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Maryland Wastewater Treatment Operator Class 4
Practice ACTUAL EXAM QUESTIONS AND CORRECT
DETAILED ANSWERS LATEST UPDATE THIS YEAR
The exam covers: Activated sludge process control, sedimentation, disinfection (chlorine/UV),
solids handling (digestion, dewatering), nutrient removal (nitrogen/phosphorus), laboratory
procedures (BOD, TSS, DO, microscopy), safety (confined space, H2S, lockout/tagout), pumps
& hydraulics, Maryland specific regulations (MDE permits, Chesapeake Bay nutrient caps),
collection systems, and calculations (detention time, SRT, F/M, wasting rates).
Section 1: Activated Sludge Process Control (Q 1-30)
1. (Coverage: Activated sludge F/M ratio control)
The F/M ratio in activated sludge is primarily controlled by:
a) Influent flow rate
b) Waste activated sludge (WAS) rate
c) Return activated sludge (RAS) rate
d) Aeration basin DO concentration
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Correct Answer: b) Waste activated sludge (WAS) rate
Rationale: Wasting removes biomass, directly lowering MLSS and increasing F/M. Influent flow
is variable, RAS affects solids inventory but wasting is the direct control lever.
2. (Coverage: SRT calculation and operational meaning)
If a plant has 10,000 lbs MLSS in the aeration basin and wastes 500 lbs/day of solids, the SRT is:
a) 5 days
b) 10 days
c) 20 days
d) 50 days
Correct Answer: c) 20 days
Rationale: SRT = Total MLSS (lbs) ÷ WAS (lbs/day) = 10,000 ÷ 500 = 20 days. Longer SRT means
older sludge.
3. (Coverage: Microscopic indicators of sludge health)
Which protozoa indicate a healthy, well-operating activated sludge with long SRT?
a) Flagellates
b) Free-swimming ciliates
c) Stalked ciliates (e.g., Vorticella)
d) Amoebae
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Correct Answer: c) Stalked ciliates
Rationale: Stalked ciliates dominate in low F/M, stable sludge. Flagellates appear in high F/M
(young sludge). Free-swimmers indicate intermediate conditions.
4. (Coverage: Filamentous bulking causes and remedies)
The most common cause of filamentous bulking (high SVI) in conventional activated sludge is:
a) Low DO (<0.5 mg/L)
b) High F/M ratio
c) Low temperature only
d) High pH
Correct Answer: a) Low DO (<0.5 mg/L)
Rationale: Low dissolved oxygen favors filamentous organisms like Sphaerotilus natans.
Maintaining DO >1.5 mg/L prevents this.
5. (Coverage: Sludge Volume Index interpretation)
An SVI of 180 mL/g indicates:
a) Excellent settling
b) Good settling
c) Poor settling (bulking)
d) Very old sludge
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Correct Answer: c) Poor settling (bulking)
Rationale: SVI <100 = good, 100-150 = fair, >150 = bulking (filaments). High SVI causes solids
washout.
6. (Coverage: RAS rate adjustment for blanket control)
If the secondary clarifier sludge blanket is rising rapidly, the operator should FIRST:
a) Increase RAS rate
b) Decrease RAS rate
c) Increase wasting rate
d) Turn off aeration
Correct Answer: a) Increase RAS rate
Rationale: Increasing RAS removes solids faster from the clarifier, lowering blanket depth.
Wasting is a longer-term control.
7. (Coverage: Denitrification in final clarifiers)
Floating sludge in a secondary clarifier is often caused by:
a) High DO in the clarifier
b) Denitrification producing nitrogen gas
c) Low influent BOD
d) Excessive polymer addition
Maryland Wastewater Treatment Operator Class 4
Practice ACTUAL EXAM QUESTIONS AND CORRECT
DETAILED ANSWERS LATEST UPDATE THIS YEAR
The exam covers: Activated sludge process control, sedimentation, disinfection (chlorine/UV),
solids handling (digestion, dewatering), nutrient removal (nitrogen/phosphorus), laboratory
procedures (BOD, TSS, DO, microscopy), safety (confined space, H2S, lockout/tagout), pumps
& hydraulics, Maryland specific regulations (MDE permits, Chesapeake Bay nutrient caps),
collection systems, and calculations (detention time, SRT, F/M, wasting rates).
Section 1: Activated Sludge Process Control (Q 1-30)
1. (Coverage: Activated sludge F/M ratio control)
The F/M ratio in activated sludge is primarily controlled by:
a) Influent flow rate
b) Waste activated sludge (WAS) rate
c) Return activated sludge (RAS) rate
d) Aeration basin DO concentration
, Page 2 of 98
Correct Answer: b) Waste activated sludge (WAS) rate
Rationale: Wasting removes biomass, directly lowering MLSS and increasing F/M. Influent flow
is variable, RAS affects solids inventory but wasting is the direct control lever.
2. (Coverage: SRT calculation and operational meaning)
If a plant has 10,000 lbs MLSS in the aeration basin and wastes 500 lbs/day of solids, the SRT is:
a) 5 days
b) 10 days
c) 20 days
d) 50 days
Correct Answer: c) 20 days
Rationale: SRT = Total MLSS (lbs) ÷ WAS (lbs/day) = 10,000 ÷ 500 = 20 days. Longer SRT means
older sludge.
3. (Coverage: Microscopic indicators of sludge health)
Which protozoa indicate a healthy, well-operating activated sludge with long SRT?
a) Flagellates
b) Free-swimming ciliates
c) Stalked ciliates (e.g., Vorticella)
d) Amoebae
, Page 3 of 98
Correct Answer: c) Stalked ciliates
Rationale: Stalked ciliates dominate in low F/M, stable sludge. Flagellates appear in high F/M
(young sludge). Free-swimmers indicate intermediate conditions.
4. (Coverage: Filamentous bulking causes and remedies)
The most common cause of filamentous bulking (high SVI) in conventional activated sludge is:
a) Low DO (<0.5 mg/L)
b) High F/M ratio
c) Low temperature only
d) High pH
Correct Answer: a) Low DO (<0.5 mg/L)
Rationale: Low dissolved oxygen favors filamentous organisms like Sphaerotilus natans.
Maintaining DO >1.5 mg/L prevents this.
5. (Coverage: Sludge Volume Index interpretation)
An SVI of 180 mL/g indicates:
a) Excellent settling
b) Good settling
c) Poor settling (bulking)
d) Very old sludge
, Page 4 of 98
Correct Answer: c) Poor settling (bulking)
Rationale: SVI <100 = good, 100-150 = fair, >150 = bulking (filaments). High SVI causes solids
washout.
6. (Coverage: RAS rate adjustment for blanket control)
If the secondary clarifier sludge blanket is rising rapidly, the operator should FIRST:
a) Increase RAS rate
b) Decrease RAS rate
c) Increase wasting rate
d) Turn off aeration
Correct Answer: a) Increase RAS rate
Rationale: Increasing RAS removes solids faster from the clarifier, lowering blanket depth.
Wasting is a longer-term control.
7. (Coverage: Denitrification in final clarifiers)
Floating sludge in a secondary clarifier is often caused by:
a) High DO in the clarifier
b) Denitrification producing nitrogen gas
c) Low influent BOD
d) Excessive polymer addition
