TEXAS WASTEWATER CLASS A CERTIFICATION EXAM 2025
COMPREHENSIVE PRACTICE EXAMINATION WITH VERIFIED SOLUTIONS
SECTION 1: ACTIVATED SLUDGE PROCESS CONTROL AND OPERATIONS
1. A "Racetrack" is what type of Activated Sludge treatment?
A. Conventional activated sludge
B. Extended air
C. Contact stabilization
D. Step feed aeration
Answer: B. Extended air
Rationale: In Texas, an oxidation ditch treatment plant is referred to as a "Racetrack," which
is a form of extended air activated sludge process. This configuration provides long
detention times (18-20+ hours) and complete nitrification .
2. Which form of activated sludge must have primary clarifiers?
A. Extended air
B. Contact stabilization
C. Conventional
D. Oxidation ditch
Answer: C. Conventional
Rationale: Conventional activated sludge systems require primary clarifiers ahead of the
aeration basin to remove settleable solids and reduce the organic loading. Other processes
like extended air may function without primary clarification .
3. What is the relationship between F/M ratio and MCRT?
A. Direct; if one goes up, the other goes up
B. Indirect; if one goes up, the other goes down
C. No relationship exists
D. They are identical measurements
Answer: B. Indirect; if one goes up, the other goes down
Rationale: Food-to-Microorganism (F/M) ratio and Mean Cell Residence Time (MCRT) have
an inverse relationship. As MCRT increases (sludge age increases), the F/M ratio decreases
because there are more microorganisms available to consume the same amount of food .
,4. Which activated sludge process does not achieve complete nitrification?
A. Extended air
B. Complete mix
C. Contact stabilization
D. Oxidation ditch
Answer: C. Contact stabilization
Rationale: Contact stabilization is designed primarily for BOD removal and does not
achieve complete nitrification in a single stage. The short contact time and sludge
reaeration configuration limit the growth of nitrifying bacteria .
5. Which form of activated sludge does the better job of handling shock or toxic loads?
A. Conventional plug flow
B. Step feed
C. Complete mix
D. Contact stabilization
Answer: C. Complete mix
Rationale: Complete mix activated sludge systems provide the best protection against
shock loads because incoming wastewater is immediately diluted with the entire contents
of the aeration basin, minimizing the impact of toxic or high-strength influent .
6. Testing of mixed liquor indicates the MLSS is 2500 mg/L and the settling test shows
that a 1000 ml sample settles to 400 mls after 30 minutes. What is the SVI?
A. 100
B. 120
C. 160
D. 200
Answer: C. 160
Rationale: SVI = (Settled volume in ml/L) / (MLSS in mg/L) × 1000. (400/2500) × 1000 = 160
mL/g. An SVI above 150 typically indicates bulking sludge conditions .
7. In the problem above, what does the sludge quality indicate?
A. Good settling
B. Pin floc
C. Bulking
D. Rising sludge
Answer: C. Bulking
Rationale: With an SVI of 160, this indicates bulking sludge. Bulking is characterized by
,poor settling, high SVI values (>150), and typically caused by filamentous organism
overgrowth .
8. A complete mix activated sludge plant has a fast settling sludge and high amounts
of pin floc leaving the clarifier. One method of correcting the problem is to:
A. Decrease the F/M ratio by wasting less
B. Increase F/M ratio by wasting more
C. Increase the return sludge rate
D. Decrease aeration tank DO
Answer: B. Increase F/M ratio by wasting more
Rationale: Pin floc formation is often associated with low F/M ratios (over-oxidation).
Increasing the F/M ratio by wasting more sludge encourages floc formation and improves
settling characteristics .
9. A predominance of filaments in an activated sludge plant indicates what?
A. High DO and good settling
B. Low DO, poor settling, and nutrient imbalance
C. Excellent treatment efficiency
D. High sludge age only
Answer: B. Low DO, poor settling, and nutrient imbalance
Rationale: Filamentous organism overgrowth is typically caused by low dissolved oxygen,
nutrient deficiencies, or other adverse conditions. It results in poor sludge settling and
effluent quality .
10. Which activated sludge process has 18 to 20+ hours detention time in the aeration
basin?
