MA Combined Wastewater Treatment Plant
Operator Grade 5 Exam Questions And Correct
Answers with Explanation.
SECTION 1: ACTIVATED SLUDGE & PROCESS CONTROL
1. An operator notices a rising sludge blanket in the secondary clarifier. What is
the most likely cause?
A) Excessive return activated sludge (RAS) flow rate causing short-circuiting
B) Denitrification occurring in the clarifier due to low dissolved oxygen in the
aeration tank
C) Low mixed liquor suspended solids (MLSS) concentration in the aeration basin
D) High waste activated sludge (WAS) pumping rate removing too much biomass
Answer: B) Denitrification occurring in the clarifier due to low dissolved oxygen
in the aeration tank
Rationale: Denitrification in the clarifier produces nitrogen gas bubbles that attach
to sludge floc, causing it to rise. This occurs when dissolved oxygen in the aeration
tank is too low, allowing nitrate to be converted to nitrogen gas in the clarifier.
2. The Food-to-Microorganism (F/M) ratio is a key process control parameter in
activated sludge systems. What does a low F/M ratio typically indicate?
A) The system is underloaded with insufficient food for the microorganisms
B) The system is overloaded with excessive organic loading
C) The return activated sludge rate is too high
D) The dissolved oxygen concentration is too low
Answer: A) The system is underloaded with insufficient food for the
microorganisms
,Rationale: A low F/M ratio indicates the system is underloaded, meaning there is
not enough organic food (BOD) available for the microorganisms relative to their
population. This can lead to sludge bulking as microorganisms starve.
3. The Sludge Volume Index (SVI) is a critical measure of sludge settleability. A
typical SVI value for a healthy activated sludge system with good settling
characteristics is:
A) Less than 50 mL/g
B) Between 50 and 150 mL/g
C) Between 150 and 250 mL/g
D) Greater than 250 mL/g
Answer: B) Between 50 and 150 mL/g
Rationale: An SVI between 50 and 150 mL/g indicates good settling sludge. Values
above 150 suggest bulking sludge (poor settling), while values below 50 indicate a
dense, rapidly settling sludge that may cause solids loss. SVI is calculated by
dividing the settled sludge volume (mL/L) after 30 minutes by MLSS (mg/L) ×
1000.
4. In the activated sludge process, the solids retention time (SRT), also known as
sludge age, is calculated by:
A) Dividing the aeration tank volume by the influent flow rate
B) Dividing the mass of solids in the aeration tank by the mass of solids wasted per
day
C) Dividing the return sludge flow rate by the influent flow rate
D) Dividing the clarifier solids inventory by the aeration tank solids inventory
Answer: B) Dividing the mass of solids in the aeration tank by the mass of solids
wasted per day
, Rationale: SRT is the average time solids remain in the system. It is calculated by
dividing the total mass of solids in the aeration tank by the mass of solids removed
from the system each day (waste sludge + effluent solids).
5. Filamentous bulking in activated sludge is often caused by:
A) High dissolved oxygen concentration
B) Low food-to-microorganism (F/M) ratio and low dissolved oxygen
C) High return activated sludge flow
D) Excessive waste activated sludge removal
Answer: B) Low food-to-microorganism (F/M) ratio and low dissolved oxygen
Rationale: Filamentous organisms proliferate under low F/M ratios (starvation
conditions) and low dissolved oxygen concentrations. These conditions favor
filamentous growth over floc-forming bacteria, leading to poor settling and
bulking sludge.
6. Which of the following conditions is most likely to cause dispersed growth
(non-settling floc) in the activated sludge process?
A) Over-aeration causing floc shearing
B) Low dissolved oxygen concentration
C) High return activated sludge flow
D) Excessive sludge wasting
Answer: A) Over-aeration causing floc shearing
Rationale: Over-aeration can cause the shearing of biological floc, breaking it into
small, dispersed particles that do not settle well. This results in high effluent TSS
and turbidity.
7. What is the most appropriate first step to control an episode of foam in an
activated sludge aeration tank?
