a wastewater wet
well is:
A. High noise levels
B. Oxygen deficiency and/or presence of toxic gases (H₂S, CH₄)
C. Extreme heat
D. Electrical shock from submerged pumps
, Correct Answer: B.
Explanation: Confined spaces in wastewater collection can have
atmospheres deficient in oxygen and/or enriched with Hydrogen Sulfide
(H₂S), Methane (CH₄), and Carbon Dioxide (CO₂). A proper confined
space entry permit and atmospheric testing are required.
9. Rotating Biological Contactors (RBCs) achieve treatment by:
A. Passing wastewater through a stationary fixed film.
B. Providing a surface for microbial growth that rotates in and out of the
wastewater.
C. Using fine bubble diffusers for aeration.
D. Rapidly mixing chemicals with the flow.
Correct Answer: B.
Explanation: RBCs are a fixed-film treatment process. Discs coated
with microbial biomass rotate, alternately exposing the microbes to the
wastewater (for food) and the air (for oxygen).
10. The primary gas produced in a properly operating anaerobic
digester is:
A. Hydrogen Sulfide (H₂S)
B. Oxygen (O₂)
C. Methane (CH₄)
D. Nitrogen (N₂)
Correct Answer: C. Methane (CH₄)
Explanation: Anaerobic digestion breaks down organic matter in the
absence of oxygen, producing a biogas that is typically 60-70% methane
(CH₄) and 30-40% carbon dioxide (CO₂), with trace amounts of H₂S.
11. A condition where activated sludge solids do not settle well,
resulting in a cloudy supernatant with pin floc, is called:
A. Rising Sludge
B. Bulking Sludge
