Wastewater Treatment Operator
Certification Examination Tests 2 & 3 –
Grade / Class 2 & 3 – Association of
Boards of Certification (ABC) / State
Environmental Agency
[DOMAIN A: Preliminary & Primary Treatment (Questions 1-12)]
Q1. A primary clarifier has a detention time of 2.5 hours and a surface overflow rate of 800 gpd/sq ft.
Which adjustment would improve solids removal if effluent TSS is elevated?
A. Increase weir loading rate
B. Decrease surface overflow rate (reduce flow or increase surface area) [CORRECT]
C. Increase detention time by reducing tank depth
D. Increase influent flow rate
Correct Answer: B
Rationale: Lower SOR allows more time for solids settling. Option A (increase weir loading) worsens
performance. Option C (reduce depth) reduces detention time. Option D (increase flow) increases SOR.
Q2. A bar screen removes debris from influent wastewater. The primary purpose of screening is to:
A. Remove grit and sand
B. Protect downstream equipment from damage and clogging [CORRECT]
C. Remove dissolved organic matter
D. Disinfect the wastewater
Correct Answer: B
Rationale: Screens remove large solids to protect pumps, pipes, and process equipment. Option A is grit
removal. Option C is biological treatment. Option D is disinfection.
,Q3. Grit removal is important because:
A. Grit is organic matter that causes odors
B. Inorganic grit damages equipment and occupies space in tanks [CORRECT]
C. Grit contains pathogens that must be removed
D. Grit is the primary source of BOD
Correct Answer: B
Rationale: Grit (sand, gravel, eggshells) is inorganic, abrasive, and reduces effective tank volume. Option
A is incorrect (grit is inorganic). Option C is incorrect. Option D is incorrect (BOD is organic).
Q4. A primary clarifier has a volume of 250,000 gallons and receives a flow of 2.5 MGD. The detention
time is:
A. 1.0 hour
B. 2.4 hours [CORRECT]
C. 4.0 hours
D. 6.0 hours
Correct Answer: B
Rationale: Detention time (hrs) = Volume (gal) ÷ (Flow (MGD) × 1,000,000 ÷ 24) = 250,000 ÷ 104,167 =
2.4 hours. Option A is too low. Options C/D are too high.
Q5. The typical detention time for a primary clarifier is:
A. 15-30 minutes
B. 1.5-2.5 hours [CORRECT]
C. 4-6 hours
D. 12-24 hours
Correct Answer: B
Rationale: Primary clarifiers typically operate at 1.5-2.5 hours detention. Option A is too short. Options
C/D are secondary treatment or lagoon detention times.
Q6. A comminutor is used to:
A. Remove grit from wastewater
,B. Grind large solids into smaller particles [CORRECT]
C. Add oxygen to wastewater
D. Remove grease and oils
Correct Answer: B
Rationale: Comminutors grind screenings for easier handling. Option A is grit chambers. Option C is
aeration. Option D is skimming.
Q7. The weir overflow rate for a rectangular clarifier is calculated as:
A. Flow ÷ Surface area
B. Flow ÷ Weir length [CORRECT]
C. Volume ÷ Flow
D. Depth × Width
Correct Answer: B
Rationale: Weir overflow rate = Flow (gpd) ÷ Weir length (ft). Option A is surface overflow rate. Option C
is detention time. Option D is volume calculation.
Q8. A primary clarifier removes approximately what percentage of suspended solids?
A. 10-20%
B. 40-60% [CORRECT]
C. 80-90%
D. 95-99%
Correct Answer: B
Rationale: Primary clarifiers typically remove 40-60% TSS and 25-40% BOD. Options A is too low. Options
C/D are secondary treatment efficiencies.
Q9. Aerated grit chambers differ from conventional grit chambers by:
A. Using air to separate organic matter from grit [CORRECT]
B. Being deeper than conventional chambers
C. Not requiring grit removal equipment
D. Operating at higher temperatures
, Correct Answer: A
Rationale: Aeration keeps organics in suspension while grit settles. Options B/C/D are incorrect
descriptions.
Q10. The velocity through a grit chamber should be maintained at approximately:
A. 0.5 ft/sec
B. 1.0 ft/sec [CORRECT]
C. 2.5 ft/sec
D. 5.0 ft/sec
Correct Answer: B
Rationale: 1.0 ft/sec allows grit to settle while keeping organics suspended. Options A is too slow.
Options C/D are too fast.
Q11. A primary clarifier sludge blanket depth of 3.5 feet indicates:
A. Normal operation
B. Potential sludge buildup requiring increased removal [CORRECT]
C. Excellent settling conditions
D. Need to decrease sludge withdrawal
Correct Answer: B
Rationale: Normal blanket is 1-2 ft; 3.5 ft suggests excessive accumulation. Options A/C/D misinterpret
the condition.
Q12. Pre-aeration of wastewater is used to:
A. Remove all BOD before primary treatment
B. Freshen septic wastewater and promote grease separation [CORRECT]
C. Disinfect the wastewater
D. Remove nitrogen
Correct Answer: B
Rationale: Pre-aeration adds oxygen to septic flows and aids grease flotation. Option A is incorrect.
Option C is disinfection. Option D is biological nutrient removal.
