TCEQ Wastewater Collection
System Operator Gravity Sewer
Systems Exam Questions and
Correct Answers with Detailed
Rationales | 2026/2027 Edition |
Exam Prep Study Guide PDF
1. What is the primary purpose of a gravity sewer system?
A. To pressurize wastewater throughout the collection network
B. To transport wastewater using natural gravitational flow
C. To disinfect wastewater before treatment
D. To remove dissolved solids from wastewater
Answer: B. To transport wastewater using natural gravitational
flow
Rationale: Gravity sewer systems are designed so wastewater
flows from higher elevations toward lower elevations primarily
because of gravity. Proper pipe slopes, elevations, and hydraulic
design provide sufficient velocity to move wastewater without
continuous pumping. Unlike force mains, gravity sewers normally
operate without internal pressure under routine conditions.
2. Which feature is most important for maintaining flow in a
gravity sewer?
A. Adequate pipe slope
B. High pipe pressure
,C. Continuous chemical dosing
D. Maximum pipe diameter regardless of flow
Answer: A. Adequate pipe slope
Rationale: Pipe slope provides the elevation difference that allows
wastewater to flow by gravity. If the slope is too flat, wastewater
may move too slowly, increasing the potential for solids
deposition and blockages. If the slope is excessively steep, flow
velocities may become unnecessarily high and can contribute to
operational problems.
3. In a gravity sewer, what does the term “invert elevation”
refer to?
A. The elevation of the ground surface above the pipe
B. The elevation at the top of the pipe
C. The elevation of the inside bottom of the pipe
D. The elevation of the water surface in a manhole
Answer: C. The elevation of the inside bottom of the pipe
Rationale: The invert is the lowest inside point of a pipe or
channel. In sewer systems, invert elevations are essential for
determining pipe slope, connecting sewer segments correctly,
and establishing whether wastewater can flow by gravity between
structures.
4. What is the main reason gravity sewer pipes are installed
with a designed slope?
A. To increase wastewater temperature
B. To provide hydraulic head for gravity flow
,C. To eliminate the need for manholes
D. To prevent all infiltration
Answer: B. To provide hydraulic head for gravity flow
Rationale: A difference in elevation along the pipe creates the
hydraulic driving force necessary for gravity flow. The slope must
be sufficient to transport wastewater and suspended solids while
avoiding unnecessarily high velocities. Proper hydraulic design
balances these considerations.
5. Which condition is most likely to cause solids to settle in a
gravity sewer?
A. Excessive flow velocity
B. Insufficient flow velocity
C. Excessive pipe slope
D. Increased hydraulic gradient
Answer: B. Insufficient flow velocity
Rationale: Wastewater carries suspended solids that must remain
sufficiently mobile to travel through the collection system. When
velocity becomes too low, heavier materials can settle onto the
pipe bottom. Repeated deposition can reduce the effective pipe
capacity and eventually contribute to blockages.
6. What is meant by “self-cleansing velocity” in a gravity
sewer?
A. The velocity at which all wastewater becomes disinfected
B. The minimum velocity needed to help prevent excessive solids
deposition
, C. The maximum velocity allowed in a force main
D. The velocity produced by a lift station pump
Answer: B. The minimum velocity needed to help prevent
excessive solids deposition
Rationale: Self-cleansing velocity is a design concept referring to
sufficient flow velocity to reduce the tendency of solids to settle
and accumulate. The required velocity depends on factors such
as pipe characteristics, wastewater properties, solids
concentration, and hydraulic conditions.
7. Which type of sewer is normally designed to operate with an
open water surface rather than under full pressure?
A. Gravity sewer
B. Force main
C. Pressure sewer
D. Pump discharge line
Answer: A. Gravity sewer
Rationale: Gravity sewers normally operate as partially filled
conduits with a free surface. The wastewater flows because of the
elevation difference between upstream and downstream points.
Force mains, by contrast, are pressurized pipelines typically
supplied by pumps.
8. What is a major operational concern when a gravity sewer
has excessive infiltration and inflow?
