Nevada Wastewater Treatment Plant
Operator Grade IV Practice Exam
Questions And Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A | Instant Download Pdf
1. Which of the following is the most common method for measuring
biochemical oxygen demand (BOD) in wastewater?
a. Chemical titration
b. Membrane filtration
c. 5-day incubation method
d. Spectrophotometry
Rationale: The 5-day BOD test (BOD₅) is standard for assessing the
oxygen demand of wastewater by microorganisms over five days at
20°C.
,2. The primary purpose of preliminary treatment in a wastewater plant is
to:
a. Remove nutrients
b. Remove large solids and debris
c. Reduce BOD
d. Disinfect wastewater
Rationale: Preliminary treatment, such as screening and grit
removal, prevents large solids from damaging downstream
equipment.
3. In activated sludge processes, the mixed liquor suspended solids
(MLSS) concentration typically ranges between:
a. 50–200 mg/L
b. 200–500 mg/L
c. 1,500–3,000 mg/L
d. 4,000–6,000 mg/L
Rationale: MLSS is maintained at levels that provide adequate
microbial population for effective treatment.
4. Which type of bacteria is primarily responsible for nitrification in
wastewater treatment?
a. Denitrifying bacteria
b. Nitrosomonas and Nitrobacter
c. Coliform bacteria
d. Sulfate-reducing bacteria
, Rationale: Nitrosomonas oxidizes ammonia to nitrite, and
Nitrobacter oxidizes nitrite to nitrate.
5. What is the minimum dissolved oxygen (DO) concentration typically
required for effective nitrification?
a. 0.5 mg/L
b. 1 mg/L
c. 2 mg/L
d. 5 mg/L
Rationale: Nitrifying bacteria require sufficient oxygen to oxidize
ammonia efficiently, generally at least 2 mg/L DO.
6. Which sludge treatment process reduces pathogens and odor while
stabilizing the sludge?
a. Sedimentation
b. Thickening
c. Anaerobic digestion
d. Screening
Rationale: Anaerobic digestion breaks down organic matter,
reducing pathogens and odor in sludge.
7. In wastewater treatment, chlorine is primarily used for:
a. Coagulation
b. Filtration
c. Disinfection
d. Aeration
, Rationale: Chlorine is a strong oxidizing agent used to kill pathogenic
microorganisms in effluent.
8. What is the typical range for effluent pH in a municipal wastewater
treatment plant?
a. 3–5
b. 5–6
c. 6.5–8.5
d. 9–11
Rationale: Effluent pH is maintained within 6.5–8.5 to prevent
corrosion, protect aquatic life, and comply with regulations.
9. In a sequencing batch reactor (SBR), which phase comes immediately
after aeration?
a. Fill
b. Aeration
c. Settling
d. Decanting
Rationale: After aeration, the mixed liquor is allowed to settle to
separate treated water from sludge.
10. The purpose of a weir in a clarifier is to:
a. Increase aeration
b. Control effluent flow and maintain uniform water level
c. Filter solids
d. Add chemicals
Operator Grade IV Practice Exam
Questions And Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A | Instant Download Pdf
1. Which of the following is the most common method for measuring
biochemical oxygen demand (BOD) in wastewater?
a. Chemical titration
b. Membrane filtration
c. 5-day incubation method
d. Spectrophotometry
Rationale: The 5-day BOD test (BOD₅) is standard for assessing the
oxygen demand of wastewater by microorganisms over five days at
20°C.
,2. The primary purpose of preliminary treatment in a wastewater plant is
to:
a. Remove nutrients
b. Remove large solids and debris
c. Reduce BOD
d. Disinfect wastewater
Rationale: Preliminary treatment, such as screening and grit
removal, prevents large solids from damaging downstream
equipment.
3. In activated sludge processes, the mixed liquor suspended solids
(MLSS) concentration typically ranges between:
a. 50–200 mg/L
b. 200–500 mg/L
c. 1,500–3,000 mg/L
d. 4,000–6,000 mg/L
Rationale: MLSS is maintained at levels that provide adequate
microbial population for effective treatment.
4. Which type of bacteria is primarily responsible for nitrification in
wastewater treatment?
a. Denitrifying bacteria
b. Nitrosomonas and Nitrobacter
c. Coliform bacteria
d. Sulfate-reducing bacteria
, Rationale: Nitrosomonas oxidizes ammonia to nitrite, and
Nitrobacter oxidizes nitrite to nitrate.
5. What is the minimum dissolved oxygen (DO) concentration typically
required for effective nitrification?
a. 0.5 mg/L
b. 1 mg/L
c. 2 mg/L
d. 5 mg/L
Rationale: Nitrifying bacteria require sufficient oxygen to oxidize
ammonia efficiently, generally at least 2 mg/L DO.
6. Which sludge treatment process reduces pathogens and odor while
stabilizing the sludge?
a. Sedimentation
b. Thickening
c. Anaerobic digestion
d. Screening
Rationale: Anaerobic digestion breaks down organic matter,
reducing pathogens and odor in sludge.
7. In wastewater treatment, chlorine is primarily used for:
a. Coagulation
b. Filtration
c. Disinfection
d. Aeration
, Rationale: Chlorine is a strong oxidizing agent used to kill pathogenic
microorganisms in effluent.
8. What is the typical range for effluent pH in a municipal wastewater
treatment plant?
a. 3–5
b. 5–6
c. 6.5–8.5
d. 9–11
Rationale: Effluent pH is maintained within 6.5–8.5 to prevent
corrosion, protect aquatic life, and comply with regulations.
9. In a sequencing batch reactor (SBR), which phase comes immediately
after aeration?
a. Fill
b. Aeration
c. Settling
d. Decanting
Rationale: After aeration, the mixed liquor is allowed to settle to
separate treated water from sludge.
10. The purpose of a weir in a clarifier is to:
a. Increase aeration
b. Control effluent flow and maintain uniform water level
c. Filter solids
d. Add chemicals
