NPDES Wastewater Treatment
Operator Certification Exam Practice
Questions And Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A | Instant Download Pdf
1. What is the primary purpose of preliminary treatment in a wastewater
plant?
A. Biological nutrient removal
B. Removal of large debris and grit
C. Disinfection of effluent
D. Sludge digestion
The correct answer is B because preliminary treatment removes large
solids and grit to protect downstream equipment from damage and
clogging.
2. Which parameter is most commonly used to measure organic
pollution in wastewater?
A. pH
, B. Biochemical Oxygen Demand (BOD)
C. Turbidity
D. Dissolved oxygen
BOD measures the amount of oxygen required by microorganisms to
decompose organic matter, making it a key indicator of organic
pollution.
3. What is the main function of primary clarifiers?
A. Disinfection
B. Aeration
C. Settling of suspended solids
D. Chlorine removal
Primary clarifiers allow solids to settle out by gravity, reducing
suspended solids before biological treatment.
4. What does nitrification refer to in wastewater treatment?
A. Conversion of nitrate to nitrogen gas
B. Conversion of ammonia to nitrate
C. Removal of phosphorus
D. Chlorine reaction with ammonia
Nitrification is a biological process where ammonia is oxidized first to
nitrite and then nitrate by bacteria.
5. Which organism group is primarily responsible for biological treatment
in activated sludge systems?
A. Fungi
, B. Bacteria
C. Protozoa
D. Viruses
Bacteria are the primary microorganisms that break down organic
matter in activated sludge systems.
6. What is the purpose of return activated sludge (RAS)?
A. To remove grit
B. To disinfect effluent
C. To maintain microbial population in aeration tanks
D. To reduce pH
RAS recycles microorganisms back into the aeration tank to sustain
treatment efficiency.
7. Which gas is most commonly produced during anaerobic digestion?
A. Oxygen
B. Nitrogen
C. Methane
D. Chlorine
Methane is produced by anaerobic bacteria during the breakdown of
organic matter in oxygen-free environments.
8. What is the purpose of secondary clarifiers?
A. Chemical dosing
B. Separation of biological solids from treated wastewater
C. Grit removal
, D. pH adjustment
Secondary clarifiers settle out biological floc after aeration before
effluent discharge.
9. What does MLSS stand for?
A. Maximum Liquid Sediment Standard
B. Mixed Liquid Solid System
C. Mixed Liquor Suspended Solids
D. Microbial Loading System Standard
MLSS refers to the concentration of suspended solids in the aeration
tank, including microorganisms.
10. Which process removes nitrogen gas from wastewater?
A. Nitrification
B. Sedimentation
C. Denitrification
D. Coagulation
Denitrification converts nitrate into nitrogen gas, which is released
into the atmosphere.
11. What is the typical pH range for optimal biological treatment?
A. 2–4
B. 4–5
C. 6.5–8.5
D. 9–12
Operator Certification Exam Practice
Questions And Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A | Instant Download Pdf
1. What is the primary purpose of preliminary treatment in a wastewater
plant?
A. Biological nutrient removal
B. Removal of large debris and grit
C. Disinfection of effluent
D. Sludge digestion
The correct answer is B because preliminary treatment removes large
solids and grit to protect downstream equipment from damage and
clogging.
2. Which parameter is most commonly used to measure organic
pollution in wastewater?
A. pH
, B. Biochemical Oxygen Demand (BOD)
C. Turbidity
D. Dissolved oxygen
BOD measures the amount of oxygen required by microorganisms to
decompose organic matter, making it a key indicator of organic
pollution.
3. What is the main function of primary clarifiers?
A. Disinfection
B. Aeration
C. Settling of suspended solids
D. Chlorine removal
Primary clarifiers allow solids to settle out by gravity, reducing
suspended solids before biological treatment.
4. What does nitrification refer to in wastewater treatment?
A. Conversion of nitrate to nitrogen gas
B. Conversion of ammonia to nitrate
C. Removal of phosphorus
D. Chlorine reaction with ammonia
Nitrification is a biological process where ammonia is oxidized first to
nitrite and then nitrate by bacteria.
5. Which organism group is primarily responsible for biological treatment
in activated sludge systems?
A. Fungi
, B. Bacteria
C. Protozoa
D. Viruses
Bacteria are the primary microorganisms that break down organic
matter in activated sludge systems.
6. What is the purpose of return activated sludge (RAS)?
A. To remove grit
B. To disinfect effluent
C. To maintain microbial population in aeration tanks
D. To reduce pH
RAS recycles microorganisms back into the aeration tank to sustain
treatment efficiency.
7. Which gas is most commonly produced during anaerobic digestion?
A. Oxygen
B. Nitrogen
C. Methane
D. Chlorine
Methane is produced by anaerobic bacteria during the breakdown of
organic matter in oxygen-free environments.
8. What is the purpose of secondary clarifiers?
A. Chemical dosing
B. Separation of biological solids from treated wastewater
C. Grit removal
, D. pH adjustment
Secondary clarifiers settle out biological floc after aeration before
effluent discharge.
9. What does MLSS stand for?
A. Maximum Liquid Sediment Standard
B. Mixed Liquid Solid System
C. Mixed Liquor Suspended Solids
D. Microbial Loading System Standard
MLSS refers to the concentration of suspended solids in the aeration
tank, including microorganisms.
10. Which process removes nitrogen gas from wastewater?
A. Nitrification
B. Sedimentation
C. Denitrification
D. Coagulation
Denitrification converts nitrate into nitrogen gas, which is released
into the atmosphere.
11. What is the typical pH range for optimal biological treatment?
A. 2–4
B. 4–5
C. 6.5–8.5
D. 9–12