A. Reduced wastewater volume entering the treatment plant
B. Increased hydraulic loading of downstream facilities
System Operator Gravity Sewer
Systems Exam Questions and
Correct Answers with Detailed
Rationales | 2026/2027 Edition |
Exam Prep Study Guide PDF
1. What is the primary purpose of a gravity sewer system?
A. To pressurize wastewater throughout the collection network
B. To transport wastewater using natural gravitational flow
C. To disinfect wastewater before treatment
D. To remove dissolved solids from wastewater
Answer: B. To transport wastewater using natural gravitational
flow
Rationale: Gravity sewer systems are designed so wastewater
flows from higher elevations toward lower elevations primarily
because of gravity. Proper pipe slopes, elevations, and hydraulic
design provide sufficient velocity to move wastewater without
continuous pumping. Unlike force mains, gravity sewers normally
operate without internal pressure under routine conditions.
2. Which feature is most important for maintaining flow in a
gravity sewer?
A. Adequate pipe slope
B. High pipe pressure
,C. Continuous chemical dosing
D. Maximum pipe diameter regardless of flow
Answer: A. Adequate pipe slope
Rationale: Pipe slope provides the elevation difference that allows
wastewater to flow by gravity. If the slope is too flat, wastewater
may move too slowly, increasing the potential for solids
deposition and blockages. If the slope is excessively steep, flow
velocities may become unnecessarily high and can contribute to
operational problems.
3. In a gravity sewer, what does the term “invert elevation”
refer to?
A. The elevation of the ground surface above the pipe
B. The elevation at the top of the pipe
C. The elevation of the inside bottom of the pipe
D. The elevation of the water surface in a manhole
Answer: C. The elevation of the inside bottom of the pipe
Rationale: The invert is the lowest inside point of a pipe or
channel. In sewer systems, invert elevations are essential for
determining pipe slope, connecting sewer segments correctly,
and establishing whether wastewater can flow by gravity between
structures.
4. What is the main reason gravity sewer pipes are installed
with a designed slope?
A. To increase wastewater temperature
B. To provide hydraulic head for gravity flow
,C. To eliminate the need for manholes
D. To prevent all infiltration
Answer: B. To provide hydraulic head for gravity flow
Rationale: A difference in elevation along the pipe creates the
hydraulic driving force necessary for gravity flow. The slope must
be sufficient to transport wastewater and suspended solids while
avoiding unnecessarily high velocities. Proper hydraulic design
balances these considerations.
5. Which condition is most likely to cause solids to settle in a
gravity sewer?
A. Excessive flow velocity
B. Insufficient flow velocity
C. Excessive pipe slope
D. Increased hydraulic gradient
Answer: B. Insufficient flow velocity
Rationale: Wastewater carries suspended solids that must remain
sufficiently mobile to travel through the collection system. When
velocity becomes too low, heavier materials can settle onto the
pipe bottom. Repeated deposition can reduce the effective pipe
capacity and eventually contribute to blockages.
6. What is meant by “self-cleansing velocity” in a gravity
sewer?
A. The velocity at which all wastewater becomes disinfected
B. The minimum velocity needed to help prevent excessive solids
deposition
, C. The maximum velocity allowed in a force main
D. The velocity produced by a lift station pump
Answer: B. The minimum velocity needed to help prevent
excessive solids deposition
Rationale: Self-cleansing velocity is a design concept referring to
sufficient flow velocity to reduce the tendency of solids to settle
and accumulate. The required velocity depends on factors such
as pipe characteristics, wastewater properties, solids
concentration, and hydraulic conditions.
7. Which type of sewer is normally designed to operate with an
open water surface rather than under full pressure?
A. Gravity sewer
B. Force main
C. Pressure sewer
D. Pump discharge line
Answer: A. Gravity sewer
Rationale: Gravity sewers normally operate as partially filled
conduits with a free surface. The wastewater flows because of the
elevation difference between upstream and downstream points.
Force mains, by contrast, are pressurized pipelines typically
supplied by pumps.
8. What is a major operational concern when a gravity sewer
has excessive infiltration and inflow?
A. Reduced wastewater volume entering the treatment plant
B. Increased hydraulic loading of downstream facilities