Certification Examination Tests 2 & 3 –
Grade / Class 2 & 3 – Association of
Boards of Certification (ABC) / State
Environmental Agency
[DOMAIN A: Preliminary & Primary Treatment (Questions 1-12)]
Q1. A primary clarifier has a detention time of 2.5 hours and a surface overflow rate of 800 gpd/sq ft.
Which adjustment would improve solids removal if effluent TSS is elevated?
A. Increase weir loading rate
B. Decrease surface overflow rate (reduce flow or increase surface area) [CORRECT]
C. Increase detention time by reducing tank depth
D. Increase influent flow rate
Correct Answer: B
Rationale: Lower SOR allows more time for solids settling. Option A (increase weir loading) worsens
performance. Option C (reduce depth) reduces detention time. Option D (increase flow) increases SOR.
Q2. A bar screen removes debris from influent wastewater. The primary purpose of screening is to:
A. Remove grit and sand
B. Protect downstream equipment from damage and clogging [CORRECT]
C. Remove dissolved organic matter
D. Disinfect the wastewater
Correct Answer: B
Rationale: Screens remove large solids to protect pumps, pipes, and process equipment. Option A is grit
removal. Option C is biological treatment. Option D is disinfection.
,Q3. Grit removal is important because:
A. Grit is organic matter that causes odors
B. Inorganic grit damages equipment and occupies space in tanks [CORRECT]
C. Grit contains pathogens that must be removed
D. Grit is the primary source of BOD
Correct Answer: B
Rationale: Grit (sand, gravel, eggshells) is inorganic, abrasive, and reduces effective tank volume. Option
A is incorrect (grit is inorganic). Option C is incorrect. Option D is incorrect (BOD is organic).
Q4. A primary clarifier has a volume of 250,000 gallons and receives a flow of 2.5 MGD. The detention
time is:
A. 1.0 hour
B. 2.4 hours [CORRECT]
C. 4.0 hours
D. 6.0 hours
Correct Answer: B
Rationale: Detention time (hrs) = Volume (gal) ÷ (Flow (MGD) × 1,000,000 ÷ 24) = 250,000 ÷ 104,167 =
2.4 hours. Option A is too low. Options C/D are too high.
Q5. The typical detention time for a primary clarifier is:
A. 15-30 minutes
B. 1.5-2.5 hours [CORRECT]
C. 4-6 hours
D. 12-24 hours
Correct Answer: B
Rationale: Primary clarifiers typically operate at 1.5-2.5 hours detention. Option A is too short. Options
C/D are secondary treatment or lagoon detention times.
Q6. A comminutor is used to:
A. Remove grit from wastewater
,B. Grind large solids into smaller particles [CORRECT]
C. Add oxygen to wastewater
D. Remove grease and oils
Correct Answer: B
Rationale: Comminutors grind screenings for easier handling. Option A is grit chambers. Option C is
aeration. Option D is skimming.
Q7. The weir overflow rate for a rectangular clarifier is calculated as:
A. Flow ÷ Surface area
B. Flow ÷ Weir length [CORRECT]
C. Volume ÷ Flow
D. Depth × Width
Correct Answer: B
Rationale: Weir overflow rate = Flow (gpd) ÷ Weir length (ft). Option A is surface overflow rate. Option C
is detention time. Option D is volume calculation.
Q8. A primary clarifier removes approximately what percentage of suspended solids?
A. 10-20%
B. 40-60% [CORRECT]
C. 80-90%
D. 95-99%
Correct Answer: B
Rationale: Primary clarifiers typically remove 40-60% TSS and 25-40% BOD. Options A is too low. Options
C/D are secondary treatment efficiencies.
Q9. Aerated grit chambers differ from conventional grit chambers by:
A. Using air to separate organic matter from grit [CORRECT]
B. Being deeper than conventional chambers
C. Not requiring grit removal equipment
D. Operating at higher temperatures
, Correct Answer: A
Rationale: Aeration keeps organics in suspension while grit settles. Options B/C/D are incorrect
descriptions.
Q10. The velocity through a grit chamber should be maintained at approximately:
A. 0.5 ft/sec
B. 1.0 ft/sec [CORRECT]
C. 2.5 ft/sec
D. 5.0 ft/sec
Correct Answer: B
Rationale: 1.0 ft/sec allows grit to settle while keeping organics suspended. Options A is too slow.
Options C/D are too fast.
Q11. A primary clarifier sludge blanket depth of 3.5 feet indicates:
A. Normal operation
B. Potential sludge buildup requiring increased removal [CORRECT]
C. Excellent settling conditions
D. Need to decrease sludge withdrawal
Correct Answer: B
Rationale: Normal blanket is 1-2 ft; 3.5 ft suggests excessive accumulation. Options A/C/D misinterpret
the condition.
Q12. Pre-aeration of wastewater is used to:
A. Remove all BOD before primary treatment
B. Freshen septic wastewater and promote grease separation [CORRECT]
C. Disinfect the wastewater
D. Remove nitrogen
Correct Answer: B
Rationale: Pre-aeration adds oxygen to septic flows and aids grease flotation. Option A is incorrect.
Option C is disinfection. Option D is biological nutrient removal.
