Arizona AZ L-85 Wastewater
Treatment Contractor Practice
Exam | Questions, Verified
Answers & Detailed
Rationales | Complete
Contractor License Exam Prep
Study Guide | Updated 2026
1.
What is the primary purpose of a wastewater treatment
system?
A. Increase the temperature of wastewater
B. Remove pollutants and contaminants before discharge or
reuse
C. Add organic matter to wastewater
D. Increase the solids concentration
Correct answer: Remove pollutants and contaminants
before discharge or reuse
Rationale: Wastewater treatment protects public health
and the environment by reducing solids, organic matter,
nutrients, pathogens, and other pollutants to
acceptable levels.
2.
Which wastewater characteristic is commonly measured as
BOD₅?
A. Biological oxygen demand over five days
B. Biological oxidation density
C. Bacterial oxygen dilution
D. Biochemical operating depth
,Correct answer: Biological oxygen demand over five
days
Rationale: BOD₅ estimates the amount of dissolved
oxygen microorganisms require to biologically stabilize
biodegradable organic matter during a standard five-
day test.
3.
What is the primary function of a bar screen?
A. Remove dissolved ammonia
B. Remove large debris and solids
C. Disinfect wastewater
D. Remove dissolved phosphorus
Correct answer: Remove large debris and solids
Rationale: Bar screens protect downstream pumps and
treatment equipment by removing rags, sticks, plastics,
and other large objects.
4.
What is the purpose of grit removal?
A. Remove sand, gravel, and other heavy inorganic particles
B. Remove dissolved oxygen
C. Increase biological activity
D. Remove dissolved ammonia
Correct answer: Remove sand, gravel, and other heavy
inorganic particles
Rationale: Grit can accumulate in channels, tanks,
pumps, and pipelines and cause excessive abrasion and
reduced treatment capacity.
5.
What is the primary purpose of a primary clarifier?
A. Add oxygen
B. Settle suspended solids from wastewater
,C. Destroy all pathogens
D. Convert nitrate to nitrogen gas
Correct answer: Settle suspended solids from
wastewater
Rationale: Primary clarification removes settleable
solids and some floating material before biological
treatment.
6.
Which process is primarily responsible for biological oxidation of
organic matter in an activated-sludge system?
A. Screening
B. Aeration
C. Grit removal
D. Chlorination
Correct answer: Aeration
Rationale: Aeration supplies oxygen and mixing so
microorganisms can consume biodegradable organic
matter.
7.
What is MLSS?
A. Maximum liquid settling speed
B. Mixed liquor suspended solids
C. Mechanical liquid sampling system
D. Minimum level sludge standard
Correct answer: Mixed liquor suspended solids
Rationale: MLSS represents the concentration of
suspended solids in the mixed liquor of an activated-
sludge aeration basin.
8.
What does MLVSS generally represent?
, A. The volatile portion of mixed liquor suspended solids
B. Maximum volatile liquid storage space
C. Mechanical valve liquid safety standard
D. Minimum verified sludge settling speed
Correct answer: The volatile portion of mixed liquor
suspended solids
Rationale: MLVSS is commonly used as an approximation
of the organic or biologically active portion of MLSS.
9.
What is the primary purpose of return activated sludge?
A. Return microorganisms from the clarifier to the aeration
process
B. Return grit to the headworks
C. Return disinfectant to the influent
D. Return screenings to the primary clarifier
Correct answer: Return microorganisms from the
clarifier to the aeration process
Rationale: RAS maintains an appropriate concentration
of microorganisms in the aeration basin by returning
settled activated sludge from the secondary clarifier.
10.
What is waste activated sludge?
A. Sludge intentionally removed from the biological process
B. Grit removed from the headworks
C. Floating grease
D. Untreated influent
Correct answer: Sludge intentionally removed from the
biological process
Rationale: WAS is removed to control the solids
inventory, sludge age, and biological process
conditions.
Treatment Contractor Practice
Exam | Questions, Verified
Answers & Detailed
Rationales | Complete
Contractor License Exam Prep
Study Guide | Updated 2026
1.
What is the primary purpose of a wastewater treatment
system?
A. Increase the temperature of wastewater
B. Remove pollutants and contaminants before discharge or
reuse
C. Add organic matter to wastewater
D. Increase the solids concentration
Correct answer: Remove pollutants and contaminants
before discharge or reuse
Rationale: Wastewater treatment protects public health
and the environment by reducing solids, organic matter,
nutrients, pathogens, and other pollutants to
acceptable levels.
2.
Which wastewater characteristic is commonly measured as
BOD₅?
A. Biological oxygen demand over five days
B. Biological oxidation density
C. Bacterial oxygen dilution
D. Biochemical operating depth
,Correct answer: Biological oxygen demand over five
days
Rationale: BOD₅ estimates the amount of dissolved
oxygen microorganisms require to biologically stabilize
biodegradable organic matter during a standard five-
day test.
3.
What is the primary function of a bar screen?
A. Remove dissolved ammonia
B. Remove large debris and solids
C. Disinfect wastewater
D. Remove dissolved phosphorus
Correct answer: Remove large debris and solids
Rationale: Bar screens protect downstream pumps and
treatment equipment by removing rags, sticks, plastics,
and other large objects.
4.
What is the purpose of grit removal?
A. Remove sand, gravel, and other heavy inorganic particles
B. Remove dissolved oxygen
C. Increase biological activity
D. Remove dissolved ammonia
Correct answer: Remove sand, gravel, and other heavy
inorganic particles
Rationale: Grit can accumulate in channels, tanks,
pumps, and pipelines and cause excessive abrasion and
reduced treatment capacity.
5.
What is the primary purpose of a primary clarifier?
A. Add oxygen
B. Settle suspended solids from wastewater
,C. Destroy all pathogens
D. Convert nitrate to nitrogen gas
Correct answer: Settle suspended solids from
wastewater
Rationale: Primary clarification removes settleable
solids and some floating material before biological
treatment.
6.
Which process is primarily responsible for biological oxidation of
organic matter in an activated-sludge system?
A. Screening
B. Aeration
C. Grit removal
D. Chlorination
Correct answer: Aeration
Rationale: Aeration supplies oxygen and mixing so
microorganisms can consume biodegradable organic
matter.
7.
What is MLSS?
A. Maximum liquid settling speed
B. Mixed liquor suspended solids
C. Mechanical liquid sampling system
D. Minimum level sludge standard
Correct answer: Mixed liquor suspended solids
Rationale: MLSS represents the concentration of
suspended solids in the mixed liquor of an activated-
sludge aeration basin.
8.
What does MLVSS generally represent?
, A. The volatile portion of mixed liquor suspended solids
B. Maximum volatile liquid storage space
C. Mechanical valve liquid safety standard
D. Minimum verified sludge settling speed
Correct answer: The volatile portion of mixed liquor
suspended solids
Rationale: MLVSS is commonly used as an approximation
of the organic or biologically active portion of MLSS.
9.
What is the primary purpose of return activated sludge?
A. Return microorganisms from the clarifier to the aeration
process
B. Return grit to the headworks
C. Return disinfectant to the influent
D. Return screenings to the primary clarifier
Correct answer: Return microorganisms from the
clarifier to the aeration process
Rationale: RAS maintains an appropriate concentration
of microorganisms in the aeration basin by returning
settled activated sludge from the secondary clarifier.
10.
What is waste activated sludge?
A. Sludge intentionally removed from the biological process
B. Grit removed from the headworks
C. Floating grease
D. Untreated influent
Correct answer: Sludge intentionally removed from the
biological process
Rationale: WAS is removed to control the solids
inventory, sludge age, and biological process
conditions.
