Wastewater Operator Class II
Licensing Exam Questions And Answers
Plus Rationales | Qs & Ans 2026 | Instant
Pdf Download
1. A Class II operator notices that the mixed liquor suspended solids
(MLSS) in the aeration basin is gradually increasing while the sludge
age remains constant. What is the most likely explanation?
A) The wasting rate has decreased
B) The influent flow rate has decreased
C) The return activated sludge (RAS) rate has increased
D) The food-to-microorganism (F/M) ratio has increased
Answer: A
Rationale: If sludge age remains constant but MLSS increases, the
wasting rate must have decreased (since sludge age = total solids under
aeration ÷ solids wasted per day). A lower wasting rate retains more
solids in the system .
2. What is the primary purpose of wasting activated sludge?
A) To increase the dissolved oxygen concentration
,B) To control the mean cell residence time (MCRT) and prevent excess
solids accumulation
C) To add nutrients to the aeration basin
D) To reduce the return sludge rate
Answer: B
Rationale: Wasting removes excess biomass from the system,
controlling MCRT (sludge age) and maintaining the desired MLSS
concentration .
3. A trickling filter is experiencing ponding on the media surface.
What is the most likely cause?
A) Low organic loading
B) Excessive sloughing and accumulation of solids
C) High recirculation ratio
D) Too much chlorine in the influent
Answer: B
Rationale: Ponding occurs when biological growth or solids accumulate
on the filter media, blocking voids and preventing drainage. This can be
caused by high organic loading, poor distribution, or inadequate
sloughing .
,4. Which microorganism is most commonly associated with
activated sludge bulking?
A) Stalked ciliates
B) Rotifers
C) Nocardia or Microthrix parvicella (filamentous bacteria)
D) Amoebae
Answer: C
Rationale: Filamentous organisms
like Nocardia and Microthrix interfere with floc settling, leading to high
sludge volume index (SVI >150) and solids carryover .
5. The SVI (Sludge Volume Index) in an activated sludge system is
calculated to be 210 mL/g. What does this indicate?
A) Excellent settling sludge
B) Bulking sludge with poor compaction
C) Young sludge with high F/M
D) Over-aerated sludge
Answer: B
Rationale: SVI = (settled sludge volume after 30 min, mL/L) ÷ (MLSS,
g/L). SVI > 150 indicates bulking sludge, typically due to filamentous
growth .
, 6. An oxidation ditch is a modification of which biological treatment
process?
A) Trickling filter
B) Activated sludge
C) Rotating biological contactor (RBC)
D) Anaerobic digester
Answer: B
Rationale: Oxidation ditches are a form of extended aeration activated
sludge, using a racetrack channel and surface aerators/rotors to circulate
mixed liquor .
7. What is the recommended dissolved oxygen (DO) concentration
in the aeration basin of a conventional activated sludge system?
A) 0.1–0.5 mg/L
B) 1.0–3.0 mg/L
C) 5.0–7.0 mg/L
D) 10.0–12.0 mg/L
Answer: B
Rationale: Maintain DO at 1.0–3.0 mg/L in the aeration basin. Below 0.5
mg/L risks anaerobic conditions; excessively high DO wastes energy
without benefit .
Licensing Exam Questions And Answers
Plus Rationales | Qs & Ans 2026 | Instant
Pdf Download
1. A Class II operator notices that the mixed liquor suspended solids
(MLSS) in the aeration basin is gradually increasing while the sludge
age remains constant. What is the most likely explanation?
A) The wasting rate has decreased
B) The influent flow rate has decreased
C) The return activated sludge (RAS) rate has increased
D) The food-to-microorganism (F/M) ratio has increased
Answer: A
Rationale: If sludge age remains constant but MLSS increases, the
wasting rate must have decreased (since sludge age = total solids under
aeration ÷ solids wasted per day). A lower wasting rate retains more
solids in the system .
2. What is the primary purpose of wasting activated sludge?
A) To increase the dissolved oxygen concentration
,B) To control the mean cell residence time (MCRT) and prevent excess
solids accumulation
C) To add nutrients to the aeration basin
D) To reduce the return sludge rate
Answer: B
Rationale: Wasting removes excess biomass from the system,
controlling MCRT (sludge age) and maintaining the desired MLSS
concentration .
3. A trickling filter is experiencing ponding on the media surface.
What is the most likely cause?
A) Low organic loading
B) Excessive sloughing and accumulation of solids
C) High recirculation ratio
D) Too much chlorine in the influent
Answer: B
Rationale: Ponding occurs when biological growth or solids accumulate
on the filter media, blocking voids and preventing drainage. This can be
caused by high organic loading, poor distribution, or inadequate
sloughing .
,4. Which microorganism is most commonly associated with
activated sludge bulking?
A) Stalked ciliates
B) Rotifers
C) Nocardia or Microthrix parvicella (filamentous bacteria)
D) Amoebae
Answer: C
Rationale: Filamentous organisms
like Nocardia and Microthrix interfere with floc settling, leading to high
sludge volume index (SVI >150) and solids carryover .
5. The SVI (Sludge Volume Index) in an activated sludge system is
calculated to be 210 mL/g. What does this indicate?
A) Excellent settling sludge
B) Bulking sludge with poor compaction
C) Young sludge with high F/M
D) Over-aerated sludge
Answer: B
Rationale: SVI = (settled sludge volume after 30 min, mL/L) ÷ (MLSS,
g/L). SVI > 150 indicates bulking sludge, typically due to filamentous
growth .
, 6. An oxidation ditch is a modification of which biological treatment
process?
A) Trickling filter
B) Activated sludge
C) Rotating biological contactor (RBC)
D) Anaerobic digester
Answer: B
Rationale: Oxidation ditches are a form of extended aeration activated
sludge, using a racetrack channel and surface aerators/rotors to circulate
mixed liquor .
7. What is the recommended dissolved oxygen (DO) concentration
in the aeration basin of a conventional activated sludge system?
A) 0.1–0.5 mg/L
B) 1.0–3.0 mg/L
C) 5.0–7.0 mg/L
D) 10.0–12.0 mg/L
Answer: B
Rationale: Maintain DO at 1.0–3.0 mg/L in the aeration basin. Below 0.5
mg/L risks anaerobic conditions; excessively high DO wastes energy
without benefit .
