Basic Wastewater Operator Practice Exam Exam 2025/2026(Questions with 100% Verified Correct Answers) for Guaranteed pass.

Basic Wastewater Operator Practice Exam Exam
2025/2026(Questions with 100% Verified Correct
Answers) for Guaranteed pass.




BASIC WASTEWATER OPERATOR

LONG • MOST TESTED • MOST TOUGH PRACTICE EXAM



QUESTION 1 – Process Flow Understanding (EXTREMELY TESTED)

A wastewater treatment plant receives raw sewage containing rags, sand, food waste, organic matter,
and disease-causing organisms. The plant includes bar screens, a grit chamber, primary clarifiers,
aeration tanks, secondary clarifiers, and chlorination.

Which sequence BEST describes the correct order of treatment?

A. Screening → Aeration → Grit Removal → Clarification → Disinfection
B. Screening → Grit Removal → Primary Clarification → Secondary Treatment → Disinfection
C. Grit Removal → Screening → Disinfection → Aeration → Clarification
D. Screening → Primary Clarification → Grit Removal → Aeration → Disinfection

✅ Correct Answer: B

Explanation:
The correct flow is:

1. Screening – removes large debris

2. Grit removal – removes sand/gravel

3. Primary clarification – removes settleable solids

4. Secondary treatment – biological removal of BOD

5. Disinfection – kills pathogens

This question tests process knowledge, not memorization.

,QUESTION 2 – BOD Meaning (VERY COMMON TRAP)

A lab report shows influent BOD of 260 mg/L and effluent BOD of 25 mg/L. A new operator states that
BOD measures how much oxygen is present in wastewater.

Which statement BEST corrects the operator?

A. BOD measures total solids in wastewater
B. BOD measures oxygen added during aeration
C. BOD measures oxygen required by microorganisms to break down organic matter
D. BOD measures how much chlorine is needed

✅ Correct Answer: C

Explanation:
BOD = Biochemical Oxygen Demand
It measures the oxygen microorganisms need, not oxygen in the water.



QUESTION 3 – Activated Sludge Purpose (MOST TESTED BIOLOGY QUESTION)

In the aeration tank, air is supplied continuously and mixed liquor appears brown and well-mixed.

What is the PRIMARY purpose of the aeration process?

A. To disinfect wastewater
B. To keep solids from settling
C. To supply oxygen so microorganisms can consume organic waste
D. To remove grit

✅ Correct Answer: C

Explanation:
Aeration provides oxygen for aerobic bacteria so they can consume BOD.



QUESTION 4 – Low Dissolved Oxygen (TOUGH OPERATOR DECISION)

An operator notices strong septic odors, dark mixed liquor, and a drop in dissolved oxygen below 1.0
mg/L in the aeration basin.

What is the MOST likely problem?

A. Excess chlorine
B. Too much grit
C. Insufficient aeration
D. Too much sludge wasting

✅ Correct Answer: C

,Explanation:
Low DO causes poor biological treatment, odors, and possible process failure.



QUESTION 5 – Secondary Clarifier Failure (VERY COMMON EXAM SCENARIO)

After heavy rainfall, the plant experiences cloudy effluent and increased solids leaving the secondary
clarifier.

What is the MOST likely cause?

A. Toxic shock
B. Low pH
C. Hydraulic overloading
D. Excess chlorine

✅ Correct Answer: C

Explanation:
Rain increases flow, causing solids washout due to hydraulic overload.



QUESTION 6 – RAS Purpose (FREQUENTLY MISUNDERSTOOD)

Why is Return Activated Sludge (RAS) pumped back to the aeration tank?

A. To reduce flow
B. To increase chlorine contact time
C. To maintain a healthy population of microorganisms
D. To remove grit

✅ Correct Answer: C

Explanation:
RAS keeps microorganisms in the system, allowing continuous treatment.



QUESTION 7 – Disinfection Reality Check (TRICK QUESTION)

Effluent chlorine residual is high, but fecal coliform counts still exceed permit limits.

What is the MOST likely reason?

A. Too much chlorine
B. Inadequate contact time
C. Low dissolved oxygen
D. High grit levels

✅ Correct Answer: B

, Explanation:
Disinfection effectiveness depends on CT (Concentration × Time), not just chlorine level.



QUESTION 8 – Sludge Bulking (HIGH-FAIL QUESTION)

An operator observes fluffy sludge, poor settling, and rising sludge blankets in the secondary clarifier.

What is the MOST likely cause?

A. Excess dissolved oxygen
B. Filamentous bacteria growth
C. Too much grit
D. Low chlorine dose

✅ Correct Answer: B

Explanation:
Filamentous bacteria cause bulking sludge and poor settling.



QUESTION 9 – Safety & Confined Spaces (ALWAYS TESTED)

Before entering a wet well, which action is MOST important?

A. Turn off pumps
B. Test the atmosphere
C. Wear rubber boots
D. Bring tools

✅ Correct Answer: B

Explanation:
Confined spaces may contain hydrogen sulfide, methane, or low oxygen.
Atmospheric testing is critical to survival.



QUESTION 10 – Hydrogen Sulfide Danger (EXAM FAVORITE)

Why is hydrogen sulfide especially dangerous?

A. It smells pleasant
B. It is lighter than air
C. It deadens the sense of smell at high concentrations
D. It improves treatment efficiency

✅ Correct Answer: C