A. Conventional
B. Contact stabilization
C. Extended air
D. Step feed
Answer: C. Extended air
Rationale: Extended air systems are designed with long hydraulic retention times (18-24
hours) to achieve complete oxidation of organic matter and nitrification .
SECTION 2: DISINFECTION AND CHLORINATION
11. What is the most effective chemical disinfecting agent produced when free
chlorine gas is dissolved in water at a pH below 7.0?
, A. Hypochlorite ion (OCl⁻)
B. Hypochlorous acid (HOCl)
C. Ammonium chloride (NH₄Cl)
D. Chlorine dioxide (ClO₂)
Answer: B. Hypochlorous acid (HOCl)
Rationale: Hypochlorous acid is the primary germicidal agent in free chlorine systems. It is
a neutral molecule that penetrates bacterial cell walls rapidly and is significantly more
effective than hypochlorite ion .
12. Disinfection with chlorine is the most effective at a pH of:
A. 4.0
B. 6.0
C. 8.0
D. 10.0
Answer: B. 6.0
Rationale: Chlorine disinfection is most effective at lower pH levels (around 6.0) because
hypochlorous acid (HOCl) predominates. At higher pH values, the less effective
hypochlorite ion (OCl⁻) becomes dominant .
13. What is the standard chlorine residual and minimum contact time on Texas
wastewater permits?
A. 0.5 mg/L for 10 minutes
B. 1.0 mg/L for a minimum of 20 minutes at peak flow
C. 2.0 mg/L for 15 minutes at average flow
D. No residual required if detention time exceeds 24 hours
Answer: B. 1.0 mg/L for a minimum of 20 minutes at peak flow
Rationale: Texas surface water discharge permits typically require a minimum chlorine
residual of 1.0 mg/L at peak flow with a contact basin detention time of at least 20 minutes
to ensure pathogen destruction .
14. What is the maximum chlorine residual typically allowed on Texas permits?
A. 2.0 mg/L
B. 3.0 mg/L
C. 4.0 mg/L
D. 5.0 mg/L
Answer: C. 4.0 mg/L
Rationale: The maximum chlorine residual allowed on Texas permits is typically 4.0 mg/L to
prevent toxicity to receiving waters and ensure environmental compliance .
COMPREHENSIVE PRACTICE EXAMINATION WITH VERIFIED SOLUTIONS
SECTION 1: ACTIVATED SLUDGE PROCESS CONTROL AND OPERATIONS
1. A "Racetrack" is what type of Activated Sludge treatment?
A. Conventional activated sludge
B. Extended air
C. Contact stabilization
D. Step feed aeration
Answer: B. Extended air
Rationale: In Texas, an oxidation ditch treatment plant is referred to as a "Racetrack," which
is a form of extended air activated sludge process. This configuration provides long
detention times (18-20+ hours) and complete nitrification .
2. Which form of activated sludge must have primary clarifiers?
A. Extended air
B. Contact stabilization
C. Conventional
D. Oxidation ditch
Answer: C. Conventional
Rationale: Conventional activated sludge systems require primary clarifiers ahead of the
aeration basin to remove settleable solids and reduce the organic loading. Other processes
like extended air may function without primary clarification .
3. What is the relationship between F/M ratio and MCRT?
A. Direct; if one goes up, the other goes up
B. Indirect; if one goes up, the other goes down
C. No relationship exists
D. They are identical measurements
Answer: B. Indirect; if one goes up, the other goes down
Rationale: Food-to-Microorganism (F/M) ratio and Mean Cell Residence Time (MCRT) have
an inverse relationship. As MCRT increases (sludge age increases), the F/M ratio decreases
because there are more microorganisms available to consume the same amount of food .
,4. Which activated sludge process does not achieve complete nitrification?
A. Extended air
B. Complete mix
C. Contact stabilization
D. Oxidation ditch
Answer: C. Contact stabilization
Rationale: Contact stabilization is designed primarily for BOD removal and does not
achieve complete nitrification in a single stage. The short contact time and sludge
reaeration configuration limit the growth of nitrifying bacteria .
5. Which form of activated sludge does the better job of handling shock or toxic loads?
A. Conventional plug flow
B. Step feed
C. Complete mix
D. Contact stabilization
Answer: C. Complete mix
Rationale: Complete mix activated sludge systems provide the best protection against
shock loads because incoming wastewater is immediately diluted with the entire contents
of the aeration basin, minimizing the impact of toxic or high-strength influent .
