MASSACHUSETTS WASTEWATER OPERATOR GRADE 6
EXAMINATION COMPLETE QUESTIONS AND DETAILED
SOLUTIONS LATEST UPDATE THIS YEAR JUST RELEASED
1. During an extended period of increasing organic loading, which activated-
sludge process-control change should an experienced operator evaluate first?
A. Immediately decrease return activated sludge flow
B. Evaluate aeration capacity, dissolved oxygen, loading, and sludge age
C. Stop wasting all biological solids
D. Eliminate secondary clarification
Answer: B
Rationale: Increasing organic loading can increase oxygen demand and biological
growth. The operator should evaluate loading, DO, aeration capacity, sludge age,
and related process-control indicators before making corrective adjustments.
2. Why is maintaining an appropriate dissolved oxygen concentration especially
important within an aerobic activated-sludge aeration basin?
A. It prevents all solids from settling in the basin
B. It supports aerobic microorganisms responsible for biological treatment
C. It eliminates the need for secondary clarification
D. It directly removes grit from incoming wastewater
Answer: B
Rationale: Aerobic microorganisms require oxygen for biological oxidation of
biodegradable pollutants. Insufficient DO can impair treatment, promote
undesirable organisms, and contribute to poor settling.
,3. A secondary clarifier suddenly develops rising sludge caused by
denitrification; what operating condition should receive immediate attention?
A. Excessive sludge detention in the clarifier
B. Excessive grit concentration in preliminary treatment
C. Influent screening size
D. Primary scum collection frequency
Answer: A
Rationale: Denitrification occurring in a secondary clarifier can produce nitrogen
gas bubbles that attach to sludge and cause it to rise. Excessive solids detention
can worsen the problem.
4. Which measurement provides the most direct indication of the amount of
suspended biological solids present within an activated-sludge aeration basin?
A. MLSS concentration
B. Influent pH
C. Effluent temperature
D. Chlorine residual
Answer: A
Rationale: Mixed liquor suspended solids, or MLSS, measures the concentration
of suspended solids in aeration-basin mixed liquor and is a fundamental
activated-sludge process-control parameter.
5. What operational problem can occur when an activated-sludge plant
maintains an excessively low sludge age for nitrifying microorganisms?
A. Excessive grit accumulation
B. Loss of nitrification because nitrifiers are washed from the system
,C. Immediate elimination of ammonia
D. Increased primary sedimentation efficiency
Answer: B
Rationale: Nitrifying organisms generally grow more slowly than ordinary
heterotrophic organisms. If sludge age becomes too low, nitrifiers may not remain
in the biological system at sufficient concentrations.
6. When calculating the food-to-microorganism relationship, which two process
characteristics are most directly involved in the calculation?
A. Influent biodegradable substrate and biological solids concentration
B. Chlorine dose and UV intensity
C. Grit volume and screen opening
D. Effluent pH and chlorine residual
Answer: A
Rationale: The F/M relationship compares available biodegradable organic
substrate with the mass of microorganisms responsible for biological treatment.
7. A high sludge volume index is observed repeatedly in an activated-sludge
facility; what condition should the operator investigate?
A. Improved sludge compaction
B. Bulking or poor settling characteristics
C. Excessive grit removal
D. Excessive chlorine contact time
Answer: B
, Rationale: Elevated SVI generally indicates that activated sludge occupies more
volume and may settle poorly. Filamentous organisms, low DO, nutrient
imbalance, and other conditions can contribute.
8. What is the principal purpose of returning activated sludge from the
secondary clarifier to the aeration basin?
A. Increase chlorine residual
B. Maintain an appropriate concentration of active microorganisms
C. Remove grit from wastewater
D. Reduce influent hydraulic flow
Answer: B
Rationale: Return activated sludge, or RAS, recycles settled biological solids back
to the aeration process to maintain the desired microorganism concentration.
9. Why must wasting activated sludge be carefully controlled instead of
continuously removing the maximum possible quantity?
A. Excessive wasting can reduce the biological solids inventory and process
stability
B. Wasting always increases dissolved oxygen
C. Wasting removes only inorganic grit
D. Wasting eliminates the need for aeration
Answer: A
Rationale: Excessive wasting can reduce MLSS and sludge age below desired
operating conditions, potentially impairing biological treatment and nitrification.