Operator Grade 5 Exam Questions And Correct
Answers with Explanation.
SECTION 1: ACTIVATED SLUDGE & PROCESS CONTROL
1. An operator notices a rising sludge blanket in the secondary clarifier. What is
the most likely cause?
A) Excessive return activated sludge (RAS) flow rate causing short-circuiting
B) Denitrification occurring in the clarifier due to low dissolved oxygen in the
aeration tank
C) Low mixed liquor suspended solids (MLSS) concentration in the aeration basin
D) High waste activated sludge (WAS) pumping rate removing too much biomass
Answer: B) Denitrification occurring in the clarifier due to low dissolved oxygen
in the aeration tank
Rationale: Denitrification in the clarifier produces nitrogen gas bubbles that attach
to sludge floc, causing it to rise. This occurs when dissolved oxygen in the aeration
tank is too low, allowing nitrate to be converted to nitrogen gas in the clarifier.
2. The Food-to-Microorganism (F/M) ratio is a key process control parameter in
activated sludge systems. What does a low F/M ratio typically indicate?
A) The system is underloaded with insufficient food for the microorganisms
B) The system is overloaded with excessive organic loading
C) The return activated sludge rate is too high
D) The dissolved oxygen concentration is too low
Answer: A) The system is underloaded with insufficient food for the
microorganisms
,Rationale: A low F/M ratio indicates the system is underloaded, meaning there is
not enough organic food (BOD) available for the microorganisms relative to their
population. This can lead to sludge bulking as microorganisms starve.
3. The Sludge Volume Index (SVI) is a critical measure of sludge settleability. A
typical SVI value for a healthy activated sludge system with good settling
characteristics is:
A) Less than 50 mL/g
B) Between 50 and 150 mL/g
C) Between 150 and 250 mL/g
D) Greater than 250 mL/g
Answer: B) Between 50 and 150 mL/g
Rationale: An SVI between 50 and 150 mL/g indicates good settling sludge. Values
above 150 suggest bulking sludge (poor settling), while values below 50 indicate a
dense, rapidly settling sludge that may cause solids loss. SVI is calculated by
dividing the settled sludge volume (mL/L) after 30 minutes by MLSS (mg/L) ×
1000.
4. In the activated sludge process, the solids retention time (SRT), also known as
sludge age, is calculated by:
A) Dividing the aeration tank volume by the influent flow rate
B) Dividing the mass of solids in the aeration tank by the mass of solids wasted per
day
C) Dividing the return sludge flow rate by the influent flow rate
D) Dividing the clarifier solids inventory by the aeration tank solids inventory
Answer: B) Dividing the mass of solids in the aeration tank by the mass of solids
wasted per day
, Rationale: SRT is the average time solids remain in the system. It is calculated by
dividing the total mass of solids in the aeration tank by the mass of solids removed
from the system each day (waste sludge + effluent solids).
5. Filamentous bulking in activated sludge is often caused by:
A) High dissolved oxygen concentration
B) Low food-to-microorganism (F/M) ratio and low dissolved oxygen
C) High return activated sludge flow
D) Excessive waste activated sludge removal
Answer: B) Low food-to-microorganism (F/M) ratio and low dissolved oxygen
Rationale: Filamentous organisms proliferate under low F/M ratios (starvation
conditions) and low dissolved oxygen concentrations. These conditions favor
filamentous growth over floc-forming bacteria, leading to poor settling and
bulking sludge.
6. Which of the following conditions is most likely to cause dispersed growth
(non-settling floc) in the activated sludge process?
A) Over-aeration causing floc shearing
B) Low dissolved oxygen concentration
C) High return activated sludge flow
D) Excessive sludge wasting
Answer: A) Over-aeration causing floc shearing
Rationale: Over-aeration can cause the shearing of biological floc, breaking it into
small, dispersed particles that do not settle well. This results in high effluent TSS
and turbidity.
7. What is the most appropriate first step to control an episode of foam in an
activated sludge aeration tank?
