1
Wastewater Treatment Operator Certification
Examination Tests 2 & 3 – Grade / Class 2 & 3 –
Core Treatment Process Knowledge
1. What is the typical BOD removal efficiency in a primary clarifier?
A) 10 to 15%
B) 40 to 60%
C) 20 to 50%
D) 95 to 99%
Correct Answer✅ C – Primary clarifiers are designed for physical sedimentation, removing around 20–
50% of the incoming Biochemical Oxygen Demand (BOD) load.
2. What does the "F" in the F/M ratio represent?
A) The microorganism population in the aeration tank
B) The amount of filamentous bacteria in the activated sludge process
C) The MLVSS in the aeration tank
D) The food for the microorganisms flowing into the aeration tank, in lbs/day
Correct Answer✅ D – The Food to Microorganism (F/M) ratio is a key control parameter, where "F"
stands for the food (BOD) entering the system.
3. Which of the following describes the appearance of healthy activated sludge?
A) Thick, greasy, dark tan foam
B) Brown with a moderate amount of crisp, white foam
C) Clear liquid with clear foam
D) Gray liquid with no foam
Correct Answer✅ B – Healthy activated sludge typically has a brown, flocculent appearance with
some light, crisp, white foam on the surface.
4. What is accomplished by the activated sludge process?
A) Anaerobic bacteria use SO4 to eat incoming volatile solids
B) Remove soluble or finely divided suspended materials
C) Anoxic microorganisms destroy incoming BOD
D) Zoogleal breaks down influent organics
Correct Answer✅ B – The activated sludge process is a biological treatment method designed to
remove soluble and finely divided suspended organic matter that was not removed during primary
treatment.
5. What is the most common imbalance in an anaerobic digester?
A) Excess methane formation
B) Excess nitrogen formation
,2
C) Excess acid formation
D) Excess carbon dioxide formation
Correct Answer✅ C – Excess acid formation, which can lower the pH and inhibit methane-forming
bacteria, is the most frequent upset in anaerobic digestion.
6. How can the efficiency of a trickling filter be improved?
A) Raise the distributor arm
B) Reduce the size of the ventilation ports
C) Decrease the amount of the filter media
D) Add recirculation
Correct Answer✅ D – Recirculating a portion of the treated effluent back through the filter increases
contact time and improves treatment efficiency.
7. What is sludge bulking in an activated sludge process?
A) When mixed liquor solids settle slowly and compact to a limited extent
B) When old sludge denitrifies and causes sludge to rise
C) When sludge forms large aggregates and settles quickly
D) When compaction in the secondary clarifier is improved
Correct Answer✅ A – Sludge bulking is a condition where the sludge settles slowly and compacts
poorly, often due to an overgrowth of filamentous bacteria.
8. What could happen if the dissolved oxygen content in the aeration tank is too low?
A) Pinpoint floc will develop
B) Filamentous organisms could thrive and prevent the sludge floc from settling properly
C) Settling will improve in the secondary clarifier
D) Sludge bulking will be eliminated
Correct Answer✅ B – Low dissolved oxygen (DO) levels favor the growth of filamentous bacteria,
leading to poor settling in the secondary clarifier.
Process Control & Troubleshooting
9. What is used to determine the amount of microorganisms in the aeration tank?
A) Mixed Liquor Suspended Solids (MLSS)
B) Influent TSS
C) BOD
D) Mixed Liquor Volatile Suspended Solids (MLVSS)
Correct Answer✅ D – MLVSS measures the organic, or volatile, portion of the mixed liquor, which
represents the active biological mass.
10. What could cause the slime growth on a rotating biological contactor to appear black?
A) It could indicate solids or organic overloading
B) High DO in the plant effluent
,3
C) Lack of BOD in the influent
D) Low hydraulic loading
Correct Answer✅ A – Black slime on an RBC is a sign that the unit is overloaded, leading to anaerobic
conditions in the biological film.
11. What is a "dead spot" in an activated sludge process?
A) An area in the secondary clarifier with rising sludge
B) An area in the aeration tank that has insufficient mixing
C) An area in the aeration tank with high concentration of DO
D) An area in the secondary clarifier with dead viruses
Correct Answer✅ B – Dead spots are areas in the aeration tank with poor mixing where solids can
accumulate and go septic, negatively impacting the process.
12. What could be the cause of ponding in a trickling filter?
A) Hydraulic loading is too low
B) Clogging of distributor orifices
C) Excessive organic loading
D) Denitrification
Correct Answer✅ C – Ponding (surface pooling) is often the result of excessive organic loading, which
causes rapid biological growth that clogs the filter media.
