New Jersey Wastewater Treatment
Operator S2 Exam Practice Questions
And Correct Answers (Verified Answers)
Plus Rationales 2026 Q&A | Instant
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Topic 1: Activated Sludge Process (Questions 1-15)
Question 1: In an activated sludge system, what does the Sludge Volume Index
(SVI) measure?
A. The concentration of dissolved oxygen in the aeration basin
B. The settleability and compaction characteristics of activated sludge
C. The amount of organic matter entering the system
D. The pH of the mixed liquor
Correct ,,,answer,,,: B. The settleability and compaction characteristics of
activated sludge
Rationale: SVI indicates how well sludge settles and compacts in the secondary
clarifier. An SVI below 100 indicates good settling; above 150 indicates bulking
sludge. This is critical for proper return activated sludge (RAS) management.
Question 2: Which biological process is primarily responsible for removing
nitrogen in wastewater treatment?
,A. Coagulation and flocculation
B. Nitrification and denitrification
C. Anaerobic digestion only
D. Phosphorus precipitation
Correct ,,,answer,,,: B. Nitrification and denitrification
Rationale: Nitrification converts ammonia to nitrate (aerobic), and denitrification
reduces nitrate to nitrogen gas (anoxic). These sequential biological processes
remove nitrogen from wastewater.
Question 3: What is the primary purpose of the aeration basin in an activated
sludge system?
A. To settle out solids before discharge
B. To provide oxygen for microorganisms that consume organic matter
C. To disinfect the effluent
D. To remove grit and sand
Correct ,,,answer,,,: B. To provide oxygen for microorganisms that consume
organic matter
Rationale: The aeration basin supplies dissolved oxygen (typically 1-3 mg/L) to
aerobic bacteria that metabolize organic pollutants (BOD/COD). Without adequate
oxygen, the system becomes septic and treatment fails.
Question 4: What does F/M ratio stand for in activated sludge process control?
A. Flow to Mixed Liquor ratio
B. Food to Microorganism ratio
,C. Filamentous to Microorganism ratio
D. Floc to Mixed Liquor ratio
Correct ,,,answer,,,: B. Food to Microorganism ratio
Rationale: F/M ratio = (BOD entering aeration basin) ÷ (MLVSS in aeration
basin). A low F/M (<0.2) can cause pin floc and poor settling; high F/M (>0.6) can
cause dispersed growth and turbid effluent.
Question 5: How is Sludge Volume Index (SVI) calculated?
A. MLSS ÷ Settled sludge volume after 30 min × 1000
B. Settled sludge volume after 30 min (mL/L) ÷ MLSS (mg/L) × 1000
C. MLVSS ÷ RAS concentration × 100
D. BOD ÷ MLSS × 1000
Correct ,,,answer,,,: B. Settled sludge volume after 30 min (mL/L) ÷ MLSS
(mg/L) × 1000
Rationale: SVI = (Settled sludge volume in mL/L after 30 min) × 1000 ÷ MLSS
(mg/L). Normal SVI range is 80-120. High SVI indicates bulking sludge
(filamentous organisms).
Question 6: What is a common operational problem in activated sludge systems
indicated by dispersed growth in the effluent?
A. Too high F/M ratio (overloaded system)
B. Too low F/M ratio (underloaded system)
C. Excessive sludge wasting
D. Low dissolved oxygen only
, Correct ,,,answer,,,: A. Too high F/M ratio (overloaded system)
Rationale: Dispersed growth (turbid effluent with fine, non-settling floc) occurs
when the system is hydraulically or organically overloaded, causing high F/M ratio
and insufficient time for floc formation.
Question 7: The activated sludge process falls under which category of wastewater
treatment?
A. Primary treatment
B. Secondary treatment (biological)
C. Tertiary treatment
D. Preliminary treatment
Correct ,,,answer,,,: B. Secondary treatment (biological)
Rationale: Secondary treatment removes soluble and colloidal organic matter
using biological processes (activated sludge, trickling filters, RBCs). Primary
treatment is physical sedimentation.
Question 8: In an activated sludge system, what does the term "bulking sludge"
refer to?
