TCEQ Wastewater Class D License
Exam Prep 2026/2027 — 300
Questions with Rationales Aligned
with the Official Certification
Blueprint||Guaranteed Pass |Graded
A+
1. How many gallons are in one cubic foot of water?
• Answer: 7.48 gallons
• Rationale: This is a fundamental constant in wastewater math. It is
used continuously to convert tank volumes calculated in cubic feet
into gallons for capacity and loading calculations .
2. What is the weight of one gallon of water?
• Answer: 8.34 lbs
• Rationale: This constant (8.34 lbs/gal) is essential for calculating
chemical feed rates and organic loadings, using the formula: lbs =
MGD x mg/L x 8.34 .
3. Convert a flow of 500 gallons per minute (gpm) to million gallons
per day (MGD).
• Answer: 0.72 MGD
• Rationale: 500 gpm x 1,440 minutes/day = 720,000 gallons/day.
Divide by 1,000,000 to get 0.72 MGD .
4. A wet well is 8 ft by 8 ft with water 6 ft deep. How many gallons
does it hold?
• Answer: 2,872 gallons
, • Rationale: Volume = Length x Width x Depth = 8 x 8 x 6 = 384
cubic feet. Convert to gallons by multiplying by 7.48: 384 x 7.48 =
2,872.3 gallons .
5. What is the detention time in hours for a 100,000-gallon tank
receiving a flow of 200,000 gallons per day?
• Answer: 12 hours
• Rationale: Detention time = Volume / Flow. 100,000 gal / 200,000
gal/day = 0.5 day. Multiply by 24 hours/day to get 12 hours .
6. A circular clarifier has a diameter of 40 feet. What is its surface
area (use pi = 3.14)?
• Answer: 1,256 square feet
• Rationale: Area = π x r². Radius is half the diameter: 40/2 = 20 ft.
Area = 3.14 x 20² = 3.14 x 400 = 1,256 square feet .
7. What percentage is equivalent to 250 mg/L?
• Answer: 0.025%
• Rationale: To convert mg/L to percent, divide by 10,000 (since 1%
= 10,000 mg/L). Therefore, ,000 = 0.025% .
8. If a pump delivers 300 gallons per minute, how many gallons
does it deliver in 8 hours?
• Answer: 144,000 gallons
• Rationale: 300 gpm x 60 min/hr = 18,000 gal/hr. Then, 18,000
gal/hr x 8 hours = 144,000 gallons .
9. What is the minimum velocity required to keep solids in
suspension in a collection system?
• Answer: 2 ft/sec
• Rationale: Maintaining a minimum velocity of 2 feet per second at
average daily flow helps prevent solids from settling and
depositing in the sewer lines .
10. What is the minimum required velocity for an inverted siphon?
, • Answer: 3 ft/sec
• Rationale: Inverted siphons require a higher minimum velocity of
3 feet per second to ensure solids are scoured and do not settle in
the depressed portion of the pipe .
11. Which two types of pipes are most commonly used in
wastewater collection systems?
• Answer: Clay and PVC
• Rationale: Both vitrified clay pipe (VCP) and polyvinyl chloride
(PVC) are durable and corrosion-resistant materials that are widely
used for gravity sewer lines.
12. According to OSHA, at what depth is shoring, sloping, or trench
boxes required?
• Answer: 5 feet or deeper
• Rationale: OSHA standards mandate protective systems for any
excavation 5 feet or deeper to prevent cave-ins and protect
workers .
13. At what depth is a portable ladder or other means of exit
required in a trench?
• Answer: 4 feet or deeper
• Rationale: A ladder or other safe exit must be provided for worker
safety in trenches 4 feet deep or more .
14. A ladder in a trench must extend how far above the trench and
be secured?
• Answer: 3 feet
• Rationale: The ladder must extend at least 3 feet above the
excavation edge to provide a safe handhold for entering and
exiting.
15. Ladders must be located within how many feet of trench
workers?
• Answer: 25 feet, and not more than 50 feet apart.
, • Rationale: This ensures quick and safe access to an exit in an
emergency.
16. Excavation spoil (dirt) must be placed at least how far from the
edge of the trench?
• Answer: 2 feet
• Rationale: Keeping spoil piles at least 2 feet back reduces the load
on the trench walls and prevents material from falling back into the
excavation.
17. What is the most common type of pump used in lift stations?
• Answer: Centrifugal Pump
• Rationale: Centrifugal pumps are preferred for their efficiency,
simplicity, and ability to handle the varying flows and solids
common in wastewater applications .
18. Installation distance between a water line and a wastewater line?
• Answer: 9 feet
• Rationale: A minimum horizontal separation of 9 feet is required
by TCEQ to prevent contamination of potable water supplies in the
event of a leak from the sewer line.
19. How often should flow measuring devices be calibrated?
• Answer: Annually
• Rationale: Annual calibration ensures the accuracy of flow data,
which is critical for regulatory reporting and process control.
20. Primary clarifiers should remove what percentage of the
incoming BOD?
• Answer: 35%
• Rationale: Primary clarification is designed to remove settleable
solids and a portion of the organic matter, typically accounting for
30-40% of the total BOD .
21. Primary clarifiers should remove what percentage of settleable
solids?