13. What is the only reliable method of detecting hydrogen sulfide?
A) Sense of smell
B) Atmospheric monitoring
C) Color titration
D) Lab test
Correct Answer✅ B – Hydrogen sulfide (H₂S) is a toxic and colorless gas that quickly deadens the
sense of smell; therefore, electronic atmospheric monitoring is the only safe and reliable detection
method.
14. What causes primary effluent to become black and odorous?
A) Excessive sludge pumping
B) Oil leak from the sludge pump
C) Crack in the launder
D) Improper sludge removal from the clarifier
Correct Answer✅ D – If sludge is not removed frequently enough from the primary clarifier, it can
become septic (anaerobic), producing hydrogen sulfide and turning the effluent black and odorous.
Laboratory & Sampling
15. What is the incubation period for the BOD test?
A) Five days +/- 3 hours
, 4
B) Five days +/- 1 hour
C) Five days +/- 4 hours
D) Five days +/- 2 hours
Correct Answer✅ A – The standard Biochemical Oxygen Demand (BOD) test requires a 5-day
incubation period in the dark at 20°C, with a tolerance of plus or minus 3 hours.
16. Which of the following is a collection of individual samples obtained at regular intervals?
A) Grab
B) Culmination
C) Comprehensive
D) Composite
Correct Answer✅ D – A composite sample is formed by combining multiple grab samples taken over a
period of time (e.g., 24 hours), providing a flow-proportional representation of the wastewater.
17. Where is the proper sampling location for an effluent from a wastewater treatment plant?
A) Next to the chlorinator equipment
B) At the end of a solids handling facility
C) Prior to discharge into receiving waters
D) At the head of a chlorine contact chamber
Correct Answer✅ C – The final effluent sample should be collected after all treatment processes have
been completed and just before the water is discharged to the receiving stream.
18. What does a properly operating pond look like?
A) Brown
B) Deep green color
C) Clear
D) Tan
Correct Answer✅ B – A healthy, properly operating stabilization pond has a deep green color,
indicating a robust population of algae that produces oxygen for the system.
Safety & Chemical Handling
19. When is a confined space considered oxygen deficient?
A) When oxygen content is less than 19.5% by volume
B) When oxygen content is less than 23.5% by volume
C) When oxygen content is more than 23.5% by volume
D) When oxygen content is less than 29.5% by volume
Correct Answer✅ A – OSHA defines an oxygen-deficient atmosphere as one containing less than
19.5% oxygen by volume.
20. Which of the following is not true about chlorine gas?
A) 2.5 times heavier than air
Wastewater Treatment Operator Certification
Examination Tests 2 & 3 – Grade / Class 2 & 3 –
Core Treatment Process Knowledge
1. What is the typical BOD removal efficiency in a primary clarifier?
A) 10 to 15%
B) 40 to 60%
C) 20 to 50%
D) 95 to 99%
Correct Answer✅ C – Primary clarifiers are designed for physical sedimentation, removing around 20–
50% of the incoming Biochemical Oxygen Demand (BOD) load.
2. What does the "F" in the F/M ratio represent?
A) The microorganism population in the aeration tank
B) The amount of filamentous bacteria in the activated sludge process
C) The MLVSS in the aeration tank
D) The food for the microorganisms flowing into the aeration tank, in lbs/day
Correct Answer✅ D – The Food to Microorganism (F/M) ratio is a key control parameter, where "F"
stands for the food (BOD) entering the system.
3. Which of the following describes the appearance of healthy activated sludge?
A) Thick, greasy, dark tan foam
B) Brown with a moderate amount of crisp, white foam
C) Clear liquid with clear foam
D) Gray liquid with no foam
Correct Answer✅ B – Healthy activated sludge typically has a brown, flocculent appearance with
some light, crisp, white foam on the surface.
4. What is accomplished by the activated sludge process?
A) Anaerobic bacteria use SO4 to eat incoming volatile solids
B) Remove soluble or finely divided suspended materials
C) Anoxic microorganisms destroy incoming BOD
D) Zoogleal breaks down influent organics
Correct Answer✅ B – The activated sludge process is a biological treatment method designed to
remove soluble and finely divided suspended organic matter that was not removed during primary
treatment.
5. What is the most common imbalance in an anaerobic digester?
A) Excess methane formation
B) Excess nitrogen formation
,2
C) Excess acid formation
D) Excess carbon dioxide formation
Correct Answer✅ C – Excess acid formation, which can lower the pH and inhibit methane-forming
bacteria, is the most frequent upset in anaerobic digestion.
6. How can the efficiency of a trickling filter be improved?
A) Raise the distributor arm
B) Reduce the size of the ventilation ports
C) Decrease the amount of the filter media
D) Add recirculation
Correct Answer✅ D – Recirculating a portion of the treated effluent back through the filter increases
contact time and improves treatment efficiency.
