California Certified Wastewater Operator
Grade I–II Exam | Questions and answers |
2026 verified Version
1.
Which process in wastewater treatment primarily removes large floating debris such as rags and
sticks?
A. Sedimentation
B. Screening
C. Aeration
D. Disinfection
Correct Answer: B
Rationale:
Screening is the first step in wastewater treatment and removes large objects like rags, plastics,
and sticks using mechanical or bar screens. Sedimentation removes settleable solids, aeration
supports biological treatment, and disinfection kills pathogens.
2.
The main purpose of primary sedimentation tanks in wastewater treatment is to:
A. Remove dissolved organic matter
B. Remove settleable solids and floatable materials
,C. Kill pathogens
D. Increase oxygen levels
Correct Answer: B
Rationale:
Primary clarifiers allow heavier solids to settle and oils/grease to float for removal. Dissolved
organic matter is treated biologically in secondary treatment.
3.
Biochemical Oxygen Demand (BOD) is best defined as:
A. Total solids in wastewater
B. Oxygen required for chemical reactions only
C. Oxygen used by microorganisms to decompose organic matter
D. Oxygen dissolved in water
Correct Answer: C
Rationale:
BOD measures the amount of oxygen microorganisms consume while decomposing organic
matter. It is a key indicator of wastewater strength.
4.
Which wastewater treatment stage mainly removes dissolved and colloidal organic material?
A. Preliminary treatment
B. Primary treatment
,C. Secondary treatment
D. Sludge digestion
Correct Answer: C
Rationale:
Secondary treatment uses microorganisms (activated sludge or biofilm processes) to degrade
dissolved and colloidal organic matter.
5.
Activated sludge treatment relies primarily on:
A. Chemical oxidation
B. Biological microorganisms
C. Mechanical filtration
D. Ultraviolet radiation
Correct Answer: B
Rationale:
Activated sludge systems use aerobic microorganisms that metabolize organic pollutants in
wastewater.
6.
A sudden decrease in dissolved oxygen in an aeration basin is most likely caused by:
A. Excess sludge wasting
B. Increased organic loading
, C. Low sludge age
D. Reduced influent flow
Correct Answer: B
Rationale:
High organic loads increase microbial respiration, which consumes oxygen rapidly and reduces
dissolved oxygen levels.
7.
The term “MLSS” in activated sludge systems stands for:
A. Mixed Liquor Suspended Solids
B. Maximum Liquid Suspended Solids
C. Microbial Level Sludge System
D. Mechanical Liquid Sediment System
Correct Answer: A
Rationale:
MLSS measures the concentration of suspended solids (microorganisms and organic material) in
the aeration basin.
8.
Which parameter indicates how long microorganisms remain in the biological treatment system?
A. Sludge Volume Index
B. Hydraulic Retention Time
Grade I–II Exam | Questions and answers |
2026 verified Version
1.
Which process in wastewater treatment primarily removes large floating debris such as rags and
sticks?
A. Sedimentation
B. Screening
C. Aeration
D. Disinfection
Correct Answer: B
Rationale:
Screening is the first step in wastewater treatment and removes large objects like rags, plastics,
and sticks using mechanical or bar screens. Sedimentation removes settleable solids, aeration
supports biological treatment, and disinfection kills pathogens.
2.
The main purpose of primary sedimentation tanks in wastewater treatment is to:
A. Remove dissolved organic matter
B. Remove settleable solids and floatable materials
,C. Kill pathogens
D. Increase oxygen levels
Correct Answer: B
Rationale:
Primary clarifiers allow heavier solids to settle and oils/grease to float for removal. Dissolved
organic matter is treated biologically in secondary treatment.
3.
Biochemical Oxygen Demand (BOD) is best defined as:
A. Total solids in wastewater
B. Oxygen required for chemical reactions only
C. Oxygen used by microorganisms to decompose organic matter
D. Oxygen dissolved in water
Correct Answer: C
Rationale:
BOD measures the amount of oxygen microorganisms consume while decomposing organic
matter. It is a key indicator of wastewater strength.
4.
Which wastewater treatment stage mainly removes dissolved and colloidal organic material?
A. Preliminary treatment
B. Primary treatment
,C. Secondary treatment
D. Sludge digestion
Correct Answer: C
Rationale:
Secondary treatment uses microorganisms (activated sludge or biofilm processes) to degrade
dissolved and colloidal organic matter.
5.
Activated sludge treatment relies primarily on:
A. Chemical oxidation
B. Biological microorganisms
C. Mechanical filtration
D. Ultraviolet radiation
Correct Answer: B
Rationale:
Activated sludge systems use aerobic microorganisms that metabolize organic pollutants in
wastewater.
6.
A sudden decrease in dissolved oxygen in an aeration basin is most likely caused by:
A. Excess sludge wasting
B. Increased organic loading
, C. Low sludge age
D. Reduced influent flow
Correct Answer: B
Rationale:
High organic loads increase microbial respiration, which consumes oxygen rapidly and reduces
dissolved oxygen levels.
7.
The term “MLSS” in activated sludge systems stands for:
A. Mixed Liquor Suspended Solids
B. Maximum Liquid Suspended Solids
C. Microbial Level Sludge System
D. Mechanical Liquid Sediment System
Correct Answer: A
Rationale:
MLSS measures the concentration of suspended solids (microorganisms and organic material) in
the aeration basin.
8.
Which parameter indicates how long microorganisms remain in the biological treatment system?
A. Sludge Volume Index
B. Hydraulic Retention Time