A. Sludge that is too dense and settles too rapidly
B. Sludge that contains excessive filamentous bacteria and settles poorly
C. Sludge that has been properly wasted to digestion
D. Sludge with optimal SVI between 80-120
Correct ,,,answer,,,: B. Sludge that contains excessive filamentous bacteria and
settles poorly
Operator S2 Exam Practice Questions
And Correct Answers (Verified Answers)
Plus Rationales 2026 Q&A | Instant
Download Pdf
Topic 1: Activated Sludge Process (Questions 1-15)
Question 1: In an activated sludge system, what does the Sludge Volume Index
(SVI) measure?
A. The concentration of dissolved oxygen in the aeration basin
B. The settleability and compaction characteristics of activated sludge
C. The amount of organic matter entering the system
D. The pH of the mixed liquor
Correct ,,,answer,,,: B. The settleability and compaction characteristics of
activated sludge
Rationale: SVI indicates how well sludge settles and compacts in the secondary
clarifier. An SVI below 100 indicates good settling; above 150 indicates bulking
sludge. This is critical for proper return activated sludge (RAS) management.
Question 2: Which biological process is primarily responsible for removing
nitrogen in wastewater treatment?
,A. Coagulation and flocculation
B. Nitrification and denitrification
C. Anaerobic digestion only
D. Phosphorus precipitation
Correct ,,,answer,,,: B. Nitrification and denitrification
Rationale: Nitrification converts ammonia to nitrate (aerobic), and denitrification
reduces nitrate to nitrogen gas (anoxic). These sequential biological processes
remove nitrogen from wastewater.
Question 3: What is the primary purpose of the aeration basin in an activated
sludge system?
A. To settle out solids before discharge
B. To provide oxygen for microorganisms that consume organic matter
C. To disinfect the effluent
D. To remove grit and sand
Correct ,,,answer,,,: B. To provide oxygen for microorganisms that consume
organic matter
Rationale: The aeration basin supplies dissolved oxygen (typically 1-3 mg/L) to
aerobic bacteria that metabolize organic pollutants (BOD/COD). Without adequate
oxygen, the system becomes septic and treatment fails.
Question 4: What does F/M ratio stand for in activated sludge process control?
A. Flow to Mixed Liquor ratio
B. Food to Microorganism ratio
,C. Filamentous to Microorganism ratio
D. Floc to Mixed Liquor ratio
Correct ,,,answer,,,: B. Food to Microorganism ratio
Rationale: F/M ratio = (BOD entering aeration basin) ÷ (MLVSS in aeration
basin). A low F/M (<0.2) can cause pin floc and poor settling; high F/M (>0.6) can
cause dispersed growth and turbid effluent.
Question 5: How is Sludge Volume Index (SVI) calculated?
A. MLSS ÷ Settled sludge volume after 30 min × 1000
B. Settled sludge volume after 30 min (mL/L) ÷ MLSS (mg/L) × 1000
C. MLVSS ÷ RAS concentration × 100
D. BOD ÷ MLSS × 1000
Correct ,,,answer,,,: B. Settled sludge volume after 30 min (mL/L) ÷ MLSS
(mg/L) × 1000
Rationale: SVI = (Settled sludge volume in mL/L after 30 min) × 1000 ÷ MLSS
(mg/L). Normal SVI range is 80-120. High SVI indicates bulking sludge
(filamentous organisms).
Question 6: What is a common operational problem in activated sludge systems
indicated by dispersed growth in the effluent?
A. Too high F/M ratio (overloaded system)
B. Too low F/M ratio (underloaded system)
C. Excessive sludge wasting
D. Low dissolved oxygen only
, Correct ,,,answer,,,: A. Too high F/M ratio (overloaded system)
Rationale: Dispersed growth (turbid effluent with fine, non-settling floc) occurs
when the system is hydraulically or organically overloaded, causing high F/M ratio
and insufficient time for floc formation.
Question 7: The activated sludge process falls under which category of wastewater
treatment?
A. Primary treatment
B. Secondary treatment (biological)
C. Tertiary treatment
D. Preliminary treatment
Correct ,,,answer,,,: B. Secondary treatment (biological)
Rationale: Secondary treatment removes soluble and colloidal organic matter
using biological processes (activated sludge, trickling filters, RBCs). Primary
treatment is physical sedimentation.
Question 8: In an activated sludge system, what does the term "bulking sludge"
refer to?
A. Sludge that is too dense and settles too rapidly
B. Sludge that contains excessive filamentous bacteria and settles poorly
C. Sludge that has been properly wasted to digestion
D. Sludge with optimal SVI between 80-120
Correct ,,,answer,,,: B. Sludge that contains excessive filamentous bacteria and
settles poorly
