ABC Wastewater Treatment Operator Class II Exam | Latest Verified Questions and Detailed Answers

1|Page



ABC Wastewater Treatment Operator Class II Exam | Latest
Verified Questions and Detailed Answers

OVERVIEW DESCRIPTION
This Comprehensive set of multiple choice questions is designed for the ABC
Wastewater Treatment Operator Class II Exam. The exam assesses an operator’s
intermediate-level ability to manage, monitor, and evaluate wastewater treatment
processes, equipment, and laboratory analyses. Content is organized into four key
areas: Treatment Process Monitoring, Evaluation & Adjustment; Equipment Operation,
Evaluation & Maintenance; Security, Safety, Compliance & Administration; and
Laboratory Analysis & Sampling. Questions span Recall, Application, and Analysis
complexity levels, ensuring thorough preparation for the certification examination
administered by the Association of Boards of Certification (ABC).


1. Treatment Process: Monitoring, Evaluation & Adjustment (75
Questions)

QUESTION 1
What is the typical detention time range for a primary clarifier?
A. 0.5 – 1 hour
B. 1 – 2 hours
C. 4 – 6 hours
D. 8 – 12 hours
CORRECT ANSWER: B
EXPERT RATIONALE: Primary clarifiers are designed for 1-2 hours of detention to allow
settleable solids to separate. Longer times risk septic conditions, shorter times reduce
removal efficiency.

QUESTION 2
An operator increases the waste activated sludge (WAS) rate. Which parameter is most
directly reduced?
A. Aeration tank dissolved oxygen

,2|Page


B. Secondary clarifier sludge blanket depth
C. Mean cell residence time (MCRT)
D. Return activated sludge flow
CORRECT ANSWER: C
EXPERT RATIONALE: Wasting sludge removes biomass, shortening the MCRT (sludge
age). Higher WAS rate decreases the solids inventory in the system.

QUESTION 3
Which growth type is exhibited by a trickling filter?
A. Suspended growth
B. Attached growth
C. Facultative growth
D. Anaerobic granular growth
CORRECT ANSWER: B
EXPERT RATIONALE: Microorganisms grow as a biofilm attached to media (rock or
plastic) in a trickling filter. Suspended growth processes, like activated sludge, keep
biomass in suspension.

QUESTION 4
What is the F/M ratio for an aeration tank with 2500 mg/L MLVSS, volume 0.5 MG,
inflow 1.0 MGD, and BOD 200 mg/L?
A. 0.16
B. 0.32
C. 0.48
D. 0.80
CORRECT ANSWER: A
EXPERT RATIONALE: BOD load = 1.0 × 200 × 8.34 = 1668 lb/day; MLVSS mass = 0.5 ×
2500 × 8.34 = 10,425 lb; F/M = ≈ 0.16.

QUESTION 5
A sludge volume index (SVI) of 180 mL/g typically indicates:
A. Young, poorly settling sludge

,3|Page


B. Old, well-settling sludge
C. Normal activated sludge
D. Inorganic sludge
CORRECT ANSWER: A
EXPERT RATIONALE: SVI > 150 mL/g suggests filamentous or bulking sludge that
settles poorly. An SVI of 80-120 is considered good settling.

QUESTION 6
In an oxidation ditch, what is the primary function of the rotors?
A. Primary clarification
B. Aeration and mixing
C. Sludge dewatering
D. Chemical addition
CORRECT ANSWER: B
EXPERT RATIONALE: Rotors or brushes simultaneously aerate and maintain horizontal
velocity to keep mixed liquor in suspension. They provide both oxygen transfer and
mixing.

QUESTION 7
Nitrification in activated sludge is most efficient when:
A. DO < 1 mg/L and temperature < 10°C
B. DO > 2 mg/L and temperature > 15°C
C. pH < 6.0
D. High F/M ratio
CORRECT ANSWER: B
EXPERT RATIONALE: Nitrifying bacteria require sufficient dissolved oxygen (>2 mg/L)
and warmer temperatures (>15°C) for optimal growth. Low DO or pH inhibits
nitrification.

QUESTION 8
A sequencing batch reactor (SBR) operates through a cycle. Which phase immediately
follows the “React” phase?

, 4|Page


A. Fill
B. Settle
C. Idle
D. Decant
CORRECT ANSWER: B
EXPERT RATIONALE: The typical SBR cycle is Fill, React, Settle, Decant, and sometimes
Idle. After biological reaction, mixed liquor is allowed to settle before decanting.

QUESTION 9
What is the main purpose of the return activated sludge (RAS) flow?
A. To remove phosphorus
B. To maintain biomass inventory in the aeration tank
C. To increase waste sludge concentration
D. To dilute influent BOD
CORRECT ANSWER: B
EXPERT RATIONALE: RAS returns settled biomass from the secondary clarifier to the
aeration tank to sustain the desired MLSS concentration and F/M ratio.

QUESTION 10
A primary clarifier removes approximately what percentage of influent TSS?
A. 10-20%
B. 30-40%
C. 50-70%
D. 90-95%
CORRECT ANSWER: C
EXPERT RATIONALE: Primary sedimentation typically removes 50-70% of settleable
solids and about 30-40% of BOD. Overall TSS removal is in the 50-70% range.

QUESTION 11
The weir overflow rate for a secondary clarifier is too high. This condition most likely
causes:
A. Improved sludge settling