Exam Prep 2026/2027 — 300
Questions with Rationales Aligned
with the Official Certification
Blueprint||Guaranteed Pass |Graded
A+
1. How many gallons are in one cubic foot of water?
• Answer: 7.48 gallons
• Rationale: This is a fundamental constant in wastewater math. It is
used continuously to convert tank volumes calculated in cubic feet
into gallons for capacity and loading calculations .
2. What is the weight of one gallon of water?
• Answer: 8.34 lbs
• Rationale: This constant (8.34 lbs/gal) is essential for calculating
chemical feed rates and organic loadings, using the formula: lbs =
MGD x mg/L x 8.34 .
3. Convert a flow of 500 gallons per minute (gpm) to million gallons
per day (MGD).
• Answer: 0.72 MGD
• Rationale: 500 gpm x 1,440 minutes/day = 720,000 gallons/day.
Divide by 1,000,000 to get 0.72 MGD .
4. A wet well is 8 ft by 8 ft with water 6 ft deep. How many gallons
does it hold?
• Answer: 2,872 gallons
, • Rationale: Volume = Length x Width x Depth = 8 x 8 x 6 = 384
cubic feet. Convert to gallons by multiplying by 7.48: 384 x 7.48 =
2,872.3 gallons .
5. What is the detention time in hours for a 100,000-gallon tank
receiving a flow of 200,000 gallons per day?
• Answer: 12 hours
• Rationale: Detention time = Volume / Flow. 100,000 gal / 200,000
gal/day = 0.5 day. Multiply by 24 hours/day to get 12 hours .
6. A circular clarifier has a diameter of 40 feet. What is its surface
area (use pi = 3.14)?
• Answer: 1,256 square feet
• Rationale: Area = π x r². Radius is half the diameter: 40/2 = 20 ft.
Area = 3.14 x 20² = 3.14 x 400 = 1,256 square feet .
7. What percentage is equivalent to 250 mg/L?
• Answer: 0.025%
• Rationale: To convert mg/L to percent, divide by 10,000 (since 1%
= 10,000 mg/L). Therefore, ,000 = 0.025% .
8. If a pump delivers 300 gallons per minute, how many gallons
does it deliver in 8 hours?
• Answer: 144,000 gallons
• Rationale: 300 gpm x 60 min/hr = 18,000 gal/hr. Then, 18,000
gal/hr x 8 hours = 144,000 gallons .
9. What is the minimum velocity required to keep solids in
suspension in a collection system?
• Answer: 2 ft/sec
• Rationale: Maintaining a minimum velocity of 2 feet per second at
average daily flow helps prevent solids from settling and
depositing in the sewer lines .
10. What is the minimum required velocity for an inverted siphon?
, • Answer: 3 ft/sec
• Rationale: Inverted siphons require a higher minimum velocity of
3 feet per second to ensure solids are scoured and do not settle in
the depressed portion of the pipe .
11. Which two types of pipes are most commonly used in
wastewater collection systems?
• Answer: Clay and PVC
• Rationale: Both vitrified clay pipe (VCP) and polyvinyl chloride
(PVC) are durable and corrosion-resistant materials that are widely
used for gravity sewer lines.
12. According to OSHA, at what depth is shoring, sloping, or trench
boxes required?
• Answer: 5 feet or deeper
• Rationale: OSHA standards mandate protective systems for any
excavation 5 feet or deeper to prevent cave-ins and protect
workers .
13. At what depth is a portable ladder or other means of exit
required in a trench?
• Answer: 4 feet or deeper
• Rationale: A ladder or other safe exit must be provided for worker
safety in trenches 4 feet deep or more .
14. A ladder in a trench must extend how far above the trench and
be secured?
• Answer: 3 feet
• Rationale: The ladder must extend at least 3 feet above the
excavation edge to provide a safe handhold for entering and
exiting.
15. Ladders must be located within how many feet of trench
workers?
• Answer: 25 feet, and not more than 50 feet apart.
, • Rationale: This ensures quick and safe access to an exit in an
emergency.
16. Excavation spoil (dirt) must be placed at least how far from the
edge of the trench?
• Answer: 2 feet
• Rationale: Keeping spoil piles at least 2 feet back reduces the load
on the trench walls and prevents material from falling back into the
excavation.
17. What is the most common type of pump used in lift stations?
• Answer: Centrifugal Pump
• Rationale: Centrifugal pumps are preferred for their efficiency,
simplicity, and ability to handle the varying flows and solids
common in wastewater applications .
18. Installation distance between a water line and a wastewater line?
• Answer: 9 feet
• Rationale: A minimum horizontal separation of 9 feet is required
by TCEQ to prevent contamination of potable water supplies in the
event of a leak from the sewer line.
19. How often should flow measuring devices be calibrated?
• Answer: Annually
• Rationale: Annual calibration ensures the accuracy of flow data,
which is critical for regulatory reporting and process control.
20. Primary clarifiers should remove what percentage of the
incoming BOD?
• Answer: 35%
• Rationale: Primary clarification is designed to remove settleable
solids and a portion of the organic matter, typically accounting for
30-40% of the total BOD .
21. Primary clarifiers should remove what percentage of settleable
solids?