EXAMINATION COMPLETE QUESTIONS AND DETAILED
SOLUTIONS LATEST UPDATE THIS YEAR JUST RELEASED
1. During an extended period of increasing organic loading, which activated-
sludge process-control change should an experienced operator evaluate first?
A. Immediately decrease return activated sludge flow
B. Evaluate aeration capacity, dissolved oxygen, loading, and sludge age
C. Stop wasting all biological solids
D. Eliminate secondary clarification
Answer: B
Rationale: Increasing organic loading can increase oxygen demand and biological
growth. The operator should evaluate loading, DO, aeration capacity, sludge age,
and related process-control indicators before making corrective adjustments.
2. Why is maintaining an appropriate dissolved oxygen concentration especially
important within an aerobic activated-sludge aeration basin?
A. It prevents all solids from settling in the basin
B. It supports aerobic microorganisms responsible for biological treatment
C. It eliminates the need for secondary clarification
D. It directly removes grit from incoming wastewater
Answer: B
Rationale: Aerobic microorganisms require oxygen for biological oxidation of
biodegradable pollutants. Insufficient DO can impair treatment, promote
undesirable organisms, and contribute to poor settling.
,3. A secondary clarifier suddenly develops rising sludge caused by
denitrification; what operating condition should receive immediate attention?
A. Excessive sludge detention in the clarifier
B. Excessive grit concentration in preliminary treatment
C. Influent screening size
D. Primary scum collection frequency
Answer: A
Rationale: Denitrification occurring in a secondary clarifier can produce nitrogen
gas bubbles that attach to sludge and cause it to rise. Excessive solids detention
can worsen the problem.
4. Which measurement provides the most direct indication of the amount of
suspended biological solids present within an activated-sludge aeration basin?
A. MLSS concentration
B. Influent pH
C. Effluent temperature
D. Chlorine residual
Answer: A
Rationale: Mixed liquor suspended solids, or MLSS, measures the concentration
of suspended solids in aeration-basin mixed liquor and is a fundamental
activated-sludge process-control parameter.
5. What operational problem can occur when an activated-sludge plant
maintains an excessively low sludge age for nitrifying microorganisms?
A. Excessive grit accumulation
B. Loss of nitrification because nitrifiers are washed from the system
,C. Immediate elimination of ammonia
D. Increased primary sedimentation efficiency
Answer: B
Rationale: Nitrifying organisms generally grow more slowly than ordinary
heterotrophic organisms. If sludge age becomes too low, nitrifiers may not remain
in the biological system at sufficient concentrations.
6. When calculating the food-to-microorganism relationship, which two process
characteristics are most directly involved in the calculation?
A. Influent biodegradable substrate and biological solids concentration
B. Chlorine dose and UV intensity
C. Grit volume and screen opening
D. Effluent pH and chlorine residual
Answer: A
Rationale: The F/M relationship compares available biodegradable organic
substrate with the mass of microorganisms responsible for biological treatment.
7. A high sludge volume index is observed repeatedly in an activated-sludge
facility; what condition should the operator investigate?
A. Improved sludge compaction
B. Bulking or poor settling characteristics
C. Excessive grit removal
D. Excessive chlorine contact time
Answer: B
, Rationale: Elevated SVI generally indicates that activated sludge occupies more
volume and may settle poorly. Filamentous organisms, low DO, nutrient
imbalance, and other conditions can contribute.
8. What is the principal purpose of returning activated sludge from the
secondary clarifier to the aeration basin?
A. Increase chlorine residual
B. Maintain an appropriate concentration of active microorganisms
C. Remove grit from wastewater
D. Reduce influent hydraulic flow
Answer: B
Rationale: Return activated sludge, or RAS, recycles settled biological solids back
to the aeration process to maintain the desired microorganism concentration.
9. Why must wasting activated sludge be carefully controlled instead of
continuously removing the maximum possible quantity?
A. Excessive wasting can reduce the biological solids inventory and process
stability
B. Wasting always increases dissolved oxygen
C. Wasting removes only inorganic grit
D. Wasting eliminates the need for aeration
Answer: A
Rationale: Excessive wasting can reduce MLSS and sludge age below desired
operating conditions, potentially impairing biological treatment and nitrification.
