Wastewater Treatment Plant Operator Test
Bank and Study Guide 2026/2027– Complete
Exam-Style Questions with Correct Answers &
Detailed Rationales | Verified & Reliable
Question 1
A wastewater treatment plant receives an average flow of 2.5
MGD. The influent BOD₅ concentration is 220 mg/L, and the
primary effluent BOD₅ concentration is 165 mg/L.
Approximately what percentage of the influent BOD₅ is
removed in primary treatment?
A. 15%
B. 20%
C. 25%
D. 33%
Answer: C. 25%
The BOD₅ removal is calculated as [(influent − effluent) ÷
influent] × 100. Thus, [(220 − 165) ÷ 220] × 100 = 25%. Flow is
not required for calculating percentage removal when both
concentrations are measured on the same basis.
,Question 2
Which treatment unit is primarily responsible for removing
settleable organic and inorganic solids before biological
treatment?
A. Secondary clarifier
B. Primary clarifier
C. Chlorine contact tank
D. Aeration basin
Answer: B. Primary clarifier
Primary clarification removes settleable solids through gravity
sedimentation before the wastewater enters biological
treatment. Secondary clarifiers primarily separate biological
solids from treated wastewater after the biological process.
Question 3
An activated-sludge process has 4,000 mg/L MLSS in a 1.2-
million-gallon aeration basin. Approximately how many pounds
of MLSS are contained in the basin?
A. 4,000 lb
B. 20,000 lb
C. 40,032 lb
D. 48,000 lb
,Answer: C. 40,032 lb
The standard conversion is pounds = MGD × mg/L × 8.34.
Therefore, 1.2 × 4,000 × 8.34 = 40,032 lb. This calculation is
useful when determining solids inventories and sludge-age-
related operating parameters.
Question 4
Which condition most directly favors filamentous bulking in a
conventional activated-sludge system?
A. Excessively high dissolved oxygen with very short sludge age
B. Low dissolved oxygen combined with conditions favorable
to filamentous organisms
C. Complete absence of suspended solids
D. Excessively high chlorine residual in the aeration basin
Answer: B. Low dissolved oxygen combined with conditions
favorable to filamentous organisms
Filamentous organisms can gain a competitive advantage under
certain low-DO, nutrient-imbalanced, septic, or readily
biodegradable-substrate conditions. Bulking is characterized by
poor sludge settling and an elevated sludge volume index (SVI),
although the specific cause must be determined from process
data and microscopic examination.
, Question 5
A secondary clarifier has a sludge blanket that is steadily rising
while effluent TSS is increasing. Which operational response
should be considered first?
A. Increase chlorine dosage immediately
B. Stop all return activated sludge pumping
C. Evaluate solids loading, return activated sludge flow,
wasting, and sludge settling characteristics
D. Increase influent screening
Answer: C. Evaluate solids loading, return activated sludge
flow, wasting, and sludge settling characteristics
Rising sludge blankets can result from excessive solids
inventory, poor settling, inadequate return sludge removal,
hydraulic overload, or other process problems. Operators should
evaluate the relevant process measurements before selecting a
corrective adjustment rather than changing disinfection or
unrelated preliminary-treatment equipment.
Question 6
What is the primary biological function of nitrifying bacteria in
an activated-sludge process?
Bank and Study Guide 2026/2027– Complete
Exam-Style Questions with Correct Answers &
Detailed Rationales | Verified & Reliable
Question 1
A wastewater treatment plant receives an average flow of 2.5
MGD. The influent BOD₅ concentration is 220 mg/L, and the
primary effluent BOD₅ concentration is 165 mg/L.
Approximately what percentage of the influent BOD₅ is
removed in primary treatment?
A. 15%
B. 20%
C. 25%
D. 33%
Answer: C. 25%
The BOD₅ removal is calculated as [(influent − effluent) ÷
influent] × 100. Thus, [(220 − 165) ÷ 220] × 100 = 25%. Flow is
not required for calculating percentage removal when both
concentrations are measured on the same basis.
,Question 2
Which treatment unit is primarily responsible for removing
settleable organic and inorganic solids before biological
treatment?
A. Secondary clarifier
B. Primary clarifier
C. Chlorine contact tank
D. Aeration basin
Answer: B. Primary clarifier
Primary clarification removes settleable solids through gravity
sedimentation before the wastewater enters biological
treatment. Secondary clarifiers primarily separate biological
solids from treated wastewater after the biological process.
Question 3
An activated-sludge process has 4,000 mg/L MLSS in a 1.2-
million-gallon aeration basin. Approximately how many pounds
of MLSS are contained in the basin?
A. 4,000 lb
B. 20,000 lb
C. 40,032 lb
D. 48,000 lb
,Answer: C. 40,032 lb
The standard conversion is pounds = MGD × mg/L × 8.34.
Therefore, 1.2 × 4,000 × 8.34 = 40,032 lb. This calculation is
useful when determining solids inventories and sludge-age-
related operating parameters.
Question 4
Which condition most directly favors filamentous bulking in a
conventional activated-sludge system?
A. Excessively high dissolved oxygen with very short sludge age
B. Low dissolved oxygen combined with conditions favorable
to filamentous organisms
C. Complete absence of suspended solids
D. Excessively high chlorine residual in the aeration basin
Answer: B. Low dissolved oxygen combined with conditions
favorable to filamentous organisms
Filamentous organisms can gain a competitive advantage under
certain low-DO, nutrient-imbalanced, septic, or readily
biodegradable-substrate conditions. Bulking is characterized by
poor sludge settling and an elevated sludge volume index (SVI),
although the specific cause must be determined from process
data and microscopic examination.
, Question 5
A secondary clarifier has a sludge blanket that is steadily rising
while effluent TSS is increasing. Which operational response
should be considered first?
A. Increase chlorine dosage immediately
B. Stop all return activated sludge pumping
C. Evaluate solids loading, return activated sludge flow,
wasting, and sludge settling characteristics
D. Increase influent screening
Answer: C. Evaluate solids loading, return activated sludge
flow, wasting, and sludge settling characteristics
Rising sludge blankets can result from excessive solids
inventory, poor settling, inadequate return sludge removal,
hydraulic overload, or other process problems. Operators should
evaluate the relevant process measurements before selecting a
corrective adjustment rather than changing disinfection or
unrelated preliminary-treatment equipment.
Question 6
What is the primary biological function of nitrifying bacteria in
an activated-sludge process?
