Wastewater Operator Class I Licensing Exam
Questions And Answers Plus Rationales | Qs &
Ans 2026 | Instant Pdf Download
1. What is the primary purpose of a bar screen in a wastewater
treatment plant?
A) To remove dissolved organic matter
B) To equalize flow
C) To remove large solids and debris
D) To add chlorine
Answer: C
Rationale: Bar screens capture large solids (rags, sticks, plastics) to
protect downstream equipment from damage or clogging.
2. Which type of solids are removed during primary treatment?
A) Colloidal solids
B) Dissolved solids
C) Settleable solids
D) Volatile solids
,Answer: C
Rationale: Primary treatment relies on sedimentation to remove
settleable solids (≈40–60% of suspended solids).
3. What does BOD measure?
A) Amount of bacteria in water
B) Oxygen demand by microorganisms to decompose organic matter
C) Chemical oxygen demand
D) Dissolved oxygen concentration
Answer: B
Rationale: Biochemical Oxygen Demand (BOD) is the oxygen used by
microbes to stabilize organic waste under specified conditions.
4. A high dissolved oxygen (DO) level in the aeration tank indicates:
A) The system is overloaded
B) Nitrification is complete
C) Adequate oxygen for microorganisms
D) Sludge bulking is occurring
Answer: C
Rationale: DO > 2 mg/L generally ensures sufficient oxygen for aerobic
bacteria; too low can cause poor treatment.
,5. What is sludge age (mean cell residence time)?
A) Time solids spend in primary clarifier
B) Average time activated sludge solids stay in the system
C) Age of the wastewater in the headworks
D) Time since last sludge removal
Answer: B
Rationale: Sludge age = (total solids in aeration tank) / (wasted solids
per day) – key for process control.
6. Which condition favors denitrification in an activated sludge
process?
A) High DO, low carbon source
B) Low DO, presence of nitrate and organic carbon
C) High temperature, high pH
D) Presence of chlorine
Answer: B
Rationale: Denitrification occurs under anoxic conditions (DO <0.5
mg/L) with nitrate and organic carbon available.
, 7. What is the typical color of healthy activated sludge?
A) Black
B) Dark brown to tan
C) Bright green
D) Grayish white
Answer: B
Rationale: Healthy, well-oxygenated sludge is typically tan to light
brown; black indicates septic conditions.
8. Which gas is produced during anaerobic digestion?
A) Oxygen
B) Carbon monoxide
C) Methane and carbon dioxide
D) Hydrogen sulfide only
Answer: C
Rationale: Anaerobic digestion produces biogas (≈60–70% methane,
30–40% CO₂), plus trace H₂S.
9. What is the purpose of a weir in a clarifier?
A) To mix chemicals
B) To control flow and collect effluent evenly
Questions And Answers Plus Rationales | Qs &
Ans 2026 | Instant Pdf Download
1. What is the primary purpose of a bar screen in a wastewater
treatment plant?
A) To remove dissolved organic matter
B) To equalize flow
C) To remove large solids and debris
D) To add chlorine
Answer: C
Rationale: Bar screens capture large solids (rags, sticks, plastics) to
protect downstream equipment from damage or clogging.
2. Which type of solids are removed during primary treatment?
A) Colloidal solids
B) Dissolved solids
C) Settleable solids
D) Volatile solids
,Answer: C
Rationale: Primary treatment relies on sedimentation to remove
settleable solids (≈40–60% of suspended solids).
3. What does BOD measure?
A) Amount of bacteria in water
B) Oxygen demand by microorganisms to decompose organic matter
C) Chemical oxygen demand
D) Dissolved oxygen concentration
Answer: B
Rationale: Biochemical Oxygen Demand (BOD) is the oxygen used by
microbes to stabilize organic waste under specified conditions.
4. A high dissolved oxygen (DO) level in the aeration tank indicates:
A) The system is overloaded
B) Nitrification is complete
C) Adequate oxygen for microorganisms
D) Sludge bulking is occurring
Answer: C
Rationale: DO > 2 mg/L generally ensures sufficient oxygen for aerobic
bacteria; too low can cause poor treatment.
,5. What is sludge age (mean cell residence time)?
A) Time solids spend in primary clarifier
B) Average time activated sludge solids stay in the system
C) Age of the wastewater in the headworks
D) Time since last sludge removal
Answer: B
Rationale: Sludge age = (total solids in aeration tank) / (wasted solids
per day) – key for process control.
6. Which condition favors denitrification in an activated sludge
process?
A) High DO, low carbon source
B) Low DO, presence of nitrate and organic carbon
C) High temperature, high pH
D) Presence of chlorine
Answer: B
Rationale: Denitrification occurs under anoxic conditions (DO <0.5
mg/L) with nitrate and organic carbon available.
, 7. What is the typical color of healthy activated sludge?
A) Black
B) Dark brown to tan
C) Bright green
D) Grayish white
Answer: B
Rationale: Healthy, well-oxygenated sludge is typically tan to light
brown; black indicates septic conditions.
8. Which gas is produced during anaerobic digestion?
A) Oxygen
B) Carbon monoxide
C) Methane and carbon dioxide
D) Hydrogen sulfide only
Answer: C
Rationale: Anaerobic digestion produces biogas (≈60–70% methane,
30–40% CO₂), plus trace H₂S.
9. What is the purpose of a weir in a clarifier?
A) To mix chemicals
B) To control flow and collect effluent evenly