6. Testing of mixed liquor indicates the MLSS is 2500 mg/L and the settling test shows
that a 1000 ml sample settles to 400 mls after 30 minutes. What is the SVI?
A. 100
B. 120
C. 160
D. 200
Answer: C. 160
Rationale: SVI = (Settled volume in ml/L) / (MLSS in mg/L) × 1000. (400/2500) × 1000 = 160
mL/g. An SVI above 150 typically indicates bulking sludge conditions .
7. In the problem above, what does the sludge quality indicate?
A. Good settling
B. Pin floc
C. Bulking
D. Rising sludge
Answer: C. Bulking
Rationale: With an SVI of 160, this indicates bulking sludge. Bulking is characterized by
,poor settling, high SVI values (>150), and typically caused by filamentous organism
overgrowth .
8. A complete mix activated sludge plant has a fast settling sludge and high amounts
of pin floc leaving the clarifier. One method of correcting the problem is to:
A. Decrease the F/M ratio by wasting less
B. Increase F/M ratio by wasting more
C. Increase the return sludge rate
D. Decrease aeration tank DO
Answer: B. Increase F/M ratio by wasting more
Rationale: Pin floc formation is often associated with low F/M ratios (over-oxidation).
Increasing the F/M ratio by wasting more sludge encourages floc formation and improves
settling characteristics .
9. A predominance of filaments in an activated sludge plant indicates what?
A. High DO and good settling
B. Low DO, poor settling, and nutrient imbalance
C. Excellent treatment efficiency
D. High sludge age only
Answer: B. Low DO, poor settling, and nutrient imbalance
Rationale: Filamentous organism overgrowth is typically caused by low dissolved oxygen,
nutrient deficiencies, or other adverse conditions. It results in poor sludge settling and
effluent quality .
10. Which activated sludge process has 18 to 20+ hours detention time in the aeration
basin?
A. Conventional
B. Contact stabilization
C. Extended air
D. Step feed
Answer: C. Extended air
Rationale: Extended air systems are designed with long hydraulic retention times (18-24
hours) to achieve complete oxidation of organic matter and nitrification .
SECTION 2: DISINFECTION AND CHLORINATION
11. What is the most effective chemical disinfecting agent produced when free
chlorine gas is dissolved in water at a pH below 7.0?
, A. Hypochlorite ion (OCl⁻)
B. Hypochlorous acid (HOCl)
C. Ammonium chloride (NH₄Cl)
D. Chlorine dioxide (ClO₂)
Answer: B. Hypochlorous acid (HOCl)
Rationale: Hypochlorous acid is the primary germicidal agent in free chlorine systems. It is
a neutral molecule that penetrates bacterial cell walls rapidly and is significantly more
effective than hypochlorite ion .
12. Disinfection with chlorine is the most effective at a pH of:
A. 4.0
B. 6.0
C. 8.0
D. 10.0
Answer: B. 6.0
Rationale: Chlorine disinfection is most effective at lower pH levels (around 6.0) because
hypochlorous acid (HOCl) predominates. At higher pH values, the less effective
hypochlorite ion (OCl⁻) becomes dominant .
13. What is the standard chlorine residual and minimum contact time on Texas
wastewater permits?
A. 0.5 mg/L for 10 minutes
B. 1.0 mg/L for a minimum of 20 minutes at peak flow
C. 2.0 mg/L for 15 minutes at average flow
D. No residual required if detention time exceeds 24 hours
Answer: B. 1.0 mg/L for a minimum of 20 minutes at peak flow
Rationale: Texas surface water discharge permits typically require a minimum chlorine
residual of 1.0 mg/L at peak flow with a contact basin detention time of at least 20 minutes
to ensure pathogen destruction .
14. What is the maximum chlorine residual typically allowed on Texas permits?
A. 2.0 mg/L
B. 3.0 mg/L
C. 4.0 mg/L
D. 5.0 mg/L
Answer: C. 4.0 mg/L
Rationale: The maximum chlorine residual allowed on Texas permits is typically 4.0 mg/L to
prevent toxicity to receiving waters and ensure environmental compliance .