7. What is sludge bulking in an activated sludge process?
A) When mixed liquor solids settle slowly and compact to a limited extent
B) When old sludge denitrifies and causes sludge to rise
C) When sludge forms large aggregates and settles quickly
D) When compaction in the secondary clarifier is improved
Correct Answer✅ A – Sludge bulking is a condition where the sludge settles slowly and compacts
poorly, often due to an overgrowth of filamentous bacteria.
8. What could happen if the dissolved oxygen content in the aeration tank is too low?
A) Pinpoint floc will develop
B) Filamentous organisms could thrive and prevent the sludge floc from settling properly
C) Settling will improve in the secondary clarifier
D) Sludge bulking will be eliminated
Correct Answer✅ B – Low dissolved oxygen (DO) levels favor the growth of filamentous bacteria,
leading to poor settling in the secondary clarifier.
Process Control & Troubleshooting
9. What is used to determine the amount of microorganisms in the aeration tank?
A) Mixed Liquor Suspended Solids (MLSS)
B) Influent TSS
C) BOD
D) Mixed Liquor Volatile Suspended Solids (MLVSS)
Correct Answer✅ D – MLVSS measures the organic, or volatile, portion of the mixed liquor, which
represents the active biological mass.
10. What could cause the slime growth on a rotating biological contactor to appear black?
A) It could indicate solids or organic overloading
B) High DO in the plant effluent
,3
C) Lack of BOD in the influent
D) Low hydraulic loading
Correct Answer✅ A – Black slime on an RBC is a sign that the unit is overloaded, leading to anaerobic
conditions in the biological film.
11. What is a "dead spot" in an activated sludge process?
A) An area in the secondary clarifier with rising sludge
B) An area in the aeration tank that has insufficient mixing
C) An area in the aeration tank with high concentration of DO
D) An area in the secondary clarifier with dead viruses
Correct Answer✅ B – Dead spots are areas in the aeration tank with poor mixing where solids can
accumulate and go septic, negatively impacting the process.
12. What could be the cause of ponding in a trickling filter?
A) Hydraulic loading is too low
B) Clogging of distributor orifices
C) Excessive organic loading
D) Denitrification
Correct Answer✅ C – Ponding (surface pooling) is often the result of excessive organic loading, which
causes rapid biological growth that clogs the filter media.
13. What is the only reliable method of detecting hydrogen sulfide?
A) Sense of smell
B) Atmospheric monitoring
C) Color titration
D) Lab test
Correct Answer✅ B – Hydrogen sulfide (H₂S) is a toxic and colorless gas that quickly deadens the
sense of smell; therefore, electronic atmospheric monitoring is the only safe and reliable detection
method.
14. What causes primary effluent to become black and odorous?
A) Excessive sludge pumping
B) Oil leak from the sludge pump
C) Crack in the launder
D) Improper sludge removal from the clarifier
Correct Answer✅ D – If sludge is not removed frequently enough from the primary clarifier, it can
become septic (anaerobic), producing hydrogen sulfide and turning the effluent black and odorous.
Laboratory & Sampling
15. What is the incubation period for the BOD test?
A) Five days +/- 3 hours
, 4
B) Five days +/- 1 hour
C) Five days +/- 4 hours
D) Five days +/- 2 hours
Correct Answer✅ A – The standard Biochemical Oxygen Demand (BOD) test requires a 5-day
incubation period in the dark at 20°C, with a tolerance of plus or minus 3 hours.
16. Which of the following is a collection of individual samples obtained at regular intervals?
A) Grab
B) Culmination
C) Comprehensive
D) Composite
Correct Answer✅ D – A composite sample is formed by combining multiple grab samples taken over a
period of time (e.g., 24 hours), providing a flow-proportional representation of the wastewater.
17. Where is the proper sampling location for an effluent from a wastewater treatment plant?
A) Next to the chlorinator equipment
B) At the end of a solids handling facility
C) Prior to discharge into receiving waters
D) At the head of a chlorine contact chamber
Correct Answer✅ C – The final effluent sample should be collected after all treatment processes have
been completed and just before the water is discharged to the receiving stream.
18. What does a properly operating pond look like?
A) Brown
B) Deep green color
C) Clear
D) Tan
Correct Answer✅ B – A healthy, properly operating stabilization pond has a deep green color,
indicating a robust population of algae that produces oxygen for the system.
Safety & Chemical Handling
19. When is a confined space considered oxygen deficient?
A) When oxygen content is less than 19.5% by volume
B) When oxygen content is less than 23.5% by volume
C) When oxygen content is more than 23.5% by volume
D) When oxygen content is less than 29.5% by volume
Correct Answer✅ A – OSHA defines an oxygen-deficient atmosphere as one containing less than
19.5% oxygen by volume.
20. Which of the following is not true about chlorine gas?
A) 2.5 times heavier than air
